Plasmonic effect of diffused Ag nanoparticles in EB evaporated Ag/TiO2 bilayer thin films and role of oxygen pressure

Abstract

Oxygen pressure mediated interfacial diffusion of silver up to top surface of thick (∼120 nm) TiO2 layer in e-beam evaporated Ag/TiO2 bilayer thin films has been observed. This effect has been investigated in oxygen pressure range of 5.9 × 10−5-5.8 × 10−4 mbar during evaporation. The amount of Ag diffusion (5–24%) has been found to be correlated with the induced porosity of TiO2 layer. Both compositional and structural characterizations confirm Ag0 chemical state of diffused silver. Scanning Electron Microscopy revealed Ag aggregates having an approximate average diameter of ∼4 nm and ∼24 nm on top surface. Spectroscopic ellipsometry analysis demonstrates that plasmonic absorption caused by these Ag nanoparticles manifests significant modulation of effective complex refractive indices (ECRI) of TiO2 layer. Specifically, the imaginary part of ECRI of TiO2 exhibits substantial non-zero value (∼6.5 × 10−2) in the visible wavelength range which is otherwise close to zero (<10−5) for intrinsic TiO2 thin film. Finally, the optical role of diffused silver on wavelength and peak of interference absorption in Ag/TiO2 bilayer thin film system has been demonstrated. This investigation gives insight on plasmonic contribution of diffused Ag nanoparticles and its consequence on ECRI of dielectric thin film due to in-situ e-beam evaporation without any post deposition treatment.