Hydrophilic ultra-thin porous films for color variation with humidity by using oblique angle deposition.

Abstract

Conventionally, in thin-film optics, optical interference can be obtained simply by the quarter wavelength condition (λ/4n). In recent years, ultra-thin film structures using highly absorbent materials overcoming the typical condition have been studied [1]. By thin-film coating an absorbent semiconductor material on the metal, due to non-trivial phase change of reflected waves, the strong optical interference can exhibit a color change with the film thickness in visible wavelengths. However, for thin film coloration, these ultra-thin structures have some difficulties in color purity and color tunability due to reasons such as high complex refractive indices and limited material selectivity. Recently, we have studied to improve color tunability and color purity of highly absorbing ultra-thin film using porosity (Pr) [2]. In our previous work, the color of the ultra-thin film can be tuned by controlling the porosity using Oblique angle deposition (OAD). Basically, the effective refractive index of the ultra-thin porous film is determined in consideration of the ratio between the air and the medium. However, in case of that pores are filled with other material instead of air, the effective refractive index can be changed, consequently, causing color variation. For a multiplicity of color variation, in this study, we propose a hydrophilic ultra-thin porous film that can be filled with vapor in pores. To implement our proposed structure, we apply a hydrophilic treatment to a thin-Ge on Au with Pr by OAD. For optical design and analysis, effective indices with Prs and humidity are calculated by the equations based on the volume averaging theory (VAT) and the reflectances of the proposed structures are calculated by the rigorous coupled-wave analysis (RCWA) method. The calculation results show that the minimum dip of the reflection changes with Pr and humidity. To verify the result of the calculation, an electron beam evaporator is used to deposit the Ge film onto an Au coated substrate on an inclined sample holder. Furthermore, to compare the color change with humidity, CIE color coordinates are employed to plot chromatic information. The chromatic value can be converted by the tristimulus color function from the reflectance in the visible wavelengths. From these results, we also discuss the color sensitivity and responsiveness of the proposed structure for humidity.