Molecular surface science of organic monolayers

Abstract

The molecular structure of organic monolayers adsorbed on transition metal surfaces at low pressures can be studied by a combination of surface science techniques that include low energy electron diffraction (LEED) - surface crystallography, high resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). Most of the studies focussed on small alkenes and aromatic molecules that were adsorbed on flat, low Miller Index metal surfaces. The investigations were carried out in a wide temperature range and at various coverages. The results of these studies revealed that the surface chemical bonds of these molecules are very similar to those found in organometallic clusters. There is also temperature dependent rearrangements of the molecules leading to sequential bond breaking and to the formation of stable fragments at elevated temperatures. Adsorbate induced restructuring of the metal surfaces has also been observed. Co-adsorption of two types of molecules can induce two-dimensional ordering if one of the adsorbates is a donor and the other is an acceptor. Co-adsorption can also modify the nature of bonding of the adsorbate to the metal by charge transfer through the metal. The reaction intermediates during catalyzed hydrocarbon reactions are different from the stable organic adsorbates that bind strongly to the metal surface. Nevertheless, these tenaceous overlayers play significant roles in catalysis for hydrogen transfer and for providing sites for desorption. They also have significant roles in lubrication. New surface science techniques can interrogate the solid-liquid and solid-solid interfaces as well. These include second harmonic and sum frequency generation that uses nonlinear laser optics and the scanning tunneling and atomic force microscopes. Studies of organic monolayers using these techniques will also be discussed. © 1988, Walter de Gruyter. All rights reserved.