Surface plasmon resonance, photoluminescence and surface enhanced Raman scattering behaviour of Ag/ZnO, ZnO/Ag and ZnO/Ag/ZnO thin films

Abstract

The surface plasmon resonance (SPR) of Ag thin films in ZnO/Ag, Ag/ZnO bilayers and ZnO/Ag/ZnO multilayers produced by vacuum thermal evaporation has been studied. The role of thickness of the individual layers in the range of 5–50 nm was investigated. Detailed studies were carried out by fixing the thickness of the plasmon active layer and varying the dielectric layer thickness and vice versa. These studies reveal that the SPR of Ag is very sensitive to the geometry of the structures and as a consequence it has a strong effect on the photoluminescence (PL) as well as Surface enhanced Raman Scattering (SERS) behaviour. The PL peak of 5 nm thickness pure ZnO films which was centered at around 370 nm, was red shifted at higher thickness of 50 nm. Further, this peak was blue shifted to 324 nm when a 5 nm thickness Ag layer is introduced underneath the ZnO layer. The SERS behaviour of these structures was tested using R6G as the analyte. The enhancement factor for a 5 nm thickness pure Ag film was found to be 5 × 103. The highest enhancement factor for ZnO/Ag, Ag/ZnO and ZnO/Ag/ZnO trilayer films was found to be 7.4 x 106, 4.4 x 105 and 8.9 x 104, respectively.