Fabrication of Optical Switching Patterns with Structural Colored Microfibers
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Structural color was generated using electrospinning and hydrothermal growth of zinc oxide (ZnO). An aligned seed layer was prepared by electrospinning, and the hydrothermal growth time control was adjusted to generate various structural colors. The structural color changed according to the angle of the incident light. When the light was parallel to the direction of the aligned nanofibers, no pattern was observed. This pattern is referred to as an “optical switching pattern.” Replication using polydimethylsiloxane (PDMS) also enabled the generation of structural colors; this is an attractive approach for mass production. Additionally, the process is quite tunable because additional syntheses and etching can be performed after the patterns have been fabricated.
KeywordsElectrospinning Hydrothermal growth Nanostructure Structural color ZnO
Scanning electron microscopy
Structural color has many advantages over pigment (chemical) color. For example, it may be eco-friendly and does not suffer from photochemical degradation. Also, because the color changes according to the observing angle, it is possible to produce various patterns that cannot be produced with conventional pigment colors. These attributes have rendered structural colors of great interest to the textiles, paints, cosmetics, security, and sensors [1, 2, 3, 4, 5, 6, 7]. A variety of coloring principles explain the expression of structural color, and recent studies have shown that zinc oxide (ZnO) nanostructures express color by quasi-ordered scattering .
Quasi-ordered scattering is determined by the size and spacing of the nanostructures and is colored when the size of the nanostructure is similar and the spacing is constant. Although the diffuse reflectance is presumed to be the main coloring principle of quasi-ordered scattering, the principle of precise coloring has not yet been clarified, and blue, green, and purple are mainly observed .
A seed layer is required to fabricate ZnO nanostructures. Hydrothermal growth occurs in the region where the seed layer forms, which is also where structural color is expressed [9, 10, 11, 12, 13, 14]. Hydrothermal growth refers to the synthesis of nanostructures in water at 40–80 °C. Therefore, the shape of the pattern is defined by the region of the seed layer. To fabricate optical switching patterns, a nanofiber seed layer is required that is aligned in one direction. To accomplish this, we used electrospinning, which is the most commonly used method for fabricating nanofibers [15, 16, 17, 18]. However, collected electrospun nanofibers are usually randomly aligned. Research has been conducted to align nanofibers to minimize the net torque of electrostatic forces applied to the fiber ends . In this way, the nanofibers can be aligned in a floating state (the nanofibers are aligned in the air between the electrodes), and an aligned seed layer can be fabricated by transferring the fabricated nanofibers to the target substrate. In order to produce the wire pattern of microscale without using electrospinning, a complicated patterning process using photoresist must be performed, which is a process that is not only difficult to realize mass production and large-scale as well as increase the process cost.
The fabricated seed layer was made from nanofibers having specific dimensions obtained through hydrothermal growth after heat treatment. ZnO is a highly suitable material for fabricating patterns because of its high refractive index (n = 2.0034) and ease of synthesis in various forms. The method of fabrication of structural color patterns using aligned ZnO nanofibers proposed in this study can be applied to create visual patterns, or in sensors for detecting various gases [20, 21, 22].
Polyvinylpyrrolidone (PVP; AR grade, M.W. 1,300,000) powder was purchased from Alfa Aesar. Ammonia solution (AR grade, 28.0–30.0% (mol/mol)), zinc chloride (AR grade), and zinc nitrate hexahydrate (AR grade) were purchased from Junsei Chemical Co., Ltd. Hydrochloric acid (AR grade) and N,N-dimethylformamide (DMF; AR grade) were purchased from Sigma–Aldrich. All reagents were used as-received and without further purification.
Electrospinning was performed at room temperature and low humidity (relative humidity, 15–20%). A solution in DMF of 500 mM Zn(NO3)2 and 0.2 g/mL of PVP (final concentrations) was prepared. The gap between the tip and collector was fixed at 50 mm, and the applied voltage was 6.5 kV. To obtain aligned microwires, parallel aluminum electrodes were fabricated with dimensions of 3 cm in width and 2 cm in height. The nanofibers collected in parallel by an electric field were transferred to a target substrate (glass or silicon wafer).
ZnO Nanostructure Fabrication
To fabricate a ZnO nanostructure that exhibits structural color, a ZnO seed layer must be prepared by heat treatment (500 °C) of the nanofibers prepared in the previous step. Hydrothermal growth was then used to fabricate nanostructures on the seed layer. To fabricate the 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. Ammonia (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 precipitated out of solution, which led to the nucleation and growth of ZnO nanostructures. To induce nanostructure synthesis at a constant rate, the reaction was carried out at pH > 10, and the pH of the solution decreased due to a dehydration reaction. Hydrothermal growth can be achieved by further growth of the nanostructures after patterning.
Patterning of ZnO Microwires
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. The masking tape was patterned using a paper cutter (Silhouette Cameo) to cut it into the desired shapes.
The morphology of the ZnO nanostructures was observed by scanning electron microscopy (SEM) using a TESCAN LYRA 3 XMH instrument. Microwires were studied using an optical microscope (model D800; Nikon) equipped with a digital camera (model LV-150; Nikon). A white LED was used as the light source.
Replication of Pattern Using PDMS
The final fabricated ZnO nanostructure is used as a master mold for replication. Replication is carried out using polydimethylsiloxane (PDMS), which is characterized by being inexpensive, flexible, and optically transparent. First, pre-polymer base is mixed with curing agent 10: 1 and bubbles are removed in a vacuum chamber for 1 h to remove bubbles. Pour over the master mold and cure for 1 h at 65 °C in the oven to complete the replication process.
Results and Discussion
We fabricated an optical switching pattern using ordered structural coloring nanostructures. The fabricated nanostructures are colored according to the principle of quasi-ordered scattering. Controlling the reaction time affects the size of the nanostructures and thereby the observable colors. We also used electrospinning, which is the most common method for fabricating nanofibers, to form an aligned seed layer to fabricate the alignment pattern. Our fabrication process is highly flexible, because the electrospinning process controlling the position and size of the pattern and the hydrothermal growth controlling the size of ZnO nanostructure can be modified independently. After the process is completed, the pattern can be modified by additional synthesis or etching, and the completed pattern can be mass-produced through replication using PDMS. Large color-changing patterned areas can be produced, for which the color changes according to the viewing direction and the light transmission direction. We successfully fabricated an optical switching pattern, for which the pattern was seen only on one side by aligning the nanofibers along one direction. We expect that our pattern-making method will find widespread applications in applications such as gas sensors and anti-tampering tags.
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (NO. 2015R1A2A1A14027903; NO. 2018R1A2A2A05023037). The English in this document has been checked by at least two professional editors, both native speakers of English. For a certificate, please see: http://www.textcheck.com/certificate/cJpHky.
Availability of Data and Materials
All datasets are presented in the main paper.
GHK carried out the experiment and prepared the manuscript. GHK, TA, and GL participated in the experiment and discussion of the results. TA and GL analyzed the data and helped modify the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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