Color is one of the major means by which humans perceive the world and process information. In the information age, color science has significant applications in a range of fields, including decorative printing [1], flat panel displays [2], information storage [3], sensing and detection [4], etc. Color presentation can be divided into two categories: structural color based on the interaction between microscopic physical structure and natural light and chemical color based on the selective absorption of dyes/pigments. Among others, structural color has the advantages of high saturation, high stability, and environmental friendliness, becoming a research hot spot [5–7]. The formation mechanisms of structural colors are diverse, including surface plasmon resonance and guided mode resonance [8–11]. However, the fabrication of nanostructures typically requires expensive lithography equipment (such as focused ion beam milling and electron beam lithography) and complicated processes (including coating, exposure, development, and etching). In addition, the structural color with plasmonic resonance is highly susceptible to both polarization state [12] and incident angle [13] due to momentum matching requirements. Furthermore, a majority of devices, once designed, are unable to generate dynamic colors, which severely constrains the potential applications in color display and other fields [14–19].

Various functional materials [20–22], including liquid crystals, electrochromic polymers, and phase change materials (PCMs) [23–27], have been widely employed to achieve dynamic color generation. Among others, PCMs have received considerable attention due to the giant optical changes when switching between amorphous and crystalline phases. For instance, Zhou et al. [28] fabricated a novel Pt/Ge2Sb2Te5/ITO sandwiched optical coating using Ge2Sb2Te5 as a functional material, capable of achieving dynamic color display as well as relatively low fabrication cost. However, current PCMs, including Ge2Sb2Te5 thin film, exhibit only a low refractive index change and corresponding small spectral shift of ∼35nm in the visible wavelength range when switching between amorphous and crystalline states [1,29–31]. Interestingly, Sb2S3 has a larger bandgap and lower visible absorption [32], which can be used for color-changing devices with lower optical loss. It is reported that the amorphous phase of Sb2S3 is transformed into polycrystalline structure when the temperature is increased from 25°C to 270°C [33]. As temperature increases, the refractive index and absorption coefficient of Sb2S3 film also evolve continuously, while its bandgap decreases from 2 eV to 1.7 eV [33]. This makes Sb2S3 attractive in tunable photonics because of its remarkable response to reversible phase transition in the visible spectrum. When used in resonator structures, Sb2S3 is stable at room temperature and can undergo a high-speed reversible switching between amorphous and crystalline states at elevated temperature [34]. The reversible switching can be cycled more than 1000 times [35], making Sb2S3 an attractive phase transition material. So far, crystallization degree induced multilevel color has yet been reported in Sb2S3-based devices.

Here, an Al/Sb2S3/SiO2-stacking structure is proposed for color printing due to its low cost and simple preparation process. The addition of SiO2 film as a protective layer can prevent the oxidation of Sb2S3 to a large extent while Al film acts as a full reflective layer. The influence of SiO2 thickness on Sb2S3 volatilization is investigated. The color gamut of the device and color tunability are further estimated by adjusting thickness and crystallization degree of Sb2S3 thin film. Finally, reversible color printing is demonstrated via a combination of picosecond laser erasing and nanosecond laser writing.

Al, Sb2S3, and SiO2 films were deposited onto Si and flexible PI (polyimide) substrates via the magnetron sputtering technique (PVD, Shanghai Najing MP-550III). Before sputtering, the base pressure of chamber was below 6.5×10−4Pa. The Al, Sb2S3, and SiO2 targets all had the purity of 99.99%. The working pressures of Al, Sb2S3, and SiO2 were set to 0.6, 0.8, and 0.4 Pa, respectively. Corresponding sputtering powers were 50, 20, and 100 W, respectively. After deposition, atomic force microscopy (AFM, Swiss Nanosurf AG Core AFM) was employed to determine the film thickness and surface morphologies. The maximum scanning area was 100μm×100μm with a resolution of 0.1 nm. It is worth mentioning that the film thickness was obtained by measuring the steps between the film and substrate. In this case, the deposition rates of Al, Sb2S3, and SiO2 films were 6.6, 3.6, and 2.5 nm/min, respectively.

To perform thermal annealing of films, a heating platform (LINKAM HFS600E- PB4) was used, where the heating rate was 20°C/min and the temperature range was 25°C–500°C. In parallel, N2 gas was adopted to prevent the films from pollutions. Raman spectra were collected by a Thermo Scientific DXR 2Xi system equipped with a 532 nm wavelength laser. The crystallization process and crystalline structures were investigated by X-ray diffraction (XRD, Bruker D8 Advance) using a copper target as an X-ray source with the operating voltage of 40 kV, the current of 40 mA, scanning step of 0.025°, and 2θ range of 20°–60°.

Reflectance spectra and optical images were tested with an Olympus BX51 type optical microscope equipped with a microspectrophotometer (Ideaoptics, PG2000). Angle-dependent reflectance spectra were determined by an R1 type angle-resolved spectrometer. Optical constants of Al, Sb2S3, and SiO2 thin films were measured using a spectroscopic ellipsometer (M-2000DI with the wavelength ranging from 193 to 1690 nm). Delta (Δ) and Psi (ψ) were measured in the wavelength range of 400 nm to 800 nm at an incident angle of 60°, 65°, and 70°, respectively. The data of Sb2S3, SiO2, and Al films were then fitted with Tauc–Lorentz/Gaussian, Cauchy, and Lorentz models, respectively, thus obtaining refractive indices and extinction coefficients. The fitting results are listed in Table 1, all exhibiting good agreement between experimental and fitting results with very low root mean square error (MSE).Table 1.

Fitting Parameters of Al, SiO₂, and Amorphous and Crystalline Sb₂S₂ Thin Films

Reflectance spectra of designed color devices were simulated using the finite element method based on COMSOL Multiphysics software. Corresponding color plates were obtained by the CIE color-matching equation, and the simulation details have been reported elsewhere [36]. Thermal field distributions were further calculated based on the heat transfer model to understand the formation mechanism of high-resolution pixels.

Color images were printed onto the device by a direct laser writing instrument with laser wavelength of 405 nm, pulse width of 10 ns, spot diameter of 0.6 μm, and laser power of 0–30 mW. The image erasing was performed by a 532-nm picosecond laser system with a single pulse duration of 13 ps, energy fluence of 0−2mJ/cm2, and repeating frequency of 1 Hz.

Figure 1 shows the schematic of the color device, consisting of Al/Sb2S3/SiO2 stacking layers. SiO2, located at the top layer, serves as a protective film for the inhibition of volatilization of Sb2S3. Al thin film acts as a reflective layer to restrain the optical transmittance. The resonance wavelength (reflectance valley) of the device mainly depends on the optical constant and thickness of the Sb2S3 layer. When Sb2S3 is triggered by a long-pulse and low-intensity nanosecond laser, its crystallization process (color image printing) can take place while the amorphization (erasing image) is achieved by a short-pulse and high-intensity picosecond laser. The phase transition is reversible, and the crystallization degree is tunable. Thus, multilevel, reversible color printing can be achieved.

To restrain the antimony/sulfur loss and surface oxidization during the phase change process, SiO2 film is selected as a protective layer owing to its high thermal stability. The optical images of Sb2S3/SiO2 stacking layers at amorphous and crystalline states are shown in Fig. 2(a). Figure 2(b) shows the influence of SiO2 thickness on thickness reduction of the Sb2S3 layer, where the thickness reduction is defined as the ratio of thickness difference between amorphous and crystalline films to the thickness of amorphous film. A corresponding schematic of film structures is also displayed in the inset of Fig. 2(b). One can see that the thickness reduction gradually becomes smaller with increasing SiO2 thickness, suggesting that SiO2 thin film can effectively restrain the volatilization of antimony/sulfur elements. When the SiO2 thickness reaches 20 nm, the Sb2S3 thickness reduces by approximately 10%, which is nearly the same with the increment of Sb2S3 density after crystallization [33]. However, a larger thickness reduction can be observed at SiO2 thickness lower than 20 nm due to the volatilization of Sb2S3. Particularly, 20-nm-thick Sb2S3 film without SiO2 almost volatilizes completely at annealing temperature of 350 °C for 20 min, and the film thickness is hardly detected. Thus, the thickness reduction of Sb2S3 film reaches 100%. This phenomenon has also been reported elsewhere [37,38]. Therefore, the optimal SiO2 thickness should be higher than 20 nm for different thicknesses of Sb2S3 thin film. In Fig. 2(a(i)), the surface of amorphous Sb2S3 thin film is clear and uniform while the crystalline Sb2S3 thin film presents nonuniform morphology with obvious grain boundary. The addition of SiO2 protective layer appears to suppress the grain size of Sb2S3, presenting uniform surface morphology [Fig. 2(a(ii–v))]. To understand this phenomenon, Fig. 2(c) further shows XRD patterns of crystalline Sb2S3 films with/without SiO2 layer. The crystalline Sb2S3 films both present diffraction peaks corresponding to (200), (201), (103), (203), (211), (212), and (402) crystalline planes (indexed as PDF#73-0393). However, the peaks become weaker after adding the SiO2 layer, indicating the inhibition of grain growth. The reasons can be the Sb2S3-SiO2 interfacial energy influencing nucleation and growth of Sb2S3 film; that is, the SiO2 layer changes the surface energy of Sb2S3 via the formation of new chemical bonds, thus affecting crystallization kinetics [38]. The smaller interfacial energy difference can restrain the rapid grain growth of Sb2S3. In addition, morphology and interfacial stress from the SiO2 layer also influence grain growth [39–41]. Thus, the crystalline Sb2S3 films with different thicknesses of SiO2 all have more uniform surface morphologies.

Figure 3(a) displays optical constants of Al, Sb2S3, and SiO2 films. The as-deposited Sb2S3 film possesses high refractive index and low extinction coefficient, beneficial for color display application. The refractive index of crystalline film increases obviously. The difference of refractive indices between as-deposited (aSb2S3) and crystalline states (cSb2S3) can reach 1, helpful for enhancing the color-changing ability. In addition, the as-deposited SiO2 thin film is transparent in visible region without optical absorption. Based on the measured optical constants, reflectance spectra of Al/Sb2S3/SiO2-based color device are simulated and corresponding experimental results are obtained in Fig. 3(b). The thicknesses of Al and SiO2 thin films are fixed at 100 nm and 20 nm, respectively. The device generates red, blue, and green colors when the Sb2S3 thicknesses are 24 nm, 119 nm, and 133 nm, respectively. The reflectance valley (resonant wavelength) is gradually red shifted. The resonant absorption is attributed to the nontrivial interface reflective phase shifts in the device [42]. The measured peak valley positions of reflectance spectra are in good agreement with the simulated results, indicating that the built physical model is relatively accurate. There remains a discrepancy between the simulated and experimental results, likely stemming from the inconsistency in optical constants across varying thicknesses, whereas a uniform optical constant is employed for color simulation irrespective of thickness difference [43]. The color gamut of the device with aSb2S3 and cSb2S3 films is further simulated in Figs. 3(c)–3(e). Obviously, wide color gamut covering from red and green to blue is realized by tuning Sb2S3 thickness, whether in aSb2S3 or cSb2S3 thin film. Moreover, the color contrast between amorphous and crystalline states is obvious, which is beneficial for implementing multilevel color tuning via the phase transition. In addition, the influence of the SiO2 layer on the color gamut of the device is illustrated in Fig. 3(f). Results reveal that fixing the thickness of the Sb2S3 layer at 20 nm and increasing the SiO2 thickness from 20 nm to 200 nm, the reflectance valley in reflectance spectra is slightly shifted but the color gamut is predominantly in the yellow region due to the small refractive index and thus optical path difference of SiO2 film. This means that the color evolution of the device is primarily influenced by the Sb2S3 layer.

Once the device structure is fixed, multilevel colors can be realized by adjusting the crystallization degree of Sb2S3. The color contrast between amorphous and crystalline states is further quantitatively calculated in Fig. 4(a) by the formula [44] ΔE=ΔL*2+Δa*2+Δb*2,(1)where the thickness of SiO2 is fixed at 20 nm. The significant color contrast can be observed for the devices with 20-nm and 90-nm-thick Sb2S3 films, where the color difference exceeds 60. In view of light source and color perception by human eye, 90-nm-thick Sb2S3 film is selected for color tuning. Figure 4(b) shows the color tones of the device annealed at different temperatures. The device presents four kinds of color tones, from red, orange–red, and dark-red to dark blue with gradually increasing annealing temperature. To quantitatively evaluate the color evolution, corresponding reflectance spectra are given in Fig. 4(c). The reflectance valley is red shifted continuously with annealing temperature, that is, from 500 nm to 600 nm wavelength, resulting in different colors. To elucidate the structural origin of color changes, XRD patterns of device at different temperatures are obtained in Fig. 4(d). Obviously, the device without thermal annealing presents three diffractive peaks corresponding to Al (PDF#85-1327) and Si substrate (PDF#72-1088). With increasing annealing temperature, diffraction peaks corresponding to Sb2S3 phase (PDF#73-0393) occur and gradually become stronger. For instance, one can see only one diffraction peak indexed at the (203) plane of Sb2S3 phase at 240°C. However, more diffraction peaks are observed at 245°C, indicating the higher crystallization degree of Sb2S3 film. At 250°C, much denser diffraction peaks occur and further become stronger, revealing the continuous increment of crystallization degree. Therefore, multilevel colors are successfully implemented by adjusting crystallization degree of the Sb2S3 layer.

Various color images are directly printed by the magnetron sputtering method. A pattern mask is first pressed onto Si substrate. Subsequently, Al, Sb2S3, and SiO2 films are successively deposited onto Si substrate containing the pattern mask. After deposition, the mask is removed and color images are finally obtained. Figures 5(a)–5(c) show different color images with 130-nm-thick Sb2S3 and 20-nm-thick SiO2 layers, exhibiting bird, butterfly, and eagle images with green hues. Moreover, yellow, blue, and purple images are also obtained via adjusting the Sb2S3 thickness to 18 nm, 118 nm, and 105 nm, respectively, shown in Figs. 5(d)–5(f). It is noted that the color difference between the edge and center regions originates from the edge effect of the pattern mask, which leads to different thicknesses at the pattern edge and the center. Furthermore, increasing annealing temperature and thus changing the crystallization degree can also generate different color images, from red and red–blue to blue butterfly, as illustrated in Figs. 5(g)–5(i). The wing color of butterfly is not uniform in Fig. 5(h), attributable to the high sensitivity of device color to annealing temperature. Specifically, the device annealed at 245°C exhibits an intermediate state, and even minor temperature fluctuation can trigger color evolution between the as-deposited state and the state annealed at 250°C. Simultaneously, the variation in temperature distribution across a large area leads to a noticeable evolution in color. Nevertheless, the designed device possesses excellent color-changing performance, and color image printing can be implemented by the pattern mask-deposition method.

To demonstrate the applicability of color devices to flexible photonics, PI (polyimide) substrate is selected for the deposition of the Al/Sb2S3/SiO2-stacking layer. Before deposition, a pattern mask with butterfly is first pressed onto PI substrate. After that, 100 nm Al, 125 nm Sb2S3, and 20 nm SiO2 are successively deposited on PI substrate. The cyan butterfly image is finally obtained on PI substrate after removing the pattern mask. Moreover, bending tests are performed and reflectance spectra after 10-time bending and folding are given in Fig. 6(a). The color of the prepared butterfly pattern remains largely unchanged after being bent and folded, as shown in insets (i)–(iii). The spectral lines of the reflectance spectra and reflectance valley nearly remain the same, indicating that the flexible device is stable without fading and deterioration. The angle-dependent reflectance spectra in Fig. 6(b) also imply that the reflectance valley (resonant wavelength) is located at the same position when the incident angle is ranged from 10º to 60º. This suggests that the designed device exhibits superior angular insensitivity, which is ascribed to the high refractive index of Sb2S3 thin film, reducing the refraction angle inside the multilayer film according to Snell’s law and leading to a very small shift of resonance position with increasing incident angle. Moreover, accumulated propagation phase shifts within the device are very small since the Sb2S3 thickness is much smaller than the incident wavelength [45,46]. Consequently, it is expected to have a wide range of applications in various fields, including color printing, flexible displays, and wearable optoelectronic devices.

To realize the reversibility of color devices, a picosecond laser is employed for the amorphization of Sb2S3 film while a nanosecond laser is utilized to crystalize the film. Before the amorphization, the prepared color device is first annealed at 300°C for the crystallization of Sb2S3 film, as shown in Fig. 7(a). One can see the color changing from red–brown to dark blue after thermal annealing crystallization (I-II). After picosecond laser irradiation, the dark-blue region returns to red–brown (III). After that, a nanosecond laser is further used for the crystallization, generating dark-blue color, again (IV), which can be returned to red–brown after picosecond laser irradiation (V). Figure 7(b) shows corresponding reflectance spectra to quantitatively estimate the color changes. The reflectance spectra of picosecond laser-induced amorphous state are nearly the same, which is slightly different from that of as-deposited state. This may be attributed to the divergence of amorphization degree. On the other hand, the reflectance valley of the thermal-annealing state is also slightly red shifted compared to that of laser crystallization. This originates from the fact that thermal annealing leads to higher crystallization degree than laser irradiation.

Raman spectra are further obtained to understand the structural evolution, as shown in Fig. 7(c). For the as-deposited state, the Raman spectrum exhibits two broad bands at ∼290 and ∼150cm−1, corresponding to the vibrations of Sb–Sb bonds in the S2Sb−SbS2 structural unit and SbS₃ pyramid, respectively [47,48]. After thermal annealing, the Raman spectrum presents five peaks located at 57, 120, 149, 280, and 307cm−1, attributing to Ag stretching modes of the Sb–S bond [49]. This is consistent with Raman features reported in crystalline Sb2S3 films [49]. After picosecond laser irradiation, the Raman features are similar to those of the as-deposited state, both displaying two broad peaks. However, the Raman peak at ∼150cm−1 is slightly shifted toward the lower direction, which may be related to amorphization degree. On the other hand, the peak intensity of the laser-irradiated sample becomes weaker compared to that of the thermal annealing sample, indicating lower crystallization degree at laser irradiation. The structural evolutions in Fig. 7(c) corroborate the reflectance spectra and color changes observed in Figs. 7(a) and 7(b) during the phase transition of Sb2S3 thin film.

The ability of reversible color changes is crucial for technical applications. The prepared color device can be repeatedly crystallized and amorphized. Reflectance spectra in amorphous and crystalline states are shown in Fig. 7(d). Figure 7(e) further illustrates the resonance wavelength as a function of cycle time. The reversible color switching is successfully realized in the device, despite the existence of minor deviations in resonance wavelength and peak intensity. The deviations may be attributed to the fluctuation of irradiation conditions during device operation, thickness variations after crystallization process, etc.

Based on the reversible color feature of the device, rewritable color printing is further implemented, as shown in Fig. 8(a). The prepared color device is first crystallized via thermal annealing and then amorphized by the picosecond laser for initialization. After that, laser color image printing is readily realized. Color flowers are printed by a nanosecond laser at the power of 2.3 mW, which can be completely erased by four picosecond pulses at the energy fluence of 1.1mJ/cm2. New flowers with different colors are further printed onto the same device at the power of 2.0 mW. Figure 8(b) shows flowers with three kinds of colors at the laser power of 0.3, 0.8, and 1.5 mW, respectively, indicating that the multilevel color printing can be realized. The color flowers are further erased by four picosecond pulses. Hence, the designed device is promising for rewritable multilevel color printing.

To investigate the feasibility of high-resolution printing, dot array patterns with various periods are fabricated onto the devices by the direct laser writing method, as shown in Fig. 8(c). The smaller and denser the individual pixel dot, the higher the resolution of the device. Although the dot period reduces from 1.2 to 0.6 μm, the pixel dot arrays are highly uniform and clear. At the period of d=0.6μm, the size of the pixel dot is close to 300 nm, far smaller than the spot size (that is, D=1.22λ/NA=1.22×405/0.8≈600nm), with the resolution of higher than 42,000 DPI, indicating that the color device can achieve nanoscale resolution.

To understand the formation mechanism of high-resolution pixel dots, photothermal simulation is performed to obtain the temperature field distribution of color device at nanosecond laser pulse. A three-dimensional transient pre-study is conducted using the solid heat transfer module of COMSOL software. The input parameters include the pulse width (Rect=10ns), spot radius (w0=0.3μm), laser wavelength (λ=405nm), and power (P=6mW). The films are considered as isotropic materials with uniform heat transfer properties. The relevant parameters of materials used are provided in Table 2, where refractive indices and extinction coefficients are measured while other parameters are from Refs. [50–52]. The initial and ambient temperatures are set to 293.15 K. The time-dependent transient heat transfer problem is described by the following equations: ρCp∂T∂t=k∇2T+Qheat(r,z),(2)where ρ is the mass density, Cp is the heat capacity, k is the thermal conductivity, T is the temperature, and Qheat(r,z) is the laser-induced heat quantity in radial (r) and thickness (z) directions. In this model, the heat quantity is expressed as Qheat(r,z)=α×I(r,z)×Rect,(3)where α is the absorption coefficient, defined as 4πk/λ, and k is the extinction coefficient. Rect denotes the width of the laser pulse. I(r,z) is the laser intensity in r and z directions, which can be expressed as I(r,z)=2Pπw2(z)exp[−2r2w2(z)].(4)Table 2.

Measured Refractive Index (n) and Extinction Coefficient (k) at 405 nm Wavelength, Heat Capacity (Cp), Thermal Conductivity (k) and Mass Density (ρ) of Al, aSb₂S₃, and SiO₂ Thin Films

The w(z) is the beam waist radius in the z direction, further obtained by w(z)=w01+z/z02.(5)z0 is the Rayleigh length, determined by z0=πw02/λ.(6)

According to Eqs. (2)–(6), the thermal distribution of the color device in r and z directions is simulated. Figures 9(a) and 9(b) suggest that the heat is mainly located at the center of Sb2S3 and SiO2 films in thickness and radial views. When the temperature reaches 240°C, the crystallization process of Sb2S3 takes place. The temperature profile in Fig. 9(c) indicates that the size of heat spot is as low as 250 nm, in good agreement with the experimental results. The high-resolution pixel is thus achievable by utilizing the crystallization threshold effect of Sb2S3.

A high-resolution, multilevel, and reversible color device is realized using the Al/Sb2S3/SiO2-stacking layer. The addition of SiO2 film can effectively restrain the volatilization of antimony/sulfur elements. Multilevel color images with wide gamut are successfully obtained by adjusting either Sb2S3 thickness or its crystallization degree. Furthermore, the color device can be patterned by direct laser writing and erased by a picosecond laser system, possessing good reversible cyclability stability. A high-resolution pixel higher than 42,000 DPI is further implemented by controlling the crystallization threshold effect of Sb2S3. Moreover, the design has minimal substrate requirements and maintains satisfactory compatibility with flexible PI substrate. After 10-time bending and folding, the flexible device is quite stable without fading, exhibiting incident-angle insensitivity. Therefore, the device is suitable for wide applications in the fields of color printing, flexible displays, wearable optoelectronic devices, and so forth.