Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Gold and cadmium selenide (CdSe) nanoparticles capped with oligothiophenes Sih, Bryan Christian 2007

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-ubc_2007-319291.pdf [ 19.46MB ]
Metadata
JSON: 831-1.0059686.json
JSON-LD: 831-1.0059686-ld.json
RDF/XML (Pretty): 831-1.0059686-rdf.xml
RDF/JSON: 831-1.0059686-rdf.json
Turtle: 831-1.0059686-turtle.txt
N-Triples: 831-1.0059686-rdf-ntriples.txt
Original Record: 831-1.0059686-source.json
Full Text
831-1.0059686-fulltext.txt
Citation
831-1.0059686.ris

Full Text

GOLD AND CADMIUM SELENIDE (CdSe) NANOPARTICLES CAPPED WITH OLIGOTHIOPHENES by B R Y A N CHRISTIAN SIH B.Sc, Chemistry Simon Fraser University, 2002 A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in The Faculty of Graduate Studies (CHEMISTRY) THE UNIVERSITY OF BRITISH COLUMBIA JULY 2007 © Bryan Christian Sih, 2007 Abstract The preparation and characterization of hybrid materials composed of oligothiophene-capped Au and CdSe nanoparticles with novel chemical, structural, electronic and optical properties are reported. a-Phosphino-oligothiophenes (12-15 and 23) and thiol-substituted oligothiophenes (26, 29, 32) were prepared by metal-catalyzed coupling reactions and studied using absorption and emission spectroscopy, and cyclic voltammetry. These functionalized oligothiophenes were used to passivate the surface of Au (16-19) and CdSe (CdSe-26, CdSe-29, CdSe-32) nanoparticles. Oligothiophene-capped Au nanoparticles were prepared directly by reducing a Au salt in the presence of the phosphino-oligothiophene. Attachment to the A u nanoparticles has little effect on the electronic structure of the oligothiophene as determined from the absorption spectra. On the other hand, the oligothiophenes appear to affect the electronic structure of the Au nanoparticle, as observed via a red-shift in the surface plasmon absorption. Electrochemical oxidation of the phosphino-terthiophene capped Au nanoparticles lead to crosslinking where the nanoparticles are linked both structurally and electronically by observed increases in conjugation, conductivity and plasmon coupling relative to the unlinked particles. The oligothiophene bridge linking the Au nanoparticles is shown to facilitate plasmon coupling between adjacent nanoparticles. The crosslinked material also demonstrates tunable conductivity where the conductivity in the material can be increased by oxidative doping of the 7r-conjugated bridge. Oligothiophene-capped CdSe nanoparticles were prepared through an exchange reaction between thiol-substituted oligothiophenes and trioctylphosphine oxide-capped CdSe nanoparticles. Attachment of the oligothiophenes to the CdSe nanoparticle has little effect on ii the electronic structure of the oligothiophene as determined from the absorption spectra. However, the optical properties are significantly affected where the oligothiophene emission is quenched after attachment to the CdSe surface due to either an energy or electron transfer mechanism. Depending on the number of oligothiophenes attached to the CdSe surface, the optical properties of the CdSe nanoparticles are affected differently. An excess number of thiols act as hole traps leading to quenching of the nanoparticle emission. Attempts to electrochemically crosslink these oligothiophene-capped CdSe nanoparticles were unsuccessful possibly due to the intrinsic resistivity in the particles. C6H-13 23 x = 0, 26 x = 1,29 x = 2,32 in x = 0, CdSe-26 x = 1, CdSe-29 x = 2, CdSe-32 iv Table of Contents A B S T R A C T ii T A B L E OF CONTENTS v LIST OF T A B L E S viii LIST OF FIGURES ix LIST OF SCHEMES xv LIST OF S Y M B O L S OF ABBREVIATIONS xvi A C K N O W L E D G E M E N T S xxi CO-AUTHORSHIP STATEMENT xxiii CHAPTER 1 1 1.1 O V E R V I E W 1 1.2 TT-CONJUGATED MATERIALS 2 1.2.1 Structure of 7T-Conjugated Polymers and Oligomers 2 1.2.2 Synthesis of Poly- and Oligothiophenes 5 1.2.3 Electronic and Optical Properties of Poly- and Oligothiophenes 8 1.3 N A N O P A R T I C L E S 11 1.3.1 Defining a Nanoparticle 11 1.3.2 Properties of Metal Nanoparticles 12 1.3.3 Properties of Semiconductor Nanoparticles 14 1.4 M E T A L N A N O P A R T I C L E - C O N J U G A T E D P O L Y M E R NANOCOMPOSITES 16 1.4.1 Preparation 16 1.4.2 Electronic Properties 23 1.4.3 Optical Properties 29 1.5 SEMICONDUCTING N A N O P A R T I C L E - C O N J U G A T E D P O L Y M E R NANOCOMPOSITES 34 1.5.1 Preparation 34 1.5.2 Electronic and Optical Properties 37 1.6 G O A L S A N D SCOPE OF PRESENT S T U D Y 42 1.7 REFERENCES 44 CHAPTER 2 52 2.1 INTRODUCTION 52 2.2 E X P E R I M E N T A L 55 2.2.1 General 55 2.2.2 Synthesis 56 2.3 RESULTS 59 2.3.1 Synthesis 59 v 2.3.2 UV-vis Absorption Spectra 62 2.3.3 Transmission Electron Microscopy 65 2.3.4 X-ray Photoelectron Spectroscopy / Thermogravimetric Analysis 67 2.3.5 Determination of Average Molecular Formula 69 2.3.6 Cyclic Voltammetry 70 2.4 CONCLUSION 71 2.5 REFERENCES 73 CHAPTER 3 76 3.1 INTRODUCTION 76 3.2 E X P E R I M E N T A L : 79 3.2.1 General 79 3.2.2 Electrodeposition 80 3.2.3 Solid-State Conductivity Measurements 80 3.2.4 Microelectrode Array Fabrication 81 3.2.5 Conductivity versus Electrochemical Doping 85 3.2.6 Spectroelectrochemistry 86 3.3 RESULTS 86 3.3.1 Electrodeposition/Cyclic Voltammetry 86 3.3.2 X-ray Photoelectron Spectroscopy/Energy Dispersive X-ray Spectroscopy 90 3.3.3 Electron Microscopy 91 3.3.4 UV-vis-NIR Absorption Spectroscopy 92 3.3.5 Solid-state Conductivity 93 3.3.6 Conductivity as a Function of Electrochemical Doping 96 3.3.7 Spectroelectrochemistry 99 3.4 CONCLUSIONS 101 3.5 REFERENCES 103 CHAPTER 4 107 4.1 INTRODUCTION 107 4.1.1 Theory 108 4.2 E X P E R I M E N T A L 110 4.2.1 General 110 4.2.2 Synthesis of NPs and Electrodeposition 110 4.2.3 U V - 0 3 Irradiation of Oligothiophene-Linked Au NP Films I l l 4.2.4 Thermal Annealing of Oligothiophene-Linked Au NP Films 111 4.2.5 Varying the Dielectric Medium in Oligothiophene-Linked Au NP Films 111 4.3 RESULTS 112 4.3.1 Preparation of Oligothiophene-Linked Au NP Films 112 4.3.2 Effects of UV-O3 Irradiation on Coupled Surface Plasmons 113 4.3.3 Effects of Thermal Annealing on Coupled Surface Plasmons 116 4.3.4 Effects of Dielectric Medium on Coupled Surface Plasmons 117 4.4 DISCUSSION 121 4.5 CONCLUSIONS 123 4.6 REFERENCES 125 CHAPTER 5 127 v i 5.1 INTRODUCTION 127 5.1.1 Theory 127 5.1.2 Literature Review 132 5.2 E X P E R I M E N T A L 133 5.2.1 Sample Preparation 133 5.2.2 Kretschmann Optical Set-up for SPR Sensing 134 5.2.3 Contact Angle Measurements 136 5.3 RESULTS 136 5.3.1 SPR sensing of Electropolymerized Polythiophene 136 5.3.2 SPR sensing of Electrodeposited OT-linked Au NP Films 138 5.4 DISCUSSION 140 5.5 CONCLUSIONS 142 5.6 REFERENCES 143 CHAPTER 6 145 6.1 INTRODUCTION 145 6.2 E X P E R I M E N T A L 147 6.2.1 General 147 6.2.2 Synthesis 148 6.3 RESULTS 155 6.3.1 Synthesis 155 6.3.2 Transmission Electron Microscopy 163 6.3.3 UV-vis Absorption Spectra 164 6.3.4 Excitation and Emission Spectra 167 6.3.5 Cyclic Voltammetry 174 6.3.6 Density Functional Theory Calculations 177 6.4 DISCUSSION 179 6.5 CONCLUSIONS 180 6.6 REFERENCES 182 CHAPTER 7 186 7.1 G E N E R A L CONCLUSIONS 186 7.2 SUGGESTIONS FOR F U T U R E W O R K 188 7.3 REFERENCES 192 vii List of Tables Table 2-1 Chemical shifts of peaks in the 3 1 P - N M R spectra and UV-vis absorption maxima of phosphino-oligothiophenes (12-15) and phosphino-oligothiophenes capped AuNPs (16-19) 61 Table 2-2 XPS and T G A analysis of 16,17,18 and 19 68 Table 3-1 E D X and XPS analysis of poly-18 and poly-19 90 Table 3-2 Room temperature electrical conductivities for 18, 19, poly-18 and poly-19 96 Table 4-1 XPS (S 2p) derived relative abundance of sulfone relative to sulfur in an electrodeposited poly-18 film as a function of O3 exposure 115 Table 5-1 SPR minimum (60), thickness (d), differential reflectivity (ARMAX) and real and imaginary part of dielectric constant for the electrodeposited layer (e^ ) from fitting using Fresnel's equations before and after exposure to organic vapors 138 Table 6-1 UV-vis absorption maxima of capping groups (23, 26, 29, 32), TOPO-capped CdSe NPs (CdSe-TOPO) and oligothiophene-capped CdSe NPs (CdSe-26, CdSe-29, CdSe-32) in CHC1 3 165 Table 6-2 Excitation and emission maxima of capping groups (23, 26, 29, 32), TOPO-capped CdSe NPs (CdSe-TOPO) and oligothiophene-capped CdSe NPs (CdSe-26, CdSe-29, CdSe-32) in CHC1 3 and in the solid-state 168 viii List of Figures Figure 1-1 Several examples of 71-conjugated polymers 3 Figure 1-2 Location of the a- and P-positions on thiophene 4 Figure 1-3 Selected examples of P-substituted polythiophenes 4 Figure 1-4 The three possible regiochemical couplings in poly(3-alkylthiophene) 5 Figure 1-5 Selected example of an oxidative chemical reaction to prepare octathiophene 5 Figure 1-6 Examples of metal catalyzed cross-coupling reactions to prepare oligothiophenes 7 Figure 1-7 Evolution of the energy band diagram of polythiophene with increasing chain length 9 Figure 1-8 Structural diagrams of a (a) polaron and (b) bipolaron on a polythiophene chain. The corresponding energy band diagrams for each are also shown 10 Figure 1-9 Schematic diagram of the ideal structures for metal nanoparticles where the metal atoms are arranged in a close packed arrangement. (Reprinted from reference 44 with permission from Elsevier.) 12 Figure 1-10 Schematic diagram of a gold nanoparticle experiencing surface plasmon oscillations highlighting the displacement of the surface electrons relative to the nuclei 13 Figure 1-11 Depiction of the changes in energy levels of a semiconductor due to size 15 Figure 1-12 The simultaneous formation of gold NPs and sexithiophene units on functionalized PVP (Adapted from reference 61 with permission from the American Chemical Society) 17 Figure 1-13 Electroactive cationic polymer templates 19 Figure 1-14 Schematic diagram of conducting polymer where the metal nanoparticles are (a) surface-confined and (b) distributed throughout the polymer matrix 21 Figure 1-15 Thiophene derivatized Au nanoparticles copolymerized electrochemically with 3-«-octylthiophene 22 Figure 1-16 Bifunctional 7r-conjugated molecules used to link Au nanoparticles via self-assembly 24 ix Figure 1-17 Activation energies for covalently and non-covalently linked A U 5 5 clusters of varying inter-particle distances. (Adapted from reference 9 7 with permission from Wiley-VCH) 27 Figure 1-18 Partially conjugated linkers and their non-conjugated analogues. (Adapted from reference 9 8 ) 28 Figure 1-19 UV/vis spectra of Au sol after the addition of 1,4-phenylene diisocyanide (1,4-PDI) with a final concentration of (a) 4.0 x 10"6, (b) 1.6 x 10"5, (c) 3.2 x 10"5, and (d) 3.2 x 10"4 M . (Reprinted from reference 16 with permission from American Chemical Society) 30 Figure 1-20 Charge distribution for (a) dipole-dipole plasmon coupling, (b) quadrupolar mode due to shorting of dipole interactions and (c) strong conductive overlap causing 'dumbbell' shaped dipolar resonance 31 Figure 1-21 UV-visible spectra for dimers linked by structure 11 (upper trace) and silver particle 'monomers' (lower trace). The monomer spectrum was acquired under solvent conditions identical to those of the dimers but in the absence of a linker. (Reprinted from reference 71 with permission from American Chemical Society) 33 Figure 1-22 Schematic diagram of the self-assembly of a CdSe/PPV nanocomposite using electrostatic attraction 35 Figure 1-23 Structure of oligo-(p-phenylethynylene dibenzylthiol dipropyl ether), an example of a bifunctional conjugated crosslinker where n = 0, 1, and 3 36 Figure 1-24 Schematic energy level diagram for a semiconductor-based (a) solar cell and (b) light emitting diode 37 Figure 1-25 Energy band diagrams for MEH-PPV/CdSe blends illustrating the routes for generating electron-hole charge separation, (a) Absorption in MEH-PPV followed by electron transfer onto the nanoparticle; (b) Absorption in the polymer followed by exciton transfer to the nanoparticle and subsequent hole transfer onto the polymer; (c) Absorption in the nanoparticle followed by hole transfer onto the polymer. (Reprinted from reference 1 2 2 with permission from American Physical Society) 39 Figure 1-26 Electronic energy alignment for CdSe nanoparticles relative to (a) oligoaniline and (b) oligothiophenes (T3 = terthiophene, T5 = pentathiophene) 41 Figure 2-1 Solution UV-visible spectra of 16-19. Spectra are offset for clarity. Inset: surface plasmon absorption of 17-19 at higher concentration 64 x Figure 2-2 Transmission electron microscopy images of NPs (a) 16, (b) 17, (c) 18 and (d) 19 dropcasted onto a carbon coated T E M grid from CHCI3. The scale bar is 20 nm 66 Figure 2-3 Size distribution for NPs (a) 16, (b) 17, (c) 18 and (d) 19 determined from T E M images. 150 particles were measured. The mean diameter (dmean) for each sample is shown on the graph 67 Figure 2-4 Cyclic voltammetry of (a) 18 and (b) 19 in CH2CI2 containing 0.1 M (n-Bu) 4 NPF 6 . Scanned from -0.5 to 1.6 V , scan rate = 100 mV s"1 71 Figure 3-1 Schematic diagram of microelectrode architecture after (a) first and (b) second lithographic steps 84 Figure 3-2 Cyclic voltammetry of (a) 18 and (b) 19 in CH2CI2 containing 0.1M (n-Bu) 4 NPF 6 . Multiple scans from -0.5 to 1.6 V vs. SCE, scan rate = 100 mV s"1 88 Figure 3-3 Cyclic voltammetry of (a) poly-18 and (b) poly-19 deposited on a Pt working electrode in CH 2 C1 2 containing 0.1M («-Bu) 4NPF 6. Scan rate = 100 mV s"1 89 Figure 3-4 Transmission electron microscopy images of (a) poly-18 and (b) poly-19 91 Figure 3-5 Scanning electron micrograph of poly-18 92 Figure 3-6 UV-vis spectra of (a) 18 (in CH2CI2 and on glass) and poly-18 film and (b) 19 (in CH2CI2 and on glass) and poly-19 film 93 Figure 3-7 Current-potential response of (a) 18 and 19 at room temperature on an IDA electrode and of (b) poly-18 and poly-19 sandwiched between two electrodes 94 Figure 3-8 (a) Photograph of a fabricated microelectrode. S E M images of microelectrodes (b) before electrodeposition, (c) after 10 electrodeposition cycles with no electrical contact between microelectrodes and (d) after 20 electrodeposition cycles with an electrical link between microelectrodes. Electrodes A and B are marked on the figure 96 Figure 3-9 (a) Electrodeposition of 18 on microelectrode A and B for 5 scans. Cyclic Volatmmetry of poly-18 with 50 mV offset between microelectrode A and B (b) without electrical contact and (c) with electrical contact between electrodes, (d) Plot of poly-18 conductance between microelectrode A and B as a function of an applied electrochemical potential 98 Figure 3-10 In situ UV-vis-NIR absorption spectrum of a poly-18 film on ITO as the applied potential is (a) increased and (b) decreased, and a poly-19 film on ITO as the applied potential is (c) increased and (d) decreased 100 xi Figure 4-1 UV-Vis-NIR absorption spectra of as-deposited OT-linked poly-18 films and with exposure to U V generated ozone. Inset: Difference absorption spectra of as-grown poly-18 subtracted from absorption spectra of O3 exposed poly-18 113 Figure 4-2 (a) C Is and (b) S 2p XPS spectra of an as-deposited poly-18 film and (c) C Is and (d) S 2p XPS spectra after exposure to 1.5 h of ozone. 114 Figure 4-3 T E M images of a poly-18 film (a) as-deposited and annealed at (b) 150 °C and (c) 250 °C for 3 h 116 Figure 4-4 UV-Vis-NIR absorption spectrum of a poly-18 film annealed at varying temperatures for 3 h 117 Figure 4-5 UV-vis-NIR absorption spectra of electrodeposited (a) poly-18 and (b) poly-19 in various solvents. Schematic diagrams of the linkers are shown beside the spectra 118 Figure 4-6 Non-linear curve fits of UV-vis-NIR absorption spectra of as-deposited poly-18 in (a) air, (b) water and (c) toluene 119 Figure 4-7 Collective surface plasmon band absorption maximum (E m a x ) of a poly-18 film as a function of the static dielectric constant of the medium. The data is obtained from the non-linear square fit of the UV-vis absortion spectra 121 Figure 5-1 Schematic diagram of (a) a propagating surface plasmon (PSP) on the interface of a thin metal film and dielectric medium, and (b) Kretschmann type configuration to increase the wave vector of incident light 128 Figure 5-2 Theoretical % reflectivity (%R) vs. angle of incidence (6) plot obtained from a Kretschmann type configuration with two different values for ej 130 Figure 5-3 Reflection and refraction of a plane wave vector (k) at the boundary of two dielectric media 131 Figure 5-4 Reflection and refraction of a plane wave vector (k) for three dielectric media 132 Figure 5-5 Photograph of the Kretschmann optical set-up used for the SPR measurements constructed on an optical table in the physics lab of Prof. Jeff Young (Dept. of Physics and Astronomy, UBC) 135 Figure 5-6 Schematic diagram of the electrodeposited poly-18 in the Kretschmann type configuration fitted with a glass port for vapour sensing 135 Figure 5-7 SPR plots of reflected intensity as a function of the angle of incidence for (a) a 50 nm thick gold film (0) and a 7 nm thick PT film on a 50 nra gold film in air (•). PT on a 50 nm gold film before (•) and after (A) exposure to , (b) methanol, (c) ethanol and (d) toluene. Solid line is the theoretical fit obtained xii from Fresnel's equations with variable film thickness and dielectric constant, using a least square algorithm 137 Figure 5-8 SPR plots of reflected intensity as a function of the angle of incidence for a 50 nm thick gold film (0) , a -60 nm thick poly-18 film on a 50 nm gold film in air (•) and poly-18 film on a 50 nm gold film after exposure to selected vapors (A), (a) methanol and (b) ethanol. Solid line is the theoretical fit obtained from Fresnel's equations with variable film thickness and dielectric constant, using a least square algorithm 139 Figure 6-1 Side product from substitution reactions between oligothienyl N-hydroxysuccinimide esters (25, 28, 31) with cysteamine 157 Figure 6-2 ' H N M R spectra of 32 and CdSe-32 dissolved in CDC1 3 (300 MHz). The (•) indicates the proton signal of residual CHCI3 present in the solvent 162 Figure 6-3 Transmission electron microscopy (TEM) images of (a) CdSe-TOPO and (b) CdSe-32 drop-cast onto a carbon-coated T E M grid 163 Figure 6-4 Transmission electron microscopy (TEM) image of CdSe-TOPO dropcast onto a carbon coated T E M grid after exposure to a 80 keV electron beam for (a) 0, (b) 7.5 and (c) 12.5 min 164 Figure 6-5 Normalized UV-vis absorption spectra of capping groups (23, 26, 29 and 32) in CHCI3 165 Figure 6-6 UV-vis absorption spectra of (a) CdSe-TOPO, (b) CdSe-26, (c) CdSe-29 and (d) CdSe-32 in CHCI3. Insets: difference absorption spectra between each oligothiophene-capped CdSe NP and CdSe-TOPO 167 Figure 6-7 Excitation (—) and emission (—) spectra of the capping groups (a) 23, (b) 29 and (c) 32 in CHCI3 excited at 367, 326 and 363 nm, respectively 169 Figure 6-8 Emission spectra of (a) CdSe-TOPO, (b) CdSe-26, (c) CdSe-29 and (d) CdSe-32 excited at 350 nm in CHC1 3. For CdSe-26, CdSe-29, and CdSe-32, the emission spectrum of CdSe-TOPO at the same optical density is plotted on the same graph for comparison 171 Figure 6-9 Solid-state emission spectra of (a) CdSe-29 and (b) CdSe-32 172 Figure 6-10 Excitation spectrum of CdSe-TOPO subtracted from excitation spectra of (a) CdSe-29 and (b) CdSe-32 (emission wavelength = 562 nm) 173 Figure 6-11 HOMO and L U M O energy potential diagram for 29 and 32. Band edge values for CdSe are based on ionization potentials from reference 43 and the band gap is from UV-vis absorption of CdSe-TOPO 174 Figure 6-12 Cyclic voltammetry of (a) 29 and (b) 32 in CH 2 C1 2 with 0.1 M (C 4H 9)4NPF 6 175 xiii Figure 6-13 Electrodeposition of (a) 32 and (b) CdSe-32 by repeated potential scanning (5 cycles) from 0 to 1.3 V in CH2CI2 containing 0.1 M (C 4H9) 4NPF 6 . The arrows indicate the direction of current change with each successive scan 176 Figure 6-14 Calculated frontier orbitals of 26, 29 and 32 and energy level diagram depicting the HOMO and L U M O levels of 26, 29 and 32 178 xiv List of Schemes Scheme 1-1 8 Scheme 2-1 ...60 Scheme 2-2 62 Scheme 3-1 87 Scheme 6-1 155 Scheme 6-2 156 Scheme 6-3 158 Scheme 6-4 158 Scheme 6-5 160 Scheme 6-6 162 Scheme 7-1 189 Scheme 7-2 190 Scheme 7-3 191 xv List of Symbols and Abbreviations Abbreviation Description A Angstrom A absorbance Z angle Anal. analysis a. u. arbitrary units P electron coupling term BE binding energy br broad Bu butyl © copyright c speed of light CB conduction band CV cyclic voltammogram °C degrees Celsius Cp* pentamethylcyclopendienyl CT charge transfer A difference 5 chemical shift (ppm) 5 inter-particle distance d doublet d thickness dmean mean diameter dd doublet of doublets (NMR) ddd doublet of doublets of doublets (NMR) dppp (1,3-diphenylphosphine)propane AE difference in energy AG* free enthalpy of activation xvi AR difference in reflectivity p density D F T density functional theory ° degrees s dielectric constant E energy E a activation energy E O X peak potential, oxidation process (V) E r e d peak potential, reduction process (V) E A elemental analysis E D C A^-(3-dimethylamino-propyl)-A^'-ethylcarbodiimide hydrochloride E D O T 3,4-ethylenedioxythiophene E D X energy dispersive X-ray analysis E I electron ionization E Q E external quantum efficiency E S I electrospray ionization r peak width h Planck's constant h hour(s) H O M O highest occupied molecular orbital H T head-to-tail I current I D A interdigitated array i.e. id est I R infrared I T O indium tin oxide cp volume concentration J magnetic coupling constant, N M R coupling constant k wavevector X wavelength (nm) ^Em emission wavelength (nm) xvii A,Ex excitation wavelength (nm) Xmax wavelength at band maximum (nm) LED light emitting diode LSP localized surface plasmon L U M O lowest unoccupied molecular orbital ix mobility M molarity (molL"1) M + molecular ion peak m multiplet (NMR) m/z mass-to-charge ratio MEH-PPV poly[2-methoxy-5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene] min(s) minute(s) mmol millimole M O molecular orbital MS mass spectra mV millivolts M W molecular weight mol mole n refractive index NHS JV-hydroxy succinimide NIR near-infrared nm nanometer N M R nuclear magnetic resonance NP nanoparticle OD optical density OT oligothiophene P pressure P60ME poly(3-(6-methoxyhexyl)thiophene) PAT poly(3-alkylthiophene) PECVD plasma enhanced chemical vapor deposition PEDOT poly(3,4-ethylenedioxythiophene) xviii Ph phenyl PPh 3 triphenylphosphine ppb parts per billion ppm parts per million PPV poly(j9-phenylenevinylene) PSP propagating surface plasmon PT polythiophene PT 2-diphenylphosphino-thiophene P T 2 2-diphenylphosphino-5,2'-bithiophene P T 3 2- diphenylphosphino-5,2': 5' ,2" -terthiophene PVP poly(4-vinylpyridine) q quartet Qabs absorption efficiency ORE quasi reference electrode r radius R universal gas constant R reflectivity CT electrical conductivity s singlet S Siemens S A M self assembled monolayer SEM scanning electron microscopy SET single electron tunnelling SFU Simon Fraser University sh shoulder SCE saturated calomel electrode SPR surface plasmon resonance STS scan tunneling spectroscopy 9 angle t triplet T temperature X I X T3 terthiophene T5 pentathiophene TDPA tetradecylphosphonic acid T E M transmission electron microscopy TGA thermogravimetric analysis TLC thin layer chromatography THF tetrahydrofuran TOP trioctylphosphine TOPO trioctylphosphine oxide UBC University of British Columbia U V ultraviolet V Volts V volume V B valence band vis visible v/v volume to volume ratio CO radial frequency X halogen XPS X-ray photoelectron spectroscopy XX Acknowledgements There are many people to thank and acknowledge that made this thesis possible. First, I want to thank my supervisor, Dr. Mike Wolf, for his mentorship and wisdom in chemistry, the research process and career choices. His confidence in me allowed me to explore and learn several different areas outside chemistry such as optical physics and photolithography. I also appreciate our thought filled discussions about science and life. I am grateful to all my co-workers in the Wolf group for their support, past and present, you celebrated with me in my achievements and helped make the frustrating days bearable. I specifically want to thank Dr. Tracey Stott who had the daunting task of teaching synthetic techniques to a self-described 'non-synthetic chemist'. I am indebted to Tim Kelly for all his useful discussions and help with the DFT calculations in this thesis. I also would like to acknowledge the other member of Team M B , Matt Roberts, thanks for putting up with my mess in the fume hood and listening to all my ideas and rants. I would like to express gratitude to Dr. Mark Maclachlan for reading my entire thesis and allowing me to use his lab equipment and chemicals when needed. I would like to thank all the people who taught me lithographic techniques and all the cleanroom operating procedures: Dr. Mario Beaudoin, A l i Izadi-Najafabadi and Dr. Andras Pattantyus-Abraham. I am grateful to Dr. Jeff Young for letting me use his lab and optical equipment to perform my sensor experiments and to David Jarvis for his help in instrumental set-up and data fitting. A good deal of this work would not have been possible without the help of the support staff in our department. The N M R and mass spectrometry staff were always helpful with any concerns and with the analysis of data. The electronics, mechanical and glass blowing shops were helpful whenever something broke down or when a new experiment needed specialized equipment. Dr. Elaine Humphrey and xxi Garnett Huygens provided a lot of helpful advice and expertise working with their electron microscopes at the U B C Bioimaging facility. I also want to acknowledge Dr. Ken Wong who carried out the X-ray photoelectron spectroscopy on all my samples. I am completely grateful to my parents who have always supported me in whatever I pursued. I know I could not have done this without them. Their guidance, advice and insight have made me the person I am today. I thank my God, Jesus Christ, whose grace and love has gotten me through all the tough times and continues to give me purpose and hope for the future. Finally, I want to thank NSERC, the Canadian Space Agency (CSA) and UBC for all the scholarships and funding. xxii Co-Authorship Statement The identification and design of my research topic was performed by my research supervisor, Prof. Michael Wolf and I. I solely carried out the chemical synthesis, chemical and physical characterization, and data analyses presented in this thesis. One exception to this is the work presented in Chapter 5 where David Jarvis assisted in the assembly of the surface plasmon detector used and also wrote the program used to fit the experimental data. My research supervisor and I prepared all the published manuscripts presented here. xxiii Chapter 1 Introduction 1.1 Overview 'Materials chemistry' emerged as a distinct discipline during the late 1980s spurring the American Chemical Society (ACS) and Royal Society of Chemistry (RSC) to introduce international journals (Chemistry of Materials in 1989 and Journal of Materials Chemistry in 1991) where findings in this area can be published. The discipline has sustained rapid growth in the years following and currently has 178 journals classified in the category (Materials Science, Multidisciplinary) but still lacks a formal definition. This led the RSC in 2006 to sponsor a workshop to come up with a working definition for the field and, they concluded that materials chemistry is "the chemistry of the design, synthesis and characterization of assemblies of molecules whose properties arise from interactions between them ... and exploitation of compounds that have useful or potentially useful properties and applications." The definition implies a highly inter-disciplinary approach to chemistry that requires technical knowledge in other scientific disciplines such as physics, biology, and engineering in addition to organic, inorganic and physical chemistry. This thesis' goals are rooted in chemical synthesis and characterization, and numerous ideas and experimental techniques from physics and electronic engineering are adopted. Another goal of this work was to discover new functional properties that can be used for device applications. 7t-Conjugated materials have potential applications in molecular electronics due to their ability to conduct electricity along a 7i-system delocalized on the molecular backbone.4'5 This has led to extensive work on preparing and characterizing ^-conjugated materials with the most 1 common derivatives being of polyaniline, polypyrrole, polythiophene (PT) and poly(p-phenylenevinylene) (PPV). 6 Oligothiophenes were selected for this study due to the ease of chemical modification of thiophene moieties and their stability under ambient conditions.7 The optical absorption and emission of oligothiophenes also occurs in the visible region of the electromagnetic spectrum making it useful as a probe of electronic states. Our group has done extensive work on attaching transition metal groups to oligothiophenes to create hybrid materials with novel structural, chemical and electronic properties.8"13 Knowing that metal atoms modulate the properties of oligothiophenes led to the hypothesis that a collection of atoms (i.e. between a single atom and the bulk) such as in a nanoparticle-oligothiophene hybrid might also produce interesting optical and electronic properties. Groundwork in this area prior to the start of this project indicated nanoparticles can interact with ^-conjugated materials to yield modified electronic 1 4 ' 1 5 and optical 1 6" 1 8 properties and deserved further investigation. This thesis describes the preparation and characterization of oligothiophene-capped Au and CdSe nanoparticles. The effect of the nanoparticles on the electronic and optical properties of the oligothiophene and vice versa is discussed. Potential applications of these materials arising from nanoparticle/oligothiophene interactions are also considered. 1.2 n-Conjugated materials 1.2.1 Structure of ^-Conjugated Polymers and Oligomers Organic 7t-conjugated polymers and oligomers are a class of materials that attract a lot of attention due to their electronic and optical properties which are finding applications in molecular electronics such as organic transistors, solar cells and light emitting diodes (LEDs). 4" 6 The functional properties of 7i-conjugated polymers arise from alternating double and single 2 bonds or linked aromatic rings and forms a delocalized 7t-system along the polymer backbone. Figure 1-1 illustrates a few examples of ^-conjugated polymers. f O - sL - t O 4 * poly(p-phenylenevinylene) polyaniline Figure 1-1 Several examples of 7i-conjugated polymers. Poly- and oligothiophenes are formed by linking thiophene rings at the a- or (3- positions (Figure 1-2). PT with a,a-linkages has more planar conformations resulting in better 7i-orbital overlap between adjacent thiophene rings and superior electronic properties relative to a,(3- or P,P - l inkages . 7 However, longer oligothiophene chains (n > 7) are insoluble and alkyl chains must be attached to the P-position to increase solubility in organic solvents (Figure 1-3).19 These polymers are known as poly(3-alkylthiophenes) (PATs). The presence of alkyl chains in PATs introduces more complexity to the polymer structure due to the asymmetry of the monomer where there are 3 regiochemically distinct a,a-couplings possible (Figure 1-4). The planarity of the polymer backbone is highly dependent on the regioregularity of PATs, which affects conjugation, packing in the solid-state and conductivity.19 Regioregular head-to-tail (HT) PATs have been shown to have longer conjugation lengths and higher conductivities compared to other regiorandom analogues.19 H polythiophene polypyrrole 3 Figure 1-2 Location of the a- and p-positions on thiophene. R = alkyl R d o \ / poly(3-alkylthiophene) poly(3,4-ethylenedioxythiophene) Figure 1-3 Selected examples of P-substituted polythiophenes. Another derivative of PT that has garnered a lot of attention are polyalkoxythiophenes where there are oxygen atoms attached to the P-positions of the ring. The oxygen atoms inductively donate electron density to the n-system reducing the bandgap, making the polymer more stable towards chemical degradation, and has no detrimental steric effects on conjugation. The most studied example of polyalkoxythiophenes is poly(3,4-ethylenedioxythiophene) or PEDOT (Figure 1-3). PEDOT is currently sold under the trade name BAYTRON® P and used commercially in conductive or antistatic coatings, as a hole injection layer in organic LEDs and 21 as flexible organic wires. 4 R = alkyl substituent Head-to-Tail Head-to-Head Tail-to-Tail F i g u r e 1-4 The three possible regiochemical couplings in poly(3-alkylthiophene). 1.2.2 Synthesis of Poly- and Oligothiophenes There is a vast amount of literature available on synthetic procedures for the preparation of poly- and oligothiophenes making it difficult to comprehensively cover the whole field here. Only the most commonly used pathways are outlined here including the ones used in this study. •7 A book chapter written by Bauerle in The Handbook of Oligo- and Polythiophenes presents a more complete treatment of this subject for the interested reader. Preparative methods of poly- and oligothiophenes can be grouped into 3 categories: chemical oxidative coupling, metal-catalyzed cross-coupling and electrochemical polymerization. Chemical oxidative coupling is the most straightforward technique for the preparation of longer oligothiophenes but suitable only when symmetric monomers are used or when regioregularity is not of concern. Figure 1-5 is an example of the preparation of 22 octathiophene by oxidizing tetrathiophene using iron (III) chloride. F i g u r e 1-5 Selected example of an oxidative chemical reaction to prepare octathiophene. 5 Metal catalyzed cross-coupling reactions are very versatile reactions where substituted thiophene monomers are linked with both selectivity and high regioregularity. Kumada coupling is a general cross-coupling reaction that involves an aryl halide and a Grignard reagent in the presence of a catalyst such as NiCl2(dppp) (dppp = (l,3-diphenylphosphine)propane). Figure l-6a shows an example of Kumada coupling where 2,5-dibromothiophene and 2 equivalents of a Grignard reagent yield terthiophene.23 Suzuki coupling is a high-yielding reaction that utilizes an aryl halide and a boronic ester for C-C bond formation with a Pd catalyst such as Pd(PPh3)4. The major advantage of Suzuki coupling is that it has a much higher reactivity for iodo groups relative to other halides such that it is widely used to prepare regioregular oligomers of 3-alkylthiophenes.24"26 Figure l-6b is an example of a Suzuki coupling between 2-bromo-3-hexyl-5-iodo-thiophene and a 3-hexyl substituted thienyl boronic ester.24 The coupling is selective towards iodo groups leaving the bromo groups unreacted which can then be converted into boronic esters for additional Suzuki coupling reactions resulting in regioregular HT-coupled oligo(3-hexylthiophene). The last metal catalyzed cross-coupling reaction to be highlighted here is Stille coupling. It is a Pd-catalyzed cross-coupling between an aryl halide and an organotin reagent. The Stille reaction between tributyl(2-thienyl)stannane and 2-iodothiophene in the 97 presence of Pd(PPh3)2Cl2 is depicted in Figure l-6c. Stille coupling although still widely used was avoided in this study because it tends to give lower yields compared to Kumada and Suzuki reactions in addition to the fact that tin reagents are toxic. 6 (a) Kumada Coupling : (b) Suzuki Coupling : (c) Stille Coupling : Figure 1-6 Examples of metal catalyzed cross-coupling reactions to prepare oligothiophenes. Electrochemical polymerization to prepare PT is a convenient and controlled method to deposit polymer on a conducting surface. It occurs first by oxidation of thiophene or an oligothiophene that creates a radical cation where the radical is localized at the a-position of the no ring (Scheme 1-1). The second step involves the coupling of two radicals to form first a dication dimer, and subsequently bithiophene, after the loss of two protons and rearomatization. This is followed by bithiophene forming a radical and undergoing further radical coupling due its lower oxidative potential relative to thiophene. The same radical mechanism is repeated until the oligomer or polymer formed becomes insoluble and is deposited onto the electrode surface. Varying the concentration of monomer or the length of time used for deposition can control the amount of material deposited. 7 Scheme 1-1 © 1.2.3 Electronic and Optical Properties of Poly- and Oligothiophenes Poly- and oligothiophenes exhibit unique properties compared to non-conjugated polymers due to a delocalized 7i-electron system along the polymer chain. The overlap of n-orbitals between several adjacent aromatic rings generates filled valence and empty conduction bands separated by a band gap similar to the band structure of a semiconductor. The valence and conduction bands are composed of bonding and antibonding orbitals, respectively. The energy band diagram shown in Figure 1-7 depicts the increase in bonding and antibonding orbitals as the number of linked thiophene rings is increased eventually forming the band structure. It is valuable to note from Figure 1 -7 that the oligothiophene chain length greatly affects the HOMO-L U M O gap and will therefore influence the optical and electronic properties. As the number of thiophenes is increased, the H O M O - L U M O gap decreases resulting in a bathochromic shift in 8 both the absorbance and luminescence of the oligomer.7 The oligothiophene length also influences the HOMO energy where longer chains result in lower oxidation potentials.7 Aside from the chain length, the planarity of the oligothiophene backbone also influences the optical properties where addition of bulky substituents at the p-position causes twisting in the oligomer backbone. This results in poorer Ti-orbital overlap and shorter conjugation lengths that blue-shift the observed absorbance and emission 19 Energy 71-M O s n = 1 n = 2 n = 3 Conduct ion Band Va lence Band n = co Figure 1-7 Evolution of the energy band diagram of polythiophene with increasing chain length. PT is also electrically conductive when it is oxidatively doped due to an increase in the number of charge carriers.29 The conductivity measured for />-doped regioregular HT-PATs (>1000 S/cm) is comparable to that of bulk copper.30'31 The process of ^ -doping is the removal 9 of an electron from the 7i-orbitals of PT resulting in the formation of a polaron (radical cation) and distortion of PT from a benzenoid to a quinoid structure (Figure l-8a). The quinoid structure is less stable than the benzenoid structure and its energy level appears in a band diagram above the valence band. Removal of a second electron from PT creates a bipolaron (dication) where the HOMO (quinoid energy level) in Figure l-8b is now empty. Bipolarons are believed to be the primary charge carriers in conductive PT. 2 9 Figure 1-8 Structural diagrams of a (a) polaron and (b) bipolaron on a polythiophene chain. The corresponding energy band diagrams for each are also shown. The oxidative doping of PT not only increases the conductivity but alters the optical properties as well. This is because the introduction of polaron and bipolaron energy levels into the band structure both introduces charge carriers and changes the band gap of the material (Figure 1-8). For example, PT in the undoped state has an absorption maximum between 450 to 530 nm from the n—>TI* transition giving it a dark red color. Oxidizing PT chemically or applying an oxidative potential of + 4.05 V vs. L i causes the n—>n* absorption to disappear while two new absorption peaks appear at 827 and 1907 nm. The new peaks are transitions from 10 the valence band to the two subgap energy levels introduced during polaron and bipolaron formation (Figure 1-8). This gives oxidatively doped PT a green color. Hence, application of a positive potential to PT changes its color making the material electrochromic. 1.3 Nanoparticles 1.3.1 Defining a Nanoparticle Nanoparticles (NPs) are particles with at least one dimension in the range of 1-10 nm that exhibit new or enhanced size-dependent properties when compared to larger particles of the same material.32 The novel properties that arise as the particle size approaches the nanometer 33 regime make them applicable for a wide range of possible applications such as catalysis, sensing,34 single electron transistors35 and solar cells. 3 6 It is an increase in the surface-to-volume ratio and quantum confinement in nanoparticles that generates these functional properties. Larger surface-to-volume ratios are important in catalytic applications where reactions take place at crystallographic facets on the nanoparticle surface33 and surface plasmon resonance, which are collective oscillations of electrons at the metal/dielectric interface (discussed further in Section 1.3.2).34 Quantum confinement is the quantization of energy levels that can be populated by electrons as the particle size decreases. This is analogous to the quantization of energy of an electron confined in a 1-dimensional box. Quantum.confinement gives gold nanoparticles the ability to charge one electron at a time (single electron transistors) and gives II-VI semiconductor nanoparticles discrete energy levels for absorbance and emission (discussed further in Section 1.3.3).37 Currently, very active areas of study in this field include sub-fields such as carbon nanotubes and magnetic nanoparticles; only metal and semiconductor nanoparticles, which are 11 applicable to this study, are introduced in detail here. The discussion will be mainly focused on the optical and some electronic properties of these particles. 1.3.2 Properties of Metal Nanoparticles The size that nanoparticles adopt is important in the understanding of their inherent properties. Unlike the bulk, however, metal nanoparticles only exist with certain diameters due to the close packed arrangement of metal atoms, thus generating a sequence of energetically optimal sizes (Figure 1-9). The precise cluster sizes that metal nanoparticles adopt are sometimes called 'magic numbers'. The surface of the clusters are not naked but are passivated by an organic capping group to prevent aggregation of the metal nanoparticles. Many different capping groups such as thiols,38'39 phosphines,40 amines,41 citrate42 and polymers43 have been used to passivate the gold nanoparticle surface. Full-Shell 'Magic Number" Clusters Number of shells Number of atoms in cluster Mi3 M55 M-I47 M309 ^ 6 1 Percentage surface atoms 92% 76% 63% 52% 45% Figure 1-9 Schematic diagram of the ideal structures for metal nanoparticles where the metal atoms are arranged in a close packed arrangement. (Reprinted from reference 44 with permission from Elsevier.) The higher percentage of surface atoms in metal nanoparticles compared to the bulk gives these materials different optical properties. Bulk metals such as gold and silver reflect 12 visible light giving the familiar luster of these precious metals. However at particle diameters between 2 - 5 0 nm, the surface electrons absorb electromagnetic radiation at a particular energy resulting in a surface plasmon oscillation (Figure 1-10). Typical absorption maxima for Ag and Au nanoparticles are -380 and -520 nm, respectively. The intensity and frequency of absorption is dependent on the size, size distribution, shape and dielectric environment.45' 4 6 The surface plasmon oscillations in an individual particle can also interact with adjacent particles through dipole-dipole interactions.47 The proximity of the nanoparticles to each other influences the strength of this interaction where the coupled surface plasmon effect increases as the particles are brought closer together. It should be noted that as the number of atoms in a nanoparticle is increased, the percentage of surface atoms decreases and at a certain point the particle's optical properties begin to resemble those of the bulk. Where this transition takes place from nanoparticle to bulk properties is still a widely debated topic in the literature. Figure 1-10 Schematic diagram of a gold nanoparticle experiencing surface plasmon oscillations highlighting the displacement of the surface electrons relative to the nuclei. The electronic properties of metal nanoparticles also differ from the bulk. Bulk metals (i.e. Au, Ag, Cu) are electrically conductive while metal nanoparticles are insulating. The 13 insulating nature of metal nanoparticles is due to the localization of the electrons in quantized energy levels, unlike the complete derealization in the bulk. 4 8 Monolayers of Au nanoparticles are also insulating because there is no close contact between the particles.49 Changing the capping group on the nanoparticle surface to smaller groups decreases the inter-particle distance and improves the conductivity because of the shorter tunneling distance between particles.50 More interesting is the dramatic resistance drop as the number of metal nanoparticle layers is increased.51 This has been explained as a change in the electron transfer mechanism from tunneling to delocalized metallic behavior. 1.3.3 Properties of Semiconductor Nanoparticles Semiconductor nanoparticles have very different properties compared to the same material in the bulk. Excitation of a bulk semiconductor results in the formation of an exciton (electron and hole pair) where the distance separating the charges is called the Bohr radius (nanometer length scale). The minimum energy required for formation of this exciton is known as the band gap of the material. When the size of a semiconductor particle is less than the Bohr radius, the charge carriers in the particle are confined resulting in the quantization of energy levels. Figure 1-11 shows the evolution of quantized states as the size of the semiconductor is reduced from the bulk to the nanometer regime. This quantum confinement greatly affects the band gap of semiconductors where the band gap increases as the nanoparticle size decreases.52 Since conductivity, exciton formation, and emission frequency in semiconductors are direct products of the band gap energy, the reduction of the particle size greatly affects these properties as well. . 14 Band G a p Energy V B Bulk Semiconductor 30 nm 2 nm Nanopart ic les < 1 nm Cluster Figure 1-11 Depiction of the changes in energy levels of a semiconductor due to size. There are numerous types of semiconductor nanoparticles but the II-VI type in particular has attracted a lot of attention due to ease of preparation, a direct band-gap and its chemical versatility by attachment of different functional groups to the particle surface.53 The II-VI type is formed from elements in groups II and VI of the periodic table. The band gap in the bulk and Bohr radius for these materials ranges from 3.68 eV and 2 nm, respectively, for ZnS, to overlapping valence and conduction bands for HgTe with a Bohr radius of -40 nm. The energy of the band gap can be further tuned by adjusting the size of the nanoparticles. CdSe is particularly interesting because the band gap energy is in the visible and changing the particle 52 diameter yields photon emission over the entire visible spectrum. It should be noted that polydispersities in size and shape,54 surface defects and poor crystallinity also affects the observed emission 55 15 1.4 Metal Nanoparticle-Conjugated Polymer Nanocomposites 1.4.1 Preparation Chemically synthesized nanocomposites Several chemical approaches have been developed to prepare nanocomposites of metallic nanoparticles and conducting polymers. Generally, these follow one of two routes: a) a "one-pot" approach where the monomer or polymer acts as a reductant for the metal or b) preparation of the nanoparticles followed by either chemical polymerization of the polymer around the particles or dispersion of the nanoparticles into a polymer matrix. The advantage of the "one-pot" approach is the lack of intermediate purification steps, which reduce yield. Nanocomposites of polyaniline derivatives and gold nanoparticles have been prepared in this, way, using 2-methoxyaniline or o-anisidine as reducing agents for gold salts. In the resulting nanocomposite the polymer remains doped. 5 6 ' 5 7 A similar approach was used by Chattopadhyay et al, using hydrogen peroxide to both reduce a gold salt and simultaneously oxidatively polymerize aniline. The nanocomposites prepared in this way are two orders of magnitude more conductive than polyaniline, with nanoparticles -100 nm in diameter.5 8'5 9 A one-pot approach has also been utilized by Advincula to prepare sexithiophene linked nanoparticles. Either a tetraalkylammonium functionalized terthiophene60 or a poly(4-vinylpyridine) (PVP) 6 1 with pendant terthiophene groups was used in this work as a reductant for solutions of HAuCLt (Figure 1-12). The ammonium salt forms a polyelectrolyte complex with polystyrene sulfonate, and subsequent addition of the HAuCU results in formation of gold nanoparticles (>15 nm). In the PVP case, multilayers of the functionalized PVP and poly(acrylic acid) are deposited on substrates, which are then dipped in a solution of HAuCL resulting in 6-16 100 nm gold nanoparticles. In both cases, nanoparticle formation is accompanied by the simultaneous formation of coupled sexithiophene linkers. Similarly, L i et al. prepared composite materials using 3,4-ethylenedioxythiophene (EDOT) as a reductant. The resulting PEDOT/nanoparticle composite was found to self-assemble via TT—TC interactions giving aggregates. It is also possible to use a conducting polymer as the reductant; the groups of Naka and Chujo have accomplished this, generating nanocomposites of poly(dithiafulvalene) and Pd, Au and Pt nanoparticles.63'64 Figure 1-12 The simultaneous formation of gold NPs and sexithiophene units on functionalized PVP (Adapted from reference 61 with permission from the American Chemical Society). 17 The other major chemical route to nanoparticle composites is to synthesize the particles first, followed by either dispersion in a polymer matrix or chemical polymerization of monomeric groups on the particle surface. This approach has been used for a number of different types of metal nanoparticles. Athawale prepared Pd nanoparticles (-20-30 nm) by y-radiolysis of a heated solution containing PdCk and substituted aniline monomers.65 These nanoparticles were then oxidatively polymerized using ammonium persulfate solutions. They have also prepared copper nanoparticles using a similar approach, however in this case the aniline was added after the particles were synthesized.66 This nanocomposite was used as a catalyst for Wacker oxidation of 1-decene. McCullough has prepared gold nanocomposites by reducing HAuCU with sodium borohydride in the presence of regioregular poly(3-hexylthiophene).67 This approach requires addition of excess tetraoctylammonium bromide to prevent oxidation of the poly(3-hexylthiophene). Thin films of these composites were cast and studied by A F M and T E M . Photopolymerization has also been used to obtain nanocomposites. Sadik prepared films 68 by photopolymerization of pyrrole at 254 nm in the presence of copper, silver or gold salts. The mechanism is believed to involve photoreduction of the metal salt to give the nanoparticles, oxidation of the pyrrole monomer followed by polymerization to give polypyrrole. Zhou formed silver-polydiacetylene nanocomposites from a mixture of 10,12-pentacosadiynoic acid and A g + , irradiated to cause photoreduction of the silver and simultaneous polymerization of the 18 diacetylene. An elegant approach to form nanowires of Au nanocomposites was demonstrated by Feldheim.6 9 In this approach gold nanoparticles are drawn into a porous alumina membrane which is then exposed to a solution of iron perchlorate on one side of the membrane, and pyrrole 18 monomer on the other. Chemical polymerization occurs within the membrane, which can then be dissolved to reveal the interconnected arrays. Feldheim et al. have also used thioacetyl 70 71 functionalized phenylacetylenes to make gold and silver nanocluster dimers and trimers. ' Electrochemically synthesized nanocomposites As discussed in Section 1.2.2, electrochemical synthesis is a well-established technique for the preparation of conducting polymer films via oxidative coupling of monomers. The technique is very versatile and polymers with functional side groups can be synthesized by modifying the monomer prior to electro-oxidation. A wide variety of anions can also be incorporated as counter ions, usually from the electrolyte used in the synthesis. In recent years, electrochemical methods have also proven to be effective in incorporating metal nanoparticles in either pre-deposited polymers or in growing polymer films. Depending on the metal, the desired metal nanoparticle size and the type of polymer, different techniques have been developed. poly-1 poly-2 Figure 1-13 Electroactive cationic polymer templates. 19 Initial attempts to electrochemically impregnate conducting polymers with metal nanoparticles employed a two-step electrochemical process. In the first step, a conducting polymer such as polyaniline7 2, polypyrrole73 or poly(methylthiophene)74 is deposited onto an electrode by electro-oxidation of the appropriate monomer. Films of the polymers are then dipped in a solution containing metal species such as PtCU 2", A g + or C u 2 + followed by electrochemical reduction yielding Pt, Ag or Cu clusters embedded in the polymer. Longer electrolysis time leads to growth of larger particles as opposed to nucleating new ones, so the metal clusters formed are generally large (> 100 nm), have a wide distribution in size, are confined largely to the polymer surface and are unevenly distributed within the polymer matrix. To improve the size distribution and reduce the particle size, cations can be attached to the polymer backbone such as in poly-1 and -2, (Figure 1-13) where the metal complex acts as a template to facilitate metal particle nucleation.75"77 After polymerization, these films were immersed in an aqueous solution of metal anions (Pd, Ru or Cu) resulting in anion exchange. This was followed by electro-reduction to grow the metallic particles. The electrogenerated Pt, Ru and Cu particles generated in this way are much smaller (< 6 nm) and the size distribution narrower than those prepared by electrochemical impregnation. The size and distribution of particles in the matrix are highly dependent on numerous factors such as the immersion time for anion exchange, the applied voltage or current, and the solution concentration. 20 Support Polymer Electrode Film Support Polymer Electrode Film (a) Surface-confined metal particles (b) Embedded metal particles Figure 1-14 Schematic diagram of conducting polymer where the metal nanoparticles are (a) surface-confined and (b) distributed throughout the polymer matrix. An alternative approach to electrochemically incorporating metal nanoparticles into a polymer matrix was first carried out by Bose et al.18 in which presynthesized nanoparticles are present as a colloidal dispersion during electropolymerization of the monomer. This leads to the nanoparticles being trapped within the growing polymer (Figure l-14b) instead of being confined to the surface (Figure 1-14a) as is typically observed when metal particles are deposited onto pre-deposited polymers by reduction of metal ions. Since a large variety of metal nanoparticles can be synthesized and their size controlled,79 this method has the advantage that significant control of embedded nanoparticle size can be exerted. The three-dimensional distribution of metal particles within the matrix also facilitates improved charge shuttling throughout the material. A disadvantage of this approach, however, is that the nanoparticles used are passivated by thiols, amines or citrate which prevent the nanoparticles from being directly chemically bound to the polymer. Leaching of the nanoparticles from the matrix is thus difficult to prevent. 21 0.1 M N0>Bu)4CIO4 Nitrobenzene + 1.8 V 4 (CH 2 )7CH 3 (CH 2)7CH3 3 poiy-(3)n(4) m Figure 1-15 Thiophene derivatized Au nanoparticles copolymerized electrochemically with 3-ft-octylthiophene. Several different methods to chemically attach pre-synthesized NPs to a polymer film during the electrodeposition process have been developed. Cioffi et al.80'81 have prepared Pd nanoparticles with weakly passivating tetraoctylammonium bromide ([N(CgHi7)4]Br) on the surface. As the nanoparticles are trapped during electrodeposition, some [N(CgHi7)4]Br molecules are displaced from the surface and polypyrrole nitrogens partly passivate the Pd nanoparticles as shown by XPS chemical shifts.81 The Pd nanoparticles carry a slight negative charge due to Pd(Br)42" groups surrounding a Pd(0) core, resulting in some ionic interaction between the partially oxidized polypyrrole and the nanoparticle. Peng et al.82 used a different approach by covalently tethering Au nanoparticles to the conjugated polymer backbone. Au nanoparticles passivated by 2-mercapto-3-«-octylthiophene (3) were synthesized and then electrochemically co-polymerized with 3-n-octylthiophene (4), forming the random copolymer poly-(3)n(4)m, (Figure 1-15). The Au nanoparticles in poly-(3)n(4)m are distributed randomly in the polymer but the long alkyl tether between the Au nanoparticle and polythiophene backbone prevents efficient shuttling of charge throughout the material. 22 1.4.2 Electronic Properties The electronic properties of metal nanoparticles have stimulated significant interest due to possible applications in single electron transistors.83 There is also interest in the enhanced conductivity that may be attained by embedding nanoparticles in conducting polymer matrices.59' 6 8 When nanoparticles are sandwiched between metal contacts, electrons can only be transferred one-by-one via quantum tunneling where the probability of tunneling is controlled by an applied external voltage. This phenomenon, called single electron tunneling (SET), has been demonstrated for individual NPs at room temperature by Brousseau et al.84 and Houbertz et al.85 using scanning tunneling spectroscopy (STS). However, for these particles to find utility in nanoelectronic devices, organization into functional architectures is needed. Metallic nanoparticles have been assembled into two- and three-dimensional structures using electrostatic interactions,86'87 hydrogen bonding56 and saturated organic linkages.38'50'57 When non-conjugated linkers connect the particles, electron tunneling between particles is the g o predominant mechanism for electrical conduction. The electrical conductivities of these materials is expected to decrease as the length of the linker is increased due to longer tunneling distances, and this has been observed experimentally. Electron transport between nanoparticles linked with saturated groups is well studied; however systems with improved inter-particle charge transport using either a 7t-conjugated matrix to embed the NPs or using a 7i-conjugated molecule to link them are of interest. A 7t-conjugated polymer or molecule between the NPs can serve as a 'molecular wire' and enhance coupling between adjacent particles.14 23 H 3 C H 2 C R = CN, SH 8 Figure 1-16 Bifunctional 7t-conjugated molecules used to link Au nanoparticles via self-assembly. Four-point probe conductivity measurements of Au nanoparticles embedded in 80 SO r»7 polypyrrole, polyaniline and PT demonstrate an increase of approximately two orders of magnitude in conductivity compared to similar polymers without nanoparticles present. Detailed information on the role of the metal particles in the films is difficult to obtain, due to problems which include: (1) the distribution of nanoparticles is random and challenging to control, (2) the nature of the bonding and the electronic link between the polymer and nanoparticle is ill-defined and (3) it is feasible for charge transport to occur without involving the nanoparticle by charge percolation through the polymer alone. Model systems where nanoparticles are linked by 71-conjugated molecules of well-defined length are useful as they may be used to probe the effect of the 'molecular wire' on the efficiency of charge transport between nanoparticles. 24 Initial efforts to construct structures where nanoparticles are linked by Tt-conjugated molecules used self-assembly to form three-dimensional networks by mixing the two components in solution. Bifunctional molecules (Figure 1-16) are used to link the nanoparticles via cyanide90' 9 1 or thiol functionalities.14'92 Although initial attempts to ascertain the effect of the conjugated crosslinker compared to unlinked Au particles gave mixed results,14'90-92 Andres et al. were able to find evidence that Au nanoparticles linked by 6 led to local reordering of the array suggesting a possible electrical link between the nanoparticles. Bourgoin et al.15 carried out the same experiment using 9 as the linking molecule and found three orders of magnitude better conductivity for nanoparticles linked by 9 than for unlinked nanoparticles. The 9-linked particles also had a lower activation energy (EA) for inter-particle tunneling suggesting 9 acts as a 'molecular wire'. Au NPs have also been crosslinked through self-assembly with conjugated dithiols (7 and 10)92' 9 3 where Maeda et al93 have shown that at room temperature the predominant pathway for electronic conduction through these materials appears to be tunneling. Snow et al.94 used different crosslinkers to compare the effect of a saturated linker (alkanedithiols) to a partially conjugated linker (8). Solutions of 8 and Au nanoparticles were used to build up a layered structure of crosslinked particles by successive dipping. The conductivity of the sample after each dipping cycle was determined and even after 10 cycles, nanoparticles linked by 8 showed higher conductivities compared to those linked by alkane dithiols. Au nanoparticles passivated by different molecules (an alkanethiol vs. 2 -phenylethanethiol)94"96 were also compared and the Au nanoparticles capped with 2-phenylethanethiol showed a conductivity enhanced by a factor of two. The higher conductivity correlates to the presence of the aromatic group in the shell, which lowers the average barrier between clusters. 25 During the time that research in this thesis was being carried out, two influential and 97 98 97 related works by Torma et al. and Wessels et al. were published. Torma et al. used a variety of conjugated and non-conjugated molecules to link nanoparticles and get a better understanding of the charge transfer mechanism between Au clusters (Figure 1-17). The conductivity was measured between 200 and 300 K to determine E A , the energy which must be overcome to create mobile charge carriers. For non-conjugated linked nanoparticles, a linear relationship was found (Figure 1-17) between E A and the intercluster distance. Since the distance between nanoparticles governs the hopping energy between them, it is clear that the chemical nature of the linker is unimportant because there are no electronic interactions between nanoparticles via the linker. Nanoparticles linked by conjugated molecules, on the other hand, show lower E A ' s relative to non-conjugated linkers of equivalent length. This suggests that the molecular orbitals of the linking molecules play a role in the charge transfer and provide a different pathway for electrons other than tunneling through the gap. 0.3 • 3 0.15 > 0.2 0) 0.25 0.1 0.05 0 1 2 Distance (nm) 3 4 26 Covalently Linked System Distance (nm) E a (eV) 0 r'1 0 • 1.1 0.095 1.6 0.11 2.3 0.12 (0 CP \-{^P (0 1.5 0.14 Non-Covalently Linked System Distance (nm) E a (eV) 0.7 0.16 H 2 ^ j ^ N ' S 0 3 H ^ 1.9 0.2 ^gp-s°*tjP—°—O—v / « » ^ Q < f ^ • 2.8 0.23 ^ 5 f o W r y * 0 o cTb 3.1 0.26 Figure 1-17 Activation energies for covalently and non-covalently linked A1155 clusters of varying inter-particle distances. (Adapted from reference 9 7 with permission from Wiley-VCH). 27 no Wessels et al. studied the effect of conjugation on inter-particle electronic coupling using a variety of partially conjugated linkers with a phenyl group in the backbone and non-conjugated linker analogues by substituting a cyclohexyl group for the phenyl group (Figure 1-18). A l l three of the phenyl-containing linkers showed an order of magnitude higher conductivity, and lower activation energies than their cyclohexyl-containing counterparts. Most interesting are the nanoparticles linked by PDBT for which a plot of conductivity as a function of T 1 has a positive slope indicating metallic behavior. The metallic behavior has been attributed to the high degree of conjugation in the linker and good Au nanoparticle/molecule contact leading to overlap of adjacent nanoparticle electronic wavefunctions. Linker Length (A) AE (HOMO-LUMO) BDMT cHDMT HS HS SH SH 7.7 8.2 4.0 5.0 DMAAB DMAAcH H . N — ( \ — N H ° SH HN HS NH 14.8 14.9 3.5 4.7 PBDT cHBDT HN—V \ — N H HN- NH 10.7 9.6 2.7 3.2 Figure 1-18 Partially conjugated linkers and their non-conjugated analogues. (Adapted from reference 9 8 ) . 28 1.4.3 Optical Properties The optical properties of both conducting polymers and metal nanoparticles were discussed in Section 1.2.3 and 1.3.2, respectively. In nanocomposites, the effect of metal nanoparticles on the n-n* absorption of conducting polymers, and the effect of conducting polymers on the coupling between nanoparticles are a subject of debate. Understanding and tuning such effects could lead to hybrid optical devices based on these nanocomposites with improved optical properties. Au nanoparticles with diameters of 5-50 nm show a surface plasmon absorption band in the visible region between 520-530 nm." This plasmon absorption is sensitive to the environment surrounding the NPs such that different solvents with varying refractive indices cause the plasmon band to red-shift ~10 nm.46 The effect of a surrounding conducting polymer on the surface plasmon of Au nanoparticles embedded in conjugated PEDOT or poly(dithiafulvalene)100 has been studied. A red-shift in the plasmon absorption band of 30 nm after passivation of the nanoparticle with the conducting polymer was observed by both groups. This red-shift has been ascribed to the lowering of the work function of the Au nanoparticle by the conducting polymer, lowering the position of the surface resonance state and thus causing a red-shift in the absorbance. Such shifts in the surface plasmon band suggests overlap between the electronic wavefunctions of the nanoparticle surface and conducting polymer. The energy of the 71-71* absorption is one method of estimating the conjugation length in conducting polymers; red-shifts in the 7t-7r* absorption indicate an increase in conjugation length. Studies by Zhou et al.18' 8 9 on the effect of metal nanoparticles on the conjugation of poly(diacetylene) showed a red-shift in the n-n* absorption of the polymer as Ag nanoparticles / 7 increased in size upon UV irradiation. Masuhara et al. reported a similar effect for Ag 29 nanoparticles surrounded by poly(l,6-di(JV-carbazolyl)-2,4-hexadiyne) where the 71-71* absorption red-shifted -20 nm more than would be expected without the Ag particles. More interesting is the disappearance of the Ag surface plasmon band upon encapsulation in the CP, attributed to strong interaction between the electronic state of the Ag plasmon band and excitons of the conducting polymer. 1.4-PDI (1 mtnj 1,4-PDI <5 min) 1,4-PDI H O r n l r a l 0.0 4C0 2.0— 2.'C>-__R • 8 c -e o tn ja 1XH Ala : sd • - - Am r.al ->• 1.4-PDI (1 in-*i) A u pol •» 1 ,4*01 (5 .iron) A u SEI 1,4-PDI (10 miriii 500 600- 700 W a v e l e n g t h ( n m ) 800 -wxi - A u B d 1 A u B L * » 1,4-PDI <1 mVi) • A u eo4l-» 1,4-PDI <S nilrt) - A u s o t - ' 1,4-PDI <10minf 0.0 2.0-. 8 - _ I.O. o GO 0.6 0-'l> 4O0 500 6O0- 700 8O0> 400 W a v e l e n g t h ( n m ) S00 600 700 .800 W a v e l e n g t h ( n m ) — — A » i aoi AJII aol t- ijt-POr. (1 rnjn> Au sol *• 1,4-PCN I'S mln) Am so l <- 1.4-PEHi (10 mai j 500 600 700 800 W a v e l e n g t h ( n m ) Figure 1-19 UV/vis spectra of A u sol after the addition of 1,4-phenylene diisocyanide (1,4-PDI) with a final concentration of (a) 4.0 x 10"6, (b) 1.6 x 10"5, (c) 3.2 x 10"5, and (d) 3.2 x 10"4 M . (Reprinted from reference 16 with permission from American Chemical Society). Model systems have been constructed where nanoparticles are linked by 7i-conjugated molecules to illustrate the effect of the 'molecular wire' on the surface plasmon absorption of inter-linked particles. K im et al.16 assembled a network of A u nanoparticles linked together by 1,4-phenylene diisocyanide (1,4-PDI, Figure 1-19). UV-vis spectra of Au nanoparticles in 30 solution show a substantial decrease in the surface plasmon band at 522 nm as more linker molecules are introduced. This result is comparable to Masuhara's work described above supporting their claim of electronic interactions between the metal plasmon and the conducting polymer. Kim also observed growth of a new band at 720 nm as 1,4-PDI was added to the solution of Au nanoparticles. This new band is a result of coupling of inter-particle surface plasmons101 upon aggregation of nanoparticles with 1,4-PDI. The type and degree of coupling between surface plasmons of nanoparticles is dependent on two factors: (1) the inter-particle spacing and (2) the conductivity between particles. The former is a dipole resonance effect, where a shorter inter-particle distance produces a larger dipole-dipole coupling energy and causes a red-shift in the surface plasmon band relative to an individual nanoparticle (Figure l-20a). The latter gives rise to two absorption bands due to contributions from both dipolar and quadrupolar resonances. Initial conductive contact between nanoparticles effectively shorts the dipole interaction and the symmetry and geometry of a 'dumbbell-like' configuration favors quadrupolar resonance (Figure l-20b). As the conductive link and physical overlap of the nanoparticles increases, the charge distribution rearranges again to the dipolar mode (Figure l-20c) as observed for ellipsoidal nanoparticles. The intensity ratio of these two resonances is dependent on the degree of conductive overlap between the particles. (a) G O Dipole-dipole coupling Figure 1-20 Charge distribution for (a) dipole-dipole plasmon coupling, (b) quadrupolar mode due to shorting of dipole interactions and (c) strong conductive overlap causing 'dumbbell' shaped dipolar resonance. (b) conductive bridge (c) + Quadrupole resonance Dipolar resonance 31 Whether a conducting polymer or a -^conjugated molecule linking adjacent nanoparticles can provide a conductive link between nanoparticles and short the dipole moments, and result in perturbation of the coupled surface plasmons is still under debate. Marinakos et al69 reported little effect on the Au nanoparticle surface plasmon absorption with or without a polypyrrole coating. Studies on model systems by Osifchin et al92, Chen et al.103 and Musick et al.104 using phenylacetylene and phenyl oligomers to bridge Au nanoparticles showed red-shifts (-30 nm) upon linking compared to unlinked particles. This red-shift appears to be mostly due to dipole-dipole plasmon coupling. However, studies on phenylacetylene oligomers bridging Ag and Au 71 nanoparticles by Novak et al. give contrary evidence. UV-vis absorption of Ag nanopaticles in solution before and after introduction of the -^conjugated bridging molecule 11 is shown in Figure 1-21. After 11 is introduced, there are two new peaks that appear in the spectrum at 370 71 and 600 nm corresponding to quadrupolar and dipole resonances of the linked nanoparticles. ' 1 0 5 This is what would be expected from cpnductively linking nanoparticles and shorting the dipole moments.47 Since the quadrupolar resonance peak at 370 nm is much larger than the dipolar resonance at 600 nm, this ratio is indicative of a weak conductive link between the nanoparticles. 32 300 350 40f> 450 500' 55$ 600 650-Waveleogfih (mm.) Figure 1-21 UV-visible spectra for dimers linked by structure 11 (upper trace) and silver particle 'monomers' (lower trace). The monomer spectrum was acquired under solvent conditions identical to those of the dimers but in the absence of a linker. (Reprinted from reference 71 with permission from American Chemical Society). During the time the work in this thesis was being carried out, Wessels et al.98 observed an even larger effect in the UV-vis absorption when Au nanoparticles were linked by PBDT. The surface plasmon band due to the individual nanoparticles nearly disappears and the linked particles exhibit a strong absorbance in the near-IR comparable to that observed for gold films. Metallic absorption has been previously observed for Au nanoparticles linked by 1,3-propanedithiol106 and 2-mercaptoethylamine.51' 1 0 4 However, the inter-particle distances for these saturated linkers are quite short (about half the length of PBDT) and metallic absorption has been attributed to close proximity of the particles that allows overlap of the metal nanoparticle wavefunctions. For PBDT-linked particles where the inter-particle spacing is ~1 nm, the metallic absorption is due to overlap of the molecular orbitals of the linker and metal wavefunctions causing the formation of a resonant state affecting the absorption of the material. This is remarkable noting the large distance between particles and suggests that -^conjugated molecules can provide a conductive link between nanoparticles, short the dipole moments and 33 perturb the plasmon resonances. This conclusion is supported by conductivity measurements on these same materials, discussed previously (Section 1.4.2.). 1.5 Semiconducting Nanoparticle-Conjugated Polymer Nanocomposites 1.5.1 Preparation Chemically synthesized nanocomposites Several methods have been developed to prepare semiconductor/conducting polymer nanocomposites. The most popular method is also the simplest, where the conducting polymer and nanoparticles are dissolved in the same solvent, mixed and cast (drop or spin) onto a substrate. Adjusting the composition of the solution controls the amount of nanoparticles in the nanocomposite. This method has been used to incorporate CdSe107"113 and CdS 1 1 4 in PPV, 1 0 7' n o " 1 1 2 poly(A/-vinylcarbazole) (PVK) 1 1 0 ' 1 1 1 and P T . 1 0 7 " 1 1 1 1 3 ' 1 1 4 While this method has proven to be quite versatile, nanocomposites formed by this method have poor electronic interactions between the particle and the polymer due to insulating organic groups on the nanoparticle surface. Chandrakanthi et al.115 used polyaniline to directly passivate the CdS particle surface and increase interactions between both materials. This was accomplished by preparing the CdS nanoparticles in the presence of polymer in solution. This procedure has also been used to prepare CdS and CdSe in poly(3-hexylthiophene).115 34 0 o s o f . 6 k ^ s , -o o -k ©' n p = i0" 2 mbar S-o -Negatively charged CdSe nanoparticle Pre-PPV P P V F i g u r e 1-22 Schematic diagram of the self-assembly of a CdSe/PPV nanocomposite using electrostatic attraction. The in situ preparation of nanoparticles in the presence of the polymer gave rise to a new set of problems where the particles prepared have a wide size distribution and there is no way to prevent aggregation. Liu et al.116 found a way to circumvent this problem by preparing PT with a functional group (amine) attached at the end of the polymer that can be used to passivate the nanoparticle surface. Semiconductor nanoparticles with a narrow size distribution can then be prepared by conventional methods and a simple exchange reaction can be used to replace the passivating group with the functionalized PT. The nanocomposite prepared in this way shows strong electronic interactions between the particle and the polymer due to the insulating passivating groups being removed from the nanoparticle surface plus the added benefit of having precise control of particle size. A similar procedure has also been used successfully to attach PPV onto CdSe." 7 Self-assembly of conducting polymers and semiconductors into 3-D architectures has 118' • • also been achieved. Gao et al. employed electrostatic attractions to assemble negatively charged CdSe nanoparticles and a positively charged PPV precursor (pre-PPV) into a 35 nanostructure (Figure 1-22). Heating of the CdSe/pre-PPV composite at 130 °C under vacuum for 11 hours yielded the desired CdSe/PPV nanocomposite. Javier et al.119 used a different approach for self-assembly employing a bifunctional conjugated oligomer, 11 (Figure 1-23). By mixing 11 with CdSe nanoparticles in toluene, the two thiols on 11 crosslink the particles and an assembled network of CdSe/11 precipitates out of solution. SH I SH 11 Figure 1-23 Structure of oligo-(/>phenylethynylene dibenzylthiol dipropyl ether), an example of a bifunctional conjugated crosslinker where n = 0, 1, and 3. Electrochemically synthesized nanocomposites Electrochemical synthesis of semiconductor nanoparticle/conducting polymer composites 120 121 has also been explored. ' The preparation of the composites uses preformed semiconductor particles that are present in solution during electropolymerization of the monomer. This leads to the nanoparticles being trapped within the growing conducting polymer. This electrochemical method is not widely used because of poor electronic interactions between the particle and the polymer due to the insulating capping groups on the nanoparticle surface. The distribution and location of particles in the polymer matrix are also difficult to control in this method. 36 1.5.2 Electronic and Optical Properties Electronic interactions between semiconductor nanoparticles and conducting polymers are interesting because of the enhanced optical properties of these materials for photovoltaic and light emitting diode (LED) applications.36 Figure 1-24 illustrates how a semiconductor-based solar cell and light emitting diode functions. For photovoltaic applications, the key to generating large currents is by creating electron-hole charge separation through absorption of light and transporting these charge carriers to the electrodes. In LEDs both electrons and holes are injected into a semiconductor material to stimulate radiative recombinations and light is emitted. (a) A A A / V anode Semiconductor (b) cathode Semiconductor Figure 1-24 Schematic energy level diagram for a semiconductor-based (a) solar cell and (b) light emitting diode. Semiconductor nanoparticles are efficient in producing charge separated species but these tend to recombine both radiatively and non-radiatively. Greenham et al.122 have shown that creating a blend of semiconductor nanoparticle with poly[2-methoxy-5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene] (MEH-PPV) aligns the energy levels resulting in a material that is conducive to charge separation with fewer recombinations. Figure 1-25 illustrates the three 37 possible mechanisms for electron-hole separation in MEH-PPV/CdSe nanoparticle blends. MEH-PPV is also an efficient hole conductor useful in shuttling the holes from the material to the electrode. In semiconductor nanoparticles with an insulating passivating group on the surface, electron and hole transfer mechanisms are suppressed and only exciton energy transfer is possible (Figure l-25b). Photovoltaic devices made from MEH-PPV/CdSe nanoparticle blends have external quantum efficiencies (EQEs) as high as 12% compared to 0.014% without 122 108 the nanoparticles present. Blends of CdSe nanoparticles with poly-(3-hexylthiophene) and 123 a polyfluorene copolymer have also been achieved with measured EQEs as high as 54 and 44 %, respectively. 38 electron (a) ^ — \ transfer hv i A A A / V | MEH-PPV CdSe (b) * e TJ MEH-PPV CdSe (c) e hv TJ CdSe Figure 1-25 Energy band diagrams for MEH-PPV/CdSe blends illustrating the routes for generating electron-hole charge separation, (a) Absorption in MEH-PPV followed by electron transfer onto the nanoparticle; (b) Absorption in the polymer followed by exciton transfer to the nanoparticle and subsequent hole transfer onto the polymer; (c) Absorption in the nanoparticle followed by hole transfer onto the polymer. (Reprinted from reference 2 2 with permission from American Physical Society). Semiconductor nanoparticles have also been exploited as LEDs due to the high fluorescence quantum yield (-50%) of these materials.124 The problem with semiconductor nanoparticles is their poor hole conductivity which creates a buildup of holes on the anode e e MEH-PPV CdSe MEH-PPV CdSe 39 surface. This forces the electrons to travel through the material only to recombine non-radiatively with a hole at anode surface producing LEDs that are not very bright with high turn-on voltages. Colvin et al.125 have shown that by placing a PPV layer on top of the CdSe nanoparticles sandwiched between electrodes, the devices are brighter with turn-on voltage as low as 4 V. The lower operating voltage is due to improved hole injection into the device that have a higher probability of recombining radiatively at the heteroj unction between PPV and CdSe. Another appealing quality of these devices is the tunability of color with voltage where an applied voltage of 4 V results in emission of yellow light while voltages greater than 8 V results in green light. The color tunability is due to different sources of electroluminescence; at lower voltages, the emission originates from the CdSe nanoparticle but at higher voltages 19 S emission is from the polymer. PPV/CdSe nanoparticle blends have also been made into white light emitting LEDs. 1 1 7' 1 2 6 However, it has recently been shown that substantial nanoparticle aggregation occurs in PPV/CdSe nanoparticle blends which limits the interfacial contact between the polymer and limits the charge transfer pathways that lead to luminescence.117' 1 2 6 Aggregation also leads to nanoparticle self-quenching that results in decreased light output. 126 Skaaf et al. have shown that by attaching the PPV directly onto the nanoparticle instead of simply blending the materials together, the solid-state fluorescence emission emanates only from the particle where the PPV emission is quenched. This is due to more efficient charge transfer from the polymer to the nanoparticles as a result of the increased contact between the two materials. This result was later confirmed by Odoi.1 1 7 40 (b) L U M O L U M O L U M O C B C B E H O M O H O M O V B V B H O M O C d S e Ol igo-anil ine T3 C d S e T5 Figure 1-26 Electronic energy alignment for CdSe nanoparticles relative to (a) oligoaniline and (b) oligothiophenes (T3 = terthiophene, T5 = pentathiophene). To better understand the interactions of conjugated polymers with semiconductor nanoparticles and build more efficient devices, model compounds have also been synthesized. A variety of conjugated oligomers have been attached to semiconductor nanoparticle surfaces such 117 126 127 128 130 131 as oligo-(p-phenylenevinylenes), ' ' oligothiophenes " and oligoanilines. The alignment of energy levels in these materials is significant in determining the optical properties. An oligoaniline attached to semiconductor nanoparticles quenches the fluorescence of both 131 species. The alignment of electronic energies in this material is shown in Figure l-26a, light absorption in the oligomer is followed by electron transfer into the conduction band (CB) of the nanoparticle while light absorption by the nanoparticle is followed by hole injection into the HOMO of the oligomer. Both quenching mechanisms make this material efficient at electron-hole charge separation and potentially useful in photovoltaics. In oligothiophene capped 129 semiconductor nanoparticles, Milliron et al. found that both terthiophene (T3) and pentathiophene (T5) moieties attached to the nanoparticle quenches the fluorescence of the 41 oligothiophenes by electron transfer from oligothiophene to the nanoparticle (Figure l-26b). On the other hand, fluorescence from the CdSe nanoparticle is quenched by T5 but not T3. T5 quenches the nanoparticle fluorescence by hole injection from its valence band (VB) onto the HOMO of the nanoparticle. The HOMO of T3 is lower than the VB of CdSe making hole injection into the VB of the CdSe not viable. Therefore, oligothiophenes at least 5 units long are required for charge separation in photovoltaic devices while shorter oligomers that undergo electron transfer to the nanoparticle but do not quench CdSe fluorescence would be useful for LEDs. 1.6 Goals and Scope of Present Study The goals of this thesis were: (1) to synthesize new oligothiophene capped Au and CdSe nanoparticles, (2) to investigate the electronic and optical properties of these new hybrid materials, (3) to assemble these nanoparticles into three-dimensional architectures using electrochemical deposition and (4) to interpret the observed properties for possible materials applications. The initial goal of this study was to directly attach oligothiophenes to the surface of Au and CdSe nanoparticles. The oligothiophene in close proximity to the nanoparticle should interact both chemically and electronically. The second goal involved the physical characterization of these materials and investigation of the electronic and optical properties. The length of the oligothiophene was varied to change the electronic energy levels (HOMO and LUMO) and conjugation length (optical absorption and emission) of the oligomers7 to give possible insight on factors that might affect the strength of the interactions. The third goal is the electrochemical crosslinking of oligothiophene-capped nanoparticles to build three-dimensional architectures. The resulting change in the electronic and optical properties of these materials as a result of electrochemical crosslinking is also of interest. The final goal was to establish 42 structure/property relationship in oligothiophene-capped Au and CdSe nanoparticles that can be exploited for use in device applications. 43 1.7 References 1. ISI Web of Knowledge, Journal citation reports (Materials Science, Multidisciplinary). http://portal.isiknowledge.com/portal.cgi?DestApp=JCR&Func=Frame 2. Day, P. Towards defining materials chemistry, http://www.iupac.org/proiects/2005/2005-001-l-200.html 3. Brazil, R. RSC Materials Chemistry Workshop Report. http://www.rsc.org/images/Defining%20Materials%20Chemistrv%20Final%20Report%2 0for%20web tcml8-67089.pdf 4. Heeger, A . J., Angew. Chem. 2001, 40, 2591. 5. MacDiarmid, A . G., Angew. Chem. 2001, 40, 2581. 6. Skotheim, T. A. ; Elsenbaumer, R. L.; Reynolds, J. R., Handbook of Conducting Polymers. 2nd ed.; Marcel Dekker: New York, 1998. 7. Bauerle, P., The Synthesis of Oligothiophenes. In Handbook of Oligo- and Polythiophenes, Fichou, D., Ed. Wiley-VCH: Weinheim, 1999; pp 89. 8. Clot, O.; Akahori, Y . ; Moorlag, C ; Leznoff, D. B.; Wolf, M . O.; Batchelor, R. J.; Patrick, B. O.; Ishii, M . , Inorg. Chem. 2003, 42, 2704. 9. Clot, O.; Wolf, M . O.; Patrick, B. O., J. Am. Chem. Soc. 2000, 122, 10456. 10. Clot, O.; Wolf, M . O.; Patrick, B. O., J. Am. Chem. Soc. 2001, 123, 9963. 11. Moorlag, C ; Wolf, M . O.; Bohne, C ; Patrick, B. O., J. Am. Chem. Soc. 2005, 127, 6382. 12. Stott, T. L.; Wolf, M . O.; Patrick, B. O., Inorg. Chem. 2005, 44, 620. 13. Wolf, M . O., Adv. Mater. 2001, 13, 545. 14. Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V . R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G., Science 1996, 273, 1690. 44 15. Bourgoin, J.-P.; Kergueris, C ; Lefevre, E . ; Palacin, S., Thin Solid Films 1998, 327-329, 515. 16. Kim, H. S.; Lee, S. J.; Kim, N . H.; Yoon, J. K.; Park, H . K. ; Kim, K., Langmuir 2003, 19, 6701. 17. Masuhara, A. ; Kasai, H. ; Okada, S.; Oikawa, H.; Terauchi, M . ; Tanaka, M . ; Nakanishi, H. , Jpn. J. Appl. Phys., Part 2 2001, 40, LI 129. 18. Zhou, H. S.; Wada, T.; Sasabe, H. ; Komiyama, H. , Synth. Met. 1996, 81, 129. 19. McCullough, R. D., Adv. Mater. 1998, 10, 93. 20. Groenendaal, L . B.; Jonas, F.; Freitag, D.; Pielartzik, H . ; Reynolds, J. R., Adv. Mater. 2000, 12, 481. 21. B A Y T R O N product page, http://www.baytron.com/index.php7page id=602 22. Noma, N . ; Tsuzuki, T.; Shirota, Y. , Adv. Mater. 1995, 7, 647. 23. Chiem Van, P.; Burkhardt, A. ; Shabana, R.; Cunningham, D. D.; Mark, H. B., Jr.; Zimmer, H. , Phosphorus, Sulfur Silicon Relat. Elem. 1989, 46, 153. 24. Cremer, J.; Mena-Osteritz, E . ; Pschierer, N . G.; Muellen, K. ; Baeuerle, P., Org. Biomol. Chem. 2005, 3, 985. 25. Kirschbaum, T.; Azumi, R.; Mena-Osteritz, E . ; Bauerle, P., New J. Chem. 1999, 23, 241. 26. Kirschbaum, T.; Briehn, C. A. ; Bauerle, P., Perkin 1 2000, 1211. 27. Crisp, G. T., Synth. Commun. 1989, 19, 307. 28. Roncali, J., Chem. Rev. 1992, 92, 711. 29. Heeger, A . J.; Kivelson, S.; Schrieffer, J. R.; Su, W. P., Rev. Mod. Phys. 1988, 60, 781. 30. Chen, T.-A.; Rieke, R. D., Synth. Met. 1993, 60, 175. 31. Chen, T.-A.; Wu, X . ; Rieke, R. D., J. Am. Chem. Soc. 1995, 117, 233. 32. Wilcoxon, J. P.; Abrams, B. L., Chem. Soc. Rev. 2006, 35, 1162. 45 33. Narayanan, R.; El-Sayed, M . A. , J. Phys. Chem. B 2005, 109, 12663. 34. Hutter, E.; Fendler, J. H. , Adv. Mater. 2004, 16, 1685. 35. Shipway, A . N . ; Katz, E.; Willner, I., ChemPhysChem 2000, 1,18. 36. Godovsky, D. Y. , Adv. Polym. Sci. 2000, 153, 163. 37. Gubin, S. P.; Kataeva, N . A. ; Khomutov, G. B., Russ. Chem. Bull. 2005, 54, 827. 38. Brust, M . ; Bethell, D.; Kiely, C. J.; Schiffrin, D. J., Langmuir 1998, 14, 5425. 39. Brust, M . ; Walker, M . ; Bethell, D.; Schiffrin, D. J.; Whyman, R., J. Chem. Soc, Chem. Commun. 1994, 801. 40. Weare, W. W.; Reed, S. M . ; Warner, M . G.; Hutchison, J. E., J. Am. Chem. Soc. 2000, 122, 12890. 41. Aslam, M . ; Fu, L. ; Su, M . ; Vijayamohanan, K. ; Dravid, V . P., J. Mater. Chem. 2004, 14, 1795. 42. Turkevich, J.; Stevenson, P. C ; Hillier, J., Discuss. Faraday Soc. 1951, No. 11,55. 43. Walker, C. H. ; St. John, J. V. ; Wisian-Neilson, P., J. Am. Chem. Soc. 2001, 123, 3846. 44. Aiken, J. D., I l l ; Finke, R. G., J. Mol. Catal. A: Chem. 1999, 145, 1. 45. Mulvaney, P., Langmuir 1996, 12, 788. 46. Underwood, S.; Mulvaney, P., Langmuir 1994, 10, 3427. 47. Atay, T.; Song, J.-H.; Nurmikko, A. V. , Nano Lett. 2004, 4, 1627. 48. Chi, L. F.; Hartig, M . ; Drechsler, T.; Schwaack, T.; Seidel, C ; Fuchs, H.; Schmid, G., Appl. Phys. A 1998, 66, S187. 49. Brust, M . ; Bethell, D.; Schiffrin, D. J.; Kiely, C. J., Adv. Mater. 1995, 7, 795. 50. Bethell, D.; Brust, M . ; Schiffrin, D. J.; Kiely, C , J. Electroanal. Chem. 1996, 409, 137. 51. Musick, M . D.; Keating, C. D.; Keefe, M . H. ; Natan, M . J., Chem. Mater. 1997, 9, 1499. 52. Murray, C. B.; Norris, D. J.; Bawendi, M . G., J. Am. Chem. Soc. 1993, 115, 8706. 46 53. Trindade, T.; O'Brien, P.; Pickett, N . L., Chem. Mater. 2001, 13, 3843. 54. Wang, Q.; Pan, D.; Jiang, S.; Ji, X . ; An, L.; Jiang, B., J. Cryst. Growth 2006, 286, 83. 55. Lifshitz, E.; Glozman, A. ; Litvin, I. D.; Porteanu, H., J. Phys. Chem. B 2000, 104, 10449. 56. Boal, A . K. ; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V . M . , Nature 2000, 404, 746. 57. von Werne, T.; Patten, T. E., J. Am. Chem. Soc. 2001, 123, 7497. 58. Sarma, T. K. ; Chattopadhyay, A. , J. Phys. Chem., A 2004, 108, 7837. 59. Sarma, T. K. ; Chowdhury, D.; Paul, A. ; Chattopadhyay, A. , Chem. Commun. 2002, 1048. 60. Youk, J. H. ; Locklin, J.; Xia, C ; Park, M.-K. ; Advincula, R., Langmuir 2001, 17, 4681. 61. Patton, D.; Locklin, J.; Meredith, M . ; Xin, Y . ; Advincula, R., Chem. Mater. 2004, 16, 5063. 62. L i , X . ; L i , Y . ; Tan, Y . ; Yang, C ; L i , Y. , J. Phys. Chem. B 2004, 108, 5192. 63. Zhou, Y . ; Itoh, H. ; Uemura, T.; Naka, K.; Chujo, Y. , Chem. Commun. 2001, 613. 64. Zhou, Y . ; Itoh, H. ; Uemura, T.; Naka, K. ; Chujo, Y. , Langmuir 2002, 18, 277. 65. Athawale, A . A. ; Bhagwat, S. V. ; Katre, P. P.; Chandwadkar, A . J.; Karandikar, P., Mater. Lett. 2003, 57, 3889. 66. Athawale, A . A . ; Bhagwat, S. V. , J. Appl. Polym. Sci. 2003, 89, 2412. 67. Zhai, L.; McCullough, R. D., J. Mater. Chem. 2004, 14, 141. 68. Breimer, M . A. ; Yevgeny, G.; Sy, S.; Sadik, O. A. , Nano Lett. 2001, 1, 305. 69. Marinakos, S. M . ; Brousseau, L. C , III; Jones, A. ; Feldheim, D. L., Chem. Mater. 1998, 10, 1214. 70. Brousseau, L. C , III; Novak, J. P.; Marinakos, S. M . ; Feldheim, D. L. , Adv. Mater. 1999, 11,447. 71. Novak, J. P.; Feldheim, D. L., J. Am. Chem. Soc. 2000, 122, 3979. 47 72. Kost, K. M.; Bartak, D. E.; Kazee, B.; Kuwana, T., Anal. Chem. 1988, 60, 2379. 73. Holdcroft, S.; Funt, B. L., J. Electroanal. Chem. Interfacial Electrochem. 1988, 240, 89. 74. Tourillon, G.; Dartyge, E.; Fontaine, A.; Jucha, A., Phys. Rev. Lett. 1986, 57, 603. 75. Coche, L.; Moutet, J. C , J. Am. Chem. Soc. 1987, 109, 6887. 76. Sigaud, M.; Li, M.; Chardon-Noblat, S.; Santos Aires, F. J. C ; Soldo-Olivier, Y.; Simon, J. P.; Renouprez, A.; Deronzier, A., J. Mater. Chem. 2004, 14, 2606. 77. Zouaoui, A.; Stephan, O.; Carrier, M.; Moutet, J.-C, J. Electroanal. Chem. 1999, 474, 113. 78. Bose, C. S. C ; Rajeshwar, K., J. Electroanal. Chem. 1992, 333, 235. 79. Katz, E.; Shipway, A. N.; Willner, I., Nanoscale Mater. 2003, 5. 80. Cioffi, N.; Torsi, L.; Losito, I.; Sabbatini, L.; Zambonin, P. G.; Bleve-Zacheo, T., Electrochim. Acta 2001, 46, 4205. 81. Cioffi, N.; Torsi, L.; Sabbatini, L.; Zambonin, P. G.; Bleve-Zacheo, T., J. Electroanal. Chem. 2000, 488, 42. 82. Peng, Z.; Wang, E.; Dong, S., Electrochem. Commun. 2002, 4, 210. 83. Simon, U., Adv. Mater. 1998, 10, 1487. 84. Brousseau, L. C , III; Zhao, Q.; Shultz, D. A.; Feldheim, D. L., J. Am. Chem. Soc. 1998, 120, 7645. 85. Houbertz, R.; Feigenspan, T.; Mielke, F.; Memmert, U.; Hartmann, U.; Simon, U.; Schoen, G.; Schmid, G., Europhys. Lett. 1994, 28, 641. 86. Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M., Adv. Mater. 1997, 9, 61. 87. Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I., Langmuir 2000, 16, 8789. 48 88. Joseph, Y . ; Besnard, I.; Rosenberger, M . ; Guse, B.; Nothofer, H.-G.; Wessels, J. M . ; Wild, U . ; Knop-Gericke, A. ; Su, D.; Schloegl, R.; Yasuda, A . ; Vossmeyer, T., J. Phys. Chem. B 2003, 107, 7406. 89. Zhou, H. S.; Wada, T.; Sasabe, H. ; Komiyama, H. , Appl. Phys. Lett. 1996, 68, 1288. 90. Andres, R. P.; Datta, S.; Dorogi, M . ; Gomez, J.; Henderson, J. I.; Janes, D. B.; Kolagunta, V . R.; Kubiak, C. P.; Mahoney, W.; et al., J. Vac. Sci. Technol, A 1996, 14, 1178. 91. Janes, D. B.; Kolagunta, V . R.; Osifchin, R. G.; Bielefeld, J. D.; Andres, R. P.; Henderson, J. I.; Kubiak, C. P., Superlattices Microstruct. 1995, 18, 275. 92. Osifchin, R. G.; Andres, R. P.; Henderson, J. I.; Kubiak, C. P.; Dominey, R. N . , Nanotechnology 1996, 7, 412. 93. Maeda, S.; Ogawa, T., Int. J. Nanosci. 2002, 1, 557. 94. Snow, A . W.; Ancona, M . G.; Kruppa, W.; Jernigan, G. G.; Foos, E. E.; Park, D., J. Mater. Chem. 2002, 12, 1222. 95. Chaki, N . K. ; Aslam, M . ; Gopakumar, T. G.; Sharma, J.; Pasricha, R.; Mulla, I. S.; Vijayamohanan, K. , J. Phys. Chem. 5 2003, 107, 13567. 96. Chaki, N . K. ; Gopakumar, T. G.; Maddanimath, T.; Aslam, M . ; Vijayamohanan, K., J. Appl. Phys. 2003, 94, 3663. 97. Torma, V. ; Vidoni, O.; Simon, U . ; Schmid, G., Eur. J. Inorg. Chem. 2003, 1121. 98. Wessels, J. M . ; Nothofer, H.-G.; Ford, W. E.; von Wrochem, F.; Scholz, F.; Vossmeyer, T.; Schroedter, A. ; Weller, H. ; Yasuda, A. , J. Am. Chem. Soc. 2004, 126, 3349. 99. Kamat, P. V. , J. Phys. Chem. B 2002, 106, 7729. 100. Zhou, Y . ; Itoh, H. ; Uemura, T.; Naka, K. ; Chujo, Y . , Langmuir 2002, 18, 5287. 49 101. Su, K. H. ; Wei, Q. H.; Zhang, X . ; Mock, J. J.; Smith, D. R.; Schultz, S., Nano Lett. 2003, 3, 1087. 102. Link, S.; Mohamed, M . B.; El-Sayed, M . A. , J. Phys. Chem. B 1999, 103, 3073. 103. Chen, S., Langmuir 2001, 17, 2878. 104. Musick, M . D.; Keating, C. D.; Lyon, L. A. ; Botsko, S. L.; Pena, D. J.; Holliway, W. D.; McEvoy, T. M . ; Richardson, J. N . ; Natan, M . J., Chem. Mater. 2000, 12, 2869. 105. Jensen, T.; Kelly, L. ; Lazarides, A. ; Schatz, G. C , J. Cluster Sci. 1999, 10, 295. 106. Brust, M . ; Kiely, C. J., Colloids Surf, A 2002, 202, 175. 107. Han, L. ; Qin, D.; Jiang, X . ; Liu, Y. ; Wang, L. ; Chen, J.; Cao, Y. , Nanotechnology 2006, 17, 4736. 108. Huynh, W. U . ; Dittmer, J. J.; Alivisatos, A . P., Science 2002, 295, 2425. 109. Huynh, W. U . ; Peng, X . ; Alivisatos, A . P., Adv. Mater. 1999, 11, 923. 110. Kucur, E.; Riegler, J.; Urban, G.; Nann, T., J. Chem. Phys. 2004, 121, 1074. 111. Kucur, E.; Riegler, J.; Urban, G. A. ; Nann, T., J. Chem. Phys. 2004, 120, 1500. 112. Park, J. H. ; Park, S.-L; Kim, T.-H.; Park, O. O., Thin Solid Films 2007, 515, 3085. 113. Physics, P. C. C ; Matter, P. R. B. C ; Sun, B.; Greenham, N . C , Phys. Chem. Chem. Phys. 2006, 8, 3557. 114. Malik, S.; Batabyal, S. K. ; Basu, C ; Nandi, A . K. , J. Mater. Sci. Lett. 2003, 22, 1113. 115. Chandrakanthi, R. L. N . ; Careem, M . A. , Thin Solid Films 2002, 417, 51. 116. Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A . P.; Frechet, J. M . J., J. Am. Chem. Soc. 2004, 126, 6550. 117. Odoi, M . Y . ; Hammer, N . I.; Sill, K. ; Emrick, T.; Barnes, M . D., J. Am. Chem. Soc. 2006, 128,3506. 118. Gao, M . ; Richter, B.; Kirstein, S., Adv. Mater. 1997, 9, 802. 50 119. Javier, A. ; Yun, C. S.; Sorena, J.; Strouse, G. F., J. Phys. Chem. B 2003, 107, 435. 120. Gaponik, N . P.; Talapin, D. V. ; Rogach, A . L.; Eychmuller, A. , J. Mater. Chem. 2000, 10,2163. 121. Pethkar, S.; Patil, R. C.; Kher, J. A. ; Vijayamohanan, K. , Thin Solid Films 1999, 349, 105. 122. Greenham, N . C ; Peng, X . ; Alivisatos, A . P., Phys. Rev. B 1996, 54, 17628. 123. Wang, P.; Abrusci, A. ; Wong, H. M . P.; Svensson, M . ; Andersson, M . R.; Greenham, N . C., NanoLett. 2006, 6, 1789. 124. Blanton, S. A. ; Hines, M . A. ; Schmidt, M . E.; Guyot-Sionnest, P., J. Lumin. 1996, 70, 253. 125. Colvin, V . L.; Schlamp, M . C ; Alivisatos, A . P., Nature 1994, 370, 354. 126. Skaff, H. ; Sill , K. ; Emrick, T., J. Am. Chem. Soc. 2004, 126, 11322. 127. Liang, Z.; Dzienis, K. L. ; Xu, J.; Wang, Q., Adv. Funct. Mater. 2006, 16, 542. 128. Locklin, J.; Patton, D.; Deng, S.; Baba, A. ; Millan, M . ; Advincula, R. C., Chem. Mater. 2004,16,5187. 129. Milliron, D. J.; Alivisatos, A . P.; Pitois, C.; Edder, C.; Frechet, J. M . J., Adv. Mater. 2003, 15, 58. 130. Querner, C.; Benedetto, A. ; Demadrille, R.; Rannou, P.; Reiss, P., Chem. Mater. 2006, 18, 4817. 131. Querner, C.; Reiss, P.; Bleuse, J.; Pron, A. , J. Am. Chem. Soc. 2004, 126, 11574. 51 Chapter 2 Synthesis and Characterization of Gold Nanoparticles Capped with Phosphino-Oligothiophenes 2.1 Introduction The synthesis of Au nanoparticles (NPs) is rich in history and can be traced as far back as the 5th to 4th century B.C. where they were used as a dye to color ceramics and glass. Since then, the number of methods that have been developed to prepare Au NPs is large and can be broken down into two main routes: a physical and chemical route. The physical route involves the 1 9 generation of Au NPs by removing atoms from bulk gold via evaporation, laser ablation or Ostwald ripening. However, the size distribution of the particles prepared by these methods is quite broad and usually another step such as thermal annealing is required to improve the size distribution. The chemical route involves the reduction of gold salts into Au NPs in the presence of a stabilizer that binds to the NP surface to prevent coalescence. The resulting NPs have narrower size distributions compared to particles prepared by the physical route, and the chemical route affords the ability to use numerous stabilizers such as citrate, thiols, phosphines, amines, carboxylates, micelles or polymers to passivate the NP surface.4 The two most commonly used chemical methods to synthesize Au NPs are the Turkevich5 and the Brust-Schiffrin6 methods. The former involves the citrate reduction of HAuCU in water but requires an initiation by either heating or UV irradiation. This method produces larger NPs (9 to 120 nm) where the diameter of the particle can be controlled by changing the gold to citrate ratio.7 The second method proceeds at room temperature through the A portion of this chapter has been published. Sih, B. C , Teichert, A. and Wolf, M . O. (2004) Electropolymerized 7t-Conjugated Gold Nanoparticle Polymers. Chemistry of Materials. 16: 2712-2718. 52 reduction of HAuCU by NaBFf.4 in a two-phase toluene/water mixture with a phase transfer catalyst ([N(C8Hi7)4]Br). The reduction is carried out with an alkanethiol present in the mixture to passivate the suface of the Au NPs once they are formed, thus preventing coalescence into larger particles. This results in particles generated using the Brust method to be generally smaller in diameter (1-5 nm). Since the choice of the passivating molecule on the Au NP surface is critical to their solubility, electronic and optical properties, and chemical reactivity, the Brust-Schiffrin method has proven to be the more versatile of the two because numerous functional groups can be linked to a thiol and subsequently attached to the Au NP surface.4 A slight modification to the Brust-Schiffrin method was advanced by Weare8 et al. using PPh3 instead of an alkanethiol to passivate the Au NP surface. The ease of synthesizing phosphine capped Au NPs is attractive as our research group has worked on phosphine-oligothiophene systems. Our group has shown that various phosphino-oligothiophenes are able to complex with metals such as Au, Pd and Ru to form complexes where the presence of the metal atom affects the electronic and optical properties of the oligothiophenes.9"11 Weare's procedure provides a suitable approach to incorporating conjugated oligothiophene capping groups to a Au NP via phosphine coordination. Hybrid materials where conjugated oligomers are attached to a Au NP surface have attracted attention in the past few years. Such hybrid materials are attractive because conjugated oligomers have remarkable electronic and optical properties which include high conductivities, electrochromism, electroluminescence and chemosensitivity.12 Coupling the properties of conjugated oligomers with the unique electronic, optical and catalytic properties of Au NPs could result in some very useful materials. Initial attempts were carried out by Hata et al.u and Peng et al.14 where pyrrole and thiophene groups, respectively, were present on the NP surface by means of an alkyl tether. While the presence of the conjugated molecule does lend some 53 interesting properties to the NPs such as the ability to polymerize the NPs together, the alkyl tether keeps the conjugated molecule too far away from the Au surface to allow any electronic or optical interactions. Au NPs have also been capped with mixed saturated/conjugated passivating groups such as arenethiolates where the conjugated material is much closer to the Au surface resulting in a substantial improvement in the inter-particle conductivity.15 The improvement in inter-particle conductivity was attributed to the aryl groups providing an aromatic tunneling bridge and thus lowering the activation carrier to inter-particle charge hopping. Therefore, it was of interest to synthesize Au NPs with oligothiophenes closely passivating the surface to see how Au NPs affect the electronic and optical properties of the conjugated oligomer and vice versa. Chart 2-1 PT 3(14) PT3(dioxy)(15) In this Chapter, the synthesis and characterization of a series of phosphino-oligothiophene-capped Au NPs is reported. The phosphino-oligothiophenes prepared for this study contain a diphenylphosphine capping group attached to the a, or the 2 - position of the 54 terminal thiophene ring (Chart 2-1) and contain one to three thiophene units to probe the effect of conjugation length on the properties of the oligothiophene capped Au NPs. Ethylenedioxy substituents are attached to the central thiophene ring in 15 to probe whether increased electron density on the oligomer backbone has any effect on the electronic structure of the resulting NPs. UV-Vis absorption spectra and cyclic voltammetry are used to probe the electronic structure of the Au NP/oligothiophene materials. A portion of the work presented in this Chapter has been published.16 2.2 Experimental 2.2.1 General All reactions were performed using standard Schlenk techniques with dry solvents under a nitrogen atmosphere. Chemicals were purchased from Aldrich except for HAuCU^FfiO and 3,4-ethylenedioxythiophene which were obtained from Strem Chemicals and Bayer, respectively. Compound PT (12) was obtained from Dr. T. Stott. The following compounds 17 17 were made by published procedures: 2,2'-bithiophene, 2,2 ^  5 \2''-terthiophene, 3',4'-ethylenedioxy-2,2':5',2"-terthiophene,18 PT 2 (13),19 and PT3 (14).20 Transmission infrared spectra were obtained on a Bomem MB-series spectrometer using KBr pellets. 'H and 3 1P NMR spectra were acquired on a Bruker AV-300 spectrometer, and spectra were referenced to residual 1 31 solvent ( H) or external 85% H3PO4 ( P). Electrochemical measurements were conducted using a Pine AFCBP1 bipotentiostat using a Pt disk working electrode, Pt coil wire counter electrode and a silver wire as the reference electrode. An internal reference (decamethylferrocene) was added to correct the measured potentials with respect to saturated calomel electrode (SCE). [(«-Bu)4N]PF6 was used as a supporting electrolyte and was purified by triple crystallization from 55 ethanol and dried at 90 °C under vacuum for three days. Methylene chloride used for cyclic voltammetry was purified by passing through an activated alumina tower. Electronic absorption spectra were obtained on a Varian Cary 5000 UV-vis/NIR spectrometer in C H 2 C I 2 . Energy dispersive X-ray (EDX) analysis was performed on a Kevex Quantum light element X-ray detector equipped with a Quartz Xone X-ray analyzer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Leybold MAX200 equipped with an Al Ka source with a pass energy of 192 eV, the sampling area was 2x4 mm. Transmission electron microscopy (TEM) images were taken using a Hitachi H7600 electron microscope operating at 80-100 kV. NPs were dropcast from C H 2 C I 2 solutions onto form var coated 300-mesh copper grids. The particle sizes were measured using the image processing program Quartz PCI 5 where the edge-to-edge distance for a total of 150 particles were individually measured resulting in a mean core size. 2.2.2 Synthesis 5-diphenylphosphino-3', 4 '-ethylenedioxy-2,2 ':5', 2' '-terthiophene (15) A solution of «-butyllithium in hexanes (12.8 mL, 1.6 M, 20.5 mmol) was added dropwise to a stirring solution of 3 ',4 '-ethylenedioxy-2,2':5 ',2 "-terthiophene (6.19 g, 18.8 mmol) in T H F (100 mL) at -78 C. The mixture was stirred for 1 hours at -78 C and distilled PPI12CI (5.1 mL, 28.1 mmol) was added dropwise. The reaction was then allowed to warm to room temperature and stirred for another 30 min, after which time 1 M HC1 was added to quench the reaction. The organic layer was separated, washed with water, and dried over M g S 0 4 . The solvent was removed to yield the crude product, which was purified by chromatography on silica gel with hexanes/methylene chloride (7/3 v/v). Removal of the solvent gave 4 as a yellow powder. Yield: 5.1 g (52%). 'H NMR (300 M H z , CDC13): 8 7.38 (m, 10H, Ph), 7.21 (s, 1H, 4-56 H), 7.20 (m, 4H, Th), 7.01 (dd, 1H, 4"-H), 4.33 (m, 4H, CH2). 31P{'H} NMR (121.5 MHz, CDC13): 5 -17.67 (s). MS(EI): m/z = 520. Anal. C26Hi 90 2PS 3 requires C, 63.65; H, 3.90. Found: C, 63.76; H, 4.04%. 2-diphenylphosphinothiophene capped Au NPs (16) Phosphine 12 (0.46 g, 1.7 mmol) was added to a solution of HAuCLf3H20 (0.62 g, 1.6 mmol) and tetraoctylammonium bromide (0.63 g, 1.1 mmol) in a nitrogen sparged distilled water/toluene (20/26 mL) mixture. The yellow solution was stirred vigorously for 20 min. NaBH4 (0.7 g, 18.5 mmol) was dissolved in 5 mL of distilled water and added dropwise to the yellow solution. The solution rapidly turned black and was stirred for an additional 3 hours. The solvent was then removed in vacuo to yield crude NPs, which were purified by washing with concentrated NaHS03 (250 mL) followed by a 3:2 water/methanol solution (250 mL) on a frit. The washed solid was futher purified by reprecipitation (8 times) from a solution of methylene chloride using pentane. The pure product was obtained as a dark black powder. After purification no residual phase transfer catalyst or starting materials were observed by NMR spectroscopy. 5-diphenylphosphino-2,2'-bithiophene capped Au NPs (17) This material was prepared according to the same procedure used for the preparation of 16 using phosphine 13 instead of 12. The amounts used were as follows: phosphine 13 (1.52 g, 4.3 mmol), HAuCl4.3H20 (0.49 g, 1.2 mmol), tetraoctylammonium bromide (0.63 g, 1.1 mmol) and NaBH4 (0.7 g, 18.5 mmol). Pure 17 was obtained as a dark black powder. After purification no residual phase transfer catalyst or starting materials were observed by NMR spectroscopy. 5-diphenylphosphino-2,2': 5',2' '-terthiophene capped Au NPs (18) 57 This material was prepared according to the same procedure used for the preparation of 16 using phosphine 14 instead of 12. The amounts used were as follows: phosphine 14 (1.5 g, 3.5 mmol), HAUCI4.3H2O (0.4 g, 1.0 mmol), tetraoctylammonium bromide (0.63 g, 1.1 mmol) and NaBlTj (0.55 g, 14.6 mmol). Pure 18 was obtained as a dark black powder. After purification no residual phase transfer catalyst or starting materials were observed by N M R spectroscopy. 5-diphenylphosphino-3', 4 '-ethylenedioxy-2,2 ':5', 2' '-terthiophene capped Au NPs (19) This material was prepared according to the same procedure used for the preparation of 16 using phosphine 15 instead of 12. The amounts used were as follows: phosphine 15 (1.69 g, 3.4 mmol), HAuCl4.3rJ.2O (0.39 g, 1.0 mmol), tetraoctylammonium bromide (0.63 g, 1.1 mmol) and N a B H 4 (0.55 g, 14.6 mmol). Purification was by reprecipitation (6 times) from a solution of methylene chloride using hexanes, followed by reprecipitation from methylene chloride with diethyl ether (5 times). Pure 19 was obtained as a dark brown powder. After purification no residual phase transfer catalyst or starting materials were observed by N M R spectroscopy. 5-(diphenylphosphine oxide)-2,2':5',2"-terthiophene (20) 14 (0.476 g, 1.1 mmol) was dissolved in a 7:3 mixture of CHC^acetone and an excess of 30% H2O2 was added. After the mixture was stirred for 30 minutes, it was dried with MgS04 and vacuum filtered. The solvent from the filtrate was removed by rotary evaporation which was purified by chromatography on silica gel with hexanes/acetone (6/4 v/v). Removal of the solvent gave 20 as a yellow powder. Yield: 0.46 g (93%). ' H N M R (300 MHz, CDC1 3): 5 7.80 - 7.71 (m, 4H, Ph), 5 7.61 - 7.45 (m, 6H, Ph), 7.36 (dd, 1H, 4-H), 7.28 (dd, 1H, 5 "-H), 7.20 (dd, 1H, 3-H), 7.17 (dd, 1H, 3"-H), 7.11 (d, 1H, 3 ' - / / ) , 7.06 (d, 1H, 4'-H), 7.01, (dd, 1H, 4"-H). 3 1 P{'H} 58 NMR (121.5 MHz, CDC13): 5 23.73 (s). MS (EI): m/z = 448. Anal. C^H^C-PSa requires C, 64.26; H, 3.82. Found: C, 63.90; H, 3.50%. 5-(diphenylphosphine oxide)-3',4'-ethylenedioxy-2,2':5',2''-terthiophene(21) This material was prepared according to the same procedure used for the preparation of 20 using phosphine 15 (0.5 g, 1 .Ommol) instead of 14. Yield: 0.45 g (90%). *H NMR (300 MHz, CDCI3): 5 7.79 - 7.70 (m, 4H, Ph), 5 7.58 - 7.42 (m, 6H, Ph), 7.33 (dd, 1H, 4-H), 7.22 (m, 3H, 3,3", 5"-H), 7.01 (dd, 1H, A"-H), 4.33 (m, 4H, CH2). 31P{'H} NMR (121.5 MHz, CDCI3): 5 22.91 (s). MS (EI): m/z = 502. 2.3 Results 2.3.1 Synthesis Phosphines 12-15 were prepared by lithiation of the respective oligothiophene followed by quenching with chlorodiphenylphosphine (Scheme 2-1). Analysis of the crude products by 31 P NMR spectroscopy showed the presence of three different phosphine species. Two of the peaks had negative chemical shifts, one was assigned to the diphosphine product as the chemical shift matched well with previously made diphosphino-oligothiophenes21 resulting from doubly lithiating the oligothiophene. The peak with a positive chemical shift was assigned to the phosphine oxide. This was confirmed by synthesizing and isolating the phosphine oxide analogues of 14 and 15 (20 and 21, respectively). The remaining peak belongs to the desired product. Column chromatography was used to isolate the desired phosphines. The yields were low due to the other side products formed during the reaction. 59 Scheme 2-1 n = 1, PT (12) 1) n -BuL i , T H F , -78 °C n = 2, PT2(13) PT3(dioxy) (15) Au NPs capped with 12-15 (NPs 16-19, respectively) were prepared using a modified Brust-Schiffrin procedure developed by Weare (Scheme 2-2). HAUCI4.3H2O and the appropriate phosphine (12-15) were dissolved in a two-phase toluene/water mixture with a phase transfer catalyst, tetraoctylammonium bromide. A large excess of NaBH4 was added to reduce the Au salt, which resulted in the solution turning black almost immediately. The black solution was stirred for three hours before work-up. The NPs were first purified by washing with concentrated NaHS03 (250 mL) followed by a 3:2 water/methanol solution (250 mL) on a glass frit to remove any Au(phosphine)Cl salt made during the reduction. The purification process was continued with numerous reprecipitations from CH2CI2 using pentane until the ! H NMR and 31 P NMR spectra showed no evidence for phase transfer catalyst and free phosphine, respectively. The absence of a triplet at 5 ~ 3.4 in the 'H NMR spectrum provides evidence for the removal of the phase transfer catalyst. From the 3 1P NMR spectrum, there were also no 60 unbound phosphine capping groups (12-15) left which have a peak at 8 -18 (Table 2-1). An impurity that could not be completely removed from 18 and 19 is the Au salt of each phosphino-o oligothiophene (i.e. AuCl(14) and AuCl(15)) which are intermediates in the NP synthesis. Gold phosphino-oligothiophene salts have a characteristic 3 1P-NMR chemical shift at 8 20.10 All the NPs are soluble in toluene, CH2CI2 and CHCI3 but 16 is noticeably less soluble and requires prolonged sonication to completely dissolve. The *H NMR spectra of the Au NPs show broadening of the phenyl and oligothiophene peaks relative to the free phosphino-oligothiophene. This broadening of the peaks is expected after the phosphines bind to the Au NP surface and are in a sterically-confined environment that limits orientational freedom.22 The 3 1P NMR spectra of the Au NPs show a shift downfield relative to the free phosphine, evidence of phosphorous binding to the Au NP surface. The chemical shifts for 16-19 are between 8 41.9 - 45.8 which is close to the reported value of 8 42.7 for PPh3 capped Au NPs.23 T 1 Table 2-1 Chemical shifts of peaks in the P-NMR spectra and UV-vis absorption maxima of phosphino-oligothiophenes (12-15) and phosphino-oligothiophenes capped Au NPs (16-19). Compound 3 1 P NMR resonance, 8 (ppm)a UV-vis absorption, Xmax (nm) 12 -18(s)b 265 13 -18.7 (s)c 330 14 -18.7 (s)d 250, 374 15 -17.7 (s)e 390 16 43.4 (s), 43.0 (s) -250, 500-550 17 44.5 (s), 44.0 (s) 329, 500-550 18 45.1 (s), 44.9 (s), 44.7 (s), 42.2 (s), 20.7 (s)e 378, 500-550 19 45.8 (s), 45.2 (s), 41.9 (s), 20.1 (s)e 395, 500-550 spectra were referenced to external 85% H3PO4. bref. 10. cref. 19. dref. 20.e ref. 16. 61 Scheme 2-2 19 2.3.2 UV~vis Absorption Spectra The UV-vis absorption spectra of the nanoparticles (16-19) in solution are shown in Figure 2-1 and the data collected in Table 2-1. A l l the nanoparticles have two clearly distinct absorptions. The absorption band at higher energies (250 - 395 nm) is assigned to the TZ—>n* absorption of the corresponding phosphine capping group attached to the Au NP surface which matches the A ,m ax values for free phosphines in solution (Table 2-1). The n—>n* absorptions bathochromically shift as the number of repeat units in the oligothiophene increases from 1 to 3 (16-18, respectively). As the number of thiophenes increase, the conjugation length of the oligothiophene also increases leading to the observed red-shift. The n—>n* absorption of 19 is 62 also red-shifted relative to 18 even though the number of repeat units is the same, because the ethylenedioxy groups attached to the oligothiophene backbone in 19 inductively donate electron density to the oligothiophene. This increases the HOMO energy level of 19 compared to 18 and explains the observed red-shift (confirmed in Section 2.3.6). The n—»7i* transition of 19 also appears broader compared to 18 with a shoulder at -425 nm possibly due to different conformations in the oligothiophene introduced by the presence of the ethylenedioxy group.24'25 Comparison of the energy of the TI—»7i* absorption in the capping groups prior to and after attachment to the NP surface revealed no significant difference except for a slight blue-shift observed for 12 after it is attached to the NP surface. This hypsochromic shift observed when the phosphino-oligothiophene is attached to the Au surface decreases as the length of the oligothiophene chain increases, as is generally seen in substituted oligothiophene where the effects of the substituent decreases as the chain length increases.21 This result is not too surprising in light of work from Stott et al.I0'26, where Au salts were prepared with the same a-phosphino-oligothiophene ligands. Stott et al. observed the same hypsochromic shift when 12 was coordinated to Au with a decreasing blue-shift as the length of the oligothiophene chain increased. This observation was attributed to the coordination of the lone pair on phosphorus to the metal, which DFT calculations showed contributes less to the 7i-orbitals of the oligothiophene as the chain length increased. This suggests that attachment of the phosphino-oligothiophenes to the Au NP surface, like coordination to a Au atom, has little effect on the electronic structure and conjugation of the oligothiophene. 63 300 400 500 600 700 800 Wavelength (nm) Figure 2-1 Solution UV-visible spectra of 16-19. Spectra are offset for clarity. Inset: surface plasmon absorption of 17-19 at higher concentration. The weaker absorption band at lower energy (500 - 550 nm) corresponds to the surface plasmon absorption of the Au NPs. These are quite weak showing no clear maxima. Alvarez et "2.1 al. have shown that A u particles with diameters < 2 nm have no surface plasmon absorption due to scattering from the particle surface exceeding the scattering from the bulk. From T E M images (Section 2.3.3), the average diameter of NPs 16-19 are less than 2 nm; particles of this size should not exhibit absorption. The origin of the weak surface plasmon absorption in the UV-vis spectrum observed here is due therefore to a small number of NPs larger than 2 nm in the NP size distribution. 64 The maximum absorption (Xmax) of the surface plasmon band of Au NPs should be at about 520 nm according to a theoretical calculation.28 This predicted value is observed for 16 but the maximum red-shifts slightly to -535 nm as the length of the oligothiophene chain in the capping groups increases (Figure 2-1, inset). This bathochromic shift of the surface plasmon band caused by the presence of the capping groups has been observed previously in Au NPs capped with a conducting polymer. ' Li et al. suggests that the conducting polymer effectively lowers the work function of the Au NP surface, lowering the energy of the surface resonance state. Hence, the lowering of the plasmon resonance state observed here indicates that the longer phosphino-oligothiophenes are able to contribute electron density to the Au NPs surface and suggests possible overlap of the wavefunctions of the oligothiophene and NP. 2.3.3 Transmission Electron Microscopy Transmission electron microscopy (TEM) was used to determine the sizes of the synthesized NPs. The TEM micrographs of the particles dropcast onto a carbon coated TEM grids are shown in Figure 2-2. The edge-to-edge distance for 150 particles were individually measured for each sample using an imaging program, Quartz PCI 5, by calibrating the pixel size to the scale bar of the images. The diameter measured corresponds only to the size of the Au core. The capping groups are not visible because there is not enough contrast compared to the amorphous carbon substrate. The NPs have narrow size distributions, which are shown in Figure 2-3. The mean diameters (dmean) for NPs 16,17,18 and 19 are 2.0 ± 0.4 nm, 1.7 ± 0.3 nm, 1.7 ± s 0.3 nm and 1.8 ± 0.4 nm, respectively. 65 66 1 1.2 1.4 1.6 1.8 2 2.2 2 .4 2 .6 2.8 3 3.2 3.4 3.6 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 Particle Diameter (nm) Particle Diameter (nm) 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 Partide Diameter (nm) Particle Diameter (nm) Figure 2-3 Size distribution for NPs (a) 16, (b) 17, (c) 18 and (d) 19 determined from TEM images. 150 particles were measured. The mean diameter (dmean) for each sample is shown on the graph. 2.3.4 X-ray Photoelectron Spectroscopy / Thermogravimetric Analysis All NP samples were characterized by X-ray photoelectron spectroscopy (XPS) (Table 2-2) and thermogravimetric analysis (TGA). XPS analysis shows Au 4f7/2 peaks between 84.2 and 84.4 eV for all NP samples comparable to metallic gold (83.8 eV). The P 2pi/2 peaks all appear at 131.5 - 131.8 eV as expected for a coordinated phosphine.31 The position of the S 2fi/2 peaks may provide some insight into the nature of the coordination of the S atoms in 16-19 to the Au in the NPs. Previous work on self-assembled monolayers (SAMs) of thiophene and oligothiophenes has shown that S atoms interact with a gold surface and the nature of the interaction differs depending on the nature and length of the oligomeric chain. Thiophene chemisorbs perpendicularly to the Au surface resulting in a Au-S interaction with a S 2pm binding energy of 162.0 eV while substituted thiophenes or longer oligothiophenes are 67 physisorbed parallel to the Au surface with a S 2pm binding energy of 163.4 eV. ' Recently, 3,4-ethylenedioxythiophene (EDOT) was used as the reductant in the preparation of Au NPs from HAuCU resulting in the formation of the polymer (PEDOT) coated Au NPs.29 XPS was used to characterize the PEDOT coated NPs and the S 2pm BE was found to be 163.4 eV indicating the S in PEDOT is bound to Au and physisorbed parallel to the Au NP surface. For all the NPs 16-19, the S 2py2 peak appears between 164.2 to 164.4 eV, close to the binding energy of S in free oligo-34 or polythiophene35 (163.8 eV) strongly supporting the conclusion that the thiophene S of capping groups 12-15 does not interact or bond with Au in the NPs. Table 2-2 XPS and TGA analysis of 16,17,18 and 19. Sample Element Binding energy Atom ratio Atom ratio Mass % from TG (eV)° relative to P relative to Au 16 Au 4f7/2 84.3 4.5 1 74.7 P 2p 3/2 131.8 1 0.2 S 2p 3/2 164.2 0.9 0.2 CI 2p 3/2 n/a* 0 0 17 Au 4f7/2 84.2 2.6 1 57.1 P 2p 3/2 131.5 1 0.4 S 2p 3/2 164.3 1.9 0.7 CI 2p 3/2 n/a* 0 0 18 Au 4f7/2 84.2 2.5 1 52.7 P 2p 3/2 131.5 1 0.4 S 2p 3/2 164.4 2.9 1.6 CI 2p 3/2 n/a* 0 0 19 Au 4f7/2 84.4 1.8 1 48.3 P 2p 3/2 131.6 1 0.5 S 2p 3/2 164.4 3 1.6 CI 2p 3/2 n/a* 0 0 "Binding energies are measured with respect to adventitious carbon, C Is = 284.8 eV. *No CI was detected. Quantitative analysis of the XPS data using a curve fitting program, XPSPEAK v. 4.1, and integrating the peak fitted curves reveals the atomic ratio (Au:S:P) for all the prepared NP 68 samples (Table 2-2). No significant amount of CI was detected in any case. The ratio of S to P of 1:1, 2:1, 3:1 and 3:1 for NPs 16-19, respectively, is as expected from the elemental ratios of the corresponding capping ligands used. Multiplying the Au to P ratio of 16 from XPS by the molecular weight of Au and 12, respectively, reveals a mass ratio of 77 % Au and 23 % ligand. This is confirmed by thermogravimetric analysis (TGA), which shows ,75 % by mass of the sample is gold. TGA was also performed on 17-19 and gave results that were in agreement with the XPS data. 2.3.5 Determination of Average Molecular Formula The average molecular formula for the NPs can be calculated using the method developed by Schaaff et al. Given the average particle size of Au NPs, the average number of Au atoms per particle can be calculated using the following equation: NAU = (59 nm' 2)(7c/6)(dmeanf (Equation 2-1), where NAU is the average number of Au atoms in the NP and dmea„ is the average particle diameter. From TEM, the dmean of 16 is 2.0 ± 0.4 nm which gives 247 as the average number of gold atoms per NP. Using 247 Au atoms and knowing the mass % of Au in 16 from the TGA data, the average number of capping ligands in 16 is calculated to be 61. An average molecular formula of Au247(12)6i is determined for 16. Using the same process, an average molecular formula of Aui46(13)6i, Aui46(14)s6 and Au2oi(15)g6 is calculated for 17, 18 and 19, which are similar to a previously reported ratios of Au225(ligand)57, Aui4o(ligand)s3 and Au2oi(ligand)85.39 69 2.3.6 Cyclic Voltammetry Cyclic voltammetry (CV) was done on all Au NP samples in CH2CI2 containing 0.1 M («-Bu)4NPF6 using a Pt working electrode. Nanoparticles 16 and 17 showed no oxidation or reduction in the CH2CI2 solvent window. The oxidation of the capping ligands 12 and 13 requires a higher potential (> +2 V vs. SCE) for oxidation. In contrast, 18 and 19 were oxidized in the CH2CI2 solvent window and the voltammograms are plotted in Figure 2-4. The CV of 18 shows the presence of a single irreversible anodic wave at +1.25 V vs. SCE. This anodic wave has two cathodic features associated with it on the return scan. The wave at +0.93 V is due to the reduction of the surface bound material formed upon oxidation while the wave at higher potential (+1.18 V) likely results from reduction of shorter oligomers formed.40 The CV of 19 (Figure 2-4b) in CH2CI2 appears very similar to 18 except that oxidation occurs at a lower potential (+1.0 V). The lower oxidation potential is a result of increased electron density inductively donated by the ethylenedioxy group to the 7t-system terthiophene backbone 4 1 This stabilizes the HOMO energy level making it easier to oxidize relative to 18. 70 o T3 CD o o CO O 0.5 1.0 Potential (V) Figure 2-4 Cyclic voltammetry of (a) 18 and (b) 19 in CH2CI2 containing 0.1 M (n-Bu)4NPF6. Scanned from -0.5 to 1.6 V, scan rate = 100 mV s'1. 2.4 Conclusion In this Chapter, the preparation of a series of phosphino-oligothiophene capped Au NPs was described. The phosphino-oligothiophenes were shown to attach to the Au NP surface via the phosphine group. UV-vis absorption spectroscopy of the NPs showed two features: (1) a surface plasmon band corresponding to the Au NP and (2) the n—>n* band corresponding to the phosphino-oligothiophene capping group. The energy of the 71—>n* absorption red-shifts as the length of the oligothiophene moiety in the series increases. There was no significant difference in the energy of 7t—»7t* absorption and hence conjugation in the capping groups before and after 71 attachment to the NP surface. This suggests that attachment of the phosphino-oligothiophenes to Au NP surface has little effect on the electronic structure and conjugation of the oligothiophene. On the other hand, the Xms>i of the surface plasmon absorption for Au NPs slightly red-shifted compared to the predicted value. This is attributed to the lowering of the plasmon resonance state by phosphino-oligothiophenes on the Au NPs surface thus suggesting possible overlap of the wavefunctions of the oligothiophene and NP. The average particle size and average number of capping groups for all NP samples synthesized were also investigated. From TEM and TGA data, an average molecular formula of Au247(12)6i, Aui46(13)6i, Aui46(14) 56 and Au2oi(15)g6 was calculated for 16, 17, 18 and 19, respectively. 72 2.5 References 1. Blackborow, J. R.; Young, D., Metal Vapour Synthesis. Springer-Verlag: New York, 1979. 2. Mafune, F.; Kohno, J.; Takeda, Y . ; Kondow, T., J. Phys. Chem. B 2002, 106, 7575. 3. Stoeva, S.; Klabunde, K. J.; Sorensen, C. M . ; Dragieva, I., J. Am. Chem. Soc. 2002, 124, 2305. 4. Daniel, M . - C ; Astruc, D., Chem. Rev. 2004, 104, 293. 5. Turkevich, J.; Stevenson, P. C ; Hillier, J., Discuss. Faraday Soc. 1951, No. 11, 55. 6. Brust, M . ; Walker, M . ; Bethell, D.; Schiffrin, D. J.; Whyman, R., J. Chem. Soc, Chem. Commun. 1994, 801. 7. Kimling, J.; Maier, M . ; Okenve, B.; Kotaidis, V. ; Ballot, H. ; Plech, A. , J. Phys. Chem. B 2006, 110, 15700. 8. Weare, W. W.; Reed, S. M . ; Warner, M . G.; Hutchison, J. E., J. Am. Chem. Soc. 2000, 122, 12890. 9. Clot, O.; Akahori, Y . ; Moorlag, C ; Leznoff, D. B.; Wolf, M . O.; Batchelor, R. J.; Patrick, B. O.; Ishii, M . , Inorg. Chem. 2003, 42, 2704. 10. Stott, T. L.; Wolf, M . O.; Patrick, B. O., Inorg. Chem. 2005, 44, 620. 11. Moorlag, C ; Wolf, M . O.; Bohne, C ; Patrick, B. O., J. Am. Chem. Soc. 2005, 127, 6382. 12. Skotheim, T. A. ; Elsenbaumer, R. L.; Reynolds, J. R., Handbook of Conducting Polymers. Marcel Dekker: New York, N .Y. , 1998. 13. Hata, K.; Fujihara, H. , Chem. Commun. 2002, 2714. 14. Peng, Z.; Wang, E.; Dong, S., Electrochem. Commun. 2002, 4, 210. 15. Wuelfing, W. P.; Murray, R. W., J. Phys. Chem. B 2002, 106, 3139. 73 16. Sih, B. C ; Teichert, A. ; Wolf, M . O., Chem. Mater. 2004, 16, 2712. 17. Chiem Van, P.; Burkhardt, A. ; Shabana, R.; Cunningham, D. D.; Mark, H. B., Jr.; Zimmer, H., Phosphorus, Sulfur Silicon Relat. Elem. 1989, 46, 153. 18. Zhu, Y . ; Wolf, M . O., J. Am. Chem. Soc. 2000, 122, 10121. 19. Field, J. S.; Haines, R. J.; Lakoba, E. I.; Sosabowski, M . H. , J. Chem. Soc, Perkin Trans. 1 2001, 3352. 20. Clot, O.; Wolf, M . O.; Patrick, B. O., J. Am. Chem. Soc. 2001, 123, 9963. 21. Stott, T. L. Synthesis and study of phosphinothiophene compounds. 2005. 22. Terrill, R. H. ; Postlethwaite, T. A. ; Chen, C.-h.; Poon, C.-D.; Terzis, A. ; Chen, A. ; Hutchison, J. E.; Clark, M . R.; Wignall, G.; et al., J. Am. Chem. Soc. 1995, 117, 12537. 23. Petroski, J.; Chou, M . H. ; Creutz, C , Inorg. Chem. 2004, 43, 1597. 24. DiCesare, N . ; Belletete, M . ; Marrano, C ; Leclerc, M . ; Durocher, G., J. Phys. Chem. A 1998, 102,5142. 25. DiCesare, N . ; Belletete, M . ; Marrano, C ; Leclerc, M . ; Durocher, G., J. Phys. Chem. A 1999, 103,795. 26. Stott, T. L.; Wolf, M . O., J. Phys. Chem. B 2004, 108, 18815. 27. Alvarez, M . M . ; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M . N . ; Vezmar, I.; Whetten, R. L.,J. Phys. Chem. B 1997, 101, 3706. 28. Dalmia, A. ; Lineken, C. L.; Savinell, R. F., J. Colloid Interface Sci. 1998, 205, 535. 29. L i , X . ; L i , Y . ; Tan, Y . ; Yang, C ; L i , Y . , J. Phys. Chem. B 2004, 108, 5192. 30. Zhou, Y . ; Itoh, H. ; Uemura, T.; Naka, K.; Chujo, Y. , Langmuir 2002, 18, 5287. 31. Muilenberg, G. E., Handbook of X-ray Photoelectron Spectroscopy. Perkin-Elmer Corporation: New York, 1979. 32. Noh, J.; Ito, E.; Araki, T.; Hara, M . , Surf. Sci. 2003, 532-535, 1116. 74 33. Matsuura, T.; Shimoyama, Y., Eur. Phys. J. E: Soft Matter 2002, 7, 233. 34. Glenis, S.; Benz, M.; LeGoff, E.; Kanatzidis, M. G.; DeGroot, D. C.; Schindler, J. L.; Kannewurf, C. R., Synth. Met. 1995, 75, 213. 35. Tourillon, G.; Jugnet, Y., J. Chem. Phys. 1988, 89, 1905. 36. Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I . ; Whetten, R. L.; Cullen, W. G.; First, P. N.; Wing, C ; Ascensio, J.; Yacaman, M. J., J. Phys. Chem. B 1997, 101, 7885. 37. Cheng, P. P. PL; Silvester, D.; Wang, G.; Kalyuzhny, G.; Douglas, A.; Murray, R. W., J. Phys. Chem. B 2006, 110, 4637. 38. Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W., J. Am. Chem. Soc. 2000, 122, 11465. 39. Huang, T.; Murray, R. W., Langmuir 2002, 18, 7077. 40. Clot, O.; Wolf, M. O.; Patrick, B. O., J. Am. Chem. Soc. 2000, 122, 10456. 41. Groenendaal, L. B.; Zotti, G.; Aubert, P.-H.; Waybright, S. M.; Reynolds, J. R., Adv. Mater. 2003, 15, 855. 75 Chapter 3 Electrodeposition and Characterization of Gold Nanoparticles Linked with Phosphino-Oligothiophenes 3.1 Introduction Nanoscale metal and semiconductor particles are of interest due to the unique catalytic, optical and electronic properties arising from their size. ' These materials have been proposed for use in sensor, biomedical and nanoelectronic devices and many such applications require organization of these nanoparticles (NPs) into controlled and functional architectures. Considerable effort has been focused on creating two- and three-dimensional networks of NPs using electrostatic interactions,3' 4 hydrogen bonds5 and saturated organic linkages.6"8 Where particles are linked by non-conjugated linkers, electron tunneling is the predominant mechanism for electrical conduction.9 The electrical conductivity (a) depends on the charge carrier population (n), the electronic coupling term (/?, A"1) and the activation energy barrier (EA, kJ mol"1) to electron transfer (Equation 3-1 and 3-2), a = a0 exp[~m exp [ - £ V R n (Equation 3-1) cr0 = neju (Equation 3-2) where 5 is the average inter-particle distance (A), R the universal gas constant, T the temperature (K), e the elementary charge of an electron and ju the mobility of the charge carriers. Thus the electrical conductivity of these materials is expected to decrease as the length of the linker is increased due to higher activation energies; this has been observed experimentally.10 It A portion of this chapter has been published. Sih, B. C , Teichert, A. and Wolf, M . O. (2004) Electropolymerized ^-Conjugated Gold Nanoparticle Polymers. Chemistry of Materials. 16: 2712-2718. 76 is therefore of interest to prepare systems with improved inter-particle electrical conductivity with fixed particle-particle distances by varying ft and EA-Previous studies have shown that conjugated linkers between an electrode and a molecular site provides improved electron coupling (Jj = 0.4 to 0.6 A " 1 ) 1 1 " 1 3 relative to purely saturated linkers (J3= 0.8 to 1.0 A" 1 ) . Recently, Torma et al.XA used partially conjugated linkers to link Au NPs and showed that these materials have substantially lower EA values compared to Au NPs linked by non-conjugated linkers. The lower activation energies have been attributed to a different mechanism for inter-particle electron transfer, which involves charge propagation through the organic bridging molecules. Bourgoin et al}5 obtained similar results by comparing the electrical conductivities of a monolayer of dodecanethiol capped Au NPs with a monolayer of the same material where some of the ligands were exchanged with a terthiophene linker. The terthiophene linked monolayer exhibited a conductivity several orders of magnitude better than the unlinked Au monolayer. These studies suggest it would be of significant interest to link NPs using fully conjugated groups in three dimensions, thus allowing improved inter-particle charge transfer and electronic interaction. Datta et al.16 predicted that Au NPs linked together by 'molecular wires' such as polythiophene (PT) or polyphenylenevinylene (PPV) would function as molecular ribbons useful in generating an interconnected network. To this end, NPs have been embedded in 7t-conjugated 17 20 matrices " and conductivity measurements on these hybrid materials show conductivities several orders of magnitude greater than the corresponding polymers without NPs present.18' 2 0 These results demonstrate that gold NPs interact with the 7t-conjugated matrix; however, the electronic interactions and distribution of NPs in the matrix are ill-defined. Recently, gold 77 nanoparticles have also been capped with arenethiolates and the mixed saturated/conjugated 21 ligand substantially improved the inter-particle conductivity. Linking Au NPs with -^conjugated groups also offers the possibility of tuning the conductivity of the material after assembly. Three-dimensional networks of NPs linked by saturated bridges have fixed conductivities dependent on inter-particle distance. Once the network has been assembled, the linkers are fixed and hence the conductivity cannot be varied. However, it is well known that the electrical conductivity of 7t-conjugated polymers such as polyacetylene and PT can be controlled via the degree of chemical or electrochemical doping of these materials.22' 2 3 Therefore, in a network of Au NPs linked by 7i-conjugated bridges, in situ control of inter-particle interactions and conductivity via doping of the polymer may be possible. It was reasoned a suitable approach to such hybrid materials was to link the Au NPs capped with phosphino-oligothiophenes by coupling of the conjugated units. NPs functionalized with electroactive pyrrolyl or ferrocenyl groups attached via an alkanethiolate group have been linked, but in these systems the linked NPs are separated by an insulating alkyl tether.24'25 Prior work in our group has shown that phosphines may be used to coordinate transition metals to conjugated oligothiophenes, and these complexes can be electropolymerized into -^conjugated polymers.27'28 In this Chapter, the electrodeposition and characterization of phosphino-oligothiophene capped Au NPs are reported. UV-vis spectroscopy and cyclic voltammetry are used to probe the electronic structure of the crosslinked Au NP/oligothiophene materials. Conductivity of the crosslinked Au NPs are measured and the effect of electrochemical doping on the conductivity are also probed. A portion of the work presented in this Chapter has been published.29'30 78 3.2 Experimental 3.2.1 General Electrochemical measurements were conducted using a Pine AFCBP1 bipotentiostat. The working electrode was either a Pt disk, an indium tin oxide (ITO) thin film on glass or Au (1000 A ) deposited on Si using a Cr (50 A ) adhesion layer. The counter electrode was a Pt coil wire and the reference electrode a silver wire. An internal reference (decamethylferrocene) was added to correct the measured potentials with respect to saturated calomel electrode (SCE). [(«-Bu)4N]PF6 was used as a supporting electrolyte and was purified by triple crystallization from ethanol and dried at 90 °C under vacuum for three days. Methylene chloride used for cyclic voltammetry was purified by passing through an activated alumina tower. Solution electronic absorption spectra were obtained on a Varian Cary 5000 UV-vis/NIR spectrometer in C H 2 C I 2 and solid state absorption spectra were acquired on films deposited on ITO. Energy dispersive X-ray (EDX) analysis was performed on a Kevex Quantum light element X-ray detector equipped with a Quartz Xone X-ray analyzer. Elemental ratios for electrodeposited films on ITO/glass were compared to those of films of unlinked Au NPs cast from CH2C12 solution onto ITO/glass. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Leybold MAX200 equipped with an Al Ka source with a pass energy of 192 eV, the sampling area was 2 x 4 mm. Transmission electron microscopy (TEM) images were taken using a Hitachi H7600 electron microscope operating at 80-100 kV. Linked NPs were electrodeposited directly onto 2000-mesh gold grids. Scanning electron microscopy (SEM) images were taken using a Hitachi S4700 electron microscope operating at 5 kV. Films were imaged directly on ITO/glass electrodes used for electrodeposition. All inter-particle distances used in this paper were either 79 from literature or calculated using Hyperchem software via a semi-empirical (AMI) geometrical optimization. 3.2.2 Electrodeposition Electrochemical deposition was carried out in dry methylene chloride using a sealed glass three-electrode electrochemical cell. A silver wire (reference electrode), platinum wire (counter electrode) and either a platinum disk, ITO/glass or Au/Si (working electrode) were used. The Pt disk working electrode was polished prior to the experiment using a 0.05 um alumina slurry, followed by sonication in water to remove traces of alumina from the Pt surface, washed with water and dried. For ITO/glass or Au/Si working electrodes, the surface was cleaned with acetone and dried in vacuum before use. The deposition solution consisted of 15-20 mg of 18 or 19 dissolved in 6 mL of methylene chloride containing 0.1 M [(n-Bu)4N]PF6. Deposition was carried out by scanning from -0.5 to +1.6 V vs. SCE at 100 mV s"1. 3.2.3 Solid-State Conductivity Measurements Solid-state conductivity measurements for unlinked Au NPs 18 and 19 were carried out using interdigitated array electrodes (IDAs) kindly provided by Prof. Tim Swager (each array consists of 100 Au fingers with a 2 um gap between fingers, 2005 um finger length and 3.5 um finger height). For calculation of conductivities, the IDAs are treated as parallel plate electrodes, using the total area of all the electrode fingers facing each other across the 2 urn gap. The total area (ATOTAL) was calculated using Equation 3-3, AOTAL = AF,NGER (N ~L) (Equation 3-3) 80 where N is the number of IDA fingers.31 Unlinked NPs 18 and 19 were dissolved in methylene chloride and dropcast onto the clean IDA electrodes from solution and then dried. The conductivity of the cast NPs were measured using linear potential sweeps, which produces linear current-potential (I-V) curves. The conductivity of 18 and 19 increased as more material was cast onto the IDAs and eventually leveled off; measurements were taken once conductivities no longer increased. Conductivities (CTRT) were measured from the slope (AI/AV) of the I-V curve between ± 1000 mV and were calculated from Equation 3-4, dAI ATOTAIAV (Equation 3-4), where d is the IDA gap (2 urn). I-V scans were initiated at -1000 mV and scanned to 1000 mV at 100 mV s"1, the I-V curves remained linear with no evidence of hysteresis. At least five measurements were taken for each sample and the standard deviations found to be less than 1%. Solid-state conductivity measurements on linked NPs poly-18 and poly-19 were carried out by electrodeposition of the samples directly onto Au/Si substrates. Au/Pd (60/40) electrodes were sputter-coated on top of the films through a mask with an area between 16-25 mm2 and silver epoxy was used to attach a Cu wire to the Au/Pd contact. The thickness of the samples was measured using stylus profilometry (Tencor Alpha-Step 100). Conductivities (CTRT) of the electrodeposited NPs were measured using linear potential sweeps and were measured from the slope (AI/AV) of the I-V curve between ± 500 mV using Equation 3-4. Voltage scans were initiated at -500 mV and scanned to 500 mV at 100 mV s"1, the I-V curves remained linear with no evidence of hysteresis over this range. 3.2.4 Microelectrode Array Fabrication 81 A different microelectrode array was also fabricated at UBC for conductivity vs. electrochemical doping measurements with only 4 Au finger electrodes (Experimental Section 3.2.5). The fabrication of the microelectrode arrays was carried out in the UBC Nanofabrication facility, a class 1000 clean room. A two-mask process was used where the first mask was used for a metal lift-off procedure to form the microelectrodes, leads and contact pads. The second mask was used to pattern a S i3N 4 insulator coating over the leads leaving the microelectrodes and contact pads exposed. w-Doped Si wafers of <100> orientation, 3-in. diameter, with a 300 nm thermal SiC>2 coating were used as substrates for the microelectrodes. The Si wafers were first RCA cleaned by immersion in hot aqueous N H 4 O H (14% by volume) and H2O2 (14% by volume), immersion in hot aqueous HC1 (12.5% by volume) and H 202 (12.5%) by volume), rinsed with deionized water and dried with a stream of N 2 . Hexamethyldisilazane was spin-cast onto the wafers at 5000 rpm for 40 s and the wafers were then air dried for 60 s. Two-thirds of the wafer surface was then covered with AZ4110 photoresist starting from the center of the wafer and spun at 4000 rpm for 40 s. The wafer was then baked at 90 °C for 10 minutes. A Canon PLA-50IF mask aligner was used to expose the photoresist using the 405 nm line from a 250 W Hg lamp through a lithographic mask. The lithographic mask had 8 separate circuits designed to fit onto one 3-in. wafer. Each circuit had four separate 2 x 2 cm contact pads connected to one 500 x 4 um microelectrode. The microelectrodes were separated from each other by a 2 um gap (Figure 3-la). An exposure time of 12 s was used. The photoresist was developed for 60 s in AZ400K developer diluted 1:4 with deionized water. This was immediately followed by rinsing with deionized water and drying with a stream of N2. 82 A bilayer metallization was performed in a home built e-beam evaporation system. A 20 nm Cr adhesion layer was first evaporated followed by 100 nm of Au, which was monitored using a Quartz crystal microbalance. The Cr/Au thickness was measured using a Tencor Alpha-Step 200 profilometer and determined to be 121 nm. Cr/Au is in contact with the SiC>2 substrate only in the areas that form the electrodes, leads and contact pads. Cr/Au was also deposited everywhere else but on top of photoresist. The photoresist/Cr/Au was removed by sonication in acetone, sonication in deionized water and dried with N2 to finish the first lithographic step (Figure 3-la). 83 (a) Side-view: Top-view: Side-view: Top-view: Figure 3-1 Schematic diagram of microelectrode architecture after (a) first and (b) second lithographic steps. The Si3N4 passivation layer was then deposited on the whole wafer using plasma enhanced chemical vapor deposition (PECVD). The thickness of the Si3N4 layer was determined to be 355 nm using a profilometer. The photoresist spin-coating process was then repeated using the second mask where the leads are covered in photoresist but leaving the microelectrodes uncovered. This was accomplished by lining up alignment marks on the second mask with lithographed alignment marks from the first mask with the Canon PLA-501F aligner. The Si3N4 84 was then etched off using PECVD at room temperature for ~ 3 mins. The etching was monitored by profilometry measurements every 30 s and etching was stopped when the 355 nm of S13N4 had been etched off the electrodes and contact pads. The remaining photoresist was then removed by sonication in acetone, immersion in hot aqueous N H 4 O H (14% by volume) and H2O2 (14%) by volume), rinsing with deionized water and drying with N2. Individual chips were then scribed, separated and used as is (Figure 3-lb). 3.2.5 Conductivity versus Electrochemical Doping Prior to use of the microelectrodes, each electrode was tested with an ohmmeter to make sure there were no shorts with the other electrodes. Each electrode was also tested by recording the cyclic voltammogram (CV) of a solution of 0.01 M Fe(NO3)2/0.1 M [(/i-Bu^ TSTJPFe/CrfeCfe. Once the electrodes were shown to perform properly, they were washed with copious amounts of CH2CI2 and sonicated three times in CH2CI2 to remove any trace Fe(N03)2. Gold NPs were then deposited on the first microelectrode surface using 5 mg of 18 in 10 mL of 0.1 M [(n-Bu)4N]PF6 in CH2CI2 and scanning 25 times from 0 to +1.8 V vs. Ag wire. To link the first and second electrode, gold nanoparticles were deposited onto the second electrode by scanning 15 times at 5 scan increments from 0 to +1.4 V vs. Ag. After each 5 scan increment, the electronic link between the first and second electrode was tested by scanning the CV of both electrodes simultaneously with a 50 mV offset between them. After a total of 15 growth scans on the second electrode, the two electrodes were observed to be electronically linked. The conductivity versus electrochemical doping was determined by measuring the current passing between the first and second electrode in a monomer-free 0.1 M [(n-Bu)4N]PF6/CFf.2Ci2 solution as the potential applied to the working electrode was increased from 0 to +1.6 V vs. Ag with a 50 mV offset between them. The scan rate was 5 mV/s to ensure that 85 the measured current was due to current flow between the two electrically linked microelectrodes and not to oxidation of the film. 3.2.6 Spectroelectrochemistry A Cary 5000 UV-Vis-NIR spectrometer was used with a Pine AFCBP1 bipotentiostat for potential control. A 1-cm path length quartz cell was used with a Pt wire counter electrode and a Ag wire quasi-reference electrode (QRE). Electrodeposited samples of oligothiophene-capped Au NPs on ITO/glass were used as the working electrode and placed perpendicular to the beam path. An area of the working electrode coated with deposited material was defined with a mask and placed in the beam path. The cell was filled with 0.1 M [(n-Bu)4N]PF6 in dry CH2CI2 and the top of the cell fitted with a Teflon cap with three contacts for the electrodes. A reference spectrum was taken of solvent and electrolyte with a blank ITO working electrode in the beam path. The selected potential was applied to the working electrode for 5-10 mins to allow the film, to reach steady state and the absorption spectrum was then measured. The applied potential was increased at 200 mV increments from 0 V to + 1.4 V vs. Ag QRE and subsequently decreased with the same potential intervals to check for reversibility. 3.3 Results 3.3.1 Electrodeposition/Cyclic Voltammetry Electrodeposition of 18 and 19 is carried out in CH2CI2 containing 0.1 M (n-Bu)4NPF6 at a Pt electrode scanned repeatedly from -0.5 to +1.6 V vs. SCE (Figure 3-2). The CV of 18 shows the presence of a single irreversible anodic wave at +1.25 V vs. SCE (Figure 3-2a, first scan). This anodic wave has two cathodic features associated with it on the return scan. The 86 wave at +0.93 V is due to the reduction of the surface bound material formed upon oxidation while the wave at higher potential (+1.18 V) likely results from reduction of shorter oligomers 2 7 formed. Repeated scanning from 0 to +1.4 V results in the emergence of a new redox wave at +1.0 V along with an increase in current indicating the formation of an extended conjugated system, poly-18, an electrochromic film attached to the electrode surface (Scheme 3-1). Scheme 3-1 87 I 1 1 ' 1 ' 1 ' 1 1 1 -0.5 0.0 0.5 1.0 1.5 2.0 Potential (V) Figure 3-2 Cyclic voltammetry of (a) 18 and (b) 19 in CH2C12 containing 0.1M («-Bu)4NPF6. Multiple scans from -0.5 to 1.6 V vs. SCE, scan rate = 100 mV s"1. The electrodeposition process resulting in poly-18 is well known and proceeds through an oxidative coupling mechanism. ' By analogy to the preparation of conducting polymers using this coupling process, the electrodeposited materials are referred to as "polymers". The films are reddish brown in the neutral state and are dark green when fully oxidized. Similar electrochromic behavior is observed for many conjugated polymers and oligomers.34 The CV of 19 (Figure 3-2b) in C H 2 C I 2 appears very similar to 18 except that oxidation occurs at a lower potential (+1.0 V) and leads to the formation of poly-19, a dark red-orange film when neutral and 88 nearly black when oxidized. The lower oxidation potential for ethylenedioxy-containing thiophene oligomers is well known.35 i 1 1 • 1 1 1 • 1 i • 1 1 1 • 1 • 1 • 1 0.0 0.5 1.0 1.5 2.0 -0.5 0.0 0.5 1.0 1.5 2.0 Potential (V) vs SCE Potential (V) vs SCE Figure 3-3 Cyclic voltammetry of (a) poly-18 and (b) poly-19 deposited on a Pt working electrode in CH2CI2 containing 0.1M («-Bu)4NPF6. Scan rate = 100 mV s"1. After the electrodeposition of poly-18 or poly-19 on a Pt working electrode, the electrode was rinsed copiously with CH2CI2 and immersed in a monomer-free solution of 0.1 M [(«-Bu)4N]PF6. The CV of the electrodeposited materials was then obtained (Figure 3-3). Both poly-18 and poly-19 showed two quasi-reversible redox waves corresponding to the first and second one^ electron oxidation of the sexithiophene moieties, which result from the electrochemical coupling of the terthiophene capping groups. The second one-electron oxidation of the sexithiophene moiety occurs at a similar potential as the oxidation of the terthiophene capping groups in the uncoupled NPs. Therefore, it is entirely possible that the second redox wave observed for the CVs of poly-18 and poly-19 could also have a partial contribution from residual uncoupled terthiophene groups present in the crosslinked material. 89 3.3.2 X-ray Photoelectron Spectroscopy/Energy Dispersive X-ray Spectroscopy The elemental composition of the poly-18 and poly-19 films were determined by both energy-dispersive X-ray (EDX) and XPS analyses (Table 3-1). The XPS S to Au ratio is 1.3:1 and 1.1:1 for poly-18 and poly-19, respectively. The Au to P ratio is unreliable due to the presence of residual («-Bu)4NPF6 even after copious washing of the samples after electrodeposition. The S to Au ratios in the electrodeposited films are lower relative to 18 and 19 (Table 2-2) indicating loss of ligand in the films. The loss of ligand after crosslinking is confirmed by EDX. XPS analysis of both films shows a slightly higher energy Au 4/7/2 peak at 84.5 eV compared with the unlinked NPs. The higher binding energy for poly-18 and poly-19 suggests the electrodeposited films may be partially oxidized even when removed from solution atO V. Table 3-1 EDX and XPS analysis of poly-18 and poly-19. Sample Element Binding energy Atom ratio relative Atom ratio relative (eV)' to Au from XPS to Au from EDX poly-18 Au 4/ 7 / 2 84.5 1 1 P 2 p i / 2 131.7 & 136.0 c 0.5 d 0.49 ± 0.02rf S 2p3/2 164.3 1.3 1.08 ±0 .03 C\2p3/2 n/a* 0 0 poly-19 Au 4/ 7 / 2 84.5 1 1 131.6 & 134.0 c 0.7^ 0.64 ±0.05^ S 2p3/2 164.3 1.1 1.30 ±0 .07 C\2p3/2 n/a* 0 0 "Binding energies are measured with respect to adventitious carbon, C Is = 284.8 eV. ~*No CI was detected. cTwo phosphorus species provided the best fit to the data. ^Ratios relative to P atom in the film are unreliable due to presence of residual («-Bu)4NPF6. Fluorine was also observed in these films. 90 3.3.3 Electron Microscopy Figure 3-4 Transmission electron microscopy images of (a) poly-18 and (b) poly-19. Poly-18 was electrodeposited directly on a gold transmission electron microscope (TEM) grid and a T E M image of the film was obtained (Figure 3-4a). It shows that after electrodeposition the individual NPs are still intact and are now crosslinked. The bridged NPs form larger (-200 nm) sized clusters, which are linked together into a three-dimensional network as seen in a scanning electron micrograph of the film (Figure 3-5). Poly-18 forms strand-like structures quite different from the 'cauliflower-like' appearance of polythiophene films.36 The morphology of poly-19 is similar to poly-18 and Figure 3-4b shows the individual NPs, 19, linked together to form a three-dimensional network. Analysis of NP sizes in poly-18 and poly-19 reveal no significant difference in mean diameter compared to the unlinked NPs prior to electrochemical coupling. 91 Figure 3-5 Scanning electron micrograph of poly-18. 3.3.4 UV-vis-NIR Absorption Spectroscopy The UV-vis spectrum of a film of poly-18 shows a broad absorption peak at 435 nm, red shifted by 55 nm from the maximum absorption for 18 (Figure 3-6a). This is consistent with longer conjugation lengths in poly-18 than in 18. Moreover, the absorption maximum is comparable to that of sexithiophene (432 nm) 3 7 consistent with a structure for poly-18 consisting predominantly of sexithiophene units linking the Au NPs. The absorbance also increases as the number of C V scans used to grow the film is increased, consistent with thicker films being formed. More interesting is the red shift of the plasmon shoulder of 18 to a very broad absorption at higher wavelengths (> 700 nm) for poly-18. Previously, red-shifts in the plasmon band have been theoretically attributed to close contact of optically absorbing NPs where metal NPs can induce plasmon oscillations in other neighboring metal NPs via near-field electrodynamic interactions.38"40 This plasmon coupling interaction has also been observed experimentally in Au NPs assembled into ordered superlattice structures.4' 4 1 ' 4 2 In the case of poly-18, the absorption at lower energy (near-IR) is due to plasmon coupling between adjacent 92 Au NPs where the oligothiophene linker acts as a bridge facilitating near-field coupling interactions between adjacent NPs. 3 0 This nature of this coupling interaction is probed and discussed fully in Chapter 4 . (a) 18 in C H 2 C I 2 18 on glass poly-18 on ITO (b) 19 in C H 2 C I 2 19 on glass poly-19 on ITO 300 400 500 600 700 800 300 400 500 600 700 800 Wavelength (nm) Wavelength (nm) Figure 3-6 UV-vis spectra of (a) 18 (in C H 2 C I 2 and on glass) and poly-18 film and (b) 19 (in CFf^Cband on glass) and poly-19 film. The UV-vis absorbance in poly-19 is very similar to poly-18 such that it also undergoes a red-shift in the n—>n* absorption relative to 19, which is a result of increased conjugation after electrochemical coupling (Figure 3-6b). Unfortunately, the absorption maximum of the n—>n* band in this material cannot be identified as the band is very broad and overlaps with the surface plasmon peak of the Au NPs. 3.3.5 Solid-state Conductivity The electrical conductivity of dried films of unlinked Au NPs 18 and 19 were measured on interdigitated array (IDA) electrodes received from Prof. Tim Swager (MIT). The I-V curves observed for both NPs were linear (Figure 3-7a) and electrical conductivities at room 93 temperature (CTRT) are given in Table 3-2. For unlinked particles, the predominant mechanism of electrical transport is tunneling as observed in alkylthiol capped NPs and for these materials conductivity is dependent on inter-particle distance. 9 ' ' 4 3 Wuelfing et al9 calculated the Au inter-NP distance to be 1.2 times the length of the alkylthiol capping group assuming that the alkyl chains between neighboring NPs are interdigitated. The A u inter-NP distance for the particles in this study has to be somewhere between full interdigitation (12.7 A , length of capping group) and zero interdigitation (25.4 A ) . Since oligothiophenes are known to self-assemble into a herringbone structure comparable to the interdigitation of alkyl chains, 4 4 ' 4 5 it is presumed the same packing arrangement occurs with 18 and 19 as for alkylthiol-capped NPs. The lengths of the capping ligands 14 and 15 (both 12.7 A ) are closest to decanethiol capped NPs where the length of the ligand is calculated to be 12.7 A . 9 The inter-particle distance is then expected to be comparable. It is not surprising, therefore, that the room temperature conductivity of decanethiol capped NPs 9 (10"5 S cm"1) is quite similar to that measured for 18 and 19. Figure 3-7 Current-potential response of (a) 18 and 19 at room temperature on an IDA electrode and of (b) poly-18 and poly-19 sandwiched between two electrodes. 94 The electrical conductivity of the electrodeposited NPs 18 and 19 on gold substrates were also measured at room temperature (Figure 3-7b) by sputtering an Au/Pd electrode onto the film surface. The measured electrical conductivities for poly-18 and poly-19 are three orders of magnitude greater than the conductivity for the unlinked NPs (Table 3-2) even though the inter-particle distance is larger between NPs in poly-18 and poly-19 (24.6 and 24.7 A, respectively, calculated using the dimerized forms of 18 and 19). The electrical conductivities reported for similarly sized Au NPs linked by similarly sized alkyl linkers are significantly lower than for poly-18 and poly-19, for example 1,10-decanethiol capped NPs linked by Cu 2 + ions (2 x 10"6 S cm"1)46 or NPs linked by the dithiol linker 1,16-hexadecanedithiol (NPs 21.55 A apart, 4.81 x 10" 5 S cm"1).47 For NPs linked by saturated alkyl chains, the predominant mechanism for electrical conductivity is electron tunneling from particle to particle.46'47 Thus, the substantial increase of <JRT for similar inter-particle distances in poly-18 and poly-19 suggests that the main mechanism of electrical conductivity involves propagation through the 7t-conjugated bridging molecules as observed for partially conjugated bridges.14 A similar increase in conductivity has been observed previously with a monolayer of dodecanethiol capped Au NPs where a few of the capping ligands have been exchanged with a terthiophene linker.15 The conductivities of poly-18 and poly-19 are similar, suggesting that the length of the conjugated bridge plays a greater role than changes in electron density on the bridge resulting from substituents. 95 Table 3-2 Room temperature electrical conductivities for 18, 19, poly-18 and poly-19. Sample aRT(S cm"1) Inter-particle distance (A) 18 (6.16 ± 0.02) x 10"5 ~15.3a 19 (7.20 ± 0.02) x 10"5 ~15.3a poly-18 (3.0 ± 0.5) x 10"2 24.6* poly-19 (8 ± 3 ) x 10"2 24.7* aInter-particle distance based on calculated 1.2 x capping ligand length. This assumes interdigitation and packing for 18 and 19 is similar to thiol capped Au NPs . 9 ' 4 6 Calculated using the dimerized forms of capping ligands 14 and 15. 3.3.6 Conductivity as a Function of Electrochemical Doping Figure 3-8 (a) Photograph of a fabricated microelectrode. S E M images of microelectrodes (b) before electrodeposition, (c) after 10 electrodeposition cycles with no electrical contact 9 6 between microelectrodes and (d) after 20 electrodeposition cycles with an electrical link between microelectrodes. Electrodes A and B are marked on the figure. Electrodeposition was performed on the prepared microelectrodes (Figure 3-8a) using the same conditions used for the bulk electrode and varying the number of growth cycles of 18 to control the amount of material deposited on the microelectrodes. At relatively few cycles (< 10), the amount of poly-18 deposited is small enough that the individual electrodes are coated with polymer but are not electrically connected (Figure 3-8c). Figure 3-9a shows the cyclic voltammetry (CV) of 18 during electrodeposition for the first 5 scans. Polymer growth on the microelectrodes is similar to growth on a bulk Pt electrode as shown previously (Section 3.2.2). The CVs of poly-18 on electrode A and B are shown in Figure 3-9b with a 50 mV offset between the two electrodes. Since there is no electrical contact between electrodes A and B, there is no current flow between the electrodes and both CVs show positive current. As the number of electrodeposition cycles and increased, eventually sufficient material is deposited to electrically connect the microelectrodes (Figure 3-8d). When the microelectrodes are connected with poly-18, the CVs of two electrodes with a 50 mV offset between them is shown in Figure 3-9c. The current for the oxidation of poly-18 on electrode B is reversed (i.e. negative current) relative to electrode A due to current flow from electrode B to A. This clearly shows that both electrodes are electronically connected by poly-18. 97 c QJ 3 o 400-350- (c) 300-< 250-c 200-c CD 150-Cur 100-50-0--50--100-0.5 1.0 1.5 P o t e n t i a l ( V ) v s A g m i c r o e l e c t r o d e A m i c r o e l e c t r o d e B o!o 0^ 5 "L0 P o t e n t i a l ( V ) v s A g — i — 1.5 < O 140 120 100-1 80 60 40-| 20 0 -20 -40 (b) 2.0 2.0 - m i c r o e l e c t r o d e A m i c r o e l e c t r o d e B 0.0 0.5 1.0 1.5 P o t e n t i a l ( V ) v s A g 2.0 0.5 1.0 P o t e n t i a l ( V ) v s A g Figure 3-9 (a) Electrodeposition of 18 on microelectrode A and B for 5 scans. Cyclic Volatmmetry of poly-18 with 50 mV offset between microelectrode A and B (b) without electrical contact and (c) with electrical contact between electrodes, (d) Plot of poly-18 conductance between microelectrode A and B as a function of an applied electrochemical potential. Figure 3-9d shows that the conductivity of poly-18 immersed in 0.1 M (n-Bu4)NPF6 depends on the electrochemical potential (i.e. doping) applied to the polymer. This was measured by measuring the current flow between electrode A and B as the potential at both electrodes is increased from 0 to +1.6 V with a 50 mV offset between them. A very slow scan rate of 5 mV/s was to used to ensure that the current being measured was due to current flow between the two electrically linked microelectrodes and not to oxidation of poly-18. The conductivity of poly-18 is constant at low applied potentials (< + 0.8 V) but increases as the 98 applied potential is further increased. The potential at which the conductivity increases in poly-18 is at the same potential where it is oxidized (Figure 3-3a). This suggests that the increase in conductivity is due to an increase in charge carriers introduced by oxidation of the oligothiophene capping groups in poly-18. Polythiophene exhibits a similar increase in conductivity when it is electrochemically oxidized due to formation of polaron and bipolaron charge carriers.48 Although one can observe the increase in conductivity as poly-18 is oxidized, a quantitative value for conductivity cannot be determined using the microelectrodes due to the nature of poly-18 growth. Poly-18 grows in a cluster-like manner, which leaves a lot of gaps between microelectrodes A and B (Figure 3-8d) such that the amount of poly-18 between electrode A and B cannot be accurately determined. 3.3.7 Spectroelectrochemistry To investigate the source of the increased conductivity in poly-18 at applied potentials higher than + 0.8 V, the optical absorbance of the oxidized material was measured (Figure 3-10). Electrolysis potentials starting at 0 mV and increasing by 200 mV increments to 1400 mV vs. Ag QRE were applied to poly-18 and poly-19 on ITO while measuring the UV-vis-NIR spectrum in situ. Poly-18 showed no change in the optical spectrum at low applied voltages (< 400 mV) but as the potential is increased further, the n—*n* absorption peak at 463 nm blue- shifts and disappears while two distinct peaks grow in at 750 and 1200 nm marked 'Pi and P2' (Figure 3-10a). The observed spectroelectrochemical behavior is quite similar to the behavior observed for sexithiophene49' 5 0 where the two new absorptions are associated with the radical cation species of the sexithiophene moiety. At an applied voltage of 1400 mV, these two peaks appear to coalesce into one broad peak centered at -1000 mV marked 'B' corresponding to the 99 oligothiophene dication species. The oxidative potentials needed for the formation of the polaron and bipolaron species in poly-18 match well with the one and two electron oxidations observed in the corresponding C V (Figure 3-3). 3 0 0 6 0 0 9 0 0 1200 Wavelength (nm) 1500 3 0 0 6 0 0 9 0 0 1200 Wavelength (nm) 1500 3 0 0 6 0 0 9 0 0 1 2 0 0 Wavelength (nm) 1 5 0 0 300 6 0 0 9 0 0 1200 Wavelength (nm) 150C F i g u r e 3-10 In situ UV-vis-NIR absorption spectrum of a poly-18 film on ITO as the applied potential is (a) increased and (b) decreased, and a poly-19 film on ITO as the applied potential is (c) increased and (d) decreased. When the electrochemically oxidized film was reduced in 200 mV increments to 0 mV, the bands in the NIR disappeared and the TI—>K* peak reappeared at 463 nm (Figure 3-10b) 100 showing that electrochemical doping of poly-18 is reversible. The spectroelectrochemistry of poly-19 is very similar to poly-18 except formation of cation and dication species occurs at lower potential (Figure 3-10c) due to the electron donating ethylenedioxy groups on the oligothiophene backbone. This is also consistent with CV data for poly-19. Hence, the observed increase in conductivity as poly-18 is electrochemically doped is due to an increase in the polaron and/or bipolaron charge carriers on the oligothiophene bridge. This is interesting because it demonstrates the in situ control of inter-particle interactions and conductivity via doping of Au NPs linked with a 7t-conjugated group. This is not possible in Au NPs networks linked by saturated bridges, which have fixed conductivities dependent on inter-particle distance. Once these networks have been assembled, the linkers are fixed and hence the conductivity cannot be varied. 3.4 Conclusions We have prepared films of electrodeposited Au NPs bridged by 7t-conjugated linkers from Au NPs 18 and 19. The NPs in poly-18 and poly-19 are linked both structurally and electronically by observed increases in conjugation, conductivity and plasmon coupling relative to the unlinked particles. The conjugated linker results in higher conductivities for poly-18 and poly-19 relative to unlinked NPs or networks consisting of particles linked with saturated alkyl groups. The increase in electrical conductivity is the result of lowering the electronic coupling term (j3) between the Au NPs by introducing another pathway for inter-particle conduction, involving the Tt-conjugated bridge. Au NPs bridged by Tc-conjugated linkers were also shown to have a tunable conductivity where the conductivity in the material can be increased by electrochemical oxidative doping. 101 This increase in conductivity has been shown to be due to an increase in the polaron and/or bipolaron charge carriers on the oligothiophene bridge as the material is oxidized. This shows the in situ control of inter-particle interactions and conductivity via doping of Au NPs linked with a 7r-conjugated group is possible. This control is not possible in Au NP networks linked by saturated bridges, which have fixed conductivities. 102 3.5 References 1. Schmid, G., Chem. Rev. 1992, 92, 1709. 2. Shipway, A . N . ; Katz, E . ; Willner, I., ChemPhysChem 2000, 1,18. 3. Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L. ; Geer, R. E . ; Shashidhar, R.; Calvert, J. M . , Adv. Mater. 1997, 9, 61. 4. Shipway, A . N . ; Lahav, M . ; Gabai, R.; Willner, I., Langmuir 2000, 16, 8789. 5. Boal, A . K. ; Ilhan, F.; DeRouchey, J. E . ; Thurn-Albrecht, T.; Russell, T . P.; Rotello, V . M . , Nature 2000, 404, 746. 6. Brust, M . ; Bethell, D.; Kiely, C. J.; Schiffrin, D. J., Langmuir 1998, 14, 5425. 7. Bethell, D.; Brust, M ; Schiffrin, D. J.; Kiely, C , J. Electroanal. Chem. 1996, 409, 137. 8. von Werne, T.; Patten, T. E . , J. Am. Chem. Soc. 2001, 123, 7497. 9. Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E . ; Murray, R. W., J. Am. Chem. Soc. 200.0, 122, 11465. 10. Simon, U . ; Flesch, R.; Wiggers, PL; Schon, G.; Schmid, G., J. Mater. Chem. 1998, 8, 517. 11. Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton, M . D.; Feldberg, S. W.; Chidsey, C. E . D., J. Am. Chem. Soc. 1997, 119, 10563. 12. Creager, S.; Yu, C. J.; Bamdad, C ; O'Connor, S.; MacLean, T.; Lam, E . ; Chong, Y. ; Olsen, G. T.; Luo, J.; Gozin, M . ; Kayyem, J. F., J. Am. Chem. Soc. 1999, 121, 1059. 13. Holmlin, R. E . ; Haag, R.; Chabinyc, M . L. ; Ismagilov, R. F.; Cohen, A . E . ; Terfort, A. ; Rampi, M . A. ; Whitesides, G. M . , J. Am. Chem. Soc. 2001, 123, 5075. 14. Torma, V . ; Vidoni, O.; Simon, U . ; Schmid, G., Eur. J. Inorg. Chem. 2003, 1121. 103 15. Bourgoin, J.-P.; Kergueris, C ; Lefevre, E.; Palacin, S., Thin Solid Films 1998, 327-329, 515. 16. Datta, S.; Janes, D. B.; Andres, R. P.; Kubiak, C. P.; Reifenberger, R. G., Semicond. Sci. Technol. 1998, 13, 1347. 17. Zhou, Y . ; Itoh, H. ; Uemura, T.; Naka, K.; Chujo, Y. , Chem. Commun. 2001, 613. 18. Sarma, T. K. ; Chowdhury, D.; Paul, A. ; Chattopadhyay, A. , Chem. Commun. 2002, 1048. 19. Peng, Z.; Wang, E.; Dong, S., Electrochem. Commun. 2002, 4, 210. 20. Breimer, M . A . ; Yevgeny, G.; Sy, S.; Sadik, O. A. , Nano Lett. 2001, 1, 305. 21. Wuelfing, W. P.; Murray, R. W., J. Phys. Chem. B 2002, 106, 3139. 22. MacDiarmid, A . G., Angew. Chem. Int. Ed. 2001, 40, 2581. 23. Heeger, A . J., Angew. Chem. Int. Ed. 2001, 40, 2591. 24. Hata, K. ; Fujihara, Ff., Chem. Commun. 2002, 2714. 25. Yamada, M . ; Tadera, T.; Kubo, K.; Nishihara, H. , J. Phys. Chem. B 2003, 107, 3703. 26. Clot, O.; Akahori, Y . ; Moorlag, C ; Leznoff, D. B.; Wolf, M . O.; Batchelor, R. J.; Patrick, B. O.; Ishii, M . , Inorg. Chem. 2003, 42, 2704. 27. Clot, O.; Wolf, M . O.; Patrick, B. O., J. Am. Chem. Soc. 2000, 122, 10456. 28. Clot, O.; Wolf, M . O.; Patrick, B. O., J. Am. Chem. Soc. 2001, 123, 9963. 29. Sih, B. C ; Teichert, A. ; Wolf, M . O., Chem. Mater. 2004, 16, 2712. 30. Sih, B. C ; Wolf, M . O., J. Phys. Chem. B 2006, 110, 22298. 31. Wooster, T. T.; Longmire, M . L.; Zhang, H.; Watanabe, M . ; Murray, R. W., Anal. Chem. 1992, 64, 1132. 32. Roncali, J., Chem. Rev. 1992, 92, 711. 33. Roncali, J.; Gorgues, A. ; Jubault, M . , Chem. Mater. 1993, 5, 1456. 34. Zhu, Y. ; Wolf, M . O., J. Am. Chem. Soc. 2000, 122, 10121. 104 35. Groenendaal, L. B.; Zotti, G.; Aubert, P.-H.; Waybright, S. M.; Reynolds, J. R., Adv. Mater. 2003, 15, 855. 36. Ugalde, L.; Bernede, J. G.; Del Valle, M. A.; Diaz, F. R.; Leray, P., J. Appl. Polym. Sci. 2002, 84, 1799. 37. Bauerle, P., The Synthesis of Oligothiophenes. In Handbook of Oligo- and Polythiophenes, Fichou, D., Ed. Wiley-VCH: Weinheim, 1999; pp 89. 38. Quinten, M.; Kreibig, U., Surf. Sci. 1986, 172, 557. 39. Su, K. H.; Wei, Q. PL; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz, S., Nano Lett. 2003, 3, 1087. 40. Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht, B.; Aussenegg, F. R., Opt. Commun. 2003, 220, 137. 41. Lazarides, A. A.; Schatz, G. C., J. Phys. Chem. B 2000, 104, 460. 42. Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J., Langmuir 2002, 18, 7515. 43. Terrill, R. PL; Postlethwaite, T. A.; Chen, C.-h.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; et al., J. Am. Chem. Soc. 1995, 117, 12537. 44. Soukopp, A.; Glockler, K.; Kraft, P.; Schmitt, S.; Sokolowski, M.; Umbach, E.; Mena-Osteritz, E.; Bauerle, P.; Hadicke, E., Phys. Rev. B 1998, 58, 13882. 45. Spano, F. C , Annu. Rev. Phys. Chem. 2006, 57, 217. 46. Zamborini, F. P.; Leopold, M. C ; Hicks, J. F.; Kulesza, P. J.; Malik, M. A.; Murray, R. W., J. Am. Chem. Soc. 2002, 124, 8958. 47. Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H.-G.; Wessels, J. M.; Wild, U.; Knop-Gericke, A.; Su, D.; Schloegl, R.; Yasuda, A.; Vossmeyer, T., J. Phys. Chem. B 2003, 107,7406. 105 48. Thackeray, J. W.; White, H. S.; Wrighton, M. S., J. Phys. Chem. 1985, 89, 5133. 49. Nakanishi, H.; Sumi, N.; Ueno, S.; Takimiya, K.; Aso, Y.; Otsubo, T.; Komaguchi, K.; Shiotani, M.; Ohta, N., Synth. Met. 2001, 119, 413. 50. Otsubo, T.; Aso, Y.; Takimiya, K., Bull. Chem. Soc. Jpn. 2001, 74, 1789. 106 Chapter 4 Probing the Coupled Surface Plasmon Resonance Interactions in Oligothiophene-linked Gold Nanoparticles 4.1 Introduction Surface plasmon based photonics (plasmonics) has been an area of increasing focus due I 2 to the possibility of integrating optical components into electronic circuits. ' Limitations imposed by the diffraction of light relegate optical devices to a minimum size of a few hundred nm,1 while electronic circuits may be presently fabricated with dimensions below 100 nm. Plasmonics presents a possible solution to this challenge where optical signals are converted to electromagnetic waves propagated on the surface of a nanostructured metal. Metal nanoparticles (NPs) interact strongly with visible light resulting in the collective oscillation of free electrons in the conduction band, a phenomenon known as surface plasmon resonance (SPR).3 The dipole field resulting from SPR of a metal NP can induce plasmon oscillations in other neighboring metal NPs via near-field electrodynamic interactions.4 An electromagnetic waveguide with a linear array of closely spaced metal NPs where the lateral width is much less than the wavelength of light may be envisioned.5 This has recently been demonstrated experimentally.6 The ability of plasmon-based waveguides to transmit information between neighboring NPs is dependent on the strength of the near-field coupling between NPs.1 For example, the strength of this interaction is influenced by the inter-particle distance where SPR coupling between Au NPs has been shown to decrease as the distance between the particles increases 4 Thus, further probing this coupling interaction may lead to a better understanding of this A version of this chapter has been published. Sih, B.C. and Wolf, M.O. (2006) Dielectric Medium Effects on Collective Surface Plasmon Coupling Interactions in Oligothiophene-Linked Gold Nanoparticles. Journal of Physical Chemistry B, 110: 22298-22301. 107 interaction and ultimately assist in the design of improved plasmonic wires. SPR coupling between Au NPs may be investigated using UV-vis absorption spectroscopy, as SPR coupling 7 8 results in a broad band at 600-800 nm, red-shifted from the SPR band of isolated Au NPs. Recently it has been demonstrated that using conjugated organic groups to connect adjacent NPs may provide a conductive link9'1 0 resulting in the perturbation of the coupled surface plasmon.10 The Tt electrons of the conjugated linker interact with the NP dipoles resulting in perturbation of the surface plasmon.10 In Chapter 3, the preparation of thin films where Au NPs are linked by conjugated oligothiophene (OT) groups was discussed." Absorption spectra of these NP films revealed, in addition to the 7t—»TC* band, a broad band in the near-IR (> 800 nm) (Figure 3-6). No coupling of SPR modes from dipole-dipole interactions of proximal NPs is expected when the crosslinker dimension is close to that of the NP diameter. Since the average particle diameter (dmean = 1.7 ± 0.4 nm) is smaller than the inter-particle spacing (-2.5 nm), the observed absorption in the near-IR cannot be attributed to dipole-dipole interactions arising from proximity effects. Larger particles (dmean = 2.6 ± 0.3 nm) were also electrodeposited however the absorption spectra showed no significant difference from those of the smaller NPs. This suggests the possibility that the OT linker facilitates dipole interactions between adjacent NPs resulting in a collective SPR band. 4.1.1 Theory The dielectric environment is another variable that could possibly affect the strength of the coupled SPR. Effects of the dielectric environment on the SPR of individual metal NPs were modeled by Mie 1 3 in the early 20th century. Mie solved Maxwell's equation for the scattering of electromagnetic radiation by spherical particles and showed the absorption efficiency (Qabs) of metal nanospheres is dependent on two parameters m and x. The first m is the magnitude of 108 refractive index mismatch between particle and the surrounding medium expressed as the ratio of the refractive indices: n m = — (Equation 4-1) where n and nm are the refractive index of particle and the surrounding medium, respectively. The parameter x accounts for the size of the particle according to the equation, l7mmr ,^ „ „ x = (Equation 4-2) X where r is the radius of the particle and X is the wavelength of radiation. Mie's calculations show that changing the size of the nanosphere has an effect on the absorption maximum. Similarly, Mie predicted that increasing the dielectric constant of the surrounding medium will result in a red-shift in the wavelength of maximum absorption. Mie theory has been used effectively to fit the SPR absorption data of unlinked Au NPs dissolved in various dielectric media.14' 1 5 There has also been some preliminary work on the effects of the dielectric environment on the collective SPR of coupled Au NPs1 6 but attempts to employ Mie theory to explain the results have been unconvincing. This is because Mie theory only provides solutions for light scattering by an isolated isotropic sphere embedded in a homogeneous medium and does not account for dipole-dipole interactions occurring between adjacent Au NPs. Therefore, the effects of the dielectric medium on a Au NP film where coupled SPR is present are studied here and classical electrodynamic theory used to explain the observed behavior. In this Chapter, the SPR coupling interaction between neighboring NPs linked by conjugated oligothiophenes is probed by breaking the conjugation using UV generated O 3 and thermal annealing. The effect of the dielectric environment on the near-field coupling 109 interactions between adjacent NPs is also examined by immersing the Au NPs linked with oligothiophenes in various solvents. All of the work in the Chapter has been published.17 4.2 Experimental 4.2.1 General Electrochemical measurements and electrodeposition were conducted using a Pine AFCBP1 bipotentiostat. The working electrode was either an indium tin oxide (ITO) thin film on glass or Au (1000 A ) TEM grids. An internal reference (decamethylferrocene) was added to correct the measured potentials with respect to saturated calomel electrode (SCE). [(«-Bu)4N]PF6 was used as a supporting electrolyte and was purified by triple crystallization from ethanol and dried at 90 °C under vacuum for three days. Methylene chloride used for cyclic voltammetry was purified by passing through an activated alumina tower. Electronic absorption spectra were obtained on a Varian Cary 5000 UV-Vis/NIR spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Leybold MAX200 equipped with an Al Koc source with a pass energy of 192 eV; the sampling area was 2x4 mm. 4.2.2 Synthesis of NPs and Electrodeposition Oligothiophene-capped Au NPs (18) were prepared as described in Chapters 2.11 Electrochemical deposition was carried out using a Pine AFCBP1 bipotentiostat in dry methylene chloride using a sealed glass three-electrode electrochemical cell. A silver wire (reference electrode), platinum wire (counter electrode) and ITO/glass (working electrode) were used. The ITO/glass working electrode surface was cleaned with acetone and dried with N2 before use. The deposition solution consisted of 10 mg of 2-diphenylphosphinoterthiophene-capped Au NPs dissolved in 10 mL of methylene chloride containing 0.1 M [(n-Bu)4N]PF6. 110 Films were grown by repeated scanning from 0 to +1.8 V at 100 mV s" . After electrodeposition, samples were washed copiously with CH2CI2 to remove traces of unlinked NPs and electrolyte. 4.2.3 UV-O3 Irradiation of Oligothiophene-Linked Au NP Films An as-deposited poly-18 film was fitted with a circular optical mask (diameter = 1 cm) to ensure that UV-Vis-near-IR spectra of an identical area were measured. The spectrum of the film was taken before exposure to O 3 and then the film was exposed to UV generated O 3 (concentration of O 3 is between 50 to 600 ppb)18 at 1.5 h increments for 6 h. The absorption spectrum was measured in 1.5 h increments. The change in the sulfur and carbon atomic concentrations and binding energy shifts in films of poly-18 exposed to UV generated O 3 (as-grown, 1.5, 3, 4.5 and 6 h exposure times) were also studied by XPS. XPS spectra were fit using XPS Peak (version 4.1). 4.2.4 Thermal Annealing of Oligothiophene-Linked Au NP Films As-deposited films of poly-18 on ITO/glass substrates were annealed in a Thermolyne 47900 furnace for 3 h at temperatures between 150 to 500 °C. The UV-vis-NIR spectra of these films were also measured using a circular optical mask (diameter = 1 cm) to ensure that the same size area was measured. To study the annealed films using a transmission electron microscope, the films were deposited onto a gold TEM grid directly and these grids were annealed under the same conditions as the films grown on ITO/glass. 4.2.5 Varying the Dielectric Medium in Oligothiophene-Linked Au NP Films 111 The UV-vis-NIR spectra of as-deposited poly-18 films on ITO/glass substrate were measured as the films were immersed in a cuvette containing different solvents. After the absorbance measurement, the films were retrieved from the cuvette and dried with a flow of pressurized air before being immersed in another cuvette with a different solvent. A square optical mask (1 cm2) was used to ensure that the same size area was measured each time. The absorption spectra were fitted using the data analysis software Microcal Origin (version 6.0) where two Gaussian curves were employed to give good fits of the experimental data. 4.3 Results 4.3.1 Preparation of Oligothiophene-Linked Au NP Films The poly-18 films were prepared by oxidative electrochemical coupling as described in Chapter 3, and were removed from solution in a reduced (0 V) state." Absorption spectra of these as-deposited poly-18 films showed two peaks that were discussed in Section 3.3.4: (1) a 7i—>7t* band and (b) a broad band in the near-IR (> 800 nm) due to plasmon coupling between adjacent Au NPs (Figure 4-1). Since oxidized OTs typically show absorption in the near-IR (NIR),19 there is the possibility that the low energy absorption band is due to residual oligothiophene radical cations. Spectroelectrochemistry discussed in the Chapter 3 (Figure 3-10) shows the formation of polarons and subsequently bipolarons in the film with increasing applied voltage. No absorption due to polarons was observed below an applied potential of 400 mV. Based on this experiment, it is concluded that the absorption band in the NIR cannot be attributed to residual radical cations from the electrodeposition. 112 1.0 0.8 O rz 05 - Q i _ O _Q < 0.64 OA A 0.2 0.00 J> -0.05 < / V _ - - .»'•' 400 600 800 1000 1200 1400 W a v e l e n g t h (nm) as-deposited OT-linked Au NPs 0 3 exposure of 1.5 h 0 3 exposure of 3 h 0 3 exposure of 4.5 h 0 3 exposure of 6 h 400 600 800 1000 Wavelength (nm) 1200 1400 Figure 4-1 UV-Vis-NIR absorption spectra of as-deposited OT-linked poly-18 films and with exposure to U V generated ozone. Inset: Difference absorption spectra of as-grown poly-18 subtracted from absorption spectra of O 3 exposed poly-18. 4.3.2 Effects of UV-O3 Irradiation on Coupled Surface Plasmons To explore the possibility that the polarizability of the conjugated linker facilitates dipole interactions between adjacent NPs, the conjugated bridge was disrupted in situ while maintaining the same inter-particle spacing. UV-generated ozone was used to oxidize the OT groups linking the Au NPs. Polythiophene (PT) has been oxidized in this fashion, resulting in the formation of sulfone and carboxyl groups and a decrease in conjugation.18 These harsh conditions result in 113 irreversible chemical modifications of the polymer backbone, rather than the reversible introduction of charge carriers by mild electrochemical doping. (a) (b) Raw Sum of fit Background 290 288 286 284 282 280 170 168 166 164 162 160 Binding Energy (eV) Binding Energy (eV) (c) Raw Sum of fit Background (d) Raw Sum of fit Background 292 290 288 286 284 282 280 Binding Energy (eV) 88 86 84 Binding Energy (eV) F i g u r e 4-2 (a) C Is and (b) S 2p XPS spectra of an as-deposited poly-18 film and (c) C Is and (d) S 2p XPS spectra after exposure to 1.5 h of ozone Figure 4-1 shows the UV-vis -NIR absorption spectra of the film as a function of oxidation time. Two significant changes are observed: first the 7i—>7i* band at 460 nm (A) blue shifts and decreases in intensity and second, band B decreases in intensity. The observed changes to the 7i->7t* band are attributed to the disruption of conjugation in the OT linker where 114 there is cleavage of the oligomer backbone as a result of oxidation. XPS analysis of oxidized poly-18 films confirms this (Figure 4-2). Before oxidation, only one peak is observed in the C Is and S 2p regions at binding energies (BE) of 285.0 and 164.4 eV, respectively. When the film is oxidized for 1.5 h with O 3 , a small peak appears in the C Is region at higher BE (288.7 eV) attributed to formation of carboxyl groups18 indicating C=C bond cleavage in thiophene.20 A much larger peak (168.7 eV) also appears in the S 2p region after O 3 exposure corresponding to the formation of sulfones. The degree of oxidation can be monitored from the XPS ratios of thienyl sulfur relative to sulfone (Table 4-1). The amount of sulfone increases up to 4.5 h of oxidation. After 6 h, the amount of sulfone present starts to decrease, likely because full oxidation of sulfur to SO2 is occurring concomitantly, liberating the gas and thus rendering it undetectable by XPS. 2 0 The loss of sulfur in the thienyl groups is consistent with disruption of conjugation in the OT linker due to bond cleavage. Table 4-1 XPS (S 2p) derived relative abundance of sulfone relative to sulfur in an electrodeposited poly-18 film as a function of O 3 exposure. Relative Abundance (S=Q) Relative Abundance (S) as-grown 0.000 1.000 1.5 h exposure 0.431 0.569 3 h exposure 0.500 0.500 4.5 h exposure 0.625 0.375 6 h exposure 0.351 0.649 More interesting is the decrease in intensity of band B in the UV-vis-NIR spectra with increasing exposure to O 3 . XPS data for Au before and after exposure to O 3 showed no change indicating that no Au oxidation occurred. The decrease in the intensity of the coupled SPR band B occurs concurrently with the disruption of conjugation in the linker. This suggests that near-115 field coupling interactions between Au NPs is dependent on the degree of conjugation in the OT linker where breaking the conjugation leads to weaker near-field coupling. 4.3.3 Effects of Thermal Annealing on Coupled Surface Plasmons The changes in the collective surface plasmon absorption of the poly-18 films after U V -O 3 exposure are in stark contrast to the effects of thermal annealing of the films. Poly-18 films were annealed at temperatures between 150 to 500°C for 3 h at each temperature and their U V -vis-NIR spectra are shown in Figure 4-4. With an increase in annealing temperature, there is a decrease in the maximum wavelength of the i t — > T Z * band similar to the samples exposed to U V -O 3 ; however, that is where the similarities end. The collective surface plasmon band (B) blue shifts to 579 nm and sharpens. The decrease in the T C - > T C * band in the annealed films is attributed to thermal decomposition of the OT linker. The changes in B for the annealed films 9 1 result from increases in the size of the Au NPs (Figure 4-3) and the blue-shift indicates the SPR 22 band is due to isolated uncoupled NPs. After annealing at 500 °C for 3 h, most if not of all the organic material has been combusted and a thin reflective gold film remains. Figure 4-3 T E M images of a poly-18 film (a) as-deposited and annealed at (b) 150 °C and (c) 250 °C for 3 h. 116 1.0 as-deposited 1 5 0 ° C 2 5 0 ° C 3 5 0 ° C 5 0 0 ° C 400 500 600 700 800 900 1000 Wavelength (nm) Figure 4-4 UV-Vis-NIR absorption spectrum of a poly-18 film annealed at varying temperatures for 3 h. 4.3.4 Effects of Dielectric Medium on Coupled Surface Plasmons The dielectric medium's influence on the coupled SPR interactions of poly-18 films was investigated. The solvent dependence of the SPR absorption for unlinked individual A u 1 4 ' 1 5 and Ag NPs has been previously studied, but only two previous reports of solvent effects on coupled plasmon absorptions of Au NPs exist. 1 6 ' 2 4 In unlinked Au NPs, 1 4 ' 1 5 the absorbance increases and red-shifts as the dielectric constant of the solvent is increased consistent with the 1 C ")£ predictions of Mie theory for the behavior of isolated non-interacting particles. ' A similar result is observed for Ag NPs immersed in various dielectric environments.23 For solvent effects on coupled plasmon absorptions of closely spaced Au NPs, 1 6 Yamada reports no change in absorbance intensity and only minor shifts to the absorption maximum as the dielectric medium 117 was varied. The shifts reported were slightly larger than predicted by Mie theory, and this was attributed to the inter-particle interactions affecting the plasmon oscillations. Zamborini et al.24 showed that for weakly coupled linked Au NPs the surface plasmon band decreases in intensity and slightly blue-shifts as the linked NP film is immersed in acetone and CH2CI2 (relative to air). This response was explained as a partitioning of the organic solvent into the film (swelling), which increases the inter-particle distance. Recent work by Zou et al.27 has shown that for Ag NPs that are spaced by the wavelength of light, the red-shift in the plasmon absorption when exposed to higher dielectric medium is amplified compared to the individual NPs due to mixing of the photonic resonances with the plasmon resonances. Here, the effect of the dielectric medium on Au NPs experiencing strong inter-particle coupling interactions is specifically probed and the intensity of the near-IR band in the poly-18 films is found to vary significantly upon immersion of the film in different solvents and as a dry film in air (Figure 4-5). cu o c CD -Q O « . O < (a) • S — — — air water •. _ acetone . ~. ... hexanes - O ^ ^ CH2CI2 OHOli toluene (b) I air \"-S*bK [ water '' \ ' ' acetone \ hexanes \ •*ss. C H 2 C I 2 \ . CHCI3 toluene o o o o 300 600 900 1200 Wavelength (nm) F i g u r e 4 -5 UV-vis-NIR absorption spectra of electrodeposited (a) poly-18 and (b) poly-19 in various solvents. Schematic diagrams of the linkers are shown beside the spectra. 118 3.5 3.0 2.5 2.0 1.5 1.0 Energy (eV) F i g u r e 4-6 Non-linear curve fits of UV-vis-NIR absorption spectra of as-deposited poly-18 in (a) air, (b) water and (c) toluene. Non-linear square fits of the absorption spectra are shown in Figure 4-6 where three Gaussian curves were used to fit the absorption data corresponding to the coupled surface plasmon band (lowest energy band), T T — > T Z * absorption (intermediate energy band) and n—>TC* absorption of the phenyl groups (highest energy band). The coupled surface plasmon band from 119 the non-linear fits blue shift as it is immersed in different solvents. The absorption maximum shifts from 1.301 eV when the spectrum is measured in air compared to 1.445 eV when the spectrum is measured in water and 1.556 eV in toluene. This effect is completely reversible; comparison of the absorption spectrum of a film after immersion in solvent and dried is identical to the spectrum of the dry film prior to immersion. Plotting the absorption maximum (Emax) as a function of the static dielectric constant of the medium reveals a dependence of the coupled surface plasmon absorption on the static dielectric constant of the solvent (Figure 4-7) where a higher dielectric medium leads to weaker coupling. The static dielectric constant of the solvents are plotted on the abscissa of Figure 4-7 because according to the Franck-Condon principle electronic transitions occur much faster than the movement of nuclei and therefore the solvent molecules are considered static during the UV-vis absorption measurement (ie. no solvent orientation polarization contribution). This specific phenomenon where the strength of the coupled surface plasmon is dependent on the dielectric of the medium has not been observed before. 120 1.0 1.1 1.2 1.3 1.4 1.5 Static dielectric constant of medium Figure 4-7 Collective surface plasmon band absorption maximum (Emax) of a poly-18 film as a function of the static dielectric constant of the medium. The data is obtained from the non-linear square fit of the UV-vis absortion spectra. 4.4 Discussion The O 3 exposure of poly-18 films revealed a decrease in the intensity of the coupled SPR band B that occurs concurrently with the disruption of conjugation in the linker. This is in stark contrast to what is observed for thermally annealed films where the coupled SPR band B blue shifted and sharpened due to Au NP aggregation. The 03-treated films do not show a sharp band at 579 nm indicating that the Au NPs do not aggregate under these conditions. Thus, the decrease in the coupled SPR band B occurring simultaneously with the disruption of conjugation in the linker suggests that prior to O 3 oxidation, adjacent NPs are interacting via a mechanism 121 involving the conjugated bridge. A similar dependence of the coupled SPR band on the type of bridge (partially conjugated or non-conjugated) linking Au NPs has been reported.10 In this case, all three partially conjugated linkers showed red-shifted SPR bands due to stronger coupling relative to their non conjugated-containing counterparts. One material in particular, where the Au NPs are linked by l,4-phenylene-bis(dithiocarbamate), showed strong SPR coupling interactions with a broad absorption in the near-IR (>800 nm). This strong electrodynamic coupling between Au NPs that are spaced by approximately 1 nm was explained by overlap of the molecular orbitals of the conjugated bridge with the Au NP wavefunctions. It is possible that a similar mechanism is occurring here facilitating near-field coupling. Probing the dielectric medium dependence of the coupled SPR band revealed that increasing the dielectric constant of the environment decreases the strength of the surface plasmon coupling between adjacent Au NPs. Earlier work on the dielectric medium dependence of the surface plasmon band for individual14'15'23 and weakly coupled16'27 metal NPs all showed red-shifts with increasing dielectric constant of the medium and these dependences have all been explained using Mie theory. The dielectric medium dependence observed here is opposite to the trend predicted by Mie theory; here the plasmon band blue-shifts as the dielectric constant of the medium is increased. Mie theory can only predict the effect of the dielectric constant of the medium on the optical resonance wavelength (SPR ^ max) of isolated spherical particles. Since band B is attributed to coupling interactions; Mie theory is not able to adequately explain the observed changes and classical electrodynamics is used instead to explain the results. Classically, placing a dielectric material between two electrical point charges reduces the force 9R between them, equivalent to increasing the distance between the charges. It has been shown that increasing the distance between NPs reduces the dipole-dipole interaction between them and 122 results in a blue-shift of the collective SPR band. Thus, the introduction of a higher dielectric constant medium between SPR coupled NPs reduces the dipole coupling between adjacent particles producing an equivalent result to moving the NPs further apart. The proposition that a higher dielectric constant medium impedes dipole-dipole interaction is based on the assumption film swelling from solvent absorption has a relatively minor effect. The observed blue-shift (> 100 nm) and decrease in absorbance (AA = 0.4) in the coupled surface plasmon band for the oligothiophene linked Au NPs is much larger compared to the slight changes (~10 nm and AA « 0.15) due to film swelling reported by Zamborini.24 In these films, alkyl linkers are used and the observed effects attributed to the flexibility of these chains. Although oligothiophene-linked Au NP films do show some film swelling when exposed to solvent vapors,29 the rigidity of the oligothiophene linkers suggests that this swelling does not result in significant changes in inter-particle distances. Furthermore, film swelling is highly dependent on the nature of the capping group attached to the NP where the capping group dictates the solubility of the NPs in a specific solvent,30 here the same solvent dependence of the coupled SPR band is observed when the Au NPs are linked with an oligothiophene linker containing ethylenedioxythiophene groups (Figure 4-5b). Although the contribution of film swelling to the observed shifts is impossible to determine from these results, it appears from these comparisons that the contribution is minimal. 4.5 Conclusions The role of a conjugated oligothiophene linker on the plasmon coupling interaction between Au NPs has been probed and it was found that the coupled plasmon resonance shifts to a lower wavelength as the conjugation in the linker is disrupted. This indicates that the oligothiophene linker acts as a bridge facilitating near-field coupling between adjacent NPs. 123 This SPR coupling between NPs was found to be dependent on the dielectric constant of the medium where a higher dielectric medium leads to weaker coupling. This phenomenon was explained using classical electrodynamic theory. 124 4.6 References 1. Maier, S. A.; Atwater, H. A., J. Appl. Phys. 2005, 98, 011101/1. 2. Ozbay, E., Science 2006, 311,189. 3. Hutter, E.; Fendler, J. H., Adv. Mater. 2004, 16, 1685. 4. Atay, T.; Song, J.-H.; Nurmikko, A. V., Nano Lett. 2004, 4, 1627. 5. Maier, S. A.; Kik, P. G.; Atwater, H. A., Phys. Rev. B 2003, 67, 205402/1. 6. Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A., Nat. Mater. 2003, 2, 229. 7. Sih, B. C.; Wolf, M. O., Chem. Commun. 2005, 3375. 8. Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I., Langmuir 2000, 16, 8789. 9. Torma, V.; Vidoni, O.; Simon, U.; Schmid, G., Eur. J. Inorg. Chem. 2003, 1121. 10. Wessels, J. M.; Nothofer, H.-G.; Ford, W. E.; von Wrochem, F.; Scholz, F.; Vossmeyer, T.; Schroedter, A.; Weller, H.; Yasuda, A., J. Am. Chem. Soc. 2004, 126, 3349. 11. Sih, B. C.; Teichert, A.; Wolf, M. O., Chem. Mater. 2004, 16, 2712. 12. Musick, M. D.; Keating, C. D.; Lyon, L. A.; Botsko, S. L.; Pena, D. J.; Holliway, W. D.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J., Chem. Mater. 2000, 12, 2869. 13. Mie, G., Ann. Phys. (Leipzig) 1908, 25, 377. 14. Okamoto, T.; Yamaguchi, I.; Kobayashi, T., Opt. Lett. 2000, 25, 372. 15. Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P., J. Phys. Chem. B 2000, 104, 564. 16. Yamada, M.; Nishihara, PL, ChemPhysChem 2004, 5, 555. 17. Sih, B. C.; Wolf, M. O., J. Phys. Chem. B 2006, 110, 22298. 125 18. Heeg, J.; Kramer, C ; Wolter, M.; Michaelis, S.; Plieth, W.; Fischer, W. J., Appl. Surf. Sci. 2001, 180,36. 19. Nakanishi, Ff.; Sumi, N.; Ueno, S.; Takimiya, K.; Aso, Y.; Otsubo, T.; Komaguchi, K.; Shiotani, M.; Ohta, N., Synth. Met. 2001, 119, 413. 20. Barsch, U.; Beck, F., Electrochim. Acta 1996, 41, 1761. 21. Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A., J. Phys. Chem. B 2006, 110, 7238. 22. Su, K. H.; Wei, Q. H.; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz, S., Nano Lett. 2003, 3, 1087. 23. Jensen, T. R.; Duval, M. L.; Kelly, K. L.; Lazarides, A. A.; Schatz, G. C.; Van Duyne, R. P., J. Phys. Chem. B 1999, 103, 9846. 24. Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. J.; Malik, M. A.; Murray, R. W.,J.Am. Chem. Soc. 2002, 124, 8958. 25. Mulvaney, P., Langmuir 1996, 12, 788. 26. Schmitt, J.; Maechtle, P.; Eck, D.; Moehwald, H.; Helm, C. A., Langmuir 1999, 15, 3256. 27. Zou, S.; Schatz, G. C., J. Chem. Phys. 2004, 121, 12606. 28. Griffiths, D. J., Introduction to Electrodynamics. 3rd ed.; Prentice Hall: Upper Saddle, 1999. 29. Sih, B. C.; Wolf, M. O.; Jarvis, D.; Young, J. F., J. Appl. Phys. 2005, 98, 114314/1. 30. Joseph, Y.; Krasteva, N.; Besnard, I.; Guse, B.; Rosenberger, M.; Wild, U.; Knop-Gericke, A.; Schloegl, R.; Krustev, R.; Yasuda, A.; Vossmeyer, T., Faraday Discuss. 2003, 125, 77. 126 Chapter 5 Propagating Surface Plasmon (PSP) Sensing with Electrodeposited Polythiophene and Oligothiophene-linked Au Nanoparticle Films 5.1 Introduction In recent years, there has been growing interest in developing techniques to selectively detect alcohol vapors for applications in the chemical, biomedical and food industries. Currently, the most widely used methods for detecting alcohol vapors are infrared spectroscopy1' 2 and fuel cell based sensors, which measure changes in current flow to determine the alcohol concentration.3' 4 Both of these methods have limitations: infrared spectroscopy is not very selective and is susceptible to interference from other organic compounds such as toluene and xylenes.5' 6 Fuel cell based detection is quite selective but suffers from a lack of sensitivity (detection limit ~ 200 ppm).4 Propagating surface plasmon (PSP) resonance is a method of vapor detection that promises both high sensitivity (< 100 ppm)7 and tunable selectivity.8"10 5.1.1 Theory A version of this chapter has been published. Sih, B.C., Jarvis, D., Young, J.F. and Wolf, M.O. (2005) Surface-Plasmon Resonance Sensing of Alcohol with Electrodeposited Polythiophene and Gold Nanoparticle-Oligothiophene Films. Journal of Applied Physics. 98: 114314-114318. 127 (a) (b) p-polar ized light Metal film (£m) Dielectric medium (ed) PSP glass Metal film (em)" Dielectric ^ medium (ed) glass' Figure 5-1 Schematic diagram of (a) a propagating surface plasmon (PSP) on the interface of a thin metal film and dielectric medium, and (b) Kretschmann type configuration to increase the wave vector of incident light. PSPs are the collective oscillations of electrons that propagate along a thin metal film surface such as gold or silver in contact with a dielectric medium (Figure 5-la).11 This should not be confused with the localized surface plasmons (LSP) discussed previously in Chapters 2 to 4, which are primarily localized on a nanoparticle. PSPs can have different energies that depend on the dielectric constant of the metal (sm) and medium (sd) according to the following equation: where kpsp is the wave vector of the PSP and oVc is the wave vector of light in a vacuum. PSPs cannot be generated directly by light on a thin gold film, because given light of the same energy, the wave vector of light is shorter (&%/,, = cc/c) and momentum is not conserved. Therefore to generate PSPs using light, the wave vector of light must be increased. This can be accomplished by using a semi-circular prism attached to a thin gold film (50 nm) using a 1-2 nm Cr adhesion 1 9 layer as described by Kretschmann (Figure 5-lb). Under these conditions and using p-polarized light, the wave vector component of light along the metal surface becomes: (Equation 5-1), 128 (Equation 5-2), where ka is the attenuated wave vector of light under Kretschmann configuration conditions, sp is the dielectric constant of the prism and 6 is the angle of incidence of light on the metal film. Equation 5-2 can be written in terms of the wavelength (X) of light used shown below: The magnitude of ka changes when the angle or wavelength of the incident light changes. Thus, light is reflected off the Au surface at nearly all angles of incidence except for a certain angle where ka equals kpsp and incoming incident light causes the surface electrons to resonate. The excitation of PSPs on the Au surface results in a loss of energy and a reduction in the intensity of the reflected light. This shows up as a minimum in a plot of % reflectivity vs. angle of incidence (Figure 5-2) where the angle with the lowest reflectivity is known as the surface plasmon resonance angle (f%). (Equation 5-3). 129 £d= 1-0 £d=^.5 00 0o F i g u r e 5-2 Theoretical % reflectivity (%R) vs. angle of incidence (9) plot obtained from a Kretschmann type configuration with two different values for sd. This technique is useful because 9Q is highly sensitive to the value of sd (Equation 5-1). When the dielectric medium is air and ed = 1, there is a specific value for 9Q and this can be considered the baseline. As the Au surface is exposed to a different dielectric medium or functionalized with a molecule with a larger dielectric constant than air, kpsp would increase leading to a shift in the 6. (Figure 5-2). 130 Reflected light Transmitted light Light source Figure 5-3 Reflection and refraction of a plane wave vector (k) at the boundary of two dielectric media. The amount of electromagnetic radiation reflected and refracted at the boundary of two dielectric media can be modeled using Fresnel's equations.13 Both layers are postulated to be infinitely thick. The amount of /(-polarized light reflected is called the reflection coefficient (Rp) and is equal to: nx cos #2 -n2 cos 6^ , P  = nx cos 92 + n2 cos #, (Equation 5-4), where 6i is the angle of incidence, 02 is the angle of transmission (Figure 5-3), rii and ri2 are the refractive indices of medium 1 and 2, respectively. Fresnel's equation also works with 3 dielectric layers where layer 2 of finite thickness (d) is sandwiched between two layers of infinite thickness (Figure 5-4). The light transmitted from the interface of medium 1 and 2 acts as the incident light wave for the interface between medium 2 and 3. This can be indefinitely extended to N layers where the calculation is reiterated for each additional layer. The mathematical calculations become exponentially longer with each additional layer so a method using matrices has been developed.13 131 Reflected light Light source d Transmitted light Figure 5-4 Reflection and refraction of a plane wave vector (k) for three dielectric media. 5.1.2 Literature Review Deposition of an organic polymer on top of a thin gold film increases the selectivity of a SPR-based sensor relative to bare gold, taking advantage of the solubilities of different vapors in the organic layer. The resulting variations of refractive index and thickness after exposure to a vapor give rise to a selective response, dependent on the specific chemical properties of the organic layer. For example, layers of isoprene rubber,10 poly(methyl methacrylate)8 or polyethylene glycol14 result in selectivity towards hydrocarbons, benzene and alcohols, respectively. Improvements in the sensitivity of a SPR sensor with respect to the response from a bare gold layer may also be attained by attaching metal nanoparticles (NPs) to the surface of the thin 132 metal film.15"17 NPs may be attached to the metal film by means of a bifunctional crosslinker such as a dithiol (on a Au film), and multilayers can be achieved by successively dipping in solutions of NP and crosslinker.18 Such layer-by-layer assembly is tedious and the build-up of multiple layers may take several hours or even days depending on the thickness desired and the efficiency of the NP and crosslinker adsorption. The presence of the NPs results in a larger plasmon angle shift and changes in reflectivity (i?),16 which enhances the detection sensitivity of the SPR device.19' 2 0 The enhanced sensitivity has been attributed to interactions between the LSP of the Au NPs and the PSP of the Au substrate.16'17 Thin films of NPs and organic layers deposited on the metal layer of SPR detectors thus improve the sensitivity and selectivity of SPR sensors, respectively, so combining these approaches by embedding NPs in an organic polymer may be advantageous. A novel approach to preparation of such NP thin films is electrodeposition, which is much faster than layer-by-21 2 3 layer assembly. " As discussed previously in Chapter 3, networks of gold NPs (dmean ~ 1.7 nm) can be readily electrodeposited onto a conducting substrate using conjugated oligothiophene linkers. The thickness of the deposited NP film can be controlled via the deposition time and the solution concentration, and deposition of a 1-2 um thick film takes a few seconds or less. After deposition, a crosslinked conjugated network surrounds the NPs. Here we probe the SPR response of these films to solvent vapors. All the work presented in this Chapter has been published.24 5.2 Experimental 5.2.1 Sample Preparation 133 The thin gold films were prepared by first cleaning glass slides with piranha (1 H2O2 : 2 H2SO4) solution (caution: piranha solution should be handled with extreme care), rinsing with deionized water and drying with a stream of N2. Deposition of 1-2 nm of Cr followed by 50 nm of Au using an evaporation system was carried out on the clean glass substrate. Poly-18 was electrodeposited on the 50 nm gold film following the procedure outlined in Chapter 3. A deposition time of 100 m at 2 V was used for a solution containing 1 mg of 18 per 5 mL of CH2CI2 and 0.1 M tetrabutylammonium hexafluorophosphate (M-B114NPF6). Polythiophene (PT) was electropolymerized onto a 50 nm gold film with a deposition time of 3 s at 1.5 V using a solution containing 2 mg of 2,2'-bithiophene per 5 mL of CH2CI2 and 0.1 M /7-BU4NPF6. The freshly electrodeposited films were held in the same solution at 0 V for 1 minute to electrochemically reduce them. These conditions gave a -60 nm thick poly-18 film (thickness determined by AFM) and a -7 nm thick PT film, respectively. 5.2.2 Kretschmann Optical Set-up for SPR Sensing The Kretschmann optical set-up12 used for the SPR measurements was constructed on an optical table in the lab of Prof. Jeff Young (Dept. of Physics and Astronomy, UBC) and is shown in Figure 5-5. Surface plasmons were excited with a/^ -polarized He-Ne laser (k - 632.8 nm). A neutral density filter was used to attenuate the laser source so as not to saturate the detector. Lenses were used to focus the laser source onto the gold sample and also to focus the reflected signal onto the detector. The prism and sample were mounted on a 6-26 rotation platform (resolution of 0.1°). A silicon photodiode connected to a HP 34401A multimeter was used to detect the intensity of reflected light. A small glass chamber with an access port was sealed to the back of the prism using silicone cement (Figure 5-6). The port was fitted with a rubber septum to permit exposure of the modified gold film to different vapors. The experimental SPR 134 curves were fitted to Fresnel's equations with variable film thickness and dielectric constant, 1 -3 using a least square algorithm. Figure 5-5 Photograph of the Kretschmann optical set-up used for the SPR measurements constructed on an optical table in the physics lab of Prof. Jeff Young (Dept. of Physics and Astronomy, UBC). Figure 5-6 Schematic diagram of the electrodeposited poly-18 in the Kretschmann type configuration fitted with a glass port for vapour sensing. 135 5.2.3 Contact Angle Measurements Static contact angle measurements were carried out using the equipment of Prof. Hogan Yu (Department of Chemistry, Simon Fraser University). A VCA-Optima Surface analysis system made by AST products, Inc. was used to photograph the static distilled water droplet on top of an electrodeposited poly-18 film on Au/glass substrate. The VCA Optima Series software was used to digitally measure the contact angle of the imaged water droplet. 5.3 Results 5.3.1 SPR sensing of Electropolymerized Polythiophene The reflectivity as a function of angle of incidence for a 50 nm thick unmodified gold film (A) and the same film with a ~7 nm PT film electrodeposited on the surface (B) is shown in Figure 5-7a. The minimum in the reflectance curve (6o) shifts towards larger angle and the peak absorption width LTW) is broader for B compared to the unmodified film A. The experimental SPR curves were fitted to Fresnel's equations with variable film thickness and dielectric constant, using a least square algorithm (Figure 5-7).25 The fit to B yields a dielectric constant of the PT layer (£* P T) of 2.40 + 0.12/, similar to the literature value (2.33+ 0.04/).26 136 4 2 4 7 52 42 47 52 A n g l e of I n c i d e n c e A n g l e of Inc idence 42 47 52 4 2 4 7 52 A n g l e of Inc idence A n g l e of I n c i d e n c e Figure 5-7 SPR plots of reflected intensity as a function of the angle of incidence for (a) a 50 nm thick gold film ( 0 ) and a 7 nm thick PT film on a 50 nm gold film in air (•). PT on a 50 nm gold film before (•) and after (A) exposure to , (b) methanol, (c) ethanol and (d) toluene. Solid line is the theoretical fit obtained from Fresnel's equations with variable film thickness and dielectric constant, using a least square algorithm. The SPR response of the PT modified layer B was tested upon exposure to vapors of five solvents (hexanes, toluene, ethanol, methanol and water). Several drops of liquid solvent were introduced to the glass chamber via syringe. After ten minutes of equilibration to allow a saturated atmosphere to form, the SPR response of B was measured. Exposure to hexanes or water resulted in no change in the SPR response. On the other hand, methanol, ethanol and 137 toluene (Figure 5-7b-d) resulted in shifts of QQ to a slightly higher angle but the minimum reflectivity (RMIN) did not change. Fitting the data shows that the thickness of the PT layer increases with exposure to either of the alcohols or toluene but only a small change in the dielectric constant was observed (Table 5-1). Table 5-1 SPR minimum (Oo), thickness (d), differential reflectivity (ARMAX) and real and imaginary part of dielectric constant for the electrodeposited layer (e<j) from fitting using Fresnel's equations before and after exposure to organic vapors. Sensor Organic 6 Oo d d' ARMAX £d £d e/ s/ Type Vapor (deg) (deg) (nm) (nm) (img) (img) PT/50 nm Au toluene 47.8 methanol 47.8 ethanol 47.8 Poly-18 /50nm methanol 50.6 51.2 68.6 77.3 0.08 1.23 0.13 1.25 0.15 Au ethanol 50.6 51.4 60.8 72.5 0.07 1.23 0.16 1.26 0.15 5.3.2 SPR sensing of Electrodeposited OT-linked Au NP Films The reflectivity as a function of angle of incidence for a 50 nm thick gold film with a ~60 nm poly-18 electrodeposited on the surface (C) is shown in Figure 5-8a. The minimum reflectivity increases significantly when poly-18 is present. The measured dielectric constant of the poly-18 film (Table 5-1) agrees well with the value predicted by Maxwell-Garnett theory 48.1 7.2 8.4 0.12 .2.39 0.15 2.40 0.12 48.4 7.2 10.2 0.32 2.39 0.12 2.40 0.12 48.0 7.1 8.3 0.14 2.41 0.08 2.42 0.09 138 (1.21 + 0.13/), which assumes that dielectric constant (£cak) is simply a weighted average dielectric constant of the two components, £cak = (1 - <P)£PT + <P£AU (Equation 5-5), 9 ^ where <p is the volume concentration of Au and EPT, £AU are the dielectric constants of Oft 9 7 electropolymerized polythiophene and gold, respectively. This approximation is expected to 9 8 9 0 hold for homogenously distributed NPs. ' Electron microscopy of the electrodeposited film 9"2 confirms that this is the case. PT and the crosslinked oligothiophenes used here are expected to have similar dielectric constants. Angle of Incidence Angle of Incidence Figure 5-8 SPR plots of reflected intensity as a function of the angle of incidence for a 50 nm thick gold film (0), a ~60 nm thick poly-18 film on a 50 nm gold film in air (•) and poly-18 film on a 50 nm gold film after exposure to selected vapors (A), (a) methanol and (b) ethanol. Solid line is the theoretical fit obtained from Fresnel's equations with variable film thickness and dielectric constant, using a least square algorithm. The SPR response of the poly-18 modified layer C was tested upon exposure to the same five solvent vapors tested with B. In this case, exposure to hexane, toluene and water resulted in 139 no change in the SPR response. On the other hand, both methanol (Figure 5-8a) and ethanol (Figure 5-8b) resulted in shifts of do to a higher angle (0.6° and 0.8° shift, respectively). Rmin also increased with exposure to methanol and slightly with exposure to ethanol. Fitting the data shows that the thickness of the poly-18 layer increases, and the dielectric constant increases slightly with exposure to methanol or ethanol (Table 5-1). 5.4 Discussion Increases in the SPR minimum angle and reflectivity after exposure of a dielectric layer to organic vapors has been previously attributed primarily to changes in the thickness of the dielectric medium (film swelling). Very small changes in refractive index caused by vapor TO adsorption may also contribute. Recently, a poly(3-(6-methoxyhexyl)thiophene) (P60ME) film spin cast onto a SPR gold substrate was used to detect organic vapors.9 A 0.2° shift in do was reported after exposure to toluene due to an increase in film thickness and a slight change in dielectric constant. This shift is similar to the 0.3° shift in do and slight change in dielectric constant observed for B after exposure to toluene. However, P60ME also showed a response to hexanes^ whereas B did not. The response depends on the solubility of a vapor in the film, which is related to the polarity of the vapor and polymer. The alkyl groups present in the P60ME film increase the solubility of hexanes in this medium relative to electropolymerized PT. The selectivity of B and C towards alcohols can be attributed to the partially oxidized nature of the electrodeposited film. Oxidative crosslinking results in positive charge remaining on the conjugated polymer or linkers (compensated by the presence of negative PF6~ ions in the film). Prior to use, the films are reduced electrochemically; however, this is an incomplete process and some charges remain. The partially oxidized nature of the poly-18 film is evidenced 140 by the presence of phosphorus and fluorine from the PF6- ions in energy-dispersive X-ray (EDX) analysis of the films and the higher binding energy of the Au 4/7/2 peak in the X-ray photoelectron spectrum (XPS) of poly-18 relative to unlinked NPs. The static contact angle o measured for a water drop on the surface of C was (64 ± 3) . This indicates that C is slightly 31 more hydrophilic than oxidized polythiophene, suggesting a possible explanation why B also responds to the less polar solvent toluene. The lack of response to water for both B and C could o be due to either low vapor pressure (23.74 mmHg at 25 C) for water compared to either alcohol, or to poor solubility of the highly polar water molecules in the polymer. Previous work by Lyon et al.19' 2 0 demonstrated that a Au NP layer adsorbed on a gold film results in changes to do, rw> and RMIN and these changes are linked to enhanced detection sensitivities. Roy et al.17 calculated SPR plots for analyte detection and predicted larger 60 shifts and improved detection sensitivities when Au NPs are present. According to Roy,17 addition of Au NPs to a dielectric material such as in C should introduce LSP and PSP interactions leading to increased sensitivity. Although a larger increase in Oo for C (Ado = +0.8°) in response to ethanol compared to B (A60 = +0.2°) and a larger increase in RMIN for C compared to B in response to methanol was observed, lower alcohol detection sensitivities for C compared to B can be seen from the respective SPR plots (Figure 5-7 and Figure 5-8). This is because from a practical detector standpoint, it is not the total transformation of the curve that is important in SPR sensing but rather the maximum change in reflectivity that can be observed at a fixed angle, R'—R MLMAX = z. (Equation 5-6), R where R and 7?' are the reflectivity before and after exposure to an analyte, respectively. Table 5-1 tabulates the values of ARMAX for B and C , and B is shown to have a larger detectable 141 response to the organic vapors. We therefore do not find evidence to support improved detection sensitivities from incorporation of Au NPs into PT for use in a SPR.device. Although no evidence was found for an increase in sensitivity from incorporating Au NPs into PT, there does appear to be an improvement in selectivity. The selectivity of C towards alcohols and lack of response towards toluene give it an advantage over traditional infrared detection of alcohols, which is susceptible to interference. For example, Intoxilyzer 5000©, a widely used instrument for measuring ethanol in motorist's breath, is susceptible to reporting false positive readings for ethanol in the presence of methyl substituted aromatics such as toluene and xylenes.5 This is because methyl substituted aromatics have a similar IR absorption as ethanol at 3.48 and 3.39 um where the instrument is calibrated to detect ethanol. Since C is selective towards alcohols due to its partially oxidized nature, it appears to be immune to interference from non-polar aromatic solvents. The information delivered by C in an SPR sensor is complementary to Intoxilyzer 5000© and could eliminate false positives introduced by methyl substituted aromatics. 5.5 Conclusions The responses of electrodeposited PT and poly-18 films employed in a Kretschmann type SPR configuration were found to be selective when exposed to organic vapors. There is a detectable response to ethanol, methanol or toluene for PT and ethanol or methanol for poly-18. The films show changes in do, Rmm, and dielectric constant after exposure to alcohol vapors. Although we do not find a significant improvement in sensor sensitivity in incorporating Au NPs in this application, it is possible that the general approach for a one-step embedding of metal NPs into dielectric materials may prove useful in other SPR sensors. 142 5.6 References 1. Goldberger, B. A . ; Caplan, Y . H. , J. Forens. Sci. 1986, 31,16. 2. Harding, P. M ; Laessig, R. H. ; Field, P. H., J. Forens. Sci. 1990, 35, 1022. 3. Huck, H. , Fresen. Z. Anal. Chem. 1974, 270, 266. 4. Kim, K . C ; Cho, S. M . ; Choi, H. G., Sens. Actuators B 2000, B67, 194. 5. Caldwell, J. P.; Kim, N . D., J. Forens. Sci. 1997, 42, 1080. 6. Jones, A . W.; Andersson, L.; Berglund, K. , J. Anal. Toxicol. 1996, 20, 522. 7. Podgorsek, R. P.; Sterkenburgh, T.; Wolters, J.; Ehrenreich, T.; Nischwitz, S.; Franke, H. , Sens. Actuators B 1997, B39, 349. 8. Capan, R.; Ray, A . K. ; Hassan, A . K.; Tanrisever, T., J. Phys. D: Appl. Phys. 2003, 36, 1115. 9. Chaure, S.; Yang, B.; Hassan, A . K.; Ray, A . K.; Bolognesi, A. , J. Phys. D: Appl. Phys. 2004,37, 1558. 10. Urashi, T.; Arakawa, T., Sens. Actuators B 2001, B76, 32. 11. Smith, E. A. ; Corn, R. M . , Appl. Spectrosc. 2003, 57, 320A. 12. Kretschmann, E., Z. Physik 1971, 241, 313. 13. Yeh, P., Optical Waves in Layered Media. John Wiley & Sons, Inc.: New York, 1988. 14. Miwa, S.; Arakawa, T., Thin Solid Films 1996, 281, 466. 15. Hutter, E.; Cha, S:; Liu, J. F.; Park, J.; Y i , J.; Fendler, J. H. ; Roy, D., J. Phys. Chem. B 2001, 105, 8. 16. Hutter, E.; Fendler, J. H. ; Roy, D., J. Phys. Chem. B 2001, 105, 11159. 17. Roy, D.; Fendler, J., Adv. Mater. 2004, 16, 479. 143 18. Musick, M. D.; Keating, C. D.; Lyon, L. A.; Botsko, S. L.; Pena, D. J.; Holliway, W. D.; McEvoy, T. M ; Richardson, J. N.; Natan, M. J., Chem. Mater. 2000, 12, 2869. 19. Lyon, L. A.; Musick, M. D.; Natan, M. J., Anal. Chem. 1998, 70, 5177. 20. Lyon, L. A.; Pena, D. J.; Natan, M. J., J. Phys. Chem. B 1999, 103, 5826. 21. Ikarashi, A.; Patton, D.; Locklin, J.; Baba, A.; Shinbo, K.; Kato, K.; Kaneko, F.; Advincula, R. C , Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2005, 46, 389. 22. Onishi, K.; Locklin, J.; Fulghum, T.; Patton, D.; Advincula, R. C , PMSE Preprints 2004, 90,591. 23. Sih, B. C ; Teichert, A.; Wolf, M. O., Chem. Mater. 2004, 16, 2712. 24. Sih, B. C ; Wolf, M. O., J. Phys. Chem. B 2006, 110, 22298. 25. Vukusic, P. S.; Sambles, J. R.; Wright, J. D., J. Mater. Chem. 1992, 2, 1105. 26. Jakob, T.; Knoll, W., J. Electroanal. Chem. 2003, 543, 51. 27. Lide, D. R., CRC Handbook of Chemistry and Physics, Internet Version 2005, <htlp://www.hbcpnetbase.com>. 85th ed.; CRC Press: Boca Raton, FL, 2005; p 12. 28. Garnett, J. C. M., Phil. Trans. Roy. Soc. 1904, 203, 385. 29. Garnett, J. C. M., Phil. Trans. Roy. Soc. 1906, 205, 237. 30. Nabok, A. V.; Hassan, A. K.; Ray, A. K., J. Mater. Chem. 2000, 10, 189. 31. Zhang, Z.; Qu, L.; Shi, G., J. Mater. Chem. 2003, 13, 2858. 144 Chapter 6 Synthesis and Characterization of CdSe Nanoparticles Capped with Oligothiophenes 6.1 Introduction Semiconductor nanoparticles (NPs) have attracted significant research attention due to size-tunable optical and electronic properties' that are promising for various applications in biological labeling,2, 3 photovoltaics4' 5 and light emitting diodes (LEDs). 6 ' 7 Early methods developed to prepare semiconductor NPs involved pyrophoric organometallic reagents such as Cd(CH 3 ) 2 which are injected rapidly into hot (300-360 °C) tri-«-octylphosphine oxide (TOPO).8' 9 This reaction is quite dangerous and is also very sensitive to the rate of addition where slightly slower injections of Cd(CH3)2 yields NPs of different size. 1 0 These problems recently led Peng et al.10 to develop a procedure that uses a non-pyrophoric starting material (CdO), which is much safer to use. In addition, the reaction can be carried out at lower temperatures (270 °C) resulting in slower nucleation rates and therefore more reproducible size distributions. An important step for employing semiconductor NPs in many applications is the replacement of the as-prepared TOPO capping groups on the NP surface with functionalized organic molecules. For photovoltaic and LED applications, this step is crucial because TOPO acts as an insulating layer that prevents efficient charge transfer into or out of the NP. 4 Attaching conducting polymers directly onto the semiconductor NP surface has been shown to result in more efficient charge transfer.4' " ' 1 2 To better understand this charge transfer mechanism, shorter model compounds such as oligoaniline,13 oligo-(p-phenylenevinylenes)12'14 and oligothiophenes1 5'1 6 have been attached semiconductor NPs. In the case of oligothiophene, the focus has been on longer oligomers (> 3 thiophene rings) where these capping groups quench 145 the fluorescence of the CdSe NP due to electron transfer from the HOMO of the oligothiophene, which is higher in energy, into the valence band of the CdSe NP. This is useful in photovoltaics where efficient charge separation of electron and holes is required for improved device performance. Previous work on oligothiophenes attached to CdSe NPs used phosphonic acid groups as the anchoring group. 1 5 , 1 6 One problem with the phosphonic acid group is that it has similar affinity for the CdSe surface as the TOPO capping groups found on the as-prepared particles and displacing the TOPO becomes challenging. The TOPO has to be first displaced by heating in neat pyridine and then the phosphonic acid groups can subsequently displace the bound pyridines, which have a lower affinity for the CdSe surface.15, 1 7 One way to eliminate this extra pyridine treatment is to use thiol anchoring groups that are known to have a stronger attraction to the CdSe surface relative to TOPO. 1 8 The shape of semiconductor NPs is important. Rod-shaped particles have anisotropic charge transport properties5 and show polarized emission.19' 2 0 Various methods have been developed to control the growth mechanism of semiconductor NPs to generate different shapes.21 22 Peng et al. showed that using CdO as a precursor, rod-shaped NPs can also be formed. These rod-shaped CdSe NPs have been blended with poly(3-hexylthiophene) ( P 3 H T ) and.incorporated into a solar cel l . 5 ' 2 3 ' 2 4 These devices based on rod-shaped CdSe N P s / P 3 H T blends have better power conversion efficiencies and output compared to devices which employ spherical CdSe NPs /P3HT. Hyunh et al.5 concluded from their work that the external quantum efficiency in these devices can still be increased by improving the polymer/NP interface leading to less charge recombination. An aspect limiting the efficient charge separation in Hyunh's work is the TOPO layer passivating the CdSe NPs which prevent efficient charge transfer from the CdSe NP into the conducting polymer. One approach to improve the polymer/NP interface is to directly attach the conducting polymer to the CdSe surface. Oligothiophenes attached to rod-shaped CdSe NPs 146 have not been previously prepared and novel properties may result from such a material. Electrochemical crosslinking of oligothiophene-capped CdSe NPs, analogous to the oligothiophene-capped Au NPs described in Chapter 3, would also be of interest because this could lead to a hybrid material similar to Hyunh's CdSe NP/P3HT blend but without any insulating TOPOs present. In this Chapter, the preparation of oligothiophenes with 1-3 thiophene rings attached to thiol and phosphine oxide anchoring groups is described. These functionalized oligothiophenes are then used to passivate the surface of rod-shaped CdSe NPs by displacement of TOPO in as-prepared CdSe NPs. The optical properties of the oligothiophene-capped CdSe NPs are studied and attempts to electropolymerize the NPs are discussed. 6.2 Experimental 6.2.1 General Chemicals were purchased from Aldrich except for CdO, selenium, trioctylphosphine and trioctylphosphine oxide, which are from Alfa Aesar and tetradecyl phosphonic acid which is from Poly Carbon Industries. 2,2 ':5 ',2 "-terthiophene,25 (diethylamino)phosphinous dichloride,26 (n-octyl)2PCl,27 2-bromo-3-hexylthiophene,28 4,3'-dihexyl-2,2'-bithiophene-5-90 • 90 carboxylic acid and 4,3',3"-trihexyl-2,2';5,2"-terthiophene-5-carboxylic acid benzyl ester were prepared according to literature procedures. All reactions were performed using standard Schlenk techniques with dry solvents under a nitrogen atmosphere and unless otherwise noted, all reagents were used without further purification. Electrochemical measurements were conducted using a Pine AFCBP1 bipotentiostat. The working electrode was a Pt disk, the counter electrode was a Pt coil wire and the reference electrode a silver wire. An internal 147 reference (decamethylferrocene) was added to correct the measured potentials with respect to saturated calomel electrode (SCE). [(«-Bu)4N]PF6 was used as a supporting electrolyte and was purified by triple crystallization from ethanol and dried at 90 °C under vacuum for three days. Methylene chloride used for cyclic voltammetry was purified by passing through an activated alumina tower. Solution electronic absorption spectra were obtained on a Varian Cary 5000 UV-vis/NIR spectrometer in C H C I 3 and solid state absorption spectra were acquired on films deposited on glass. Solution emission and excitation spectra were obtained using a Photon Technology International Quantamaster fluorimeter and solid state emission spectra were acquired on films deposited on glass. 'H NMR spectra were acquired on a Bruker AV-300 spectrometer, and spectra were referenced to residual solvent. Transmission electron microscopy (TEM) images were taken using a Hitachi H7600 electron microscope operating at 80 kV. NPs were dropcast from C H C I 3 solutions onto carbon coated 300-mesh copper grids. The particle sizes were measured using the image processing program Quartz PCI 5 where a total of 150 particles were counted resulting in a mean core size. Density functional theory (DFT) calculations were performed using a B3LYP/6-31G*(d,p) basis set implemented using the Gaussian 03 program.30 6.2.2 Synthesis (Diethylamino)-di-(n-octyl)phosphine (22) Powdered Mg (3.60 g, 0.150 mol) and a small amount of I2 were stirred in 125 mL of diethyl ether and 1-bromooctane (23.2 g, 0.120 mol) was added dropwise. The solution was then heated to reflux for 2 h. In a separate flask, (diethylamino)phosphinous dichloride26 (8.37 g, 48.1 mmol) was dissolved in 50 mL of diethylether and cooled to -33 °C in dry ice/ethanol bath. 148 The Grignard solution was added dropwise to the (diethylamino)phosphinous dichloride diethylether solution through a glass frit. After complete addition, the solution was warmed to room temperature and stirred overnight. The following morning 20 mL of dioxane was added to the solution and heated to reflux for 1 h to precipitate MgBrCl. After cooling, the salt was filtered off, and the solvent removed from the filtrate in vacuo to yield the crude product. The crude product was distilled (0.4 Torr, 168-175 °C) to yield 22 (5.0 g, 32 %) as a clear liquid. 'H NMR (300 MHz, CDC13): 5 3.04 (dq, 4H, NCH?CHA 1.60-1.56 (m, 28H, -CH2-), 1.10 (t, 6H, NCH2CH3), 0.87 (t, 6H, -CH3). 3 1P NMR (121.5 MHz, CDCI3): 8 53.7 (s). 5-(Dioctylphosphine oxide)-2,2': 5',2"-terthiophene (23) A solution of /7-butyllithium in hexanes (4.8 mL, 1.6 M, 7.7 mmol) was added dropwise to a stirring solution of 2,2':5',2 "-terthiophene (1.81 g, 7.3 mmol) in THF (60 mL) at -78 C. The mixture was stirred for 1 h at -78 C and distilled (rc-octyl^PCl (2.25 g, 7.7 mmol) was added dropwise. The reaction was then allowed to warm to room temperature and stirred for another 2 h, after which time 1 M HC1 was added to quench the reaction. The organic layer was separated, washed with water, and dried over MgSCV The solvent was removed and the residue dissolved in a 7:3 mixture of CHCi2:acetone (40 mL). Excess H 2 O 2 was added to the solution and stirred at room temperature for 30 min. The solution was washed with distilled water (3 times) and then dried over MgS04. Removal of the solvent yielded the crude product, which was purified by chromatography on silica gel with hexanes/ethyl acetate (1/2 v/v). Removal of the solvent gave 23 (2.03 g, 53 %) as a yellow powder. *H NMR (300 MHz, CDC13): 8 7.44 (dd, 1H, 4-H), 7.23 (dd, 1H, 5"-H), 7.22 (dd, 1H, 3-H), 7.20 (dd, 1H, 3"-H), 7.15 (d, 1H, 3'-H), 7.10 (d, 1H, 4'-H), 7.03, (dd, 1H, 4"-H), 2.08-1.62 (m, 8H, a,B-CH2), 1.44-1.17 (m, 20H, -CH2-), 149 0.86 (t, 6H, -CH3). 31P{'H} NMR (121.5 MHz, CDC13): 8 38.5 (s). MS (EI): m/z = 520.2. Anal. C28H41OPS3 requires C, 64.57; H, 7.94. Found: C, 64.78; H, 8.11. 3-Hexylthiophene-2-carboxylic acid (24) A solution of 2-bromo-3-hexylthiophene (2.5 g, 10.1 mmol) in 50 mL of dry THF was cooled to -78 °C and «-BuLi (6.32 mL, 1.6 M, 10.1 mmol) was added dropwise while stirring. The mixture was stirred for 1.5 h and excess solid CO2 was added. The mixture was stirred for an additional 2 h and allowed to warm up to room temperature. The mixture was quenched with 1M HC1 and extracted with 50 mL of diethyl ether (3 times). The combined organic layers were washed with 1M HC1 and deionized water followed by drying over MgSCV After evaporation of the solvent, the residue was purified with column chromatography over silica gel (CH2O2-ethyl acetate [80:20]) yielding the product 24 (1.97 g, 92%) as a white solid. 'H NMR (300 MHz, C D C I 3 ) : 5 7.49 (d, J = 5.0 Hz, 1H, 5-H), 6.99 (d, J = 5.0 Hz, 1H, 4-H), 3.02 (t, J = 7.6 Hz, 2H, a-CH2), 1.70-1.57 (m, 2H, (3- CH2), 1.42-1.29 (m, 6H, -CH2-), 0.90 (t, J = 6.9 Hz, 3H, -CH3). MS (EI): m/z = 268. EA: CnHi 6 0 2 S requires C, 62.23; H, 7.60. Found: C, 62.63; H, 7.95. 3-Hexyl-thiophene-2-carboxylic acid-2,5-dioxo-pyrrolidin-l-yl ester (25) To a solution of 24 (0.5 g, 2.4 mmol) in 30 mL dry CH2CI2, ./V-hydroxysuccinimide (0.298 g, 2.59 mmol) and A -^(3-dimethylamino-propyl)-A '^-ethylcarbodiimide hydrochloride (0.50 g, 2.59 mmol) was added. The mixture was stirred at room temperature overnight. The solvent was removed and the residue purified by column chromatography over silica gel (CH2CI2) and yielded the product 25 (0.60 g, 82%) as a clear oil. *H NMR (300 MHz, CDC13): 5 7.60 (d, J = 5.0 Hz, 1H, 5-H), 7.05 (d, J = 5.0 Hz, 1H, 4-H), 2.99 (t, J = 7.8 Hz, 2H, a-CH2), 2.89 (s, 4H, OCH2CH2CO), 1.69-1.56 (m, 2H, B-CH2), 1.39-1.23 (m, 6H, -CH2-), 0.87 (t, J = 6.7 Hz, 150 3H, -CH3). MS (EI): m/z = 309. EA: Ci 5H,9N0 4S requires C, 58.23; N, 4.53; H , 6.19. Found: C, 58.36; N, 4.80; H , 6.04. 3-Hexylthiophene-2-carboxylic acid-(2-mercapto-ethyl)-amide (26) To a solution of 25 (1.7 g, 5.5 mmol) in 15 mL dry C H 2 C I 2 , 5 mL of triethylamine and cysteamine (0.47 g, 6.1 mmol) were added. The mixture was stirred for 24 h at room temperature. The solvent was removed and the residue was purified by column chromatography over silica gel (CH2Cl2-ethylacetate [20:1]), yielding 26 (0.91 g, 60%) as a clear oil. * H NMR (300 MHz, CDCI3): 5 7.27 (d, J = 5.0 Hz, 1H, 5-H), 6.94 (d, J = 5.0 Hz, 1H, 4-H), 6.25 (br, 1H, -NH), 3.60 (dt, J = 6.2 Hz, 2H, -NCH2-), 2.93 (t, J = 7.9 Hz, 2H, a-CH2), 2.77 (m, 2H, -CH2S-), 1.70-1.57 (m, 2H, J3-CH2), 1.37-1.22 (m, 6H, -CH2-\ 0.88 (t, J = 7.0 Hz, 3H, -CH3). MS (EI): m/z = 271. EA: Ci 3H 2iNOS 2 requires C, 57.52; H, 7.80. Found: C, 57.90; H, 7.90. 4,3'-dihexyl-2,2'-bithiophene-5-carboxylic acid-2,5-dioxo-pyrrolidin-l-yl ester (28) Following the same synthetic method as for 25, 4,3'-dihexyl-2,2'-bithiophene-5-carboxylic acid (26) (1.26 g, 3.35 mmol) was reacted with A -^hydroxysuccinimide (0.424 g, 3.69 mmol) and A -^(3-dimethylaminopropyl)-A '^-ethylcarbodiimide hydrochloride (0.706 g, 3.69 mmol). After chromatography (silica gel, C H 2 C I 2 ) , 28 was obtained (1.35 g, 85%) as a yellow oil. 'H NMR (300 MHz, CDCI3): 5 7.26 (d, J = 5.0 Hz, 1H, 5'-H), 7.05 (s, 1H, 3-H), 6.96 (d, J = 5.0 Hz, 1H, 4'-H), 2.98 (t, J = 7.7 Hz, 2H, a'-CH2), 2.89 (s, 4H, OCH 2CH 2CO), 2.79 (t, J = 7.9 Hz, 2H, a-CH2) 1.71-1.58 (m, 4H, /3,/3'-CH2), 1.42-1.24 (m, 12H, -CH2-), 0.88 (t, J = 6.4 Hz, 6H, -CH3). MS (ESI): m/z [M + Na+] = 498. EA: C25H33NO4S2 requires C, 63.13; N, 2.95; H, 6.99. Found: C, 63.10; N, 3.20; H, 7.13. 4,3 '-dihexyl-2,2 '-bithiophene-5'-carboxylic acid (2-mercapto-ethyl)-amide (29) 151 Following the same synthetic method as for 26, 28 (1.1 g, 2.3 mmol) was reacted with 2.4 mL of triethylamine and cysteamine (0.20 g, 2.6 mmol). After chromatographic work-up (silica gel, CH2Cl2-hexanes [80:20]), 29 was obtained (0.43 g, 43%) as a yellow oil. * H NMR (300 MHz, CDC13): 5 7.20 (d, J = 5. 1 Hz, 1H, 5'-H), 6.94 (s, 1H, 3-H), 6.94 (d, J = 5.1 Hz, 1H, 4'-H), 6.23 (br, 1H, -NH), 3.61 (dt, J = 6.1 Hz, 2H, -NCH2-\ 2.93 (t, J = 7.7 Hz, 2H, a'-CH2), 2.83-2.71 (m, 4H, a-CH2, -CH2S-), 1.72-1.57 (m, 4H, p,/3'-CH2), 1.38-1.21 (m, 12H, -CH2-), 0.88 (t, J = 6.3 Hz, 6H, -CH3). MS (ESI): m/z [M + H+] = 438. EA: C23H35NOS3 requires C, 63.11; N, 3.20; H, 8.06. Found: C , 63.46; N, 3.39; H, 8.11. 4,3',3"-trihexyl-2,2';5,2"-terthiophene-5-carboxylic acid (30) 4,3',3"-trihexyl-2,2';5,2"-terthiophene-5-carboxylic acid benzyl ester29 (1.01 g, 1.60 mmol) and [Bu4N]OH.30 H 2 0 (2.60 g, 1.19 mmol) were dissolved in 250 mL of dry THF and heated to reflux for 3 h. The mixture was cooled to room temperature and acidified with 1M HC1. Diethylether (300 mL) was added to the mixture and then washed 3 times with 50 mL of 1M HC1 and finally with water. The organic phase was dried over MgS04 and the solvent was removed. The residue was purified using column chromatography over silica gel (CH2C12-ethylacetate [80:20]) yielding 30 (0.55 g, 63%) as a yellow-orange solid. *H NMR (300 MHz, CDCI3): 5 7.18 (d, J = 5.1 Hz, 1H, 5"-H), 7.01 (s, 1H, 4'-H), 6.96 (s, 1H, 3-H), 6.94 (d, J = 5.1 Hz, 1H, 4"-H), 3.01 (t, 7.8 Hz, 2H, a-CH2), 2.83-2.75 (m, 4H, a',a"-CH2), 1.72-1.59 (m, 6H, PJ3',j3"-CH2\ \ A1-\21 (m, 18H, -CH2-), 0.94-0.83 (m, 9H, -CH3). MS (ESI): m/z [M + Na+] = 567.5. EA: C3iH4402S3 requires C , 68.33; H, 8.14. Found: C, 68.53; H, 8.31. 4,3',3"-trihexyl-2,2';5,2"-terthiophene-5-carboxylic acid-2,5-dioxo-pyrrolidin-l-yl ester (31) Following the same synthetic method as for 25, 30 (0.59 g, 1.08 mmol) was reacted with AMiydroxysuccinimide (0.137 g, 1.19 mmol) and ^-(S-dimethylamino-propyl)- '^-152 ethylcarbodiimide hydrochloride (0.229 g, 1.19 mmol). After chromatographic work-up (silica gel, CH2C12), 31 was obtained (0.67 g, 88%) as an orange oil. 'H NMR (300 MHz, CDC13): 7.20 (d, J = 5.2 Hz, 1H, 5"-H), 7.06 (s, 1H, 4'-H), 6.97 (s, 1H, 3-H), 6.94 (d, J = 5.2 Hz, 1H, 4"-H), 2.99 (t, J = 7.7 Hz, 2H, a"-CH2), 2.90 (s, 4H, OCH 2CH 2CO), 2.83-2.74 (m, 4H, a',a"-CH2), 1.73-1.59 (m, 6H, PP',p"-CH2), 1.44-1.25 (m, 18H, -CH2-), 0.93-0.83 (m, 9H, -CH3). MS (ESI): m/z [M + Na+] = 664.4. EA: C35H47NO4S3 requires C, 65.48; H, 7.38. Found: C, 65.56; H, 7.40. 4,3 ',3 "-trihexyl-2,2 ';5,2 "-terthiophene-5-carboxylic acid (2-mercapto-ethyl)-amide (32) Following the same synthetic method as for 26, 31 (0.63 g, 0.98 mmol) was reacted with 1 mL of triethylamine and cysteamine (0.085 g, 1.10 mmol). After chromatography work-up (silica gel, CH2C12), 32 was obtained (0.252 g, 43%) as an orange solid. 'H NMR (300 MHz, CDCI3): 7.17 (d, J = 5.1 Hz, 1H, 5"-H), 6.96 (s, 1H, 4'-H), 6.94 (s, 1H, 3-H), 6.93 (d, J = 5.2 Hz, 1H, 4"-H), 6.24 (br, 1H, -NH), 3.61 (dt, J = 6.1 Hz, 2H, -NCH2-), 2.93 (t, J = 7.7 Hz, 2H, a"-CH2), 2.83-2.71 (m, 6H, a',a-CH2,-CH2S-), 1.74-1.58 (m, 6H, p,p\P"-CH2), 1.46-1.23 (m, 19H, -CH2-,-SH), 0.93-0.82 (m, 9H, -CH3). MS (ESI): m/z [M + Na+] = 626.5. EA: C23H35NOS3 requires C, 65.62; N, 2.32; H, 8.18. Found: C, 65.51; N, 2.41; H, 8.45. Trioctylphosphine oxide capped CdSe nanorods (CdSe-TOPO) Synthesis of CdSe nanorods followed procedures by Peng and co-workers.10 CdO (0.13 g, 1.0 mmol) was dissolved in a solution of trioctylphosphine oxide (9.442 g, 24.42 mmol) and tetradecylphosphonic acid (0.56 g, 2.1 mmol) heated to 300-320 °C. The temperature was lowered to 270 °C and a 0.2 M solution of Se/trioctylphosphine (6.3 mL, 1.25 mmol) was injected quickly. After injection, the nanorods were allowed to grow for 4 mins and then the heat source was removed to stop the growth process. The nanorod mixture was cooled to 60 °C and then 10 mL of degassed CHCI3 was added. The mixture was centrifuged to remove 153 unreacted CdO and Se that was left after decanting the solution. The nanorods were precipitated out of solution by adding degassed methanol and centrifuged. The filtrate was discarded and the reprecipitations repeated until no more trioctylphosphine oxide was detected in the washings by 3 1P NMR spectroscopy. Oligothiophene capped CdSe nanorods (CdSe-26, CdSe-29, CdSe-32) Capping groups on the CdSe nanorod surface were replaced by addition of 0.4 mmol of 26 in degassed CHCI3 to a solution of 50 mg of CdSe-TOPO dissolved in 2 mL of degassed CHCI3. The mixture was allowed to stir for 24 h at room temperature. The CdSe nanorods capped with 26 (CdSe-26) were precipitated by addition of degassed methanol. The precipitate was collected by centrifugation and the mother liquor was discarded. CdSe-26 was redissolved in CHC13, precipitated with methanol and centrifuged to remove any excess unbound 26. The reprecipitations were repeated until 26 was no longer detectable in the washings by TLC. The same exchange reactions were repeated to obtain 29-capped CdSe nanorods (CdSe-29) and 32-capped CdSe nanorods (CdSe-32). CdSe-29 and CdSe-32 were reprecipitated with degassed methanol from hexanes to remove any excess unbound capping groups. Reprecipitations were repeated 20 times for both samples. However, even after 20 reprecipitations, there was still some unbound capping groups detectable in the TLC of the washings, suggesting that the capping groups are not tightly bound to the surface of the CdSe nanorods and some dissociate 1 T1 during the reprecipitation process. All three samples gave similar H and P NMR spectra which are: 'H NMR (300 MHz, CDC13): 8 7.5-6.7 (br), 2.1-0.2 (br). 3 1 P NMR (121.5 MHz, CDCI3): no detectable signal suggesting most if not all the TOPO capping groups have been exchanged. 154 6.3 Results 6.3.1 Synthesis Two classes of oligothiophene capping groups were prepared to bind to the CdSe nanoparticle surface. The first class utilizes a phosphine oxide attached to the ot-position of bind to the surface of CdSe nanoparticles. The second class of compounds utilizes a thiol functionality attached to an oligothiophene through an amide linkage (26, 29, 32). Thiols have been also been shown to bind strongly to the surface of CdSe nanoparticles.18'31 To prepare the phosphine oxide capping group, («-octyl)2PCl first had to be prepared where the «-octyl groups are present to promote the solubility of the phosphine oxide-capped CdSe nanoparticles. Scheme 6-1 outlines the synthetic procedure used to prepare («-octyl)2PCl. Using the procedure of Whitaker et al, PCI3 was first reacted with 1 mole equivalent of diethylamine. The resulting (Et2N)PCl2 was then reacted with 2.5 equivalents of «-octylmagnesium bromide to give 22. Crude 22 was purified by first heating to reflux with 1,4-dioxane to precipitate the Mg salt formed followed by fractional distillation. 22 was deprotected with ethereal HC1 to yield P(«-octyl)2Cl following a procedure by Fogg.27 terthiophene (23). Phosphine oxides 4 and phosphonic acids 15, 16 have been previously shown to Scheme 6-1 CI 1) 2NH(Et) 2 , -78 °C, ether ( E t ) 2 ' ] 1 I 2) 3 hrs, 25 °C *~ p. 1)2.5r>-octyl-MgBr, -33 °C, ether (Et) 2N CI 2) 25 °C, overnight CI 3) Dioxane, reflux CgHi7 CgH-17 22 CI 1M HCI, ether CgHi7 CsHi7 155 Phosphine oxide 23 was prepared by lithiation of terthiophene followed by quenching with P(«-octyl)2Cl (Scheme 6-2). The as-prepared phosphine was oxidized with excess 30% H2O2. Column chromatography was used to isolate the desired phosphine oxide 23 as a yellow 1 ^ I solid. Analysis of all the fractions from chromatography by H- and P-NMR showed the presence of unreacted terthiophene, 23 and a-substituted diphosphine oxide terthiophene. The disubstituted terthiophene side product and unreacted terthiophene explains the low yields observed. Scheme 6-2 1) /7-BuLi, T H F , -78 °C Jl \ - ^ S \_f S 2) P[/7-octyl]2CI 3) a c e t o n e / C H 2 C I 2 , excess H 2 0 2 The second class of compounds is composed of regioregular (head-to-tail) oligothiophenes with a thiol attached at one of the a-positions through an amide linkage. Hexyl chains were attached to the 3-position of the thiophene rings to aid in the solubility of the oligothiophene-capped CdSe nanoparticles. Regioregular oligothiophenes (1-3 repeating units) were synthesized because they have been shown to have better 71-orbital overlap between adjacent thiophenes resulting in longer conjugation lengths compared to regiorandom oligothiophenes.32 Carboxylic acid derivatives of the regioregular oligothiophenes (24, 27, 30) were first synthesized as outlined in Scheme 6-3. The thiophene analogue 24 was prepared by lithiating 2-bromo-3-hexylthiophene28 followed by quenching with solid CO2 and protonation 156 with HC1. Regioregular bithiophene (27) and terthiophene (30) analogues were prepared using Figure 6-1 Side product from substitution reactions between oligothienyl N-hydroxysuccinimide esters (25, 28, 31) with cysteamine. Thiol functionalities were then attached to the oligothiophenes via the carboxylic acid end groups as shown in Scheme 6-4. Instead of directly reacting the cysteamine with a carboxylic acid, an amine-reactive iV-hydroxy succinimide (NHS) ester was initially prepared. This approach has been previously shown to increase the overall yield of the desired amide product. 3 3 The carboxylic acids were reacted with A -^(3-dimethylamino-propyl)-7V'-ethylcarbodiimide hydrochloride (EDC) to form an amine-reactive O-acylisourea intermediate which in the presence of NHS undergoes substitution to yield the NHS ester (25, 28, 31). The subsequent substitution reaction of cysteamine with NHS leaving groups yields the desired thiol functionalized oligothiophenes (26, 29, 32). These compounds were purified using column chromatography and two major fractions were isolated in each case. The side products from these reactions were isolated and accounted for ~35 mole % of the products. 1H NMR spectroscopy of these side products showed two equivalents of oligothiophene for each cysteamine moiety and the most likely structures are shown in Figure 6-1. 157 Scheme 6-3 ! = 2 c eHi3 x = 2 30 Scheme 6-4 x = 0, 24 x = 1, 27 x = 2, 30 x = 0, 25 x = 1, 28 x = 2, 31 x = 0, 26 x = 1, 29 x = 2, 32 10 CdSe nanorods (CdSe-TOPO) were prepared using a method developed by Peng et al. Q that uses non-pyrophoric CdO and Se precursors (Scheme 6-5) instead of the Cd(CH3)2 that was 158 used previously. Peng et al. have shown that CdSe nanoparticles prepared using this method results in nanorods instead of spherical particles. Reddish brown CdO powder was dissolved in a mixture of trioctylphosphine oxide (TOPO) and tetradecylphosphonic acid (TDPA), and heating the mixture to 300 °C generated a colorless solution. Se powder dissolved in trioctylphosphine (TOP) was injected into the Cd solution and the evolution of CdSe nanoparticles observed as the color of the solution changed from clear to red. The change in color concomitant with the growth of the nanoparticles corresponds to a decreasing band gap in the semiconductor.8 Removing the heat stops the growth process. After cooling, the bright red CdSe-TOPO were purified by multiple precipitations with methanol from CHC13. 3 1P NMR spectroscopy of the washed NPs showed no evidence of residual TOPO, TOP or TDPA. To prepare oligothiophene-capped CdSe nanoparticles, an exchange reaction between the TOPO passivating the surface of CdSe-TOPO and the prepared oligothiophene capping groups is required. Bawendi17 and Emrick 3 4' 3 5 have shown that efficient exchange of functionalized phosphine oxides for TOPO requires the TOPO to be first displaced by pyridine. This results in pyridine-capped CdSe nanoparticles where the pyridine can be displaced by a functionalized phosphine oxide. To completely replace the TOPO molecules with pyridine, CdSe-TOPO was dissolved in pyridine and stirred at 70 °C overnight. After cooling, the pyridine-capped CdSe rods, which are poorly soluble, were precipitated out of solution with hexanes. The pyridine-capped CdSe NPs were stirred in CHCI3 with 23 at 55 °C overnight in an attempt to replace the pyridine on the NP surface with 23. The 23-capped CdSe NPs were precipitated repeatedly from CHCI3 solution with methanol to remove unbound 23 and displaced pyridine. TLC was used to monitor the washings and the reprecipitations were halted once free 23 is no longer detected. Unfortunately, 'H NMR spectroscopy and UV-vis absorption of the product showed no evidence 159 of any 23 attached to the CdSe surface. It appears that all the 23 present during the exchange reaction was removed during the reprecipitations. This procedure was repeated 3 more times at slightly different temperatures and in THF but the same results were obtained. This suggests that 23 has very poor affinity for the CdSe NP surface, and that a stronger binding group is needed. Scheme 6-5 CdSe -TOPO Thiols have been shown to have higher affinities for CdSe surfaces compared to phosphine oxides and stirring a thiol in the presence of TOPO-capped CdSe NPs will replace TOPO without the need to prepare the pyridine-capped CdSe NPs first.18'31 Scheme 6-6 outlines the procedure used for attaching thiol functionalized oligothiophenes (26, 29, 32) to the CdSe NP surface. The oligothiophenes 26, 29, 32 were dissolved in CHCI3 with CdSe-TOPO and stirred at room temperature overnight. Nanoparticles capped with 26 (CdSe-26) were purified by repeated precipitation with methanol from solutions of CHCI3. The precipitations were halted when 26 was no longer observed via the TLC of the washings. CdSe NPs capped with 29 and 32 (CdSe-29 and CdSe-32, respectively) were more soluble than CdSe-26 and stayed dissolved in CHCI3 even as excess methanol was added. CdSe NPs capped with 29 and 32 were purified by numerous precipitations with methanol from hexane solutions. The washings were also 160 examined by TLC but even after 20 precipitations, trace free thiol was still detected. Previous studies have shown that thiolates attached to the surface of CdSe NPs can readily dissociate into solution where an equilibrium is established. As CdSe-29 and CdSe-32 are dissolved in hexanes or CH2CI2, the capping groups (29 and 32, respectively) dissociate from the surface and equilibrate with solution, this is observed as an increase in emission from unbound capping group versus time (discussed further in Section 6.3.4). Capping groups 29 and 32 are brightly T 1 fluorescent and even a diminutive amount of dissociation is evident via TLC. P NMR spectra of the washed thiol-capped NPs CdSe-26, CdSe-29 and CdSe-32 show no peaks indicating the complete exchange of thiol for TOPO on the surface of CdSe-TOPO. 'H NMR spectroscopy of CdSe-26, CdSe-29 and CdSe-32 shows broadening of the oligothiophene proton resonances due to restricted rotation of the oligothiophenes attached to the CdSe NP surface. Figure 6-2 shows the 'if NMR spectrum of the oligothiophene protons of CdSe-32. These signals are much broader when compared to the spectrum of unbound 32. Some of the proton signals have shifted downfield after attachment to the NP surface. This downfield shift has been observed previously TO in related systems, and was attributed to re-electron density being inductively drawn to the CdSe surface causing deshielding of the oligothiophene protons. It is also interesting to note that there are no sharp peaks due to unbound 32 present in the spectrum of CdSe-32. 161 Scheme 6-6 CdSe-TOPO x = 0, CdSe-26 x = 1, CdSe-29 x = 2, CdSe-32 Figure 6-2 ' H N M R spectra of 32 and CdSe-32 dissolved in CDC1 3 (300 MHz). The (•) indicates the proton signal of residual CHCI3 present in the solvent. 162 6.3.2 Transmission Electron Microscopy Transmission electron microscopy (TEM) images of CdSe-TOPO dropcast onto carbon coated T E M grids are shown in Figure 6-3a. The CdSe nanoparticles are rod shaped with dimensions of ~2 x 15 nm similar to the CdSe particles prepared by Peng et al. using the same procedure.22 After the exchange reaction of 32 with CdSe-TOPO, T E M images of CdSe-32 show no apparent change in the size and shape of the particles (Figure 6-3b). Figure 6-3 Transmission electron microscopy (TEM) images of (a) CdSe-TOPO and (b) CdSe-32 drop-cast onto a carbon-coated T E M grid. Interesting behavior was observed when the CdSe NPs were irradiated for extended periods of time under the electron beam of the T E M . Figure 6-4 shows a change in shape of CdSe-TOPO from rods to spheres after several minutes of electron-beam irradiation at 80 keV. E-beam induced shape changes have not been observed previously for CdSe nanorods although e-beam induced decomposition of bulk II-VI semiconductors has been previously reported.39' 4 0 Loginov et al. explained that exposure of II-VI semiconductors to a 100 keV e-beam causes void/defect formation that leads to precipitation of Cd(0) and the breakdown of the local 163 structure. The e-beam induced shape change of CdSe-TOPO is therefore possibly due to collapse of the rod structure as Cd(0) precipitates out of the CdSe NPs. Figure 6-4 Transmission electron microscopy (TEM) image of CdSe-TOPO dropcast onto a carbon coated T E M grid after exposure to a 80 keV electron beam for (a) 0, (b) 7.5 and (c) 12.5 min. 6.3.3 UV-vis Absorption Spectra The UV-vis spectra of the capping groups (23, 26, 29, 32) are shown in Figure 6-5 and the data is tabulated in Table 6-1. The capping groups have one major absorption band corresponding to the n - » 7 i * transition for 26 and to the TC—>7t* of 23, 29 and 32 (discussed further in Section 6.3.6). The absorption bathochromically shifts as the number of repeat units in the oligothiophene is increased from 1 to 3 (26, 29, 32, respectively) resulting from an increase in the conjugation length and decrease in the H O M O - L U M O gap. Capping groups 23 and 32 which both have 3 oligothiophene rings possess very similar n->n* absorption energies indicating there is a relatively small contribution to the HOMO or L U M O energies from the phosphine oxide and thiol functional groups, respectively. 164 <D O C 03 .Q i _ O CO < 300 400 500 Wavelength (nm) 600 Figure 6-5 Normalized UV-vis absorption spectra of capping groups (23, 26, 29 and 32) in CHCI3. Table 6-1 UV-vis absorption maxima of capping groups (23, 26, 29, 32), TOPO-capped CdSe NPs (CdSe-TOPO) and oligothiophene-capped CdSe NPs (CdSe-26, CdSe-29, CdSe-32) in CHCI3. Sample UV-vis absorption A, m a x (nm) 23 367 26 258 29 326 32 363 CdSe-TOPO 547 CdSe-26 266,555 CdSe-29 319,544 CdSe-32 362, 544 165 The UV-vis absorption spectrum of the TOPO-capped CdSe NPs are shown in Figure 6-6a. The band edge energy of CdSe-TOPO is 547 nm (2.27 eV). After capping group exchange reactions, the absorption spectra of the oligothiophene-capped CdSe NPs show two peaks in their absorption spectra (Figure 6-6b-d). The lower energy peak corresponds to the band edge of the NPs while the peak at higher energy corresponds to the n—>n* and n—>TC* absorptions of the oligothiophene capping groups. The insets of Figure 6-6b-d shows the UV-vis absorption spectrum of CdSe-TOPO subtracted from the absorption spectra of each oligothiophene-capped CdSe NP (CdSe-26, CdSe-29, CdSe-32) confirming this assignment. The difference spectra are identical to the absorption spectra of the corresponding capping group used (26, 29, 32). 166 300 400 500 600 700 800 300 400 500 600 700 800 Wavelength (nm) Wavelength (nm) 300 400 500 600 700 800 300 400 500 600 700 800 Wavelength (nm) Wavelength (nm) Figure 6-6 UV-vis absorption spectra of (a) CdSe-TOPO, (b) CdSe-26, (c) CdSe-29 and (d) CdSe-32 in CHCI3. Insets: difference absorption spectra between each oligothiophene-capped CdSe NP and CdSe-TOPO. 6.3.4 Excitation and Emission Spectra The shortest capping group (26) is non-emissive consistent with the lack of emission in thiophene at room temperature.41 The excitation and emission spectra of the longer oligothiophene capping groups (23, 29, 32) in CHCI3 are shown in Figure 6-7 with the data tabulated in Table 6-2. The emission maxima bathochromically shift as the number of repeat units increases from 2 to 3 due to increased conjugation and a decrease in the HOMO-LUMO 167 gap. Solid-state emission spectra for the capping groups show a bathochromic shift compared to solution spectra (Table 6-2). A similar red-shift has been observed in the solid-state emission of bithiophene and terthiophene compared to solution42 and this has been attributed to increased planarity in the solid-state compared to free-rotation in solution.43 Table 6-2 Excitation and emission maxima of capping groups (23, 26, 29, 32), TOPO-capped CdSe NPs (CdSe-TOPO) and oligothiophene-capped CdSe NPs (CdSe-26, CdSe-29, CdSe-32) in CHC13 and in the solid-state. Sample Excitation maxima in Emission maxima in Solid-state emission CHCI3 [emission CHCI3 [excitation maxima [excitation wavelength] (nm) wavelength] (nm) wavelength] (nm) 23 368 [442] 442 [367] 500 [367] 26 no emission no emission [258] no emission 29 335 [414] 414 [326] 448 [399] 32 367 [468] 468 [363] 485 [450] CdSe-TOPO broad 562 [350] 569 [350] CdSe-26 broad 560 [350] 566 [350] CdSe-29 broad 415,562 [350] 407, 566 [350] CdSe-32 broad 468, 565 [350] 465, 567 [350] 168 300 400 500 600 Wavelength (nm) Figure 6-7 Excitation (—) and emission (—) spectra of the capping groups (a) 23, (b) 29 and (c) 32 in CHCI3 excited at 367, 326 and 363 nm, respectively. The emission spectra of the CdSe NPs in CHCI3 excited at 350 nm are plotted in Figure 6-8. The TOPO-capped NPs (CdSe-TOPO) have an emission maximum at 562 nm exhibiting a large energy difference from the excitation wavelength (350 nm). The large energy difference is due to the large number of excitonic transitions that non-radiatively decay to the L U M O , emitting at the energy of the first excitonic peak (HOMO-LUMO gap).44 When oligothiophenes are exchanged for the TOPO on the CdSe NP surface interesting results are observed. Figure 6-8b shows the emission spectra of CdSe-26 and CdSe-TOPO at the same optical density. This shows that the presence of 26 on the CdSe surface quenches the NP fluorescence. This is in 169 contrast to CdSe-29 and CdSe-32 where no significant quenching is observed for the NP fluorescence (Figure 6-8c and d). The emission of the capping groups 29 and 32 attached to the CdSe surface in CdSe-29 and CdSe-32 is quenched. The emission peak observed in CdSe-29 and CdSe-32 at shorter wavelength corresponds to unbound 29 and 32 present in solution that has dissociated from the NP surface. Since 29 and 32 are much more fluorescent than CdSe-TOPO, a small number of molecules dissociating from the NP surface appears to dwarf the NP emission. As CdSe-29 and CdSe-32 are left in solution, the intensity of the emission from unbound capping groups increases due to more dissociation from the CdSe surface. After four hours, the growth of the capping group emission halts as equilibrium is established between dissociating capping groups and free capping groups reattaching onto the particle surface. This is confirmed by the solid-state emission spectra of CdSe-29 and CdSe-32 (Figure 6-9) where dissociation from the NP surface does not occur and the emission at shorter wavelength originating from capping groups is much weaker than the emission from the CdSe NP. 170 Z5 3-c 13 O O CdSe-TOPO CdSe-26 500 550 600 Wave leng th (nm) 650 500 550 600 Wave leng th (nm) 650 co -*—> c o O CdSe-TOPO CdSe-29 to -4—' c 3 o O (d) CdSe-TOPO CdSe-32 350 400 450 500 550 600 650 Wave leng th (nm) 350 4 0 0 4 5 0 500 550 600 6 5 0 W a v e l e n g t h (nm) Figure 6-8 Emission spectra of (a) CdSe-TOPO, (b) CdSe-26, (c) CdSe-29 and (d) CdSe-32 excited at 350 nm in CHCI3. For CdSe-26, CdSe-29, and CdSe-32, the emission spectrum of CdSe-TOPO at the same optical density is plotted on the same graph for comparison. 171 400 450 500 550 600 650 400 450 500 550 600 650 Wave leng th (nm) Wave leng th (nm) Figure 6-9 Solid-state emission spectra of (a) CdSe-29 and (b) CdSe-32. The excitation spectra of the oligothiophene-capped CdSe NPs are equally interesting. The excitation spectrum of CdSe-26 at an emission wavelength of 562 nm is identical to that of CdSe-TOPO demonstrating that no energy or electron is transferred from 26 to the NP. On the other hand, the excitation spectra of CdSe-29 and CdSe-32 are quite different from that of CdSe-TOPO. Figure 6-10 shows the excitation spectrum of CdSe-TOPO subtracted from the excitation spectra of CdSe-29 and CdSe-32. The difference spectra for CdSe-29 and CdSe-32 have maxima at 355 and 383 nm, respectively. These difference spectra have the same shape as the excitation of the unbound capping groups in solution except that the maxima have been red-shifted from the excitation maxima of 29 and 32 by 20 nm in CdSe-29 and 16 nm in CdSe-32. This red-shift in the excitation spectra may be due to increased planarity in the oligothiophene backbone that results from close packing between adjacent molecules at the NP surface. A similar red-shift was observed in the solid-state absorbance spectra of bithiophene and terthiophene compared to solution42 due to increased planarity in the solid-state.43 The solid-172 state absorbance spectra of 29 and 32 both display a 15 nm red-shift compared to the absorption in solution. This demonstrates that in CdSe-29 and CdSe-32 excitation of the oligothiophene capping groups bound to the NP surface results in emission from the NPs at 562 nm. 300 350 400 450 500 550 300 350 400 450 500 550 W a v e l e n g t h (nm) W a v e l e n g t h (nm) Figure 6-10 Excitation spectrum of CdSe-TOPO subtracted from excitation spectra of (a) CdSe-29 and (b) CdSe-32 (emission wavelength = 562 nm). The band edge of a CdSe nanoparticle and the HOMO and LUMO energies of 29 and 32 determined from UV-vis absorption spectra and cyclic voltammetry (Section 6.3.5) are plotted in Figure 6-11. Since the LUMO of 29 and 32 are higher in energy than the lower edge of the nanoparticle's conduction band, electron transfer is possible from excited 29 or 32 into the CdSe NP. The transferred electron can then radiatively relax to the CdSe valence band where the emitted photon's energy equals the CdSe band edge energy. A similar electron transfer mechanism has been observed in a blend of polyvinylpyrrolidone/CdSe nanoparticles where the energy levels are similar to this system.46,47 Another possible mechanism is Forster resonance energy transfer (FRET) where the excited oligothiophene undergoes non-radiative transfer of 173 energy to the CdSe NP giving radiative emission from the CdSe NP. Examples of FRET from a conjugated polymer to a CdSe NP has also been observed previously.48'49 L U M O L U M O C B V B H O M O H O M O 29 C d S e 32 Figure 6-11 HOMO and LUMO energy potential diagram for 29 and 32. Band edge values for CdSe are based on ionization potentials from reference 43 and the band gap is from UV-vis absorption of CdSe-TOPO. 6.3.5 Cyclic Voltammetry The HOMO levels of the oligothiophene capping groups were determined by cyclic voltammetry (CV) and voltammograms are plotted in Figure 6-12. The shortest oligomer 26 shows no oxidation below the solvent's potential limit of CH2CI2 (2.0 V vs SCE). The higher oxidation potential of 26 compared to 29 and 32 is due to a lower HOMO energy due to its shorter -^conjugated system (Section 6.3.6). As the conjugation length is increased by an additional thiophene ring, the oxidation potential decreases (1.48 V for 29) due to an increase in the HOMO energy (Section 6.3.6). There is however no clear reduction peak associated with the oxidation wave indicating that the oxidation is irreversible. Repeated potential cycling of 29 S C E (V) 2.0 1.5 1.0 0.5 0 0.5 1.0 1.5 2.0 174 from 0 to 1.7 V does not show any new peaks or peak growth indicating that no material is electrodeposited onto the working electrode. Extending the 7i-conjugated system with a third thiophene ring decreases the oxidation potential even more (1.09 V for 32) with two reduction peaks at 0.99 and 0.76 V. The wave at 0.99 V is due to the reduction of oxidized 32 species while the wave at lower potential (0.76 V) corresponding to surface bound material formed upon oxidation, possibly dimerized 32. With successive cycling of the potential from 0 to 1.3 V, the peak at 0.76 V grows in intensity with each successive scan (Figure 6-13a), consistent with electrodeposition of conductive material possibly dimerized 32. o ~o o c < c O o ~o o TO o - 0 . 5 (a) 2 nA ^^^---^j (b) 2 nA Av_yy 0 . 0 0 . 5 1 . 0 Potential (V) vs S C E 1 . 5 2 . 0 Figure 6-12 Cyclic voltammetry of (a) 29 and (b) 32 in CH2C12 with 0.1 M (C4Ho)4NPF6. 175 I 1 1 1 1 ' 1 1 1 1 1 -0.5 0.0 0.5 1.0 1.5 2.0 Potential (V) vs S C E Figure 6-13 Electrodeposition of (a) 32 and (b) CdSe-32 by repeated potential scanning (5 cycles) from 0 to 1.3 V in CH2CI2 containing 0.1 M (C4Ha)4NPF6. The arrows indicate the direction of current change with each successive scan. Electrodeposition of oligothiophene-capped CdSe NPs (CdSe-26, CdSe-29, CdSe-32) by oxidative coupling was attempted by potential cycling and electrolysis. However, no electrodeposited was detected on the working electrodes for any of the samples. Nanoparticles CdSe-26 and CdSe-29 showed no oxidation peaks in the CH2CI2 solvent window and therefore no coupling could result. Electrochemical potential cycling of CdSe-32 did show a small oxidation at -1.1 V (Figure 6-13b) with a small broad reduction peak associated with it on the reverse scan at -0.7 V. There was a slight increase in current on the second scan compared to the first but subsequent scans after that showed no further increase in current. This indicates that oxidative coupling may have occurred on the first scan but potential cycling after that results in 176 no further crosslinking. This is in contrast to terthiophene-capped gold NPs (18), which can be electrochemically polymerized to films up to 1 micron thick (Chapter 3). This difference could be related to the inherent conductivity of the nanoparticle core where Au NPs are more conductive than CdSe NPs. 5 0' 5 1 When CdSe-32 is electrochemically deposited onto the working electrode after the first scan, the particles act as an insulating layer between the working electrode and other CdSe-32 particles in solution thus preventing further oxidative crosslinking to occur. On the other hand, poly-18 deposited onto a working electrode is sufficiently conductive that NPs are electrochemically accessible and further crosslinking can occur. 6.3.6 Density Functional Theory Calculations DFT calculations on 26, 29 and 32 were carried out in Gaussian 0330 using a B3LYP/6-31G*(d,p) basis set and the results are shown in Figure 6-14. The calculated HOMO-LUMO gaps for 26, 29 and 32 are similar to the experimentally measured HOMO-LUMO gaps from the UV-vis absorption and follow the same trend. The calculations indicate that the thiol contributes less orbital density to the HOMO as the length of the oligothiophene increases. In the shortest oligothiophene 26, the HOMO is mostly localized on the thiol, and the absorption band in the UV-vis spectrum (Figure 6-5) is assigned as an n—Mr* transition. For the longer oligothiophenes, the HOMO has larger thiophene n character and the absorption band in the UV-vis spectra is assigned as a TC—»rc* transition. With an increase in the number of thiophenes, the relative change in the HOMO-LUMO gap with each additional ring decreases. This is typical of conjugated oligomers where the red-shift decreases with increasing chain length, and has been previously observed for other oligothiophenes. ' The increase of the calculated HOMO energy level as the length of the oligothiophene increases also agrees well with the electrochemistry data discussed in Section 6.3.5. 177 Molecule HOMO 32 7* 29 26 Energy relative to vacuum (eV) 1.0 2.0 3.0 - 4 . 0 5.0 6.0 7.0 L U M O H O M O 26 L U M O H O M O 29 L U M O H O M O 32 Figure 6-14 Calculated frontier orbitals of 26, 29 and 32 and energy level diagram depicting the HOMO and L U M O levels of 26, 29 and 32. 178 6.4 Discussion According to the cyclic voltammetry measurements, the HOMOs of 26, 29 and 32 are all lower in energy relative to the valence band edge of the CdSe NP and therefore cannot act as hole traps. This explains why no significant quenching of the CdSe NP luminescence is observed in CdSe-29 and CdSe-32, but does not explain the quenching of the CdSe NP luminescence observed in CdSe-26 since the HOMO of the 26 is also lower than the HOMOs of both 29 and 32. A possible explanation for the anomalous behavior of CdSe-26 is based on consideration of the number of thiolate groups on the NP surface. Bullen et al.54 showed that the choice of functional group used to attach to the CdSe surface greatly affects CdSe NP luminescence. Thiolates in particular have been shown to quench CdSe NP fluorescence. Wuister et al. theorize that quenching occurs due to thiolates acting as hole traps (electron transfer from the HOMO of the thiolate into the CdSe particle).55 However, not all thiolate capped NPs show luminescence quenching. Jeong et al. have shown that low thiolate concentrations result in slightly enhanced NP fluorescence.56 They showed that CdSe NP surfaces have intrinsic electron trap states that are deactivated by electron-donating thiolates at low concentrations. As the number of thiolates on the CdSe NP surface exceeds the number of electron trapping sites, the excess thiolates become hole trapping sites that quench fluorescence. The average number of capping groups on the surface of CdSe-26, CdSe-29 and CdSe-32 can be calculated from UV-vis absorption spectra and the molar extinction coefficients. The molar extinction coefficients for capping groups are s = 8190, 14860 and 20200 L mol"1 cm"1 for 26, 29 and 32, respectively. The extinction coefficient for the CdSe NPs was estimated to be 8 = 94850 L mol"1 cm"1 from the first excitonic peak position.43 Deconvoluting the absorption 179 spectra (Figure 6-6) for the contribution of the capping group and CdSe NP gives an average of 69, 29 and 47 capping groups present in each CdSe-26, CdSe-29 and CdSe-32 nanoparticle, respectively. The smaller number of capping groups for CdSe-29 and CdSe-32 compared to CdSe-26 are possibly due to the steric bulk of the longer oligothiophene capping groups. The number of thiolate capping groups on the NP surface correlates with the NP fluorescence as in Jeong's work. Nanoparticle CdSe-26 which has the largest average number of capping groups on the surface shows quenching of the NP fluorescence. Here, a large number of thiolates relative to electron trap sites results in 26 acting as hole trapping states that quench fluorescence. On the other hand, CdSe-29 and CdSe-32 with fewer capping groups present do not have excess thiolate acting as hole trapping states and fluorescence is not quenched. To test this hypothesis, an excess amount of thiol was added to CdSe-29 in solution. Dodecanethiol was used instead of 29 since strong emission from 29 completely swamps out any CdSe NP emission. Furthermore, dodecanethiol is less bulky compared to 29 and may be able to insert between molecules of 29 already tethered to the surface of CdSe-29. The introduction of dodecanethiol to CdSe-29 in CHCI3 results in the immediate quenching of the fluorescence by half, and after 30 minutes, complete quenching is observed. The dodecanethiol added to CdSe-29 in solution are able to bind to the NP surface and quenches the fluorescence from the CdSe NP. As time elapses, more dodecanethiol molecules are able to bind to the CdSe surface and eventually no more emission from the NP is observed. This observation is consistent with the proposed explanation that the presence of excess thiolate on the NP surface act as hole traps and quench the NP fluorescence. 6.5 Conclusions 180 A series of thiol substituted-oligothiophenes were used to functionalize the surface of CdSe nanoparticles. Attachment of the oligothiophenes to the CdSe nanoparticles has little effect on the electronic structure of the oligothiophene as shown by the lack of change in the absorption spectra. However, the optical properties are significantly affected where the oligothiophene emission is quenched after attachment to the CdSe surface due to either an electron or energy transfer mechanism. Depending on the number of oligothiophenes attached to the CdSe surface, the optical properties of the CdSe nanoparticles are affected differently where an excess number of thiols act as hole traps leading to quenching of the NP emission. Attempts to electrochemically crosslink these oligothiophene-capped CdSe nanoparticles were unsuccessful possibly due to the intrinsic resistivity in the particles. 181 6.6 References 1. Gaponenko, S. V., Optical Properties of Semiconductor Nanocrystals. Cambridge University Press: Cambridge, UK, 1998. 2. Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P., Science 1998, 281, 2013. 3. Chan, W. C. W.; Nile, S., Science 1998, 281, 2016. 4. Greenham, N. C ; Peng, X.; Alivisatos, A. P., Phys. Rev. B 1996, 54, 17628. 5. Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P., Science 2002, 295, 2425. 6. Coe, S.; Woo, W.-K.; Bawendi, M.; Bulovic, V., Nature 2002, 420, 800. 7. Lee, J.; Sundar, V. C ; Heine, J. R.; Bawendi, M. G.; Jensen, K. F., Adv. Mater. 2000, 12, 1102. 8. Murray, C. B.; Norris, D. J.; Bawendi, M. G., J. Am. Chem. Soc. 1993, 115, 8706. 9. Peng, X.; Wickham, J.; Alivisatos, A. P., J. Am. Chem. Soc. 1998, 120, 5343. 10. Peng, Z. A.; Peng, X., J. Am. Chem. Soc. 2001, 123, 183. 11. Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Frechet, J. M. J., J. Am. Chem. Soc. 2004, 126, 6550. 12. Odoi, M. Y.; Hammer, N. I.; Sill, K.; Emrick, T.; Barnes, M. D., J. Am. Chem. Soc. 2006, 128, 3506. 13. Querner, C ; Reiss, P.; Bleuse, J.; Pron, A., J. Am. Chem. Soc. 2004, 126, 11574. 14. Skaff, H.; Sill, K.; Emrick, T., J. Am. Chem. Soc. 2004, 126, 11322. 15. Locklin, J.; Patton, D.; Deng, S.; Baba, A.; Millan, M.; Advincula, R. C , Chem. Mater. 2004, 16, 5187. 182 16. Milliron, D. J.; Alivisatos, A. P.; Pitois, C ; Edder, C ; Frechet, J. M. J., Adv. Mater. 2003, 15,58. 17. Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G., J. Chem. Phys. 1997, 106, 9869. 18. Wisher, A. C.; Bronstein, I.; Chechik, V., Chem. Commun. 2006, 1637. 19. Hikmet, R. A. M.; Chin, P. T. K.; Talapin, D. V.; Weller, H., Adv. Mater. 2005, 17, 1436. 20. Hu, J.; Li, L.-s.; Yang, W.; Manna, L.; Wang, L.-w.; Alivisatos, A. P., Science 2001, 292, 2060. 21. Kumar, S.; Nann, T., Small 2006, 2, 316. 22. Peng, Z. A.; Peng, X., J. Am. Chem. Soc. 2002, 124, 3343. 23. Huynh, W. U.; Dittmer, J. J.; Libby, W. C ; Whiting, G. L.; Alivisatos, A. P., Adv. Funct. Mater. 2003, 13, 73. 24. Sun, B.; Greenham, N. C , Phys. Chem. Chem. Phys. 2006, 8, 3557. 25. Chiem Van, P.; Burkhardt, A.; Shabana, R.; Cunningham, D. D.; Mark, H. B., Jr.; Zimmer, H., Phosphorus, Sulfur Silicon Relat. Elem. 1989, 46, 153. 26. Whitaker, C. M.; Kott, K. L.; McMahon, R. J., J. Org. Chem. 1995, 60, 3499. 27. Fogg, D. E.; Radzilowski, L. H.; Blanski, R.; Schrock, R. R.; Thomas, E. L., Macromolecules 1997, 30, 417. 28. Hoffmann, K. J.; Carlsen, P. H. J., Synth. Commun. 1999, 29, 1607. 29. Kirschbaum, T.; Azumi, R.; Mena-Osteritz, E.; Bauerle, P., New J. Chem. 1999, 23, 241. 30. Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C ; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, 183 J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X . ; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C ; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C ; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C ; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C ; and Pople, J. A. Gaussian 03, Revision C.02, Gaussian, Inc.: Wallingford CT, 2004. 31. Hikmet, R. A. M.; Talapin, D. V.; Weller, H., J. Appl. Phys. 2003, 93, 3509. 32. McCullough, R. D., Adv. Mater. 1998, 10, 93. 33. Grabarek, Z.; Gergely, J., Anal. Biochem. 1990, 185, 131. 34. Skaff, H.; Emrick, T., Chem. Commun. 2003, 52. 35. Skaff, H.; Ilker, M. F.; Coughlin, E. B.; Emrick, T., J. Am. Chem. Soc. 2002, 124, 5729. 36. Aldana, J.; Wang, Y. A.; Peng, X . , J. Am. Chem. Soc. 2001, 123, 8844. 37. Aldana, J.; Lavelle, N . ; Wang, Y.; Peng, X . , J. Am. Chem. Soc. 2005, 127, 2496. 38. Querner, C ; Benedetto, A.; Demadrille, R.; Rannou, P.; Reiss, P., Chem. Mater. 2006, 18, 4817. 39. Loginov, Y. Y.; Brown, P. D., Phys. Status Solidi A 1992, 132, 323. 40. Loginov, Y. Y.; Brown, P. D.; Thompson, N., Phys. Status Solidi A 1991, 126, 63. 41. Becker, R. S.; de Melo, J. S.; Macanita, A. L.; Elisei, F., J. Phys. Chem. 1996, 100, 18683. 184 42. Gombojav, B.; Namsrai, N.; Yoshinari, T.; Nagasaka, S.-i.; Itoh, H.; Koyama, K., J. Solid State Chem. 2004, 177, 2827. 43. Yassar, A.; Horowitz, G.; Valat, P.; Wintgens, V.; Hmyene, M.; Deloffre, F.; Srivastava, P.; Lang, P.; Gamier, F., J. Phys. Chem. 1995, 99, 9155. 44. Bagga, A.; Chattopadhyay, P. K.; Ghosh, S., Los Alamos National Laboratory, Preprint Archive, Condensed Matter 2005, 1. 45. Li, Y.; Zhong, H.; Li, R.; Zhou, Y.; Yang, C.; Li, Y., Adv. Fund Mater. 2006, 16, 1705. 46. Kucur, E.; Riegler, J.; Urban, G.; Nann, T., J. Chem. Phys. 2004, 121, 1074. 47. Kucur, E.; Riegler, J.; Urban, G. A.; Nann, T., J. Chem. Phys. 2004, 120, 1500. 48. Anni, M.; Manna, L.; Cingolani, R.; Valerini, D.; Creti, A.; Lomascolo, M., Appl. Phys. Lett. 2004, 85,4169. 49. Javier, A.; Yun, C. S.; Sorena, J.; Strouse, G. F., J. Phys. Chem. B 2003, 107, 435. 50. Chen, S., Anal. Chim. Acta 2003, 496, 29. 51. Pradhan, S.; Chen, S.; Wang, S.; Zou, J.; Kauzlarich, S. M.; Louie, A. Y., Langmuir 2006, 22, 787. 52. Demanze, F.; Cornil, J.; Gamier, F.; Horowitz, G.; Valat, P.; Yassar, A.; Lazzaroni, R.; Bredas, J.-L., J. Phys. Chem. B 1997, 101, 4553. 53. Tabet, A.; Schroeder, A.; Hartmann, H.; Rohde, D.; Dunsch, L., Org. Lett. 2003, 5, 1817. 54. Bullen, C ; Mulvaney, P., Langmuir 2006, 22, 3007. 55. Wuister, S. F.; De Donega, C ; Meijerink, A., J. Phys. Chem. B 2004, 108, 17393. 56. Jeong, S.; Achermann, M.; Nanda, J.; Ivanov, S.; Klimov, V. I.; Hollingsworth, J. A., J. Am. Chem. Soc. 2005, 127, 10126. 185 Chapter 7 Conclusions and Future Work 7.1 General Conclusions This thesis describes the preparation and characterization of a series of a-substituted oligothiophene derivatives, and their application as capping groups on metal and semiconductor nanoparticle surfaces. Oligothiophene-capped Au and CdSe nanoparticles were synthesized and the chemical, electronic and optical properties of the hybrid materials were investigated to understand the effect of the nanoparticles and oligothiophenes on one another. This section summarizes the results of this study with general conclusions in reference to the initial objectives of this thesis. Considering the results presented in this thesis, attachment of oligothiophenes to the surface of Au or CdSe nanoparticles via a tethering group in the a-position has little effect on the electronic structure of the oligothiophene deduced from the nominal change in absorption spectra. However, the optical properties are significantly affected: the oligothiophene emission is quenched after attachment to either Au or CdSe nanoparticle surfaces. In the oligothiophene-capped Au nanoparticles the singlet-excited states are quenched through an energy-transfer mechanism.1 In the oligothiophene-capped CdSe nanoparticles quenching is due to either an electron or energy transfer mechanism from the nanoparticle to the HOMO of the capping oligothiophene. Attaching oligothiophenes to Au and CdSe nanoparticle affects the electronic structure of the nanoparticles differently. Oligothiophenes on the Au nanoparticle surface results in a red-shifted surface plasmon absorption. This is attributed to reduction of the plasmon resonance 186 state energy of the Au nanoparticles from overlap of the oligothiophene with the nanoparticle wavefunctions. Connecting the a-phosphino-terthiophene capped Au nanoparticles together in 3-dimensions by electrochemical oxidative coupling results in nanoparticles linked both structurally and electronically. Linking the Au nanoparticles together with a conjugated bridge results in the surface plasmon absorption of the Au nanoparticles red-shifting into the near-IR. This red-shift is due to the conjugated oligothiophene acting as a bridge facilitating near-field coupling between adjacent nanoparticles. Au nanoparticles bridged by 7i-conjugated linkers also have higher conductivities compared to unlinked Au nanoparticles due to lowering of the electronic coupling term (JJ) between the Au nanoparticles resulting from introduction of another pathway for inter-particle conductivity. The crosslinked Au nanoparticles also have tunable conductivity where the conductivity in the material can be increased by oxidative doping. The Au nanoparticles do not affect the electrochemical doping of the sexithiophene moieties bridging the nanoparticles and the material behaves electrochemically similar to molecular sexithiophene. Thiol-functionalized oligothiophenes attached to CdSe nanoparticles influence the optical properties of the CdSe nanoparticles: an excess number of thiols attached to the surface act as hole traps leading to quenching of the NP emission. Attempts to electrochemically crosslink these oligothiophene-capped CdSe nanoparticles were unsuccessful possibly due to the intrinsic resistivity in the particles. In conclusion, the work in this thesis has contributed to better understanding of optical and electronic interactions in Au and CdSe nanoparticles capped with conjugated materials. Better understanding of these interactions and the novel properties that arise are essential in the design and development of hybrid nanoparticle/conjugated polymer materials for molecular and conjugated polymer devices. 187 7.2 Suggestions for Future Work There are several new ideas that could be further developed stemming from this study. P- Phosphino-oligothiophenes could be prepared and used to passivate the Au nanoparticle surface (Scheme 7-1). Previous work in our lab has shown that Au atoms complexed to 0-phosphino-oligothiophenes significantly affect the electronic properties of the n-system compared to complexation to a-phosphino-oligothiophenes, which does not appear to influence the Tc-system.2'3 Au nanoparticles attached to the P-position of the phosphino-oligothiophene may interact more strongly with the oligothiophene Tc-system and modify their electronic and optical properties. Electrochemical crosslinking of 33 where there are two a-positions available for oxidative coupling in each capping group would result in a material more similar to polythiophene with Au nanoparticles tethered to the polymer backbone. This is expected to affect the mechanism of charge transfer through the material possibly giving very different conductivities. In connection to the failed attempt to crosslink the oligothiophene-capped CdSe nanoparticles prepared in this thesis, oligothiophenes substituted with a thiol at the P-position could be prepared and attached to CdSe nanoparticles (Scheme 7-2). Nanoparticle CdSe-34 should be electrochemically polymerizable regardless of the conductivity of the CdSe nanoparticle due to the oligothiophene capping group 34 having two a-positions open to oxidative coupling compared to 32. Photovoltaic devices could be made by evaporating Al onto electrochemically polymerized poly-(CdSe-34) on top of an indium tin oxide (ITO) substrate. Blends of poly(3-hexylthiophene) and CdSe nanoparticles have been used in photovoltaic devices where poly(3-hexylthiophene) was shown to be a good hole conductor that facilitates charge separation and hole conduction to the electrodes.4 A problem with blends is that the 188 CdSe nanoparticles are coated with an insulating tri-«-octylphosphine oxide (TOPO) layer that inhibits charge transfer into the conducting polymer and hence prevents efficient charge separation.5 A material where the poly-/oligothiophene is directly attached to the CdSe nanoparticle as in poly-(CdSe-34) should enhance charge transfer and result in better charge separation. S c h e m e 7-1 The preparation of other oligothiophene-capped metal nanoparticles such as Pt is also of interest. Pt nanoparticles have shown potential as catalysts for the electrooxidation of methanol for direct methanol fuel cells.6 The preparation of Pt nanoparticles protected with thiols have recently been demonstrated ' and using the thiol-substituted oligothiophenes (26, 29, 32) prepared in this thesis, oligothiophene-capped Pt nanoparticles could be prepared (Scheme 7-3). 189 Electropolymerization of the oligothiophene-capped Pt nanoparticles could lead to improved electroactive methanol catalysts for use as an anode material in methanol fuel cells. Scheme 7-2 190 S c h e m e 7-3 x = 0 x = 1 x = 2 191 7.3 References 1. Thomas, K. G.; Ipe, B. I.; Sudeep, P. K., Pure Appl. Chem. 2002, 74, 1731. 2. Clot, O.; Akahori, Y.; Moorlag, C ; Leznoff, D. B.; Wolf, M. O.; Batchelor, R. J.; Patrick, B. O.; Ishii, M., Inorg. Chem. 2003, 42, 2704. 3. Stott, T. L.; Wolf, M. O.; Patrick, B. O., Inorg. Chem. 2005, 44, 620. 4. Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P., Science 2002, 295, 2425. 5. Greenham, N. C ; Peng, X.; Alivisatos, A. P., Phys. Rev. B 1996, 54, 17628. 6. Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y., J. Phys. Chem. B 2004, 108, 8234. 7. Chen, S.; Kimura, K., J. Phys. Chem. B 2001, 105, 5397. 8. Yee, C ; Scotti, M.; Ulman, A.; White, H.; Rafailovich, M.; Sokolov, J., Langmuir 1999, 15,4314. 192 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0059686/manifest

Comment

Related Items