Nanoparticle Metal−Semiconductor Charge Transfer in ZnO/PATP/Ag Assemblies by Surface-Enhanced Raman Spectroscopy

The ZnO/PATP/Ag sandwich structure and its reverse Ag/PATP/ZnO were fabricated using ZnO nanorods and silver nanoparticles with functional molecules 4-aminothiophenol (PATP) through layer-by-layer self-assembly techniques. Under near-IR excitation (1064 nm, far from the surface plasmon resonance of the Ag nanoparticles in the assemblies), the enhancement of surface-enhanced Raman spectra (SERS) from the charge-transfer mechanism can be readily observed only in the former structure. The considerably larger enhancement of the b 2 modes, relative to those of the a 1 modes, is ascribed to the charge transfer between the silver nanoparticles and ZnO tunneling through the interconnecting PATP molecules. This is not observed in the second structure. The result demonstrates for the first time that directional charge transfer between nanoscale metal and semiconductor tunneling through the interconnecting molecules may be examined by SERS.