Critical Interplay of Defect Engineering and Plasmonics in Hybrid Nanostructures for Ultrasensitive Photo-Enhanced Raman Spectroscopy

Abstract

Photoinduced enhanced Raman spectroscopy (PIERS) selectively amplifies the chemical enhancement mechanism of surface-enhanced Raman spectroscopy (SERS) by leveraging the electron dynamics across the metal-semiconductor junction upon light irradiation. Previous studies have demonstrated the critical role of oxygen vacancies in facilitating the PIERS mechanism, particularly in semiconductor substrates with pre-existing defect states. However, other pathways of electron transfer across the Schottky-type junction can also be triggered by light and the inherent plasmonic properties of the metal nanoparticles. In this work, we hypothesized that by synthesizing TiO2 nanomaterials with minimal intrinsic defects, the exact mechanism of electron dynamics can be elucidated. Specifically, we aim to determine whether the PIERS effect is a universal response of TiO2 to UV irradiation or whether it fundamentally requires the presence of intrinsic vacancy defects to facilitate charge transfer and surface enhancement. To validate this, we systematically investigated three distinct TiO₂ nanomaterial samples, where two samples exhibit intrinsic defect states and one nearly defect-free or buried defect states. The sol-gel technique resulted in the creation of intrinsic defect states, whereas an advanced ultrasonic spray pyrolysis technique (USP) was utilized to prepare nearly defect-free TiO₂ nanoparticles. Optical and electronic spectroscopic measurements revealed that only defect-rich TiO₂ nanomaterials effectively promote vibronic coupling at the metal-semiconductor interface, whereas the defect-free TiO₂ nanomaterial fails to support PIERS enhancement. These results underscore the fundamental role of pre-existing oxygen vacancies in optimizing TiO₂-based PIERS nanomaterials and provide valuable insights into defect engineering strategies for advanced Raman spectroscopy-based sensing applications.