Antireflective Optical Properties of Colloidal Subwavelength Nanostructured Surfaces
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1256-N11-22
Antireflective Optical Properties of Colloidal Subwavelength Nanostructured Surfaces
Bo-Tau Liu and Sheng-Jie Tang Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin, Taiwan 64002
ABSTRACT Colloidal subwavelength nanostructured surfaces were fabricated by the deposition of uniform silica nanoparticles on a glass substrate by means of electrostatic attraction between charged colloidal particles and charged polyelectrolyte multilayers. The effects of surface morphology via the variation of nanoparticles on the antireflective properties of the nanostructured surfaces were investigated by the analysis of the reflection spectra and the SEM images. The Maxwell’s equations were solved by a rigorous coupled-wave analysis (RCWA) to evaluate the experimental results. It was found that the reflective properties revealed by the simulation analysis were similar to the experimental results. The nanostructured surfaces with particles of ~120 nm in diameter yielded the most suitable performance for antireflection with respect to the visible-light region. In addition, the nanostructured surfaces showed the good antiscratch when the nanoparticles were bound by polyethoxysiloxane. INTRODUCTION For glass and most plastics, about 4-5% of incident light is reflected from the surface. To eliminate disturbance of the external light and increase light transmission to enhance the clarity of display images and the performance of optical components, antireflection (AR) coatings are usually applied to optical lenses, solar cells, displays, thermochromic windows, eye glasses, camera lenses, etc. The main AR method for most practical applications is a layer construction yielding destructive interference in light reflected from interfaces between the different refractive-index layers. If the surface would like to achieve zero reflection, the refractive index of the AR coating, for the case of a single-layer AR coating, must be equal to the square root of the refractive index of the substrate. Because the refractive indices of glass and most plastics are ~1.5, the required refractive index of the coating must be ~1.22. This refractive index is so low that no known bulk materials can meet the criterion. Therefore, many approaches, such as a multilayer structure with layers of varying refractive indices [1], a surface-relief structure made by phase-separation [2], selective dissolution [3], or lithography technologies [4], and homogenous porous coatings prepared by sol-gel processes [5], have been reported. Among these methods, subwavelength-nonstructured surfaces are usually prepared. The antireflection properties of subwavelength-nanostructured surfaces have been found in the cornea of night-flying moths, and so is called the “moth-eye” effect [6]. This characteristic is also found in other insects, such as the compound eye of a fly. Hattori [7] deposited a single layer of silica nanoparticles (NPs) on glass substrates by electrostatic attraction [8] between charged colloida
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