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...

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Titanium dioxide nanoparticles were produced by the controlled hydrolysis of titanium tetraisopropoxide (TTIP) in the presence of reverse micelles formed in CO2 with the surfactants ammonium carboxylate perfluoropolyether (PFPECOO-+NH4) (Mw = 587) and poly(dimethyl amino ethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl methacrylate) (PDMAEMA-b-PFOMA). Based on dynamic light scattering measurements, the amorphous TiO2 particles formed by injection of TTIP are larger than the reverse micelles, indicating surfactant reorganization. The size of the particles and the stability of dispersions in CO2 were affected by the molar ratio of water to surfactant headgroup (w(o)), precursor concentration, and injection rate. The amorphous particle size did not change upon depressurization and redispersion in CO2. PDMAEMA-b-PFOMA provided greater stability against particle aggregation at higher reactant concentration compared with PFPECOO-+NH4. The crystallite size after calcination, which was examined by X-ray diffraction and transmission electron microscopy, increased with w(o).

A new concept is introduced in which a surfactant, poly(dimethylsiloxane)-b-poly(methacrylic acid) (PDMS-b-PMA) (Mw ) 5500 g/mol PDMS, 900 g/mol PMA), is utilized to stabilize an organic latex in either a nonpolar medium, dense CO2, or water. The latex particles, in this case poly(methyl methacrylate) (PMMA), were synthesized by dispersion polymerization in supercritical carbon dioxide. In CO2, the PDMS block provides steric stabilization while the PMA block adsorbs to the particle surface. Upon transfer to water, the PDMS block collapses onto the surface and the PMA block ionizes for pH > 5 to stabilize the latex by electrostatic repulsion, as shown by zeta potential measurements. The surfactant is "ambidextrous" in that it provides stabilization in either CO2 or water, by different mechanisms in each medium. Smaller more uniform particles were produced in CO2 with a mixture of the commercially available surfactant, PDMS-g-pyrrolidonecarboxylic acid (PDMS-g-PCA) (Mw ) 8500 g/mol, ∼2 PCA groups) and PDMS-b-PMA.

We observe the time-dependent structural reorganization of monolayers of gold nanocrystals deposited from liquid carbon dioxide. Compressed carbon dioxide does not exhibit dewetting instabilities, allowing deposition of spatially continuous monolayers at fast evaporation rates. Comparison of the translational and orientational correlation functions and a translational order parameter with a computer simulated equilibrium state indicates that the nanocrystal organization kinetics are slower than single particle diffusion limited assembly and are likely dominated by ensemble reorganization.

It has recently been shown that thin polymer films in the nanometer thickness range exhibit anomalous swelling maxima in supercritical CO 2 (Sc-Co 2 ) in the vicinity of the critical point of CO 2 . The adsorption isotherm of CO 2 on carbon black, silica surfaces, porous zeolites, and other surfaces, is known to exhibit anomalous maxima under similar CO 2 conditions. It is believed that because CO 2 possesses a low cohesive energy density, there would be an excess amount of CO 2 at the surfaces of these materials and hence the CO 2 /polymer interface. This might cause excess CO 2 in the polymer films near the free surface, and hence the swelling anomaly. In addition, an excess of CO 2 would reside at the polymer/substrate and polymer/CO 2 interfaces for entropic reasons. These interfacial effects, as have been suggested, should account for an overall excess of CO 2 in a thin polymer film compared to the bulk, and would be responsible for the anomalous swelling. In this study, we use in situ spectroscopic ellipsometry to investigate the role of interfaces on the anomalous swelling of polymer thin films of varying initial thicknesses, h 0 , exposed to Sc-CO 2 . We examined three homopolymers, poly(1,1 0 -dihydroperflurooctyl methacrylate) (PFOMA), polystyrene (PS), poly(ethylene oxide) (PEO), that exhibit very different interactions with Sc-CO 2 , and the diblock copolymer of PS-b-PFOMA. We show that the anomalous swelling cannot be solely explained by the excess adsorption of CO 2 at interfaces. V V C 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45:

Porous polyethylene oxide-b-polyfluorooctylmethacrylate (PEO-b-PFOMA) diblock copolymer films were drop cast onto substrates from Freon (1,1,2-trichlorotrifluoroethane) in a humid atmosphere. The pores in the films exhibit long range hexagonal order in some cases, depending on the PFOMA-to-PEO molecular weight ratio. Films with the best ordered pores were formed with PFOMA-to-PEO ratios of 70 kDa:2 kDa. The pores in the polymer films derive from water droplets that condense as Freon evaporates. The polymer stabilizes the water droplets, or "breath figures," which act as an immiscible template that molds the porous film. Increased polymer hydrophobicity reduces the water wettability of the air/Freon interface, which in turn decreases water droplet nucleation, thus influencing the final pore size and spatial order in the polymer films. We describe how water droplet nucleation influences the final pore size and packing order in the polymer films.