Preparation method and application of electric field responsive photonic crystal for yellow-green switching camouflage device

Abstract

The application relates to a preparation method and application of an electric field response type photonic crystal for a yellow-green switching camouflage device. The application aims to solve the problems of poor fusion of the discoloring of the electric field response crystal prepared by the existing method and a typical background color, and the need for continuous application of an electric field to obtain a specific color, and the too short color steady state time after power-off. Method: I. Preparing monodisperse PVP modified CdS nanospheres; II. Preparing an electric field response type photonic crystal. The electric field response type photonic crystal for the yellow-green switching camouflage device is used for assembling a yellow-green switching camouflage device. Compared with a solid colloidal crystal, the volume fraction of the colloidal particles in the liquid colloidal crystal system is lower, the interval is larger, and the electric field response type photonic crystal prepared by the application often has a wider adjustment range, a faster response speed and higher sensitivity.

Description

Technical Field

[0001] This invention relates to a method for preparing a photonic crystal and its application. Background Technology

[0002] Electrochromism refers to the phenomenon where the color or optical properties of a material change reversibly under the influence of an applied electric field. It has been widely used in fields such as smart windows, electronic paper, flexible displays, and adaptive camouflage. In adaptive camouflage, devices are required to adapt to various terrains such as woodlands and deserts, or to backgrounds with mixed terrain features, and their colors are mostly yellow-green combinations.

[0003] Currently, electrochromic materials mainly consist of three types: field-responsive liquid crystal materials, traditional electrochromic devices, and field-responsive photonic crystals (ERPCs). Compared with other materials, photonic crystals offer an attractive platform for electromagnetic wave control due to their ease of color control through structure, stable structural colors, simple fabrication processes, and unique designability, making adaptive camouflage a possibility. One of the main requirements for responsive photonic crystals is their ability to dynamically change their structural colors in response to external stimuli, which has become a research hotspot in this field. For example, Baumberg et al. prepared electroresponsive photonic crystals based on PSMMA colloidal particles. The structural color could be modulated through the balance between the electric field and the repulsive force of the surface charge of the microspheres; however, the modulation range was only 80 nm, requiring an excessively high voltage. Meanwhile, researchers dispersed Fe3O4@SiO2 particles in propylene carbonate (PCb) to form a uniform dispersion system sandwiched between two transparent electrodes, preparing an electroresponsive photonic crystal whose structural color could change from red to blue with voltage changes. Furthermore, Ge's team concentrated monodisperse SiO2 colloidal particles in a mixture of ethoxylated trimethylolpropane triacrylate and PCb to prepare photonic crystal "electronic ink," enabling multicolor printing and grayscale control. Therefore, ERPC is a promising material for adaptive camouflage devices, offering better manufacturing processes and color saturation. However, the main problems with ERPC are: poor integration of the color change with the typical background color, and the need for continuous application of an electric field to obtain a specific color, raising concerns about signal exposure, energy consumption, and signal stability.

[0004] Research on stable color conversion devices based on ERPC is scarce because achieving a stable color switching state is a key challenge in this field. To date, only Ge et al. have achieved a 300-second stable state by adding a viscous liquid to the system. Therefore, there is a need to develop stable color conversion devices with high color saturation and longer stable-state times after power failure. Summary of the Invention

[0005] The purpose of this invention is to solve the problems of poor integration between the color change and typical background color of electric field responsive crystals prepared by existing methods, the need for continuous application of electric field to obtain a specific color, and the short color steady-state time after power is turned off. The invention provides a method for preparing an electric field responsive photonic crystal for yellow-green switching camouflage devices and its application.

[0006] A method for fabricating an electric field-responsive photonic crystal for yellow-green switching camouflage devices is carried out according to the following steps:

[0007] I. Preparation of monodisperse PVP-modified CdS (PMCdS) nanospheres:

[0008] ① Add polyvinylpyrrolidone to diethylene glycol and stir at room temperature until polyvinylpyrrolidone dissolves to obtain a PVP diethylene glycol solution;

[0009] ② Transfer the PVP diethylene glycol solution to a three-necked flask, install a condenser, heat to 75℃~80℃, and then add thiourea and cadmium nitrate tetrahydrate under stirring. After the solid is completely dissolved, heat to 155℃~165℃, continue stirring the reaction, and finally cool to room temperature to obtain the reaction product.

[0010] ③ Add anhydrous ethanol to the reaction product, then transfer it to a centrifuge tube and centrifuge. Discard the supernatant and wash the solid product several times with anhydrous ethanol to obtain monodisperse PMCdS nanospheres. Finally, disperse them in anhydrous ethanol to obtain a monodisperse PMCdS nanospheres in anhydrous ethanol.

[0011] II. Fabrication of Electric Field-Responsive Photonic Crystals:

[0012] Propylene carbonate was added to a monodisperse anhydrous ethanol dispersion of PMCdS nanospheres and ultrasonically dispersed to obtain a uniform suspension. The uniform suspension was then heated to remove some of the solvent, resulting in an electric field-responsive photonic crystal for yellow-green switching camouflage devices.

[0013] An electric field-responsive photonic crystal is used to assemble a yellow-green switching camouflage device.

[0014] The principle of this invention:

[0015] I. The wavelength of reflected light from the normal incident direction is calculated using Bragg's law:

[0016]

[0017] Where d is the lattice spacing, n eff Where n is the effective refractive index, d is the particle diameter, and n1 and n2 are the refractive indices of the particle and the medium, respectively. This refers to the volume fraction of negatively charged particles near a positively charged electrode. Based on Bragg's diffraction law, this invention fabricates an electro-responsive photonic crystal with low angle dependence and distinct structural color (an electric field-responsive photonic crystal for yellow-green switching camouflage devices) to address the problem of insufficient color saturation in traditional electro-responsive photonic crystals. Studies have shown that the reflection intensity of an electric field-responsive photonic crystal decreases with increasing electric field strength or application time. Simply put, this is due to the interference of the electric field on colloidal assembly, leading to a reduction in the number of ordered structure periods. The number of ordered structure periods in the photonic crystal... When the number of coherent periods is less than the maximum, the reflection intensity decreases. For electrically tunable devices, disturbances in the electric field within the component are unavoidable; therefore, using materials with a large difference in dielectric constant is the best solution to this problem. According to the Bragg diffraction principle, designing a photonic crystal with a high refractive index can achieve this goal. This invention selects monodisperse PMCdS microspheres as the structural unit of the electrically responsive photonic crystal. This material has a relatively large refractive index among currently reported electrically responsive photonic crystals: CdS (2.51), while propylene carbonate (PCb) has a refractive index of 1.42. Due to the large difference in refractive index between CdS and PCb, the prepared liquid photonic crystal has a wide photonic bandgap, a strong reflection peak signal, and exhibits a highly saturated structural color. By using direct observation, the PMCdS-based yellow-green switching camouflage device displays almost the same color at different angles, with applied voltages of -3.5V, 0V, and -3.5V.

[0018] Advantages of this invention:

[0019] This invention proposes a photonic crystal with stable coloring after power is turned off. The entanglement-locking effect of long-chain polyvinylpyrrolidone (PVP) balances electrostatic repulsion, further freezing the Brownian motion of particles and stabilizing the ordered structure. According to Bragg's diffraction law, enhancing the structural color rendering effect of electric field-responsive photonic crystals usually requires a high refractive index difference between the colloidal particles and the surrounding medium. According to the subtractive color theorem, in order to simulate the typical ground background color change of "woodland-desert", the initial liquid colloidal solution preferably contains either green or ochre, and then a ochre or light green liquid photonic crystal is obtained by superimposing the structural color and the initial subtractive color. Compared with solid colloidal crystals, the liquid colloidal crystal system has a lower volume fraction of colloidal particles and a larger spacing, often with a wider adjustment range, faster response speed and higher sensitivity. Attached Figure Description

[0020] Figure 1 The XRD pattern, FTIR spectrum and Zeta potential in anhydrous ethanol of the monodisperse PMCdS nanospheres prepared in Example 1 are shown in the figure. (a) is the XRD pattern, (b) is the FTIR spectrum and (c) is the Zeta potential.

[0021] Figure 2 SEM images and particle size distribution diagrams of the PMCdS nanospheres prepared in Examples 1-3 are shown in the figures. (a) shows the PMCdS nanospheres with an average particle size of 115 nm prepared in Example 1, (b) shows the PMCdS nanospheres with an average particle size of 180 nm prepared in Example 2, and (c) shows the PMCdS nanospheres with an average particle size of 235 nm prepared in Example 3.

[0022] Figure 3 This is a schematic diagram of the yellow-green switching camouflage device prepared in Example 7 and its working mechanism;

[0023] Figure 4 Optical images of the yellow-green switching camouflage device prepared in Example 7 from different perspectives;

[0024] Figure 5 This is the microscopic mechanism of the reversible multilevel behavior of electric field-induced lattice compression and recovery in low-concentration photonic crystals in Example 7;

[0025] Figure 6 (a) CIE 1931 color coordinates of the yellow-green switching camouflage device prepared in Example 7 in the range of -3.5V to +3.5V. Figure 6 (b) Digital photographs of ERPCs under different applied voltages;

[0026] Figure 7 The effect of PMCdS nanosphere concentration and applied voltage on the reflection spectra of electric field-responsive photonic crystals for yellow-green switching camouflage devices prepared in Examples 4-7 is shown in the figure. (a) is Example 4, (b) is Example 5, (c) is Example 7, and (d) is Example 6.

[0027] Figure 8 The reflection spectrum changes of the yellow-green switching camouflage device prepared in Example 7 are shown in the figure. (a) shows the reflection spectrum after applying a voltage of -3.5V, (b) shows the reflection spectrum after applying a voltage of -3.5V for 5 minutes and then removing it, (c) shows the reflection spectrum after applying two voltages continuously for 30 minutes, and (d) shows the reflection wavelength changes when ±3.5V voltage is applied alternately for 15 cycles.

[0028] Figure 9 A simulated visual image of a simple camouflage device against a background color typical of ground vegetation. Detailed Implementation

[0029] Specific Implementation Method 1: This implementation method describes a method for preparing an electric field-responsive photonic crystal for yellow-green switching camouflage devices, which is specifically completed according to the following steps:

[0030] I. Preparation of monodisperse PMCdS nanospheres:

[0031] ① Add polyvinylpyrrolidone to diethylene glycol and stir at room temperature until polyvinylpyrrolidone dissolves to obtain a PVP diethylene glycol solution;

[0032] ② Transfer the PVP diethylene glycol solution to a three-necked flask, install a condenser, heat to 75℃~80℃, and then add thiourea and cadmium nitrate tetrahydrate under stirring. After the solid is completely dissolved, heat to 155℃~165℃, continue stirring the reaction, and finally cool to room temperature to obtain the reaction product.

[0033] ③ Add anhydrous ethanol to the reaction product, then transfer it to a centrifuge tube and centrifuge. Discard the supernatant and wash the solid product several times with anhydrous ethanol to obtain monodisperse PMCdS nanospheres. Finally, disperse them in anhydrous ethanol to obtain a monodisperse PMCdS nanospheres in anhydrous ethanol.

[0034] II. Fabrication of Electric Field-Responsive Photonic Crystals:

[0035] Propylene carbonate was added to a monodisperse anhydrous ethanol dispersion of PMCdS nanospheres and ultrasonically dispersed to obtain a uniform suspension. The uniform suspension was then heated to remove some of the solvent, resulting in an electric field-responsive photonic crystal for yellow-green switching camouflage devices.

[0036] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the mass ratio of polyvinylpyrrolidone to diethylene glycol in step one ① is (3.0g~8.0g):150mL. The other steps are the same as in Specific Implementation Method One.

[0037] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the mass ratio of thiourea to polyvinylpyrrolidone in step one ② is (0.6g~1.2g):(3.0g~8.0g). The other steps are the same as in Specific Implementation Method One or Two.

[0038] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that: the mass ratio of cadmium nitrate tetrahydrate to polyvinylpyrrolidone in step one (②) is (2.3g~4.6g):(3.0g~8.0g); after the solid in step one (②) is completely dissolved, the temperature is raised to 155℃~165℃, and the reaction is continued with stirring for 2h~5h. Other steps are the same as in Specific Implementation Methods One to Three.

[0039] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that: the volume fraction of the monodisperse PMCdS nanospheres in anhydrous ethanol dispersion described in step one ③ is 0.5% to 2.0%, wherein the density of the PMCdS nanospheres is 4.8 g / cm³. 3 The other steps are the same as those in implementation methods one through four.

[0040] Specific Implementation Method Six: The difference between this implementation method and Specific Implementation Methods One to Five is that the volume ratio of propylene carbonate to monodisperse PMCdS nanospheres in step two is (191 μL ~ 195 μL): (5 μL ~ 9 μL). The other steps are the same as in Specific Implementation Methods One to Five.

[0041] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that: in step two, the uniform suspension is placed in a forced-air drying oven at a temperature of 80℃ to 100℃ and heated for 2 to 4 hours; the volume fraction of PMCdS nanospheres in the electric field-responsive photonic crystal used for yellow-green switching camouflage devices described in step two is 2.5% to 5.0%. Other steps are the same as in Specific Implementation Methods One to Six.

[0042] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that: an electric field-responsive photonic crystal is used to assemble the yellow-green switching camouflage device. The other steps are the same as in Specific Implementation Methods One to Seven.

[0043] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: a method for assembling a yellow-green switching camouflage device using an electric field-responsive photonic crystal is completed according to the following steps:

[0044] An electric field-responsive photonic crystal for the yellow-green switching camouflage device is cast onto the edge of a conductive glass or conductive film. Then, another piece of conductive glass or conductive film with 3M double-sided adhesive glued to its edge is placed over it. The device is then placed under low pressure to allow liquid to enter and fill the space between the two pieces of conductive glass or conductive film. Finally, the two pieces of conductive glass or conductive film are sealed to obtain the yellow-green switching camouflage device. Other steps are the same as in embodiments one through eight.

[0045] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One to Nine in that: the distance between the two conductive glass pieces or conductive films is 50–200 μm; the pressure of the low-pressure environment is 10–60 kPa; the two conductive glass pieces or conductive films are sealed with photocurable adhesive; the conductive glass is ITO glass, FTO glass, or AZO glass; the conductive film is a PET-ITO film or a silver nanowire film. Other steps are the same as in Specific Implementation Methods One to Nine.

[0046] The beneficial effects of the present invention are verified using the following embodiments:

[0047] Example 1: A method for preparing a monodisperse PMCdS nanosphere anhydrous ethanol dispersion, specifically carried out according to the following steps:

[0048] I. Preparation of monodisperse PMCdS nanospheres:

[0049] ① Add 5g of polyvinylpyrrolidone to 150mL of diethylene glycol and stir at room temperature until the polyvinylpyrrolidone dissolves to obtain a PVP diethylene glycol solution.

[0050] ② Transfer the PVP diethylene glycol solution to a 250mL three-necked flask, install a condenser, heat to 80℃, and then add 0.76g thiourea and 3.08g cadmium nitrate tetrahydrate under stirring. After the solids are completely dissolved, heat to 160℃ and continue stirring at 160℃ for 2 hours. Finally, cool to room temperature to obtain the reaction product.

[0051] ③ Add anhydrous ethanol to the reaction product, then transfer it to a centrifuge tube and centrifuge. Discard the supernatant and wash the solid product three times with anhydrous ethanol to obtain monodisperse PMCdS nanospheres. Finally, disperse them in anhydrous ethanol to obtain a monodisperse PMCdS nanospheres in anhydrous ethanol.

[0052] The monodisperse PMCdS nanospheres in anhydrous ethanol dispersion described in step 1, ③, have a volume fraction of 1.0%, and the density of the PMCdS nanospheres is 4.8 g / cm³. 3 ;

[0053] The average particle size of the PMCdS nanospheres in the anhydrous ethanol dispersion of monodisperse PMCdS nanospheres described in step 1③ is 115 nm.

[0054] Example 2: The difference between this example and Example 1 is that in step 1, ②, 0.95g of thiourea and 3.85g of cadmium nitrate tetrahydrate were added under stirring conditions. All other steps and parameters are the same as in Example 1.

[0055] The average particle size of the PMCdS nanospheres in the anhydrous ethanol dispersion of monodisperse PMCdS nanospheres prepared in Example 2 was 180 nm.

[0056] Example 3: The difference between this example and Example 1 is that step 1, ②, involves continuing the stirring reaction at 160°C for 4 hours. All other steps and parameters are the same as in Example 1.

[0057] The average particle size of the PMCdS nanospheres in the anhydrous ethanol dispersion prepared in Example 3 was 235 nm.

[0058] Figure 1 The XRD pattern, FTIR spectrum and Zeta potential in anhydrous ethanol of the monodisperse PMCdS nanospheres prepared in Example 1 are shown in the figure. (a) is the XRD pattern, (b) is the FTIR spectrum and (c) is the Zeta potential.

[0059] from Figure 1 As shown in (a), the PMCdS nanospheres are crystalline, and the diffraction peak positions and relative intensities of the obtained product are consistent with the standard cubic phase PMCdS spectrum, with the standard card being JCPDS NO.75-1545. This gives PMCdS a large dielectric constant and a high refractive index as a basic material. More importantly, the cubic phase crystal form results in a lemon-yellow color for the initial liquid colloidal crystals, which is more conducive to achieving the color variation of woodland-desert.

[0060] Figure 1 (b) is the infrared spectrum of a typical sample prepared. Where 1101 cm⁻¹... -1 The position of the characteristic Cd-S peak at 1282 cm⁻¹ indicates the presence of CdS. In the spectrum, this peak is located at 1282 cm⁻¹. -1 1633cm -1 2938cm -1 These peaks represent the characteristic peaks of -CN, -C=O-, and -CH, respectively, confirming the presence of PVP in the sample. The 3000–3500 cm⁻¹ peaks... -1 The strong and broad absorption bands between them are considered to be the absorption peaks of the stretching vibration of the -OH group and the deformation vibration of the OH group in physically adsorbed water. This type of water is considered to be water adsorbed by PVP molecules.

[0061] The zeta potential of PMCdS particles in ethanol is shown in Figure 1 (c) The particle carries a positive charge due to the coating of PVP molecules, and measurements show that its surface zeta potential is +5 mV. This value is significantly lower than the critical value of -30 mV for colloidal particles to maintain stability in solution. For electrostatically repulsive colloidal particles, if their zeta potential is less than -30 mV, they cannot be stably dispersed in solution and may even precipitate. However, PMCdS particles exhibit good dispersion stability and can remain dispersed in ethanol for a long time. This is because the suspension stability of the particles comes from the steric hindrance generated by the nonionic polymer PVP on the surface.

[0062] Figure 2 SEM images and particle size distribution diagrams of the PMCdS nanospheres prepared in Examples 1-3 are shown in the figures. (a) shows the PMCdS nanospheres with an average particle size of 115 nm prepared in Example 1, (b) shows the PMCdS nanospheres with an average particle size of 180 nm prepared in Example 2, and (c) shows the PMCdS nanospheres with an average particle size of 235 nm prepared in Example 3.

[0063] from Figure 2It can be seen that the size distribution of a typical PMCdS nanosphere with an average particle size of 115 nm, plotted based on the statistical results of scanning electron microscopy (SEM) images, deviates from the average diameter by approximately 8.6%. The narrow size distribution, tunable particle size, and lemon yellow as the primary color make PMCdS particles an ideal component of colloidal photonic crystals.

[0064] Example 4: The preparation method of the yellow-green switching camouflage device is completed according to the following steps:

[0065] I. Fabrication of electric field-responsive photonic crystals:

[0066] Propylene carbonate was added to the monodisperse PMCdS nanospheres in anhydrous ethanol dispersion prepared in Example 1, and ultrasonically dispersed to obtain a uniform suspension. The suspension was then placed in a forced-air oven at 90°C and heated for 4 hours to remove some of the solvent, resulting in an electric field-responsive photonic crystal for yellow-green switching camouflage devices.

[0067] The volume ratio of propylene carbonate to monodisperse PMCdS nanospheres in anhydrous ethanol dispersion in step one is 390 μL: 1.0 mL.

[0068] The volume fraction of PMCdS nanospheres in the electric field-responsive photonic crystal used for yellow-green switching camouflage devices described in step one is 2.5%.

[0069] 2. Cast the electric field-responsive photonic crystal used for the yellow-green switching camouflage device onto the edge of the ITO glass, then cover it with another piece of ITO glass with 3M double-sided adhesive glued to the edge, and then place it in a low-pressure environment to allow liquid to enter and fill the space between the two pieces of ITO glass. Finally, use light-curing adhesive to seal the two pieces of ITO glass to obtain the yellow-green switching camouflage device.

[0070] The distance between the two ITO glass pieces mentioned in step two is 100 μm.

[0071] Example 5: The difference between this example and Example 4 is that the volume ratio of propylene carbonate to monodisperse PMCdS nanospheres in anhydrous ethanol in step one is 386 μL: 1.4 mL; the volume fraction of PMCdS nanospheres in the field-responsive photonic crystal used for yellow-green switching camouflage devices in step one is 3.5%. All other steps and parameters are the same as in Example 4.

[0072] Example 6: The difference between this example and Example 4 is that the volume ratio of propylene carbonate to monodisperse PMCdS nanospheres in anhydrous ethanol in step one is 382 μL: 1.8 mL; the volume fraction of PMCdS nanospheres in the field-responsive photonic crystal used for yellow-green switching camouflage devices in step one is 4.5%. All other steps and parameters are the same as in Example 4.

[0073] Example 7: The difference between this example and Example 4 is that the volume ratio of propylene carbonate to monodisperse PMCdS nanospheres in anhydrous ethanol in step one is 384 μL: 1.6 mL; the volume fraction of PMCdS nanospheres in the field-responsive photonic crystal used for yellow-green switching camouflage devices in step one is 4%. All other steps and parameters are the same as in Example 4.

[0074] Figure 3 This is a schematic diagram of the yellow-green switching camouflage device prepared in Example 7 and its working mechanism;

[0075] This invention successfully prepared various monodisperse PMCdS microspheres with high dielectric constants. Furthermore, liquid colloidal crystals were prepared via volatilization-induced self-assembly. Figure 3 Because PMCdS colloidal particles carry a positive charge, under low voltage, they can move and aggregate along the electric field direction towards the negative electrode, leading to a decrease in interparticle and lattice spacing, and consequently, a change in structural color. The resulting EPRCs exhibit good stability, a simple preparation process, and a reversible color change from light green to yellowish-brown. The successful preparation of these photonic crystals provides new ideas for developing color-changing camouflage materials with stable colors and high controllability.

[0076] Figure 4 Optical images of the yellow-green switching camouflage device prepared in Example 7 from different perspectives;

[0077] from Figure 4It can be seen that the yellow-green switching camouflage device prepared in Example 7 displays almost the same color at different angles, with applied voltages of -3.5V, 0V, and -3.5V respectively. When a voltage is applied to the electrode, the positively charged PMCdS particles move towards the cathode in the electric field. The particles are more closely arranged on the negative electrode until the electrostatic repulsion between particles and the spatial steric hindrance between PVP molecules increase to a new equilibrium with the electrophoretic force. Compared to the initial state, the lattice constant of the photonic crystal shrinks, causing its reflection peak to shift blue, thus causing a corresponding change in the structural color. Based on the subtractive color superposition theory, the negative electrode color rendering effect of the device is the superposition of the initial lemon yellow color and the structural color. When an electric field of -3.5V is applied, the color is the superposition of green and lemon yellow: ochre; correspondingly, when an electric field of +3.5V is applied, the negative electrode color is the superposition of the blue-green structural color and lemon yellow: green.

[0078] The microscopic mechanism of the reversible multilevel behavior of lattice compression and restoration induced by the electric field in Example 7 is as follows: Figure 5 As shown, this invention hypothesizes that the entire excitation process can be divided into three stages: Stage I, Stage II, and Stage III.

[0079] Stage I: After loading a colloidal solution between two ITO electrodes, particles spontaneously precipitate from propylene carbonate (PCb) to form a liquid photonic crystal, thereby minimizing the entropy of the entire colloidal system. In this invention, the low-concentration liquid photonic crystal mainly consists of dispersed colloidal particles and aggregated microcrystals, while previously reported high-concentration liquid photonic crystals are mainly composed of microcrystals.

[0080] Stage II: Under voltage, both individual colloidal particles and microcrystals move towards the negative electrode. The driving force acting on the microspheres (differences in electric field force, weight, and electrophoretic resistance) is less than the driving force acting on the individual colloidal particles. Therefore, most individual colloidal particles preferentially reach the electrode surface, resulting in the formation of a photonic crystal (denoted as n). p The regularity of the regularity and the effective refractive index (n in formula (1)) eff (n1 = 2.51, n2 = 1.42) is lower than that of microcrystals (denoted as n m The photonic crystal formed. Since the volume fraction of CdS in the particles is less than the volume fraction in the crystallites, therefore n p Less than n m Therefore, compared with classical electroresponsive photonic crystals, the electroresponsive photonic crystal in this invention exhibits lower reflectivity and greater regularity. Previous studies on electroresponsive photonic crystals mainly demonstrated structural color variations, while our study is characterized by subtractive color superposition. It can be inferred that low-concentration photonic crystals remain the primary cause of subtractive color superposition.

[0081] Stage III: Once the electric field is removed, the electrostatic repulsion between adjacent colloidal particles and the steric hindrance between PVP molecules trigger a certain lattice expansion of the photonic crystal. The liquid photonic crystal in equilibrium gradually transforms into microcrystals, resulting in a subtractive superposition of the original lemon yellow and structural red (ochre). After voltage removal within a short period (within 3 hours), the entanglement-locking effect of the long-chain PVP balances the electrostatic repulsion, further freezing the Brownian motion of the particles and stabilizing the ordered structure. With prolonged voltage removal (more than 3 hours), the electroresponsive photonic crystal slowly returns to its initial state.

[0082] Figure 5 This is the microscopic mechanism of the reversible multilevel behavior of electric field-induced lattice compression and recovery in low-concentration photonic crystals in Example 7;

[0083] This invention utilizes the CIE color space to evaluate visually perceived colorimetric changes, and the relevant data is plotted on... Figure 6 (a). Interestingly, the yellow-green switching camouflage device prepared in Example 7 exhibits color switching capability between yellow and green (including ochre, light yellow, yellow-green, and light green) at different voltages from -3.5V to +3.5V, such as... Figure 6 As shown in (b), this makes it very suitable for military camouflage applications.

[0084] Figure 6 (a) CIE 1931 color coordinates of the yellow-green switching camouflage device prepared in Example 7 in the range of -3.5V to +3.5V. Figure 6 (b) Digital photographs of ERPCs under different applied voltages;

[0085] Table 1 shows the CIE 1976L values ​​for the yellow-green switching camouflage device prepared in Example 7 within the range of -3.5V to +3.5V. * a * b * Color coordinates. At -3.5V and 3.5V, the color coordinates of the photonic crystal are respectively... =71.47and Color difference (ΔE) * ab The distance between two points in a color space can be determined using the following formula:

[0086]

[0087] According to reports, ΔE * ab Values ​​less than 3.0 are generally considered almost imperceptible, while values ​​between 3.0 and 6.0 may be less noticeable. When the ΔE value exceeds 6.0, the human eye can more easily perceive color changes. The ΔE between the device's yellowish-brown state at -3.5V and its light green state at 3.5V was measured.* ab The value of 33.6 indicates that the yellow-green switching camouflage device of the present invention exhibits a significant color change. Furthermore, ΔE for different shades of green or yellow under different voltages was calculated. * ab Values. Different yellow values ​​for ΔE within the ranges of -3.5V to -3.0V and -3.0V to 0V. * ab The values ​​reached 10.3 and 15.3 respectively. The ΔE values ​​for different shades of green in the ranges of 0V–3.0V and 3.0V–3.5V were also observed. * ab The values ​​are 10.7 and 36.1, respectively. Different yellow-green colors can be distinguished under different applied voltages, ΔE. * ab The values ​​all exceed 6.0. In summary, the subtractive color mixing principle of structural color and initial color in photonic crystals can achieve reversible switching between ground yellow and light green in a single display unit under different voltages.

[0088] Table 1. CIE color coordinates of the yellow-green switching camouflage device prepared in Example 7 at different potentials.

[0089]

[0090] Note: The data in the table were obtained using a colorimeter, and the observation angle for the test was 10 degrees. ΔE * ab This represents the distance between the point corresponding to -3.5V (the color of an electric field-responsive photonic crystal is tan) and a point in the color space.

[0091] Figure 7 The effect of PMCdS nanosphere concentration and applied voltage on the reflection spectra of electric field-responsive photonic crystals for yellow-green switching camouflage devices prepared in Examples 4-7 is shown in the figure. (a) is Example 4, (b) is Example 5, (c) is Example 7, and (d) is Example 6.

[0092] from Figure 7 It can be seen that as the concentration of microspheres used to prepare ERPCs increases from 3.5% to 4.5%, the position of the maximum reflection peak at -3.5V remains almost unchanged, while the peak intensity tends to increase. 2.5% ERPCs did not exhibit any identifiable electroresponsive behavior. Due to the high viscosity and complex packaging requirements associated with ERPCs, the optimal concentration of the crystals described above was determined to be 4.5%. It is worth noting that, as... Figure 7As shown in (d), the reflection peak intensity of ERPCs based on PMCdS nanospheres does not change significantly in the voltage range of 0 to -2.5V. However, the reflection peak shifts to some extent when the voltage reaches -3.0V. Furthermore, when the maximum voltage is -3.5V, the reflectivity peak continues to shift from 720nm to nearly 560nm. Therefore, the optimal concentration for preparing ERPCs using 115nm PMCdS nanospheres is determined to be 3.5%–4.5%.

[0093] Electrical response behavior is an important parameter for evaluating the performance of ERPCs. Its main electrical response performance parameters include voltage, response time (on-state response time and off-state response time), stability, and reversibility.

[0094] Figure 8 The reflection spectrum changes of the yellow-green switching camouflage device prepared in Example 7 are shown in the figure. (a) shows the reflection spectrum after applying a voltage of -3.5V, (b) shows the reflection spectrum after applying a voltage of -3.5V for 5 minutes and then removing it, (c) shows the reflection spectrum after applying two voltages continuously for 30 minutes, and (d) shows the reflection wavelength changes when ±3.5V voltage is applied alternately for 15 cycles.

[0095] First, this invention uses a -3.5V voltage as the test voltage to determine the electrical response rate; such as Figure 8 As shown in (a), the electric field-responsive photonic crystal used for the yellow-green switching camouflage device reaches the equilibrium reflection peak shift (90% of the maximum reflectivity) about 30s after the voltage is applied, and the intensity of the reflection peak gradually increases in the following 30s. Due to the symmetry of the device, when the same absolute value of voltage is applied, the time required for the front side to transition from the initial state to yellowish-brown is equivalent to the time required for the back side to transition from the initial state to light green.

[0096] When an electric field of -3.5V is applied for 5 minutes, the shift of the reflection peak of ERPCs after the voltage is removed can be divided into two stages: (1) a rapid redshift stage and (2) a gradual redshift stage. In the first stage, the reflection peak of ERPCs undergoes a rapid shift, with its peak position redshifting by 125nm within 5s (magnified). Figure 8 (b) In this stage, the electrostatic repulsion between microspheres and the steric hindrance of PVP molecules facilitate the movement of colloidal particles. In the second stage, the peak position gradually shifted 10 nm towards the equilibrium position, which took approximately 3 minutes to achieve, and then stabilized within 3 hours. This phenomenon is attributed to the entanglement of PVP molecular chains after prolonged exposure to the electric field, with some microspheres becoming anchored. This entanglement-locking effect of long-chain PVP balances the electrostatic repulsion, maintaining the distance between colloidal particles in the photonic crystal assembly within a certain range. The migration velocity of charged particles under the influence of an electric field can be represented by an electrophoresis equation, which is used to analyze the electroresponse rate of ERPCs. The equation can be written as:

[0097] U=ζεE / Kπηd Formula (3);

[0098] Where ζ is the colloidal potential, ε is the dielectric constant, E is the voltage between the two electrodes, η is the liquid viscosity, and d is the distance between the two electrodes. Therefore, a larger dielectric constant and colloidal potential can lead to faster migration and electric field response rates of the liquid photonic crystal under an electric field, which is mainly related to the selectivity of the colloidal particles. During device assembly, using a shorter electrode distance can also improve the response rate of the electroresponsive photonic crystal. Under the same conditions, the viscosity of the liquid with a non-volatile organic solvent is negatively correlated with the electric field response rate of the device.

[0099] Furthermore, the stability of electric field-responsive photonic crystals (ERPCs) under an electric field is crucial for evaluating device performance. ERPCs were subjected to -3.0V and -3.5V for 5 minutes, respectively, and their reflection spectra were recorded every 10 seconds. Figure 8 As shown in (c), the reflection peak shift of ERPCs rapidly reaches its equilibrium state after an electric field of -3.0V is applied, and remains almost unchanged under the subsequent 5 minutes of voltage application. A hypothesis is proposed that the low surface charge of the microspheres and the steric hindrance between PVP molecules may be the reason why the applied electric field cannot further compress the distance between colloidal lattice particles. Under a voltage of -3.5V, the reflection peak shift of ERPCs reaches its maximum value and tends to decrease after 2 minutes, reaching an equilibrium state. In the above process, the blue shift of the reflection peak originates from the lattice contraction caused by the electrophoretic motion of PMCdS nanospheres in the liquid photonic crystal cell under the influence of the external electric field, while the red shift is due to the strong electrostatic repulsion between colloidal particles after excessive lattice contraction and the steric hindrance between PVP molecules.

[0100] The reversible cyclic response of the yellow-green switching camouflage device (ERPC) as a color switching unit under an electric field was investigated. The ERPCs operated for 15 cycles at -3.5V and +3.5V (80s per cycle at ±3.5V, 40s per cycle at ±3.5V). The position and intensity changes of the reflection peaks of the ERPCs were continuously recorded. Figure 8(d) shows that during a single electric field switching process, applying a -3.5V voltage causes the reflection peak to blue shift to approximately 567nm, followed by applying a +3.5V voltage, which shifts the peak to approximately 495nm. The applied reverse voltage (+3.5V) causes the PMCdS microsphere clusters to collectively sink, resulting in a synchronous decrease in reflectivity. After 15 cycles, no attenuation of the reflection peak shift was observed, indicating that the positions of the reflection peaks of ERPCs remain essentially unchanged in the "ochre" and "light green" states. It is evident that the PMCdS-based yellow-green switching camouflage device can undergo 15 cycles in a short period, achieving a reversible color change between ochre and light green. It is worth noting that in the case of camouflage color-changing devices, frequent color adjustments in a short period are generally unnecessary.

[0101] In fact, the crucial requirement is the ability to maintain a relatively constant color performance after power is cut off. Traditional ERPCs change color immediately after power is cut off and gradually return to their initial color, while the ERPCs in this invention exhibit a particularly stable state. Table 1 shows the CIE color coordinates of the PMCdS-based yellow-green switching camouflage device after applying a -3.5V voltage for 5 minutes and removing it. The vast majority of color difference values ​​(ΔE) within 3 hours after voltage removal... * ab The color difference was consistently below 4.0, except for one isolated instance whose color difference was 5.8 after 15 minutes of voltage removal. These results indicate that color changes in ERPCs are not easily detectable by the naked eye, especially on camouflage devices at a distance of hundreds of meters. The entanglement between PVP molecules balances the electrostatic repulsion, which further allows ERPCs to retain their earthy yellow color for extended periods without the need for frequent short-term voltage applications (detailed explanation is available in...). Figure 5 (This will be explained in Phase III).

[0102] Figure 9 A simulated visual image of a simple camouflage device against a background color typical of ground vegetation;

[0103] The electric field-responsive photonic crystal used in the yellow-green switching camouflage device exhibits excellent electric field response performance and closely resembles a typical ground background. By combining microfabrication and microelectronics technologies, this display device holds promise for further miniaturization, enabling the production of high-resolution practical devices and their assembly into simple camouflage devices mimicking typical ground vegetation background colors. Figure 9 This image shows a simple simulation of a camouflage device against a typical ground vegetation background, where all colors shown are experimentally prepared and extracted from [source missing]. Figure 6These devices combine applied electric fields at each pixel to create camouflage digital patterns by adjusting voltage, and are capable of simulating various typical ground backgrounds, including forests, wilderness, and deserts. Therefore, the fabrication strategy for the yellow-green reversibly tunable photonic crystal proposed in this study can provide a simple and practical method for the development of adaptive military camouflage photonic crystals.

[0104] In summary, this invention proposes the fabrication of novel electric field-responsive photonic crystals using monodisperse PMCdS microspheres with high refractive index. Based on the subtractive color theory of structural and initial colors, ERPCs are prepared by dispersing PMCdS particles in propylene carbonate via evaporation-induced self-assembly. Lattice contraction induced by an external electric field causes a blue shift in the reflection peak of the ERPCs. Thanks to the use of microspheres with high refractive index, the electric field-responsive photonic crystal exhibits strong reflective color, thereby enhancing the color saturation of the device. The electric field-responsive photonic crystal based on PMCdS microspheres (for yellow-green switching camouflage devices) has a simple fabrication process, good stability, can maintain a yellowish-brown state for a long time, and can reversibly change color from light green to yellowish-brown. The aforementioned device has high application prospects in the field of adaptive camouflage.

Claims

1. A method for fabricating an electric field-responsive photonic crystal for yellow-green switching camouflage devices, characterized in that... The preparation method is specifically carried out according to the following steps: I. Preparation of monodisperse PVP-modified CdS nanospheres: ① Add polyvinylpyrrolidone to diethylene glycol and stir at room temperature until polyvinylpyrrolidone dissolves to obtain a PVP diethylene glycol solution; ② Transfer the PVP diethylene glycol solution to a three-necked flask, install a condenser, heat to 75℃~80℃, and then add thiourea and cadmium nitrate tetrahydrate under stirring. After the solid is completely dissolved, heat to 155℃~165℃, continue stirring the reaction, and finally cool to room temperature to obtain the reaction product. ③ Add anhydrous ethanol to the reaction product, then transfer it to a centrifuge tube and centrifuge. Discard the supernatant and wash the solid product several times with anhydrous ethanol to obtain monodisperse PMCdS nanospheres. Finally, disperse them in anhydrous ethanol to obtain a monodisperse PMCdS nanospheres in anhydrous ethanol. II. Fabrication of Electric Field-Responsive Photonic Crystals: Propylene carbonate was added to a monodisperse PMCdS nanosphere anhydrous ethanol dispersion and ultrasonically dispersed to obtain a uniform suspension. The uniform suspension was heated to remove part of the solvent, resulting in an electric field-responsive photonic crystal for yellow-green switching camouflage devices. In step two, the volume fraction of PMCdS nanospheres in the electric field-responsive photonic crystal used for yellow-green switching camouflage devices is 3.5% to 4.5%.

2. The method for fabricating an electric field-responsive photonic crystal for a yellow-green switching camouflage device according to claim 1, characterized in that... In step 1①, the mass ratio of polyvinylpyrrolidone to diethylene glycol is (3.0g~8.0g):150mL.

3. The method for fabricating an electric field-responsive photonic crystal for a yellow-green switching camouflage device according to claim 1, characterized in that... The mass ratio of thiourea to polyvinylpyrrolidone mentioned in step 1② is (0.6g~1.2g):(3.0g~8.0g).

4. The method for fabricating an electric field-responsive photonic crystal for a yellow-green switching camouflage device according to claim 1, characterized in that... The mass ratio of cadmium nitrate tetrahydrate to polyvinylpyrrolidone in step 1 and 2 is (2.3g~4.6g):(3.0g~8.0g); after the solid in step 1 and 2 is completely dissolved, the temperature is raised to 155℃~165℃ and the reaction is continued with stirring for 2h~5h.

5. The method for fabricating an electric field-responsive photonic crystal for a yellow-green switching camouflage device according to claim 1, characterized in that... The volume fraction of the monodisperse PMCdS nanospheres in anhydrous ethanol dispersion described in step 1, ③, is 0.5%–2.0%, wherein the density of the PMCdS nanospheres is 4.8 g / cm³. 3 .

6. The method for fabricating an electric field-responsive photonic crystal for a yellow-green switching camouflage device according to claim 1, characterized in that... The volume ratio of propylene carbonate to monodisperse PMCdS nanospheres in step two is (191 μL~195 μL):(5 μL~9 μL).

7. The method for fabricating an electric field-responsive photonic crystal for a yellow-green switching camouflage device according to claim 1, characterized in that... In step two, the uniform suspension is placed in a forced-air drying oven at a temperature of 80℃~100℃ and heated for 2h~4h.

8. The application of an electric field-responsive photonic crystal for a yellow-green switching camouflage device prepared by the preparation method described in claim 1, characterized in that... An electric field-responsive photonic crystal is used to assemble a yellow-green switching camouflage device.

9. The application of an electric field-responsive photonic crystal for a yellow-green switching camouflage device according to claim 8, characterized in that... An electric field-responsive photonic crystal for assembling a yellow-green switching camouflage device is provided by the following steps: An electric field-responsive photonic crystal for yellow-green switching camouflage devices is cast onto the edge of a conductive glass or conductive film. Then, another piece of conductive glass or conductive film with 3M double-sided adhesive glued to its edge is covered. The device is then placed in a low-pressure environment to allow liquid to enter and fill the space between the two pieces of conductive glass or conductive film. Finally, the two pieces of conductive glass or conductive film are sealed to obtain the yellow-green switching camouflage device.

10. The application of an electric field-responsive photonic crystal for a yellow-green switching camouflage device according to claim 9, characterized in that... The distance between the two conductive glass pieces or conductive films is 50~200μm; the pressure of the low-pressure environment is 10~60kPa; the two conductive glass pieces or conductive films are sealed with photocurable adhesive; the conductive glass is ITO glass, FTO glass or AZO glass; the conductive film is PET-ITO film or silver nanowire film.

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