ENHANCING STM IMAGES WITH OPTICAL TECHNIQUES

Experimental efforts will be aimed at improving techniques for using scanning tunneling microscopy (STM) to image molecules which can be absorbed on or bonded to surfaces. In particular, methods will be tested and developed for enhancing the information content available from a molecular STM image by using light "naturally" emitted from the molecule during the scanning process. Improved imaging of biologically important adsorbates is expected to have a significant impact on efforts to relate the structure of large biomolecules and biomolecular assemblies to their function in animal systems. The main thrust of this program is the collection and spectral analysis of light (luminescence) emanating from the tunneling junction of an operating scanning tunneling microscope. Excitation of the optically emitting adsorbate and surface states is brought about by the inelastic scattering of a small number of the STM electrons. The scattering process is enhanced when localized tip-induced plasmons interact with the tunneling electrons. High spatial resolution is maintained by monitoring the intensity and frequency of the emitted light as a function of the tip position. This optical/STM technique will be developed with a number of prototype molecular systems. These include organic and inorganic molecules, such s metal porphyrins and phthalocyanines evaporated onto suitable substrates, and self-assembled monolayers of long chain alkyl thiol molecules terminated with a variety of photoactive functional groups such as ferrocene moieties. In addition, this wedding of optical and STM techniques could easily be extended to the study of Langmuir-Blodgett monolayer and bilayer lipid systems which mimic biological membranes.