Theoretical study on the adsorption of organic compounds on semiconductor surfaces

Abstract

1. Atomic and electronic structure of pyrrole on Ge(100): The adsorption structures of pyrrole $(C_4H_5N)$ on a Ge(100) surface at various coverages have been investigated using ab initio density functional theory (DFT) calculations. The most stable configuration is that the adsorbed pyrrole molecules with H dissociated form bridges between two down-Ge atoms of neighboring Ge dimer rows through N-Ge bonding and β-carbon-Ge interaction. The simulated image from this optimized geometry shows the flower-like feature with a dark side, which is consistent with the most dominant feature observed in the STM images. On the other hand, a similar configuration to the most stable one, but with a dissociated H out of the region of interest, can explain the experimental images having the flower-like feature without a dark side. Finally, a C- and N-end-on configuration, where pyrrole molecule is located on one dimer row, may be identified as bright protrusions in the STM images. 2. Theoretical study of acetic acid adsorption on Ge(100): We have performed ab initio pseudopotential density-functional calculations in order to investigate the atomic and electronic structure of acetic acid adsorbed on a Ge(100) surface. Due to its acidity, acetic acid dissociates and the resulting electron-rich acetate group reacts with the electron-deficient down Ge atom through nucleophilic addition (mono-dentate structure). Further reaction between electron-rich up-Ge atom and carboxylic oxygen atom results in bidentate bridged structures, which are energetically most favorable among tested geometries. The existence of the dissociated H atom on the neighboring Ge atoms and the buckling of Ge dimers are found to affect the feasibility of the formation of bidentate structures. Simulated STM images from the optimized mono-dentate or end-bridged bidentate explain experimental observations well: Multiple attachment of acetate on the Ge surface shows different STM features depending upon the ...