Antireflection coating with enhanced anti-scratch property from nanoporous block copolymer template
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].where 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.
References (21)
- et al.
Surf. Coat. Technol.
(2005) - et al.
Thin Solid Films
(2008) - et al.
Opt. Mater.
(2005) Thin-film Optical Filters
(1986)- et al.
Science
(1999) - et al.
Adv. Mater.
(2004) - et al.
Macromolecules
(2008) - et al.
Small
(2007) - et al.
Langmuir
(2007) - et al.
Appl. Phys. Lett.
(2009)
Cited by (30)
Asymmetric polymer materials: Synthesis, structure, and performance
2022, PolymerCitation Excerpt :Additionally, the reflectance and transmittance properties also depended on the spin-coating rate, solvent and nonsolvent nature [244]. To further improve the life span of AR coatings, a silica could be incorporated on top of the BCP template to enhance its scratch resistance [245]. Although the development of the asymmetric structure has continued for decades, there are still many challenges to overcome.
Characterization of spectral selectivity of tin-doped In<inf>2</inf>O<inf>3</inf> thin films using spectroscopic ellipsometry
2015, Surface and Coatings TechnologyCitation Excerpt :To date, little has been done on computing the spectral transmittance and reflectance of TCO-AR stack with different TCO thickness in the wide wavelength range from ultraviolet to infrared. Additionally, it is noted that most of the reported TCO-AR stacks only aimed for the highest visible transmittance [6,7] without considering the whole solar transmittance. When this kind of TCO-AR stack acting as a selectively transmitting layer on the cover glass of a solar heat collector, the thermal efficiency is not optimized since the visible light only accounts for 47% of the total solar energy.
Antireflective films with Si-O-P linkages from aqueous colloidal silica: Preparation, formation mechanism and property
2014, Solar Energy Materials and Solar CellsPreparation of hydrophobic and abrasion-resistant silica antireflective coatings by using a cationic surfactant to regulate surface morphologies
2014, Solar EnergyCitation Excerpt :The hardness test was performed by the pencil scratch method, i.e., using pencils with hardness ranging from 6 B to 6 H. The flat end of the pencil was placed on the coating at a 45° angle to the surface with a loading of 9.8 N (Joo and Kim, 2011). According to the ASTM Standard D 3363-00, the process is conducted under the observation of the microscopes.
Synthesis and characterization of nanostructured poly(methyl methacrylate) for antireflection coating
2014, Applied Surface Science