Single‐Step Self‐Organization of Ordered Macroporous Nanocrystal Thin Films

Abstract

Nanocrystals, 20 to 100 in diameter, exhibit unique size dependent electronic, optical, and magnetic properties. [1,2] Assembly with spatial control on various length scales presents a critical challenge in nanocrystal processing. Although translational order on the nanometer-and submicrometer-scale is readily achieved in self-organized superlattices, [3] some applications such as photonics, separations, catalysis, and sensors require organization into more spatially complex macroporous structures where the lattice parameter is on the order of a micrometer or more. Herein, we show the formation of ordered macroporous thin films of nanocrystals by dispersion evaporation of a volatile solvent in a relatively humid environment. Water droplets condense on the evaporating dispersion and self-organize into an ordered array that templates the deposition of the hydrophobic nanocrystals. Upon complete evaporation, an ordered macroporous thin film made of close-packed nanocrystals remains. Interfacially active nanocrystals that adsorb at the solvent±water interface prevent water droplet coalescence and preserve the structure of the macroporous film during drying. The ligands and the nanocrystal cores may be chosen independently to form a wide variety of novel porous films. Macroporous materials are typically fabricated in a threestep process of template formation, matrix infusion, and template disintegration. Prefabricated templates of colloidal crystals, [4] emulsions, [5] or microemulsions [6] are infused with a material of choice, such as metals, [7] semiconductors, [8] inorganics, [4] and polymers, [9] and then subsequently removed to reveal arrays of holes. In some cases, nanocrystals have been infused into colloidal crystal templates. [7,8] Often the collapse, or loss of shape, of the templated media occurs during thermally or chemically harsh template removal steps. The evaporative self-assembly of macroporous nanocrystal films demonstrated here represents a single-step method to achieve films with ordered micrometer-size holes. The nanocrystals in the films remain as distinct individual particles with the potential to retain their size-dependent properties. Figure 1 shows scanning electron microscopy (SEM) images of macroporous gold nanocrystal thin films consisting of a continuous network of perfluoropolyether thiol-coated (PFPE-SH, (F(CF(CF 3 )CF 2 O) 3 CF(CF 3 )CONH(CH 2 ) 2 SH)) 5 nm diameter gold nanocrystals. The self-organized porous array of submicrometer-size holes extends over thousands of square micrometers of substrate. The films in Figure 1 were formed by drop casting the PFPE-capped Au nanocrystals in 1,1,2-trichlorotrifluoroethane (Freon) under ambient conditions at 60 % relative humidity. The Freon evaporates quickly due to its high vapor pressure, leading to a significant temperature drop and water condensation at the air±liquid interface. The water droplets grow by molecular condensation until they reach a self-limiting narrow size distribution [10] and ultimately organize at high surface coverage...

Breath Figures as a Dynamic Templating Method for Polymers and Nanomaterials

“…Korgel and co-workers and Chen and co-workers have investigated the BF formation of dodecanethiol or perfluoro-thiol (29) stabilized gold nanoparticles (5 nm) cast from Freon-type solvents or CS 2 . [54,55] from chloroform was explored as well. Hexagonal bubble arrays with bubble dimensions from 350 nm to 5 lm formed upon evaporation in a humid environment.…”

Self-assembly of nanoparticles at interfaces

“…Korgel et al for example created self-organized superstructures from 5 nm gold particles. 42,43 Analogous experiments using CdSe, CdTe and HgTe nanoparticles were described by Bawendi et al 44 First studies on the nanoparticle self-assembly at curved (droplet) interfaces in Pickering emulsions were recently published by Lin et al, 10,45,46 Mann et al, 47 Dai et al 48 and Mö hwald and coworkers. [11][12][13] 2.…”

Nanoporous gold assemblies of calixarene-phosphine-capped colloids

“…The basic physical chemistry behind their formation follows that articulated by Hildebrand, 13 which is also based on observations of Pickering, 14 and Ramsden, 15 spanning a century of research into solid colloidal-particle stabilization of oil/ water interfaces. More recently, similar phenomena have been captured by the elegant work of Korgel and coworkers, 11,12 when describing the breath-figure templating of gold nanoparticles by water droplets, which are formed upon condensation in an organic fluorocarbon medium (which evaporates during the kinetic templating process). In our case, we invoke H 2 bubbles as the equivalent of an oil phase in a conventional oil droplet in water microemulsion.…”

“…In our case, we invoke H 2 bubbles as the equivalent of an oil phase in a conventional oil droplet in water microemulsion. Akin to water bubbles in breath-figure templating, 11,12 we hypothesize the H 2 bubbles act as templates.…”

“…This gold-gold bonding between clusters must be sufficient so as to retain a stable and unaltered nanoporous gold network during both drying of organic solvent and ligand removal (vide supra). It differentiates our materials from those previously synthesized using breath-figure templating of gold nanoparticles, 11,12 in which there does not appear to be bonding between gold nanoparticles (i.e. distinct 3 nm nanoparticles comprise the assembly pore wall).…”