Preparation of porous silica films in a binary template system for double-layer broadband antireflective coatings

Abstract

Porous silica films with a refractive index varying from 1.44 to 1.15 were prepared and applied to realize a double-layer broadband AR coating. The porous silica films were obtained using tetraethylorthosilicate (TEOS) as precursor and a binary template as porogen which was composed of cetyltrimethylammonium bromide (CTAB) and polypropylene glycol (PPG). The molar ratio of the two templates was optimized, and a hypothetical mechanism for the cooperative assembly of silicates with the binary template system was proposed. The two layers had the same skeleton despite different porosity with the top layer much more porous than the bottom one. The double-layer double-wavelength AR coating had excellent optical properties with transmittances of 99.6% and 99.8% at 532 nm and 1064 nm, respectively.

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Introduction

Antireflective (AR) coatings have been widely used in optical devices and energy-related applications, such as solar cells, windscreens and high power lasers, to suppress the undesired interfacial Fresnel reflections.^1–7 Conventional single-layer quarter-wave AR coatings, which only work at a single wavelength, have been seriously limited in some special applications because of this inherent property. For instance, in a high power laser system that involves harmonic converter crystals to convert laser light to a lower harmonic wavelength, a double-wavelength broadband AR coating works simultaneously at 1064 nm and 532 nm to accomplish the frequency conversion effectively.⁸ For achieving effective broadband coatings, a thin film with ultralow refractive index is of great importance. Many efforts have been devoted to searching for film materials with tunable refractive index, including etching,^9,10 surfactant-templating,^11–14 phase separation,^15,16 approaches etc.

AR coatings can be prepared by both physical and chemical approaches. The former is called physical vapor deposition (PVD) which the process is time-consuming and high cost. Nowadays, wet chemical methods have been widely studied such as layer-by-layer (LbL) assembly, sol–gel process, and so on. Among these methods, the coatings prepared by sol–gel process are well known as nano-porous with much higher LIDT (laser induced damage threshold).⁸ Thomas⁸ used sol–gel process to prepare a double-wavelength broadband AR coating for laser system with transmittances of 99.4% and 99.6% at 532 nm and 1064 nm, respectively. The relatively low transmittance is probably due to the limited availability of coating materials. T. Y. Tan et al.¹⁷ reported an LBO/xLHML/AIR multi-layer frequency-doubled AR coating with reflectance 0.07% and 0.11% at 1064 nm and 532 nm, using the electron beam evaporation method. The process is time-consuming and expensive. We¹⁸ previously reported an organically modified silicate film to prepare double-layer coatings. The coatings had high transmittances over a broad wavelength region.

In this paper, a novel and simple route to prepare double-layer broadband silica antireflective film was investigated. The nano-porous layer materials with tunable refractive index were obtained by sol–gel process coupled with a binary-template method, and a double-layer double-wavelength AR coating was obtained with high transmittances at 532 nm and 1053 nm simultaneously.

Experimental section

Materials

Tetraethylorthoxylsilicane (TEOS) was purchased from Sigma-Aldrich. Cetyltrimethylammonium bromide (CTAB) was purchased from Alfa Aesar. Polypropylene glycol (PPG, M_n = 2000) was purchased from Sigma-Aldrich. Ethanol and concentrated hydrochloric acid were purchased from Kelong Chemical Reagents Factory. The water was deionized. All chemicals were used without further purification.

Preparation of template-based acid-catalyzed silica sol

Tetraethyl silicate (104 g) was mixed with anhydrous ethanol (860 g) and water (36 g) that contained concentrated hydrochloric acid (0.2 g). The final molar ratio of TEOS : EtOH : H₂O : HCl was 1 : 37.5 : 4 : 0.004 and the concentration of equivalent SiO₂ was 3 wt%. The solution was stirred in a closed glass container at 30 °C for 2 h, and then aged at room temperature for 7 days.

The proportion of two templates was first optimized. The two templates were added into the aged sol at the same time. The solution was stirred at 30 °C until the completely dissolution of the templates and then cooled down at room temperature before deposition. The molar ratio of CTAB/PPG varied from 0.6–1.4, and the total amount of two templates was maintained at 7 wt%. Next, the molar ratio of 1.0 was chosen as the optimum ratio for the following experiments, and the templates/TEOS molar ratio was varied from 0 (namely the pure acid-catalyzed sol) to 0.14. Before deposition, the sols with the templates/TEOS molar ratio ranged from 0.10 to 0.14 were diluted with quarter weight of anhydrous ethanol.

Film preparation

BK-7 substrates (Φ = 35 mm, d = 3 mm) were cleaned by ultrasonication in acetone for 10 min and wiped carefully before dip-coating. The sols were deposited on the well-cleaned BK-7 substrates under ambient condition of 30% RH (relative humidity) and 30 °C by dip coating. The double-layer film was deposited by a sequential dip-coating process. The thickness of films was finely controlled by tuning the withdraw rate. The withdraw rates for bottom and top layers were about 300 mm min⁻¹ and 160 mm min⁻¹, respectively. The deposited coatings were firstly pre-heating at 100 °C for 1 h, and then 400 °C for 2 h to completely remove the organic species. Both layers were prepared and treated as the above process.

Characterization

The experimental transmittance spectrum was measured with an UV-vis spectrophotometer (Mapada, UV-3100PC) over a spectral range of 400–1100 nm. The refractive indices and thickness of AR coatings were determined by ellipsometry (SENTECH SE850 UV). The surface topography of the coatings was studied with atomic force microscopy (AFM, SEIKO SPA-400). The surface root-mean-square (RMS) roughness values were obtained from the analysis of atomic force microscopy (AFM) images. For scanning electron microscopy (SEM) observations, the coatings were coated on Si wafer for its good electrical conductivity, and then surface morphologies and cross-sections of the AR coatings were investigated by scanning electrons microscope (SEM, JSM-5900LV).

Results and discussion

Design of double-layer double-wavelength broadband AR coatings

For the double-layer coating system, λ/4–λ/2 (λ is the central wavelength) and λ/4–λ/4 film designs are usually adopted. In terms of optical property, the λ/4–λ/4 design that refers to two layers having the same optical thickness of λ/4 is better than the other one.

In this scenario, the refractive indices are stacked as follows, n_s > n_bot > n_top > n₀, where n_s is the refractive index of substrate, i.e. BK-7 (n_s = 1.52) and n₀ is the air (n₀ = 1). For two quarter-wavelength coatings, the optimal refractive indices of each layer in a double-layer coating system can be determined by¹⁹

n_top³ = n₀²n_s (1)

n_bot³ = n₀n_s² (2)

For the case of a double-layer AR coating in air, the required refractive indices, determined using eqn (1) and (2), are n_top = 1.15 and n_bot = 1.32.

If the phase thickness at λ₁ (1064 nm) is δ, to obtain minimum reflection at λ₁ and λ₂ (λ₂ = λ₁/2, 532 nm) simultaneously, the phase thickness at λ₂ will be π − δ. Consequently, the relationship between λ₀ (central wavelength), λ₁ and λ₂ can be obtained as follow,

By calculating the above two equations, the simplified equation can be obtained.

λ₀ = 2λ₁/3 = 710 nm (5)

The double-layer AR coating described above was verified with the aid of thin film design software TFCalc™. The refractive index and thickness of double-layer AR coatings were optimized and listed in Table 1. The optical thickness of both layers is designed as quarter-wave. The central wavelength for these coatings is set at 710 nm. Simulation results are in accordance with those of theoretical analysis. Double-layer coatings achieve 100% transmittances simultaneously at 1064 nm and 532 nm with the refractive index of bottom layer being 1.32 and top layer being 1.15.

Table 1 Refractive indexes, thicknesses and transmittances at 1064 nm and 532 nm of broadband AR coatings on BK-7 substrates modeled by TFCalc™

Sample number	Bottom layer		Top layer		T1064 nm (%)	T532 nm (%)
	n1	Thickness (nm)	n2	Thickness (nm)
C1	1.41	126	1.22	145	98.96	98.95
C2	1.37	130	1.22	145	99.31	99.30
C3	1.32	134	1.22	145	99.51	99.47
C4	1.37	129	1.20	147	99.47	99.44
C5	1.32	134	1.20	147	99.75	99.71
C6	1.32	134	1.18	150	99.89	99.89
C7	1.32	134	1.16	153	99.99	99.98
C8	1.32	134	1.15	154	100.00	100.00

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The effects of the binary template system on the properties of AR coatings

The materials widely used as AR coatings are dielectric materials such as silica, titania, and alumina with refractive indices of 1.45, 2.3 and 1.65, respectively. As aforementioned, a thin film with an ultralow refractive index n = 1.15 is needed in preparation of high quality double-wavelength broadband AR coatings. Since the refractive index of a material is related to its density, the refractive index of the film can be reduced by introducing porosity, provided that the pore sizes are much smaller than the electromagnetic wavelengths of interest. The porosity and index of refraction are related by²⁰

n_p² = (n_d² − 1)(1 − P) + 1 (6)

where P is the volume fraction of non-scattering porosity, n_p is the effective refractive index of nano-porous materials and n_d is the refractive index of the related bulk materials. As a result of tunable refractive index properties, porous silica is considered to be a good antireflective coating material.

The template-based synthesis technique is a common method for preparing nano-porous films. Template is added to the sol and decomposed at high temperature to create pores in the resulting film. As to coatings with single template, an excess of template may resulting in a collapse of the porous structure during calcination. When another kind of template is introduced, the newly-formed interaction force assists in controlling the nanostructure more regular.¹³ So the film can bear a greater amount of templates during calcination, and thus owns a lower refractive index.

In this study, a binary template composed of CTAB and PPG was used as porogen. Before further exploration about the relationship between porosity and templates amount, the ratio of the two templates was optimized. As the increase in the concentration of templates, the porosity of the film increases. This may lead to a partial collapse of the porous structure during calcination. The film may become heterogeneous and even crack may occur. Therefore, we choose almost the largest concentration of templates 7 wt% to optimize the molar ratio of the two templates, and thus the conclusion would be suitable for other weight percent. The properties of films prepared with various CTAB/PPG molar ratios are shown in Table 2. The maximum transmission can be used to evaluate the refractive index of the coatings according to the simplified Fresnel formula,²¹ and then the thickness (d = 0.25λ/n_c, λ is the central wavelength, n_c is the refractive index) of the film can be calculated. As all the coatings had almost the same thickness, and thus the comparison of coatings' surface quality was fair. The digital images of the five films listed in Table 2 are given in the ESI (Fig. S1†). A uniform and crack-free coating was obtained when the molar ratio of CTAB/PPG is 1.0. The templates concentrations in silica sols could not only influence the transmittance of coatings but also alter the surface morphology.

Table 2 The properties of single-layer coatings made by sols with the molar ratio of two templates CTAB/PPG ranged from 0.6–1.4

Sample	Maximum transmission (%)	Central wavelength (nm)	Refractive index	Thickness (nm)	Surface
CTAB/PPG = 0.6	99.34	589	1.16	127	Bad (crack)
CTAB/PPG = 0.8	99.41	591	1.17	126	Fair (crack)
CTAB/PPG = 1.0	99.29	586	1.16	126	Good (no crack)
CTAB/PPG = 1.2	99.21	610	1.16	131	Fair (crack)
CTAB/PPG = 1.4	99.15	598	1.15	130	Bad (crack)

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To explain this phenomenon, it is helpful to analyze the composition of the sol. In the SiO₂ sol, the pH is about 6, which is much higher than the isoelectric point (∼2), yielding negatively charged silicates particles.²² Therefore, silicates are subjected to two forces: one is electrostatic interaction of cationic surfactant CTA⁺, Br⁻ and negative charged silicates; the other is hydrogen bond interaction between PPG and silicates. At the dip-coating stage, preferential evaporation of ethanol concentrates the nonvolatile templates and silica species, and hence strength the two forces mentioned above, which result in the formation of SiO₂-PPG-CTAB supramolecular assemblies.²³

The timescale of film deposition process which is established by the thickness of the entrained film and the evaporation rate is typically several seconds. There is little time available for reacting species to adopt the minimum energy configurations,²⁴ and an optimized ratio of CTAB/PPG is needed to balance the electrostatic interaction and hydrogen-bonded interaction. The molar ratio of 1.0 for CTAB/PPG may be a better one to promote the forming of supramolecular assemblies in sol, and thereby forming homogeneous films. As illustrated in Fig. 1, a hypothetical mechanism for the cooperative assembly of silicates with the binary template system is proposed. The structural control is essentially ascribed to the balance between the ordered assembly of anionic silicates and a cationic surfactant through an electrostatic interaction and an affinity with PPG through the hydrogen bonds.

Fig. 1 Schematic model for the cooperative assembly of silicates with the binary template system.

Fig. 2 shows the change of refractive index of AR coatings as a function of templates/TEOS molar ratio. The refractive index initially decreases very fast with increasing the templates concentration, then slows down and finally reaches a stable value of 1.15, corresponding to the porosity increases from 5% to 71%. Thin films with refractive index of 1.32 for the bottom layer and 1.15 for the top layer can be obtained with templates/TEOS molar ratios of 0.01 and 0.12, respectively. The scanning electron microscopy (SEM) images of bottom layer and top layer are shown in Fig. 3. The bottom layer exhibits a smooth and dense surface morphology for its quite low porosity. However, as the templates concentration increases, the top layer is becoming rougher and more porous.

Fig. 2 Change in refractive index and porosity of silica thin films as a function of templates/TEOS molar ratio.

Fig. 3 The SEM surface images of the films with (a) templates/TEOS = 0.01 for bottom layer, (b) templates/TEOS = 0.12 for top layer.

Optical property of double-layer AR coating

In sol–gel process, film thickness is sensitive to deviation from room temperature and humidity, so the ambient condition should be well controlled. Meanwhile, viscosity of a sol is an influencing factor for the thickness of films.²⁵ The viscosity of the sol for top layer is quite high, and thus the withdraw rate would be very small to obtain films with desired thickness. In dip-coating process, the coating machine is not stable enough to obtain a uniform coating if the withdrew rate is too big or too small. So the sol employed for the top layer was diluted with quarter weight of anhydrous ethanol to get a proper withdraw rate (the proper withdraw rate can be very different as the sols or ambient environment changes. In our experiments, the proper withdraw rates are about 80 mm min⁻¹ to 350 mm min⁻¹). In this work, each layer's thickness was designed as a quarter of the center wavelength, which was carried out mainly by adjusting the withdraw speed. Owning to the great viscosity, The double-layer film was deposited by a sequential dip-coating process. The sol with templates/TEOS molar ratio of 0.01 was used as precursor for bottom layer and 0.12 for top layer. The cross-sectional SEM image of a prepared double-layer AR coating is shown in Fig. 4. The cross-section image shows a distinguishable boundary between the top layer and bottom layer, it could be seen that the film thicknesses of the top and bottom layers agreed well with the design value, and the top layer possesses a higher porosity than the bottom layer.

Fig. 4 Cross-sectional SEM image of the double-layer antireflective film.

The transmittance spectrum of the resultant double-wavelength broadband AR coating is shown in Fig. 5. Compared with the spectra of modeled one (C8 in Table 1) and typical single-layer quarter-wave AR coating by computer simulation, the advantage of the double-layer broadband AR coating over the single-layer quarter-wave AR coating is apparent. The double-layer AR coatings can afford the substrate high transmittance of 99.6% and 99.8% at 532 nm and 1064 nm, simultaneously.

Fig. 5 Transmittance spectra of a single-layer quarter-wave AR coating and an experimental and modeled double-wavelength AR coating.

The experimental transmittance spectrum is in good agreement with the theoretical one with observation of a slightly decrease in the short-wavelength regions. In the theoretical analysis and simulation, it has been assumed that the refractive index of a film is constant, and the refractive index at 550 nm was adopted as the refractive index for the entire range of spectrum. However, in practical, the refractive index of silica film decreases slightly with the increase of wavelength due to the dispersion of the refractive index in the wavelength region,^26,27 which leads to the observation of difference between the experimental and theoretical transmission spectrum.

Surface morphology of the films

The surface topographies of the film determine the quality of the coating in terms of their optical transmission.²⁸ Optical transmission through a rough surface is considerably affected by scattering of light whose wavelengths close to the magnitude of the surface features. AFM patterns of the surface morphologies for bottom layer, top layer and double-layer are showed under the same scale in Fig. 6. As to acid-catalyzed sol, the growth of clusters can be described as a RLCA (Reaction Limited Cluster–cluster Aggregation) model,²⁹ resulting in a structure of linear chains. Therefore, the formed film tends to be dense and exhibits a smooth surface. The surface morphology of bottom layer film is almost the same as the pure acid-catalyzed film due to the quite low porosity, and the root-mean-square (RMS) roughness (R_q) value is only 0.66 nm (Fig. 6a). For the top layer film, as the void space increases between the particles, the surface becomes rougher with the R_q value of 3.25 nm (Fig. 6b). The surface morphology of the double-layer film is similar to the top layer film. Since R_q value of all these films is less than 20 nm, this roughness is small enough not to cause any intense surface light scattering as long as the wavelength is longer than 200 nm.³⁰

Fig. 6 Surface topography of the film by AFM: (a) bottom layer, (b) top layer, (c) double-layer.

Conclusions

A double-layer double-wavelength AR coating with 100% transmittance at both 1064 nm and 532 nm can be designed using thin-film design software (TFCalc). Nano-porous silica coatings with refractive index ranged from 1.44 to 1.15 were obtained by sol–gel process coupled with a binary-template method, and the coatings were used for the preparation of doubled-wavelength broadband AR coatings. The AR coatings with excellent optical property were successfully realized and the transmittances at 532 nm and 1064 nm are 99.6% and 99.8%, respectively. This double-wavelength silica AR coating prepared by sol–gel process can find applications in high power laser system.

Acknowledgements

The authors gratefully acknowledge the support from the Natural Science Foundation of China “Project J1103315” supported by NSFC.

Notes and references

1. A. L. Pénard, T. Gacoin and J. P. Boilot, Acc. Chem. Res., 2007, 40, 895.
2. L. Zhang, Z. A. Qiao, M. Zheng, Q. S. Huo and J. Q. Sun, J. Mater. Chem., 2010, 20, 6125.
3. H. Ye, X. X. Zhang, Y. Zhang, L. Q. Ye, B. Xiao, H. B. Lv and B. Jiang, Sol. Energy Mater. Sol. Cells, 2011, 95, 2347.
4. S. Cai, Y. L. Zhang, H. L. Zhang, H. W. Yan, H. B. Lv and B. Jiang, ACS Appl. Mater. Interfaces, 2014, 6, 11470.
5. K. Manabe, S. Nishizawa, K. H. Kyung and S. Shiratori, ACS Appl. Mater. Interfaces, 2014, 6, 13985.
6. I. Sta, M. Jlassi, M. Hajji, M. F. Boujmil, R. Jerbi, M. Kandyla, M. Kompitsas and H. Ezzaouia, J. Sol-Gel Sci. Technol., 2014, 72, 421.
7. Y. J. Wang, X. Y. Li, M. M. Li, B. Shi, Y. Liu and L. Q. Mao, J. Sol-Gel Sci. Technol., 2014, 70, 1.
8. I. M. Thomas, Proc. SPIE, 1997, 3136, 215.
9. L. Yao and J. H. He, Langmuir, 2013, 29, 3089.
10. J. Xiong, S. N. Das, J. P. Kar, J. H. Choi and J. M. Myoung, J. Mater. Chem., 2010, 20, 10246.
11. L. Q. Ye, S. M. Zhang, Q. Wang, L. H. Yan, H. B. Lv and B. Jiang, RSC Adv., 2014, 4, 35818.
12. F. T. Chi, L. H. Yan, H. B. Lv and B. Jiang, Mater. Lett., 2011, 65, 1095.
13. K. Ikari, K. Suzuki and H. Imai, Langmuir, 2006, 22, 802.
14. B. B. Xia, Q. H. Zhang, S. Y. Yao, Y. L. Zhang, B. Xiao and B. Jiang, J. Sol-Gel Sci. Technol., 2014, 71, 291.
15. S. Walheim, E. Schaffer, J. Mlynek and U. Steiner, Science, 1999, 283, 520.
16. X. Li, X. H. Yu and Y. C. Han, J. Mater. Chem. C, 2013, 1, 2262.
17. T. Y. Tan, J. Shan, W. Wu, Y. X. Guo, J. D. Shao and Z. X. Fan, J. Wuhan Univ. Technol., Mater. Sci. Ed., 2009, 24, 849.
18. X. X. Zhang, S. Cai, D. You, L. H. Yan, H. B. Lv, X. D. Yuan and B. Jiang, Adv. Funct. Mater., 2013, 23, 4361.
19. H. A. Macleod, Thin-Film Optical Filters, Adam Hilger Ltd, Bristol, 1986.
20. B. E. Yoldas, Appl. Opt., 1980, 19, 1425.
21. B. G. Prevo, Y. Hwang and O. D. Velev, Chem. Mater., 2005, 17, 3642.
22. J. Limmer, S. Seraji, Y. Wu, P. T. Chou, C. Nguyen and G. Z. Cao, Adv. Funct. Mater., 2009, 12, 59.
23. C. J. Brinker, Y. F. Lu, A. Sellinger and H. Y. Fan, Adv. Mater., 1999, 11, 579.
24. C. J. Brinker, A. J. Hurd, P. R. Schunk, G. C. Frye and C. S. Ashley, J. Non-Cryst. Solids, 1992, 147–148, 424.
25. D. Grosso, J. Mater. Chem., 2011, 21, 17033.
26. D. Bouhafs, A. Moussi, A. Chikouche and J. M. Ruiz, Sol. Energy Mater. Sol. Cells, 1998, 52, 79.
27. S. Y. Lien, D. S. Wuu, W. C. Yeh and J. C. Liu, Sol. Energy Mater. Sol. Cells, 2006, 90, 2710.
28. M. Ulmeanu, A. Serghei, I. N. Mihailescu, P. Budau and M. Enachescu, Appl. Surf. Sci., 2000, 165, 109.
29. M. J. Vold, J. Colloid Interface Sci., 1963, 18, 684.
30. Y. Xu, W. H. Fan, Z. H. Li, D. Wu and Y. H. Sun, Appl. Opt., 2003, 42, 108.

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Footnote

† Electronic supplementary information (ESI) available: Digital images of the films. See DOI: 10.1039/c4ra17141a

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