Temperature-Dependent Surface Structural Change in Self-Assembled Monolayers Studied with Vibrational Sum-Frequency...

Area of Science:

• Surface science and nanotechnology
• Interface phenomena
• Molecular engineering

Background:

• Self-assembled monolayers (SAMs) are vital for surface engineering, impacting applications like area-selective atomic layer deposition (AS-ALD) and perovskite solar cell stabilization.
• Vibrational sum-frequency generation (VSFG) spectroscopy is a key technique for molecular-level interfacial analysis.
• Understanding molecular orientation's effect on VSFG is essential for advancing surface technologies.

Purpose of the Study:

• To investigate the influence of terminal methyl group orientation in alkanethiolate SAMs on VSFG spectra.
• To quantitatively simulate temperature-dependent VSFG spectra by correlating molecular orientation with optical responses.
• To establish a framework for interpreting VSFG data and guiding molecular design for interfacial layers.

Main Methods:

• Molecular dynamics (MD) simulations to determine polar and azimuthal tilt angles of terminal CH3 groups.
• Density functional theory (DFT) calculations to obtain angle-dependent nonlinear susceptibilities (χ(2)) for CH3 stretching modes.
• Angle-weighted integration of MD and DFT results for quantitative simulation of VSFG spectra.

Main Results:

• Temperature-induced changes in methyl group orientation were quantified.
• Simulated VSFG spectra showed distinct responses for symmetric and asymmetric CH3 stretching modes.
• Mode-specific sensitivity to orientational disorder was observed under specific polarization conditions.

Conclusions:

• A direct relationship between molecular orientation and mode-specific VSFG spectral features was established.
• The study provides a foundation for interpreting VSFG responses at interfaces.
• This work supports the rational design of SAM-based interfacial layers for enhanced surface engineering applications.