Epitaxial Si nanowires grown from Au seeds using the vapor-liquid-solid method begin growing normal to the Si(111) substrate atop a tapered base. After a kinetically determined length, the NWs may kink away from [111] to another crystallographic direction. The smallest NWs prefer growth along 110 while larger Si NWs choose either 111 or 112 based on whether growth conditions favor Au-free sidewalls. "Vertical" growth normal to the Si(111) substrate is obtained only for slowly growing NWs with Au-decorated sidewalls. At the fastest growth rates, single-crystal Si NWs smoothly, continuously, and randomly vary their growth directions, producing a morphology that is qualitatively different than highly kinked growth.

Vapor-liquid-solid growth of high-quality Si nanowires relies on the stability of the liquid metal seed. In situ transmission electron microscopy shows that liquid AuSi seed spreads along the sidewalls of Si nanowires for some growth conditions. This liquid thin film phase separates to form solid Au clusters as the nanowire is quenched below the solidus temperature. The length that the liquid film spreads from the seed and its thickness can be explained by considering the spreading thermodynamics of droplets on cylinders.

The presence and configuration of Au on the sidewalls of vapor-liquid-solid grown Si nanowires (NWs) was investigated using analytical (scanning) transmission and scanning electron microscopy. The relationship between growth conditions and Au/Si interface thermodynamics is shown to have a profound effect on NW growth. For some growth conditions, liquid AuSi can spread from the seed at the NW tip along the sidewalls during growth. This liquid film will phase separate and solidify, forming small Au clusters as the NW is cooled from the growth temperature. Growth conditions are correlated with the variety of Au cluster configurations found. The observed behavior can be explained by considering the thermodynamics of droplet spreading on cylinders.

Hole accumulation in Ge/Si core/shell nanowires (NWs) has been observed and quantified using off-axis electron holography and other electron microscopy techniques. The epitaxial [110]-oriented Ge/Si core/shell NWs were grown on Si (111) substrates by chemical vapor deposition through the vapor-liquid-solid growth mechanism. High-angle annular-dark-field scanning transmission electron microscopy images and off-axis electron holograms were obtained from specific NWs. The excess phase shifts measured by electron holography across the NWs indicated the presence of holes inside the Ge cores. Calculations based on a simplified coaxial cylindrical model gave hole densities of (0.4 ± 0.2) /nm(3) in the core regions.

The composition and growth direction of epitaxial SiGe alloy nanowires (NWs) grown via the Au-catalyzed vapor-liquid-solid technique can be controlled by varying growth conditions. These alloy NWs can adopt either Si-like or Ge-like characteristics. Si-like growth is characterized by Au-coated ⟨111⟩-oriented NWs for low pressure growth and Au-free ⟨112⟩-oriented NWs for higher pressure growth. Ge-like NWs always follow ⟨111⟩ and grow with Au-free sidewalls.

By spark-eroding Fe75Si15B10 in water/ethanol mixtures, spherical particles with nanostructured cores consisting of mixed amorphous and crystalline phases were produced. The relative volume fractions of the amorphous and crystalline phases were dependent on the water/ethanol ratio. In the same process, continuous oxide layers were formed on the particle surfaces. The basic mechanisms responsible for the formation of the surface oxide layers and the core nanostructures were modeled. At frequencies ranging from 1 to 100 MHz, the combination of the core nanostructures and the insulating oxide shells yielded exceptionally low-loss magnetic behavior.

Au islands grown on Si(111) substrates at substrate temperatures of 500 and 600 °C, both of which are greater than the bulk Au-Si eutectic temperature of 363 °C, are characterized using atomic force (AFM) and electron microscopy. Specific islands are imaged using AFM before and after Au dissolution using aqua regia to characterize the Au-Si interface formed as the islands solidify from the liquid phase while cooling from the growth temperature. Subsequent to Au dissolution, the islands present a craterlike morphology with a pit that may extend below the substrate surface depending on growth and annealing conditions. Craters formed beneath islands grown at a substrate temperature of 600 °C exhibit pits that penetrate below the substrate surface to a depth that is proportional to the area of the island footprint and possess a well-developed (111) facet at their base. Facets are also sometimes observed in the crater sidewalls and are more prevalent in samples slowly cooled through the solidus temperature than those that are radiatively quenched. Transmission electron micrographs of etched islands indicate the presence of segregated Au nanocrystals entrained in the crater lip.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.