Probing alkylsilane molecular structure on amorphous silica surfaces by sum frequency generation vibrational...

Area of Science:

• Surface science
• Computational chemistry
• Spectroscopy

Background:

• Sum Frequency Generation (SFG) spectroscopy is a powerful technique for studying surfaces and interfaces.
• Understanding the vibrational spectra of organic monolayers on silica is crucial for applications in materials science and nanotechnology.
• Simulating SFG spectra provides insights into molecular orientation and dynamics at interfaces.

Purpose of the Study:

• To simulate and analyze the SFG signatures of octadecyl-trichlorosilane (OTS) and dodecyl-dimethyl-chlorosilane (DDCS) monolayers on silica.
• To investigate the influence of Si-linked methyl groups in DDCS on its SFG spectra.
• To elucidate the origin of SFG signatures using theoretical models and compare simulation results with experimental data.

Main Methods:

• First principles calculations using the ONIOM approach to determine molecular properties (vibrational frequencies, IR/Raman intensities).
• Application of a three-layer model to calculate macroscopic SFG responses.
• Utilizing two chemical models: an adsorbed-model (including SiO2 surface) and an isolated-model (silane chain only).

Main Results:

• Simulations accurately reproduced experimental SFG spectra for OTS monolayers (ppp and sps configurations) with a refractive index of 1.1.
• Both OTS and DDCS showed dominant symmetric CH3 stretching vibrations (r-) over asymmetric ones (r+) in ppp and sps configurations.
• A distinct shoulder in the DDCS r- peak, attributed to Si-linked methyl groups, was identified and disappeared upon freezing these groups.

Conclusions:

• The study successfully simulated SFG spectra, highlighting the impact of molecular structure on interfacial vibrational responses.
• The presence of Si-linked methyl groups in DDCS introduces a unique spectral feature.
• Theoretical simulations provide valuable insights into the interpretation of experimental SFG data for organic monolayers on silica.