Spray-Layer-by-Layer Assembly Can More Rapidly Produce Optical-Quality Multistack Heterostructures

Abstract

Automated spray-layer-by-layer (LbL) assembly was used to create highly reflective structurally colored thin films with high reflectance at near-UV light wavelengths. Reflectance peaks were tuned by fabricating alternating stacks of high (TiO(2) nanoparticles) and low (SiO(2) nanoparticles) refractive index materials using a non-quarter-wave design. Spray-assembled multilayer heterostructures fabricated with up to 840 individual polymer or nanoparticle deposition steps presented similar roughness and refractive index values compared to Bragg stacks obtained via immersion LbL assembly. Such complex multilayer heterostructures, however, could be fabricated in significantly shorter times; the time required to deposit a complete bilayer was only about 90 s, compared to 36 min for the immersion assembly of the same system. Optimization of the experimental parameters was performed to achieve uniform coatings and relatively smooth interfaces and surfaces. We observed that the spraying times of the nanoparticle and polymer solutions are the main parameters that determine the thickness, optical properties, and uniformity of the assembled films. Ellipsometry, atomic force microscopy (AFM), UV-vis spectroscopy, and transmission electron microscopy (TEM) were used to characterize the samples. The nanoparticle thin films were iridescent and presented relatively narrow peaks of high reflectance (∼90%) at visible and near-UV wavelengths of light.

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

“…Both systems exhibit linear growth, which is typical for bilayers containing nanoparticles. 10,20,22 The relationship between the number of bilayers and RI was not linear and evolves differently in each case. The RI in PEI/VMT shows greater consistency after 10 bilayers, where it ranges from 1.65-1.66.…”

Spray‐Assisted Polyelectrolyte Multilayer Buildup: from Step‐by‐Step to Single‐Step Polyelectrolyte Film Constructions

“…The equipment allowed to demonstrate the optical properties of the double layered film deposited on a glass with an increase of the transmission of about 5% compared to the bare glass side [85] Nogeira et al used automated spray assisted LbL to prepare highly reflective structurally colored thin films with high reflectance at near-UV light wavelength [86] This was reached by alternating stacks of high and low refractive index materials. High refractive index (1.87) stacks were built by alternating several layers of poly(vinyl sulfonic acid) and 7 nm TiO 2 nanoparticles and low refractive index (1.28) stacks were obtained by alternated deposition of PAH and 7 nm SiO 2 nanoparticles.…”

Layer‐by‐Layer Processed Multilayers: Challenges and Opportunities

“…New developments in the spray-assembly of LbL coatings look very promising in this regard, and much effort is currently devoted to the development of this process. Since the initial publication [64] showing that multilayers comparable to those obtained by dip assembly can be realized much faster by simply spraying the various solutions onto surfaces, this area has expanded rapidly [61,[65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] and is quickly moving toward technologically viable commercial products [81].…”

“…Figure 2.4 shows a structural color coating created by an automated LbL spray assembly technique. This particular multilayer heterostructure was assembled with 840 individual layers by the alternate spraying of polymer, nanoparticle and rinsing solutions onto a substrate, typically with 6 s spraying times [65]. When compared to an identical multilayer heterostructure fabricated by an LbL immersion process, the optical quality and uniformity is as good or better, and the processing time is reduced from days to hours.…”