Temperature-Dependent Surface Structural Change in Self-Assembled Monolayers Studied with Vibrational Sum-Frequency Generation and QM/MD Simulation

Self-assembled monolayers (SAMs) play an essential role in surface engineering and nanotechnology, modifying interfacial properties critical for applications such as area-selective atomic layer deposition (AS-ALD) and the stabilization of perovskite solar cells. Vibrational sum-frequency generation (VSFG) has emerged as a powerful tool for probing interfacial phenomena at the molecular level. Advancing these technologies requires a detailed understanding of molecular-scale influences on the interfacial optical responses. Notably, the orientation of terminal methyl groups in alkanethiolate SAMs has a strong impact on the VSFG spectra under varying thermal conditions. In this study, molecular dynamics (MD) simulations were employed to determine the distributions of polar and azimuthal tilt angles of terminal CH₃ groups. These orientation distributions were combined with single-molecule density functional theory (DFT) calculations of angle-dependent nonlinear susceptibilities (χ⁽²⁾) for symmetric and asymmetric CH₃ stretching modes. An angle-weighted integration of these results enabled the quantitative simulation of temperature-dependent VSFG spectra. Results reveal that temperature-induced orientational changes affect the symmetric and asymmetric modes differently, revealing a mode-specific sensitivity to orientational disorder under specific polarization conditions. This framework provides a solid foundation for interpreting interfacial VSFG responses by establishing a direct relationship between molecular orientation and vibrational mode-specific spectral features and supports molecular-level design strategies for SAM-based interfacial layers in surface engineering applications.