Self-assembled dual-sided hemispherical nano-dimple-structured broadband antireflection coatings

A non-lithography-based approach is developed in this study for assembling monolayer close-packed hemispherical nano-dimple arrays on both sides of a PET film by a scalable Langmuir-Blodgett technology. The resulting gratings greatly suppress specular reflection and therefore enhance specular transmission for a broad range of visible wavelengths, resulting from a gradual change in the effective refractive index at air/PET interface. The experimental results reveal that the antireflection properties of the as-fabricated coatings are affected by the size of the nano-dimples. Moreover, both optical performances of single-sided and dual-sided nano-dimple-structured coatings have been investigated in this study. Natural nanostructures, with unique functionalities, have provided a vast range of inspirations for practical applications.1–4 For example, cicada wings are highly transparent and exhibit less than 1% reflectance over the visible spectrum, owing to the presence of hexagonal arrays of sub-200 nm conical structures on their wings.5,6 The cone arrays with a period smaller than the propagating wavelength leads to a gradient index of refraction across the interface between air and wing, substantially suppressing Fresnel's reflection.7,8 Biomimicry of the cicada wing design onto material surfaces is extensively used as an antireflection coating in optical and optoelectronic applications, including displays, solar cell systems, and vehicle dashboards.9,10 Comparing to traditional quarter-wavelength antireflection coatings, multilayer antireflection coatings, as well as porous antireflection coatings, biomimetic subwavelength conical-structured coatings give rise to a gradual change in refractive index rather than a sudden one, and display great broadband antireflection properties over a wide range of angles of incidence.11–13 However, the coatings are often composed of fragile high-aspect-ratio surface geometries and the fabrication procedures usually require complex preparation schemes.14 Moreover, current top-down lithographic technologies (e.g., photolithography, nanoimprint lithography, and laser interface lithography) for fabricating cicada-wing antireflection coatings are expensive and exhibit limited control over pattern spatial homogeneity and reproducibility.15–17 In contrast, self-assembled colloidal crystals have been used as etching masks during a reactive ion etching process to template nanostructure arrays, offering a less expensive and simpler methodology in fabricating cicada-wing gratings.18–21 Unfortunately, bottom-up self-assembly technologies are mostly used in laboratory-scale and it is challenging to scale up these processes.22–24 Recently, plasmonic antireflection coatings consisting of a subwavelength metal array and a dielectric cover layer by combination of lithography and metal deposition are designed and fabricating, showing a broadband reduction of reflection.25–28 However, the reactive ion etching process is relatively expensive and highly labor-intensive, and also requires clean room conditions.29,30 In this research, we developed a Langmuir-Blodgett-like approach to assemble large-area nano-dimple-structured antireflection coatings in a roll-to-roll compatible manner. Monodispersed silica spheres with diameters of 155 nm and 70 nm were synthesized by the Stöber method.31 The synthesized Stöber silica spheres were purified in absolute ethanol by repeated centrifugation/re-dispersion cycles to remove impurities, following by re-dispersing in non-volatile ethoxylated trimethylolpropane triacrylate (ETPTA) monomers with 1 vol. % Darocur 1173 as photoinitiator. The silica particle volume fraction was controlled to be 22.5 vol. %. Figure 1 presents a schematic illustration of the fabrication procedure for creating dual-sided hemispherical nano-dimple-structured antireflection coatings. The as-prepared silica/ETPTA suspension was added dropwise, while the ETPTA monomer-covered silica spheres spread rapidly and formed two-dimensional colloidal monolayers at the deionized water surface. A poly(ethylene terephthalate) (PET) film pre-immersed in deionized water was vertically withdrawn at a rate of ∼5 cm/min by using a double-sided tape attached to a syringe pump. The floating ETPTA monomer-covered silica colloidal monolayers were uniformly transferred onto both sides of the PET substrate. The capillary action enables the ETPTA monomers to flow and fill the empty space between the silica colloidal crystals and the PET substrate. The sample was then transferred to a pulsed UV curing system to photo-polymerize ETPTA monomers under UV radiation. The silica spheres partially embedded in ETPTA polymeric matrices were finally removed by wet etching in a 2 vol. % hydrofluoric acid aqueous solution to pattern periodic nano-dimple structures