Atomic Surface Structure Of Ch3-Ge(111) Characterized By Helium Atom Diffraction And Density Functional Theory

The atomic-scale surface structure of methyl-terminated germanium (111) has been characterized by using a combination of helium atom scattering and density functional theory. High-resolution helium diffraction patterns taken along both the <(1) over bar2 (1) over bar > and the < 01 (1) over bar > azimuthal directions reveal a hexagonal packing arrangement with a 4.00 +/- 0.02 angstrom lattice constant, indicating a commensurate (1 X 1) methyl termination of the primitive Ge(111) surface. Taking advantage of Bragg and anti-Bragg diffraction conditions, a step height of 3.28 +/- 0.02 angstrom at the surface has been extracted using variable de Broglie wavelength specular scattering; this measurement agrees well with bulk values from CH3-Ge(111) electronic structure calculations reported herein. Density functional theory showed that methyl termination of the Ge(111) surface induces a mild inward relaxation of 1.66% and 0.60% from bulk values for the first and second Ge Ge bilayer spacings, respectively. The DFT-calculated rotational activation barrier of a single methyl group about the Ge C axis on a fixed methyl-terminated Ge(111) surface was found to be approximately 55 meV, as compared to 32 meV for a methyl group on the H-Ge(111) surface, sufficient to hinder the free rotation of the methyl groups on the Ge(111) surface at room temperature. However, accurate MD simulations demonstrate that cooperative motion of neighboring methyl groups allows a fraction of the methyl groups to fully rotate on the picosecond time scale. These experimental data in conjunction with theory provide a quantitative evaluation of the atomic-scale surface structure for this largely unexplored, yet technologically interesting, hybrid organic semiconductor interface.