Demonstration of tunable Ag morphology from nanocolumns to discrete nanoislands using novel angle constrained glancing angle EB evaporation technique

In glancing angle electron beam evaporation technique, the deposition geometry has been engineered in a novel manner in this work to produce extensive tunablity of Ag morphology. A physical plate (collimator) parallel to the substrate has been suitably placed in order to constrain the angle of incoming deposition vapor flux. The distance between substrate and this collimator as well as the distance between substrate to crucible orifice has been optimized for maximum variation in Ag morphology which resulted in the discussed tunability. SEM images acquired across the sample surface on the best optimized sample show variation of Ag morphology from nearly continuous film to Ag nanocolumns (dia-135 nm) and then to Ag nanoislands of varying size (26-83 nm) with the variation in height from substrate bottom edge. Surface plasmon resonance (SPR) peak of this Ag nanostructure has been found to shift to longer wavelength and to broaden with the increase of thickness across the substrate surface. The estimated band gap values of the Ag nanostructure are non-zero which reveals its dielectric like dispersion behavior. Thickness and optical constants at various locations on the best optimized sample have been estimated through spectroscopic ellipsometry measurements and the obtained results have been validated with measured transmission spectrophotometry spectra. Three different ellipsometry models have been tried to fit the ellipsometry data of such grown ultrathin film consisting of Ag nanoislands/nanocolumns. An effective media consisting of the mixture of bulk Ag, dielectric media represented by Lorentz oscillator and air was found to be the best to describe the disperision behavior of the deposited Ag film. The dielectric like dispersion behavior of Ag ultrathin film has been ascribed to localized surface Plasmon resonance (LSPR) effect of Ag nanoislands in optical wavelength region. Due to the gradual variation of thickness and optical constants across the film, the optimized film exhibits variation in average optical transmittance (600-1000 nm) from 1.1% to 91% across 40 mm distance which demonstrates its potential for application as variable optical transmission attenuator. Finally, this novel deposition flux constrainment technique has been regarded as an effective method for fabrication of tunable Ag nano-morphologies for optical and surface plasmonic applications.