The objective of the study was to develop the operational basis for rapid and controlled deposition of crystal coatings from particles of a wide size range. We deposited such structured coatings by dragging with constant velocity a small volume of liquid confined in a meniscus between two plates. Two types of structured coatings were characterized: latex colloidal crystals and thin layers from metallic nanoparticles. The crystal deposition was sped up by use of preconcentrated suspensions. Crystal coatings larger than a few square centimeters were deposited in minutes from aqueous suspension volumes of approximately 10 microL. The governing mechanism of crystal deposition is convective assembly at high volume fractions. The two major process parameters that allow control over the coating thickness and structure were the deposition speed and particle volume fraction. The evaporation rate was not found to affect the process to a large extent. A volumetric flux balance was used to relate the deposition parameters to coating structure and properties. Operational "phase" diagrams were constructed, relating the crystal layer thickness and packing symmetry to the process parameters. These diagrams could be instrumental in transforming the convective colloidal deposition into a robust scaleable technology.

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The miniaturization of chemical and biological processes in microfluidic devices and bioarrays is a major technological achievement. Microchips performing multiphase material synthesis operations could be a future step in this trend of miniaturizing technology. Here we show how electrically controlled chips can be used for the synthesis and manipulation of new types of particles with advanced structure. The method is based on a technique that allows freely suspended droplets and particles to be entrapped and transported using electric fields. The fields that hold and guide the droplets and particles are applied through arrays of electrodes submerged in the oil. Each of the microdroplets suspended on the surface of fluorinated liquid serves as a microscopic reactor, where the particles are formed by solidification of the carrier droplets. Controlled on-chip assembly, drying, encapsulation and polymerization were used to make anisotropic 'eyeball' and striped particles, polymer capsules and semiconducting microbeads.

We present a simplified synthesis of hollow gold nanoshells 20-50 nm in diameter via the wellestablished templated galvanic replacement reaction of silver for gold. The surface plasmon resonance absorbance of nanoshells made in this fashion can be tuned using basic colloid chemistry to control the size of the silver templates. The gold nanoshells can be varied in size and shell thickness depending on silver/gold reagent ratios and have an aqueous core. The template replacement chemistry is rapid, highly scalable, uses minimal amounts of toxic reagents, and in many cases is a true `one pot' synthesis. The smallest nanoshells (20 nm diameter, 7 nm wall thickness) reach the highest temperature on irradiation with femtosecond light pulses in the near infrared and anneal to form spherical nanoparticles fastest, even though their plasmon resonance does not overlap as well as the larger nanoshells (50 nm diameter, 7 nm wall thickness) with the 800 nm wavelength excitation. Optimizing a nanoshell structure to reach the highest nanoshell temperature is not the same as optimizing the structure for maximum energy absorbance.

Convective assembly at high volume fraction was used to deposit silica nanoparticle coatings onto glass and silicon substrates. By allowing control of the film structure and thickness, this technique provides a means for making large-scale coatings with antireflective properties. The reflectance was reduced by 50% for silicon (at 600 nm) and by 70% for single glass/air surface. Microstructural investigations using SEM, AFM, profilometry, and ellipsometry provided good correlation to the observed macroscopic optical properties. By virtue of the coatings' uniformity, the reflectance and transmission spectra from both substrates could be modeled well by classical reflection relations, using a volume-averaged refractive index. Data analysis showed that the relatively high packing fraction in nanocoatings made from monodisperse spheres is responsible for the limit on antireflective capabilities. To overcome this restriction, low-density silica coatings were made from binary colloidal mixtures of different diameter SiO2 particles. The packing fraction of these coatings was further optimized to yield 88% maximal reduction in the reflectance of glass surfaces. The technique is simple, inexpensive, and scalable.