Molecular electronics offer the promise of continued scaling of devices to smaller dimensions. To integrate molecular devices into conventional electronics, the molecules must be bound to a substrate. While much work has been done toward the understanding of conduction and switching mechanisms in single molecules, the interface between molecule and substrate is much less understood. This work focuses on the interface of small aromatic molecules on Si (111)-7x7 surfaces. Most molecular electronics systems are based on organic molecules with large pi-conjugated systems. This work focuses on three smaller molecules with similar structures: phenylacetylene, styrene and biphenyl-bisacetylene.

X-ray and UV photoelectron spectroscopy (XPS, UPS), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations have been used to probe the molecule/silicon interface. Chemical attachment of the three molecules on the Si surface was verified using the three techniques. Comparison of the valence structure as measured by UPS with the theoretical density of states calculated by DFT demonstrated the binding geometry of the molecules. Phenylacetylene and biphenyl-bisacetylene were found to bind in a “[2+2] cycloaddition” mode, binding at the terminal acetylene in both cases. Styrene, though, was found to bind in a [4+2] mode; binding at the terminal C of the vinyl group, as well as a member of the phenyl ring. This binding mode breaks the aromaticity of the phenyl molecule and the pi-conjugation length does not extend to the surface.

Alignment between highest occupied molecular orbital (HOMO) and silicon valence band, as well as work function measurements were also provided by UPS. These measurements showed a small barrier between silicon valence band and molecular HOMO for all three molecules. However, from the work function measurements an interfacial dipole was found for the phenylacetylene molecules, but not for the styrene. It is hypothesized that charge transfer occurs in the phenylacetylene cases as the pi-system extends to the surface, allowing an efficient route for charge transfer; this does not hold for styrene. This work demonstrates the role of the chemical bond in charge transfer at the molecule/silicon interface.