Effects and applications of capillary condensation in ultrathin nanoparticle assemblies

Abstract

The electrostatic layer-by-layer (LbL) assembly technique can be used to make uniform, conformal multi-stack nanoparticle thin films from aqueous solution, with precise thickness and roughness control over each stack. Much of the effort in this area has focused on the assembly and characterization of novel nanostructures. However, there is a scarcity of studies addressing critical barriers to commercialization of LbL technology, such as the lack of mechanical durability and the difficulty of incorporating a diverse set of functional organic molecules into aqueous solution-based nanoparticle assemblies. The versatility of existing chemical functionalization methods are limited by requirements for particular substrate surface chemistries, compatible solvents, and concerns over uncontrolled nanoparticle deposition. Here we describe the advantageous use of capillary condensation, a well-known natural phenomenon in nanoporous materials, as a more universal functionalization strategy. Capillary condensation of solvent molecules into nanoporous LbL films was shown to bridge neighboring nanoparticles via a dissolution-redeposition mechanism to impart mechanical durability to otherwise delicate films. In situ crosslinking ability of photosensitive capillary-condensates was demonstrated. Particle size-dependence of the capillary condensation process was studied theoretically and utilized experimentally to modulate refractive index over coating thickness to achieve broadband antireflection (AR) functionality. Multi-stack AR coatings with alternating high- and low-index stacks were also made, and the influence of inter-stack and surface roughness on film transparency were studied quantitatively. The equivalent-stack approximation was utilized and presented as an enabling design tool for fabricating sophisticated solution-based optical coatings. Surface wettability could also be modified using capillary condensation - either by condensation of adventitious vapors during an aging process leading to a loss of optimized film properties, or by advantageous condensation of carefully chosen hydrophobic or hydrophilic molecules to tune wettability. Finally, preliminary Young's moduli measurements of all-nanoparticle and polymer-nanoparticle composite films were made using strain induced elastic buckling instabilities for mechanical measurements (SIEBIMM).