Science, Faculty of
Physics and Astronomy, Department of
DSpace
UBCV
Roussy, Tanya
Master of Applied Science - MASc
University of British Columbia
Developing a bottom-up understanding of the physics behind charge transfer processes on the nanometer scale will enable the focused design and synthesis of new materials which will revolutionize everything from solar cells to wearable electronics. Pushing our understanding of these processes to the nanometer scale is critical for next generation device development for two primary reasons. Firstly, modern electronic devices are fabricated ever smaller; to date IBM Research has already produced working chips using with gate widths only 14 atoms (7 nm) wide [1]. Secondly, for many devices which rely on charge transfer the important action is at the interface between materials; it is here that the energy level offset and other parameters can make or break a device. For modern organic devices, the interfacial region is in essence a nanometer-wide region: energy levels can differ by hundreds of meV only a few molecules away from an interface [2]. This thesis presents the design and execution of experiments which couple optical measurements with a scanning probe system. The marriage of optical and scanning probe systems enables simultaneous exploration of two complementary dimensions (optical and electronic) of the physics of the system under study, enabling the probing of parameters affecting charge transfer between single molecules. The custom-built system was used to explore optical and electronic properties of two prototypical organic molecules forming an acceptor-donor pair: 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and copper (II) pthalocyanine. This proof-of-concept will allow future users to explore a wide variety of systems which may offer clues to how charge transfer processes occur at the nanometer scale. In the first part of this work I describe the motivation for our experiment as well as the experimental design and set-up. In the second part I detail how we used the enhanced optical-electrical scanning probe to observe real-space energy levels, luminescence (or lack thereof), and attempted optical excitations between two single organic molecules. Analysis of scanning tunnelling spectroscopy data coupled with laser excitation as well as the results from experiments which in principle can measure sub-molecularly resolved luminescence show that the new optical system works as expected.
Coupling spatially and spectrally resolved optical measurements witha scanning probe systembyTanya RoussyB.A.Sc., The University of British Columbia, 2014A THESIS SUBMITTED IN PARTIAL FULFILLMENTOF THE REQUIREMENTS FOR THE DEGREE OFMaster of Applied ScienceinTHE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES(Engineering Physics)The University of British Columbia(Vancouver)August 2016c© Tanya Roussy, 2016AbstractDeveloping a bottom-up understanding of the physics behind charge transfer processes on thenanometer scale will enable the focused design and synthesis of new materials which will revo-lutionize everything from solar cells to wearable electronics.Pushing our understanding of these processes to the nanometer scale is critical for next generationdevice development for two primary reasons. Firstly, modern electronic devices are fabricated eversmaller; to date IBM Research has already produced working chips using with gate widths only14 atoms (7 nm) wide [1]. Secondly, for many devices which rely on charge transfer the importantaction is at the interface between materials; it is here that the energy level offset and other parameterscan make or break a device. For modern organic devices, the interfacial region is in essence ananometer-wide region: energy levels can differ by hundreds of meV only a few molecules awayfrom an interface [2].This thesis presents the design and execution of experiments which couple optical measurementswith a scanning probe system. The marriage of optical and scanning probe systems enables simul-taneous exploration of two complementary dimensions (optical and electronic) of the physics of thesystem under study, enabling the probing of parameters affecting charge transfer between singlemolecules. The custom-built system was used to explore optical and electronic properties of twoprototypical organic molecules forming an acceptor-donor pair: 3,4,9,10-perylene tetracarboxylicdianhydride (PTCDA) and copper (II) pthalocyanine. This proof-of-concept will allow future usersto explore a wide variety of systems which may offer clues to how charge transfer processes occurat the nanometer scale.In the first part of this work I describe the motivation for our experiment as well as the experimentaldesign and set-up. In the second part I detail how we used the enhanced optical-electrical scanningprobe to observe real-space energy levels, luminescence (or lack thereof), and attempted optical ex-citations between two single organic molecules. Analysis of scanning tunnelling spectroscopy datacoupled with laser excitation as well as the results from experiments which in principle can measuresub-molecularly resolved luminescence show that the new optical system works as expected.iiPrefaceThe data analyzed in this thesis is the result of a significant collaborative effort between myself,my supervisor Dr. Sarah Burke, and my colleagues Katherine Cochrane and BingKai Yuan. Manyothers played a supporting role, and all were invaluable in the creation of this thesis.My contributions and the contributions of my collaborators to the present work are as follows:• The day-to-day maintenance of the Omicron STM/AFM was carried out by myself, KatherineCochrane, and BingKai Yuan.• The sample preparation was carried out by myself and Katherine Cochrane.• The Matlab code written to analyze the data was written by myself, Katherine Cochrane,Agustin Schiffrin, and Gary Tom.• The optical spectrometer and lasers used in this thesis were specified and purchased by Dr.Burke.• The specialized electronics were all built and designed by David Tonkin and Benny Ng at theChemistry electronics shop in consultation with myself.• Debugging of all electronics and code used for optical measurements was performed/writtenby myself.• Arthur Mills provided invaluable feedback, help, and ideas during the development of theoptical spectrometer alignment process.• The concept and basic structure for the MATE scripts, for extending the raster time duringoptical spectroscopy grids and for performing KPFM grids (in the future), was offered byJu¨rgen Ko¨ble and implemented and modified by myself.• The specialized mechanical components were designed by myself and fabricated by myself orthe machinists in AMPEL and Chemistry: Harish Gautam, Pritesh Padhiar, and Des Lovrity.iii• The knudsen-cell evaporator used to deposit NaCl was built and debugged by myself (withdetailed design done by myself), with the original design concept coming from the Crommiegroup.• The LabView code written for optical spectroscopy was written by myself with some helpfrom Robert Delaney.• The data analyzed and images presented were primarily collected by myself, with major con-tributions coming from Katherine Cochrane and BingKai Yuan.• The 3D models of the Omicron were drawn by myself based on drawings from OmicronScienta.• The data analysis presented in this thesis was performed by myself.• Dr. Burke is responsible for the overall experimental plan, and provided invaluable guidance,help, and advice throughout the process.• Repairs on the Omicron STM/AFM were executed by Dr. Burke, Katherine Cochrane, BingKaiYuan and myself.• The Omicron service engineers, in particular Alex Skripnik, Ju¨rgen Ko¨ble, and Eike Schrenkanswered dozens of questions and provided valuable support in the development of the exper-iment.• The tips used for STS measurements were prepared by Katherine Cochrane. The tips used forAFM measurements were prepared by BingKai Yuan. The tips used for luminescence mea-surements were prepared by myself, based on a method developed by Floren Rubio, NandiniMukherjee, Dr. Burke, Niv Levy, and Dr. Crommie.ivTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiiList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ixGlossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxAcknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxii1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Scanning probe techniques and the power of real-space, atomically-resolved mea-surements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Molecular acceptor-donor pair: towards an understanding of charge transfer in anorganic electronic device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1 Scanning tunnelling microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.1 The tunnelling current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1.2 Scanning tunnelling spectroscopy . . . . . . . . . . . . . . . . . . . . . . 132.2 Atomic force microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.2.1 Frequency modulation atomic force microscopy . . . . . . . . . . . . . . . 212.2.2 The forces acting on the tip . . . . . . . . . . . . . . . . . . . . . . . . . . 232.2.3 Kelvin probe force microscopy . . . . . . . . . . . . . . . . . . . . . . . . 252.3 Scanning tunnelling microscope induced luminescence . . . . . . . . . . . . . . . 26v2.4 Optical excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.1 Scanning probe system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.1.1 Ultra-low vibration facility . . . . . . . . . . . . . . . . . . . . . . . . . . 303.1.2 Omicron UHV STM/AFM . . . . . . . . . . . . . . . . . . . . . . . . . . 313.1.3 Optical access in the scanning probe system . . . . . . . . . . . . . . . . . 323.2 System under study: copper (II) pthalocyanine and 3,4,9,10-perylene tetracarbox-cylic dianhydride on NaCl on metal surfaces . . . . . . . . . . . . . . . . . . . . . 353.2.1 Substrates: Ag(111) and Au(100) with bi- and tri-layer NaCl . . . . . . . . 353.2.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2.3 Tip preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.3 External optical system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.3.1 Mechanical components . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.3.2 Electrical components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.3.3 Photomultiplier setup and alignment . . . . . . . . . . . . . . . . . . . . . 503.3.4 Spectrometer setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.3.5 Laser setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 Optical excitation of an acceptor-donor pair . . . . . . . . . . . . . . . . . . . . . . 554.1 Characterization of spurious signals: the influence of the tip/substrate and thermalexpansion of the tip-sample junction . . . . . . . . . . . . . . . . . . . . . . . . . 574.1.1 The effect of the tip and specific location on the substrate . . . . . . . . . . 584.1.2 Thermal expansion of the tip-sample junction . . . . . . . . . . . . . . . . 614.2 Optical excitation of the system with a Ag(111) substrate . . . . . . . . . . . . . . 644.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.3.1 Vibrational excitation leading to possible tunnelling current changes . . . . 724.3.2 STS of CuPc-PTCDA on NaCl(2ML)/Au(100) . . . . . . . . . . . . . . . 765 Scanning tunnelling luminescence measurements . . . . . . . . . . . . . . . . . . . . 835.1 Luminescence measurements on bare Ag(111) . . . . . . . . . . . . . . . . . . . . 835.2 Luminescence measurements of CuPc-PTCDA on NaCl/Ag(111) . . . . . . . . . . 905.3 Point optical spectra collected on NaCl/Ag(111) . . . . . . . . . . . . . . . . . . . 935.4 Discussion and suggestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.4.1 The signal seen on NaCl . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.4.2 The molecular luminescence, or lack thereof . . . . . . . . . . . . . . . . 955.4.3 Suggestions for future experiments and improvement of the current experi-mental set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98vi6 Conclusions and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1006.1 Kelvin probe force microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1006.2 Optical spectroscopy on NaCl/Ag(111) . . . . . . . . . . . . . . . . . . . . . . . . 1016.3 Large area photomultiplier tube luminescence measurements for finding lumines-cent molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1016.4 Scanning tunnelling luminescence on supported molecules . . . . . . . . . . . . . 102Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103A Supporting materials: knudsen cell evaporator for epitaxial growth of NaCl: designand assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112A.1 Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113A.1.1 Evaporator body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113A.1.2 Shutter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115A.2 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117A.2.1 Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117A.2.2 Quartz cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118A.2.3 Heater wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120A.2.4 Thermocouple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120A.2.5 Radiation shield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120A.2.6 Shutter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122A.3 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124B Supporting materials: laser power study STS spectra averaged over smaller portionsof the molecule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126B.1 Masking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126B.2 STS results for masking over only part of the molecules: laser excitation study . . . 130B.3 STS results for masking over only part of the molecules: CuPc-PTCDA on Na-Cl/Au(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136viiList of TablesTable 4.1 z(t) results for 488 nm laser. Note that the first fit for 10 mW appears to bepoor, though it was within a 95% confidence interval. In addition, the initial τshould be 0, but it is likely that we are seeing the piezo drift. This means thatthe piezo drift and the laser-induced thermal expansion are both present in themeasurements taken. Even with the piezo drift, we can see that the longest timeone needs to wait before reaching steady state is 6 minutes. For the 488 nmlaser, we can conclude that ≥6 minutes is a safe time to wait after turning it onor off to any power to be confident that the tip and sample have reached thermalequilibrium, assuming piezo drift is at an acceptable value. . . . . . . . . . . . 63viiiList of FiguresFigure 1.1 Cartoon depicting the formation of an exciton . . . . . . . . . . . . . . . . . 2Figure 1.2 Energy level schematic for an exciton in the charge transfer state at an acceptor-donor interface. Eopt is the energy of the exciton, Egap is the bandgap energy ofa free electron-hole pair in the bulk, and Eb is the difference between the two:the exciton binding energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 1.3 Spatial and energetic evolution of the local density of states for a single CuPcmolecule (left) which is adjacent to a single PTCDA molecule (right) as imagedvia scanning tunnelling spectroscopy. Yellow indicates the presence of states. . 4Figure 1.4 STM constant current image (11nm2, I=5pA, Vb=-0.5V, T = 4.3K) of CuPc andPTCDA clusters on NaCl(2ML)/Ag(111). The CuPc images here as a ’cross’.The edge of the NaCl island is clearly visible with right-angled lines. . . . . . 5Figure 1.5 A model system: organic photovoltaics reduced to a molecular acceptor-donorpair. Copper (II) Pthalocyanine (CuPc) and 3,4,9,10-Perylene TetracarboxylicDianhydride (PTCDA) on NaCl/Ag(111) and NaCl/Au(100). . . . . . . . . . . 6Figure 2.1 Schematic for basic Scanning Tunnelling Microscope (STM) operation . . . . 7Figure 2.2 STM image of two different configurations of Copper (II) Pthalocyanine (CuPc)and 3,4,9,10-Perylene Tetracarboxylic Dianhydride (PTCDA) on NaCl(2ML)/Ag(111)(5x5nm2, I=5pA, Vb=-0.5V, T =4.3K ). The CuPc is imaged as a ‘cross’ here.Note the PTCDA images differently depending on its location with respect tothe CuPc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 2.3 STM image of CuPc and PTCDA clusters on NaCl(2ML)/Ag(111) taken with aPTCDA functionalized tip (15x15nm2, I=5pA, Vb=0.5V ). . . . . . . . . . . . 9Figure 2.4 Schematic of the energy levels for a metal tip (left) and a semiconducting sam-ple (right). The filled states are in darker colours, while the unfilled states arelighter. The fermi levels are offset by eV. An electron at a given energy in thetip can tunnel into an empty band in the sample if it is available, but it cannottunnel when there are no states to tunnel into. . . . . . . . . . . . . . . . . . . 14ixFigure 2.5 Comparing the Koslowski normalization scheme to taking only ∂ I∂V for an iso-lated CuPc molecule stabilized at the corner of a trilayer step. Plots shown arethe average over the individual spectra taken inside the area delineated in red asshown in the topography image. The peaks for the Koslowski method are in theexact same location as those obtained by taking only ∂ I∂V . . . . . . . . . . . . . 17Figure 2.6 Comparing the Koslowski normalization scheme to taking only ∂ I∂V for an iso-lated CuPc molecule stabilized at the corner of a trilayer step. Plots shown arethe average over the individual spectra taken inside the area delineated in red asshown in the topography image. The peaks for the Koslowski method are in theexact same location as those obtained by taking only ∂ I∂V . . . . . . . . . . . . . 18Figure 2.7 Comparing the Koslowski normalization scheme to taking only ∂ I∂V for an iso-lated PTCDA molecule. Plots shown are the average over the individual spectrataken inside the area delineated in red as shown in the topography image. Thepeaks for the Koslowski method are in the exact same location as those obtainedby taking only ∂ I∂V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 2.8 Comparing the Koslowski normalization scheme to taking only ∂ I∂V for an iso-lated PTCDA molecule. Plots shown are the average over the individual spectrataken inside the area delineated in red as shown in the topography image. Thepeaks for the Koslowski method are in the exact same location as those obtainedby taking only ∂ I∂V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Figure 2.9 Schematic of the AFM. The electronics are left out for simplicity. A piezo drivecontrols the position of the probe tip, as before. A tip is placed on an oscillatingcantilever, and tip-sample interaction forces change the resonant frequency ofthe cantilever. The frequency shift is taken as the imaging signal. . . . . . . . . 21Figure 2.10 Energy levels of the sample and tip in three cases: 1) tip and sample separatedby gap with no electrical contact, 2) tip and sample are in electrical contact, and3) an external DC bias is applied to cancel the CPD and the tip-sample electricalforce. Evac is the vacuum energy and EF are the fermi levels. Adapted from [3]. 26Figure 2.11 Schematic indicating the basic process in scanning tunnelling microscope in-duced luminescence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27xFigure 3.1 Photographs of our system. a. The Omicron STM/AFM in its ‘pod’, withoutthe optical components. The cryostat stands tall. b. Front view of the UHVsystem. The LT chamber is on the left and the prep chamber is on the right.c. The scanner head showing the optical access without the lens tube installed(indicated with white circle and red arrow) and the fins for eddy current damp-ing. d. The scanner head with no tip or sample as seen from the sample accessside. e. The system as seen in a) but with the optical table installed (indicatedwith the red arrow). Detailed location of the optical table and components withrespect to the Omicron LT chamber can be found in the 3D model. See Section3.3 for more information about the placement and assembly of the optical table. 32Figure 3.2 Photo of the scanner head indicating the optical paths available. The right pathis the one used for optical collection and excitation: our lens was placed onthe right at a distance of approximately 18mm from the tip-sample junction.The left path is used in conjunction with a camera to visualize tip and sampletransfer as well as coarse tip approaches. The central path is the one used by theexperimenter when transferring samples (or tip carriers) into and out of the head. 33Figure 3.3 Standard viewport transmission curve (kodial design). The 3dB points are at∼310nm - ∼2800nm. Image courtesy of [4]. . . . . . . . . . . . . . . . . . . 34Figure 3.4 In-vacuum viewport transmission (KG5). The 3dB points are at ∼350nm -∼620nm. Image courtesy of [4]. . . . . . . . . . . . . . . . . . . . . . . . . . 34Figure 3.5 STM images of the Ag(111) surface. a) Clean Ag(111) surface. b) CleanAg(111) surface showing the surface state scattering off of impurities and stepedges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Figure 3.6 STS spectra showing the Ag(111) surface state . . . . . . . . . . . . . . . . . 36Figure 3.7 STM images of Au(100). a) Clean Au(100) surface with screw dislocation. b)NaCl islands on Au(100). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Figure 3.8 Images of the sample holder. . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Figure 3.9 STM images of NaCl on NaCl/Ag(111). a) Ag(111) surface with bi-, tri-, andquadlayer NaCl. b) Atomic resolution on NaCl/Ag(111) showing the expectedsquare lattice and a Moire´ pattern due to the lattice constant mismatch betweenAg(111) and NaCl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Figure 3.10 STM images of Au(100). a) Clean Au(100) surface. b) Atomic resolution ofNaCl on Au(100). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Figure 3.11 STM images of CuPc and PTCDA on Ag(111). a) NaCl island and bare Agsurface with deposited molecules, before 2 minute room temperature anneal.b) CuPc and PTCDA clusters formed on NaCl/Ag(111) after a 2 minute roomtemperature anneal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39xiFigure 3.12 STM images of CuPc and PTCDA on NaCl/Au(100). Both images indicateclearly the positioning of PTCDA with respect to the underlying Au(100) sub-strate: the PTCDA adsords normal to or parallel to the Au(100) reconstruction.Image b) also indicates the positioning of CuPc with respect to the underlyingAu(100) substrate. a) CuPc and PTCDA near the edge of a NaCl island. b)CuPc and PTCDA near the edge of a NaCl island. Submolecular resolution isvisible. Image taken with a PTCDA functionalized tip. . . . . . . . . . . . . . 39Figure 3.13 STM image of CuPc and PTCDA on NaCl/Au(100). NaCl lattice is visible,indicating the positioning of both CuPc and PTCDA with respect to the under-lying NaCl layer. Image taken with a PTCDA functionalized tip. . . . . . . . . 40Figure 3.14 Solidworks model of the Omicron. . . . . . . . . . . . . . . . . . . . . . . . . 42Figure 3.15 Various views of the optical set-up, used in this case for the spectrometer, asdesigned in Solidworks. The beam path is highlighted in red. . . . . . . . . . . 42Figure 3.16 Photo of the STM/AFM with optical components installed . . . . . . . . . . . 43Figure 3.17 Photos of the optics integrated with the Omicron STM/AFM. a. Birds-eye viewof the optical table and flange optics, b. Close-up view of the beam path goinginto the spectrometer. c. View of the supports for the optical table, which usedonly the holes already present in the table and can handle considerable weightand impact. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Figure 3.18 Image of the Ag(111) sample, looking in from the camera side (where illumina-tion is coming in from the optics flange). Left, original image. Center, imagewith sample outlined with a red oval. Also outlined in red is the tip and the tip’sreflection. Right, yellow indicates the image of the optics viewport as seenreflected in the sample (with no optical components mounted on the flange).Yellow demarcates the portion of the sample to which we have optical access.The result is that only the back half of the sample offers optical access. . . . . 45Figure 3.19 PMT photon counting circuit block diagram . . . . . . . . . . . . . . . . . . . 46Figure 3.20 Schematic for the timing signals cabling and the isolator box. . . . . . . . . . 48Figure 3.21 Aligning the PMT. Images show the Ag(111) sample as seen with the camera. . 50Figure 3.22 Solidworks model of the dark box. The frame is built with aluminum t-slot ex-trusion and the panels are made from corrugated plastic. The panels are caulkedinto the frame to prevent light leaks. Note the panels are actually black, but arepurple in the model to ease visualization. . . . . . . . . . . . . . . . . . . . . 51Figure 3.23 Schematic indicating the connections and cabling for the optical spectrometerset-up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Figure 3.24 Image of the tip-sample junction as seen after the lens before the spectrometerslit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53xiiFigure 4.1 STM constant current image (5x4nm2, I=3 pA, Vb=-2 V) of the ‘XO’ configura-tion of CuPc and PTCDA on NaCl(2ML)/Ag(111). CuPc images as a ‘flower’here. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Figure 4.2 STM constant current image (5x5nm2, I=10 pA, Vb=0.5 V) of the ‘XO’ con-figuration of CuPc and PTCDA on NaCl(2ML)/Au(100). CuPc images as a‘cross’ here, which is why it is called the ‘XO’ configuration. Note this wouldbe different from a +O configuration, where the CuPc would be rotated 45◦ withrespect to the PTCDA. Submolecular resolution is visible. Image taken with aPTCDA functionalized tip. . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Figure 4.3 STS spectra averaged over the entire molecule (either CuPc or PTCDA) in theXO configuration on NaCl(2mL)/Ag(111) taken with different tips on differentspecific molecules. The location and shapes of the peaks change with differenttips and locations on the substrate. . . . . . . . . . . . . . . . . . . . . . . . . 57Figure 4.4 STS spectra averaged over the entire CuPc molecule in the XO configuration onNaCl(2mL)/Ag(111) taken with different tips on different molecules. Spectrashown correspond to the ‘0 mW’ data point for each data set shown in section 4.2 58Figure 4.5 STS spectra averaged over the entire PTCDA molecule in the XO configura-tion on NaCl(2mL)/Ag(111) taken with different tips on different molecules.Spectra shown correspond to the ‘0 mW’ data point for each data set shown insection 4.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Figure 4.6 STS spectra averaged over the NaCl surrounding the XO configuration on NaCl(2mL)/Ag(111)taken with different tips on different molecules. Spectra shown correspond tothe ‘0 mW’ data point for each data set shown in section 4.2 . . . . . . . . . . 60Figure 4.7 Schematic indicating how the laser power was ramped up and down over timeto collect the z(t) data with the 488 nm laser. Note with the 687 nm laser thebasic scheme remains the same, but the maximum power is 30 mW. . . . . . . 61Figure 4.8 Data and fit for z(t) as measured with the initial 5 mW of radiation from the 488nm laser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Figure 4.9 STM and STS images showing the bounded area over which individual STSspectra were averaged for each plot. The image shown is from the 561 nm dataset, and shows the areas taken to obtain an average over the whole molecule.Data taken on NaCl(2mL)/Ag(111). . . . . . . . . . . . . . . . . . . . . . . . 64Figure 4.10 Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 685 nm laser. Spectra are averaged over the entiremolecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65xiiiFigure 4.11 Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 640 nm laser. Spectra are averaged over the entiremolecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Figure 4.12 Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 561 nm laser. Spectra are averaged over the entiremolecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Figure 4.13 Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 519 nm laser. Spectra are averaged over the entiremolecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68Figure 4.14 Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 488 nm laser. Spectra are averaged over the entiremolecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68Figure 4.15 Spatial and energetic distribution of the LDOS, imaged via slicing the STS gridat selected energies. The spatial distribution showed little variation in each dataset. Yellow indicates the presence of a tunnelling resonance. Slices shown arefrom the 561 nm 0 mW grid (taken on NaCl(2mL)/Ag(111)). . . . . . . . . . . 69Figure 4.16 Topography of the same XO showing both the lower (left) and upper (right)PTCDA configurations, which we call ‘A’ and ‘B’. Comparing the two imagesabove, the PTCDA jumps'0.362 nm downwards, which is close to the spacingbetween adjacent Cl− top sites in the NaCl (0.353 nm) as measured from Figure3.9b. Images taken on NaCl(2ML)/Ag(111). . . . . . . . . . . . . . . . . . . 72Figure 4.17 Constant height NC-AFM images using a PTCDA-functionalized tip of the XOconfiguration on NaCl(2ML)/Ag(111). The oxygen end of the PTCDA whichis adjacent to the CuPC is closer to one isoindoline subunit of the CuPc than theother. Image size 3.2x3.2nm2; oscillation amplitude A = 15 mV. Note the imagein a is the Laplace transform of the gaussian smoothed raw data. Image takenon NaCl(2ML)/Ag(111). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73Figure 4.18 Exciting a vibrational mode in PTCDA on NaCl(2ML)/Ag(111) at 2V, showingreversible and controllable jumping between two XO configurations. Jumpingoccurs during scan 2 (down) and scan 5 (up). . . . . . . . . . . . . . . . . . . 74Figure 4.19 STM-IETS spectra for PTDCDA in the XO configuration on NaCl(2mL)/Ag(111),with no illumination. Note the resonance at 2.12 V. . . . . . . . . . . . . . . . 74Figure 4.20 Topography for all of the STS grids taken during the laser excitation study,showing how the PTCDA jumped (and the CuPc rotated) multiple times duringmost grids. Note that there was no correlation with the change in the 2.2 V peakheight and the number of jumps observed in the images above. . . . . . . . . . 75Figure 4.21 dI/dV spectra for the XO configuration on NaCl(2ML)/Au(100) . . . . . . . . 77xivFigure 4.22 dI/dV spectra for the XO configuration on NaCl(2ML)/Ag(111) . . . . . . . . 77Figure 4.23 Comparison of the gap as measured via STS of PTCDA and CuPc in the XOconfiguration on NaCl(2ML)/Au(100) vs NaCl(2ML)/Ag(111). Note the gapindicated for PTCDA on Au is an estimate, since no tunnelling resonances wereobserved below Fermi for the XO configuration (which was only explored to-2.5V) but was observed at '-3V for isolated PTCDA. . . . . . . . . . . . . . 78Figure 4.24 Schematic indicating the energy level alignment for a molecule in the STMbarrier without an applied bias, assuming the molecule is more coupled to thesubstrate than the tip. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79Figure 4.25 Cartoon indicating how the energy levels as measured with STS shift or changeunder various conditions. A, energy levels for a neutral molecule vs a chargedmolecule. When the molecule is charged, the LUMO splits across Fermi. B,energy levels for a substrate with a larger work function vs those for a smallerwork function. Energy levels shift up when measured on a substrate with alarger work function. C, energy levels for a molecule in unperturbed surround-ings vs polarized surroundings. When the surroundings are polarized, the ion-ization potential is reduced (so the HOMO moves closer to Fermi) while theelectron affinity is increased (so the LUMO moves closer to Fermi). . . . . . . 80Figure 4.26 Schematic indicating the energy level alignment and possible optical excitationpathways for molecules adsorbed on NaCl/Ag(111) vs NaCl/Au(100). The rel-ative energies are not exact. On Ag(111) we could see an excitation from theHOMO of the CuPc to the LUMO of the CuPc (process 1). In this case, thereis no driving force for charge transfer to PTCDA since the energy levels are al-most aligned (and the LUMO of PTCDA is half occupied). We could also see anexcitation from the lower Hubbard state on PTCDA to the LUMO+1/LUMO+2(process 2). In this case, there would be a large energy drop into CuPc’s LUMO,but PTCDA’s LUMO is also at this energy so it is possible that the electronwould just relax to there instead of being transferred across molecules. Thereare other possibilities as well. On Au(100), we could see an excitation fromCuPc’s HOMO to LUMO, then transfer to PTCDA’s LUMO (process 3). Thisenergy level alignment appears to be quite favourable for charge transfer be-tween the two molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81Figure 4.27 STS slices of the XO configuration on NaCl/Au(100) indicating the spatial andenergetic distribution of the local density of states (LDOS). There is only onevisible state below Fermi in the experimental energy range, on the CuPc molecule. 82xvFigure 5.1 STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=300 nA, Vb=-10 V. Note there are some tip changes which, while visible inthe topography, are quite pronounced in the PMT image. . . . . . . . . . . . . 85Figure 5.2 STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=5 nA, Vb=-3 V. Note the tip is slightly double in this image. The quenchingof the luminescence at the end of the screw dislocation is interesting. . . . . . . 86Figure 5.3 STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=500 pA, Vb=-3 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86Figure 5.4 STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=250 pA, Vb=-3 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87Figure 5.5 STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=100 pA, Vb=+3 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87Figure 5.6 Point optical spectra on bare Ag(111) (I=200 pA, Vb=+3 V, 100µm slit, andCCD exposure time = 100 s). . . . . . . . . . . . . . . . . . . . . . . . . . . . 88Figure 5.7 Bottom left, Integrated optical spectroscopy grid. Counts integrated from 450 -730 nm. Top left, Example spectra collected at first pixel in grid. Right, Asso-ciated STM image for optical grid (30x30 nm2, I=50 pA, Vb=0.03 V). Opticalgrid taken at I=50 nA, Vb=3.25 V, with 50 ms CCD exposure time. The grid wasaborted early so a scan taken at the same resolution before the grid was startedis the one shown. The area delineated in red in the STM scan corresponds tothe section plotted as the integrated optical grid. . . . . . . . . . . . . . . . . . 88Figure 5.8 STML image (left) and STM image (right) of adjacent isolated CuPc and PTCDAon NaCl(2ML)/Ag(111) (9x6 nm2, I=100 pA, Vb=-3 V). The luminescence isquenched on both molecules, the bare NaCl/Ag substrate shows more emissionthan the molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90Figure 5.9 STML image (left) and STM image (right) of adjacent CuPc and PTCDA onNaCl(2ML)/Ag(111) (4.5x6 nm2, I=100 pA, Vb=-3 V). The luminescence isquenched, the bare NaCl/Ag substrate shows more emission than the molecules. 91Figure 5.10 Point optical spectra on CuPc and PTCDA in XO configuration (I=200 pA,Vb=-3 V, 250µm slit, and CCD exposure time = 200 s). Note the measurementover the CuPc molecule shows the emission seen on bare NaCl but attenuated(see Section 5.3 for a discussion of the NaCl emission). The measurementover PTCDA shows no emission, even though both bare Ag and NaCl showedemission prior to this measurement. This is roughly consistent with what wasobserved in the PMT images; namely the luminescence is strongly quenchedover the molecules compared to the bare NaCl or Ag. . . . . . . . . . . . . . . 92xviFigure 5.11 Point optical spectra on CuPc and PTCDA in XO configuration (I=200 pA,Vb=+3 V, 250µm slit, and CCD exposure time = 200 s). Note the luminescenceis totally quenched over both molecules. Both bare Ag and NaCl were showingemission immediately prior to this measurement. . . . . . . . . . . . . . . . . 92Figure 5.12 Point optical spectra of bare NaCl taken at 550 nm center wavelength showing abimodal peak (I=200 pA, Vb=+3 V, 100µm slit, and CCD exposure time = 200 s). 93Figure 5.13 Point optical spectra of bare NaCl taken at 650 nm center wavelength showingmultiple strong peaks (I=200 pA, Vb=-3 V, 250µm slit, and CCD exposure time= 200 s). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Figure 5.14 Point optical spectra of bare NaCl taken at 650 nm center wavelength showingmultiple strong peaks (I=200 pA, Vb=+3 V, 250µm slit, and CCD exposure time= 200 s). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Figure 5.15 Schematic indicating the basic STML mechanism as described by Buker andKirczenow. Adapted from [5]. . . . . . . . . . . . . . . . . . . . . . . . . . . 96Figure 5.16 STM constant-current image of a PTCDA island grown on Ag(111) with CuPcaround the edges and on top (40x40 nm2, I=5 nA, Vb=0.35 V). . . . . . . . . . 99Figure A.1 Evaporator without heat shield or shutter . . . . . . . . . . . . . . . . . . . . 112Figure A.2 Sophomoric diagram of major evaporator components . . . . . . . . . . . . . 113Figure A.3 Drawing for one of the Macor disks. Dimensions are in millimetres. Solidworksfiles are on our internal Wiki. . . . . . . . . . . . . . . . . . . . . . . . . . . . 116Figure A.4 Drawing of the reducer flange with machined holes dimensioned. . . . . . . . 118Figure A.5 Rendering of the base flange with standoffs supporting the macor disks andcrucible. On the left, the flange has been rendered transparent to help visualizethe locations of the machined holes and other features. On the right, the macordisks have been rendered transparent to help visualize the standoffs mating witheach other. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119Figure A.6 The copper heat shield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Figure A.7 Solidworks drawing of the heat shield. All units are in mm. . . . . . . . . . . . 122Figure A.8 The shutter assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123Figure A.9 Rendering of the knudsen cell assembly in stages. In the center the heat shieldand flange are made transparent, and on the right the heat shield is removedexposing the macor/crucible/standoff assembly. . . . . . . . . . . . . . . . . . 124Figure B.1 STM and STS images showing the bounded area over which individual STSspectra were averaged for each plot. The image shown is from the 561 nm dataset, and shows the areas taken to obtain an average over the oxygen end of thePTCDA molecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127xviiFigure B.2 STM and STS images showing the bounded area over which individual STSspectra were averaged for each plot. The image shown is from the 561 nm dataset, and shows the areas taken to obtain an average over the center of the CuPcmolecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128Figure B.3 STM and STS images showing the bounded area over which individual STSspectra were averaged for each plot. The image shown is from the 561 nm dataset, and shows the areas taken to obtain an average over the perimeter of theCuPc molecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129Figure B.4 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 685 nm laser. Spectra are averaged over the perimeter of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Figure B.5 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 685 nm laser. Spectra are averaged over the centre of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Figure B.6 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 640 nm laser. Spectra are averaged over the perimeter of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132Figure B.7 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 640 nm laser. Spectra are averaged over the centre of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132Figure B.8 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 561 nm laser. Spectra are averaged over the perimeter of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133Figure B.9 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 561 nm laser. Spectra are averaged over the centre of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133Figure B.10 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 519 nm laser. Spectra are averaged over the perimeter of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134Figure B.11 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 519 nm laser. Spectra are averaged over the centre of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134Figure B.12 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 488 nm laser. Spectra are averaged over the perimeter of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135xviiiFigure B.13 Comparison of dI/dV spectra for the XO configuration under illumination fromthe 488 nm laser. Spectra are averaged over the centre of the CuPc and theoxygen end of the PTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135Figure B.14 Comparison of dI/dV spectra for the XO configuration on NaCl(2ML)/Au(100).Spectra are averaged over the perimeter of the CuPc and the oxygen end of thePTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136Figure B.15 Comparison of dI/dV spectra for the XO configuration on NaCl(2ML)/Au(100).Spectra are averaged over the centre of the CuPc and the oxygen end of thePTCDA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137xixGlossaryAFM Atomic Force MicroscopeAMPEL Advanced Materials and Process Engineering LaboratoryCPD contact potential differenceCRTC central real time controllerCT Charge TransferCU control unitCuPc Copper (II) PthalocyanineDOS density of statesKPFM Kelvin Probe Force MicroscopyLCPD local contact potential differenceLDOS local density of statesLSP localized surface plasmonLT low temperatureMO molecular orbitalOMBE Organic Molecular Beam EpitaxyPMT photomultiplier tubePTCDA 3,4,9,10-Perylene Tetracarboxylic DianhydrideSTM Scanning Tunnelling MicroscopexxSTM-IETS scanning tunnelling microscope inelastic tunnelling spectroscopySTML Scanning Tunnelling LuminescenceSTS Scanning Tunnelling SpectroscopyUHV ultra-high vacuumxxiAcknowledgmentsThank you to my supervisor Dr. Sarah Burke, for teaching me so much and giving me this opportu-nity.Thank you to Dr. Doug Bonn for being an awesome un-official co-supervisor.Thank you to our post-docs, Agustin Schiffrin and BingKai Yuan, for everything I have learnedfrom you.Thank you to my lab partner, Katherine Cochrane, for being a great person to do science with andfor always having my back.Thanks to everyone in the LAIR, for your friendship, ideas, and laughs.Thank you to Arthur Mills, you were a fantastic sounding board for all my post-Masters worries,and I don’t think I could have succeeded with the optical spectrometer experiment if it weren’t foryour help. And thank you to Dr. David Jones, for setting me up so well for the next phase of myeducation!I am extremely grateful for all the support I had from my family, Rob’s family, and Rob.Thank you to all of my friends who always stuck around for BBQ’s, roadtrips, and epic breakfastseven though I would disappear for months at a time with a strange work schedule.This thesis is dedicated to Stephanie Groethe and Kory Campbell. You are missed. Your brightsouls bring light to my heart. “Like Honey, hard to get off!”xxiiChapter 1Introduction1.1 MotivationAs the planet heats up [6], so does the search for better, cheaper and more robust solar powertechnology. Building solar panels from organic semiconductor materials would solve a number ofproblems inherent to inorganic (often silicon-based) semiconductor devices: organic materials arelow-cost, flexible, compatible with high-throughput processing techniques, and have significantlylower embedded energy than their inorganic counterparts. However, organic materials will not beadopted until their cost/kW (which involves conversion efficiency, lifetime, and manufacturing cost)is less than that of inorganic devices. For now, organic devices perform quite poorly when comparedto crystalline semiconductor devices: at the time of this writing, a multi-junction InGaP/GaAs/In-GaAs cell has an independently confirmed efficiency of 37.9%, while the best organic thin filmdevices have confirmed efficiencies of about 11% [7]. A critical difference between inorganic semi-conductors and organic semiconductors lies in the differing photoconversion mechanisms: a photonabsorbed in an inorganic semiconductor generally produces free carriers (electron and hole whichmove freely through the material), while a photon absorbed in an organic material generally resultsin the formation of an exciton: an electron-hole pair which remain bound together by the Coulombattraction.For organic semiconductor-based devices to have any energy production value, the excitons must bedissociated into free carriers. Dissociation can occur in the presence of high electric fields, at defectsites, or at the interface between two materials which have mismatched energy levels [8]. C. W.Tang famously demonstrated a two-layer organic photvoltaic cell in 1986 with the basic structureof positive electrode - donor - acceptor - negative electrode [9]. In Tang’s cell, the donor - acceptororganic interface consisted of a 500 A˚ thick perylene tetracarboxylic derivative layer evaporated1Figure 1.1: Cartoon depicting the formation of an excitononto a 300 A˚ thick copper pthalocyanine (CuPc) layer. The interface region in this cell was “crucial”in determining its photovoltaic properties. This is because the interface is responsible for generatingfree charges: dissociation at the interface consists of charge transfer between donor and acceptormaterials [9], [10]. In general, interfacial processes dominate charge generation in excitonic solarcells, and it is these interfacial processes which we seek to understand in this thesis 1.The absorption of a photon in organic semiconductors generally results in an exciton rather than freecharges for two reasons. First, the electron’s wave function is (relatively) spatially localized over its‘parent’ molecule; thus the excited electron ends up localized in the potential well of its conjugatehole. Second, the attractive potential well caused by the hole tends to be less well-localized thanthe electron’s excited-state wavefunction due to low dielectric constants of the material (so chargesare poorly screened) [11], [10]. The result is generally a localized Frenkel exciton, mobile butelectrically neutral, to first order unaffected by electric fields, with an energy corresponding to theoptical bandgap Eopt of the parent molecule.If the exciton can diffuse to then dissociate at the boundary, why are organic photovoltaic cells somuch less efficient than inorganic? There are a number of reasons. One is that an exciton producedin the ‘bulk’ can only diffuse so far. Another is that excitons which reach the boundary do notalways result in free carriers: there are a number of criteria which must be met first. The energy1Note that even though dissociation can occur at defect sites, we do not focus on this process because dissociation ata defect generally results in one trapped carrier and one free carrier and is less viable for energy production [11].2Figure 1.2: Energy level schematic for an exciton in the charge transfer state at an acceptor-donor interface. Eopt is the energy of the exciton, Egap is the bandgap energy of a free electron-hole pair in the bulk, and Eb is the difference between the two: the exciton binding energy.of an exciton, Eopt , is less than the bandgap energy of a free electron-hole pair (Egap) in the bulk,and we call the difference between the two the exciton binding energy: Eb. It follows that at theinterface the band offset must be greater than the exciton binding energy (see figure 1.2). Note thebinding energy is usually much greater than kBT (where kB is the Boltzmann constant and T is thetemperature) for organic materials, so the exciton cannot thermally dissociate.Once the exciton reaches the donor-acceptor heterojunction it is called a Charge Transfer (CT) state.The CT state consists of a Coulombically bound electron and hole residing on opposite sides of thejunction, in either neighbouring or adjacent molecules [12]. The energy of the CT state is generallyaround 0.5 eV, which exceeds the thermal energy (at ambient conditions) of 0.026 eV [13] [14],thus begging the question: how do the electron and hole manage to get away from each other andbecome truly free charges (and more importantly, a photocurrent)?The method by which a CT state overcomes its binding energy to realize true charge separationremains elusive and the subject of some debate. Local molecular ordering/crystallinity (which in-creases both carrier mobility and wavefunction delocalization) has been shown to have an effect oncharge separation at hetero-interfaces [12] [13] [15] with charge delocalization in particular playinga key role [14] [16] as it lowers the Coulombic barrier. The intricate details of the spatial properties3Figure 1.3: Spatial and energetic evolution of the local density of states for a single CuPcmolecule (left) which is adjacent to a single PTCDA molecule (right) as imaged via scanningtunnelling spectroscopy. Yellow indicates the presence of states.of the delocalized excited state wavefunction have been shown to play an important role [17], whileothers have argued that charge diffusion within an energetically disordered medium increases theentropy of the system, and thus underpins charge separation [18]. Local polarization can inducesignificant energy level shifts at the interface, either inhibiting or encouraging charge transfer statesto form in the first place [2]. Vibrational coherence has been shown to play a role [14] [19] [20],and some propose that strong and cohrerent vibronic coupling between the nuclear and electronicdegrees of freedom is what promotes the delocalization necessary to reduce the Coulomb barrier tobelow the thermal energy [20].The importance of the local characteristics of the heterojunction drives the desire to take a local viewof the system. Scanning probe techniques, with their ability to provide a sub-nanometer view of theelectronic and optical properties of a system, are not only powerful but in this case are necessaryfor expanding our understanding of organic photovoltaic systems.1.2 Scanning probe techniques and the power of real-space,atomically-resolved measurementsIn 1981 Gerd Binnig and Heinrich Rohrer introduced a revolutionary new imaging technology to theworld: the Scanning Tunnelling Microscope (STM) [21]. Five years later, Binnig, Quate and Gerberextended the concept to create the Atomic Force Microscope (AFM) [22]. Both the STM and theAFM have become indispensable tools for the materials scientist, the condensed matter physicist,the physical chemist, the biologist, and many more. The STM and the AFM both provide real spaceinformation about the structure and electronic states of a surface (or an adsorbate on a surface), fromthe confinement of surface state electrons [23] to the resolution of the chemical structure of a singlemolecule [24].The STM can be used to map out, with sub-nanometer resolution, an approximation to the localdensity of states (LDOS) via Scanning Tunnelling Spectroscopy (STS). These real-space images ofthe energy landscape of a surface and its adsorbates are critical for optimizing the heterointerfaces inorganic photocells: one can see with exquisite precision how subtle changes in molecular configura-4Figure 1.4: STM constant current image (11nm2, I=5pA, Vb=-0.5V, T = 4.3K) of CuPc andPTCDA clusters on NaCl(2ML)/Ag(111). The CuPc images here as a ’cross’. The edge of theNaCl island is clearly visible with right-angled lines.tions and conformations not only affect the spatial characteristics of the molecular orbitals, but alsohow changes affect the relative energy levels between individual molecules. When imaging withAFM, one can determine exactly how two molecules on a surface are positioned both with respectto the substrate and to each other with atomic precision. Kelvin Probe Force Microscopy (KPFM),another type of scanning probe microscopy, can be used to map out parameters related to the chargedistribution (the local contact potential difference) in the system under study, a critical piece ofinformation when trying to determine how conformation and configuration affects charge transfer[25].In addition to the direct measurements offered by scanning probe devices, one can simultaneouslyobtain complementary information by introducing optical measurements. The tip and concomitanttunnelling current act as a very local source of hot electrons; which, when tunneling inelasticallythrough the sample into the substrate, emit photons which can be detected, counted, and spectrallyresolved. This Scanning Tunnelling Luminescence (STML), as it is often called, can be used toelucidate the vibrational energy levels of a single molecule [26] or to gain insight into coherentdipole-dipole coupling in real space [27]. This technique is particularly important since a numberof studies have pointed to the pivotal role vibronic quantum coherence may play in enabling orpreventing charge separation in both biological and synthesized inorganic light harvesting systems[28], [16], [19], [14], [29]. In addition, STML can be used to visualize excitonic coupling spanningseveral molecules [27], which is crucial for charge transfer [30]. Finally, one can probe the sample5Figure 1.5: A model system: organic photovoltaics reduced to a molecular acceptor-donorpair. Copper (II) Pthalocyanine (CuPc) and 3,4,9,10-Perylene Tetracarboxylic Dianhydride(PTCDA) on NaCl/Ag(111) and NaCl/Au(100).in an excited state by coupling illumination into the tip-sample junction and imaging the chargedistribution via KPFM [25].1.3 Molecular acceptor-donor pair: towards an understanding ofcharge transfer in an organic electronic deviceIn this thesis, we look exclusively at a model acceptor-donor molecular system: small clustersof Copper (II) Pthalocyanine (CuPc) and 3,4,9,10-Perylene Tetracarboxylic Dianhydride (PTCDA)adsorbed on NaCl on metal (Ag(111) and Au(100)) substrates. Using these prototypical and often-studied molecules (and harkening back to Tang’s seminal paper) we have explored in depth oneparticular configuration, composed of a single acceptor and donor molecule adjacent to each other2,in which we have developed a molecular scale picture of the electronic states and performed proof-of-concept experiments coupling light into and out of the tunnelling junction. From here we canbegin to piece together how the spatial configuration of individual molecules, with respect to boththe substrate and other molecules, affects exciton creation and dissociation.2Using small clusters like this allows us to eliminate the complications introduced by the exciton diffusion length[31], and with a configuration this small we can to first order ignore the exciton motion itself [32] as well as have somehope that the system can be treated in an ab-initio theoretical framework.6Chapter 2Experimental techniquesIn this chapter, we review the basic theories behind the experimental techniques used in this thesis.2.1 Scanning tunnelling microscopyFigure 2.1: Schematic for basic STM operationThe STM employs quantum tunnelling and precision mechanical control of a conducting tip tomap out, in real space, the electronic properties of a material. The core of the general apparatusconsists of a conducting tip held very close (5-15 A˚) to a conducting sample. At this distance,the electron wavefunctions of the tip and sample can overlap significantly, such that electrons can7Figure 2.2: STM image of two different configurations of CuPc and PTCDA onNaCl(2ML)/Ag(111) (5x5nm2, I=5pA, Vb=-0.5V, T =4.3K ). The CuPc is imaged as a ‘cross’here. Note the PTCDA images differently depending on its location with respect to the CuPc.tunnel between the two. A bias is applied between the tip and the sample so that a single directionof electron tunnelling is favoured, giving a measurable net current. The current is amplified andtransduced into a voltage. The tip height and position are controlled by a piezodrive, which canprecisely position the tip in space with sub-nanometer resolution. If the tip height is held constant(known as constant-height operation), then the current is plotted as a function of x and y. Generally,it is preferable to use a feedback loop to keep the tunnelling current constant by adjusting the tipheight (known as constant current operation), and in this case the tip height as a function of (x,y)is plotted. The tip height is measured against a calibration where a certain voltage corresponds toa given height. The result is a sub-nanometer resolved contour plot of the equal current tunnellingsurface (see Figure 2.2) [33].In addition to constant-current (or constant-height) imaging, one can obtain an approximation to theLDOS via STS, which is explained in more detail in section 2.1.2. In an STS measurement, the tipis held at a constant location in (x,y,z) and the voltage is swept. The resulting current is recorded asa function of voltage. Doing this at each (x,y) location (pixel) in an image results in a 3 dimensionalimage in (x,y, E) which can then be processed numerically to obtain an approximation to the localdensity of states.The STM apparatus used in this thesis is held inside an ultra-high vacuum (UHV) chamber toprevent the introduction of impurities which could confound the measurement, and is kept at lowtemperatures (4.3 K) both to cool the molecules such that they will adsorb securely and won’tmove on the surface, and to improve energy resolution when taking STS measurements. In addi-tion, because mechanical noise is very undesirable (see sec 2.1.1) the entire apparatus is kept in acarefully-designed ultra-low vibration facility.Unfortunately, finding an accurate analytical relation between the measured quantity (the tunnelling8Figure 2.3: STM image of CuPc and PTCDA clusters on NaCl(2ML)/Ag(111) taken with aPTCDA functionalized tip (15x15nm2, I=5pA, Vb=0.5V ).current) and the relevant physics of the sample (say, the LDOS) is not entirely straightforward.It follows that the interpretation of the images one obtains with the STM requires some carefulconsideration of the apparatus, the physics involved, and the system under study.2.1.1 The tunnelling currentThe tunnelling current is often given to be approximately proportional to the integrated LDOS.This loose equivalence, first described by Tersoff and Hamann, is derived from Bardeen’s transferHamiltonian approach to tunnelling with additional approximations added in [33], [34], [35], [36].Analytically, this model only holds for the simplest (free-electron-type) metal surfaces, and onceone is imaging with atomic resolution or even just at ‘large’ biases (roughly ≥ |2|V) this model nolonger holds (because the approximations made to obtain it are violated). In practice, this modelis actually remarkably useful with more complex systems (like semiconducting surfaces or singlemolecules), despite its limitations.For anything but the simplest metal surfaces imaged at low bias, a more careful exploration of thetheory is warranted. One should be forewarned that there is still considerable debate about a numberof issues with STM (and AFM) imaging, and that there remains much work to be done before wehave a truly complete theoretical framework for the apparatus. Nevertheless, considerable work hasbeen done to advance our understanding of the physics at play, and it pays handsomely to considerwhat we do know before continuing on to discuss our results.9The systems explored in this thesis consist of a double-barrier tunnelling junction with a ‘semicon-ducting’ sample (the molecules) sandwiched between the barriers. The theoretical work we willreview in this section was been done with the presumption of a single barrier tunnelling junctionwith a free-electron metal sample and tip. More complex models are beyond the scope of this thesis.The most commonly used tunnelling theory is Bardeen’s transfer-Hamiltonian approach which wasreformulated for the STM by Tersoff and Hamann [34], [35], [36]. It has been pointed out that it isnot possible to deduce Bardeen’s theory “firmly from first principles”, and that Bardeen’s model isnot sufficient to treat many body effects properly [37].The most rigorous and complete description of tunnelling theory comes from Feuchtwang and Car-oli et. al.1 [37], [38], [39], [40], [41], [42]. In addition to being the most rigorous, these theoriescan handle electron-electron and electron-phonon interactions if desired. Feuchtwang and Caroli’sformalism is used in principle by theorists who model STM measurements, however there is to dateno known way to utilize their formalism to invert experimental data to get a desired quantity like theLDOS. Because of this, the transfer-Hamiltonian approach of Bardeen/Tersoff and Hamann remainsthe ‘industry standard’ (for experimentalists).Because our analysis is based on the transfer-Hamiltonian approach, including our newly-developednormalization schemes (see sec 2.1.2), it is this model which we briefly review in this section. Notethat this approach disregards electron-electron interactions, so it cannot reasonably be used forsingle-electron charging effects which we had hoped to encounter in this thesis. Schulz et al. haverecently explored many-body effects inside single molecules via scanning tunnelling spectroscopy,and their theoretical approach points to a reasonable path forward for experimentalists examiningsuch systems [43].I would urge the interested experimentalist to explore Feuchtwang’s and Caroli et. al.’s models,however, as I think that a careful consideration of their expressions for the tunnelling current willlead to a better intuitive understanding of any STM results the experimentalist may encounter, andwill give them some ideas for a path forward when they encounter more complex systems.The transfer-Hamiltionian approach to tunnellingThis section is a brief summary of Bardeen’s tunnelling theory, with some help from Gottlieb andWesoloski [44], C-T Chen’s dissertation [45], and Feuchtwang [38]. Feuchtwang in particular pro-vides a very nice overview of Bardeen’s theory in ref. [38] if the interested reader is finding theoriginal somewhat opaque. Bardeen used first-order time-dependent perturbation theory, a ‘trans-1Apparently, both theories are adaptations of a transport theory due to Keldysh, written in Russian, titled ‘Ionizationin the field of a strong electromagnetic wave’ and published in 1964.10fer Hamiltonian’, a ‘separable’ system, and Fermi’s golden rule to derive an expression for thetunnelling current.To begin, we take the full Hamiltonian for the entire system (tip + sample) to be separable intothree parts, namely: H = Htip +Hsam +Ht where Ht is a weak perturbation which couples the twosubsystems (the tip and sample, which are nearly independent).The Hamiltonians for the tip and the sample are of the usual formHtip =∑tεtc†t ct (2.1)Hsam =∑sεsc†s cs (2.2)Where c†i , ci are the creation and annihilation operators for the one particle states which characterizethe (unperturbed) tip and sample Hamiltonians (where {ci,c†i } = 0), and εi is the single particleeigen-energy.Let ψ be a many-body state of the whole system. Let ψt and ψs differ in the transfer of the electronfrom the tip to the sample, and let ψt be a solution to the time-independent Schro¨dinger equationwith Hamiltonian 2.1 but not Hamiltonian 2.2 and vice-versa. This follows from the assumptionthat the wavefunction for the quasi-particle n drops to zero on the opposite side of the barrier.The transfer Hamiltonian which couples the two states has the form:H =∑s,tTs,t{c†s ct + c†t cs} (2.3)where Ts,t is the tunnelling matrix element. This is the probability amplitude to transfer an electronfrom, say, the tip to the sample. Fermi’s golden rule states that the probability to transition per unittime from an initial state |i > to a final state | f > is given by P = 2pih¯ |< f |H ′|i > |2ρ( f ), where ρ( f )is the ‘density of final states’. Assuming that the transition rate for an electron to move from the tipto the sample is given by this rule, and taking both Ht and Hsam as perturbations to the ‘tip state’ ψt11(since ψt is not a good solution to Hsam) we findP =2pih¯|< ψs|Hsam +Ht |ψt > |2ρs(E) (2.4a)=2pih¯|< ψs|H−Htip|ψt > |2ρs(E) (2.4b)=2pih¯|< ψs|H−Wt |ψt > |2ρ(E) (2.4c)where Wt is the energy of the many-body wavefunction ψt (obtained after Htip operates on |ψt >).We recognize that t ·P (where t is time) would be our T from eq 2.3. Also, P as written above is therate at which electrons in the tip would tunnel into the sample if all the sample states were vacantand all the tip states were full (and the tip DOS is flat) and all of the vacant states were energeticallyaligned with the full states. To correct for the reality of the situation, we invoke Pauli’s exclusionprinciple and multiply by the correct fractions of occupied tip states and unoccupied sample states,which are described by the Fermi-Dirac distribution Fµ,T . We also account for the fact that a bias isapplied when expressing the Fermi-Dirac functions, since the Fermi level of the tip is higher thanthat of the sample (for a positively biased sample). We reference E to the Fermi level of the sample.Finally, we multiply by the electron charge e and a factor of two (to account for spin) and sum overall energies to get the current:I =4pieh¯ ∑n{Fµt,T (En−eV )(1−Fµs,T (En))−(1−Fµt,T (En−eV ))Fµs,T (En)}|<ψs|H−Wt |ψt > |2ρs(E)(2.5)Note that Fµ,T is the Fermi-Dirac distribution Fµt,T (E) = 11+e(E−µ)/kBT , µt and µs are the chemicalpotentials of the tip and sample, respectively, T is the temperature, and kB is Boltzmann’s constant.Multiplying out the Fermi-Dirac distributions, we note that the two ‘cross terms’ cancel:I =4pieh¯ ∑n{Fµt,T (En− eV )−Fµs,T (En)}|< ψs|H−Wt |ψt > |2ρs(E) (2.6)The sum can be approximated by an integral where instead of P = | < ψs|H−Wt |ψt > |2 we useT(E). T(E) is the average value of P over all sample states whose energy is near to E.I =4pieh¯∫ ∞−∞T (E) f1,2(E,V )ρs(E)dE (2.7)12Here, f1,2 = Fµt,T (En− eV )−Fµs,T (En).There are three things to note here. One, the above expression is only valid for single particlewavefunctions. For the more general interacting system with many-body effects, we should writethe tunnelling current in terms of the spectral functions of the two electrodes [46]. Details of thisformalism are beyond the scope of this thesis. Two, the actual tunnelling current measured willhave its features broadened by roughly 3.5kBT (in our case, ≈1.3 mV). Three, we have assumeda flat (constant) density of states (DOS) for the tip, which is not always true but is a reasonableapproximation2.One should also note that all of this assumes that the tip-sample distance is constant during themeasurement. If the tip-sample distance changes, the current magnitude changes exponentially,resulting in a drastically altered measurment of the signal (the tunnelling current). The mechanicalstability of the tip-sample junction is thus paramount, and considerable effort is put into minimizingmechanical vibrations of the tip-sample junction. This will be discussed further in Chapter 3.There are some important limitations to this theory. Some of these limits were pointed out in section2.1.1 and there are others which we will not discuss here. However, the expression derived fromthe transfer Hamiltonian approach is still valid within a certain regime and is very useful, bothconceptually and as a tool for data manipulation.Feuchtwang studied the issues with Bardeen’s approach in detail and investigated the validity of thetransfer Hamiltonian formalism [38]. Happily, through a more rigorous analysis, he found that an‘obvious generalization’ (writing the current in terms of the spectral functions of the tip and sample)of Bardeen’s expression for the tunnelling current agreed with his, and found that one can interpret“Bardeen’s matrix element of the transfer Hamiltonian as a pseudopotential representing the bound-ary conditions at the interface”. Thus, the simple expression we have for the tunnelling current (eq.2.7) is relevant to our studies3 and provides an important jumping-off point for analyzing our STSresults.2.1.2 Scanning tunnelling spectroscopyWhile we have a reasonable expression for the tunnelling current, it’s not terribly useful to thephysicist. What we really want is either a real space map of the LDOS at some energy E or theLDOS as a function of E at some location. Though we can’t measure the LDOS directly, we canobtain an approximation to it via scanning tunnelling spectroscopy.2The user can alter the tip DOS through pulsing or ‘nano spot-welding’ and can check that it is reasonably flat bytaking spectra on a surface with a known DOS3Except, of course, when electron-electron interactions are a concern; then the full many-body spectral function mustbe considered.13Figure 2.4: Schematic of the energy levels for a metal tip (left) and a semiconducting sample(right). The filled states are in darker colours, while the unfilled states are lighter. The fermilevels are offset by eV. An electron at a given energy in the tip can tunnel into an empty bandin the sample if it is available, but it cannot tunnel when there are no states to tunnel into.An STS measurement proceeds as follows: the tip is moved to a specific location in (x,y) and thefeedback loop is turned off (the z height is now constant). The voltage is varied through a specifiedrange at a specified rate, and the current is recorded as a function of voltage at a specified numberof points. This can be done at a single point (point spectroscopy) or it can be done at every pixel inan image to create a 3D ‘grid’ of I(V) in (x,y).As the bias is swept the tip electrons ‘see’ more states that they can tunnel into. For a sample withfeatures in the density of states (like a semiconductor or a molecule), this means a new tunnellingchannel will open up each time the bias becomes large enough for the filled states in the tip toaccess an empty band in the sample. When the bias is negative, it is the electrons in the samplewhich tunnel into the tip, with a new channel opening up each time the bias becomes large enoughfor a new filled band to see the empty states in the tip. In the I(V) curve, one observes a ‘kink’ eachtime a new tunnelling channel opens up. Taking the derivative makes it easier to visualize the localelectronic structure of the sample.In practice it isn’t quite so simple as taking the derivative and equating that to the local band structure(the LDOS), as detailed above . However, STS point spectra or 3D grids do allow us to extract anapproximation to the LDOS. The procedure for this extraction is detailed below.14Obtaining approximations to the local density of states from scanning tunnellingspectroscopyIn this section we mostly follow Koslowski et al. [47]. We then test their method on our data andfind that their correction term is not needed, but their general derivation remains useful.We begin with eq. 2.7, and add one small modification:I =4pieh¯∫ ∞−∞T (E,V ) f1,2(E,V )ρs(E)dE (2.8)In this case, T(E,V) has been modified to include a voltage dependence. As before, we are takingthe energy with respect to the Fermi level of the sample. In some cases, the DOS in eq 2.8 aboveshould be weighted to reflect potential selectivity of tunnelling in k space.Applying the one-dimensional WKB approximation plus a trapezoidal approximation, we have abias-dependent transmission coefficientT (E,V ) = e−z√Φ+V/2−E (2.9)where Φ is the effective barrier height, V is the applied voltage, and E is the energy. If we take thederivative of the tunnelling current with respect to V we obtain∂ I∂V≈ ρs(V )T (E =V )−∫ ∞−∞ρs(E)T (E,V ) f1,2(E,V )z4√Φ+V/2−E dE (2.10)Where the equivalence is approximate rather than exact since we approximate ∂V f1,2(E,V )≈ δ (E−V ). It is common to neglect the second term here, and assume that T (E,V is featureless, giving∂ I∂V ∝ ρs(V ). However, Koslowski et al. claim that the second term is comparable to the first4 andshould not be neglected.We then apply the generalized mean value theorem for integrals (setting the mean value to z4√Φ) andfind∂ I∂V≈ ρs(V )T (E =V )− z4√ΦI(V ) (2.11)We solve for the LDOS to obtain4At least for the tunnelling conditions explored in this thesis.15ρs(V )≈ 1T (E =V )( ∂ I∂V+z4√ΦI(V ))(2.12)If one has both an I(z) grid and an I(V) grid, one can extract all the parameters needed to solvethis equation (using z as given by the topography). In our case, we only have a few I(z) grids. Toproceed, we use an averaged Φ for each molecule from the I(z) grids we do have and implementeq 2.11 numerically (with the averaged values of Φ and T ) to obtain our LDOS. For STS gridsobtained on isolated PTCDA and isolated CuPc molecules we implemented the Koslowski schemefor obtaining ρ and compared it to the results obtained by taking simply ∂ I∂V . We found that therewas no effect on the location of the peaks. The only effect seen with the Koslowski implementationwas one of vertical scaling. Figures 2.5, 2.6, 2.7, and 2.8 show the results. Given the nature of theseresults (in terms of introducing the Koslowski correction term vs not) for the data explored in thisthesis5, the analysis of STS data was done by taking ρ ∝ ∂ I∂V . Note that in Equation 2.12 there is afactor of 1T (E=V ) out front. Even if we neglect the second term, the∂ I∂V term should take this intoaccount. Commonly, 1T (E=V )∂ I∂V is approximated by∂ I∂V /IV . Because our data shows both negativedifferential resistance and noise near 0, dividing by IV introduces infinities where I crosses 0. Thisin turn can be solved by instead dividing by I+cV , where c is some constant, but in this thesis we willuse simply use ρ ∝ ∂ I∂V .5In addition, because the majority of the STS grids collected don’t have an accompanying I(z) grid, our implementa-tion of this scheme would have been less than ideal in any case.16Figure 2.5: Comparing the Koslowski normalization scheme to taking only ∂ I∂V for an isolatedCuPc molecule stabilized at the corner of a trilayer step. Plots shown are the average over theindividual spectra taken inside the area delineated in red as shown in the topography image.The peaks for the Koslowski method are in the exact same location as those obtained by takingonly ∂ I∂V .17Figure 2.6: Comparing the Koslowski normalization scheme to taking only ∂ I∂V for an isolatedCuPc molecule stabilized at the corner of a trilayer step. Plots shown are the average over theindividual spectra taken inside the area delineated in red as shown in the topography image.The peaks for the Koslowski method are in the exact same location as those obtained by takingonly ∂ I∂V .18Figure 2.7: Comparing the Koslowski normalization scheme to taking only ∂ I∂V for an isolatedPTCDA molecule. Plots shown are the average over the individual spectra taken inside the areadelineated in red as shown in the topography image. The peaks for the Koslowski method arein the exact same location as those obtained by taking only ∂ I∂V .19Figure 2.8: Comparing the Koslowski normalization scheme to taking only ∂ I∂V for an isolatedPTCDA molecule. Plots shown are the average over the individual spectra taken inside the areadelineated in red as shown in the topography image. The peaks for the Koslowski method arein the exact same location as those obtained by taking only ∂ I∂V .202.2 Atomic force microscopyFigure 2.9: Schematic of the AFM. The electronics are left out for simplicity. A piezo drivecontrols the position of the probe tip, as before. A tip is placed on an oscillating cantilever, andtip-sample interaction forces change the resonant frequency of the cantilever. The frequencyshift is taken as the imaging signal.Early in the development of the STM it was realized that relatively strong forces act between theprobe tip and the sample [48]. This observation led Binning, Quate, and Gerber to develop a micro-scope based on the detection of these forces: the AFM. The modern AFM detects forces as small as10−18 N between the probe tip and the sample via frequency shifts in an oscillating cantilever [22],[49]. The cantilever has a small, STM-like tip attached to it which acts as the probe6. The resultingreal-space map of the force (or some function of the force) between the tip and sample reveals asurprising amount of physics, such as the chemical structure of a single molecule, the bond orderof individual bonds, or the charge state of a single atom [24], [50], [51]. In this thesis, our AFMuses a self-detecting sensor, the qPlus quartz tuning fork, and operates in non-contact frequencymodulation mode.2.2.1 Frequency modulation atomic force microscopyIn non-contact frequency modulation mode, pioneered by Albrecht et al, a constant-amplitude os-cillator’s resonant frequency will shift as the tip is exposed to different forces while it scans acrossthe surface [49].6In our system, we use an STM-like tip. In other systems, one can have a microfabricated cantilever with an integratedtip at the end.21In this mode, a cantilever is excited by a mechanical actuator to oscillate on resonance ( f0). Itis kept oscillating on resonance at a specific amplitude through controlled positive feedback. Thesensor detects its own deflection (through piezoelectricity7), and this signal is phase-shifted andamplitude-controlled then sent back to the actuator [52]. Any change in the force gradient ∂F/∂ zcauses a change in the oscillator frequency by ∆ f . A phase-locked loop determines the oscillationfrequency f = f0 +∆ f , and the frequency shift ∆ f is the imaging signal.Let us consider the relation between the imaging signal and the force acting on the AFM tip. If weconsider the cantilever to be a 1D harmonic oscillator, then the free oscillation frequency f0 is givenasf0 =12pi√km∗ (2.13)where m∗ is the effective mass of the cantilever and k is the spring constant. A force acting on thetip changes the effective spring constant by some value δk, so we find a new resonant frequencyf =12pi√k+δkm∗ (2.14)if δk = ∂ 2V/∂ z2 = −∂F/∂ z is constant over the whole range of the oscillating cantilever. If it isalso true that δk is small compared to k (the small-amplitude approximation8), we can expand thesquare root in a Taylor series and obtain∆ f = f0δk2k(2.15)This looks nice, but what if the force is not constant over the oscillation amplitude range? Thereare a few options for calculating ∆ f in this case. The most common path forward is to use firstorder perturbation theory and the Hamilton-Jacobi approach [53]. This leads to a rather complexexpression, which is simplified via integration by parts. The result is rather intuitive [48]:∆ f = f0< δk(z)>2k(2.16)where7The deflection of the quartz cantilever produces charge at the electrodes on the surfaces of the two prongs. This canbe detected electrically to give the deflection signal.8The qPlus sensors we use are generally safely in the small amplitude regime.22< δk(z)>=1A2pi/2∫ A−Aδk(z−q′)√A2−q′2dq′ (2.17)where A is the amplitude and q is the deflection of the cantilever. Note that eq 2.16 is almost thesame as eq 2.15, except that δk is replaced by a weighted average < δk >.2.2.2 The forces acting on the tipAs always, the (negative) gradient of a potential gives a conservative force. Since there is a potentialV between the tip and the sample in an AFM, we can take the z component of the gradient to find thez component of the force between the sample and the tip: Fz =−∂V/∂ z. This is then related to a tip-sample spring constant9 k = ∂F/∂ z. The force which the AFM measures is a sum of contributionsfrom a number of sources. In UHV, we can reasonably limit our consideration to 3 contributions:the Van der Waals force, the electrostatic force, and the chemical force10 [48], [54].The Van der Waals force is caused by fluctuations in the electric dipole moment of an atom inter-acting with an induced dipole moment in another atom. For a spherical tip of radius R at a distancez from the surface we can express this force as [55]:Fvdw =−Hr6z2 (2.18)Where H is the material-dependent Hamaker constant, r is the tip radius, and z is the distancebetween the tip and the surface. Note that the Van der Waals force goes like z−6 for two atomsseparated by a distance z, while the expression above is for two macroscopic bodies; in this case,the tip and sample with the tip modelled as a sphere11 [48], [54]. For a pyramidal and conical tipmodel, a 1/z force law is found [53]. The Van der Waals force is always attractive, and is often quitelarge.The electrostatic force is self-explanatory. When one has a conductive tip and conductive sample,the system can be regarded as a capacitor with a distance-dependent capacitance C(z). In this case,the electrostatic force is given byFes =12∂C∂ z(UDC− ∆φe −∆pole)2(2.19)9In the small amplitude limit.10One should also consider magnetic forces but these are not relevant to the systems and tips under study in this thesis.11The force is calculated with the Hamaker approach by making some assumptions about the geometry and integratingover the respective volumes. It is also assumed that the forces are additive and nonretarded.23Where UDC is the applied bias, ∆φ is the difference in the tip and sample work functions, and in apolarizable medium (which is relevant for our analysis, since PTCDA and CuPc are polarizable) wehave a term describing the charge induced dipole barrier ∆pol [56], [57], [58]∆pol =−∑ αq2i8piε0r4i j(2.20)where α is the polarizability of the sample, qi are the charges in the tip at distances ri j from the jthinduced dipole in the sample.Note that (classically, at least12) ∂C∂ z is always negative (and the other factor is squared), so theelectrostatic interaction is always attractive. The term ∂C∂ z depends on the tip geometry, so it cannotbe calculated exactly in every case. If we model the tip as a sphere of radius R on a truncated cone,then12∂C∂ z=−piε0 Rz (2.21)assuming that z << R, there is zero contact potential, and the tip is electrically grounded [59]. Notethat this expression diverges at 0 thus it cannot be a correct model at close tip-sample distances.Kurokawa and Sakai wrote an excellent review of both the theoretical and experimental work done(up to 1997) on the capacitance of the STM junction in ref [60], and presented their own resultsexploring the STM junction capacitance in the non-tunnelling regime in ref [61]13. In ref [61], theypoint out that the calculated capacitance of the ‘truncated cone + half sphere tip’ (eq. 2.21) reason-ably reproduces their observed C-z characteristics and its dependence on tip geometry, indicatingthat this model is a reasonable assumption to make and to use in our calculations14. In addition,Kurokawa and Sakai point out that even though the capacitance increases with decreased gap dis-tance, the sensitivity ∂C∂ z decreases with decreased gap distance, so the capacitance does not divergeas z→ 0.The so-called chemical force is generally considered to be a short-range force, and can have bothattractive and repulsive regimes. The Lennard-Jones potential is often used to describe the force[63] between a neutral pair of molecules (or atoms) at ‘close’ distances, where ‘close’ means the12The analytical form of C(z) in the tunneling regime is unknown, so it is possible that at very close distances ∂C∂ zbecomes positive.13Kurokawa and Sakai originally planned to extend their analysis into the tunnelling regime, but they were foiled bythe instability of their capacitance bridge in the tunnelling regime [62].14This is despite the fact that their results were obtained in the ’non-tunnelling regime’: they did go as close as 1nmwhich was actually a tunnelling distance - at this distance they measured a 50 pA current with a 30 mV applied bias. Ourtip is generally farther than this, since we usually scan at ∼2pA and larger bias. In addition, they observed no anomalousbehaviour at the onset of tunnelling, so any potential quantum effects on the capacitance remain elusive.24electron wavefunctions in the tip and sample overlap. This is almost by definition the regime ofSTM/AFM measurements. The Lennard-Jones potential is given asVLJ = Em[(zmz)12−2(zmz)6](2.22)Where Em is the potential minimum and zm is the equilibrium distance. Taking the negative gradientin the z direction gives us the forceFLJ = 12Emzm[(zmz)13−(zmz)7](2.23)Alternatively, one can use the Morse potential to describe the close range forces. Both the Lennard-Jones and the Morse potentials lack anisotropy, so a more complete description of chemical bondingforces is often given by the Stillinger-Webber potential [48]. Discussion of the specific potentialsused to explain atomic features in AFM images is beyond the scope of this thesis.2.2.3 Kelvin probe force microscopyKelvin probe force microscopy (KPFM), pioneered by Nonnenmacher et al [64], uses the AFMto measure the local contact potential difference (LCPD) between the sample and the tip. Thecontact potential difference (CPD) is most easily described with two metals, as in fig 2.10, whichis adapted from [3]. When two different metals are electrically connected15 but there remains avacuum barrier between them, the Fermi levels align and electrons flow to the material with thehigher work function. The result is a voltage drop V ′ = ∆Φ/e across the barrier, a positive charge onthe material with the smaller work function, a negative charge on the material with the larger workfunction, and an electrostatic force between the sample and the tip. The CPD can be compensatedby applying a DC voltage across the barrier which nullifies the tip-sample electrical force.The DC voltage required to nullify the force gives the CPD between the tip and the sample. Becausethe tip is so small at its apex, and because the piezodrive offers precise control over the tip position,with KPFM we call it the local contact potential difference (LCPD). The LCPD can be imagedquantitatively with ∼ 50 nanomter resolution16, offering detailed information about the electronicstate of a surface [57]. In practice, instead of applying a DC voltage to nullify the tip-sample force,we determine V ′ directly from ∆ f (V ) spectroscopy. Re-examining eq 2.19, we note that the ∆ f (V )spectrum is parabolic and peaks at ∆φe −∆pole . If there is no polarizability term, then the LCPD is15For example, through an external circuit.16In the FM-KPFM mode.25Figure 2.10: Energy levels of the sample and tip in three cases: 1) tip and sample separated bygap with no electrical contact, 2) tip and sample are in electrical contact, and 3) an externalDC bias is applied to cancel the CPD and the tip-sample electrical force. Evac is the vacuumenergy and EF are the fermi levels. Adapted from [3].given as the maximum of the ∆ f (V ) spectra. With a polarization term, however, the fit is slightlymore complicated - one must first obtain an estimate for the polarizability term before fitting theparabola to find the estimated LCPD.Perhaps most importantly, KPFM can be sensitive to the charge state of a surface, making it theperfect tool to image real-space charge transfer between two molecules. Adding a single electronto an atom can increase the force on the AFM tip by a few piconewtons. With the addition orremoval of an electron, the LCPD will be shifted either up or down depending on the sign of thecharge. Given that sub-molecular contrast in charge distribution has been observed [51], it shouldin principle be possible to image a change in charge state over a single molecule when the moleculeis ‘given’ a charge by an optically excited donor17.2.3 Scanning tunnelling microscope induced luminescenceScanning tunnelling microscope induced luminescence (STML) measurements merge the sub-nanometerresolution of the STM with photon collection from the tunnelling junction to produce atomically-resolved optical images of the sample.In 1976 J. Lambe and S. L. McCarthy discovered a new way to generate light [65]. Working atFord Motor company at the time, the two scientists were exploring metal-insulator-metal tunnelingjunctions with very thin oxide layers (' 30 A˚). If a voltage was applied to the junction they no-ticed visible light emanated from the entire junction. They interpreted the effect as one of inelastictunnelling exciting optically coupled surface plasmon modes.17Or, conversely, when an optically excited donor loses a charge.26Figure 2.11: Schematic indicating the basic process in scanning tunnelling microscope in-duced luminescence.With the invention of the STM, it was quickly noticed that a similar tunnelling barrier was present,only in this case with a very sharp counter-electrode, so luminescence should in principle be possibleto observe. It is indeed.In STML, the tip acts as a very localized source of hot electrons. When those electrons tunnelinelastically into the sample, they excite a localized surface plasmon (LSP) which can decay radia-tively [66]. Collecting the photons emitted from the tunnelling junction gives the user clues aboutthe optical properties of the system under study. Photon information can be collected at each pixelin a topographic image, giving the user optoelectronic information with sub-nanometer resolution.The photons collected can be either counted to give an intensity map or spectrally resolved to give a3D spectral grid (x,y,hν). Obtaining photon maps like this is quite difficult due to the low quantumefficiency of photons produced by tunnelling electrons: ∼ 10−5 to 10−3 photons per electron [67],[68], [66]. With typical tunnelling currents in our system on the order of 100−12 A (at best), andassuming 10−5 photons/electron, one could expect100−12Cs·6.242 ·1018 eC· 10−5photone≈ 600 photons/s27but due to geometrical considerations (we only collect a small solid angle of the total light emitted,which is assumed to be a point source) and the transmission efficiency of the windows, mirrors, andlenses in the beam path, we can only expect to collect about 5% of the photons emitted. This leavesus with only ∼ 30 photons per second! If we can instead assume a quantum efficiency of 10−3photons per electron, we can expect 3000 photons per second. All of this makes obtaining datawith a reasonable signal to noise ratio difficult. The photon count can be increased by increasingthe tunnelling current, which is undesirable with our samples18, or, when taking spectrally-resolveddata or using a photon-counting-type photomultiplier tube (PMT), increasing the collection time ateach pixel. Of course, background noise (in this case stray photons) becomes a serious concern withlong integration times - so care must be taken to ensure that ambient conditions are very dark.The tip plays a critical role in the generation of luminescence from the tunnelling junction. Thetip-sample junction forms a plasmonic nano-cavity, and resonance in this cavity can enhance theradiative decay rate by three to five orders of magnitude [69]. Given the low efficiency per tunnellingelectron and the low currents needed to image molecules on a thin insulating layer, achieving thiskind of resonant enhancement is critical for successful imaging. Chen et al argue that that theplasmonic nanocavity enhances the radiative decay through plasmon enhanced vacuum fluctuations,not through a stimulated emission process [69]. Because the LSP depends on the detailed geometryof the tip-sample junction, tip changes can have drastic effects on the light emission - for better andfor worse [66]. In addition, the dielectric properties of the tip will affect the radiative decay - tipswhich are fabricated from a lossy material in the wavelength range of interest will quench the photonemission. It has been shown that silver (Ag) and gold (Au) tips achieve an order or magnitude betteremission than tungsten (W) tips in the relevant wavelength regimes for systems so far studied [70],[71]. This is because both Ag and Au have a small imaginary component of their dielectric constantin the visbile [72], [73].There remains considerable debate surrounding the exact physical mechanism behind luminescencefrom supported molecules in the STM junction [69], [74], [75]. Possible models which could ex-plain our results are discussed in Chapter 5.In our experiment, a lens was installed at the focal distance away from the tip-sample junction.This lens collimates some subset of the total emitted luminescence, and sends the beam out of thevacuum (through the windows in the heat shield and the chamber) and onto a tip-tilt mirror. Thebeam is then either a) directed into a second lens and focused on the slit of a spectrometer, or b)directed onto the active area of a PMT. The signal from either the PMT or the spectrometer wasthen collected simultaneously with topography data from the STM. See Section 3.3 for more details18Stable imaging of molecules adsorbed on NaCl is usually constrained to the 2 pA, 0.5-3 V regime. Increasing thetunnel current to even 50 pA tends to result in disturbed molecules which move on the substrate, or the tip picking upmolecules and crashing/becoming unstable.28of the experimental set-up.2.4 Optical excitationIn addition to collecting light from the junction it is possible to couple light into the junction. Wehave done so in this thesis using simple solid-state diode lasers and a tip-tilt mirror mounted on thechamber flange to direct the laser. The lens inside the vacuum focuses the light onto a small spot,' 20µm, which is much larger than the tunnel junction but serves as a reasonable concentration ofthe signal. Our goal is to see if molecular acceptor-donor pairs can be excited optically to inducecharge transfer between the molecules. The results will be determined with KPFM while STS willalso be recorded to look for additional signatures of excitation and charge transfer.29Chapter 3Experimental setupIn this chapter, we review the experimental set-up in detail.3.1 Scanning probe systemAll of the measurements in this thesis were performed with a commercial low-temperature UHVSTM/AFM. The Omicron STM/AFM has a base temperature around 4.3K1, a base pressure below5 · 10−11mbar, and optical access to the tunnelling junction. The system is housed in an ultra-lowvibration facility and sits on a 36 ton concrete block which ‘floats’ on pneumatic isolators.3.1.1 Ultra-low vibration facilityOur UHV STM/AFM system is located in an ultra-low vibration facility in the basement of theAdvanced Materials and Process Engineering Laboratory (AMPEL) building (the ‘main’ building).Each instrument in our facility is housed in its own ‘pod’, a building-within-a-building designed toprotect our instruments from external mechanical vibrations. Each pod contains a large concreteblock (often called an ‘inertia block’) which ‘floats’ on pneumatic isolators which themselves reston an isolated foundation2. The Omicron’s pod, which we call ‘omegapod’, houses the OmicronSTM/AFM. The system sits directly on the inertia block - a second set of pneumatic isolators sitsunder the Omicron’s table, but a small study performed by looking at the noise in the tunnellingjunction showed that using only the isolators under the inertia block and the internal damping stageprovided the best vibration isolation. Each pod is surrounded by a double-walled enclosure. The1All of our measurements were taken at 4K, but the system will image at 77K and at room temperature as well. Inaddition, a heating stage in the STM head allows for sample heating above the base temperature while the cryostat is cold.2The foundation of each pod is connected to the surrounding building only indirectly through the earth.30inner walls are either reinforced concrete or concrete block3 anchored to the foundation of the podand the outer walls are concrete block anchored to the main building’s foundation. Acoustically ab-sorptive material is placed in between the walls to further damp any vibrations transmitted betweenthem. Access to the pods is through a double set of acoustic doors, and suspended aluminum decksact as the floor where there is no block. The pods communicate electrically with the surroundingbuilding via small feedthroughs in the walls which cables are led through. After all the cabling isfed through to the main building, the remaining space in the feedthroughs are plugged with foam.B. MacLeod describes the facility in detail as well as studies of its vibration isolation capabilities inhis thesis [76]. Our instrument control unit is housed in the main building. This unit in turn commu-nicates with a computer in the control room, where users control the microscope remotely. Insidethe microscope, additional vibration isolation is achieved by a) suspending the whole STM/AFMstage from 3 springs, and b) an eddy current damping stage.3.1.2 Omicron UHV STM/AFMThe UHV system, shown in fig 3.1, consists of two main chambers separated by a gate valve: theprep chamber and the low temperature (LT) chamber. Almost all sample preparation (sputtering,annealing, and NaCl deposition) is done in the prep chamber. This keeps the LT chamber veryclean and the base pressure very low (' 10−11 mbar). The molecules in this thesis were depositedonto the substrate at 4K, so this deposition is done in the LT chamber from an Organic MolecularBeam Epitaxy (OMBE) cell (Kentax UHV equipment). The LT chamber houses the bath cryostatand the scanner head, and this is where measurements are taken. In fig 3.1 a) the prep chamber ison the left side of the photo and the LT chamber stands tall on the right side. The height is dueto the cryostat. The cryostat is a liquid helium bath cryostat with a liquid nitrogen shield. Thescanner head is thermally connected to the cryostat through thermal braids, and is generally at∼4.3K during measurements.The system is baked after any venting to 150 C for at least 60 hours to ensure a low base pressure.Both the prep and the LT chambers are pumped with individual ion getter pumps and titaniumsublimation pumps. The prep is additionally pumped with a turbo-molecular pump (backed bya dry scroll pump) during sputtering and annealing. The sputtering is done with Argon leakedinto the chamber which is ionized then accelerated with an LK Technologies sputter gun. Thesamples are annealed with an e-beam heating stage with built-in temperature PID control. NaCl isthermally evaporated onto our metal substrates using a home-built knudsen cell (built by the author,see appendix A) mounted on the prep chamber.Samples and tips are exchanged through a high vacuum load lock, which is pumped by the same3Omegapod has 3 reinforced concrete inner walls and one concrete block inner wall due to construction constraints.31Figure 3.1: Photographs of our system. a. The Omicron STM/AFM in its ‘pod’, without theoptical components. The cryostat stands tall. b. Front view of the UHV system. The LTchamber is on the left and the prep chamber is on the right. c. The scanner head showing theoptical access without the lens tube installed (indicated with white circle and red arrow) andthe fins for eddy current damping. d. The scanner head with no tip or sample as seen from thesample access side. e. The system as seen in a) but with the optical table installed (indicatedwith the red arrow). Detailed location of the optical table and components with respect to theOmicron LT chamber can be found in the 3D model. See Section 3.3 for more informationabout the placement and assembly of the optical table.turbo-molecular pump and scroll pump as the prep chamber. Samples are brought through the prepchamber via a magnetically coupled transfer arm, and placed into a carousel in the LT which canhold up to six samples or tips. Coarse motion is achieved through a stick slip drive. Tips areapproached to the sample automatically using a piezodrive and a feedback loop, and samples areplaced into the scanner head in the LT chamber via a wobblestick. The tip is moved with respectto the sample (for scanning and small movements) via a piezotube. At 5 K the piezosensitivity in xand y is 3.6 nm/V, and in z is 1.2 nm/V.3.1.3 Optical access in the scanning probe systemThe Omicron STM/AFM offers optical access to the tip-sample junction. Figure 3.2 shows the mainoptical paths to the junction as seen when looking in on the wobblestick side. In this view, the leftside is used to mount a camera with zoom lens for coarse tip approaches and sample transfers. Theright side contains the lens and is the path we use for optical excitation and collection.32Figure 3.2: Photo of the scanner head indicating the optical paths available. The right path isthe one used for optical collection and excitation: our lens was placed on the right at a distanceof approximately 18mm from the tip-sample junction. The left path is used in conjunction witha camera to visualize tip and sample transfer as well as coarse tip approaches. The central pathis the one used by the experimenter when transferring samples (or tip carriers) into and out ofthe head.A planoconvex 10mm diameter sapphire lens (Melles-Griot PXS-10.0-15.4-S) is situated about 18mm from the tip-sample junction in the STM scanner head. This lens has a back focal distance of18.4 mm, a focal length of 20 mm, a radius of curvature of 15.4 mm, an edge thickness of 2 mm,and an f# of 2.0. This allows collection of roughly 5% of the half solid angle above the samplesurface, where luminescence will be emitted assuming a point source. It should be noted that the25◦ angle is close to the angle of maximum emission from the tip-sample junction, which is situatedroughly 30◦ with respect to the surface [77], [78]. For luminescence, collection losses are due to thegeometry just explained as well as the windows in the heat shields, the vacuum chamber, and anyoptical component encountered along the beam’s path.The wavelength range accessible to us is currently limited primarily by the windows (viewports) inthe heat shields, and secondarily through the viewports in the LT chamber. Photons originating fromthe tunnel junction pass through two heat shield viewports and one standard viewport before theyreach air (where they are then bounced of a silver mirror and pass through other optical components).The in-vacuum viewports (see fig 3.4) limit our experiments to the wavelength range4 ∼350-620nm. This is one of the biggest constraints on our system for luminescence experiments.4There is a considerable design tradeoff here: limiting the wavelength range accessible via these windows limits thesample stage’s exposure to radiation, which causes heating. Replacing the current windows with ones allowing access toa broader wavelength range would reduce our liquid helium hold time (which is currently only about 56 hours) and couldincrease the base temperature of the sample stage (which is currently about 4.3 K).33Figure 3.3: Standard viewport transmission curve (kodial design). The 3dB points are at∼310nm - ∼2800nm. Image courtesy of [4].Figure 3.4: In-vacuum viewport transmission (KG5). The 3dB points are at ∼350nm -∼620nm. Image courtesy of [4].34(a) 100x100nm2, I=10pA, Vb=0.5V (b) 100x100nm2, I=20pA, Vb=0.03VFigure 3.5: STM images of the Ag(111) surface. a) Clean Ag(111) surface. b) Clean Ag(111)surface showing the surface state scattering off of impurities and step edges.3.2 System under study: copper (II) pthalocyanine and3,4,9,10-perylene tetracarboxcylic dianhydride on NaCl on metalsurfacesThe system under study in this thesis consists of small clusters of CuPc and PTCDA adsorbed ontoNaCl (bi-, tri-, and quad-layer) on either Ag(111) or Au(100).3.2.1 Substrates: Ag(111) and Au(100) with bi- and tri-layer NaClTwo substrates are used in this thesis. The first, Ag(111) was chosen for its optical properties - theplasmon resonance should be ∼600nm and thus accessible through our heat shield viewports. Thesecond, Au(100) was chosen because it has a larger work function than Ag(111), altering the energylevel alignment between the molecules and the underlying substrate.The Ag(111) sample is a chemo-mechanically polished single crystal (MaTeck). Ag(111) has ahexagonal lattice with a lattice constant of 2.88A˚ [79]. Ag(111) also has a Shockley-type surfacestate at ∼-67mV which forms a two-dimensional electron gas at the surface which can be imagedwith STM (see Figures 3.6 and 3.5b) [80].The Au(100) sample is a chemo-mechanically polished single crystal (MaTeck). Au(100) has a 1x5surface reconstruction of hexagonal geometry on top of the bulk square geometry. The result is a35Figure 3.6: STS spectra showing the Ag(111) surface statebuckling of the surface and a characteristic ‘wavy’ appearance (see Figure 3.7) [81].(a) 75x75nm2, I=200pA, Vb=0.5V (b) 60x60nm2, I=5pA, Vb=-0.5VFigure 3.7: STM images of Au(100). a) Clean Au(100) surface with screw dislocation. b)NaCl islands on Au(100).Both samples were clamped to Omicron’s sample plates with a molybdenum foil piece. Throughholes were drilled in the sample plates, and the foil piece as well as a ‘locator’ piece were cut witha waterjet cutter. The locator piece centers the sample in the correct location on the plate. Detailsfor this sample holder including materials, screw sizes, and Solidworks files are available on theinternal LAIR group wiki, under ‘AgHolder’.36(a) Solidworks model showing the sampleholder as designed.(b) Photo of the sample holder with a de-stroyed Ag(100) crystal inside.Figure 3.8: Images of the sample holder.3.2.2 Sample preparationThe Ag(111) surface and Au(100) surface were cleaned via repeated cycles of Ar+ sputtering andsubsequent annealing. Ag(111) was annealed at 770K, and Au(100) was annealed at 700 K.NaCl powder (TraceSELECT 99.999%, Fluka), was evaporated at∼800 K onto the Ag(111) sampleheld at 370 K and the Au(100) sample held at 350K resulting in (001) bilayer, trilayer, and quadlayerislands covering ∼50% of the surface (see Figures 3.9 and 3.10b).(a) 100x100nm2, I=20pA, Vb=0.3V (b) 2x2nm2, I=100pA, Vb=1VFigure 3.9: STM images of NaCl on NaCl/Ag(111). a) Ag(111) surface with bi-, tri-, andquadlayer NaCl. b) Atomic resolution on NaCl/Ag(111) showing the expected square latticeand a Moire´ pattern due to the lattice constant mismatch between Ag(111) and NaCl.37To create the clusters we studied, PTCDA was deposited before CuPc. Both molecules were out-gassed in UHV for several hours before deposition. Degas temperatures were ramped up fromwell below evaporation temperature while watching the pressure in the chamber. Once the pressuredropped, the temperature would be increased until it reached about 50 K less than the evaporationtemperature. After the initial degas, one or two ‘fake’ depositions5 were executed to ensure themolecules were sufficiently clean prior to their actual deposition.PTCDA (98%, Alfa Aeasar) was thermally deposited at 598.15 K onto the NaCl/Ag(111) or Na-Cl/Au(100) surface held at ∼4.3 K.CuPc (99.95%, Sigma-Aldritch) was the subsequently thermally deposited at 678.15 K onto Na-Cl/Ag(111) or NaCl/Au(100) surface held at ∼4.3 K.Clusters were formed via subsequent ‘high’ temperature annealing of the sample for 2 minutes. Thiswas achieved by removing the sample from the STM/AFM stage and holding it in the wobblestickinside the UHV chamber for two minutes, then re-inserting the sample into the STM/AFM stage.The exact temperature which the sample reaches in this process is unknown.(a) 8x8nm2, I=560pA, Vb=0.3V (b) 8x8nm2, I=150pA, Vb=0.15VFigure 3.10: STM images of Au(100). a) Clean Au(100) surface. b) Atomic resolution ofNaCl on Au(100).5To perform a fake deposition, just do everything the same as for a real deposition but don’t open the heat shields.38(a) 33.3x50nm2, I=5pA, Vb=-0.5V (b) 20x20nm2, I=3pA, Vb=-2VFigure 3.11: STM images of CuPc and PTCDA on Ag(111). a) NaCl island and bare Agsurface with deposited molecules, before 2 minute room temperature anneal. b) CuPc andPTCDA clusters formed on NaCl/Ag(111) after a 2 minute room temperature anneal.(a) 20x20nm2, I=5pA, Vb=-2V (b) 20x20nm2, I=5pA, Vb=0.5VFigure 3.12: STM images of CuPc and PTCDA on NaCl/Au(100). Both images indicateclearly the positioning of PTCDA with respect to the underlying Au(100) substrate: thePTCDA adsords normal to or parallel to the Au(100) reconstruction. Image b) also indi-cates the positioning of CuPc with respect to the underlying Au(100) substrate. a) CuPc andPTCDA near the edge of a NaCl island. b) CuPc and PTCDA near the edge of a NaCl island.Submolecular resolution is visible. Image taken with a PTCDA functionalized tip.39Figure 3.13: STM image of CuPc and PTCDA on NaCl/Au(100). NaCl lattice is visible, in-dicating the positioning of both CuPc and PTCDA with respect to the underlying NaCl layer.Image taken with a PTCDA functionalized tip.3.2.3 Tip preparationIn this thesis, we used cut Ag-terminated Platinum-Iridium (Pt/Ir) tips, electrochemically etchedsilver (Ag) tips and qPlus sensors with electrochemically etched tungsten (W) tips.The Pt/Ir (Goodfellow, 0.38mm dia) tip preparation is straightforward: the tip is cut and clamped inOmicron’s custom tip holders. Pt/Ir tips were used for STS measurements on the molecular clusterson Ag(111). Pt/Ir tips were prepared by K. Cochrane.The Ag tips were etched in 40-50% w/w reagent grade citric acid (Fisher Scientific) under an applied39V AC bias (from a variac) with a cylindrical mesh electrode (counter electrode) surrounding thetip. The negative terminal was attached to the tip and the ground terminal to the counter electrode.The Ag wire (Alfa Aesar, 0.404mm dia, annealed, 99.9%) is covered in teflon tubing except for a1-2mm length which is immersed in the solution and centered in the counter electrode. The portionof the wire which is not covered by teflon tubing is where the etching occurs. We always used the‘dropped’ tip from the etching procedure (because etching terminates as soon as it drops), ratherthan the top half which remains connected to the electrical circuit and continues to etch, dulling it.Tips were imaged under an optical microscope before cleaning and clamping in the tip holder toensure they were straight and of the correct shape macroscopically. After etching, tips were cleanedin de-ionized water and methanol. Ag tips were used for luminescence collection data. Ag tips wereprepared by the author.40The AFM sensors were etched tungsten (Goodfellow, 0.025mm dia, 99.95%) glued to quartz Qplussensors. AFM sensors were prepared by B. K. Yuan, and instructions below are as described byB. K. Yuan. AFM sensors were used for all KPFM experiments and AFM imaging. The Qpluspreparation followed these general steps:1. Glue one prong of the tuning fork to the substrate on the tip holder with electrically insulatingepoxy.2. Brush electrically insulating epoxy to the end area of the free prong of the tuning fork.3. Glue tungsten wire to the electrically insulating epoxy with conductive epoxy.4. Glue a thin gold wire between the tip and the electrode for the tunnelling current.5. Brush conductive epoxy between the electrode of the tuning fork for collecting the oscillationsignal and the corresponding electrode on the tip holder.6. Etch the tip with KOH (Fisher Scientific, ∼1 mol/L) solution using the meniscus method anda 5V DC bias with the ring electrode negative with respect to the tip.3.3 External optical systemThe STM/AFM system was integrated with an external optical system. This integration requiredmechanical design, electrical design, and coding in Matlab, MATE, and LabView.3.3.1 Mechanical componentsTwo primary mechanical components were designed and fabricated: supports to hold the breadboardsecurely on the Omicron table and the ‘flange optics’ assembly to enable secure attachment ofoptical components directly to the flange. In addition, a number of mounts and other small partswere made to enable optimal mating of various components.An accurate model of the LT chamber and the Omicron table was constructed in Solidworks toensure the correct mating of all mechanical parts. All solidmodels can be found on the internalLAIR group wiki under ‘Omicron3Dmodel’. A ‘flange optics’ system was designed and fabricatedwhich allows the user to attach a tip-tilt angled mirror as well as other desired components to theflange used for optical access. Aluminum supports were designed to hold the breadboard and otheroptical components. The breadboard supports were designed to use only the threaded holes alreadypresent in the table to prevent any damage which may have occurred when drilling holes in thetable, and to allow the table to float freely on its isolators if desired. Because the breadboard41(a) Detail showing some planes and sketches (b) Detail showing paths to the tunnel junctionFigure 3.14: Solidworks model of the Omicron.Figure 3.15: Various views of the optical set-up, used in this case for the spectrometer, asdesigned in Solidworks. The beam path is highlighted in red.and spectrometer are themselves quite heavy, and there is a non-negligible chance that the entireassembly will be ‘bumped’ forcefully by the users of the pod in some sort of manner (and becausethe spectrometer itself is quite valuable and would not survive a fall from the table), the supportswere designed with a safety factor of ∼5 and are secure enough to withstand considerable abuse.42The breadboard is securely bolted to the supports which are securely bolted to the table, making fora rigid assembly which should maintain the alignment of optics even if the table is disturbed (thoughtemperature changes etc may lead to misalignment for particularly precise set-ups). Recently, thealignment of the beam into the spectrometer was preserved through a cryogen filling cycle, provingthe whole assembly is sufficiently rigid to preserve alignment through considerable disturbances.Figure 3.16: Photo of the STM/AFM with optical components installedThe beam from the sample/tip can be steered into its target via the tip-tilt mirror on the flange, andif present a second tip-tilt mirror and/or a lens mounted onto an xyz stage. The flange is slightlymisaligned with respect to the sample, so there is only optical access to the back half of the crystalswe used (see fig 3.18). Use of a smaller sample (<10mm) will further reduce the area one hasoptical access to.The photomultiplier tube was mounted to an off-the-shelf 3-axis translation stage (though only 2axes are needed) via a simple mounting plate. The lasers were mounted onto the same stage viaanother simple mounting plate. The spectrometer was placed directly onto the breadboard6, andtwo mirrors as well as a lens steered and focused the signal onto the slit.3.3.2 Electrical componentsTo take simultaneous optical and STM/AFM images, specialized electronic components were built.The specialized electronics are only needed for optical imaging with the photomultiplier tube (PMT)and the spectrometer. No specialized electronics were built for laser radiation. Future experimentscould include modulation of the laser power, which may require further electronic design.6Initially, I was concerned that the noise of the shutter opening and closing (which is loud enough to be audible)would disturb the tip. Luckily this is not the case, as far as I can tell, so the spectrometer can be bolted to the breadboardif needed.43Figure 3.17: Photos of the optics integrated with the Omicron STM/AFM. a. Birds-eye viewof the optical table and flange optics, b. Close-up view of the beam path going into thespectrometer. c. View of the supports for the optical table, which used only the holes alreadypresent in the table and can handle considerable weight and impact.The PMT used in this thesis (Hamamatsu H9305-04) was powered and amplified by a home-builtamplifier/power circuit, loaned to us from another group. There is no known documentation for thiscircuit, but operation proved relatively self-explanatory. Section 3.3.2 details a more optimal PMTselection and counting circuit which will improve signal quality.44Figure 3.18: Image of the Ag(111) sample, looking in from the camera side (where illumina-tion is coming in from the optics flange). Left, original image. Center, image with sampleoutlined with a red oval. Also outlined in red is the tip and the tip’s reflection. Right, yellowindicates the image of the optics viewport as seen reflected in the sample (with no opticalcomponents mounted on the flange). Yellow demarcates the portion of the sample to whichwe have optical access. The result is that only the back half of the sample offers opticalaccess.Over-voltage protection boxThe PMT’s amplified analog output signal was sent directly to the auxiliary input of the Omicroncontrol unit (CU) via a coaxial cable. Communications with Omicron service engineers revealedthat the auxiliary inputs of the CU do not have any over-voltage protection, and a maximum inputvoltage of ∼+/-10V, so a simple over-voltage protection box was built by Chemistry electronics.This box contains one TVS diode (Littelfuse, SA8.5CA) which is connected across the signal inputand ground of the BNC connector. The diode is bi-directional and can clamp the voltage to +/- 10.4V. The box has a single BNC connector at the output, and from there a short coax can take the signalinto the CU.Photon counting circuitInside the CU, the PMT signal is digitized via a 16 bit ADC and then can be plotted by the Matrixsoftware at each (x,y) pixel as the STM scans across the surface to create an image of the emission.The PMT used in this thesis produced an analog signal, so one could not drastically improve thesignal quality by scanning slower and collecting more photons - scanning slower will simply resultin a better average measurement of the light level over the pixel scan time and a slightly cleanerimage. In addition, relatively high currents (≥200pA) were needed to see a signal. PMT imagingsignal quality can be significantly improved by using a photon counting head instead (recommendedchoice: Hamamatsu H7828-01 or H10682-01). With a photon counting head, signal amplitude (andthus, signal to noise ratio) can be improved by scanning slowly and collecting more photons at eachpixel. This will enable PMT imaging with sensitive samples which can only withstand very low cur-45rents (∼1pA). In addition, because an absolute 7 count can be obtained, quantitative measurementsof luminescence8 can be effectively obtained, which will enable comparisons to theory and point toa better understanding of the physics at play in the STM-induced luminescence process.All of the necessary details for a photon counting circuit optimized for input to the Matrix CU’sauxiliary input have been discussed with the Chemistry electronics shop, and the circuit can be builtas soon as the part number of the photon counting head is confirmed. Figure 3.19 shows the blockdiagram for the proposed circuit.Figure 3.19: PMT photon counting circuit block diagramThis circuit is ideal for input to the Matrix CU for a number of reasons9:1. The output is analog, which is required for input to the auxiliary channel on the CU (ironically,it will be re-converted back to digital by the CU’s ADC). Off-the-shelf counting circuits fromHamamatsu output a digital count, which will need to be converted to analog before input tothe CU, and indexing the count to the pixel scan clock will be difficult with an off-the-shelfoption (see next point).7The count given is the most accurate measure of the number of photons collected given the geometric and materiallosses.8A spectrometer also gives quantitative count numbers, but the PMT simplifies matters by offering a known trans-mission function which does not depend on changing variables like centre wavelength, etc, and the beam passes thoughfewer optical components on the way to the PMT than it does on its way to the spectrometer.9In addition, the up-front cost to build this circuit is the same as an off-the-shelf option from Hamamatsu which wouldrequire post-purchase modifications to work, so the total cost is less.462. The count is automatically indexed to the pixel scan clock. Every time the tip moves to a newpixel, there is a timing signal (TTL pulse) sent from the CU which will trigger the conversionof the current count to an analog voltage and the beginning of a new count. This will ensurewe get the photon count which corresponds to each pixel. This is a very clean way of gettingan integrated photon map while imaging.3. The DAC in the circuit has the same resolution as the ADC in the CU, resulting in maximumresolution in photon counts possible (16 bits).4. The cabling for routing the timing signal from the CU as well as an isolation box to protectthe CU from electronic noise coming from the PMT have already been built and installed.5. The whole circuit will be ‘plug and play’ so that the user simply needs to connect the PMT tothe circuit and the output of the circuit (coax) to the input of the CU.106. The circuit will be very well shielded (especially compared to off-the-shelf versions), andwill be connected directly to the isolation box (to receive the TTL signal) described in section3.3.2.7. The circuit will include an interlock tied to the dark enclosure, protecting the PMT head fromaccidents where the lights are turned on in the pod during measurement, etc.8. The circuit will plug into wall power.The details of the signal flow for the auxiliary inputs can be found in the Matrix Technical Referencemanual [82].Timing signalsBoth spectrometer imaging as well as photon-count imaging require synchronization with the Ma-trix CU. This synchronization is achieved primarily through the use of timing signals (TTL pulses)originating from the CU’s central real time controller (CRTC) board. These TTL signals triggersome event in either spectrometer or photon-counting mode. In photon counting mode, TTL pulsestrigger the beginning and end of photon acquisition and subsequent conversion of the count to ananalog voltage. In spectrometer mode, a TTL pulse sent at the beginning of each pixel triggers theacquisition of a spectrum. The timing signals from the CU are brought to the spectrometer/PMTset-up via the ‘timing signals cable’. This is a DD5011 connector which has the correct pins wired10Technically, to the input of the over-voltage protection box.11It is common to refer to all connectors of this type as ‘DB’ but the correct generic name is ‘D-sub’ (for D-subminiature). The naming convention for this type of connector uses D as the prefix for the whole series, followedby A, B, C, D, or E denoting the ‘shell size’ (the ‘container’, if you will), and then a numeral indicating the number of47Figure 3.20: Schematic for the timing signals cabling and the isolator box.to coaxial lines to carry the desired signals to the isolator box (described in section 3.3.2). Theconnector is very well shielded, and the pins which are carried on each line are clearly labelled.Currently, the timing signals cable connects to pins 13, 29, and 30:• Pin 13 is the acquisition clock. This triggers every time at least one channel is acquiring data.This is the signal used for the optical spectroscopy experiments in this thesis.• Pin 29 is the line trigger. It triggers at the end of each line in a scan. This should be usefulfor optical spectroscopy collection, to save time writing data to file (currently the bottleneckin terms of time spent per pixel) - the data should be stored in a buffer and only written to filewhen it sees this trigger.• Pin 30 is the pixel scan clock. This triggers every time the tip moves, not just when scanning(ie a digital pulse is seen at each pixel while scanning, but also when the scanner walks back tothe corner of the scan area after pressing ’stop’). This pin should be used for photon counting.pins. In our case, the connector is actually a DD50 connector, not a DB50 connector [83].48In the timing signals cable, all pins are referenced to one of the ‘analog ground’ pins in the connector.Other TTL signals are accessible via the CRTC board. Preliminary testing can be done with thebreakout board purchased from Winford Engineering, and further lines can be added to the timingsignals cable by Chemistry electronics. All TTL pulses are currently 1.25µs long. The TTL pulsewidth can be modified if you connect to a web-server on the CRTC board: type 10.0.42.2 for theaddress in your preferred Web-Browser (on the Omicron computer). Then find the proper triggerand set the time in µs.Isolation boxNoise isolation is critical with STM and AFM imaging. Connecting anything directly to the CRTCboard on the Matrix CU will add 60Hz electronic noise if care is not taken to isolate the CRTC(and CU) from whatever is connected on the other side. Since the timing signals extracted from theCRTC connect to both a noisy digital i/o device, which is powered by a noisy desktop computer(see section 3.3.2) and (in the future) a photon counting circuit, an isolation box was built to protectthe CU from unwanted electronic noise. The box was built and designed by Chemistry electronicswith consultation from the author. The box is designed to allow the user to extract a timing signaland to connect that signal to any other electronic device without introducing noise from that deviceto the CU.The box is placed very close to the CU (in the back of the CU desk, beside the CRTC board) tominimize noise picked up on the coaxial cables running from the CRTC to the box. The box iswall-powered through a 5V adaptor, plugged into ‘dirty’ (beige plug inlet) power.The box is also ’plug and play’, the user simply needs to connect their timing signal on the inputand output via coax. There are currently 3 ports, so pins 13, 29, and 30 can be used simultaneouslyto control both spectrometer and PMT imaging modes.The schematic and board layout for the isolator box can be found on the LAIR internal wiki, under’IsolationBox’.NI USB digital i/o deviceTo interface the TTL pulses with LabView, a simple digital i/o device was purchased from NationalInstruments: NI USB 6501. This is by a wide margin the most affordable digital i/o device offeredwhich suits our needs (The NIUSB 6501 costs $150 compared to most DAQ’s which cost roughlya few thousand dollars). The device has 24 digital input and output lines as well as a 32 bit counter.Preliminary testing to verify if the device was ‘seeing’ the TTL pulses from the CU was done with49the ‘Counter - Count Edges’ example VI in LabView.The NI USB 6501 counter requires a minimum pulse width of 100ns (minimum high = minimumlow = 100ns). It runs at USB 2.0 full speed (12Mb/s).The timing signals from the CRTC (and isolation box) are sent to this device for integration withLabView, which uses them to trigger acquisition of spectra (and in the future writing the data to file)via the optical spectrometer. The screw terminal wiring should be 16 to 28 AWG copper conductor.The screw terminal inputs are single-ended.3.3.3 Photomultiplier setup and alignmentThe PMT used in this thesis is a Hamamatsu model H9305-04, which was loaned to us from anothergroup. For PMT measurements, the PMT was bolted to an xyz stage via a small mounting plate.The stage was positioned at the edge of the breadboard nearest to the angled mirror mounted on the‘optics flange’, and roughly aligned via xy motion. To align the tip-sample junction with the PMT,the image of the PMT as seen in the sample is used. First, the user approaches almost all the wayto the sample, using a light shining in from the camera side. Second, the user shines a second lightonto the face of the PMT. An image of the PMT will now appear in the sample. The user can thendrive the tip so that the tip-sample junction is centered on the image of the active area of the PMT.Note all of this should be done with the PMT OFF. Exposing the PMT light levels this high willruin it.(a) Tip and tip reflection visible in Ag(111)sample, with no light shining on the PMT.(b) Tip, tip reflection, and PMT image visible insample (light shining directly on PMT).Figure 3.21: Aligning the PMT. Images show the Ag(111) sample as seen with the camera.For the PMT used in this thesis, the offset of the power supply/amplifier should be set to about -9.5Vso that the PMT signal with no light (dark state) hovers around -9.5V. This will give the user the50entire +/-10 V range of the auxiliary input for any light value above the dark state.The room must be carefully darkened before using the PMT, otherwise the real signal will be indis-tinguishable from the background (and the PMT could be ruined). The control rack is darkened viaa curtain (blackout fabric from ThorLabs) drawn across the front. Any little blinking lights or otherlight sources in the room are similarly darkened. A dark enclosure was built to protect the PMT andto improve the signal integrity (and to provide additional safety when using the laser sources). Ifthe photon counting PMT is set-up, the dark box will be connected to an interlock which will poweroff the PMT head whenever the door of the enclosure is opened.Figure 3.22: Solidworks model of the dark box. The frame is built with aluminum t-slot extru-sion and the panels are made from corrugated plastic. The panels are caulked into the frameto prevent light leaks. Note the panels are actually black, but are purple in the model to easevisualization.3.3.4 Spectrometer setupThe optical spectrometer is set up to take both point spectra and optical spectroscopy grids.To align the spectrometer, the user should follow these steps:1. If this is the first time the spectrometer has been used in a while, or if the spectrometer hasbeen moved at all, perform both an intensity and a wavelength calibration using IntelliCaland the included calibration lamps.2. To begin, use the same alignment procedure as the PMT: approach the tip very close to thesample using only a light shining in from the camera side. For the spectrometer set-up, the tipneeds to be very close to tunnelling (≤100 steps away from tunnelling). Then, shine a lighton the slit and try to line the tip sample junction up with the image of the slit.3. Install a lens mounted on the xyz stage as shown in figure 3.17 b.51Figure 3.23: Schematic indicating the connections and cabling for the optical spectrometerset-up.4. You no longer need the camera. Move it out of the way and shine the light from the cameraside into the middle of the viewport.5. Use a white card to look at the image coming out of the STM after the lens which is directlyin front of the slit. You should see the tip and its reflection, as in figure 3.24.6. If you do not see the tip and its reflection centered in the circle of light, you need to move thetip. The directions to move on the remote box are exactly backwards from the direction youwant to see it go on the card (ie if you want the image of the tip to move left (-x), push right(+x). Similarly, if you want the image to move down (-y) push up (+y)).7. Once the image of the tip and its reflection are centered in the light spot, align the light spotto the spectrometer slit. Start with the slit open quite wide, and try to make the light spotdisappear as you move the xyz stage.8. Once you feel like the coarse alignment is satisfactory, move to the control room and completethe approach (turn off all lights in the room, and darken the room as you would for PMTimaging). Leave the tip tunnelling over a large clean terrace at very high current (100nA).Take a single spectrum to get a reference value (initially, you may want to set the CCDexposure time to 20s).9. Press ’backwards’ on the remote control box, so that the tip is no longer tunnelling. Don’t gointo the pod with the tip tunnelling! This can ruin a very good luminescence tip.10. Go into the pod and adjust the alignment by moving the xyz stage. Begin by moving the stage52in the direction which goes across the slit (let’s call this x). Each division on the micrometer is10µm, so pick an appropriate step size. For example for a 250 µm wide slit, 50 µm steps arereasonable. Iterate by moving the stage one step, exiting the pod, re-approaching, and takinga spectrum. You should see the signal decrease as you move one direction, then increase anddecrease again as you move the opposite direction.11. Once you have found the maximum in the x direction, position the micrometer there and takeanother reference spectra. Now iterate by moving the xyz stage in the y or z direction.12. Again, once you have found the maximum in one direction repeat for the last direction.13. It is important to align the beam in all 3 directions, since the CCD has some variability inwhere it is most efficient, so even the vertical direction can make a huge difference.14. If the beam is well-aligned, you should be able to close the slit further with no major signallosses. If you have a large loss, re-do the alignment procedure with this slit width.15. Once the alignment is complete, you can begin measuring. With all components securelybolted to the breadboard, after a cryogen filling cycle you should be able to skip steps 1 and2 and only have to do a small fine alignment.Figure 3.24: Image of the tip-sample junction as seen after the lens before the spectrometerslit.To collect an optical spectroscopy grid, one needs to use the LabView program. The LabViewprogram interfaces with both LightField, the spectrometer’s control software, and the Matrix, viaTTL pulses. The current incarnation of the program for optical spectrometer imaging takes the TTLsignal from the CRTC at the beginning of each new pixel to trigger the acquisition of an opticalspectrum. The data is saved in a flat file which is unflattened in Matlab.533.3.5 Laser setupThe lasers used in this thesis are Coherent OBIS LX and LS series lasers. They are solid statecontinuous wave lasers with simple plug-and-play packaging. The laser set-up is the same as that forthe PMT: the laser itself is mounted via a mounting plate onto an xyz stage. Preliminary alignmentcan be done by shining the laser through a target hanging from cage rod on the flange optics. Theuser first approaches the tip to the sample using only a light shining in from the camera side, then alight is shone directly on to the face of the laser so that an image of the face appears in the sample.The tip is then moved such that the tip-sample junction is aligned with the aperture of the laser. Ifthe aperture is not visible in the sample, adjust the tip-tilt mirror mounted on the flange until youcan see it.The lasers are controlled remotely via the ‘Coherent Connection’ software. The laser is connectedto the computer via an ethernet cable and USB extender. The laser power is kept inside the pod.For safety, the black box should be placed over the optical table to minimize stray beams in theroom.54Chapter 4Optical excitation of an acceptor-donorpairFigure 4.1: STM constant current image (5x4nm2, I=3 pA, Vb=-2 V) of the ‘XO’ configurationof CuPc and PTCDA on NaCl(2ML)/Ag(111). CuPc images as a ‘flower’ here.In this chapter, we explore STS results obtained in conjunction with optical excitation of the system.In particular, we explore a single adjacent CuPc-PTCDA configuration which I refer to as the ‘XO’geometry (see Figure 4.2). The long-term experimental plan is to take KPFM data to determine thecharge state of adjacent acceptor-donor molecules, and to see if charge is transferred upon opticalexcitation. STS data will be taken concomitantly, so that one can explore how the STS spectrum isaffected by having a molecule in an excited state.Unfortunately, all of the data explored in this chapter were taken with a misaligned laser. The tip-sample junction was aligned with a secondary reflection of the laser beam, not the primary beam. It55Figure 4.2: STM constant current image (5x5nm2, I=10 pA, Vb=0.5 V) of the ‘XO’ configu-ration of CuPc and PTCDA on NaCl(2ML)/Au(100). CuPc images as a ‘cross’ here, which iswhy it is called the ‘XO’ configuration. Note this would be different from a +O configuration,where the CuPc would be rotated 45◦ with respect to the PTCDA. Submolecular resolution isvisible. Image taken with a PTCDA functionalized tip.was not noticed that this was an issue until the spectrometer was being set up months later, as thespectrometer required very careful inspection of the optical path. Plans were in place to performoptical excitation with KPFM data collection on the system (with a correctly aligned laser), butseveral issues with the Omicron have deferred data collection. See Section 3.3 and Figure 3.18for details regarding the alignment issue. As such all of the data in this chapter should be takenas inconclusive. The studies performed (drift(t) as well as STS with laser illumination) should berepeated with a properly aligned laser as described in Section 3.3.5 if the future experimenter desiresconclusive data with this system. In all cases, a thorough drift(t) study will need to be completed forany study of a system under illumination, since the thermal expansion coefficients will depend on thespecific tip and sample being used. Because a secondary reflection was used instead of the primarybeam in this study optical power was still incident on the sample but it was significantly reduced. Inany case, what was attempted here provides a useful template for further studies. Discussion of theresults follows in section 4.3.56Figure 4.3: STS spectra averaged over the entire molecule (either CuPc or PTCDA) in the XOconfiguration on NaCl(2mL)/Ag(111) taken with different tips on different specific molecules.The location and shapes of the peaks change with different tips and locations on the substrate.4.1 Characterization of spurious signals: the influence of thetip/substrate and thermal expansion of the tip-sample junctionA steady tip-sample distance is critical for quality STM and STS measurements. Because laserradiation causes both the tip and sample to thermally expand, it is important to understand thetransient effects observed when one turns the laser on or off. Once the transient effects of turningthe laser on or off are understood, one knows how long they must wait before taking a measurementto ensure they are measuring a system which is (nominally) in the steady state. In addition, thespecific tip, molecules, and substrate do affect the measurement, so each set of data must be takenwith the same tip at the same location (see Figure 4.3) to be comparable. There is considerablevariation present when taking data with different tips at different locations on the substrate.57Figure 4.4: STS spectra averaged over the entire CuPc molecule in the XO configuration onNaCl(2mL)/Ag(111) taken with different tips on different molecules. Spectra shown corre-spond to the ‘0 mW’ data point for each data set shown in section 4.24.1.1 The effect of the tip and specific location on the substrateDifferent tips and specific locations on the substrate introduced various artifacts into the data whichbecame very relevant in this study1. Because it is not known what one should see in STS whenthe molecule under study is in an excited state, features like ‘side humps’ and small wiggles couldbe a real signal. The tip, substrate, and other factors can introduce very subtle wiggles and so onwhich are not related to the state of the molecule, but resemble something interesting. For anymeaningful comparison, then, one needs to collect each set of data with the same tip, on the sameset of molecules, at the same location on the substrate. Refer to Figure 4.3 and note that both theFebruary 23 PTCDA and the Feb 15 CuPc appear to have bimodal peaks around 1 V. These ‘ghostpeaks’ are red herrings, and appear in all the data collected with that given tip at that given location.This kind of variation is present in any two sets of data taken with different tips in different locations,and this kind of variation should be kept in mind for the experimenter looking for small signals. Tocontrol for this, STS grids were collected in sets with the same tip on the same 2 molecules in thesame location. Each set consisted of a grid taken at 0, 5, and 15mW of laser power. Generally, this1Despite the best effort of the experimenters to obtain a flat DOS, some very subtle tip-related effects persisted andaffected some of the measurements. It is also possible that ‘invisible’ defects were present in the substrate which affectedthe measurements.58Figure 4.5: STS spectra averaged over the entire PTCDA molecule in the XO configurationon NaCl(2mL)/Ag(111) taken with different tips on different molecules. Spectra shown corre-spond to the ‘0 mW’ data point for each data set shown in section 4.2would use up the majority of the hold time of the cryostat so that the next set would necessarily betaken with a different tip on a different two molecules, etc.59Figure 4.6: STS spectra averaged over the NaCl surrounding the XO configuration onNaCl(2mL)/Ag(111) taken with different tips on different molecules. Spectra shown corre-spond to the ‘0 mW’ data point for each data set shown in section 4.2604.1.2 Thermal expansion of the tip-sample junctionMeasurements of the drift in z as well as in (x,y) were taken with both 488 nm and 687 nm radiation.A time constant in z was extracted from the z(t) data, and an estimate for the (x,y) drift was obtainedfrom tracking the position of a specific molecule in successive images (the position of the scan boxwas set at (0,0) and was not moved, and the successive images show a molecule drifting acrossthe screen). This drift analysis only gives a lower bound on the laser-induced drift due to the lasermisalignment.For the (x,y) drift measurements, the drift without any radiation was taken prior to the drift withradiation. The difference between the two is reported as the drift due to the laser. For the 488 nmlaser at 20mW, the drift in x was found to be ≈ 5.3 pm/min, and the drift in y was found to be ≈ 2.2pm/min. For the 687 nm laser at 20 mW, the drift in x was found to be ≈ 6.7 pm/min, and the driftin y was found to be ≈ 5.3 pm/min.For z(t) measurements, the laser power was ramped up in 5 mW increments from 0 to 20 mWfor 488 nm laser and from 0 to 30 mW for 687 nm laser then back down to 0 mW at 5 minuteintervals (see Figure 4.7). For the ramp, the power was turned on for 5 minutes then off for 5minutes before the next increment. At each power, a z(t) trace was taken. The tip was positionedover NaCl/Ag(111) at (0,0).Figure 4.7: Schematic indicating how the laser power was ramped up and down over timeto collect the z(t) data with the 488 nm laser. Note with the 687 nm laser the basic schemeremains the same, but the maximum power is 30 mW.A two-term exponential model with 95% confidence bounds was fit to the data:y = aebx + cedx (4.1)Two terms were used to deal with the offset: since z(t) wasn’t centred at 0 (or since the functions61Figure 4.8: Data and fit for z(t) as measured with the initial 5 mW of radiation from the 488nm laser.don’t decay to 0) the exponential function we want to fit to should have the following formy = aebx +C (4.2)Given that the exponential is decaying ‘up’ or ‘down’ (the sign is changing on a), and there were38 traces to fit, putting all the data in a for loop with no need for initial guesses was the easiestway to proceed. As such, the Matlab function ‘exp2’ was used for the fit since one of the termsessentially acts as the constant - one of the decay constants was always very small (on the order of10−6) such that edx ≈ 1, and the associated amplitude is a good fit for the offset given the start andend points of the data. For example, for the 488 nm laser initially turned on to 5 mW the fit valueswere a =−1.057 ·10−1,b =−0.0211,c =−5.540 ·10−8,d =−5.915 ·10−7, giving a time constantof τ = 47.448 s, an offset of C= 55.404 nm, and a ‘time to steady state’ of 237.24 seconds or 3.954minutes. The data and the fit can be seen in Figure 4.8. With an exponential fit, the time constant τis the time for the function to reach 1/e of its initial value (for a decay), and 5*τ is the approximatetime before the function reaches steady state.A similar procedure was followed with the 687 nm laser. Since the results aren’t really valid, wewon’t explore them in detail. With the ramp going up to 30 mW and back down, the largest τ was62laser power [mW] τ (seconds) 5*τ (minutes)0 31.588 2.6325 47.448 3.9540 62.128 5.17710 26.797 2.2330 57.363 4.78015 71.234 5.9360 55.429 4.61920 57.083 4.7570 65.549 5.46215 64.668 5.3890 55.912 4.65910 56.374 4.6980 56.707 4.7265 55.224 4.6020 47.210 3.934Table 4.1: z(t) results for 488 nm laser. Note that the first fit for 10 mW appears to be poor,though it was within a 95% confidence interval. In addition, the initial τ should be 0, but it islikely that we are seeing the piezo drift. This means that the piezo drift and the laser-inducedthermal expansion are both present in the measurements taken. Even with the piezo drift, wecan see that the longest time one needs to wait before reaching steady state is 6 minutes. For the488 nm laser, we can conclude that ≥6 minutes is a safe time to wait after turning it on or off toany power to be confident that the tip and sample have reached thermal equilibrium, assumingpiezo drift is at an acceptable value.178.77 seconds, giving a wait time of at least 15 minutes.634.2 Optical excitation of the system with a Ag(111) substrateThe XO configuration’s response to (misaligned) laser illumination was studied in depth via thecollection of STS grids on the system under varying power levels (0, 5, and 15 mW) and wavelengthsof radiation (488 nm, 519 nm, 561 nm, 640 nm, and 685 nm). Each set of data consists of three STSgrids taken on the exact same two molecules (in the XO configuration, on NaCl(2 ML)/Ag(111))with the same tip, with one laser illuminating them at 0, 5, and 15 mW (note with the 488nmlaser, the 5mW data point is replaced with one at 10mW). The spectroscopy grids were analyzedby averaging the individual spectra over either the entire molecule or some portion of the molecule,and comparing the results for different levels of illumination.Figure 4.9: STM and STS images showing the bounded area over which individual STS spec-tra were averaged for each plot. The image shown is from the 561 nm data set, and shows theareas taken to obtain an average over the whole molecule. Data taken on NaCl(2mL)/Ag(111).The spectra shown on the following pages are averaged over the whole molecule. Because it ispossible that averaging over the whole molecule could average out an interesting signal which wasfound only on part of the molecule, averaging was also done over only the perimeter of the CuPcmolecule, the center of the CuPc molecule, and the oxygen end of the PTCDA molecule. Nosignificant differences are found for these sets of spectra compared to those obtained by averagingover an entire molecule, so they are not included in the main body of the text. See Appendix B for64these results.Figure 4.9 shows the topography and STS slice where the average was taken for the whole molecule(the area which was averaged is delineated in red, called a ‘mask’) for the entire 561 nm data set.Figure B.1 shows the topography and STS slice where the average is taken over just the oxygenends of the PTCDA. Figures B.2 and B.3 show the topography and STS slice where the average istaken over just the center or the perimeter of the CuPc.The masking for averaging was done by comparing, at a given energy, the value of ∂ I∂V at each pixelto some threshold value. The energy chosen for this comparison was selected as one offering highcontrast between the desired area and the remainder of the grid as seen in a STS slice. Matlab codewas written by K. Cochrane which goes through each pixel in a STS slice at a user defined energyand assigns a ‘1’ if the ∂ I∂V value is above the user defined threshold, or a ‘0’ if the∂ I∂V is below theuser-defined threshold2. This 2D matrix of 1’s and 0’s then multiplies each slice in the full STS gridso that only the pixels which meet the threshold criteria are kept. The ∂ I∂V data at each pixel in thenew, ‘masked’ grid is then averaged to obtain the plots shown.Figure 4.10: Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 685 nm laser. Spectra are averaged over the entire molecule.Recall that the peaks in a ∂ I∂V (E) plot are related to the tunneling resonances of the molecule. Ingeneral, the peaks below Fermi (0 V) are associated with the occupied molecular states and the peaks2Note there is actually an upper and lower bound defined by the user, but in practice one generally only uses the lowerbound.65above Fermi are associated with the unoccupied molecular states. So, for Figure 4.11, consideringonly the blue solid line for the CuPc molecule as measured with 0 mW of laser power, one finds thehighest occupied tunnelling resonance (or molecular orbital (MO)) at -2.01 V, the lowest unoccupiedtunnelling resonance at +1V, and the second lowest unoccupied tunnelling resonance at ≥2.5 V.It is probable that PTCDA is negatively charged on NaCl(2mL)/Ag(111) [2], [84]. The result isa partial occupation of the lowest unoccupied MO. When probing this state with STS, one sees asplitting of the state above and below Fermi. This is because the state is half occupied: you can seeit below Fermi because you can extract an electron from it; conversely you can also see it aboveFermi since you can inject an electron into it3. The split states are separated by the Hubbard energy.Thus, with PTCDA on NaCl(2ML)/Ag(111), instead of the straightforward highest occupied MOand lowest unoccupied MO, we see the split LUMO as well as the usual highest occupied MO andthen the second lowest unoccupied MO (which for PTCDA is expected to be nearly degenerate withthe next highest MO [85]). In addition, one cannot be confident that the Hubbard states are notoverlapped with the other states, so state identification is tentative. In any case, the peaks remainvisible and shifts in energy are detectable, even though the peaks are not easily identifiable.A discussion of the results, including what we may have expected to see, follows in Section 4.3.In addition to plotting the averaged spectra, one can plot ‘slices’ of the 3D STS grid, showing thespatial distribution of ∂ I∂V as the energy is varied. A slice is obtained by plotting the value of∂ I(E)∂Vat each pixel for a given energy E. Note that the relative intensity is the primary concern in an STSslice. In Figure 4.15 yellow indicates the presence of a tunnelling resonance. The spatial distributionof ∂ I∂V was not observed to change with illumination. Figure 4.15 is representative of all of the laserexcitation data collected4.3I conjecture that a similar effect may be observed with an optically excited molecule, but in that case two stateswould be split across Fermi: the half occupied ‘HOMO’ and the half occupied ‘LUMO’.4Aside from PTCDA jumps, that is. See Section 4.3.1 for a discussion on that.66Figure 4.11: Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 640 nm laser. Spectra are averaged over the entire molecule.Figure 4.12: Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 561 nm laser. Spectra are averaged over the entire molecule.67Figure 4.13: Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 519 nm laser. Spectra are averaged over the entire molecule.Figure 4.14: Comparison of dI/dV spectra for the XO configuration on NaCl(2mL)/Ag(111)under illumination from the 488 nm laser. Spectra are averaged over the entire molecule.68Figure 4.15: Spatial and energetic distribution of the LDOS, imaged via slicing the STS gridat selected energies. The spatial distribution showed little variation in each data set. Yellowindicates the presence of a tunnelling resonance. Slices shown are from the 561 nm 0 mWgrid (taken on NaCl(2mL)/Ag(111)).694.3 DiscussionWe note that no ‘obvious’ signal is present when comparing spectra for illumination vs no illumi-nation. For example, none of the peaks shift in energy when the sample was illuminated by themisaligned laser. No new peaks appear in illuminated vs un-illuminated samples. The peak heightschange, but not in a fashion which is correlated with the laser power. This could be due to themisalignment, or it could also be due to other factors. However, there is one notable feature in eachdata set: the amplitude of the 2.4 V peak in the PTCDA spectra changes considerably, for all lasersused:• For the 685 nm laser, and spectra averaged over the entire molecule, the 5 mW peak is thelargest, followed by the 15 mW then 0 mW peaks (see Figure 4.10). For spectra averagedonly over the oxygen end, the 5 mW peak remains the tallest, with the 0 mW and 15 mWpeaks approximately the same height (see Figure B.4).• For the 640 nm laser, and spectra averaged over the entire molecule, the 0 mW peak and 15mW peaks are the largest, followed by the 5 mW peak (see Figure 4.11). For spectra averagedonly over the oxygen end, the peaks are all approximately the same height (see Figure B.6).• For the 561 nm laser, and spectra averaged over the entire molecule, the 0 mW and 5 mWpeaks is the largest, followed by the 15 mW peak (see Figure 4.12). For spectra averaged onlyover the oxygen end, the 0 mW peak remains the tallest, then the 5 mW peak is the secondlarges, and the15 mW peak the smallest (see Figure B.8).• For the 519 nm laser, and spectra averaged over the entire molecule, the 0 mW peak is thelargest, followed by the 5 mW then the 15 mW peaks (see Figure 4.13). For spectra averagedonly over the oxygen end, the peaks are all roughly the same height (see Figure B.10).• For the 488 nm laser, and spectra averaged over the entire molecule, the 0 mW and 15 mWpeaks are the largest, followed by the 10 mW peak (see Figure 4.14). For spectra averagedonly over the oxygen end, we find the same thing (see Figure B.12).There are two main things to note from the list above. First, the height change does not correlateconsistently with the laser power. Second, the height changes are different if you average over thewhole molecule versus if you average over only part of the molecule.What could this indicate? First we should consider that, if laser excitation is happening at all, it isunlikely to be happening with all of the lasers used. Therefore, some sets of data should show asignal and some sets show none if the signal is related to laser excitation. It is possible that morethan one laser could excite a given molecule, since the lasers are separated in energy by ∼130-150meV. In particular, we would expect something close to the 640 nm laser to excite CuPc, and either70the 488nm laser or a shorter wavelength laser5 to excite PTCDA [86], [87], [88]. Considering thatthe 2.4 V peak height varies in all data sets, this may indicate that the peak height is not correlatedto a signal seen in STS from successful laser excitation.However, Yeyati and Flores calculated the tunnelling current for a small molecule such as CO or NOin the tunnel gap excited by radiation, and they found that one can expect an increase in tunnellingcurrent of about 1-3% when molecular vibration is excited by radiation [89]. They performed thiscalculation with a laser intensity of 2 kW/cm2. In our system, the laser intensity is unknown due tothe misalignment. If the laser were properly aligned, for 5mW of power and a spot size of roughly20µm, we would have an intensity of 2.5 W/cm2, three orders of magnitude less than Yeyati andFlores considered. In addition, as noted previously, none of the sets of data considered here showsthe largest peak being 15 mW (except in the case where both 15 mW and 0 mW peaks are thelargest, which clearly contradicts the premise).Grafstro¨m points out that the increased tunnelling resistance found in most adsorbate layers will beaccompanied by a decrease in Yeyati and Flores’ expected signal, which will eventually becomeundetectable [90]. This could be the case here, but that still leaves the variation in peak heightunexplained. Grafstro¨m et al. investigated the possibility that an additional tunnelling channelcould open up in an excited adsorbate (they investigated co-adsorbed layers of PTCDA and octyl-cyanobiphenyl liquid crystals on highly ordered pyrolitic graphite) [91]. They did not perform STS,but did find that a small increase in tunnelling current (100 fA at a laser intensity of 3.5 kW/cm2)was present when the PTCDA islands were irradiated6. They did not propose a specific origin fortheir signal, and only could only tentatively claim it was not due to thermal expansion.Smith and Owens also observed a laser-induced resonance effect when tunnelling into a monolayerof the J-aggregating dye 1,1’-diethyl-2,2’-cyanine bromide on a silver substrate [92]. They pro-posed that the signal originated from the nonradiative decay of the excited molecular state into thesubstrate, causing local heating and thermal expansion, which in turn caused a change in the tunnelgap width and an increase in tunnelling current. Im et al. performed a similar experiment to thatof Smith and Owens, but this time with a flattened tip, and found transient currents were inducedwhen they irradiated a thin molecular film7 deposited onto a gold substrate with pulsed light [93].They proposed that the transient currents were due to photoassisted charge injection.In general, all of these explanations are contradicted by our results, because for each set we shouldsee an increase in the laser-induced tunnelling current with increasing laser power. In our case, thatwould mean that the (2.4 V) peak height for 15 mW radiation should be the largest in amplitude5The shortest wavelength laser we were able to use in this experiment was 488 nm.6In this experiment, they used an Ar+ laser and a tunable dye laser of unspecified wavelength.7In this experiment they explored a number of films: high molecular weight hydrocarbons, polymethylmethacrylate,octadecanethiol, and anthracene.71(and 0mW the smallest). This is not what is observed.The experiments of Grafstro¨m et al., Im et al., and Smith and Owens, as well as the theory fromYeyati et al., point to a different study for this same system, however: one can simply hold the tipin place and measure the tunneling current with the laser off then measure it again at least 7 (or15) minutes after turning the laser on (when thermal expansion effects should have reached steady-state) [94]. If the current with the laser on is larger than it is with the laser off, then this shouldindicate that an additional tunnelling channel has opened up due to the adsorbate being excited bythe radiation.4.3.1 Vibrational excitation leading to possible tunnelling current changesThere is one more possibility, which Yeyati and Flores as well as Smith and Owens point to: therole of excited vibrational modes affecting the magnitude of the tunnelling current. During the laserpower study, it was noticed that the PTCDA in the XO configuration was very jumpy. It was oftenimpossible to get a full grid where the PTCDA didn’t jump; see Figure 4.20.After the laser power study was completed, it was noticed that the PTCDA was specifically jumpingbetween an ‘upper’ and a ‘lower’ configuration with respect to the CuPc; see Figure 4.16. Since‘upper’ and ‘lower’ depends on which way you are looking at the XO configuration, we named thetwo configurations A and B. Note that the CuPc also rotates slightly depending on if the PTCDA is inthe A or B position. Hopping of single molecules caused by inelastic tunnelling-induced vibrationalexcitations has been observed before with STM, for example in the work of Komeda et al. [95].Figure 4.16: Topography of the same XO showing both the lower (left) and upper (right)PTCDA configurations, which we call ‘A’ and ‘B’. Comparing the two images above, thePTCDA jumps '0.362 nm downwards, which is close to the spacing between adjacentCl− top sites in the NaCl (0.353 nm) as measured from Figure 3.9b. Images taken onNaCl(2ML)/Ag(111).72Comparing the two images in Figure 4.16, and using the CuPc molecule in the bottom left corner asa reference to adjust for piezo drift, the PTCDA jumps'0.362 nm downwards, which is close to thespacing between adjacent Cl− top sites in the NaCl (0.353 nm as measured from Figure 3.9b). Thepositioning of the PTCDA with respect to the CuPc was demonstrated clearly with AFM imaging;see Figure 4.17. The PTCDA is not symmetric with respect to the centre of the CuPc, as one mayhave expected when examining only STM images. The PTCDA jumping distance combined withthe AFM-resolved positioning of the PTCDA with respect to the CuPc center likely indicates thatthe PTCDA is jumping between adjacent Cl− top sites, and, since the PTCDA is not symmetric withrespect to the centre of the CuPc, that the CuPc is not centred on a Cl− top site.(a) Laplace transform of original image (b) Raw dataFigure 4.17: Constant height NC-AFM images using a PTCDA-functionalized tip of the XOconfiguration on NaCl(2ML)/Ag(111). The oxygen end of the PTCDA which is adjacentto the CuPC is closer to one isoindoline subunit of the CuPc than the other. Image size3.2x3.2nm2; oscillation amplitude A = 15 mV. Note the image in a is the Laplace transformof the gaussian smoothed raw data. Image taken on NaCl(2ML)/Ag(111).Finally, controlled jumping of the PTCDA between the A and B configurations was demonstratedwhen tunnelling into the centre of the molecule at +2V; see Figure 4.18. This switching wasachieved by taking an ‘STS’ point spectra, from +2V to +2V, for 200 points with a raster timeof 30 ms. This switching was also possible with a PTCDA-terminated tip.Stipe et al. pioneered single molecule vibrational spectroscopy, which they coined scanning tun-nelling microscope inelastic tunnelling spectroscopy (STM-IETS), and showed that vibrational ex-citation could result in a change in tunnelling current due to the change in the molecule’s locationwith respect to the tip [96], [97]. STM-IETS ( d2IdV 2 ) spectra for PTCDA in the XO configurationshows an STM-IETS resonance at 2.12V, see Figure 4.1973Figure 4.18: Exciting a vibrational mode in PTCDA on NaCl(2ML)/Ag(111) at 2V, showingreversible and controllable jumping between two XO configurations. Jumping occurs duringscan 2 (down) and scan 5 (up).Figure 4.19: STM-IETS spectra for PTDCDA in the XO configuration onNaCl(2mL)/Ag(111), with no illumination. Note the resonance at 2.12 V.It is possible that inelastic tunnelling into a vibrational mode at 2 eV excites the PTCDA sufficientlyto cause it to jump. This vibrational excitation could also cause a change in tunnelling current, assuggested by Yeyati and Flores (except in our case the mode is excited by tunnelling, and not ab-sorption of a photon) and demonstrated by Stipe et al.. However, Stipe et al. found a different,and very simple, mechanism for the change in tunnelling current: when the molecule rotates afterexcitation, the position of the tip changes with respect to the molecule, leading to a different tun-74nelling current. In their experiment, they excited an acetylene molecule adsorbed onto Cu(100) intoa rotational state via inelastic tunnelling. They found that the current was high when the tip wasover the plane of the molecule, then low when the molecule rotates and the tip is no longer over theplane of the molecule.Figure 4.20: Topography for all of the STS grids taken during the laser excitation study, show-ing how the PTCDA jumped (and the CuPc rotated) multiple times during most grids. Notethat there was no correlation with the change in the 2.2 V peak height and the number ofjumps observed in the images above.Because all of the STS grids so far discussed in this chapter scan the voltage through +2eV, hundreds75of times per grid, it is likely that the PTCDA jumped between the A and B configuration manytimes in each grid. If the final topography image does not show any jumps, there still may havebeen jumps in each line of STS and it was just lucky that the PTCDA jumped back to its startingconfiguration before the imaging along that line occurred. The variation in peak height, then, canbe thought of as the result of averaging over the vibrational excitations in each grid; some grids hadmore vibrational excitations (and therefore a larger amplitude 2.4 V peak) and some grids had lessvibrational excitations (and therefore a smaller amplitude 2.4 V peak). Note there is no correlationbetween the observed peak height differences for a set of data and the number of jumps observed inthe topography for the grids in that set.Another point of interest is that difference in amplitude of the 2.4 V peak in the PTCDA STSspectra is different when spectra are averaged over just the oxygen end of the PTCDA versus thewhole molecule. In general, one sees less variation in the peak height when averaging only overthe oxygen end of the PTCDA. This may indicate that the induced vibrational mode has some non-uniform spatial dependence over the molecule, or that the coupling of tunnelling electrons to thejumping of the molecule is orbital-specific [97].In our case, it is unlikely that the vibrational mode was excited by the lasers, but that cannot becompletely ruled out. Since the mode can be excited by tunnelling into PTCDA at 2V, we may alsobe able to excite the mode with a laser of '620 nm. Of course, we don’t have a laser at exactly620 nm but, if the vibrational band is wide enough, then the 640 nm laser (only 62.5 meV differentin energy from 2eV) could excite it. However, the data so far don’t indicate any special amountof variability in the 2.4 V peak in the 640 nm set of data versus the other sets, so it would appearunlikely that laser excitation of PTCDA was achieved.4.3.2 STS of CuPc-PTCDA on NaCl(2ML)/Au(100)Because the STS data taken so far imply that laser excitation was not successful with this system, theplan for future studies involves laser illumination and STS/KPFM exploration of the same molec-ular system on a different substrate: Au(100). Au(100) has a larger work function than Ag(111),and importantly the work function should be larger than PTCDA’s electron affinity, so PTCDA onNaCl(2ML)/Au(100) should not be charged. STS of the same molecular system in the same ‘XO’configuration on NaCl(2ML)/Au(100) was briefly explored and the results are intriguing. Interest-ingly, some of the tunnelling resonances in the PTCDA molecule appear to be barely shifted8 whencomparing measurements on NaCl(2ML)/ Au(100) vs Ag(111): the lowest resonance above Fermi8Since PTCDA is likely charged on one substrate, but neutral on the other, it is hard to say which states correspond towhich, so we can’t draw any firm conclusions about how much any specific state has shifted; but we can talk about thetunnelling resonances.76shifts down from 0.98 eV (as measured on Ag(111)) to 0.90 eV (as measured on Au(100)). Thetotal shift is only 0.08 eV. See Figures 4.21 and 4.22.Figure 4.21: dI/dV spectra for the XO configuration on NaCl(2ML)/Au(100)Figure 4.22: dI/dV spectra for the XO configuration on NaCl(2ML)/Ag(111)77Note that because PTCDA is charged on NaCl(2ML)/Ag(111), in STS the LUMO appears split intostates above and below Fermi [2] [84], while on NaCl(2ML)/Au(100) this is likely not the case.Meanwhile, the highest occupied tunnelling resonance disappears for PTCDA on Au(100) - at leastup to -2.5 V9. For CuPc, the lowest tunnelling resonance above Fermi is significantly shifted whenmeasured on Ag(111) vs Au(100): the lowest peak energy shifts up from 1.03 eV (as measured onAg(111)) to 1.86 eV (as measured on Au(100)); which is a shift of 0.83 eV. A similar shift up inenergy happens for the highest tunnelling resonance below Fermi, resulting in an almost unchangedgap but very different individual energy levels. In general, the spectra for CuPc looks similar onboth substrates, just shifted. See Figures 4.23, 4.21, and 4.22.Figure 4.23: Comparison of the gap as measured via STS of PTCDA and CuPc in the XOconfiguration on NaCl(2ML)/Au(100) vs NaCl(2ML)/Ag(111). Note the gap indicated forPTCDA on Au is an estimate, since no tunnelling resonances were observed below Fermifor the XO configuration (which was only explored to -2.5V) but was observed at '-3V forisolated PTCDA.The shifts of specific resonances are interesting as these change the level alignment. The gaps asmeasured by STS also change. The gap as measured on Ag(111) vs Au(100) for PTCDA changesby ' 2.3 eV, while the gap for CuPc only changes by '0.07 eV. This is likely due to the chargestate of PTCDA on the two different substrates: as noted earlier, it is likely that PTCDA is chargedon Ag(111), but it may not be on Au(100).It is important to note that a change in substrate will shift all of the energy levels as measured withrespect to Fermi for both molecules. This is because our 0 V is referenced to the Fermi level ofthe substrate. When a molecule is adsorbed onto a substrate, assuming it is strongly coupled to the9Note that the first tunnelling resonance below Fermi for PTCDA on Au is not visible in Figure 4.21, it was notmeasured for the XO configuration. It was measured for isolated PTCDA molecules on NaCl/Au(100), and was found tobe at -3 V.78substrate, then one still measures the ionization potential and electron affinity with respect to thevacuum level, but this time the vacuum level is defined with respect to the substrate, as defined bythe work function.Figure 4.24: Schematic indicating the energy level alignment for a molecule in the STM bar-rier without an applied bias, assuming the molecule is more coupled to the substrate than thetip.Because of the change in the substrate work function, we can get an estimate for how the energylevels of the molecule as measured via STS (ie, with respect to Fermi of the substrate) will changewhen measured on one substrate vs another (see Figure 4.24). Ag(111) has a work function of∼4.46eV10 [98]. If we let the electron affinity of neutral PTCDA be X (and we assume the vacuum level ofthe molecule is aligned with that of the substrate) then on Ag(111) we should see the LUMO at 4.46- X eV. On the other hand, Au(100) has a work function of ∼5.22 eV, so we should see the LUMOof neutral PTCDA at 5.22 - X [99]. All of this means that for neutral molecules we should see allof the energy levels shift ‘rigidly’ by a value close to the difference between the two substrate’swork functions when we measure them on one substrate vs another. Of course, for PTCDA this isconfused by the fact that it is charged on Ag(111). However, for CuPc we do see a fairly rigid shiftof all energy levels of about ∼0.84-0.91 eV, which is close to the difference in the two substrates’work functions (0.76 eV).One should note that even for a molecule adsorbed onto a bare metal substrate, the vacuum levelfor the molecule isn’t exactly aligned with the vacuum level of the substrate - there is some energydrop due to a dipole barrier [100]. In our case, for a molecule adsorbed onto (2ML) NaCl, the workfunction of the substrate is changed due to the surface dipole from the adsorption of NaCl. In anycase, the difference between the energy shift we observe and the difference in work functions canlikely be made up by the work function change due to the NaCl, polarization of the surroundings,10In this paper, they show that the work function approached a steady value as the sample was cleaned more and morethoroughly (by Ar+ bomardment). It was quite nice, clean physics ;)79Figure 4.25: Cartoon indicating how the energy levels as measured with STS shift or changeunder various conditions. A, energy levels for a neutral molecule vs a charged molecule.When the molecule is charged, the LUMO splits across Fermi. B, energy levels for a substratewith a larger work function vs those for a smaller work function. Energy levels shift up whenmeasured on a substrate with a larger work function. C, energy levels for a molecule inunperturbed surroundings vs polarized surroundings. When the surroundings are polarized,the ionization potential is reduced (so the HOMO moves closer to Fermi) while the electronaffinity is increased (so the LUMO moves closer to Fermi).response to the field caused by the charge (first order) and dipole moment (second order) of PTCDAon Ag(111)11, and the fact that the energy levels probably don’t stay put when a bias is applied,among other things [101]. See Figure 4.25 to see how the energy levels as measured by STS changeunder various conditions.In any case, the probable different charge state of the PTCDA, as well as the different energy levelalignments in general for both molecules with respect to the substrate, results in energy levels whichare shifted considerably compared to those on NaCl(2ML)/Ag(111). Figures 4.21, B.14, and B.1511Incidentally, I calculated the ‘stark shift’ expected from the neighbouring charged PTCDA and found that the field ofthe charge alone caused a shift of 8.7 meV, while the field of a dipole (due to the image charge in the substrate) caused ashift of 0.5 meV, both of which are relatively small - especially when one considers that the molecule is in a much largerelectric field due to the applied bias.80indicate the energy levels for the same (XO) configuration, as measured by STS on NaCl/Au(100).With this change in energy level alignment, molecules adsorbed on NaCl/Au(100) offer a muchlikelier pathway for optical excitation and charge transfer. See Figure 4.26.Figure 4.26: Schematic indicating the energy level alignment and possible optical excitationpathways for molecules adsorbed on NaCl/Ag(111) vs NaCl/Au(100). The relative energiesare not exact. On Ag(111) we could see an excitation from the HOMO of the CuPc to theLUMO of the CuPc (process 1). In this case, there is no driving force for charge transferto PTCDA since the energy levels are almost aligned (and the LUMO of PTCDA is halfoccupied). We could also see an excitation from the lower Hubbard state on PTCDA to theLUMO+1/LUMO+2 (process 2). In this case, there would be a large energy drop into CuPc’sLUMO, but PTCDA’s LUMO is also at this energy so it is possible that the electron wouldjust relax to there instead of being transferred across molecules. There are other possibilitiesas well. On Au(100), we could see an excitation from CuPc’s HOMO to LUMO, then transferto PTCDA’s LUMO (process 3). This energy level alignment appears to be quite favourablefor charge transfer between the two molecules.As before, spectra were also averaged over only part of the molecule in case any interesting featuresemerged which were averaged out otherwise. No major features of interest were noted. Thoseresults are in Appendix B.81Figure 4.27: STS slices of the XO configuration on NaCl/Au(100) indicating the spatial andenergetic distribution of the LDOS. There is only one visible state below Fermi in the exper-imental energy range, on the CuPc molecule.82Chapter 5Scanning tunnelling luminescencemeasurementsIn this chapter, we explore the scanning tunnelling luminescence (STML) measurements obtainedwith the optical system coupled to the STM/AFM. Measurements were taken on bare Ag(111) aswell as clusters of CuPc and PTCDA on NaCl(2ML)/Ag(111). In the first part of this chapter,we discuss the proof of principle experiment, where luminescence is collected from the tunnellingjunction with both the photomultiplier tube (PMT) and the optical spectrometer on bare Ag(111).In the second part of this chapter, we discuss the luminescence measurements taken on the CuPc-PTCDA-NaCl/Ag(111) system. Finally, we discuss our results and make some suggestions forfuture modifications to the experimental setup as well as suggest some experiments to try next withthe current setup.5.1 Luminescence measurements on bare Ag(111)The photomultiplier setup and the optical spectrometer setup described in Section 3.3 were used tocollect luminescence from the tunnelling junction on bare Ag(111) with a Ag tip. The PMT pro-vides a wavelength-integrated measurement, so images created with this setup show lighter pixelscorresponding to more photons collected and darker pixels corresponding to less photons collected.The spectrometer provides wavelength-resolved measurements, and 3D grids can be taken and in-tegrated over a wavelength range to give an image in (x,y) of the luminescence similar to the oneobtained with a PMT. It is important to recall that our measurement can only access emitted photonswith wavelengths between '350-650 nm due to the radiation blocking coating on the in-vacuumviewports. It is also important to note that a background subtraction was performed for the optical83spectrometer, which sometimes results in negative counts in a given spectrum.The initial proof of principle experiment was to collect luminescence from bare Ag(111) withthe PMT, since imaging on bare Ag allows for very high tunnelling currents to be used (whichshould correspond to larger photon counts) and because we knew what we should see if successful:an image resembling the topography but with the luminescence quenched at the step edges [78],[102],[103], [104], [105]. The quenched luminescence at the step was interpreted by Berndt et. al[102]. They noted that the tunnelling current density is typically peaked along the surface normal,but at a step the tunnelling current vector ceases to be peaked in this direction. When the tunnellingcurrent vector is no longer normal to the surface, it also ceases to be parallel to the dipole momentof the tip-induced plasmon mode, which gives rise to photon emission. Note that the tip-inducedplasmon mode is presumed to stay approximately unchanged at a step due to its relatively large lat-eral extent (Aizpurua et al. later quantified the lateral extent of the plasmon more precisely [104]).The vector mismatch between the tunnelling current filament and the plasmon mode dipole momentreduces the inelastic tunnelling matrix element. If one takes a simple model where the current isroughly perpendicular to the surface and that the orientation of the plasmon dipole is constant, thenthe inelastic tunnelling probability is proportional to cos2(α), where α is the angle between thesurface normal and the tip’s central (long) axis.The general method to get started with a luminescence data collection run was to bring 3 differentAg tips into vacuum, since the success rate for luminescence tips can be low. Each tip was tested byfirst ‘cleaning’ it on clean Ag(111) via dozens (likely hundreds) of high voltage pulses (+/- 10V),being careful to avoid crashing as this can ruin the tip. Once the tip was clean and imaged stably athigh (nA) currents, the tip-sample junction was aligned with the PMT active area and the room wasprepared for luminescence detection (all sources of light were removed or blocked). The initial testto see if the tip luminesced was to approach completely1, then take one step back and reapproach,watching the voltage in the Auxiliary channel as a function of time. One could approach and retractat multiple currents and voltages, as well as tunnel while changing the voltage and current, to geta sense of how responsive the luminescence signal is to changes in tunnelling parameters and todetermine which voltage provides the most luminescence. In the end, for my first batch of tips, onetip barely luminesced while the other two did. Of the two tips which did luminesce, one showeda larger voltage jump when moving from not approached to approached at the same voltage andcurrent, so this tip was taken as the ‘most luminescent.’ The other two tips were brought out ofvacuum and the experiment was begun.Because it is known that a scan over bare Ag(111) step edges should result in a photon map with1When doing this initial testing, as well as the alignment to the optical spectrometer, care should be taken to positionthe tip at (0,0) and above a large clean terrace of Ag so that surface defects, drift, or other spurious signals do not affectthe luminescence. This way you can reasonably compare each spectra / data point you take.84Figure 5.1: STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=300 nA, Vb=-10 V. Note there are some tip changes which, while visible in the topography,are quite pronounced in the PMT image.quenched luminescence at the steps, a suitable clean area of Ag with a few steps was found for PMTimaging. The PMT image is formed by the time-averaged Aux(t) channel at each pixel. Scanningmore slowly improves the noise (via averaging) but does not increase the signal amplitude, thusimprovements in the signal to noise ratio are limited to reducing the noise. The scans shown inFigures 5.1-5.5 show the results for gradually decreasing current.It is interesting to note that the step edge in Figure 5.1 displays increased luminescence rather thandecreased luminescence. Note that this image was taken at very high currents compared to any otherPMT image in this section and compared to most images in the literature. In addition, Figure 5.1was the only image taken with Vb=-10 V. The difference in luminescence at the step edge is likelybecause at these parameters we are in the field emission regime. Different processes are likely to beat play here.It is also interesting to note that while defects appear prominently in both the PMT and STM imagesin Figure 5.1, they appear prominently in the PMT image and less prominently in the STM imagein Figure 5.2. A similar phenomenon (luminescence images betraying defects and impurities whichare invisible or faint in the STM images) was noted in [78]. Note that a 20 ms traster (time spent ateach pixel) was used for Figures 5.1 - 5.2.Obtaining images at lower tunnelling currents required slower scanning to reduce the noise. A 50ms traster was used for Figures 5.3 - 5.5.A slight improvement in signal quality (in this case, amplitude) was achieved when switching from85Figure 5.2: STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2, I=5nA, Vb=-3 V. Note the tip is slightly double in this image. The quenching of the luminescenceat the end of the screw dislocation is interesting.Figure 5.3: STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=500 pA, Vb=-3 V.negative to positive bias. This can be easily checked when initially testing the tip, by toggling thebias while watching Aux(t).Once we knew that PMT imaging could work, we performed wavelength-resolved measurementswith the optical spectrometer. Preparation of the set-up and alignment followed the procedure out-lined in Section 3.3.4. Again, we knew what was expected, namely the photon emission corre-sponding to the plasmon resonance of the tip-sample junction, which should be a peak centered86Figure 5.4: STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=250 pA, Vb=-3 V.Figure 5.5: STML image (left) and STM image (right) of clean Ag(111) steps. 30x30 nm2,I=100 pA, Vb=+3 V.somewhere in the neighborhood of 600 nm [77]. We collected point spectra of the surface as wellas an optical grid, see Figures 5.6 and 5.7. Note that for point spectra on bare Ag(111), the locationand amplitude of the peak would change considerably depending on the state of the tip. The peakcould be centered anywhere between 500-600 nm, and the amplitude would vary between 30 countsand 200 counts (for +3 V, 200 pA tunnelling).Unfortunately, as one can see in Figure 5.7, the optical spectroscopy grid did not work out as ex-pected. The image formed from plotting the integrated spectra at each pixel shows that the grid87Figure 5.6: Point optical spectra on bare Ag(111) (I=200 pA, Vb=+3 V, 100µm slit, and CCDexposure time = 100 s).Figure 5.7: Bottom left, Integrated optical spectroscopy grid. Counts integrated from 450 -730 nm. Top left, Example spectra collected at first pixel in grid. Right, Associated STM im-age for optical grid (30x30 nm2, I=50 pA, Vb=0.03 V). Optical grid taken at I=50 nA, Vb=3.25V, with 50 ms CCD exposure time. The grid was aborted early so a scan taken at the sameresolution before the grid was started is the one shown. The area delineated in red in the STMscan corresponds to the section plotted as the integrated optical grid.88started out correctly, but that the steps seem to drift across the screen as the up scan continues. Theproblem was that saving the data to file took a little bit too long at some pixels: the time spent ateach pixel was set to 3 seconds, with only 0.5 seconds of that time CCD exposure time; the remain-der of the time was allocated to saving the data and running the code. The readout for the CCDshould only take 50 ms, and the code running should only add a few hundred more milliseconds.Observing the grid while it was running revealed that saving the data (writing it to file) was the mosttime-consuming aspect of each pixel. As the grid continued, every once in a while the data wouldstill be saving when the next TTL pulse was sent. This resulted in that TTL pulse being missed andthat pixel not getting a spectrum. Since this happened randomly throughout the grid, there was noway to unflatten it properly to create an image.This problem can be solved immediately by further extending the traster for scanning - the MATEscript which extends the traster can simply be edited to have a sleep time of 4 seconds instead of 3seconds (replace sleep(3000) with sleep(4000)). Of course, this is an undesirable solution since thiswill mean that optical grids take even longer than they already do - adding a second per pixel in an150x150 pixel grid adds an additional 6.25 hours to the grid running time. The better solution is tobuffer the data and only write it to file at the end of each line, when the tip is walking back to thestart of the line. This usually takes several seconds when scanning slowly, and can be adjusted byadjusting the ‘move time’ in the Matrix. The TTL pulse (line scan clock) which triggers at the endof the line is already extracted from the CRTC and is on the timing signals cable, all one has to dois adjust the LabView code to change the saving to occur after it sees the line scan clock. This willhave the additional benefit of either drastically reducing the time required to take a grid (since onewill only need about 1 second per pixel) or allowing one to take grids with longer CCD exposuretimes, improving the signal quality.Note that when running optical spectra grids, one should keep the actual traster short and adjust theMATE script to get desired time spent at each pixel. Adjust the move time as desired to ensure thatthe tip walks at a reasonable speed back to the start of the line scan. If the traster is set to be long(like 500ms), then the move time will default to being long as well- even if you ‘decouple’ themand specify it should be short (there seems to be a hard-coded setting in the Matrix where the movetime can only be half the traster). Having the tip walk back to the start of each line very slowly willmake your grid take at least 1.5 x as long.895.2 Luminescence measurements of CuPc-PTCDA on NaCl/Ag(111)Luminescence was also collected from the CuPc-PTCDA system on NaCl/Ag(111)2. Unfortunately,the molecules did not luminesce in our experimentally accessible wavelength range. Figures 5.8and 5.9 show the results for PMT imaging of both isolated CuPc and PTCDA as well as the XOconfiguration of CuPc and PTCDA. Note both images were taken at negative bias, but the resultshould be the same at positive bias since the luminescence mechanism is roughly symmetric withrespect to Fermi [5], [74]3. Note the molecules in Figures 5.8 and 5.9 appear somewhat skewed,this is due to the long scan time (traster=50 ms) combined with piezo drift.Figure 5.8: STML image (left) and STM image (right) of adjacent isolated CuPc and PTCDAon NaCl(2ML)/Ag(111) (9x6 nm2, I=100 pA, Vb=-3 V). The luminescence is quenched onboth molecules, the bare NaCl/Ag substrate shows more emission than the molecules.We also collected point optical spectra on CuPc/PTCDA clusters to see if perhaps some lumines-cence could be extracted with a little luck (even though we knew that would be unlikely due tothe PMT imaging results). All of the optical spectra discussed in this chapter were taken with the300g/mm grating (500nm blaze) with varying centre wavelength. Note that I did not perform anintensity calibration before collecting this data, but I did perform a wavelength calibration. Notperforming an intensity calibration was a mistake! See Section 5.3 for more details of how this mayhave affected the measurement. The lack of intensity calibration may mean that two of the samemeasurement at different center wavelengths could look different. Luckily, most measurementswere taken with both a 650 nm center wavelength and a 550 nm center wavelength, so there is nochance that any variability in the intensity between the two measurements could have ‘hidden’ theluminescence signal.2Spectra were collected on both bilayer and trilayer NaCl.3Changing bias either changes the states one would see transitions between (for a LUMO-LUMO+1 type transition)or changes the direction of a LUMO-HOMO transition. For PTCDA the energy gaps between HOMO and HOMO-1are almost the same as between LUMO and LUMO+1, and for both molecules the energy gap between HOMO-LUMOremains constant no matter which direction the transition between them goes. In addition, no luminescence was observedat positive bias when taking point spectra.90Figure 5.9: STML image (left) and STM image (right) of adjacent CuPc and PTCDA onNaCl(2ML)/Ag(111) (4.5x6 nm2, I=100 pA, Vb=-3 V). The luminescence is quenched, thebare NaCl/Ag substrate shows more emission than the molecules.To collect luminescence from the molecules, we tried collecting spectra at both positive and negativebias, at both 550 nm and 650 nm center wavelengths, on both bilayer and trilayer NaCl, and evenwith the molecule attached to the tip (both tunnelling over Ag and NaCl). In all cases, the moleculesdid not appear to luminesce.For example:1. At -3V, for an XO on a bilayer, the PTCDA shows no emission at all and the CuPc showsmodified NaCl emission (see Section 5.3 for a discussion of the NaCl emission). See Figure5.10 for an example of the type of ‘spectra’ we observed. Measurements were taken at both550 nm and 650 nm center wavelength.2. At +3V, for an XO on a bilayer, neither molecule showed any emission. See Figure 5.11 foran example of the type of ‘spectra’ we observed. Note the PTCDA was jumpy and no spectraon the PTCDA were obtained where the PTCDA did not jump4. The CuPc did not jump inthe spectra shown. Measurements were taken at both 550 nm and 650 nm center wavelength.3. At +3V, for isolated molecules as well as XO on trilayer, no emission was evident. Measure-ments were taken at 550 nm center wavelength.4. At +3V, we picked up both CuPc and PTCDA (separately) and tried to see if there was emis-sion when the molecule was attached to the tip. There was not. Interestingly, picking upmolecules slowly killed any emission from that tip, even when tunnelling over Ag at largercurrents. Measurements were taken at 550 nm center wavelength.4Even though we were not tunnelling into the known vibrational resonance at ∼2.1 V, there may have been anothervibrational resonance at 3 V.91Figure 5.10: Point optical spectra on CuPc and PTCDA in XO configuration (I=200 pA, Vb=-3 V, 250µm slit, and CCD exposure time = 200 s). Note the measurement over the CuPcmolecule shows the emission seen on bare NaCl but attenuated (see Section 5.3 for a discus-sion of the NaCl emission). The measurement over PTCDA shows no emission, even thoughboth bare Ag and NaCl showed emission prior to this measurement. This is roughly consistentwith what was observed in the PMT images; namely the luminescence is strongly quenchedover the molecules compared to the bare NaCl or Ag.Figure 5.11: Point optical spectra on CuPc and PTCDA in XO configuration (I=200 pA,Vb=+3 V, 250µm slit, and CCD exposure time = 200 s). Note the luminescence is totallyquenched over both molecules. Both bare Ag and NaCl were showing emission immediatelyprior to this measurement.925.3 Point optical spectra collected on NaCl/Ag(111)While we did not see any emission on the molecules, we did see an interesting signal on the bareNaCl. For weeks I thought the source of the signal was a molecule on the tip, which is why wetried picking up molecules and collecting luminescence. However, the result of that experimentshowed that the signals shown in Figures 5.13, 5.14, and 5.10 is not due to a molecule on the tip -it is a genuine signal from the bare NaCl. My oversight in not performing an intensity calibrationreally delayed me figuring out that this was a real signal - the signal is almost unremarkable whenmeasured at a center wavelength of 550 nm, but is notable when measured at a center wavelength of650 nm. I would advise the reader to be careful and always perform a full wavelength and intensitycalibration at all relevant center wavelengths if the spectrometer has been moved or has been leftunused for some time. Note that the signal is real and not simply due to the lack of intensitycalibration. If it were due only to the lack of intensity calibration, the same spurious signal wouldhave been visible on bare Ag. Bare Ag always showed the same single peak (moving, but a singlepeak, consistent for a given tip) at both 550 and 650 nm center wavelength. Even at 550 nm centerwavelength, the spectra on NaCl is not what would be expected for a ‘regular’ surface plasmon: ithas two to three side peaks which indicate something more is going on (see Figure 5.12). Whenspectra were collected at a center wavelength of 650 nm, the uniqueness of the signal becomesevident: see Figures 5.13 and 5.14.Figure 5.12: Point optical spectra of bare NaCl taken at 550 nm center wavelength showing abimodal peak (I=200 pA, Vb=+3 V, 100µm slit, and CCD exposure time = 200 s).The spectrum shown in 5.13 and 5.14 are taken with different quality tips. One can see that theemission in Figure 5.14 is stronger and cleaner than that in Figure 5.13. In either case, as well as inFigure 5.10, where the spectra was collected over CuPc revealed only the quenched NaCl emission,one can see a signal which is clearly different from that of bare Ag(111).93Figure 5.13: Point optical spectra of bare NaCl taken at 650 nm center wavelength showingmultiple strong peaks (I=200 pA, Vb=-3 V, 250µm slit, and CCD exposure time = 200 s).Figure 5.14: Point optical spectra of bare NaCl taken at 650 nm center wavelength showingmultiple strong peaks (I=200 pA, Vb=+3 V, 250µm slit, and CCD exposure time = 200 s).The potential source of this signal is discussed in Section 5.4.5.4 Discussion and suggestions5.4.1 The signal seen on NaClWhen NaCl is adsorbed on Ag(111), the Ag(111) surface state band remains, forming a two-dimensional interface state which is confined to the insulator/metal interface. Note that the interfacestate is shifted up in energy from the Ag(111) surface state. This interface state is unpopulated, so94one can inject a charge into it [106], [80]. Inelastic tunnelling into this interface state can excitea surface plasmon, which we can detect if it is coupled to the nanocavity and in our detectablewavelength range.In general, a surface plasmon would not have multiple strong peaks. The simplest explanation isthat the NaCl confines the electrons to a quantum well, and the spectrum shown corresponds toradiative transitions between energy levels in this quantum well [107].It is possible that the source of this signal is a surface plasmon polariton, whose dispersion relationis altered by the presence of the Na+ and Cl− ions on the surface, creating a periodic boundarycondition in the 2D space. Further complexity can be introduced by considering the phonon modesof the NaCl and how those modes would in turn alter the surface plasmon polariton’s dispersionrelation. One may also want to consider the relative placement of the Ag atoms to the Na+ andCl− ions at the interface, as the Moire effect may manifest here in the surface state modulation. Theelectronic structure of crystalline and thin film NaCl has been explored both theoretically and exper-imentally, for example in [108], [109], [106], [110], [111]. Ho¨vel et al. studied the modification ofthe Ag(111) surface state with STM after the adsorption of Xe, and found that a modified standingwave pattern emerges in ∂ I∂V maps in both the Ag(111) and the Xe islands [80]. Repp et al. studiedNaCl/Cu(111) and showed that the interface state band shows strong standing wave patterns whichobey Snell’s law at the NaCl island step edges [111].Note the two strong signals shown in Images 5.13 and 5.14 were both taken about 4nm away fromthe NaCl step edge. It is possible that the interface state band in our case has a dispersion relationwith more than one term, which would lead to peaks in the observed luminescence spectra. Thiscan be determined by following Repp et al.’s method carefully, and taking a more complex fit to thek vector data.There are more possibilities which could explain this signal. Unfortunately, an in-depth explorationof this signal is outside the scope of this thesis due to time constraints. In any case, this signal and itsimplications warrant further investigation once confirmed by measuring with an intensity calibratedspectrometer.5.4.2 The molecular luminescence, or lack thereofDetectable molecular luminescence in a tunnel junction is a complex interplay of a number of fac-tors, and there remains some debate about the specifics. Buker and Kirczenow along with Chen etal. are responsible for some of the most in-depth theory surrounding the phenomenon of molecularluminescence in a STM junction [112], [74] [113]. Buker and Kirczenow took a semi-classical ap-proach to the problem, and Chen et al. built on their results to introduce a full quantum theory of95molecular electroluminescence in STM junctions. In addition, Hoffmann et al. showed that the spe-cific geometry of the tip can have a significant affect on the emission spectrum, both in the shape ofthe emission peak and the location in energy of the peak [103]. Aizpurua et al. showed that increas-ing the tip-sample distance results in an overall blue shift of the emission, while the spectral shaperemains unaffected. They also note that the overall aperture (sharpness) of the tip is responsible forthe rough position of the emission peak [104].Figure 5.15: Schematic indicating the basic STML mechanism as described by Buker andKirczenow. Adapted from [5].Because the Ag(111) resonance was often seen to be between 500-600 nm, the tip-sample distanceeffect could play a role in pushing the resonance out of our experimentally accessible window. Forexample, the Ag resonance was often measured at 200 pA over bare Ag. The tip was then movedover a molecule on (2 or 3 ML) NaCl, at the same current. The tip would then be farther from theunderlying substrate, blueshifting the Ag resonance, possibly right out of the range we can detect.The best mitigation for this is to widen the bandwidth of the in-vacuum windows. But this alonedoes not answer why the CuPc and PTCDA did not luminesce even within our allowable wavelengthrange. To consider other reasons, we turn to Buker and Kirczenow along with Chen et al..The basic model is that the detectable luminescence is the product of resonant enhancement fromradiatively-coupled surface plasmons excited from inelastic tunnelling into the underlying substrateand photons due to radiative decay between the LUMO - HOMO of the molecule (or the LUMO+1 -LUMO, etc). Cavity QED indicates that coupling the molecular photons with the plasmon enhancesemission by several orders of magnitude. For this to occur, the LUMO-HOMO5 gap must be insidethe energy window defined between the tip and substrate Fermi levels for a radiative transition to bepossible, see Figure 5.15. In addition, optical transitions between the higher energy orbital and the5or the LUMO+1 - HOMO gap, or the LUMO+1 - LUMO, etc.96lower energy orbital must not be forbidden. A key aspect of Buker and Kirczenow’s theory is thatthe energy levels of the molecules are not pinned to their values at zero bias; they shift as a bias isapplied. To calculate this shift, Buker and Kirczenow assumed that the total charge of the moleculestays constant as a bias is applied, which necessitates a shifting of the energy levels. As the HOMOshifts upward towards and past Fermi, it becomes partially or wholly unoccupied, which is whatallows a transition into that orbital6.Buker and Kirczenow predicted that the photon emission rate is very sensitive to coupling asym-metries between the molecule and tip and the molecule and underlying substrate. This is predictedto lead to significant quenching of the luminescence if there is asymmetric coupling between thesubstrate and molecule vs the molecule and tip. Consider for example that the molecule is morestrongly coupled to the substrate than the tip. In this case, the electrons incoming from the tip havea relatively low probability for entering the molecule and a high probability for exiting into the sub-strate. In this case, there will be a relatively large number of surface plasmons excited but relativelyfew photons to couple to. One needs a balance of electrons entering into the substrate, excitingplasmons, and entering into the molecule, decaying radiatively from the LUMO to the HOMO, toget the kind resonant enhancement which is necessary for successful luminescence detection.The asymmetrical coupling may not be the culprit for our lack of luminescence since we exploredboth bilayers and trilayers of NaCl, but the exact coupling strengths remain unknown so we can’t sayfor sure either way. Conversely, our ‘low’ tunnelling current of 200 pA could amount to moleculeswhich are more coupled to the substrate than the tip7. What seems more likely is that the radiativetransition from LUMO - HOMO or LUMO+1 - HOMO was not well-matched to the plasmon reso-nance. If this is the case, then there will be no resonant enhancement effect, and the luminescencewhich is produced is at such a low level that it is undetectable.We have one clue from the PMT images: the substrate emission was not seen when tunnelling intothe molecules. This implies that the asymmetric coupling may not be the problem: If that werethe case we would likely see the substrate emission, and not a ‘darkness’ where the molecules lie.This is because the electron has a high probability of entering directly into the substrate and ex-citing a plasmon which would resonate with the nanocavity, as shown on bare Ag(111) and bareNaCl(2ML)/Ag(111). Also, if the molecular HOMO-LUMO gap did not lie in the Fermi windowwe would also likely see the substrate, again for the same reason. This leaves us with the plasmonresonance being mismatched to the radiative decay in the molecules, leading to little or no reso-nant enhancement of the fluorescence and an essentially undetectable signal. Rough calculations,6The luminescence found by Qiu et al. was most likely a LUMO+1 - LUMO transition, but Buker and Kirczenow’sdiscussion is centered around LUMO-HOMO transitions.7In fact, the negative charge of PTCDA on bilayer NaCl indicates that there is definitely considerable coupling betweenPTCDA and the underlying substrate.97taking the CuPc transport energy levels as equivalent to the optical energy levels8, and consideringthe LUMO-HOMO gap as well as the LUMO+1-HOMO gap, we find that the luminescence fromCuPc would be either '250nm or '400nm. Neither of these values is close to the 500-600nm res-onances we saw on Ag9. In addition, the NaCl signal may be playing a role - if the NaCl is stronglymodulating the Ag plasmon mode in some way where it can’t couple to radiation emitted from themolecules, we would similarly see a lack of detectable luminescence.A final option for explaining the observed lack of luminescence from the molecules is that themolecules actually are luminescing, and the Ag plasmon is resonating with that luminescence, butoutside of our experimentally accessible window. Note that STM-induced luminescence has beenobserved from thin films of CuPc on Au(111) [114]. The resonance was around 700 nm. Thecombination of thin films and a different substrate would have drastically shifted the energy levelscompared to small clusters on NaCl/Ag(111). In any case, adjusting all the relative energy levelsvia a change in substrate and or molecules will likely solve the problem, and widening the opticallyaccessible range will make future attempts more likely to succeed.5.4.3 Suggestions for future experiments and improvement of the currentexperimental set-upFor future studies, a change in substrate and/or molecules is warranted. Luminescence has beendemonstrated on Au(111) with a plasmon resonance at 500-600nm, which is just inside our acces-sible range. Expanding the experimentally accessible wavelength range by replacing the in-vacuumwindows with ones allowing a broader band of radiation through will open up the selection of po-tential molecules and substrates but will decrease the cryogen hold time, presenting a tradeoff tothe user. The tradeoff can be mitigated by using smaller windows, or by using the same diameterwindows with shutters which can be closed when the system is not being used for optical measure-ments.Another option is to use molecules as spacers instead of NaCl: Dong et al. demonstrated successwith this model using porphyrin molecules on Au(100) (note their resonances were in the range of600 - 700 nm) [5]. When using molecules as spacers, it is easy to explore how the STML changeswith increasing thickness, and once the desired thickness of one molecule (say, PTCDA) is foundthen one can deposit the second molecule (say, CuPc) right on top. Another advantage of thismethod is that this will more closely resemble the interface of a real thin-film organic electronicdevice. Note that instead of plasmon-mediated emission, this approach will result in molecular8The optical gap is likely smaller than the transport gap, which may push the luminescent photons into the range ofthe plasmon resonance, so this can’t be concluded for sure.9As others have found, the peak of the Ag resonance shifts considerably with tip changes.98fluorescence from hot electron injection excitation, which involves radiative decay associated withHOMO - LUMO transitions and Frank Condon transitions from the excited state to the ground state[5], [113].Figure 5.16: STM constant-current image of a PTCDA island grown on Ag(111) with CuPcaround the edges and on top (40x40 nm2, I=5 nA, Vb=0.35 V).An approach to this type of molecular layering was achieved in December of 2015, when we grewlarge PTCDA islands on Ag(111) then added in CuPc; see Figure 5.16.Iterating through growing ever thicker layers of PTCDA and testing the luminescence will be rela-tively easy since the sample won’t need to be cleaned between depositions: the new layers can beadded right on top. Once the PTCDA spacer is optimized, a single deposition of CuPc results inisolated molecules sprinkled on top of the PTCDA islands.99Chapter 6Conclusions and future directionsIn this thesis, we have demonstrated the successful coupling of optical measurements with a scan-ning probe system. New experimental capabilities now include illumination of the tunnel junctionand local detection of the potential excitation by STS and KPFM, tunnelling luminescence detectionby PMT (offering an integrated signal), and tunnelling luminescence detection by optical spectrom-eter (offering a wavelength-resolved signal). The luminescence collected can be imaged at each(x,y) pixel to create either a 2D luminescence map (directly from the PMT signal or from an inte-grated spectrometer signal) or a 3D (x,y,hν) optical grid. In principle, additional modes involvingillumination and tip enhancement of fluorescence or Raman spectroscopy are also possible with theapparatus.The system under study (CuPc and PTCDA clusters on NaCl/Ag(111)) provided no clear signal inSTS measurements for illuminated vs not illuminated molecules. This could be due to poor opticalalignment and/or energy level alignment both of which can be addressed; immediate plans involveSTS and KPFM measurements on illuminated CuPc-PTCDA clusters on NaCl/Au(100). Tunnellingluminescence measurements on molecules showed quenching of the modified plasmon resonanceseen over bare NaCl. Understanding this result, as well as the modified plasmon resonance observedover bare NaCl, will require further investigation.A number of options for future directions are offered below.6.1 Kelvin probe force microscopyThe immediate plan moving forward is to perform KPFM, which can measure local charge distri-butions, on small clusters of CuPc and PTCDA on NaCl/Au(100), both with and without optical100excitation. If this experiment works, we will have achieved real-space imaging of charge transferbetween single molecules. If this occurs, then further exploration of the specific system may bewarranted. In particular, measurements of the luminescence spectra to correlate vibronic levels withcharge transfer would be a logical next step. Because Au(100) may have a redshifted plasmon reso-nance compared to Ag(111), we may have to replace the in-vacuum viewports with windows whichallow a wider band of radiation to pass through. In addition, a tip etching procedure for Au willhave to be optimized, but there are some options in the literature which will provide a good startingpoint (for example [115], [71]). Other studies which do not require changes to the system couldinclude detailed studies of excited state wavefunctions, via simple high-resolution STM imaging, orIETS mapping using d2IdV 2 .6.2 Optical spectroscopy on NaCl/Ag(111)The signal seen on NaCl/Ag(111) is intriguing, and it is regrettable that it could not be exploredfurther in this thesis due to time constraints. A thorough literature review would be the first step.Then, if the theory warrants further measurements, obtaining several 3D (x,y,hν) grids of someNaCl islands on Ag(111)1 as well as some high-resolution point spectroscopy would be very usefulfor untangling the origin of the signal observed.6.3 Large area photomultiplier tube luminescence measurements forfinding luminescent moleculesAs suggested in Section 3.3.2, implementing the photon counting scheme as detailed will enablethe collection of high-quality large area scans which can discern if only a few clusters of moleculesare luminescing. This will allow the experimenter to ‘zoom in’ on those clusters of interest quickly,saving considerable time hunting for clusters which luminesce amongst a forest of those who don’t.In addition, the implementation of photon counting will offer (more direct) quantitative measure-ments2 of the emission for a given system, allowing for direct comparison to theories which areworking towards a better understanding of the origin of STM-induced luminescence.1Of course, any further measurements should be performed with a properly intensity and wavelength calibrated spec-trometer.2The measurements performed with a PMT are more directly quantitative than those performed with a CCD, since theCCD and optical spectrometer introduces additional modifications to the photon count.1016.4 Scanning tunnelling luminescence on supported moleculesObtaining sub-molecularly resolved STML grids is now possible. Finding a combination of sub-strate and molecules which will work given both the experimental constraints and the factors dis-cussed in Section 5.4.2 will require some careful planning as well as some luck. It is possible thatseveral substrate/molecule combinations will need to be tried before one finds success. As discussedin Section 5.4.3, one can also take the approach of using molecules both as spacers and emitters,which may work for the current substrate and molecules already in the system, and would be thequickest option to try next in terms of set-up and debugging time.102Bibliography[1] Christine Vu. IBM Research Alliance Produces Industrys First 7nm Node Test Chips.https://www-03.ibm.com/press/us/en/pressrelease/47301.wss, 2015. [Online; accessed2-June-2016]. → pages ii[2] K A Cochrane, A Schiffrin, T S Roussy, M Capsoni, and S A Burke. Pronouncedpolarization-induced energy level shifts at boundaries of organic semiconductornanostructures. 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This Knudsen cell is designed to be able todeposit a multitude of molecules at a variety of operating temperatures and currents: from 80-600◦Cand 0.7-4.0 A. A photograph of the evaporator without the heat shield and shutter is shown above.The evaporant is placed inside a quartz crucible, in the middle of the assembly. The quartz cell is112Figure A.2: Sophomoric diagram of major evaporator componentswrapped with a tungsten wire which acts as the heater coil, and a thermocouple wire is attached toa button on the glass. The temperature of the cell is controlled manually by passing current throughthe heater coil with a power supply while monitoring the temperature read by the thermocouple.The assembly is supported by two stainless steel rods, screwed into the reducer flange, and twomacor ceramic disks, held by wire to the rods. The current design can be improved further withsome minor modifications which are discussed in the body of this document.A.1 PartsA.1.1 Evaporator body1. Zero Length Reducer Flange, 1 1/3” to 2 3/4’MDC PN: 150001113Qty: 12. Mini Thermocouple and two pin copper feedthroughInsulator Seal (MDC) PN: 9392007 - 50 mil copper rods orInsulator Seal (MDC) PN: 9392015 - 100 mil copper rodsThe 50 mil feedthrough is preferred3. Quartz TubeChemglass, Inc. PN: CGQ-0800-056mm - outer diameter4mm - inner diameterWe obtain our quartz crucibles from the Chemistry glass shop.4. Tungsten Wire0.01 in (10mil). Purchase about 10 ft in to have on handQty: 1 ft5. 316 stainless steel set screw shaft collarsMcMaster-Carr PN 9943K11 - for 50 mil rodsMcMaster-Carr PN 9943K13 - for 100 mil rodsThese will be used to clamp the W wire to the copper feedthroughs, and the TC wire to itsfeedthroughs. You should purchase 10, as they get lost easily. Make sure you purchase 316SS to reduce outgassing when assembly is hot.Qty: 46. Thermocouple wireChromel:i. 10milii. Omega PN: SPCH-010-50ftAlumeli. 10milii. Omega PN: SPAL-010-50ftQty: 6in each7. 316 stainless steel threaded rod1146-32 threadingThe rods will make up the support structure for the evaporator.Qty: Two, 4in eachSuggestion: Better structural support would be provided by standoffs which hold the macorin place. The suggested design for the standoffs is explained in the construction and assemblysections. Materials required would be 316-SS non-threaded rod, approximately 5mm diame-ter for the machinist to use for constructing the standoffs.8. Copper foil - (Oxygen Free) e.g. Alfa Aesar - Stock no: 13380, Lot no: D22N385 mil thick (not important)This will be the thermal radiation shield for the evaporator. It allows for higher achievabletemperatures at lower currents.Qty: 6x6 in2, but you need a large piece to secure properly for waterjetting.9. Macor DisksWe had these machined to our specifications from a macor rod we purchased. The solidworksfiles are available on our internal wiki. Figure 3 below illustrates the general shape. The twodisks used are the same save for the centre hole: in one disk the hole is large enough for thecrucible to pass through, in the other it is not.Macor: McMaster-Carr: part no: 8489K81Macor rod - diameter: 7/8”, 3” length rodQty: one of each style of disk. Of course, get more than 2 of each machined in case any arebroken during assembly.A.1.2 Shutter1. Rotary motion feedthroughMDC Part Number: 67000Mini flange mountableQty: 1115Figure A.3: Drawing for one of the Macor disks. Dimensions are in millimetres. Solidworksfiles are on our internal Wiki.2. Double edge 2 3/4 flange with mini side portMDC Part Number: 140013-2001This is a specialty item that MDC made for us. $200Qty: 13. Stainless steel threaded rodm2 threadQty: 4-6 inBe careful to keep the rod as straight as possible, this will facilitate the assembly. The smalldiameter rod will bend easily.4. Molybdenum sheet5 mil thick (not important)This sheet will be made into the actual shutter. It will be attached to the SS rod.116Qty: 2 in2, but you need a larger piece to secure for waterjetting5. Nutsm2 threadUsed to hold shutter on rodQty: 26. Lock Washersfor m2 threadUsed to hold shutter on rodQty: 27. Set Screws4-40 threadUsed to hold rod in place on the feedthroughQty: 18. Hex screws m3 threadSocket cap hex screws are used to screw down the heat shield tabs. Note these need to bevented.Qty: 2All quantities given are the actual number used in the assembly. More than the minimum should bepurchased and cleaned in case something is dropped or lost. All steel items should be 316 SS, andany copper should be OFC.A.2 ConstructionA.2.1 BaseThe evaporator base consists of a 2 3/4 to mini reducer flange with four blind tapped holes ma-chined on the 2 3/4 side. The holes should be drilled as indicated in Figure 4. Unless you arecomfortable machining stainless steel flanges, this job is left best to the machine shop - especiallytapping of the holes and minding the knife edge on the flange. Two of these holes (m3) are used117Figure A.4: Drawing of the reducer flange with machined holes dimensioned.for mounting the copper heat shield. The stainless steel support rods (or, in an improved version,the standoffs) are screwed into the other two holes (m2.5). The mini port side is used to attach thefilament/thermocouple feedthrough.1. Machine the 2 3/4 to mini reducer flange as indicated in Figure 4.2. Cut the 6-32 SS threaded rod to the desired length to make the two support rods. Note it needsto be vented on the end which screws into the flange.Alternatively, have the machinist fabricate the SS standoffs for supporting the structure.Relevant Solidworks files for the standoffs are on the wiki. Note the standoff screws need tobe vented.3. Clean all pieces thoroughly to ensure they are UHV compatible.A.2.2 Quartz cellThe Chemistry department has a glass working shop which can make nearly any specialized pieceof glass. Write up your desired dimensions appropriately and send them to the shop for fabrication.We added ’buttons’ to the side of the crucible to wrap the thermocouple (TC) around. Wrappingthe TC around a button on the side of the crucible rather than the base of the crucible serves twopurposes. First, it allows for an improved placement of the TC - it can be placed near to the (top)surface of the matter in the crucible, which is ostensibly where the matter will be evaporating from.Second, it reduces the overall length over which the TC is shorted, improving the reading’s accu-racy. Any short along the length of a TC acts as a new junction, and it is the ’last’ short on the line118Figure A.5: Rendering of the base flange with standoffs supporting the macor disks and cru-cible. On the left, the flange has been rendered transparent to help visualize the locationsof the machined holes and other features. On the right, the macor disks have been renderedtransparent to help visualize the standoffs mating with each other.whose temperature you will read. Preventing shorts along the length or at least keeping them veryclose to the location where you would like to read the temperature is optimal. Also, shorts fromwrapping have poor electrical contact and may be unreliable or changeable - which is why using theactual (welded) junction is optimal.Each button used for a TC must have two accompanying buttons, above and below, to act as insula-tors from the filament wire. If you have only a single exposed button with your TC wrapped aroundit, the filament wire can scootch down the crucible and short onto the TC wire. Obviously, this isbad.Don’t forget to clean the crucible so that it is UHV compatible.119A.2.3 Heater wire1. Cut 1 foot of 10 mil tungsten wire.2. Wrap the middle of the wire around a screw driver with a diameter that is slightly smallerthan the diameter of the quartz cell. For 0.24 inch diameter cell use a 0.15 in screwdriver.Wrap the heater wire very tightly, this will ensure the heater wire is tightly wrapped aroundthe cell.3. Make about 10 turns4. Ensure there is at least 2 in of wire left over on either side. You will use this to make contactwith the copper feedthroughs.5. Make sure it is cleaned to be UHV compatible.A.2.4 Thermocouple1. Spot weld the chromel wire to the alumel wire on one of the ends. Sometimes spot weldingcan be difficult with very fine wires, so I used a hydrogen torch to melt the ends together. Thismade a beautiful junction which was covered in an oxide. I removed the oxide with the helpof an optical microscope and a razor blade.2. Before proceeding any further, take the time to test the thermocouple gives you appropriatereadings.3. Clean the thermocouple to be UHV compatible.A.2.5 Radiation shieldThe radiation shield is made out of a thin copper sheet, rolled into a tube as shown in Figure 6. Theshield reflects back the thermal radiation emitted by the hot tungsten filament allowing for moreconcentrated heating. The shield also minimizes radiative heating of nearby chamber walls. Theshield is secured to the flange via two tabs at the base which screw into the flange. The copper sheetwas cut with the waterjet, and a separate lid was also made to reduce the aperture width (allowingfor a smaller shutter). The relevant solidworks files for reproducing this heat shield are on the wiki.1. Use the solidworks files from the wiki to cut the shield and lid pieces using the waterjet. If youdo not know how to use the waterjet, the shield can be cut by hand or you can get someone tohelp you with the waterjet.120Figure A.6: The copper heat shield2. Roll up the copper shield along the long side to make a cylinder. Slide the 5 tabs along theone side through the holes on the other side. Bend the tabs backwards to keep the cylindersnugly closed.3. Bend the tabs on the bottom outwards, carefully as they can snap easily.4. Bend the tabs on the lid piece downwards and slide into the slots at the top of the cylinderfrom the outside (see fig. 5), then flatten on the inside of the cylinder to keep them snug.121Figure A.7: Solidworks drawing of the heat shield. All units are in mm.A.2.6 Shutter1. Contact the machine shop to perform the following step. They must be reminded to be carefulof the knife edge on the part - it is helpful to them if you bring a gasket for them to use tocover the knife edge. On the inner shaft of the rotary motion feedthrough, drill a 4-40 taphole about a 1.5mm away from the edge of the shaft. This will be the hole for the set screw.Perpendicular to this hole, as close to the end of the shaft as possible, drill an m2 tap holethrough the rod. Tap both holes appropriately.2. Cut the shutter rod to the desired length (4-6 in, depending on geometry of the crucible as-sembly).122Figure A.8: The shutter assembly3. Cut the shutter from the molybdenum sheet into the desired shape. You can cut the shutteron the waterjet, relevant solidworks files are on the LairWiki. Otherwise, you can use somescissors and a punch to create the hole where the rod feeds through.4. Clean all parts to make them UHV compatible (including the set screws, lock washers andnuts).5. Attach the threaded rod to the rotary feedthrough (by threading it in!) using the set screw tohold it in place.6. Attach the shutter to the shutter rod by sandwiching it between m2 nuts and lock washers:nut→ lock washer→ shutter→ lock washer→ nutBe sure to tighten the whole assembly well so that it does not loosen with thermal cycling.123Figure A.9: Rendering of the knudsen cell assembly in stages. In the center the heat shieldand flange are made transparent, and on the right the heat shield is removed exposing themacor/crucible/standoff assembly.A.3 Assembly1. Clean all parts to make UHV compatible2. Screw the 316-SS support rods / standoffs tightly into the base.3. Attach the feedthrough with filament and thermocouple leads to the base.4. Use a small alumel wire ’bridge’ across the SS support rods to support a macor disk about1/2” from the base. You should choose the macor piece with the smaller central hole- this oneis meant to support the base of the crucible. You will need to thread the power feedthroughsand the thermocouple feedthroughs through the holes in the disk.Alternatively, use the standoff set-up as shown in figures 5 and 9 to secure the lower (onewith smaller central hole) macor disk in place.5. Wrap the thermocouple wire tightly around a button on the crucible.6. Feed the crucible through the heater wire you constructed earlier. Fix the ends of the heaterwire to the copper feedthroughs using the sleeves and set screws. Position everything such124that the base of the crucible rests in the lower macor disk’s central hole.7. Slide the second macor disk in place, near the top 1/3 of the crucible. This one should havethe centre hole large enough to fit the crucible through. It will be held in place by either theSS rods or the standoffs. See the photo at the beginning of this document.8. You can fill the crucible at this point.9. Cut off any excess power or TC wire to avoid shorts. Check for shorts at the feedthroughsusing a multimeter.10. Slide the heat shield over the whole assembly. Bolt it to the base using the 2 tabs and m3socket head screws. Bolt the heat shield tightly to the base to ensure it is well secured.11. Check for shorts again at the feedthroughs using a multimeter.12. Place a gasket appropriately and slide the shutter assembly over the knudsen cell assembly.13. Place another gasket appropriately and slide the cover flange over the whole assembly.14. Check to make sure the shutter works as desired, and write down the ‘open’ and ‘closed’positions.125Appendix BSupporting materials: laser power studySTS spectra averaged over smallerportions of the moleculeThis appendix shows the STS results obtained when averaging the spectra over smaller subsetsof the molecule. This was done to verify that potentially small and important differences whichmay have been present in only one part of the molecule (such as the copper center of the CuPc)weren’t getting averaged out when averaging over the whole molecule. The smaller portions of themolecule averaged over in this section are: the center of the CuPc molecule, the perimter of theCuPc molecule, and the oxygen end1 of the PTCDA molecule. The results indicate that there are nomajor differences when compared to averaging over the entire molecule; except, for the case of thelaser excitation study, that the amplitude change of the PTCDA 2 V resonance is less pronouncedwhen averaging only over the oxygen end. See Section 4.3.1 for a discussion of the amplitudedifference and its possible relation to vibrational excitation.B.1 MaskingThis section shows where the masks were taken on each molecule for averaging the STS spectraover smaller parts of the molecules.1The PTCDA molecule has two oxygen ends, in this section we are referring to the end which is farthest from theCuPc molecule.126Figure B.1: STM and STS images showing the bounded area over which individual STS spec-tra were averaged for each plot. The image shown is from the 561 nm data set, and shows theareas taken to obtain an average over the oxygen end of the PTCDA molecule.127Figure B.2: STM and STS images showing the bounded area over which individual STS spec-tra were averaged for each plot. The image shown is from the 561 nm data set, and shows theareas taken to obtain an average over the center of the CuPc molecule.128Figure B.3: STM and STS images showing the bounded area over which individual STS spec-tra were averaged for each plot. The image shown is from the 561 nm data set, and shows theareas taken to obtain an average over the perimeter of the CuPc molecule.129B.2 STS results for masking over only part of the molecules: laserexcitation studyThis section shows the averaged STS spectra obtained from averaging over smaller portions of eachmolecule. No relationship of the STS spectra shown here to the applied laser power is apparent.130Figure B.4: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 685 nm laser. Spectra are averaged over the perimeter of the CuPc and the oxygen end ofthe PTCDA.Figure B.5: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 685 nm laser. Spectra are averaged over the centre of the CuPc and the oxygen end of thePTCDA.131Figure B.6: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 640 nm laser. Spectra are averaged over the perimeter of the CuPc and the oxygen end ofthe PTCDA.Figure B.7: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 640 nm laser. Spectra are averaged over the centre of the CuPc and the oxygen end of thePTCDA.132Figure B.8: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 561 nm laser. Spectra are averaged over the perimeter of the CuPc and the oxygen end ofthe PTCDA.Figure B.9: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 561 nm laser. Spectra are averaged over the centre of the CuPc and the oxygen end of thePTCDA.133Figure B.10: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 519 nm laser. Spectra are averaged over the perimeter of the CuPc and the oxygen end ofthe PTCDA.Figure B.11: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 519 nm laser. Spectra are averaged over the centre of the CuPc and the oxygen end of thePTCDA.134Figure B.12: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 488 nm laser. Spectra are averaged over the perimeter of the CuPc and the oxygen end ofthe PTCDA.Figure B.13: Comparison of dI/dV spectra for the XO configuration under illumination fromthe 488 nm laser. Spectra are averaged over the centre of the CuPc and the oxygen end of thePTCDA.135B.3 STS results for masking over only part of the molecules:CuPc-PTCDA on NaCl/Au(100)Figure B.14: Comparison of dI/dV spectra for the XO configuration on NaCl(2ML)/Au(100).Spectra are averaged over the perimeter of the CuPc and the oxygen end of the PTCDA.136Figure B.15: Comparison of dI/dV spectra for the XO configuration on NaCl(2ML)/Au(100).Spectra are averaged over the centre of the CuPc and the oxygen end of the PTCDA.137
Thesis/Dissertation
eng
Engineering Physics
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Coupling spatially and spectrally resolved optical measurements with a scanning probe system
Text