ZnO nanoparticle size-dependent excitation of surface Raman signal from adsorbed molecules: Observation of a charge-transfer resonance

By observing the Raman intensity of several molecules (4-mercaptopyridine and 4-mercaptobenzoic acid) adsorbed on the surface of ZnO nanocrystals as a function of particle size in the range of 18-31 nm diameter, we show that a large increase in intensity is observed near 28 nm. This is an indication of a size-dependent charge-transfer resonance. We interpret this effect as due to formation of a charge-transfer complex between a surface-bound exciton and the adsorbed molecule.