Material selection for pure and stable structural color of Morpho-inspired reflectors

Kyungjae Chung* and Jung H. Shin*,** * Department of Physics, KAIST, 335 Gwahak-ro, Yuseong-Gu, Daejeon, South Korea

** Graduate School of Nanoscience and Technology (WCU), KAIST, 335 Gwahak-ro, Yuseong-Gu, Daejeon, South Korea

Ⅰ. Introduction

The bright, metallic blue of Morpho butterflies are among the best kown examples of structural colors in nature. Created by interference within the multilayered ridges on the scale that cover the butterfly’s wings, the blue color is also well-known for the its seeming paradox of appearing the same at different viewing angles. Previous investigation have indicated that the internal disorder within the scales, in particular the sub-wavelength scale randomness in the spatial location of the ridges, plays the crucial role in producing such an angle-independent structural color [cite?]. Recently, using a self-assembled monolayer of randomly sized silica nanospheres as the substrate, we have demonstrated reproduction of such angle-independent structural color in TiO2/SiO2 multilayers [cite]. In this paper, we investigate the effect of the multilayer material on the range and stability of the colors that can be generated in such Morpho-inspired reflectors. We find that simple transfer matrix method (TMM) calculation of a flat multilayer can be used to successfully predict the color of corresponding Morpho-inspired reflectors. Based on an extensive series of TMM simulations, we conclude that the chitin/air multilayer found in the actual butterflies provides larger range and higher stability of colors than other inorganic multilayers such as TiO2/SiO2 or Si3N4/SiO2. In particular, we find that the intra-multilayer absorption by melanin in chitin plays a critical role in keeping the color stable against changes in the viewing angle..

Ⅱ. Results

1. Analysis tool

The complex internal structure of Morpho-inspired structures, with their nanoscale randomness render accurate calculation of their optical properties difficult. However, as Fig. 1 (a)-(c) shows, the normal reflectance of such a Morpho-inspired structure is quite similar to that of a corresponding flat multilayer with the same periodicity such that they provide colors that are quite close. As the normal reflectance of a flat multilayer can be calculated accurately using simple TMM calculations as shown in Fig. 1 (d), this indicates that we may avail ourselves of TMM simulations for a wide-ranged search for colors that can be generated by Morpho-inspired structures.

2. Color range and stability

Based on the above observation, we have calculated the range of colors that can be generated by Morpho-inspired structures using TMM calculations of multilayers with layer thicknesses ranging from 30 to 250 nm, for a total of 2025 combinations. As shown in Fig. 2 (a)-(e), the largest color gamut is obtained from the chitin/air multilayer found in actual Morpho butterflies, as the low refractive indices result in narrow stop-band in the reflectance that help create pure colors. In addition, we investigate the stability of the generated colors by introducing random variations in the thickness of the multilayers and calculating the resulting movement across the color space. This also simulates the effect of randomly varying the viewing angle. The results are summarized below in Table 1. Surprisingngly, we find that internal absorption in the chitin layer provides a crucial role in providing the color stability of Morpho butterflies. We believe that this is due to suppression of side-peaks whose positions vary rapidly with small

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changes, thus leading to color variation. The result suggest that for replicating the properties of Morpho butterflies, attention should be paid to proper selection of materials as well as to replicating their structures.

Figure 1. (a)-(c) Reflection comparison between flat multilayer and Morpho-inspired film. (d) Reflection comparison between experimentally measured data and TMM calculation.

Figure 2. Color ranges calculated by TMM. Black points are each color points of thicknesses condition, and white triangle is optimal color gamut of (a) SiO2/Si (b) TiO2/SiO2 (c) TiO2/Si3N4 (d) Si3N4/SiO2 (e) chitin/air.

Red Green Blue chitin / air 9.78 6.46 30.25 TiO2 / SiO2 107.67 29.26 84.50 Si3N4 / SiO2 21.61 17.09 38.03

chitin / air (no absorption) 25.63 21.58 38.37 Table 1: Color distance, calculated ΔE in Lab system

Ⅲ. Conclusion

The effect of material selection on the range and stability of colors generated by Morpho-inspired reflectors are investigated. We find that proper selection of materials is as important as replication of structures for generating vivid blue with angle-independency. Based on result, we suggest that low refractive index material containing absorption could lead pure and stable structural color of Morpho-inspired reflectors.

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