Bio-inspired structural colors fabricated with ZnO quasi-ordered

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Bio-inspired structural colors fabricated with ZnO quasi-ordered nanostructures Geon Hwee Kim, Taechang An, and Geunbae Lim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 22, 2017

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ACS Applied Materials & Interfaces

Bio-inspired structural colors fabricated with ZnO quasi-ordered nanostructures Geon Hwee Kim 1, Taechang An 2, * and Geunbae Lim 1, *

1. Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea 2. Department of Mechanical Design Engineering, Andong National University, Kyungbuk, 760749, Republic of Korea * Corresponding author: Geunbae Lim ([email protected]) and Taechang An ([email protected])

KEYWORDS: Structural color, hydrothermal growth, ZnO, nanostructure, MEMS (microelectro mechanical systems)

ABSTRACT

Despite their advantages in different applications, structural colors are difficult to use due to the inability to change a structural color once it is implemented, as well as their high fabrication costs; implementing multiple structural colors simultaneously on one substrate is a challenge as well. In this study, structural colors were reproduced using quasi-ordered scattering to mitigate these issues. To this end, a ZnO flower-like structure having unimodal distributions of size and

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spacing was fabricated by ZnO hydrothermal growth. This fabricated nanostructure has a thickness on the order of 103 nm and a diameter on the order of 102 nm. The thickness and diameter increase in proportion with the synthesis time (thickness growth rate: 43 nm/min, diameter growth rate: 20 nm/min). The shape of the nanostructure can be easily tuned by simply adjusting the synthesis and etching times. This method combines the advantages of top-down and bottom-up synthetic approaches in that the structural color can be continuously modified once fabricated.

INTRODUCTION Colors in nature typically arise from two primary sources: pigment (chemical) colors and structural colors. In contrast to pigment colors, structural colors are eco-friendly and do not undergo photochemical degradation. Furthermore, their colors can change in various ways based on the angle at which they are viewed. More distinct colors can also be obtained because the structural spacings in these systems are similar in size to light wavelengths.1 Recent studies have been conducted on the use of structural colors in a variety of fields including paints, cosmetics, fibers, sensors, and anti-forgery patterns.2,3 However, there are still many difficulties associated with the implementation of such structural colors. In order to implement a specific structural color, a nanostructure similar in size to the wavelength of light must be fabricated, and these small structures must be well-distributed over a large area,4 making it necessary to fabricate such nanostructures with high reproducibility. Different structural color motifs in nature include multilayer reflectors, three dimensional photonic crystals, diffraction gratings, and light scattering.5 Light scattering models include


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coherent scattering and incoherent scattering. Quasi-ordered scattering used in this study falls into the category of coherent scattering. Unlike laminar and crystal-like nanostructures in the category of coherent scattering, iridescence is absent or less conspicuous in quasi-ordered scattering.24 Quasi-ordered scattering is a phenomenon where a constructively reflected wavelength is observed when nanostructures with the same size are uniformly distributed over an irradiated area, such as in beetle shells. The colors observed in a given structure are determined by the size and spacing of the constituent nanostructures, allowing for blue, green, and purple colors to manifest.6 The detailed mechanism of quasi-ordered scattering is not clear, but it is presumed that diffuse reflectance during this process creates a strong visual signal.5 To simulate this behavior, the sizes of the nanostructures must be similar and they must be spaced out in equal distances. The nanostructure size must also be smaller than 1000 nm, to yield scattered light of high intensity and discrete wavelengths instead of white light (due to diffuse reflection). Processes for creating structural colors can be largely divided into top-down approaches and bottom-up approaches. Top-down approaches mainly uses microfabrication techniques,16,17 but these have difficulty in creating three-dimensional structures due to resolution limits, high production costs, and low production efficiency. In contrast, bottom-up approaches produce hierarchical structures using methods such as self-assembly and physicochemical interactions, such as in colloidal crystallization and dielectric layer stacking.2,4,18 Previous studies have implemented structural colors that change in response to physical stimuli by combining colloidal suspensions with a PDMS layer, moving beyond the simple stacking of colloidal suspensions.19 Furthermore, structural colors have been produced using TiO2 anodization and etching which improves their reproducibility, mechanical stability, and economic feasibility.20 Structural colors


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have also been realized using materials containing superparamagnetic colloidal nanocrystals (CNCs), referred to as M-ink,21 over a large area and on a single surface. As mentioned previously, various methods have been used to implement structural colors in different systems. However, previous studies have encountered limitations such as difficulty in implementing multiple structural colors onto one substrate, and the inability to modify structural colors once they are affixed to a surface. This study presents a structural color production method that overcomes these limitations by utilizing design motifs observed in beetles. Quasiordered scattering was implemented using zinc oxide (ZnO) flower-like structures, and various structural colors were produced on a single substrate by adjusting the growth time of these ZnO particles using lithography. The ZnO flower-like structure was synthesized by hydrothermal growth at 40-80 ° C and can be synthesized in various shapes according to the type and composition of the material added during the synthesis.10-15 In addition, ZnO has a high refractive index (n = 2.0034) which benefits its usage for the production of structural color. It also has a wide direct band gap of 3.37 eV, favorable electrical properties (it is a II-IV semiconductor at room temperature) and a high exciton binding energy (60 meV)22, which can be used for various applications.7-9 This technique is compatible with microelectromechanical system (MEMS) microfabrication in that it enables selective growth and etching of nanostructures. The process of growing and etching nanostructures falls into the category of bottom-up approaches, and the process of fabricating the shape of the entire pattern falls into the category of top-down approaches. This is the first report combining top-down and bottom-up methods in order to fabricate structural colors.


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EXPERIMENTAL SECTION Materials Polyvinylpyrrolidone (PVP, AR, M.W. 1,300,000) powder was purchased from Alfa Aesar Company, U. S. A. Ammonia solution (AR, 28.0–30.0% (m/m)), zinc chloride (AR), and zinc nitrate hexahydrate (AR) were purchased from Junsei Chemical Co., Ltd. HCl (AR), DMF (N,Ndimethylmethanamide, AR) was purchased from Sigma Aldrich. All reagents were used as received and without further purification. ZnO nanostructure fabrication The process of fabricating these ZnO nanostructures is shown in Figure 1. First, a uniform seed layer was coated on a silicon substrate. This was performed using a polymer solution containing Zn ions, which was produced by dissolving 500 mM Zn(NO3)2 and 0.1 g/mL of PVP (final concentrations) in DMF and stirring at 600 rpm with a magnetic stirrer for 2 h. This solution was then spin-coated on a silicon wafer at 5000 rpm. After spin-coating, the polymer was decomposed and calcinated in air at 500 °C to form a ZnO seed layer. Hydrothermal growth was then used to fabricate nanostructures on the seed layer. To fabricate ZnO nanostructures, ZnCl2 was dissolved in deionized water (DI) at a concentration of 10 mM, and maintained at 40–80 °C to initiate the reaction. NH4OH was added to this aqueous solution at a rate of 5 µL/mL, generating OH- and raising the pH of the solution. In this environment, the Zn2+ ions quickly precipitate out of solution, which leads to the nucleation and growth of ZnO nanostructures. Because the nanostructure formation is influenced by pH, the concentration of Zn2+ ions, and the temperature of the solution, it is vital to keep these constant in order to induce the desired reaction. In order to induce nanostructure synthesis at a


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constant rate, the reaction was carried out at pH > 10, and the pH of the solution lowered due to a dehydration reaction.23 In particular, even a small increase in temperature promotes differences in response speed, and as such it is very important to maintain a constant temperature during the reaction (60 °C in this study). Hydrothermal growth can be achieved by growing the nanostructures further after patterning (Figure 1A) or etching away unwanted portions (Figure 1B). It is also possible to confine the ZnO nanostructures by etching the seed layer (Figure 1C). Patterning of ZnO nanostructures The growth of the nanostructures can be adjusted by using lithography to alter the time during which the seed layer is exposed to the reaction solution. In this study, lithography was performed with the help of masking tape or standard MEMS lithography techniques. The masking tape was patterned by using a paper cutter (silhouette CAMEO) to cut the tape into desirable shapes. UV lithography was used for this study. AZ 5214 photoresist was spin coated at 3000 rpm for 30 s then soft baked in an oven at 100 °C for 1 min. After that, the surface was exposed to an UV light source of 150 W for 7.5 s then developed in TMAH 2.38% solution. Furthermore, the seed layer and the produced ZnO nanostructures can be selectively modified or removed using dilute HCl (0.5 mM). The structural color of the area from which the seed layer is removed does not change even with subsequent hydrothermal growth. Characterization The morphology of the ZnO nanostructures was determined using scanning electron microscopy (SEM) equipped for energy dispersive X-ray spectroscopy (EDS) with TESCAN LYRA 3 XMH. The diameter and thickness were determined using the Image J program. The reflectance was measured using the Spectra Academy program (K-MAC Co., Ltd). The reference


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sample was a bare silicon wafer, and white LED was incident at 45° to measure the reflected light.

RESULTS AND DISCUSSION Figure 2 shows the size and distribution of the nanostructures as determined by SEM. Figure 2A shows a uniform seed layer on a silicon wafer after spin coating and calcination; the ZnO structures on the single layer were formed as uniform island structures. When hydrothermal growth was performed on this seed layer, the surface progresses through Figure 2B, 2C, 2D, and 2E, which were recorded at 1, 2, 3, and 4 minutes of reaction time, respectively. The shapes of the ZnO nanostructures indicate a gradual nucleation and growth process. The diameters of the nanostructures (Figure 2F) show that reaction times of 1, 2, 3, and 4 minutes result in nanostructure diameters of 20 nm, 43 nm, 57 nm, and 83 nm, respectively (n=5), with an average growth rate of 20 nm/min. Figure 3A shows samples of structural colors on substrates made using different synthesis times. The growth time was increased in fixed amounts by masking different sections of a bare silicon wafer. The rightmost part was exposed first and underwent synthesis for 6 min. From left to right, each region was reacted for 1, 2, 3, 4, and 5 min. The leftmost silver part was very thinly coated because only the seed layer is present. As the synthesis progressed, the structural color gradually changed from blue to red. More detailed explanation of the figure 3A sample (RGB values, Commission Internationalable I'Eclairage (CIE) chromaticity coordinates) is given in figure S1 of the supplementary material. The thickness of the ZnO nanostructure was measured by cutting the wafer along the white dotted line (Figure 3A) and examining the cross-section by


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SEM. This is shown in Figure 3B, where the ZnO nanostructure is synthesized on the silicon wafer with uniform thickness and coverage. The nanostructure thickness and synthesis time have a linear relationship (Figure 3C), where the thickness increases at a rate of 43 nm/min. This suggests that the diameter and thickness of the nanostructures both grow simultaneously as the synthesis time increases, shifting the high reflectance sections toward violet (shorter) wavelengths. Nearly 100% reflectivity was measured from the reference silicon wafer in the ultraviolet (λ < 400) and infrared (λ > 760) regions, due to the low ultraviolet and infrared intensities of the white LED used for the measurements. All the results were measured using the sample prepared in the same manner as the sample in figure 3A. Lithography process can be used to modify structural colors by selective growth and etching even after initial fabrication. Figure 4A shows the synthesis of structural colors and their modifications through post-processing. Structural colors a, b, and c were obtained by increasing the synthesis time. If large nanostructures are etched with dilute HCl, the color a changes to Re (which is a bare silicon wafer state with no nanostructures), b changes to b’, and c changes to c’. In comparing b and b’, the average particle diameter decreased from 67 nm to 54 nm (n = 5) after etching. When viewed with the naked eye, the colors of b’ and a are nearly identical, as are c’ and b with one another. In analyzing the nanostructure size, the average diameters of b’ and a are very similar at 54 nm and 51 mm (n = 5), respectively. Plotting the reflection measurements for more precise analysis (Figure 4B), colors a, b, c changed due to etching when viewed with the naked eye. Colors b’ and a were made identical by adjusting the etching time, as were c’ and b. Furthermore, α, β, and γ were fabricated by further growth in order to observe color changes after the growth of a, b, and c. As a result, a changed to α, b to β, and c to γ; the reflectance also changed (Figure 4C). SEM images of these samples show that growth and etching only changed


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the sizes of the particles in the nanostructures, without actually destroying them. Figures 4D and 4E shows thickness and diameter changes during etching with diluted HCl, with average rates of 39 nm/s and 14 nm/s respectively. Based on Figures 2, 3 and 4, the sizes of the fabricated and modified particles correspond to the scale of quasi-ordered scattering.6 Various colors were designed to highlight the advantages of this technique as shown in Figure 5. Figure 5A shows a gradient pattern produced by linearly increasing the reaction time across different substrate regions. A section of masking tape was removed every 10 s, with the reaction time increasing from left to right and from top to bottom. As the reaction time increased, the blue structural color changed to violet and green. The possibility of manifesting clear beautiful patterns over a large surface area using structural colors was demonstrated by implementing a flower garden on a 3-inch wafer as shown in Figure 5B. In previous studies, it was difficult to selectively modify or remove structural colors once produced, but Figure 5C shows that it is possible to do so in specific parts of our new structural color systems using selective etching. In the text “Impossible” structured using these structural color materials, “Im” was etched with diluted HCl, leaving “possible” by itself. Figure 5D shows that if the seed layer itself is etched before the first application of the hydrothermal method, no structural color is implemented even after hydrothermal processing. Therefore, the structural colors can be manifested in only the desired areas by first adjusting the seed layer coverage. Figure 5E shows the result of fabricating a complex structure color pattern using standard MEMS lithography. The figures inserted in the black border are images that map the components based on EDS analysis. Blue and red colors indicate the distributions of Zn and O, respectively. Figure 5F shows the edge of a ZnO pattern fabricated by standard MEMS lithography, where the edge is formed well without residue or nanostructure damage.


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CONCLUSIONS A new method for obtaining biomimetic structural colors was developed, one with the ability to finely tune the completed structures. The quasi-ordered scattering model, where colors are changed by controlling the size of the nanostructure unlike other conventional structural color fabrication principles, is the most suitable model for the fabrication of flexible structural colors. ZnO was synthesized into desired shapes and colors on one substrate through selective growth and etching. This technique is capable of large-area fabrication, allows for flexible fabrication and modification of structural colors, and features reasonable fabrication costs, unlike similar previously published processes. The proposed nanostructure fabrication method is based on a bottom-up approach where the synthesized structures can be adjusted by controlling the synthesis time or reagent concentrations. In addition, this method also provides the advantages of topdown techniques in that selective growth and etching is made possible by using masking techniques. Large-area structural colors can be implemented at a resolution appropriate for reproducible structural colors. Therefore, the proposed method is expected to be applicable to several fields, including the fabrication of microelectrodes by combining this method with MEMS processing, sensors and anti-tampering tags.

ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (NO. 2015R1A2A1A14027903; NO. 2015R1A2A2A01006496)


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SUPPORTING INFORMATION Detailed explanation of the colors of each motif in figure 3A (RGB analysis and CIE chromaticity coordinates of the resulting spectra) is supplied as Supporting Information.

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FIGURES

Figure 1. Schematic illustration of the ZnO structural color fabrication process. (A) structure achieved by further growing the nanostructures after patterning. (B) structure with unwanted portions etched away. (C) structure with confined ZnO nanostructures by seed layer etching.


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Figure 2. SEM images of ZnO nanostructures based on reaction time. (A) seed layer, and after (B) 1 min, (C) 2 min, (D) 3 min, and (E) 4 min hydrothermal growth; (F) graph correlating synthesis time and nanostructure diameter (n = 5, mean ± standard error).


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Figure 3. Relationship between synthesis time, nanostructure thickness and reflectance. (A) Specimens having various structural colors, where the total synthesis time increases by one minute from left to right (leftmost image is the seed layer). (B) SEM image of the cross-section after 5 min synthesis, with a thickness of approximately 200 nm. (C) Correlation between the synthesis time and nanostructure thickness (n = 5, mean ± standard error). (D) Correlation between the nanostructure synthesis time and reflectance in the visible light range (400–800 nm).


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Figure 4. Optical and morphological differences of the structural colored surfaces based on selective etching and growth. (A) Structural color synthesis in various conditions; (a, b, c) denote increasing synthesis time from left to right, (Re, b’, c’) denote the colors after etching, and (α, β, γ) denote colors after growth. SEM images are labeled accordingly (Scale bar: 200 nm). (B) Graph of reflectance for (a, b, c) and (b’, c’) after etching. (C) Graph of reflectance for (a, b, c) and (α, β, γ) after growth. (D) Correlation between etching time and nanostructure thickness (n = 5, mean ± standard error). (E) Correlation between synthesis time and nanostructure diameter (n = 5, mean ± standard error).


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Figure 5. Various patterns produced with ZnO structural colors. (A) Gradient patterns produced by linearly increasing the reaction time. (B) A flower garden pattern produced on a large area by adjusting the synthesis time. (C) Unnecessary structural colors were partially removed by selective etching with acid. (D) A pattern grown by removing the seed layer in a moon shape and adjusting the synthesis time. (E) Complex-shaped pattern produced by UV lithography and its component analysis maps determined by EDS. (F) SEM image of the edge of the pattern produced by UV lithography.


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Bio-inspired structural colors fabricated with ZnO quasi-ordered

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