Quantum Effects Enter Semiconductor-Based SERS: Multiresonant MoO₃·xH₂O Quantum Dots Enabling Direct, Sensitive SERS...

There is keen research interest in building highly effective semiconductor-based surface-enhanced Raman scattering (SERS) platforms, due to their selectivity for many probe molecules and suitability for complex scenario applications. However, current tuning approaches have not yet been successful in creating semiconductor-based SERS sensors for small inorganic molecules, due to the challenge of creating sufficient SERS enhancement in semiconductors. Here, we demonstrate the use of MoO₃·xH₂O quantum dots (QDs), to achieve direct and sensitive fingerprinting of the inorganic species hydrazine, which is a first attempt in semiconductor-based SERS research, as well as various other probe molecules. The resulting SERS platform that uses QDs with average size of 2.2 nm could successfully detect the signal of hydrazine with a limit of detection estimated to be around 4 × 10^-5 M, significantly lowering the detectable concentration by at least 1000-fold, in sharp contrast to the weak performance of 10 and 100 nm particles, demonstrating that quantum size effect triggered by small particle size below the Bohr radius is crucially responsible for high SERS activity. The significantly enhanced SERS activity is a result of vibronically coupled multipathway, highly efficient charge-transfer resonances induced by the divergence of energy states in quantum-sized MoO₃·xH₂O. This is a proof-of-concept demonstration of the exploitation of quantum size effect, toward significantly enhanced intrinsic SERS activity in semiconductor-based SERS materials.