Size and Wavelength Dependence of the Charge-Transfer Contributions to Surface-Enhanced Raman Spectroscopy in Ag/PATP/ZnO Junctions

Abstract

By use of surface-enhanced Raman spectroscopy, we observe the degree of charge-transfer for Ag/PATP/ZnO sandwich compounds as a function of both ZnO nanoparticle size and as a function of excitation wavelength. We show that there are several likely charge-transfer resonances. The most obvious is the resonance at particle diameter of 27.7 nm for all wavelengths. In a theoretical study it has been suggested that when there is an electron acceptor on the nanoparticle surface it may form a complex with the semiconductor exciton and that this is most likely the origin of the size-dependent resonance. At the smallest size (18.2 nm) studied here, there is an increase in degree of charge-transfer (relative to adjacent sizes), indicating the possibility of another, lower-lying charge-transfer state, which also could be caused by the acceptor−exciton complex. The other resonance suggested by our data is to higher excitation energy for all particle sizes. It can be seen that the degree of charge-transfer is rising as the excitation wavelength is shortened, indicating an additional charge-transfer resonance in the ultraviolet