Fractal structure and optical properties of semicontinuous silver films

Abstract

Semicontinuous silver films, prepared by vacuum evaporation on substrate of KBr crystals, have been made with area coverage in the range 0.3–0.8. The morphology of the films were found to change with coverage p. At small or large coverage range, homogeneous films were observed, but when the coverage approaches a critical value p c ( p c ≈ 0.65), the film was inhomogeneous. The topologic parameters of films such as mean size of cluster S AV, correlation length ξ and fractal dimension D f were measured. When the area coverage was close to p c, S AV and ξ rapidly diverged and a knee point appeared in the curve of D f vs. p. The transmittance of films in mid-infrared wave-band (2.5–12.5 m) was measured. For homogeneous films, as wavelength increased, transmittance increased when p < p c but decreased when p < p c. On the other hand, transmittance was wavelength independent for inhomogeneous films. The optical percolation phenomenon is observed at a region where the fractal dimension D f of the film was kept approximately constant while correlation length diverges. These experimental results were compared with existing theory and we interpret the effect of deposition and coalescence on the percolation parameters by considering the growth mechanism of a nano-structured metal film.