Manipulating the chemical- and micro-structure of silica thin film for tailoring the refractive index by a template- and modifier-free sol-gel method.

Thin films with adjustable refractive indices are crucial for fabricating multi-layer antireflective coatings (ARCs). Conventional sol-gel methods typically require the incorporation of templates or modifiers to precisely control film porosity and refractive index. In this work, we propose a template- and modifier-free sol-gel method to precisely engineer the chemical- and micro-structure of silica nanoparticles (SNPs), enabling the fabrication of silica thin films with tunable refractive indices ranging from 1.15 to 1.38 using only tetraethoxysilane (TEOS), ethanol, ammonia water, and water as raw materials. By simply adjusting NH·HO and HO content, the hydrolysis and condensation of TEOS can be controlled to obtain SNPs with adjustable chemical- and micro-structure, which enable the control of inter-particle and intra-particle porosity. It is found that at low NH·HO content, the resulting SNPs exhibit a surface abundant in hydrophobic ethoxy groups, which effectively reduce the capillary pressure within a thin film during the drying process, thereby inhibiting the film shrinkage, increasing the porosity, and ultimately producing a hydrophobic film with a refractive index as low as 1.15. Conversely, with high NH·HO content, the resultant SNPs have few hydrophobic ethoxy groups on their surface, exhibiting excellent hydrophilicity. The resultant increased capillary pressure induces significant film shrinkage during the drying process, resulting in a thin film with low porosity and high refractive index. Finally, leveraging this tunability of refractive index, we successfully fabricated a double-layer hydrophobic broadband ARC with excellent optical performance, where the thin films prepared with high and low NH·HO contents were integrated as bottom and top layers, respectively.