Photoinduced kinetics of SERS in bioinorganic hybrid systems. A case study: Dopamine-TiO₂

The reported observation of SERS on semiconductors has confirmed the feasibility of distinguishing the chargetransfer mechanism from the electromagnetic one responsible for the enhancement of the signal in metal nanoparticles. Experimental investigation of the well characterized dopamine-TiO₂ system revealed an unexpected dependence on coverage and size. We propose here a theoretical model applicable to SERS on semiconducting substrates that explains this remarkable behavior. The model is based on a competition mechanism arising from the formation of an electron gas in the conduction band of the semiconductor due to the photoexcitation of a charge-transfer complex. Taking into account the two competing effects, a linear increase in the Raman intensity arising from increasing coverage and a quenching effect due to the photon absorption by the electron gas, provides excellent agreement between our model and the experiment for 5 nm nanoparticles. Discrepancies for the case of 2 nm nanoparticles are attributed to quantum confinement, an effect that is investigated elsewhere.