Abstract
Intelligent structural colors have received extensive attention in recent years due to their diverse applications. However, the large-area, uniform, and cost-effective fabrication of ultra-thin structural color films is still challenging. Here, for the first time, we design and employ an ultrasonic spray-coating technique with non-toxic, green nano-silica and polyvinylpyrrolidone as raw materials, to prepare structural color films on silicon wafers. Due to the addition of polyvinylpyrrolidone, the coffee-ring effect during droplet drying is suppressed and uniform composite films are formed. We further perform a detailed study of the influence of various processing parameters including silica/polyvinylpyrrolidone concentration, substrate temperature, nozzle-to-substrate distance, and number of spray-passes on film roughness and thickness. By increasing the number of spray-passes from 10 to 30, the film thickness from 120 to 340 nm is modulated, resulting in different colors, and large-area and uniform colors on commercial round silicon wafers with 15 cm diameter are achieved. The silica/polyvinylpyrrolidone composite films show strong hydrophilicity and are sensitive to humidity changes, leading to quickly tunable and reversible structural colors. Quartz crystal microbalance with dissipation demonstrates water vapor adsorption and condensation on the nanofilm when increasing environmental humidity. Thereby, ultrasonic spray-coating as a novel film fabrication technique provides a feasible scheme for large-area preparation of intelligent structural colors.
Highlights
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Structure color, which arises from the physical interaction of visible light with micro- and nano-fabricated ordered structures, has a lot of advantages, such as having good light stability, and being pollution-free and fadeless [3,4]
The surface tension of SiO2 /PVP dispersion is 68.8 mN/m (Table S1) and its contact angle on a silicon wafer is 28◦ (Figure 2d). This shows that the addition of PVP does not affect much the particle size and surface charge, and SiO2 /PVP is still stable and transparent
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Replacing chemical colors with artificial structural colors has attracted more and more attention due to contamination from chemical dyes and pigments, and easy fading of chemical colors [1,2]. Structure color, which arises from the physical interaction of visible light with micro- and nano-fabricated ordered structures, has a lot of advantages, such as having good light stability, and being pollution-free and fadeless [3,4]. The superior advantage that chemical colors can be manufactured on an industrial scale makes it difficult to find substitutes for them [5,6]. There are still enormous challenges to replace chemical colors with artificial structural ones [7]
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