Atomic Surface Structure of CH3-Ge(111) Characterized by Helium Atom Diffraction and Density Functional Theory

Zachary M. Hund, Kevin J. Nihill, Davide Campi, Keith T. Wong, Nathan S Lewis, M. Bernasconi, G. Benedek, S. J. Sibener

Research output: Contribution to journalArticle

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Abstract

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 "1I¯21I¯" and the "011I¯" 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 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.

Original languageEnglish
Pages (from-to)18458-18466
Number of pages9
JournalJournal of Physical Chemistry C
Volume119
Issue number32
DOIs
Publication statusPublished - Jul 31 2015

Fingerprint

Helium
helium atoms
Surface structure
Density functional theory
Diffraction
density functional theory
Atoms
diffraction
Scattering
Germanium
Semiconducting organic compounds
de Broglie wavelengths
Discrete Fourier transforms
Diffraction patterns
Lattice constants
Electronic structure
organic semiconductors
Chemical activation
scattering
germanium

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Electronic, Optical and Magnetic Materials
  • Surfaces, Coatings and Films
  • Energy(all)

Cite this

Atomic Surface Structure of CH3-Ge(111) Characterized by Helium Atom Diffraction and Density Functional Theory. / Hund, Zachary M.; Nihill, Kevin J.; Campi, Davide; Wong, Keith T.; Lewis, Nathan S; Bernasconi, M.; Benedek, G.; Sibener, S. J.

In: Journal of Physical Chemistry C, Vol. 119, No. 32, 31.07.2015, p. 18458-18466.

Research output: Contribution to journalArticle

Hund, ZM, Nihill, KJ, Campi, D, Wong, KT, Lewis, NS, Bernasconi, M, Benedek, G & Sibener, SJ 2015, 'Atomic Surface Structure of CH3-Ge(111) Characterized by Helium Atom Diffraction and Density Functional Theory', Journal of Physical Chemistry C, vol. 119, no. 32, pp. 18458-18466. https://doi.org/10.1021/acs.jpcc.5b05678
Hund, Zachary M. ; Nihill, Kevin J. ; Campi, Davide ; Wong, Keith T. ; Lewis, Nathan S ; Bernasconi, M. ; Benedek, G. ; Sibener, S. J. / Atomic Surface Structure of CH3-Ge(111) Characterized by Helium Atom Diffraction and Density Functional Theory. In: Journal of Physical Chemistry C. 2015 ; Vol. 119, No. 32. pp. 18458-18466.
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abstract = "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 {"}1I¯21I¯{"} and the {"}011I¯{"} azimuthal directions reveal a hexagonal packing arrangement with a 4.00 ± 0.02 {\AA} 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 {\AA} 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.",
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AU - Wong, Keith T.

AU - Lewis, Nathan S

AU - Bernasconi, M.

AU - Benedek, G.

AU - Sibener, S. J.

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AB - 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 "1I¯21I¯" and the "011I¯" 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 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.

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