Controlling the Color and Effective Refractive Index of Metal-Anodic Aluminum Oxide (AAO)–Al Nanostructures: Morphology of AAO

Abstract

Metal-anodic aluminum oxide (AAO)–Al nanostructures have been deposited by using sputtering for the metal layer deposition and a two-step anodization process in different electrolytes to produce self-ordered anodic aluminum oxide films. The effect of the morphological parameters of AAO films (such as thickness, pore diameter, interpore distance, and porosity) on the optical properties was studied. The UV–vis reflectance properties as a function of the thickness for the different electrolytes of metal-AAO–Al films were analyzed in order to obtain the color diagrams and the effective refractive indexes of the films. The effective refractive index was found to depend on the thickness and porosity of AAO films. The change in color observed in metal-AAO–Al nanostructures is due to the thickness and porosity of the AAO films. In order to verify the experimental results, UV–vis reflectance spectra of AAO–Al films were simulated using commercially available finite element simulation software.

Deterministic Model of Observed Colours on Metal-Anodic Aluminium Oxide-Al Nanostructures

“…This behaviour has already been observed in literature. [17,18] In both cases, after the deposition of 8 nm of Cr and 10 nm of Au on top of the AAO-Al films (see Figure 2b and 2c), a blue-shift and a decrease in the reflectance is observed, due to plasmonic effects [4,11,13]. The wavelength of each sample and consequently the colour that the films will exhibit can be obtained from the maximum reflectance.…”

“…Different AAO films were anodized under the same conditions (same porosity), changing only the second anodization time (from 120 to 600 s) to obtain different thicknesses (209 ± 12 nm to 380 ± 15 nm). FIB milling and FE-SEM imaging were used to accurately determine the films' thickness, as was done in a previous study [4]. The porosity, P, of the AAO templates is given by the following equation:…”

“…In a previous study carried out by our group, the dependence of effective refractive index and the thickness of AAO films for different anodization electrolytes was obtained [4]. In this work, more films and thinner films were studied, therefore it is necessary to recalculate the equation for the thickness range 209 ± 12 nm to 951 ± 19 nm.…”

“…There are different approaches in obtaining brilliant colours using AAO films, such as using new anodization electrolytes (etidronic acid) [2], via pulsed anodization [3], as well as by depositing a metal layer onto the surface of AAO-Al films [4]. Specifically, metal-AAO-Al nanostructures exhibit structural colours, which can find applications as wavelength absorbers [5], RGB display devices [6] and chemical [7] or optical sensors [8].…”

“…The colour observed in metal-AAO-Al nanostructures depends on the morphological parameters of the AAO films and on the metal's nature and thickness once deposited onto the AAO-Al films. In a previous study by our group, the effects of morphological parameters (pore diameters, interpore distance, porosity, nanostructure order) on the colours and the effective refractive index of AAO films were studied on metal-AAO-Al nanostructures [4]. Thickness and porosity are the two main structural parameters, which affect the observed colours and effective refractive index of the AAO films.…”