Chemical functionalization of silicon surfaces is of significant technological interest for surface passivation and applications in biotechnology and photocatalysis. Here we present a spectroscopic study of the molecular orientation at methyl-terminated silicon (111) surface.  By using polarization-selected vibrational sum frequency generation (SFG), we observed the symmetric and asymmetric C-H stretch modes of the methyl.  The methyl stretches show azimuthal anisotropy, namely, the 3-fold symmetry in registry with the signal from the Si (111) substrate, which indicates that the propeller-like rotation of the methyls at room temperature is hindered. On the other hand, different line widths of the asymmetric stretch recorded for different polarization combinations (SPS and PPP) indicate that the rotation does proceed on a 1-2 ps time scale. When IR-visible delay is used for background suppression, a phase-flip of the symmetric vs. asymmetric stretch is observed in the vibrational SFG spectrum due to the difference in their frequencies.

Orientational anisotropy and rotational dynamics of methyl-terminated silicon (111) surface

244th National Meeting (2012)

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