Influence of Pore and Cell Sizes on Aluminum/Nanoporous Alumina Optical Properties

Considerable attention has been paid to nanoporous anodic alumina (NAA) due to its excellent electronic and optical properties. A wide range of self-organized honeycomb structures is formed by controlling the aluminum... [ view full abstract ]

Considerable attention has been paid to nanoporous anodic alumina (NAA) due to its excellent electronic and optical properties. A wide range of self-organized honeycomb structures is formed by controlling the aluminum anodization parameters (potential, time, temperature, and electrolyte solution). The optical properties of bi-layer systems composed by Al/NAA are strongly influenced by the thickness, porosity and refractive index of the NAA. A deep knowledge of the influence of morphological characteristics of NAA on its optical performance is necessary to enlarge the film applications in optics. In the literature, the effect of film thickness and porosity on optical properties is well known. Herein, in this study, the influence of pore and cell sizes on reflectance was studied. Highly ordered structures with similar thicknesses but different cell and pore sizes were obtained at 40 V, 50 V, 55 V and 60 V by two-step anodization in oxalic acid. The pore and cell sizes increased with anodizing voltage as SEM images showed. The porosity percentages were around 5% assessed by image analysis (Image J software). Ordered hexagonal structures grouped into domains were showed by fast Fourier transform for all samples. The reflectance curves revealed that electropolished aluminum is highly reflective near 250 nm in the UV zone. While, the anodized samples worked as UV filters and showed a maximum reflectance percentage ranging from 77 to 86 % near 600 nm being the lower for 60 V condition and the higher for 40 V. As density and pores diameter change with voltage, about the same porosity percentages were obtained at all anodizing conditions. It was found that reflectance decreases as cell and pore sizes increases, despite both porosity and thickness are still about constant. Therefore, these two characteristics of the NAA films must be considered in order produce the looked-for optical properties of the multilayer systems.