Enhanced stability and electronic structure of phenylacetylene lines on the Si(100)-2x1:H surface

Abstract

A recent scanning tunneling microscopy (STM) study (J. Am. Chem. Soc. 2010, 132, 3013) reported the self-assembled growth of phenylacetylene (PA) molecular lines on the H-terminated Si(100)-(2 x 1) surface. Using first-principles density functional calculations within the generalized-gradient approximation, we investigated the proposed chain reaction pathway, thermodynamic stability, and electronic structure of the PA line. We found that the growth of the PA line with sp(2) hybridization at the surface is kinetically more facile than that of the styrene line with sp(3) hybridization, indicating that self-assembled growth of molecular lines employing alkynes rather than alkenes will be kinetically facilitated. We also found that the thermodynamic stability of the PA line is significantly enhanced compared to that of the styrene line. Analysis of the local density of states (LDOS) shows that, in the PA line, pi electrons originating from the terminal C=C bond and the benzene ring are well delocalized across the two groups, whereas in the styrene line, such extended pi conjugation is not preserved. These contrasting electronic structures of the PA and styrene lines lead to reproduce the different features observed in the STM images (simulated at a sample bias of -2.0 V): specifically, the PA line appears brighter than the styrene line. However, we predict a dramatic change of the STM images with further increasing the bias voltage downward from the Fermi level, showing that styrene becomes brighter than PA.