  Photo-Induced Enhanced Raman Spectroscopy (PIERS) is a new surface enhanced
Raman spectroscopy (SERS) modality with an order-of-magnitude Raman signal
enhancement of adsorbed analytes over that of typical SERS substrates. Despite
the impressive PIERS enhancement factors and explosion in recent demonstrations
of its utility, the detailed enhancement mechanism remains undetermined. Using
a range of optical and X-ray spectroscopies, supported by density functional
theory calculations, we elucidate the chemical and atomic-scale mechanism
behind the PIERS enhancement. Stable PIERS substrates with enhancement factors
of 10^6 were fabricated using self-organized hexagonal arrays of TiO2 nanotubes
that were defect-engineered via annealing in inert atmospheres, and silver
nanoparticles were deposited by magnetron sputtering and subsequent thermal
dewetting. We identified the key source of the enhancement of PIERS vs. SERS in
these structures as an increase in the Raman polarizability of the adsorbed
probe molecule upon photo-induced charge transfer. A balance between
crystallinity, which enhances charge transfer due to higher electron mobility
in anatase-rutile heterostructures but decreases visible light absorption, and
oxygen vacancy defects, which increase visible light absorption and
photo-induced electron transfers, was critical to achieve high PIERS
enhancements.
