A simple route to prepare crack-free thick antireflective silica coatings with improved antireflective stability

doi:10.1016/j.matlet.2011.11.043

Materials Letters

Volume 69, 15 February 2012, Pages 86-88

https://doi.org/10.1016/j.matlet.2011.11.043 Get rights and content

Abstract

In this study, a simple route was proposed to prepare crack-free thick antireflective (AR) silica coatings with improved abrasion resistance and AR stability. Using tetraethylorthosilicate (TEOS) as precursor and hydroxyl-terminated polydimethylsiloxane (PDMS) as modifier, silica AR coatings were prepared by sol–gel process with dip-coating method. It is found that the addition of PDMS extends the adjustable range of sol viscosity, and allows the preparation of thick coatings by a simple route. PDMS prevents the thick AR coating from cracking and enhances its abrasion resistance due to the enhanced particle-to-particle bond strength and the reduced stress inside the film. The hydrophobicity of the coating is also improved by the introduction of hydrophobic methyl groups, and the measured water contact angle increases from 23.4° to 75.3°. Thick crack-free AR coatings with good abrasion resistance and antireflective stability can be prepared by this simple route.

Highlights

► A simple route to prepare crack-free antireflective coatings was proposed using Sol-gel process. ► Hydroxyl-terminated polydimethylsiloxane (PDMS) was used as a modifier. ► PDMS reduced effectively the drying stress and thus prevented the thick films from cracking. ► PDMS enhanced the hydrophobicity of coatings and thus the antireflective stability.

Introduction

Antireflective (AR) coatings with high transmittance in the near-infrared (NIR) region have been utilized in transmitting optics, such as output couplers, dichroic mirrors, lenses, and windows [1]. In some cases, the optical theory requires these coatings to have thickness of more than 400 nm. Normal sol–gel process, which is an effective way to prepare AR coatings, can prepare oxide films with a maximum thickness of 100–300 nm by single-deposition. For base-catalyzed pure silica AR coatings, it is difficult to prepare thick AR coatings by single-deposition due to the relative low sol viscosity [2]. Thick pure inorganic coatings can be obtained by multi-deposition. However, these coatings often suffer from low abrasion-resistance, poor AR stability and cracking problem due to high capillary pressures developed during drying process.

It is well known that base-catalyzed AR coatings are kept intact only by point contact forces between individual particles, and the bindings within the coatings are therefore quite weak [3]. Thomas IM [4] demonstrated that the coating strength can be significantly improved by introducing polymer binders into silica sol. The polymer binder can link individual particles together by covalent or hydrogen bond and enhance the particle-to-particle bond strength. Previous research suggests also that introduction of organic groups has an effect of releasing the stresses and allows the preparation of very thick coatings [5]. Sol–gel silica AR coatings are hydrophilic and porous, which tend to absorb water from working environment. The water absorbed in the pores of coatings will increase the refractive index [6], [7] and then decrease the AR property. The introduction of hydrophobic organic groups into AR coatings is a good way to reduce the absorption of water [8], [9].

PDMS molecule possesses two hydroxyl groups and many methyl groups. The hydroxyl groups of PDMS can dehydrate with the hydroxyl groups on the silica particle surface, resulting in a covalent link among individual silica particles. As we previously reported [2], the addition of low concentration of PDMS increased the adjustable viscosity range of silica sols. In this work, we propose a modified Stöber method for preparation of organic–inorganic hybrid sol in the presence of high concentration PDMS, with a view to provide a simple one-step route to prepare crack-free thick AR coatings with improved AR stability, as well as enhanced abrasion resistance.

Section snippets

Preparation of PDMS modified silica sol and AR coatings

PDMS modified silica sols were prepared based on the Stöber method. A solution of EtOH, PDMS, TEOS, H₂O, and NH₃·H₂O (13.4 mol/L) was prepared and then immediately stirred for 2 h at 30 °C. The final molar ratio of TEOS: H₂O: EtOH: NH₃ was 1: 3.25: 37.6: 0.17. The weight ratio of PDMS to SiO₂ was varied from 0% (namely the pure base-catalyzed sol) to 30%. Finally, the resultant sols aged at room temperature for some days to reach a certain suitable sol particle size and viscosity before deposition.

IR characterization

The FTIR spectra of pure and PDMS modified xerogels, as well as PDMS, were measured to demonstrate that PDMS were successfully introduced into silica skeleton by dehydration. As shown in Fig. 1, all spectra show absorption peaks at 1062 cm^− 1 and 795 cm^− 1, attributed to the stretching modes of the Si single bond OSi bonds. After PDMS modification, two additional adsorption peaks appear at 1268 cm^− 1 and 903 cm^− 1 corresponding to the Si-C and the Si-CH₃, and with an increase of PDMS concentration, these two

Conclusion

Crack-free thick single-layer AR coatings were successfully prepared in this work by using hydroxyl terminated PDMS as organic binder. Due to the presence of sufficiently high concentration of organic PDMS, the sol viscosity could be maintained at a relatively high value, allowing to prepare thick films with good optical quality in one deposition process. The dehydration reaction between hydroxyl groups of PDMS and silica particles gives some covalent bonding among the silica particles which

Acknowledgments

The authors gratefully acknowledge the support from the Central University Foundation of China “2010scu23003”.

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A simple route to prepare crack-free thick antireflective silica coatings with improved antireflective stability

Abstract

Highlights

Introduction

Section snippets

Preparation of PDMS modified silica sol and AR coatings

IR characterization

Conclusion

Acknowledgments

Mater Lett

Mater Lett

Thin Solid Films

J Colloid Interface Sci

J Colloid Interface Sci

J Phys Chem C

Proc SPIE