Antireflective surface fabricated from colloidal silica nanoparticles

Abstract

Nanostructured antireflective surfaces were fabricated by the deposition of monodispersed silica nanoparticles on a glass substrate by electrostatic attraction between charged colloidal particles and charged polyelectrolyte multilayers. The effects of particle size and surface nanoparticle density on the antireflective properties of the nanostructured surfaces were investigated by the analysis of their reflection/transmission spectra and examination of their surface morphology. It was found that the nanostructured surfaces with particles of ∼120 nm in diameter yielded the most suitable performance for antireflection with respect to the visible-light region. The nanoparticle coatings revealed reflective properties similar to homogeneous porous layers. However, in comparing the simulation analysis to the experimental results, it was found inappropriate that an NP coating is entirely regarded as a homogenous porous layer.

Introduction

To prevent disturbances from external light and increase the transmittance of incident light, antireflective (AR) technologies are widely applied in optical components, solar cells, displays, thermochromic windows, etc. The main AR method for most practical applications is a multilayer construction yielding destructive interference in light reflected from interfaces between the different refractive-index layers. To achieve zero reflection, the refractive indices of the AR materials, for the case of a single-layer coating on substrates, should satisfy the criterion $n_c=\sqrt{n_0{n}_s}$, where n_c, n₀, and n_s are the refractive indices of the AR material, air, and substrate, respectively [1]. For glass and most plastics, the value of n_s is ∼1.5, and thus the value of n_c must be 1.22. This required value of n_c 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 [2], [3], [4], [5], a surface-relief structure made by phase-separation [6], [7], selective dissolution [8], [9], or lithography technologies [10], and homogenous porous coatings prepared by sol–gel processes [11], [12], [13], have been reported. In particular, the layer-by-layer assembled deposition of nanoparticles (NPs) has revealed excellent antireflection and other functions, e.g. antifogging, superhydrophilic, and self-cleaning [14], [15], [16], [17], [18].

Hattori [19] deposited a single layer of silica NPs on glass substrates by electrostatic attraction [20] between charged colloidal particles and charged polyelectrolyte multilayers to produce AR properties. The reflectance of one side of the coated glass was as low as 0.07% in his experiment. Although the AR properties of the single-layer NP coatings were very good [21], [22], [23], the reflective mechanism for these nanostructured surfaces fabricated by NPs was not analyzed. The optical analysis of multilayer structures with varying refractive-index materials has been well performed by the calculation of the recursive Fresnel formula or the admittance method [1]. In contrast, the analysis of nanostructured surfaces has been very limited, especially for single-layer NP-structured surfaces.

Considering the need for information gathering on antireflection in NP-structured surfaces for practical applications, performing a systematic analysis was highly desirable. In this study, the effects of the particle size and the surface NP density on the antireflection of nanostructured surfaces were studied experimentally. The optimal conditions of NP coatings with respect to AR performance were also evaluated. In addition, the validity of regarding a single-layer NP coating as a homogenous porous layer was further discussed.