Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces

Vibrational sum-frequency generation (VSFG) spectroscopy was used to investigate the orientation and azimuthal anisotropy of the C-H stretching modes for propynyl-terminated Si(111) surfaces, Si - C=C - CH₃. VSFG spectra revealed symmetric and asymmetric C-H stretching modes in addition to a Fermi resonance mode resulting from the interaction of the asymmetric C-H bending overtone with the symmetric C-H stretching vibration. The polarization dependence of the C-H stretching modes was consistent with the propynyl groups oriented such that the Si - C=C- bond is normal to the Si(111) surface. The azimuthal angle dependence of the resonant C-H stretching amplitude revealed no rotational anisotropy for the symmetric C-H stretching mode and a 3-fold rotational anisotropy for the asymmetric C-H stretching mode in registry with the 3-fold symmetric Si(111) substrate. The results are consistent with the expectation that the C-H stretching modes of a -CH₃ group are decoupled from the Si substrate due to a -C=C- spacer. In contrast, the methyl-terminated Si(111) surface, Si-CH₃, was previously reported to have pronounced vibronic coupling of the methyl stretch modes to the electronic bath of bulk Si. Vacuum-annealing of propynyl-terminated Si(111) resulted in increased 3-fold azimuthal anisotropy for the symmetric stretch, suggesting that removal of propynyl groups from the surface upon annealing allowed the remaining propynyl groups to tilt away from the surface normal into one of three preferred directions toward the vacated neighbor sites.