Thermal annealing induced strong photoluminescence enhancement in Ag-TiO2 plasmonic nanocomposite thin films

Abstract

Thin films of Ag-TiO2 plasmonic nanocomposites were fabricated on silica glass substrates by a combined approach using thermal evaporation and RF magnetron sputtering. The optical and plasmonic response of Ag-TiO2 nanocomposites were tuned with the help of thermal annealing by varying the annealing temperature from 400 °C to 600 °C. The optical absorption spectra revealed the transformation of Ag nanolayer into Ag nanoparticles upon annealing. The band gap of Ag-TiO2 nanocomposite thin films varied from 3.09 eV to 2.53 eV due to annealing. AFM studies revealed growth of nanostructures in the Ag-TiO2 nanocomposites with increase in annealing temperature. The average nanoparticle size varied from 27 nm to 230 nm upon annealing. XRD results confirmed the appearance of Ag nanoparticles along with nanostructured anatase TiO2. The photoluminescence spectra of Ag-TiO2 nanocomposites revealed significant enhancement in intensity of UV and defect emissions upon annealing at 600 °C. The observed enhancement originated from the changes in the plasmonic properties of Ag nanostructures in the nanocomposite upon annealing. The observed significant enhancement in the UV emission arises due to the resonant coupling of surface plasmons of Ag nanoparticles with the band edge emission from TiO2 which significantly improved the number of electrons in the conduction band of TiO2. The PL enhancement in the visible region is ascribed to increased creation of shallow traps in TiO2 due to the presence of Ag nanoparticles. This work provides a simple route to strong enhancement in PL intensity of nanostructured TiO2 thin film which is very promising for applications in nanophotonics.