Critical interplay of defect engineering and plasmonics in hybrid nanostructures for ultrasensitive photo-enhanced...

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 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 exhibits 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 nanomaterials fail 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.