A Density Functional Theoretical Study on the Charge-Transfer Enhancement in Surface-Enhanced Raman Scattering

Area of Science:

• Computational Chemistry
• Surface Science
• Spectroscopy

Background:

• Surface-enhanced Raman scattering (SERS) is a powerful technique for molecular detection.
• Chemical enhancement mechanisms, including ground-state charge transfer (GSCT) and photon-driven charge transfer (PDCT), significantly contribute to SERS.
• Understanding these mechanisms is crucial for optimizing SERS sensitivity.

Purpose of the Study:

• To investigate the chemical enhancement in SERS using density functional theory (DFT).
• To evaluate the contributions of GSCT and PDCT to the overall SERS enhancement factor.
• To explore how molecular modifications affect charge transfer processes and SERS signals.

Main Methods:

• Density functional theory (DFT) calculations were employed.
• Para-substituted thiophenol derivatives adsorbed on silver and gold surfaces were used as model systems.
• Off-resonance and pre-resonance SERS spectra were simulated to determine chemical enhancement factors.

Main Results:

• The GSCT enhancement factor (EF_GSCT) exhibits a near-linear correlation with (ω_TP /ω_M-TP)⁴.
• EF_GSCT depends on the HOMO-LUMO gap of the free molecule and the molecule-metal energy difference.
• The PDCT enhancement factor (EF_PDCT) is influenced by the incident light energy relative to the charge transfer excitation energy, showing an initial increase followed by a decrease.

Conclusions:

• DFT calculations provide insights into the molecular-level mechanisms of SERS chemical enhancement.
• The study quantifies the relationship between molecular properties, charge transfer, and SERS enhancement.
• Findings can guide the design of molecules and substrates for enhanced SERS performance.