A novel route to control refractive index of sol-gel derived nano-porous silica films used as broadband antireflective coatings

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Abstract

A relatively new method to adjust refractive index of nano-porous silica films rapidly and continuously is reported. The thin films were prepared with the base/acid two-step catalytic sol-gel process and the dip-coating method. The structure, reflective spectra and refractive index of the films were analyzed with TEM, AFM, SEM, spectrophotometer, and ellipsometery, respectively. The experimental results have shown that the base/acid two-step catalysis can adjust the refractive index of the nano-porous silica films from 1.18 to 1.42 rapidly and continuously. And a broadband antireflective coating made with the method has very low reflectance. The average reflectance of the coated glass in the solar energy spectrum decreases to 1.5% from ∼7% of the glass.

Introduction

Sol-gel derived nano-porous silica films with an adjustable refractive index can form a very good broadband antireflectance. It is increasingly interesting to use them into a variety of commercial purposes such as cathode ray tubes (CRTs), color monitor tubes (CMTs), video display panels (VDPs), solar energy applications, shop windows, wind screen for cars, high power [1], [2], [3] laser system and so on for anti-glare and anti-reflectance.

There are several methods to produce a broadband antireflectance [4]. Among them, the coating with an adjustable refractive index is one of the best candidates. Usually, the adjustment of refractive index of the coating is performed by controlling porosity of the coating. US Patent 4944986 has reported an etching method to form a low reflectance yet high clarity glass, the surface of which was composed of the various structures such as surface scallops, micro-porous; structure, and skeletonized silica structure. However, the process produces some unrestorable spots, brings out the environment problems and much light scattering. Instead, Yoldas et al. [5] have invented a method to achieve a silica coating with a graded degree of porosity by means of controlling proportion of water to alkoxide and a concentration of alkoxide to solution as well as a small amount of catalyst, then etching the silica coatings in order to enlarge the pore in the coatings. Similarly, US Patent 5254904 presented a process to carry out a change in light refractive index between adjacent layers by controlling the temperature, acidity and degree of hydrolysis of the starting materials. Therefore, the antireflective coating is a light refractive index gradient. Whereas Tong et al. [6] have achieved a multilayer antireflective coating by using the same starting gel materials and controlling the ageing of the silica gel, with longer ageing providing increased cross-linking and larger molecular weight for a low light refractive index. As a result, a light refractive index ranges from 1.45 (shorter ageing) to 1.18 (longer ageing).

Although these approaches can produce a silica coating with the refractive index gradient, therefore the excellent broadband antireflectance, they are overly complicated, poor reproductivity, high time-cost, and requiring precise control of the composition of the coating, and thus impractical for large scale manufacturing of consumer-type CRTs, CMTs, VDPs and other applications.

In this paper, we report a novel route to control refractive index of nano-porous silica films rapidly — a base/acid two-step catalytic sol-gel process, and investigate effect of the two-step catalysis on the nano-porous structures and refractive index using TEM, AFM, SEM, and ellipsometer. Also, we produced a refractive-index-step broadband antireflective silica coating and measured reflective properties of the coating in the solar spectrum region with spectrophotometer.

Section snippets

Experimental details

Fig. 1 shows the procedure of preparation of silica sols which were made from TEOS, NH3 · H2O, ethanol, and HCI with a base/acid two-step catalysis. In the first step, TEOS, NH3 · H2O, and ethanol were mixed at room temperature with a molar ratio of 1: 1–5: 20–40 and then aged at room temperature for ∼10 days. The sol was under reflux at 80°C for several hours afterwards, and this is called the sol A which was prepared by only a base one-step catalysis. A mixture of TEOS, water, ethanol and HCI

TEM observation

Fig. 2 shows TEM photographs of the silica sols with the different catalysis. It can be found that the network of silica sols is strongly dependent on catalytic conditions. From Fig. 2(A), a particle structure like the bead chains is observed, diameters of the particles are approximately several nanometers. In contrast, no any structures in the acid catalyzed silica sols can be observed as shown in Fig. 2(B) due to their very fine linear chains. While the silica sols made by the two-step

Discussion

The growth of silica sols may result in structures ranging from linear or randomly branched chains to discrete porous clusters to fully dense colloidal particles, greatly depending on solution conditions [7]. Consequently, the structures of the silica sols can be controlled using different proportion of the components, ageing time, temperature and other conditions as mentioned in the introduction. But there are some disadvantages in the large scale production, although they can produce the good

Conclusion

It is concluded that the base/acid two-step catalysis can adjust the refractive index of the nano-porous silica films from 1.18 to 1.42 rapidly and continuously. Also the broadband antireflective coating with average reflectance 1.5% in the solar energy spectrum has been obtained with the method.

Acknowledgements

The authors are very grateful to the National Science Foundation of China (No. 69978017, 59802007) and the National Foundation of High Technology for Young Scientists (ICF 98-15) for their financial support, and also to the Development Foundation of Science and Technology of Shanghai (98JC14003) and Shanghai Phospher Program (99QE14041) for their financial supports.

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