Preparation of Thin Silica Films with Controlled Thickness and Tunable Refractive Index

Abstract

Silica films with controlled thickness and refractive index have been formed by the sequential adsorption of a cationic polyelectrolyte and silica sols. The conditions used to prepare the sol were varied, and allowed films with refractive indices as low as 1.16 to be obtained. The sequential adsorption technique allows the thickness of these films to be controlled in increments of 5-10 nm, depending on the desired refractive index. Scanning electron microscopy revealed that a low packing density of constituent silica particles was responsible for the low indices of these films. The as-adsorbed films are thermally robust; calcination at 500 degrees C resulted in only very small decreases in film thickness (by < or =1.8%) and refractive index (to as low as 1.14). After calcination, the silica films remained hydrophilic and sorbed water vapor from the atmosphere. As a result, the refractive indices of these films increased with increasing relative humidity (RH). The dependence of the refractive index on RH was eliminated by treating the calcined films with trimethylchlorosilane.

Layer-by-layer fabrication of broad-band superhydrophobic antireflection coatings in near-infrared region

“…Since then, much more attention has been paid to the fabrication of AR coatings using LbL assembly technique. The building blocks used for fabrication of AR coatings by LbL assembly technique can be linear polyelectrolytes [13], nanoparticles or nanowires with low refractive index [14,15], co-polymer micelles [16], and so forth. Through the LbL assembly process, these building blocks would be stacked into the porous structures, which resulted in the desirable AR coatings.…”

“…The situation becomes even worse when AR coatings are used in the near infrared (NIR) region between 900 and 2500 nm, where the night vision systems and NIR spectrum analysis work [18]. Therefore, the fabrication of AR coatings with water-repellent properties is quite important [15].…”

Composite films of polycations and TiO2 nanoparticles with photoinduced superhydrophilicity

“…It is the aim of the present investigation to build up composite films containing TiO 2 (most probably anatase) nanoparticles and polyethyleneimine (PEI) by such an integrative process but not by the direct use of preformed nanoparticles. The nanoparticles will be synthesized in situ during the film build-up instead of being deposited along the LBL process [14][15][16][17]. It will imply the LBL deposition of PEI and a precursor of titanium dioxide, namely Ti(IV) bis(ammoniumlactato)dihydroxide (TiBisLac) by alternated spraying of the corresponding solutions [18][19][20] as well as by dipping the substrate in the corresponding solutions.…”

Superhydrophilic and antireflective La(OH)3/SiO2-nanorod/nanosphere films

“…Thus, for a glass substrate with n s = 1.52, n f of an AR film must be 1.23 to achieve zero reflectance. Unfortunately, because of unavailability of materials with sufficiently low refractive index, such an AR coating has commonly been induced by the multilayer thin-films containing oppositely charged nanoparticles with or without polyelectrolytes [8][9][10]. In comparison with multilayer films for which rather complex coating process is required, the single-layer AR coating can be much more simply made but has limited functionality, exhibiting the minimum reflection at a narrow wavelength band.…”