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

Dhritiman Bhattacharyya, Angelo Montenegro, Noah T. Plymale, Chayan Dutta, Nathan S. Lewis, Alexander V. Benderskii

Research output: Contribution to journalArticle


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) × 1013 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.

Original languageEnglish
Pages (from-to)5434-5439
Number of pages6
JournalJournal of Physical Chemistry Letters
Issue number18
Publication statusPublished - Sep 19 2019


ASJC Scopus subject areas

  • Materials Science(all)
  • Physical and Theoretical Chemistry

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