Rapid Sol–Gel Fabrication of High-Quality Thin-Film Stacks on Planar and Curved Substrates

Abstract

A versatile and rapid sol−gel technique for the fabrication of high quality one-dimensional photonic bandgap materials was developed. Silica/titania multilayer materials are fabricated by a sol−gel chemistry route combined with dip-coating onto planar or curved substrates. A shock-cooling step immediately following the thin-film heat-treatment process is introduced. This step was found crucial in the prevention of film crack formation especially in silica/titania alternating stack materials with a high number of layers. The versatility of this sol−gel method is demonstrated by the fabrication of various Bragg stack-type materials with fine-tuned optical properties by tailoring the number and sequence of alternating layers, the film thickness and the effective refractive index of the deposited thin films. Measured optical properties show good agreement with theoretical simulations, confirming the high quality of these sol−gel fabricated optical materials.

Sol–gel-derived thin-film stacks with high radiation stability

“…Multilayer film samples were prepared by dip-coating from silica and titania sol-gel solutions, as reported previously [11]. In short, silica precursor solution was prepared by mixing 105 mL of tetraethyl orthosilicate with 12.3 mL of deionized water, 76 mL of absolute ethanol (EtOH), and 6.3 mL of 0.1 M hydrochloric acid.…”

“…Firstly, suppression of cracking during formation of thin films and multilayer stacks, and secondly, fabrication of films with high mechanical and radiation stability [6,7]. In recent years, several methods have been developed to address the former challenge, including spin-coating from metal-peroxo solutions [8], firing processes at high temperatures (900-1000°C) [9,10], and the shock-cooling procedure of sol-gel dip-coated films developed in our group [11]. Achieving high mechanical and radiation stability is still very challenging in sol-gel methods.…”

Bio-inspired iridescent layer-by-layer assembled cellulose nanocrystal Bragg stacks

“…Despite its simple process and variety of choice in materials, sol-gel techniques suffer from issues of poor uniformity and cracking (heat treatment required for improvement) over larger areas. 18,19 In order to create a periodic structure with high uniformity, layer-by-layer (LbL) deposition was applied by assembling oppositely charged materials from water. The variety of materials that are available and the simple process (i.e., aqueous solutions under ambient conditions) makes LbL assembly a useful method to fabricate Bragg reflectors.…”

1D photonic defect structures based on colloidal porous frameworks: Reverse pore engineering and vapor sorption

“…The presence of guest molecules in the textural pores of the multilayer constituents induces a change in the effective refractive index (RI) and, hence, the structural color of the infiltrated photonic architecture. SiO 2 and TiO 2 layers derived from low-cost solegel processing routes are typically used as building blocks in 1D PCs [9,10], as they allow for a high RI contrast and well-defined Bragg reflection peaks with only a few bilayers (BLs) in the stack. A variety of modification schemes have been explored to enhance the response toward environmental changes including the variation of the morphology of the constituting layers such as porosity, size or shape of the building blocks [5,6,11,12].…”

“…The sensitivity of solegel derived architectures could thus be improved by the use of mesoporous oxides [7,13]. Another possibility is the fabrication of cavity structures whose symmetry is disrupted by a defect layer deposited either inside or on top of a regular BS (referred to as "sandwich" and "top" defect structure, respectively) [6,9,10,14]. The addition of such a layer leads to a narrow band range of allowed frequencies within the photonic stop band, and the well-defined spectral position of the narrow defect mode can be employed as a precise measure for gradually differing analyte concentrations [10].…”