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Abstract
Dynamic structural color has attracted considerable attentions due to its good tunable characteristics. Here, an ultrathin asymmetric Fabry-Perot (FP)-type structural color with phase-change material VO2 cavity is proposed. The color-switching performance can be realized by temperature regulation due to the reversible monoclinic-rutile phase transition of VO2. The various, vivid structural color can be generated by simply changing the thickness of VO2 and Ag layers. Moreover, the simple structural configuration enables a large-scale, low-cost preparation on both rigid and flexible substrates. Accordingly, a flexible dynamic structural color membrane is adhered on a cup with a curved surface to be used for temperature perception. The proposed dynamic structural color has potential applications in anti-counterfeiting, temperature perception, camouflage coatings among other flexible optoelectronic devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Dynamic color plays an important role in our daily life, include mobile phone displays, smart glass, security marks, and many other active optical components [1–5]. Key requirements for such applications are high switching speed, light and flexible, durable, cost-effective, full-color display and so on [6]. Typically, conventional dynamically tunable colors can be categorized into two kinds, namely, inorganic-type and organic-type. Inorganic tunable colors are expected to have significant advantages over organic molecules or polymeric materials, such as high cycling, good thermal and chemical stability, and long durability [7]. At present, a number of active inorganic materials with unique physical or chemical properties such as electrochromism [8], piezochromism [9], plasmonic effect [10–12], and phase-change materials [13,14] have been studied for color modulation purposes. Structural color is a research hotspot due to its excellent color and gloss stability, fine thermal and chemical resistance. Combining the inorganic material with structural color led to the development of many dynamic color generation technologies. This technology was used in liquid crystal-enabled reconfigurable structural colors [15], electrochromically switchable structural color, etc. Except that, the mechanically control of the gap distance in plasmonic nanostructures [16], and the mechanically rotating plasmonic structures with polarization-dependent spectral responses [17], can also get a good color-changing performance. However, the realization of dynamic color display with plasmonic nanostructures rely on complicated multistep fabrication process, resulting in a very high fabrication cost which drastically limit large-area implementation. The realization by the mechanical control of the gap distance in plasmonic nanostructures lacks durability, thus the color performance will be weakened significantly after a period of stretching [16]. And the operation of reversible electrodeposition required a relatively long reaction time, thus slowing the refreshing rate. [18,19]
Given the above situation, it is crucial to develop novel dynamic structural color to simplify its fabrication process and accelerate its response time. To achieve this goal, a series of typical multilayer-based structural color with the Fabry-Perot (FP)-type cavity has been proposed [20–23]. One particular example is the chemically tunable color filters with FP resonators using silk protein as the FP cavity. It exhibits a red shift of the resonance after the swelling of silk protein, which could be controlled by stimuli such as PH and alcohol concentration [24]. However, its durability when using the silk protein as the FP cavity material needs to be improved. Phase-change materials (PCMs) have been studied deeply due to their reversible phase transition, which has many extraordinary properties such as extreme scalability, fast switching speeds [25–27]. One of the most typical phase-change materials is vanadium dioxide (VO2) [28]. By taking the advantage of the reversible monoclinic-rutile phase transition of VO2, we can dynamically tune its refractive index by heating the temperature up to the transition temperature (approximately 68℃) [29]. And the phase transition in VO2 is known to occur at a picosecond timescale and can be triggered thermally, optically or electrically [30]. On this basis, we replaced the FP cavity material with VO2 to get a switchable dynamic structural color.
In this work, we propose an asymmetric ultrathin FP-type structure with VO2 cavity to realize vivid subtractive flexible structural colors with stable color switching performance, good flexibility and low sensitivity to the angle of incidence. The VO2 layer is crucial and contributes to the color-changing performance in our system, it allows for a better color modulation above and below the transition temperature. The color hue, saturation and brightness can be altered by simply changing the thicknesses of VO2 and top Ag layers. Moreover, the above switchable structural color can be easily fabricated by using standard thin-film deposition techniques on either rigid or flexible substrates. Thereby, this allows a large-scale, low-cost implementation on practical devices. To demonstrate these advantages, we experimentally realized: 1) a series of color-changing patterns on silicon substrates used for decoration, and 2) a flexible structural color membrane well adhered on the curved surfaces of a glass cup used for temperature perception. The approach of dynamic color generation based on the phase transition of VO2 can easily realize diverse color patterns, which makes it beneficial for imaging and flexible optoelectronics technology.
2. Design and fabrication
The concept of the proposed dynamic structural color system is illustrated in Fig. 1(a). It is composed of an ultracompact asymmetric FP nanocavity with a phase-change material VO2 film sandwiched between a top Ag film and a bottom Al film seated on the substrate. Here, the substrate can be either a rigid substrate (such as silicon or glass) or a flexible substrate (such as aluminum foil). In this design, the substrate is chosen as silicon. The Ag and Al layers are used as the top and bottom reflective mirrors, respectively. The refractive index of VO2 changes before and after the phase transition. Based on the above characteristics of VO2, obvious color changes will occur after heating.
Fig. 1. (a) Illustration of the proposed dynamic structural colors. (b) Schematic daigram for theoretical analysis. (c) AFM image of the top layer of the fabricated sample with 50nm Al/70nm VO2/15nm Ag. (d) Raman spectroscopy of the VO2 sample on silicon substrate at room temperature. (e) The measured complex refractive index (n, k) of VO2 at 30℃ and 100℃.
As shown in Fig. 1(b), the reflectance can be calculated as the coherent accumulation of the partial waves reflected from the FP cavity, and the resonant wavelength λ0 can be presented as [22]:
where t2 is the thickness of the VO2 layer, with ${\tilde{n}_2}\textrm{ = }{n_2} + i{k_2}$ being the complex refractive index of the VO2 layer, and θ2 being the angle of incidence, ϕ21 and ϕ23 represent the phase shift of the reflection coefficients r21 and r23 at the Ag/VO2 interface and the VO2/Al interface, and m is an integer. According to Eq. (1), the thicknesses of VO2 and Ag layers, as well as the refractive index of VO2 film, are three major factors contributing to the modulation of the spectral reflectance. The reflection phase ϕ21 and reflectivity can be changed with the varying thickness of the top Ag layer, and the peak or valley λ0 can be shifted by adjusting the refractive index or the thickness of VO2 layer. Thus, the color hue, brightness and saturation can be tuned by changing t1 and t2, and the color performance can be switched by heating up to the phase-transition temperature due to the refractive index change of VO2.The VO2 was fabricated by the magnetron sputtering, the Al and Ag layers were fabricated by the electronic beam (e-beam) evaporation. The uniformity of the fabricated films has been confirmed by the atomic force microscope (AFM) inspections (Fig. 1(c)), the roughness Ra is only 4.66nm for the sample with 50nm Al/70nm VO2/15nm Ag. Raman spectroscopy was used to verify the presence of the VO2 after the crystallization anneal, see Fig. 1(d). The Raman characteristic peaks at 195, 222, 390 and 617 cm−1 prove that this material is the VO2 at monoclinic phase state. As shown in Fig. 1(e), the refractive indexes (n, k) are apparently changed at monoclinic phase state (30℃) and rutile phase state (100℃). This enables using VO2 as a tunable optical material, and the dynamic image can be reversibly switched on/off by heating.
3. Results and discussion
3.1. Color change by heating
According to the above analysis, a palette of reflective structural colors can be easily obtained by simply changing the thicknesses of Ag and VO2 layers, as shown in Fig. 2(a). The top Ag layer plays an important role in controlling the brightness and saturation. The brightness can be significantly improved and the saturation can be decreased with the increasing thickness of Ag layer. For example, as shown in the rightmost column of Fig. 2(a), the saturation decreases from 0.3032 to 0.1576, and the brightness value increases from 0.7373 to 0.7961 with the thickness of Ag layer changing from 0 to 20nm. At the same time, the hue can be controlled by changing the thickness of VO2, as shown in the bottom row of Fig. 2(a), the color changes from violet (Hue: 283.2353°) to yellow (Hue:62.1053°) with the thickness of VO2 increasing from 40nm to 80nm. Figure 2(b) shows the color palette in Fig. 2(a) after heating up to 100℃, and the color changing performance (see Visualization 1) before and after heating can be illustrated clearly in the CIE 1931 color space. As shown in Fig. 2(c), the color switch caused a remarkable shift of the CIE color coordinates. For the sample with 60 nm VO2, the hue changes from 180.95° to 194.1°, and the hue variation was only 13.15°; and for the sample with 70nm VO2, the hue changed from 133.64° to 186.25°, resulting in a hue variation of 52.61°; while for the sample with 80 nm VO2, the hue changed from 63.21° to 149.3° with a hue variation reaching up to 86.09°. Thus, the color difference will increase with the increasing thickness of VO2. Derived from Eq. (1), the variation of the resonant wavelength λ0 is directly proportional to the VO2’s thickness, which is consistent with the above experimental phenomenon. In a word, the color hue, saturation and brightness can be controlled by changing the thickness of top Ag and VO2 layers, and the above color performance can be switched before and after heating.
Fig. 2. Recorded color palette of the reflective colors as functions of the thicknesses of Ag and VO2 layers at (a)30℃ and (b)100℃, keeping the bottom Al layer at 50nm. Each sample has an area of 6mm by 6mm. (c) CIE color coordinates for the samples with different thickness of VO2 at different temperatures.
To study the effect of film thickness on the overall optical properties, we systematically measure the reflectance spectra of the structures with different parameters. Every measurement is performed twice, one for each phase of the VO2 layer. Figures 3(a) and 3(b) show the experimental reflectance spectra versus the varied VO2 thicknesses of 40, 50, 60, 70, and 80nm, respectively, while the underlying metallic Al layer had a fixed thickness of 50 nm. In both cases, the valley position has an obvious redshift with the increasing thickness of VO2. At the same time, the spectra reflection and valley position can be altered when we vary the thickness of the top Ag layer. As shown in Figs. 3(c) and 3(d), the reflection increases with a slight blueshift in the valley position as the thickness of the Ag layer increases. This is due to the reflection phase of the top Ag layer is tuned, and less light can enter into the FP cavity with the increasing Ag thickness.
Fig. 3. Effect of the thicknesses of VO2 layer and Ag layer on the reflection spectra. Experimental reflectance spectra of samples with 0 nm Ag when the VO2 layer (t2) varies from 40 to 80 nm at (a)30℃ and (b)100℃, respectively. Measured reflectance spectra of samples with five different thicknesses (t1) of the top Ag layer (t2=70 nm, spacing=5 nm) at (c)30℃ and (d)100℃, respectively.
Then, we further use the spectra result to quantify the color switching performance. A change in the refractive index of the VO2 layer modulates the reflectance spectra of the FP cavity. The spectra are finally compared and the percentage variation in reflectivity is plotted as: |ΔR|=|(RM-RI)|/RI×100%, with RM being the reflectivity when the VO2 layer is in the rutile phase and RI being its reflectivity for the monoclinic phase. Figure 4(a) shows the reflectance spectra of two examples with 70nm VO2/0nm Ag and 70nm VO2/15nm Ag, for the sample with 0nm Ag, the valley position changes from 769.93nm to 661.34nm (the variation is 108.59nm), while for the sample with 15 nm Ag, the valley position changes from 660.6 nm to 598.9 nm (the variation is 61.7 nm). This result demonstrates that the spectrum modulation range decreases with the increase of the Ag layer thickness. Figure 4(b) displays the reflection spectra of the structure with 50 nm Al/70 nm VO2/0 nm Ag as a function of temperature. There is a distinct blueshift in the spectrum as the temperature incrementally increased from 60 to 70°C. The temperature-dependent reflection demonstrates that the proposed dynamic structural color does not only show two color states, but also reflects a color gradient as the temperature changes (see Supplement 1 for the details about the color changing performance). Figure 4(c) indicates our experimental study of the change in optical reflectivity for various thicknesses of Ag, the result indicates that the change of the sample with 50nm Al/70nm VO2/0nm Ag is the highest, and as the thickness of Ag layer increased, the variation decreased.
Fig. 4. (a) The experimental results of the reflectance spectra at 30℃ and 100℃, respectively. (b) The reflection spectra of sample (70nm VO2) changes monotonically as a function of the temperature from 60 to 70°C with an arrow indicating the increasing temperature. (c) The change in reflectivity |ΔR| for various thicknesses of the Ag layer.
3.2. Physical mechanism investigation
Here, we model the system using the finite-difference time-domain (FDTD) method to calculate properties such as reflectance and internal electrical field at visible wavelengths. Figure 5(a) shows an exceptionally good consistence between the simulated and measured reflectance spectra for the sample with 50nm Al/70nm VO2/0nm Ag. In order to reveal the physical mechanism in the proposed absorber, we get the distribution of the electric field intensity (|E|2) for the above structure illuminated at the absorption peaks of 500nm (Fig. 5(b)). The electric-field energy is mainly confined in the VO2 layer, which demonstates that the color filtering performance is caused by the FP reasonance effect. Figures 5(c), 5(d) show our computational study of optical reflectivity for various thicknesses of the VO2 and top Ag layers, respectively. The results shown in Fig. 5(c) demonstrate that the valley position has a distinct blueshift with the increasing thickness of VO2, which is corresponding with the experimental results shown in Fig. 3(a), and is consistent with the Eq. (1). The results shown in Fig. 5(d) illustrate that the reflection had a significant increase and the valley position had a slight blueshift with the increasing thickness of Ag layer, this is due to the phase change and reflection increase produced by the increased Ag layer thickness.
Fig. 5. (a) The experimental and simulated spectra of the sample with 70nm VO2 at 30℃ and 100℃, respectively. (b) Cross section (xz-plane) showing the distribution of the eletric field intensity at the peak position with 500nm. The white dashed lines are the boundaries of the two layers. (c) Contour map of the simulated VO2 layer thickness-resolved reflectance spectra at 30℃. (d) Contour map of the simulated reflectance spectra as functions of the Ag layer thickness at 30℃. The asterisks refer to the valley position, and the white line is the fitted curve.
3.3. Practical applications
The dynamic color generation technology is a research hotspot since it can realize diverse color patterns. Here, the demonstrated structural color requires no micro- or nanofabrication steps except the film deposition, thus it can be easily prepared on a large-area substrate with various stiffness properties. As described above, the color switching patterns could be realized by changing the heating temperature. Here, we fabricate the structural color sample with different thickness of VO2 on silicon substrates and put them in the auspicious cloud’s molds, as shown in Fig. 6. Before (Fig. 6(a)) and after (Fig. 6(b)) heating, the pattern color has a noticeable change. This case demonstrate that the above dynamic structural color can be potentially used for decoration and among other applications.
Fig. 6. The color patterns used for decoration before (a) and after (b) heating (see Visualization 2).
The flexible dynamic structural color has attracted much attention in flexible optoelectronic field because of its high flexibility and good color performance. Here, we prepared the Al / VO2 films on the aluminum foil substrate to get the flexible color membrane, as shown in Fig. 7(a). Due to the negligible propagation phase shift in the ultrathin VO2 cavity, the realized structural colors are insensitive to the viewing angle up to ±50° (see Supplement 1 for the details about the incident angle insensitivity). Thus, the color performance is nearly unchanged after the sample being bent (Fig. 7(a)). The flexible structural color membranes with different colors are perfectly adhered on the outer wall of the round glass cup. Figures 7(b) and 7(c) illustrate that the cup exhibits different color states before and after pouring with 100℃ hot water. The presented method proves that the flexible dynamic structural color canbe used for temperature perception.
Fig. 7. (a) The flexible dynamic structural color membrane being bent. The color-changing glass cup before (b) and after (c) pouring with 100℃ hot water.
4. Conclusion
A dynamic structural color generated by an ultrathin asymmetric FP cavity with the phase-change material was proposed. The vivid colors, high contrast ratio, large viewing angle are obtained from these structural colors. The color hue, saturation and brightness can be altered by simply changing the film thickness of VO2 and Ag. And the above color performance can be switched upon temperature variation (up to the phase-transition temperature). The good substrate adaptation makes the structural color both feasible on rigid and flexible substrates, and the simple multilayered structural configuration proved suitable for mass production on a large area. Thus, the presented structural color can be used for various applications such as temperature perception, anti-counterfeiting, decoration, and many others.
5. Experimental section
Fabrication. The VO2 film was deposited by magnetron sputtering from a vanadium target with a power of 200 W, where the chamber pressure was 5.05 mTorr with a gas flow rate of Ar/O2 maintained at 90:4. Then it was annealed for two hours with a temperature of 450 ℃. The bottom Al layer, and the top Ag layer were deposited by the e-beam evaporation at a rate ratio of 2 Å s−1 for Al, and 0.5 Å s−1 for Ag with a vacuum pressure less than 4×10−4 Pa at 25 °C.
Measurement. The reflectance of the prepared structural colors was measured using a NOVA-EX spectrometer coupled to a microscope system (Olympus-BX53) at room temperature and 100 °C, respectively. A white light source in the wavelength from 400 to 800 nm was focused by an objective onto the sample surface and the reflected light was collected by the same objective (MPlanFLN, NA = 0.45, 20x).
Optical constants of VO2. Refractive index (n) and extinction coefficient (k) were probed by spectroscopic ellipsometry measurements for single-layer films deposited on bare silicon substrates. The Raman spectra of VO2 was measured by the Microconfocal Raman spectrometer (Alpha300R).
Funding
Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20180508151936092); National Natural Science Foundation of China (51975483, 61775195, 62075196); Natural Science Foundation of Ningbo (202003N4033); Key Research and Development Projects of Shaanxi Province (2020ZDLGY01-03); Collaborative Innovation Center Project of Shaanxi Provincial Department of Education (20JY031).
Acknowledgments
The authors would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for the AFM and Microconfocal Raman Spectrometer measurement.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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