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

Purnim Dhar, Noah T. Plymale, Sergey Malyk, Nathan S Lewis, Alexander V. Benderskii

Research output: Contribution to journalArticle

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Abstract

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.

Original languageEnglish
Pages (from-to)16872-16878
Number of pages7
JournalJournal of Physical Chemistry C
Volume121
Issue number31
DOIs
Publication statusPublished - Aug 10 2017

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Vibrational spectra
Stretching
Anisotropy
anisotropy
Annealing
annealing
Substrates
spacers
baths
Spectroscopy
Vacuum
Polarization
harmonics
vibration
vacuum
polarization
electronics
spectroscopy

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films

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Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces. / Dhar, Purnim; Plymale, Noah T.; Malyk, Sergey; Lewis, Nathan S; Benderskii, Alexander V.

In: Journal of Physical Chemistry C, Vol. 121, No. 31, 10.08.2017, p. 16872-16878.

Research output: Contribution to journalArticle

Dhar, Purnim ; Plymale, Noah T. ; Malyk, Sergey ; Lewis, Nathan S ; Benderskii, Alexander V. / Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces. In: Journal of Physical Chemistry C. 2017 ; Vol. 121, No. 31. pp. 16872-16878.
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abstract = "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.",
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AB - 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.

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