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Grazing incidence small angle X-ray scattering (GISAXS) was used to study the structure and interparticle spacing of monolayers of organic ligand-stabilized iron oxide nanocrystals floating at the air-water interface on a Langmuir trough, and after transfer to a solid support by the Langmuir-Blodgett technique. GISAXS measurements of the nanocrystal arrangement at the air-water interface showed that lateral compression decreased the interparticle spacing of continuous films. GISAXS also revealed that Langmuir-Blodgett transfer of the nanocrystal layers to a silicon substrate leads to a stretching of the film, with a significant increase in interparticle spacing.

Small-angle X-ray scattering (SAXS) data reveal that superlattices of organic ligand-stabilized gold (Au) nanocrystals can undergo a series of ordered structure transitions at elevated temperature. An example is presented of a body-centered cubic superlattice that evolves into a hexagonal close-packed structure, followed by the formation of binary simple cubic AB13 and hexagonal AB5 superlattices. Ultimately the superlattice decomposes at high temperature to bicontinuous domains of coalesced Au and intervening hydrocarbon. Transmission electron microscopy revealed that the ordered structure transformations result from partial ligand desorption and controlled Au nanocrystal growth during heating, which forces changes in superlattice symmetry. These observations suggest some similarity between organic ligand-coated nanocrystals and microphase-segregated diblock copolymers, where thermally induced nanophase-segregation of Au and organic ligand influences the ordered arrangements in the superlattice.

It was observed that silicon and germanium nanowires can exhibit significant electrostatic charging and respond strongly to externally applied electric fields. This includes nanowires in air and dispersed in low-conductivity, low-dielectric-constant solvents such as hexane, toluene, and benzene. The electrostatic charging of semiconductor nanowires was investigated as a tool for nanowire manipulation. By charging a substrate, nanowires could be deposited on surfaces with very high coverage and onto selected locations of the surface. The density of deposited nanowires could be adjusted systematically by varying the strength of the electric field. Alternating electric fields, applied between two electrodes, resulted in nanowires oriented with respect to the field orientation.

The structural evolution of a body-centered cubic (bcc) superlattice of 6.6 nm diameter organic ligand-coated PbSe nanocrystals was studied in situ by small-angle X-ray scattering (SAXS) as it was heated in air from room temperature to 350 °C. As it was heated above room temperature, the superlattice contracted slightly but maintained bcc structure up to 110 °C. Once the temperature rose above 110 °C, the superlattice began to disorder, by first losing long-range translational order and then local positional order. At temperatures exceeding 168 °C, the nanocrystals sintered and oxidized, transforming into PbSeO3 nanorods.

Hexagonally close-packed monolayers of FePt nanocrystals were deposited on silicon substrates by Langmuir−Blodgett, Langmuir−Schafer, or spin-coating techniques and characterized by scanning electron microscopy (SEM) and grazing incidence small-angle X-ray scattering (GISAXS). The extent of order in each nanocrystal film is quantified based on the GISAXS data and compared. The LB technique provided the most ordered superlattice films.

Patterned monolayers and multilayers of FePt nanocrystals were printed onto substrates by first assembling nanocrystals on a Langmuir-Blodgett (LB) trough and then lifting them onto prepatterned polydimethylsiloxane (PDMS) stamps, followed by transfer printing onto the substrate. Patterned features, including micrometer-size circles, lines, and squares, could be printed using this approach. The magnetic properties of the printed nanocrystal films were also measured using magnetic force microscopy (MFM). Room-temperature MFM could detect a remanent (permanent) magnetization from multilayer (3 nanocrystals thick) films of chemically ordered L1(0) FePt nanocrystals.

Monodisperse CuInS2 nanodisks were synthesized by heating metal chlorides and thiourea in oleylamine. X-ray diffraction showed the predominant phase to be wurtzite (hexagonal) CuInS2 instead of chalcopyrite (tetragonal) or compositionally disordered sphalerite (cubic). High-resolution transmission electron microscopy, however, revealed polytypism in the nanodisks, with the wurtzite phase interfaced with significant chalcopyrite domains.

Composite electrodes composed of silicon nanowires synthesized using the supercritical fluid-liquid-solid (SFLS) method mixed with amorphous carbon or carbon nanotubes were evaluated as Li-ion battery anodes. Carbon coating of the silicon nanowires using the pyrolysis of sugar was found to be crucial for making good electronic contact to the material. Using multiwalled carbon nanotubes as the conducting additive was found to be more effective for obtaining good cycling behavior than using amorphous carbon. Reversible capacities of 1500 mAh/g were observed for 30 cycles.

Multifunctional colloidal core-shell nanoparticles of magnetic nanocrystals (of iron oxide or FePt) or gold nanorods encapsulated in silica shells doped with the fluorescent dye, Tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate (Rubpy) were synthesized. The as-prepared magnetic nanocrystals are initially hydrophobic and were coated with silica using a microemulsion approach, while the as-prepared gold nanorods are hydrophilic and were coated with silica using a Stober-type of process. Each approach yielded monodisperse nanoparticles with uniform fluorescent dye-doped silica shells. These colloidal heterostructures have the potential to be used as dual-purpose tags-exhibiting a fluorescent signal that could be combined with either dark-field optical contrast (in the case of the gold nanorods), or enhanced contrast in magnetic resonance images (in the case of magnetic nanocrystal cores). The optical and magnetic properties of the fluorescent silica-coated gold nanorods and magnetic nanocrystals are reported.

The synthesis of monodisperse chalcopyrite (tetragonal) CuInSe(2) nanocrystals is reported. The nanocrystals have trigonal pyramidal shape, and they exhibited a common crystallographic orientation when drop-cast onto carbon substrates. A crystallographic model for the nanocrystals was developed. The nanocrystals are bounded by one polar {112} surface facet and three nonpolar {114} surface facets.