Chemisorption of organic adsorbates on silicon and gold studied by scanning tunnelling microscopy

The scanning tunnelling microscope (STM) has been used to study two quite different systems. First, the unsaturated hydrocarbons ethene and ethyne, chemisorbed on a silicon substrate, and secondly, a monolayer of 4,4′-bipyridyl disulfide (PySSPy), chemisorbed on a gold single-crystal sphere, in pure water. The images of the hydrocarbons on silicon showed, at low coverages, random clusters of adsorbates, but at higher coverages, approaching saturation the molecules prefer to be adsorbed on alternate dimer sites. The in situ STM study of the PySSPy on gold, revealed that the monolayer exhibits a marked molecular alignment with specific sites on the gold surface. The periodic unidirectional array showed a separation of 46 ± 3 A with a corrugation of ca. 1.5 A.