Structural, morphological and optical properties of Ag–AgO thin films with the effect of increasing film thickness and annealing temperature

Abstract

Ag films of thickness ranging from 5 to 60nm were deposited by thermal evaporation technique followed by air annealing process with temperature varying from 50 to 250°C. Morphological properties such as particle size, shape, surface roughness and number particles density were studied by atomic force microscope (AFM). The structural transition from quasi-amorphous to nanocrystalline to crystalline upon increasing film thickness and annealing temperature were studied. Ag films with smallest particle size and surface roughness were achieved up to film thickness of 7nm. The possibility of surface oxidation of Ag on both as deposited and annealed films was studied through Raman mapping by using confocal Raman spectroscopy. Ag film was X-ray amorphous even after annealing process up to the film thickness of 7nm and above which the crystallinity reached maximum at 250°C. The surface plasmon resonance (SPR) with a symmetric line shape due to dipole–dipole interactions was found to be very strong for film thickness of 5nm at 100°C, attributed to the formation of smaller Ag NPs size of ∼22nm with least size distribution and higher particles number density of ∼1625μm−2 in a self-organized fashion. With an increase of film thickness and annealing temperature, an asymmetric broad absorption arose due to increase in damping of collective electron oscillation on bulky NPs. Theoretical absorption spectra were simulated using extended Maxwell garnet method showing a decent agreement with experimental data. The real and imaginary parts of dielectric constants were determined and plotted for different film thicknesses of as deposited Ag films. Even though the film is oxidized at the surface level, it still can be used for plasmonic sensor applications however the film thickness should be approximately 7nm for the enhanced result.