The interaction of organic adsorbate vibrations with substrate lattice waves in methyl-Si(111)-(1 × 1)

Abstract

A combined helium atom scattering and density functional perturbation theory study has been performed to elucidate the surface phonon dispersion relations for both the CH3-Si(111)-(1 × 1) and CD3-Si(111)-(1 × 1) surfaces. The combination of experimental and theoretical methods has allowed characterization of the interactions between the low energy vibrations of the adsorbate and the lattice waves of the underlying substrate, as well as characterization of the interactions between neighboring methyl groups, across the entire wavevector resolved vibrational energy spectrum of each system. The Rayleigh wave was found to hybridize with the surface rocking libration near the surface Brillouin zone edge at both the ${\rm \bar M}$M¯-point and ${\rm \bar K}$K¯-point. The calculations indicated that the range of possible energies for the potential barrier to the methyl rotation about the Si-C axis is sufficient to prevent the free rotation of the methyl groups at a room temperature interface. The density functional perturbation theory calculations revealed several other surface phonons that experienced mode-splitting arising from the mutual interaction of adjacent methyl groups. The theory identified a Lucas pair that exists just below the silicon optical bands. For both the CH3- and CD3-terminated Si(111) surfaces, the deformations of the methyl groups were examined and compared to previous experimental and theoretical work on the nature of the surface vibrations. The calculations indicated a splitting of the asymmetric deformation of the methyl group near the zone edges due to steric interactions of adjacent methyl groups. The observed shifts in vibrational energies of the -CD3 groups were consistent with the expected effect of isotopic substitution in this system.

Experimental and theoretical study of rotationally inelastic diffraction of H2(D2) from methyl-terminated Si(111)

“…Slight disagreement between theory and experiment is observed at higher beam energies, which is likely due to the strong corrugation of this system, as well as the use of a frozen-surface model for the PES, which becomes a less reasonable approximation when the energy of incident D 2 molecules nears the rotational barrier of the methyl group (∼100 meV for a surface temperature of 0 K). 14 Having established the accuracy of the PES, a quasiclassical dynamics analysis can be performed on the regions of the PES that determine the characteristics of the diffraction spectra. To establish the validity of the classical analysis, quantum and classical total elastic and rotational excitation probabilities have been compared.…”

“…This interface has been thoroughly characterized by many experimental and theoretical techniques: the surface structure and extent of methyl termination have been surveyed by elastic helium atom scattering, [13][14][15] scanning tunneling microscopy, 16,17 and synchrotron-based x-ray photoelectron spectroscopy (XPS), 18 and the vibrational dynamics were studied via high-resolution electron energy loss spectroscopy, 19 transmission infrared spectroscopy, 20,21 and inelastic helium atom scattering in conjunction with density functional perturbation theory. 14,15 This study employs rotationally inelastic diffraction and accompanying scattering calculations to further explore the surface charge density of CH 3 -Si(111) and probe the interaction potential with H 2 , broadening the understanding of the anisotropic features of this system. This potential anisotropy can serve a fundamental role in dissociative chemisorption for thin-film systems.…”

Fast resolution change in neutral helium atom microscopy

“…Particularly helium has been used extensively in studies of surface diffraction and dynamics. [1][2][3][4][5] Due to helium's excellent properties, inertness and low energy (typically less than 0.1 eV) research has been ongoing in developing a new microscope using neutral helium atoms for imaging. [6][7][8][9][10][11][12][13][14][15][16] Currently the helium microscope exists in two configurations: In the pinhole microscope, the beam is collimated using a small pinhole and is scanned across the surface.…”

Vibrational dynamics and band structure of methyl-terminated Ge(111)

“…13,14 Additionally, methylated germanane (a single-layer crystal of germanium terminated with hydrogen) has recently been synthesized, and X-ray diffraction data indicate a hexagonal spacing with a lattice constant of a = 3.96 Å. 15 Despite extensive structural characterization, the dynamics of CH 3 -Ge(111) remain largely unexplored, whereas the dynamics of a surface analog, CH 3 -Si(111), have been thoroughly studied both experimentally [16][17][18][19][20][21] and theoretically. 19,20,22 We describe herein the comprehensive characterization of the dynamical properties of CH 3 -Ge(111) to provide a foundation for understanding the thermal properties, energy accommodation at the interface, and vibrational band structure of such surfaces.…”

“…15 Despite extensive structural characterization, the dynamics of CH 3 -Ge(111) remain largely unexplored, whereas the dynamics of a surface analog, CH 3 -Si(111), have been thoroughly studied both experimentally [16][17][18][19][20][21] and theoretically. 19,20,22 We describe herein the comprehensive characterization of the dynamical properties of CH 3 -Ge(111) to provide a foundation for understanding the thermal properties, energy accommodation at the interface, and vibrational band structure of such surfaces. Specifically, a combined scattering and theoretical study has been performed to characterize the vibrational dynamics and phonon band structure of this organicfunctionalized Ge(111) interface.…”