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Published September 19, 2019 | Supplemental Material
Journal Article Open

Vibrational Sum Frequency Generation (VSFG) Spectroscopy Measurement of the Rotational Barrier of Methyl Groups on Methyl-Terminated Silicon(111) Surfaces

Abstract

The methyl-terminated Si(111) surface possesses a 3-fold in-plane symmetry, with the methyl groups oriented perpendicular to the substrate. The propeller-like rotation of the methyl groups is hindered at room temperature and proceeds via 120° jumps between three isoenergetic minima in registry with the crystalline Si substrate. We have used line-shape analysis of polarization-selected vibrational sum frequency generation spectroscopy to determine the rotational relaxation rate of the surface methyl groups and have measured the temperature dependence of the relaxation rate between 20 and 120 °C. By fitting the measured rate to an Arrhenius dependence, we extracted an activation energy (the rotational barrier) of 830 ± 360 cm^(–1) and an attempt frequency of (2.9 ± 4.2) × 10^(13) s^(–1) for the methyl rotation process. Comparison with the harmonic frequency of a methyl group in a 3-fold cosine potential suggests that the hindered rotation occurs via uncorrelated jumps of single methyl groups rather than concerted gear-like rotation.

Additional Information

© 2019 American Chemical Society. Received: May 23, 2019; Accepted: August 23, 2019; Published: August 23, 2019. This research was supported by AFOSR Grant No. FA9550-15-1-0184. N.T.P. and N.S.L. acknowledge support from the National Science Foundation Grant No. CHE-1808599. D.B. acknowledges support from the Burg Teaching Fellowship from Anton Burg Foundation. The authors declare no competing financial interest.

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August 19, 2023
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