Atomic Surface Structure of CH_3-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 ⟨1̅21̅⟩ and the ⟨011̅⟩ azimuthal directions reveal a hexagonal packing arrangement with a 4.00 ± 0.02 Å lattice constant, indicating a commensurate (1 × 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 Å at the surface has been extracted using variable de Broglie wavelength specular scattering; this measurement agrees well with bulk values from CH_3-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.
© 2015 American Chemical Society. Received: June 14, 2015; Revised: July 6, 2015; Published: July 31, 2015. S.J.S. acknowledges support from the Air Force Office of Scientific Research Grant No. FA9550-10-1-0219, and the Material Research Science and Engineering Center at the University of Chicago, NSF-DMR-14-20709. N.S.L. acknowledges support from the National Science Foundation (CHE-1214152), and the research was in part carried out at the Molecular Materials Research Center of the Beckman Institute of the California Institute of Technology. The authors declare no competing financial interest.