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Abstract
We proposed a multi-layered nanorod structure with the same tilt angle and different diameters, which has high visible transmittance and strong 3-5 µm absorption based on the principles of the gradient of the refractive index and the multi-size cavity resonances. The indium tin oxide (ITO) was selected as the target material to fabricate the structure by using a glancing angle deposition method. The experimental results show that when the deposition angle θ is 80°, swing deposition is successively done with the rotation angle φ of ±8°, ± 5°, ± 3°, and 0° on the surface of the substrate, and the quartz crystal microbalance thicknesses of ITO nanorods are 220 nm for each deposition, the average transmittance is 80.5% in the range of 400-800 nm and the integrated absorption is 86% in the 3-5 µm band. Such a simple, low-cost, and easy-to-fabricate device has potential applications in window stealth materials and other related fields.
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1. Introduction
Mid-infrared metamaterial broadband absorbers have a wide range of applications in infrared stealth [1–3], energy capture [4,5], solar cells [6], and sensors [7–10]. However, since the absorption of metamaterial absorbers depends on the resonance of their units, the absorption spectra are often narrow. Once the spectra are far away from the resonance frequencies, the absorption will be greatly attenuated, which limits the applications of the absorbers. How to effectively obtain broadband absorbers has become a research hotspot in recent years. Most of the existing broadband absorbers are composed of resonant units with different dimensions in one structural unit. When electromagnetic waves interact with such structures, their independent units can be excited to resonate, thereby generating broadband absorption. Metal/dielectric complexes are the most straight-forward way to achieve multiple resonant elements. Based on this, Hu et al [11] designed a truncated cone structure with alternating stacks of Au/ZnO, which has near-perfect absorption in the 2.7-4 µm band. Liu et al [12] fabricated a multi-layer Si/Fe stack structure, exhibiting an average absorption of over 96% in the range of 300-3000 nm. Qin et al [13] designed four Ti rings with two sizes in one structural period. The structure has multiple absorption units in the range of 7.76-14 µm, and its average absorption reaches 93.8%. In addition to relying on the design of the multi-layer stack structure, broadband absorption can also be achieved utilizing the inherent properties of materials themself. For example, Dang et al [14] proposed a method to capture about 95% light by adding a layer of epsilon-near-zero (ENZ) nano film in the periodic structure of Au/SiO2/TiN. Smith et al [15] designed and fabricated a 10-layer SiO2/ITO alternating ENZ thin film structure, which can achieve more than 90% absorption in the wavelength range of 1.5-2.4 µm. In addition, genetic algorithms and multi-mode resonances are also important methods to achieve mid-infrared broadband absorption. For example, Li et al [16] used a genetic algorithm to design a complex structure based on the tungsten material, which has an absorption rate of over 80% in the 2-4.5 µm band. Yu et al [17] proposed a five-layer metal-sphere-metal-insulator-metal structure, which can simultaneously support multiple mode resonances to achieve over 88% absorption in the 3-6 µm range.
In order to meet the design requirements of some window materials, such as air force helmets, windshields of stealth fighters and so on, the absorbers not only have strong absorption capacities in the mid-infrared band, but also require high transmittance in the visible band to meet the needs of human observation. For the high transmittance performance in the visible band, the researchers mainly use the gradient of refractive indexes from the environment to the substrate to minimize the Fresnel reflection and realize the efficient transmission of visible light. Liu et al [18] designed a bionic “moth-eye” TiO2 nanocone structure with high transmission in the visible band. Feng et al [19] used the glancing angle deposition (GLAD) technology to prepare graded index SiO2 thin films, which achieved high performance in the 400-1800 nm band. Inspired by leafhopper wings, Li et al [20] designed a hydrangea-like nanostructure with gradient refractive indexes, with an average transmittance of 91% in the visible light band. A SiO2/TiO2 bilayer anti-reflection film was prepared by Zhao et al [21] to improve the average transmittance of glass substrates in the range of 380-780 nm.
At present, the materials with high visible transmittance and strong mid-infrared absorption are widely studied, respectively, but it is rare for the researches on materials with two simultaneous properties. Recently, for this purpose, the layered nanorod array structure with different tilt angles was prepared by our group using the oblique angle deposition (OAD) technology, and the structure achieved high visible transmission and strong mid-infrared absorption. In this paper, the GLAD [22–24] technique was used to fabricate a multi-layered nanorod structure with the same tilt angle and different diameters by swinging at different rotation angles. The experimental results show that the structure has an average transmittance of 80.5% in the visible band, and an integrated absorption of 86% in the mid-infrared band of 3-5 µm. Its transmittance and absorption values are larger than those of the previous work.
2. Experiment
2.1 Materials
The substrate was conductive glass, which was bought from Huanan Xiangcheng Technology Co., Ltd. ITO (m(In2O3) : m(SnO2) = 90 : 10) was purchased from Dingwei New Material Co., Ltd. Ultrapure water (18.25 M·cm−1) was prepared by an ultrapure water machine in our laboratory. The purity of acetone and ethanol was analytically pure.
2.2 Fabrication of samples
An electron beam evaporation system (DE 500) was used to prepare samples. As shown in Fig. 1(a), shows the GLAD schematic diagram. Its main mechanisms for controlling the growth of the material are shadow effects and surface diffusion. The growth process of a 3-5 µm absorber with the high visible transmittance is shown in Fig. 1(b). Firstly, a single-sided polished conductive glass substrate and a silicon wafer used for morphology characterization were sequentially placed in acetone, alcohol, and ultrapure water for ultrasonic cleaning for five minutes. Then, the substrate and Si wafer were dried with flowing nitrogen and attached to the sample stage. Next, place the indium tin oxides (ITO) target in a crucible 50 cm below the sample stage, set the deposition angle θ to 80°, and set the rotation angle φ to ± 8°, ± 5°, ± 3°, and 0° in turn. During the process, the deposition was oscillated at a rate of 1°/s. The thickness and rate of deposition were monitored by two separate 6 MHz quartz crystal microbalance (QCMs), which were 220 nm and 0.1 nm/s, respectively. The growth rate of films was constant throughout the deposition process. The mean free path in our evaporation system was about 1.51 × 103 cm, which was larger than the source-substrate distance. Therefore, the vapour flux reached at the substrate was extremely directional. The substrate was at room temperature during the deposition. Evaporation deposition started when the vacuum level in the chamber reached below 5 × 10−7 Torr and the chamber vacuum level was not higher than 5 × 10−5 Torr throughout the experiment. The whole experimental deposition time was about 150 minutes. Finally, the samples were placed in a tube furnace for annealing at 550 °C for 30 minutes in an air environment in order to improve crystal structures and reduce their inner stresses, and the temperature was increased at a rate of 1 °C/min. The annealed structures have better stabilities.
Fig. 1. (a) Schematic diagram of GLAD technology; (b) Flow chart of fabrication of multi-layered nanorod structures with the same tilt angle and different diameters.
2.3 Characterization
In this paper, a Fourier infrared spectrometer [25] (Bruker Tensor 27) was used to measure the absorption in the mid-infrared band (3-5µm). During the measurement, the transmittance (T) of the sample was measured with air as the scanning background, and then the reflectance (R) of the sample was measured with a standard gold mirror as the scanning background. Finally, according to the formula $A\; = \; 1 - R - T$, the absorption (A) of the structure in the mid-infrared band was calculated. The transmittance (T) of the sample in the visible band was measured using an ultraviolet/visible/near-infrared spectrophotometer [26] (Lambda 950), with air as the scanning background. A field emission scanning electron microscope (SEM) (SU8010, Hitachi) was used to observe the morphology of the sample. The corresponding surface and side SEM images are shown in Fig. 2. It can be observed that the sample is flanked by multi-layer inclined nanorods and has a porous columnar structure on the surface. On the other hand, when the deposition angle θ is 80°, the actual inclination angle of the grown sample is 34°. When the nanorod QCM thicknesses for each different rotation angle are set to 220 nm, the actual growth thickness is 200 nm. The relevant data are marked in Fig. 2(b). The formation of the columnar structure is due to the fact that when the deposition angle is large, the cores will form and produce the shadow centers, which block the deposition of atoms in the shadow areas on the back sides of the cores. As the atoms continue to be deposited, the evaporated atoms land on the top of the nuclear structure, making the nuclear structure higher and higher, and eventually growing into a columnar structure in the direction of the vapor. Finally, a porous film is formed, and the pore sizes vary with the deposition angle.
Fig. 2. SEM images of the surface (a) and side (b) of the multi-layered nanorod structure with the same tilt angle and different diameters.
3. Results and discussion
Figure 3(a) shows the transmittance spectrum of the sample in the range of 400-800 nm. Evidently, the transmittance in the visible band is high. ITO is a transparent material in this band. When each layer of nanorod arrays composed of ITO was fabricated, the different refractive indexes of each layer could be obtained due to different rotation angles. The larger the rotation angles, the larger the nanorod diameters and the larger the resulting equivalent refractive indexes of the layers. From the substrate to the air background, the equivalent refractive index of each layer gradually decreases, satisfying the condition of the gradient of the refractive indexes. Hence, high transparency in the visible band can be achieved. Figure 3(b) shows the absorption spectrum of the sample in the 3-5 µm band. From the figure, we can see that the sample has strong absorption in this band. It is well known that ITO is also a mid-infrared plasmonic material. When mid-infrared light is incident on the sample, the light interacts with nanorods in the sample and excites surface plasmonic resonances, resulting in cavity resonances between different nanorods. Additionally, the cavities have different sizes in the different layers of nanorod arrays. Their resonances occur at different absorption wavelengths, leading to the broadening of absorption, which will be demonstrated in the simulations of the electric field intensity distributions.
Fig. 3. (a) Transmission spectrum in the visible band and (b) absorption spectrum in mid-infrared band of the multi-layered nanorod sample.
3.1 Influences of structural parameters on optical properties
In order to achieve high transmittance in the visible band, the effects of the rotation angle changes on the transmittance of the structure was first studied. The deposition angle θ was fixed to be 87°, the QCM thicknesses of each layer of nanorods were 300 nm, and the optimal rotation angles of each layer were selected by layer-by-layer optimization. Firstly, the rotation angles of the first layer of ITO nanorods were set to ±10°, ± 8°, and ±6°, respectively, and the rotation angles of the second to fourth layers were ±5°, ± 2.5°, and 0°. The corresponding transmittance spectra are shown in Fig. 4(a). Through the comparisons of the spectra, it is found that when the rotation angle of the first layer is ±8°, the transmittance is the highest. The first layer was thus determined to be ±8°. To detemine the rotation angle of the second layer, the rotation angles of the second-layer ITO nanorods were set to ±6°, ± 5°, and ±3°, respectively, and the rotation angles of the first, third and fourth layers were ±8°, ± 2.5°, and 0°, respectively. Comparing the transmittance spectra shown in Fig. 4(b), we found that the transmittance is the highest when the second layer φ is ±5°. Therefore, the second layer φ was determined to be ±5°. Next, the rotation angles of the third-layer ITO nanorods were set to ±3°, ± 2.5°, ± 2°, and ±1°, respectively, and the rotation angles of the first, second, and fourth layers were ±8°, ± 5°, and 0° respectively. The transmittance spectra shown in Fig. 4(c) demonstrate that the transmittance is the highest when the third layer φ is ±3°. Hence, the third layer φ was determined to be ±3°. Finally, the rotation angles of the fourth layer of ITO nanorods were set to be ±2°, ± 1°, and 0°, respectively, and the rotation angles of the first to third layers were ±8°, ± 5°, and ±3°. The corresponding transmittance spectra are shown in Fig. 4(d). It is found that when the fourth layer φ is 0°, the transmittance is the highest. Following the above analyses, the final rotation angles φ of the first to fourth layers of nanorods are optimized to ±8°, ± 5°, ± 3°, and 0°, respectively.
Fig. 4. (a) Transmittance spectra corresponding to different rotation angles φ1, where φ2 = ±5°, φ3 = ±2.5°, and φ4 = ±0°; (b) Transmittance spectra corresponding to different rotation angles φ2, where φ1 = ±8°, φ3 = ±2.5°, and φ4 = ±0°; (c) Transmittance spectra corresponding to different rotation angles φ3, where φ1 = ±8°, φ2 = ±5°, and φ4 = ±0°; (d) Transmittance spectra corresponding to different rotation angles φ4, where φ1 = ±8°, φ2 = ±5°, and φ3 = ±3°.
Under the same deposition angle, the average transmittance and integrated absorption of each layer with different layer thicknesses under the condition of the optimal rotation angles are shown in Fig. 5. It can be found from the figure that the transmission performance of visible light decreases with the increase of the thicknesses of each layer, while the absorption of mid-infrared light gradually increases. In order to meet high transmittance in the visible band and strong absorption in the mid-infrared band, the QCM thicknesses of each layer were selected to be 260 nm to explore the influences of different deposition angles on corresponding visible transmittance and mid-infrared absorption.
Fig. 5. Integral absorption in the 3-5 µm range and average transmittance in the 400-800 nm range of samples with different layer thicknesses.
The refractive indexes of each layer of the structure are directly related to the porosities of each layer of nanorods, which are caused by the different deposition and rotation angles during fabrication [19]. After preliminary exploration, we determined that the rotation angles of the first to fourth layers were ±8°, ± 5, °±3°, and 0°, respectively. We also studied the influences of the different deposition angles on optical properties. Figures 6(a) and 6(b) are the 400-800 nm transmittance spectra and 3-5 µm absorption spectra at different deposition angles. From Fig. 6(b), it can be seen that absorption decreases with increasing deposition angle. Figure 6(a) shows that transmittance spectra oscillate with the increase of deposition angle, which means that there exists an optimal deposition angle. Since the high transmittance of visible light and the strong absorption of mid-infrared light have a synergistic improvement mechanism, the deposition angle are chosen to be 80°.
Fig. 6. (a) Transmittance spectra and (b) absorbance spectra of the samples with different deposition angles
However, the transmittance seen from Fig. 6(a) is still low. Hence, we reduced the QCM thicknesses of each layer and investigated their influences on the transmittance. Under the same other conditions, the QCM thicknesses of each layer were reduced from 260 nm to 200 nm with an interval of 20 nm. The corresonding transmittance and absorption spectra are shown in Figs. 7(a) and 7(b). Their average transmittance and integral absorption are shown in Fig. 7(c). Since high transmittance and strong absorption are simultaneously met, the QCM thickness of the layer of ITO nanorods is finally selected to be 220 nm. For this case, the equivalent refractive index of each layer as a function of wavelength measured by an ellipsometric polarimeter (RC2 XI+) is shown in Fig. 8. It can be seen that the larger the rotation angle, the larger the equivalent refractive index of each layer. The gradient of the refractive indexes can be satisfied from the air to the substrate, and the designed absorber thus has high transmittance in the visible band.
Fig. 7. (a) Transmittance spectra and (b) absorpton spectra of the samples with different thicknesses. (c) Integral absorption in the 3-5 µm range and average transmittance in the 400-800 nm range of the samples with different layer thicknesses.
3.2 Simulations on structural optical properties
A multi-layered nanorod structure with the same tilt angle and different diameters, similar to the multilayer stacks shown in Figs. 9(a) and 9(b), was simulated using the finite difference time domain method (FDTD). A coordinate system was established with the vertical structure upward as the Z axis. The coordinate origin was located on the upper surface of the substrate. The length and the tilt angle of nanorods were 236 nm and 34°, respectively. The structural parameters used in the simulations were all chosen within the experimental measurement error values. The nanorods were arranged in a hexagonal lattice as shown in Fig. 9(c). Therefore, only a single periodic unit was cosidered to be simulated. The boundary surfaces perpendicular to the propagation direction of the electromagnetic waves were set as the periodic boundary conditions, and the boundary surfaces in the direction parallel to the propagation direction of the electromagnetic waves were set as the perfectly matched layer (PML) boundary conditions. The dielectric constant of ITO was expressed by the Drude-Lorentz model [27].
Fig. 9. Schematic diagram of simulated structure. (a) 3D view; (b) Vertical view; (c) X-Y cross-sectional view at z = 0.
Fig. 10. (a) Simulated transmittance spectrum in the 400-800 nm band; (b) Simulated absorbance spectrum in the 3-5 µm band.
In order to explore the principle of structural absorption, the electric field intensity distributions in the Y-Z section under the incidence of 3 µm, 4 µm, and 5 µm electromagnetic waves were simulated, respectively. The corresponding results are shown in Fig. 11. It can be seen that the electric fields distribute in the cavities between the nanorods. The resonances are strong in the range of 200-800 nm in the Z direction. When the wave with the wavelength of 3 µm is incident, the electric field is relatively weaker than those of the 4 µm and 5 µm waves. For the 4 µm incident wave, the strong resonances occur between z = 600 nm and z = 800 nm, while the strong resonances for the 5 µm incidence take place between z = 300 nm and z = 400 nm. It can be found that the strong resonance positions move inward with increasing wavelengths. The incidence of electromagnetic waves can excite the surface plasmon resonances of the ITO nanorods, thereby causing the cavity resonances between the nanorods. These cavities have different sizes formed by the diameter-graded ITO inclined nanorods, and these resonances result in carrier oscillations in the ITO at the different cavity regions, thus producing a more broadband mid-infrared light absorption.
Fig. 11. Distributions of Electric field intensities in the Y-Z planes at different incident wavelengths. (a) λ = 3 µm; (b) λ = 4 µm; (c) λ = 5 µm.
4. Conclusion
In conclusion, a multi-layered nanorod structure with the same tilt angle and different diameters was fabricated using the GLAD technique. By adjusting the diameters of each layer of nanorods, the gradient of the equivalent refractive indexes of the layers can be met, thereby achieving high transmittance of visible light. On the other hand, the multi-layered nanorod structure has cavities with different sizes formed by ITO nanorods. These cavities resonate due to the interaction with the incident light and then generate strong broad absorption in the mid-infrared band. When the deposition angle θ is 80°, the rotation angle φ are ±8°, ± 5°, ± 3°, and 0°, respectively, and the QCM thickness is 220 nm, the average transmittance in the 400-800 nm range is 80.5%, and the integrated absorption of 3-5 µm is 86%. This device with dual-band characteristics provides a reference for the design and fabrication of a new generation of stealth window materials, as well as the practical application of related devices. Through constantly adjustment of structure parameters, absorption can also be realized outside 3-5 µm, and it also has a wide range of applications.
Funding
National Natural Science Foundation of China (61771227, 62071208); Priority Academic Program Development of Jiangsu Higher Education Institutions.
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.
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