Photoinduced enhanced
Raman spectroscopy (PIERS) is a new surface
enhanced Raman spectroscopy (SERS) modality with a 680% Raman signal
enhancement of adsorbed analytes over that of SERS. Despite the explosion
in recent demonstrations, the PIERS mechanism remains undetermined.
Using X-ray and time-resolved optical spectroscopies, electron microscopy,
cyclic voltammetry, and density functional theory simulations, we
elucidate the atomic-scale mechanism behind PIERS. Stable PIERS substrates
were fabricated using self-organized arrays of TiO2nanotubes
with controlled oxygen vacancy doping and size-controlled silver nanoparticles.
The key source of PIERS vs SERS enhancement is an increase in the
Raman polarizability of the adsorbed analyte upon photoinduced charge
transfer. A balance between improved crystallinity, which enhances
charge transfer due to higher electron mobility but decreases light
absorption, and increased oxygen vacancy defect concentration, which
increases light absorption, is critical. This work enables the rational
design of PIERS substrates for sensing.