Elsevier

Thin Solid Films

Volume 519, Issue 11, 31 March 2011, Pages 3804-3808
Thin Solid Films

Antireflection coating with enhanced anti-scratch property from nanoporous block copolymer template

https://doi.org/10.1016/j.tsf.2011.01.107Get rights and content

Abstract

We prepared a sponge-like nanoporous silica film with a dense skin layer by the infiltration of silica precursor into nanoporous polystyrene-block-poly(methyl methacrylate) copolymer template followed by calcinations at high temperature. This film showed not only excellent antireflection at visible light wavelength range but also very good resistance to scratching compared with antireflection materials made of polymeric film. We expect that this film could be used for antireflection film with anti-scratching property for flat panel displays or touch panels.

Introduction

Light reflection at the interface between a transparent substrate and a transmitted medium is inevitable because of the rapid change in the refractive index (n) from air to the substrates [1], [2]. This disturbing light reflection can cause a “ghost image” or blurring of viewed images on a flat panel display or light loss which can cause low efficiency of a solar cell. This light reflection loss is avoided by using antireflection (AR) film with a proper n and a judicious control in the thickness (d) [1].

To remove light reflection completely at the interface, two requirements should be satisfied [1]: nf = (nsno)1/2, with nf, ns, and no being the n of an AR film, a substrate, and a transmitted medium, respectively, and nfd = 4λ for d of the AR film and target wavelength of light (λ). For instance, for zero reflectance at 550 nm, the values of n and d of the AR film on the glass substrate should be 1.23 and 112 nm, respectively. However, because most organic or inorganic materials have n higher than 1.23, AR film should be achieved by introducing porous structure into the film [1].

Porous polymer films have been extensively employed for AR films. Steiner and coworkers introduced porous structure by using polystyrene/polymethyl methacrylate (PS/PMMA) blend followed by removing the PMMA [2]. Some research groups fabricated AR films by colloidal assembly [3], [4], layer-by-layer assembly [5], [6], nanoimprinting method [7], and plasma treatment on polymer surface [8]. Others prepared a polymeric nanorod array based on anodized alumina template [9], [10]. Although these methods are easy and versatile, porous polymer films have very poor thermal stability. Also, it shows poor anti-scratch property which is very important for a flat panel display.

To increase the anti-scratch property as well as to enhance thermal stability, inorganic materials should be used. Hattori [11] and Tao [12] showed that an array of silica spheres with an appropriate diameter satisfying the quarter-wave optical thickness, exhibited good AR. However, because of the difficulty in controlling the pore volume fraction and the final thickness, multiple process steps of the layer-by-layer deposition were employed. Other research groups [13], [14] prepared a regularly patterned surface on a silicon substrate by using reactive ion etching with the aid of an etchant mask and this showed good AR and Saarikoski et al. [15] prepared nanoporous anodized aluminum oxide layer on a polycarbonate surface; but these methods need a patterned mask in addition to the use of a high vacuum.

In this study, we prepared a nanoporous silica film with enhanced anti-scratch property based on a nanoporous block copolymer template. The template was fabricated by spin-coating of polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) on a glass substrate without thermal annealing, followed by removing PMMA block [16]. Then, silica precursor was infiltrated into the nanoporous template and calcined at high temperature. The fabricated nanoporous silica film showed good AR at visible wavelength range and high anti-scratch property confirmed by a pencil hardness test.

Section snippets

Materials and fabrication

PS-b-PMMA with a number average molecular weight of 98,200 and a polydispersity of 1.13 was purchased from Polymer Source Inc. (#P2355-SMMA) and used as received. The volume fraction of PMMA was 0.46. Soda lime glass of n = 1.52 (Plain product #2947, Corning) was cleaned with a mixture of sulfuric acid (70 vol.%) and hydrogen peroxide (30 vol.%) for 30 min at 80 °C (piranha treatment).

Organosilicate sol was prepared according to the method in Ref. [17]. First, 0.753 mol of methyltriethoxysilane

Results and discussion

The volume fraction of the PMMA block in the PS-b-PMMA employed in this study was 0.46. We chose this volume fraction to satisfy the zero reflectance for nanoporous silica film. When the PMMA block in PS-b-PMMA becomes nanopores, and the silica sol is infiltrated into the nanopores, n of a nanoporous silica film is estimated [21].n2=nSiOx2fPMMA+nair21fPMMAwhere nSiOx and nair are the refractive indices of the silica (1.46) [1] and air (1.0), respectively, and fPMMA is the PMMA volume fraction

Conclusion

We fabricated nanoporous silica film based on nanoporous block copolymer. Since the volume fraction of PMMA block (thus the pore volume of the template) is easily tuned, various inorganic thin films could be used for excellent AR at visible wavelengths. Furthermore, since this film has a very thin and dense skin layer, excellent anti-scratch property was obtained without sacrificing the excellent AR.

Acknowledgements

This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (NRF) and the second stage of the BK 21 Program of Korea.

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