Adsorption sites and STM images of C2H2 on Si(1 0 0): a first-principles study

We investigate adsorption energetics and scanning tunnelling microscopy (STM) imaging of acetylene on Si(1 0 0)-(2×1) by first principles simulations. The groundstate of chemisorption is the di-σ configuration. We found two adsorption sites: one on top of a single dimer (adsorption energy 2.97 eV), and one bridging the ends of two dimers (2.87 eV). There are also two fourfold bonded configurations. The molecule is either perpendicular (2.00 eV) or parallel (1.20 eV) to adjacent dimers. STM images are simulated with a first principles method based on the Bardeen integral. We show that the molecule is imaged as a depression and confirm that the contour difference in our simulations between reacted and unreacted sites is equal to experimental values.