Aggregation of Silica Nanoparticles in Sol–Gel Processes to Create Optical Coatings with Controllable Ultralow Refractive Indices

Abstract

Optical coatings with controllable ultralow refractive indices are of profound significance in optical areas. However, it remains a challenge to fabricate such coatings using a simple method. Here we develop an effective and simple approach to create ultra-low-index coatings. This approach was based on a modified sol–gel process, with a key process that involved the aggregation of silica nanoparticles via the addition of a polymer surfactant (e.g., polyvinylpyrrolydone) in sols before coating. The approach involves three steps: the synthesis of silica sols under ammonia catalysis in ethanol (Stöber method), the addition of polyvinylpyrrolydone in the silica sols to induce the aggregation of the silica nanoparticles, and the formation of ultra-low-index coatings by depositing the aggregated silica sols on substrates. Through varying the aggregation extent, this approach produced coatings with controllable refractive indices ranging from 1.17 to 1.07. To the best of our knowledge, the minimum index value of 1.07 from our coating is among the lowest refractive indices ever reported. The ultra-low-index coatings demonstrated excellent optical properties, with which perfect quarter-wavelength antireflection coatings (maximum transmittance ∼100%) and broadband antireflection coatings (transmittance >98% from 400 to 1100 nm) can be prepared. One advantage of the antireflection coatings is that their transmission is less dependent on the refractive index and the thickness of the stacking layer, which make it promising in large-scale production. Moreover, the coatings can be made hydrophobic (water contact angle 136°) by exposing the coatings to a hexamethyldisilazane atmosphere, exhibiting high environmental stability in a humid environment. The aggregation of silica nanoparticles in sol–gel processes provides a scalable alternative to the current approaches for creating ultra-low-index coatings.

Chain length of bioinspired polyamines affects size and condensation of monodisperse silica particles

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Emerging Applications of Silica Nanoparticles as Multifunctional Modifiers for High Performance Polyester Composites

“…Sol-gel method is widely used in synthesis of SNs to get pure and homogeneous products under mild conditions [101][102][103]. The process involves initial hydrolysis and subsequent condensation of silicates employing mineral acids or bases as catalysts.…”

Optical and Laser-Induced Damage Characterization of Porous Structural Silicon Oxide Film with Hexagonal Period by Nanoimprint Lithography

“…The smooth surface of the optical element helps to improve its LIDT [11]. In the past, various preparation methods, such as e-beam evaporation [12], plasma-enhanced chemical vapor deposition [13][14][15], atomic layer deposition [16,17], and the sol-gel method [18], were used to fabricate laser films, but these methods are not suitable for the preparation of large-area nanostructured surfaces with high-precision patterns. Laser surface treatment can be used to process some functional surfaces, but large-area nanoscale patterning is difficult to fabricate.…”