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Self-assembly of amino acids on noble metal surfaces : morphological, chemical and electronic control of matter at the nanoscale Schiffrin, Agustin


Designing novel nanostructures which exploit the self-assembly capabilities of biomolecules yields a promising approach to control matter at the nanoscale. Here, the homochiral molecular self-assemblies of the methionine and tyrosine amino acids on the monocrystalline Ag(111) and Cu(111) surfaces are characterized by means of scanning tunneling microscopy (STM) and spectroscopy (STS), helium atom scattering (HAS), x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) in ultrahigh vacuum (UHV). On Ag(111), methionine self-assembles into supramolecular chains following the substrate axis, forming regular nanogratings with tunable periodicity. Within the nanowires, a zwitterionic dimerization scheme is revealed. STS shows that the biomolecular nanostructures act as tunable one-dimensional quantum resonators for the surface state electrons. Zero-dimensional electronic confinement is achieved by positioning single iron atoms in the molecular trenches. This shows a novel approach to control the dimensionality of surface state electrons. The nanogratings were exploited to steer the spontaneous one-dimensional ordering of cobalt and iron atoms. For T > 15 K, the metal species self-align into homogeneously distributed chains in between the biomolecular trenches with ~25 Å interatomic distace. For Co, the dynamics of the self-alignment was monitored, revealing a reduced mobility in comparison with isolated Co atoms on bare Ag(111). On Cu(111), the self-assembly of methionine is influenced by the substrate reactivity and its temperature during molecular deposition. For T < 273 K, the biomolecules assemble in anisotropic extended clusters oriented with a -10° rotation off the substrate orientations, whereas above 283 K a regularly ordered 1D phase arises with a +10° rotation off these high-symmetry axis. XPS reveals a structural transformation triggered by a thermally activated deprotonation of the zwitterionic ammonium group. On Ag(111), tyrosine self-assembles above a critical temperature into linear structures primarily following the substrate crystalline symmetry. A zwitterionic non-covalent molecular dimerization is demonstrated, NEXAFS data providing evidence of a non-flat adsorption of the phenyl ring. This recalls the geometrical pattern of methionine on Ag(111) and supports a universal self-assembling scheme for amino acids on close-packed noble metal surfaces, the different mesoscopic ordering being determined by the side chain reactivity.

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