Paper SS2-FrM9
Identifying Adsorbate Structures on Semiconductor Surfaces Using Simulated Scanning Tunneling Microscope Images

Friday, November 13, 2009, 11:00 am, Room N

Simulated Scanning Tunneling Microscopy images, based on first-principles calculations, have been used to characterize adsorption products of organic molecules on semiconductor surfaces and assign molecular structures to specific features in experimental STM images. Three examples are presented: 1) Styrene molecular lines on the H-terminated Si(100)-2x1 surface system exhibit several novel molecular conductance phenomena. First-principles calculations show that the phenyl ring orientation at chain ends are fluxional, favoring structures with the terminal ring arranged perpendicular to the molecular line. Simulated STM images show that the tunneling current depends strongly on the phenyl ring orientation, since it controls the coupling between the charged dangling bond and the styrene π system. The perpendicular orientation shows higher conductivity than the parallel one, increasing the apparent height of the molecule at the end of the row. This is consistent with experimental observations, while the simulated images of the parallel-ring geometry are not. Because such subtle changes in molecular structure control the flow of electrical currents, STM can be used to distinguish these conformations. 2) The bonding configuration of styrene attached to the bare Si(111)-7x7 surface is not known from spectroscopic measurements, though two likely possibilities have been identified. Matching simulated empty-state and filled-state STM images to new experimental observations provides strong evidence that the attachment of styrene to Si(111)-7x7 is by a [4+2] cycloaddition, involving both the external –CH=CH₂ and a C=C inside the phenyl ring. A comparison experimental images to theoretical predictions of the bias dependence for the two binding structures is critical to this identification. 3) Three unsaturated organic molecules – styrene, phenyl acetylene, and benzaldehyde – were attached to the H-terminated Si(111) surface through analogous radical chain reactions. Both simulated and experimental STM images of the three molecules show significant differences in apparent height, despite small differences in physical height. Thus STM is sensitive to the functional group used to link the molecule to the surface. The simulations also suggest a new scanning protocol that can enhance the contrast among molecules. The so-called “local constant height” STM images, which probe the spatial variation of conductance, show distinctive features for the three molecules that can be used to tell the molecules apart very easily.