9^th Nano Congress for Next Generation

Biography

Biography: Michael Hietschold

Abstract

The investigation of self-assembled periodic adsorbate structures on crystalline substrate surfaces is a classical topic of surface physics which has been dominated for a long time by diff raction techniques. Th e appearance of scanning probe microscopies – especially scanning tunneling microscopy (STM) – has opened the fascinating opportunity of direct realspace imaging with atomic or submolecular resolution. At the interface between a solution and a crystalline solid, solute (and sometimes also solvent) molecules may deposit in an ordered manner at the solid substrate surface. In-situ studies of the adsorption pattern created this way are possible by ambient STM with the tip immersed in a deposited solution droplet. As an example, trimesic acid (TMA) molecules solved in alkanoic acids may arrange in open adsorption patterns (chicken wire and flower structures) due to H bonding via carboxylic functional groups. At the liquid-solid interface, such type of polymorphism may be controlled by the nature of the solvent (especially its polarity) as well as the concentration of the solutions which opens access to further novel structures. By a controlled increase of molecular packing density of solutions of TMA in alcohols, even a surface-reaction of TMA with coadsorbed solvent molecules (monoester formation with undecanol) has been observed. Recent investigations concerning substrate temperature during deposition and replacement of trimesic acid by the nonplanar benzene-triphosphonic acid will be discussed also. Another approach is based on the self-assembly of molecules at the crystalline surface in ultra-high vacuum (UHV). Under such “ideal conditions” the local electronic structure at the adsorbatesubstrate interface can be studied in detail by scanning tunneling spectroscopy (STS)offering insighte.g.into highly localized donation-backdonation charge transfer processes. We demonstrate some examples for the adsorption of phthalocyanines and porphyrines on metal surface. As an example, shows a temperature-induced polymerization in a monolayer of brominated Cu-Tetraphenylporphyrin on a Au(111) substrate. Such kind of investigations may open a way to better understanding the conditions of structure formation and control which is permanently encountered in the biotic world and which might become extremely fruitful for future engineering of molecular architectures and devices.