Open Access
Add to BibSonomy
Abstract
Fabrication parameters of antireflection films made of spin coating hollow SiO2 nanospheres were investigated. The film thickness, film morphology and optical properties of the antireflection films were observed dependent on speed of spin coating. With a proper coating speed to produce film of desirable thickness, the antireflection film shows transmittance of 98.7% at λ = 550 nm. We applied a two-step coating to serve as an alternative way for fabrication of hollow SiO2 nanospheres antireflection films. For the antireflection films made by the two-step coating, a relatively improved uniformity in surface morphology and decrease in diffuse reflectance were found. The results suggest using the two-step coating method can be beneficial for uniformity in film morphology and optical properties of the antireflection films.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Antireflection (AR) coatings are widely used to reduce the reflection and increase transmission in a wide range of industries [1,2] such as displays, buildings, lenses, and photovoltaics. Reflection is usually undesirable because of its detrimental effects on optical systems.
Antireflection coatings can improve the performance of optical devices [3–5], visibility of windows [6,7], and efficiencies of photovoltaic systems [8,9]. The low visibility of the displays of portable phones or windows under intense light can be solved by decreasing the reflection of light. Besides, the deep-ultraviolet antireflection coatings are attractive for use in deep-ultraviolet optical systems such as high-peak-power laser systems and photolithography.
Currently, the development of low-cost antireflection coatings is an active area of scientific research in cost sensitive industries, such as photovoltaics. Sol-gel processes exhibit great promise in the fabrication of antireflection coatings [10–14]. Compared with other methods, the sol-gel process possesses the advantages of low cost, low processing temperature, and a simple operation process. These sol-gel silica antireflection coatings have been applied on poly(methyl methacrylate) (PMMA) [6,15,16], polyethylene terephthalate (PET) [4], triacetyl cellulose (TAC) [3] or polyvinyl chloride (PVC) [17] substrates.
Sol-gel process produced antireflection coatings are frequently made of a packing of silica nanoparticles [2,11,18–20]. Hollow silica nanospheres are one of the most promising materials for antireflection coatings [3,4,15,21–24], due to their ultra-low refractive index, outstanding chemical stability, corrosion and heat resistance, and significant hardness.
Prior investigations have studied the properties of fabricated antireflection films by spin coating silica nanoparticles [19], mesoporous silica nanoparticles [20] and monodisperse hollow silica nanospheres [24]. For achieving antireflection films with improved film uniformity, an optimization of the spin coating of hollow silica nanospheres is necessary [24]. Large aggregation would form when the spinning speed is too low. At relatively high spinning speed, the substrates are not uniformly covered with hollow silica nanospheres. Therefore, for antireflection films made via the spin coating of hollow silica nanospheres, the coating process needs to be optimized.
Apart from the suitable refractive index and film thickness, the uniformity of films is expected for a verity of applications. The selection of proper fabrication conditions to achieve the conformity of reflective index and film thickness over the coating region is important. Successive spin-coating of thinner layers has been applied to manufacture thicker crack-free coatings [19,25]. In a prior study on single layer antireflection coatings comprising mesoporous silica nanoparticles, the uniformity and variation of refractive index over the coating area has been studied [20].
The change of spin coating speed influences the packing density of hollow SiO2 nanospheres in the films, and thus resulted in the differences in the refractive index of the films. The change of spin coating speed also alters the film thickness. In this study, we investigated the influences of spin coating speed on the refractive index, thickness, surface morphology and optical properties of single layer antireflection coatings. In addition, we applied a two-step coating to fabricate antireflection films. In this method, a layer of hollow SiO2 nanospheres was spin-coated on glass to resemble the typical method. The second step is to apply a top coating of hollow SiO2 nanospheres upon the first layer. The results reveal significantly changed film morphology obtained for films made by the two-step method. A relatively improved uniformity in surface morphology suggests the two-step coating method may be applied to enhance film morphology uniformity and the optical properties of antireflection films.
2. Experiment method
Hexadecyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), n-hexadecane, ammonium hydroxide, isopropanol, poly(vinyl alcohol) (PVA) were purchased from Tokyo Chemical Industry (TCI). The hollow silica nanospheres were synthesized according to the method in the literature with modifications [4]. To prepare hollow silica nanospheres, 0.5 g of CTAB was dissolved in 50 ml of deionized water by ultrasonication. Then 1.25 ml of n- hexadecane was added into the solution by stirring at room temperature for 10 min. Then, 5 ml of TEOS and 12.25 mg of PVA were added, and the mixture was ultrasonicated at 40 °C for 75 min. The samples were further aged for 12 hr. to obtain the hollow silica nanospheres.
The antireflection coatings were prepared on 2.5 × 2.5 cm glass. The substrates were ultrasonically cleaned with isopropanol before usage. For the typical one-step spin coating method, the obtained hollow silica nanospheres dispersions were applied and then spin-coated on glass to deposit the particles. By varying the spin speed, the thickness of the film can be altered. The resulting coatings were thermally treated at 120 °C for 10 min in air and then allowed to cool to room temperature. For the two-step spin-coating method, the first step resembled the step of typical one-step coating. The second step was to spin coat a top coating onto the first layer (Fig. 1). For the second step, the spin coating speed was 7000 rpm. The resultant coatings were thermally treated at 120 °C for 10 min in air and then allowed to cool to room temperature.
Fig. 1. Schematic illustration of the preparation of antireflection film derived from typical one-step coating and two-step coating.
For transmission electron microscopy (TEM) observations, powder samples were observed on a HITACHI HT7700 transmission electron microscopy. Scanning electron microscopy (SEM) images were obtained by a HITACHI S-4800 Cold Field Emission Scanning Electron Microscope (FE-SEM). Transmittance, reflectance spectra and diffusion reflectance were measured by UV-vis spectroscopy on a Shimadzu UV-2600 spectrophotometer with an ISR-2600 Plus integrating sphere. The refractive index and film thickness of the deposited films were determined via the spectroscopic ellipsometry measurements (J. A. Woolam M2000-DI).
3. Results and discussion
The TEM image of the synthesized hollow silica nanospheres reveals their morphology and the dispersion, with a diameter of about 30-120 nm (Fig. 2).
The refractive index and thickness of the one-step coated films of the hollow silica nanospheres were determined via the spectroscopic ellipsometry measurements. The obtained films thicknesses and the refractive indices are as shown in Table 1. The refractive index is not significantly changed under different spin coating speeds.
Table 1. Film thicknesses, refractive indices, transmittance, and reflectance (at λ = 550 nm) of hollow silica nanospheres antireflection films as a function of spin coating speeds
Figure 3(a) shows the transmittance of glass with one-step coatings of hollow silica nanospheres on both sides. As compared with glass, the coated samples exhibit enhanced transmittance. The transmittance of the coated glass became higher in the order 1000 rpm < 5000 rpm < 4000rpm < 3000 rpm < 2000rpm (Table 1). For the sample coated with 2000rpm, the transmittance reached 98.7% atλ= 550 nm. The reflectance of the coated glass is lowered in the order 1000 rpm > 5000 rpm > 4000 rpm > 3000 rpm > 2000rpm atλ= 550 nm, as shown in Fig. 3(b) and Table 1. The increased spin coating speed leads to decreased coating thickness, resulting in a corresponding shift in the wavelength of the anti-reflectance effect and the shift of peak transmittance. This shift is observed due to single-layer quarter-wave antireflecting coatings, the maximum antireflection occurring at a wavelength λ= 4nd. While the thickness (d) was adjusted by the increasing of spinning speed, a thinner thickness was obtained, which results in the shift of maximum antireflection effect to blue side of the visible spectrum.
Fig. 3. (a) Transmittance and (b) reflectance spectra of the hollow SiO2 nanospheres coated glass as a function of spin coating speeds.
For single-layer quarter-wave antireflection coatings, it is well known when d = λ/4n and n = (nair * n sub) 1/2, the minimum reflection would occur through destructive interference, where d is the thickness of the coated layer, λ is the reference wavelength, and n, nair, nsub represent the refractive index of the coated film, the ambient air and the substrate, respectively [4,19]. The refractive index of the moderate-refractive-index glass has a refractive index of ∼1.52. Therefore, one of the optimum coating conditions would be to carefully control the films to be 1.23 in refractive index and 112 nm in film thickness, as calculated at a reference wavelength of 550 nm. The hollow silica nanospheres film coated at 2000 rpm achieved a suitable film thickness, and therefore yielded a lower reflectance and higher transmittance at λ = 550 nm.
To study the morphology of the one-step coatings, the SEM images of the surface of the coatings were examined. The improvement in the overall optical properties through size and morphology control of structures in the films is an important topic in the development of SiO2 based single layer antireflection coatings [7]. Reduction in the microscale structures can suppress light scattering, prevent opaque appearance, and achieve a higher transmittance. In this study, we examined and compared the films surface structures through SEM measurement to seek proper parameters for the fabrication of films with desirable surface morphology. The SEM images show the top surface morphology of the films (Fig. 4). For the sample prepared under 1000 rpm, the SEM images indicate a relatively rough surface obtained in this condition. For the films coated under spin coating speed of 2000 rpm, a comparatively flat surface was observed. For the samples prepared under further higher spin coating speed, increased voids and pin holes appeared on the surface.
Fig. 4. SEM surface images of the coatings obtained from different spin coating speeds: (a) 1000 rpm; (b) 2000rpm; (c) 3000 rpm; (d) 4000 rpm; (e) 5000 rpm.
The two-step spin coating method was applied to increase the film thickness and modify the surface morphology. Several samples made of two-step spin coating were examined and their properties were compared to the samples derived by typical one-step coating.
The refractive index and thickness of the coated films made by two-step method were also determined via spectroscopic ellipsometry measurements. The film thicknesses and the refractive indices of the samples are as shown in Table 2. The decrease in the refractive index for the films made by the two-step coating as compared to that made by the typical one-step coating might be due to the changed packing density of the hollow silica nanospheres.
Table 2. Film thicknesses, refractive indices, transmittance, reflectance, and reflectance (at λ = 550 nm) of two-step coated hollow silica nanospheres antireflection films as a function of spin coating speeds
Figure 5(a) shows the transmittance of glass with two-step coatings of hollow silica nanospheres. The transmittance of the coated glass became higher in the order 1000 rpm + 7000rpm < 2000rpm + 7000 rpm < 3000 rpm + 7000 rpm < 5000 rpm + 7000 rpm < 7000 rpm + 7000 rpm (Table 2). For the sample coated with 7000 rpm + 7000 rpm, the transmittance reached 98.9% at λ = 550 nm. The reflectance of the coated samples is lowered in the order 1000 rpm + 7000 rpm < 2000rpm+ 7000 rpm < 3000 rpm + 7000 rpm< 5000 rpm + 7000 rpm < 7000 rpm + 7000 rpm at λ = 550 nm (Fig. 5(c) and Table 2). The maximum antireflection occurred for the sample obtained from 7000 rpm + 7000 rpm spin coating speed.
Fig. 5. (a) Transmittance and (b) reflectance spectra of the two-step coated glass as a function of spin coating speeds.
The surface morphology and features of the samples produced via the two-step coating method were examined by the SEM images. The images show the top surface morphology of the films of hollow silica nanospheres (Fig. 6). Improved uniformity in surface morphology were found for the films compared to that obtained from one-step coating as resolved from the SEM images.
Fig. 6. SEM surface images of the two-step coatings obtained from different spin coating speeds: (a) 1000 rpm + 7000 rpm; (b) 2000rpm + 7000 rpm; (c) 3000 rpm + 7000 rpm; (d) 5000 rpm + 7000 rpm; (e) 7000 rpm + 7000 rpm.
We compared the transmittance, reflectance and diffuse reflectance between the optimal one-step and two-step coated films (Table 3). The two-step coated sample exhibited slightly higher transmittance and lower reflectance.
Table 3. Comparison of optical properties (at λ = 550 nm) between the optimal one-step and two-step coated films
We also determined the diffuse reflectance of the samples. Diffuse reflection is the reflection of light incident on the surface scattered at many angles, while the case of specular reflection is excluded. The diffuse reflectance in the samples is not significant for the samples, reflecting the low scattering loss in the hollow silica nanospheres films. The diffuse reflectance of the samples obtained by the two-step coatings was found with obvious lower diffuse reflectance and lower standard deviation of diffuse reflectance as shown in Table 3.
4. Conclusions
We studied the influences of fabrication parameters on hollow SiO2 nanospheres antireflection films. The film thickness, film morphology and optical properties of the antireflection films were observed dependent on speed of spin coating. The results show the antireflection film made of hollow SiO2 nanospheres by one-step spin coating under 2000rpm with the highest transmittance of 98.7% at λ = 550 nm.
In addition, we applied a two-step coating to fabricate antireflection films. As compared with samples derived by the typical method, a relatively improved uniformity in surface morphology observed. Decrease in diffuse reflectance was obtained for the samples made of the two-step coating. The results suggest the two-step coating method may be applied to enhance film morphology uniformity and the optical properties of the antireflection films.
Funding
Ministry of Science and Technology, Taiwan (MOST 109-2222-E-035-005); Feng Chia University (1410-D1, 110:20H00702, 1410-D1, 110:21H00702).
Acknowledgments
The authors thank the Ministry of Science and Technology, Taiwan (MOST 109-2222-E-035-005) and Feng Chia University (1410-D1, 110:20H00702; 110:21H00702) for financial support of this research.
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.
References
1. H. K. Raut, V. A. Ganesh, A. S. Nair, and S. Ramakrishna, “Anti-reflective coatings: A critical, in-depth review,” Energy Environ. Sci. 4(10), 3779–3804 (2011). [CrossRef]
2. P. Buskens, M. Burghoorn, M. C. D. Mourad, and Z. Vroon, “Antireflective Coatings for Glass and Transparent Polymers,” Langmuir 32(27), 6781–6793 (2016). [CrossRef]
3. R. Muraguchi, W. Futagami, Y. Hakoshima, K. Awaya, and S. Ida, “Preparation and Properties of Organic–Inorganic Hybrid Antireflection Films Made by a Low-Temperature Process Using Hollow Silica Nanoparticles,” ACS Omega 6(12), 8570–8577 (2021). [CrossRef]
4. L. Zhang, L. Zhang, Y. Qiu, Y. Ji, Y. Liu, H. Liu, G. Li, and Q. Guo, “Improved Performance by SiO2 Hollow Nanospheres for Silver Nanowire-Based Flexible Transparent Conductive Films,” ACS Appl. Mater. Interfaces 8(40), 27055–27063 (2016). [CrossRef]
5. J.-H. Shin, H.-J. Choi, K.-S. Han, S. Ra, K.-W. Choi, and H. Lee, “Effect of anti-reflective nano-patterns on LED package,” Current Applied Physics 13, S93–S97 (2013). [CrossRef]
6. N. Mizoshita and H. Tanaka, “Versatile Antireflection Coating for Plastics: Partial Embedding of Mesoporous Silica Nanoparticles onto Substrate Surface,” ACS Appl. Mater. Interfaces 8(45), 31330–31338 (2016). [CrossRef]
7. N. Mizoshita, M. Ishii, N. Kato, and H. Tanaka, “Hierarchical Nanoporous Silica Films for Wear Resistant Antireflection Coatings,” ACS Appl. Mater. Interfaces 7(34), 19424–19430 (2015). [CrossRef]
8. D. Wan, H.-L. Chen, T.-C. Tseng, C.-Y. Fang, Y.-S. Lai, and F.-Y. Yeh, “Antireflective Nanoparticle Arrays Enhance the Efficiency of Silicon Solar Cells,” Adv. Funct. Mater. 20(18), 3064–3075 (2010). [CrossRef]
9. J. Zhang, L. Ai, S. Lin, P. Lan, Y. Lu, N. Dai, R. Tan, B. Fan, and W. Song, “Preparation of humidity, abrasion, and dust resistant antireflection coatings for photovoltaic modules via dual precursor modification and hybridization of hollow silica nanospheres,” Sol. Energy Mater. Sol. Cells 192, 188–196 (2019). [CrossRef]
10. Z. Guo, H. Zhao, W. Zhao, T. Wang, D. Kong, T. Chen, and X. Zhang, “High-Quality Hollow Closed-Pore Silica Antireflection Coatings Based on Styrene–Acrylate Emulsion @ Organic–Inorganic Silica Precursor,” ACS Appl. Mater. Interfaces 8(18), 11796–11805 (2016). [CrossRef]
11. F. Chi, Y. Zeng, C. Liu, D. Liang, Y. Li, R. Xie, N. Pan, and C. Ding, “Aggregation of Silica Nanoparticles in Sol–Gel Processes to Create Optical Coatings with Controllable Ultralow Refractive Indices,” ACS Appl. Mater. Interfaces 12(14), 16887–16895 (2020). [CrossRef]
12. J. Sun, X. Cui, C. Zhang, C. Zhang, R. Ding, and Y. Xu, “A broadband antireflective coating based on a double-layer system containing mesoporous silica and nanoporous silica,” J. Mater. Chem. C 3(27), 7187–7194 (2015). [CrossRef]
13. D. Chen, “Anti-reflection (AR) coatings made by sol–gel processes: A review,” Sol. Energy Mater. Sol. Cells 68(3-4), 313–336 (2001). [CrossRef]
14. L. Zou, X. Li, Q. Zhang, and J. Shen, “An Abrasion-Resistant and Broadband Antireflective Silica Coating by Block Copolymer Assisted Sol–Gel Method,” Langmuir 30(34), 10481–10486 (2014). [CrossRef]
15. Y. Du, L. E. Luna, W. S. Tan, M. F. Rubner, and R. E. Cohen, “Hollow Silica Nanoparticles in UV−Visible Antireflection Coatings for Poly(methyl methacrylate) Substrates,” ACS Nano 4(7), 4308–4316 (2010). [CrossRef]
16. X. Huang, Y. Yuan, S. Liu, W. Wang, and R. Hong, “One-step sol-gel preparation of hydrophobic antireflective SiO2 coating on poly(methyl methacrylate) substrate,” Mater. Lett. 208, 62–64 (2017). [CrossRef]
17. V. Purcar, V. Rădițoiu, A. Rădițoiu, R. Manea, F. M. Raduly, G. C. Ispas, A. N. Frone, C. A. Nicolae, R. A. Gabor, M. Anastasescu, H. Stroescu, and S. Căprărescu, “Preparation and Characterization of Some Sol-Gel Modified Silica Coatings Deposited on Polyvinyl Chloride (PVC) Substrates,” Coatings 11(1), 11 (2020). [CrossRef]
18. H. Hattori, “Anti-Reflection Surface with Particle Coating Deposited by Electrostatic Attraction,” Adv. Mater. 13(1), 51–54 (2001). [CrossRef]
19. K. T. Cook, K. E. Tettey, R. M. Bunch, D. Lee, and A. J. Nolte, “One-Step Index-Tunable Antireflection Coatings from Aggregated Silica Nanoparticles,” ACS Appl. Mater. Interfaces 4(12), 6426–6431 (2012). [CrossRef]
20. J. Moghal, J. Kobler, J. Sauer, J. Best, M. Gardener, A. A. R. Watt, and G. Wakefield, “High-Performance, Single-Layer Antireflective Optical Coatings Comprising Mesoporous Silica Nanoparticles,” ACS Appl. Mater. Interfaces 4(2), 854–859 (2012). [CrossRef]
21. X. Zhang, P. Lan, Y. Lu, J. Li, H. Xu, J. Zhang, Y. Lee, J. Y. Rhee, K.-L. Choy, and W. Song, “Multifunctional Antireflection Coatings Based on Novel Hollow Silica–Silica Nanocomposites,” ACS Appl. Mater. Interfaces 6(3), 1415–1423 (2014). [CrossRef]
22. Y. He, K.-J. Kim, and C.-H. Chang, “Continuous, size and shape-control synthesis of hollow silica nanoparticles enabled by a microreactor-assisted rapid mixing process,” Nanotechnology 28(23), 235602 (2017). [CrossRef]
23. L. Ye, L. Li, X. Wang, Y. Zhang, and L. Yan, “Template-free synthesis of uniform hollow silica nanoparticles for controllable antireflection coatings,” Ceram. Int. 46(6), 7453–7458 (2020). [CrossRef]
24. T. Gao, B. P. Jelle, and A. Gustavsen, “Antireflection properties of monodisperse hollow silica nanospheres,” Appl. Phys. A 110(1), 65–70 (2013). [CrossRef]
25. J. H. Prosser, T. Brugarolas, S. Lee, A. J. Nolte, and D. Lee, “Avoiding Cracks in Nanoparticle Films,” Nano Lett. 12(10), 5287–5291 (2012). [CrossRef]
