Three-dimensional structure of helical and zigzagged nanowires using electron tomography.

Electron tomography and high-resolution transmission electron microscopy were used to characterize the unique three-dimensional structures of helical or zigzagged GaN, ZnGa2O4, and Zn2SnO4 nanowires. The GaN nanowires adopt a helical structure that consists of six equivalent <011> growth directions with the axial [0001] direction. We also confirmed that the ZnGa2O4 nanosprings have four equivalent <011> growth directions with the [001] axial direction. The zigzagged Zn2SnO4 nanowires consisted of linked rhombohedrons having the side edges matched to the <110> direction and the [111] axial direction.

Helical structure of single-crystalline ZnGa2O4 nanowires.

Two unique helical zinc gallate (ZnGa2O4) nanostructures were synthesized by thermal evaporation using the zinc selenide (ZnSe) nanowires; helical ZnGa2O4 nanowire rolls either on a straight ZnSe nanowire support or without any support. They all consist of single-crystalline cubic ZnGa2O4 crystals without any dislocation over the entire helical structure and have four equivalent growth directions of 011 with the axial direction of [001]. We suggest that the lattice matching with the ZnSe nanowires would be an important factor in determining the growth direction of the helical ZnGa2O4 nanowires.

Release of N(2) from the carbon nanotubes via high-temperature annealing.

Nitrogen (N)-doped carbon nanotubes (CNTs) were heated to 1000 degrees C under an ultrahigh vacuum. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) reveal three different N structures; graphitelike, pyridine-like, and molecular N(2). The vibrationally resolved XANES peaks of N(2) were first observed, suggesting the existence of molecular N(2) as intercalated and trapped forms. The annealing process can decrease the average N content from 6.3 at. % to 3.3 at. %, mainly by releasing molecular N(2). Electron energy-loss spectroscopy (EELS) confirms that the annealing releases molecular N(2) from the CNTs.

Comparative structure and optical properties of Ga-, In-, and Sn-doped ZnO nanowires synthesized via thermal evaporation.

ZnO nanowires doped with a high concentration Ga, In, and Sn were synthesized via thermal evaporation. The doping content defined as X/(Zn + X) atomic ratio, where X is the doped element, is about 15% for all nanowires. The nanowires consist of single-crystalline wurtzite ZnO crystal, and the average diameter is 80 nm. The growth direction of vertically aligned Ga-doped nanowires is [001], while that of randomly tilted In- and Sn-doped nanowires is [010]. A correlation between the growth direction and the vertical alignment has been suggested. The broaden X-ray diffraction peaks indicate the lattice distortion caused by the doping, and the broadening is most significant in the case of Sn doping. The absorption and photoluminescence of Sn-doped ZnO nanowires shift to the lower energy region than those of In- and Ga-doped nanowires, probably due to the larger charge density of Sn.

X-ray absorption near edge structure study of BN nanotubes and nanothorns.

Two boron nitride (BN) nanostructures, the bamboo-like nanotubes and nanothorns where the nanosize h-BN layers are randomly stacked looking like thorns, were synthesized selectively via thermal chemical vapor deposition of B/B(2)O(3) under the NH(3) flow at 1200 degrees C. Electron energy-loss spectroscopy reveals the N-rich h-BN layers with a ratio of B/N = 0.75-0.85. Angle-resolved X-ray absorption near edge structure of these two N-rich nanostructures has been compared with that of h-BN microcrystals. The pi transition in the N K-edge shifts to the lower energy by 0.8-1.0 eV from that of h-BN microcrystals, and the second-order signals of N 1s electrons become significant. We suggest that the N enrichment would decrease the band gap of nanostructures from that of h-BN microcrystals. The Raman spectrum shows the peak broadening due to the defects of N-rich h-BN layers.

Single- and double-shelled coaxial nanocables of GaP with silicon oxide and carbon.

Coaxial nanocables of gallium phosphide (GaP) core with three different-typed single and double shells (i.e., silicon oxide (SiO(x)), carbon (C), and SiO(x)/C) were exclusively synthesized by the chemical vapor deposition method. The GaP/SiO(x)) nanocables were directly grown on gold-deposited silicon substrates. Deposition of C on the GaP nanowires and GaP/SiO(x) nanocables produces the GaP/C and GaP/SiO(x)/C nanocables, respectively. The outer diameter of the nanocables is <50 nm. The thickness and crystallinity of the C outer layers were controllable by the growth conditions. X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, and photoluminescence reveal that the outer layer formation reduces the surface defects of GaP nanowires. A great enhancement of the conductivity due to the C outer layers has been measured by the four-probe method. The growth process of these nanocables has been discussed on the basis of the vapor-liquid-solid mechanism.

Synthesis and Magnetic Properties of Manganese-Doped GaP Nanowires.

We characterized the structure and magnetic properties of Mn-incorporated GaP nanowires synthesized by thermal evaporation of GaP/Mn powders. The nanowires consist of twin-crystalline zinc blende GaP grown with the [111] direction and doped with about 1 at. % Mn. They are often sheathed with the bumpy amorphous outerlayers containing high concentrations of Mn and O. The ferromagnetic hysteresis curves at 5 and 300 K and temperature-dependent magnetization provide evidence for the ferromagnetism with the Curie temperature higher than room temperature. Magnetic properties of individual nanowires have been measured, showing a large negative magnetoresistance equal to about -5% at 5 K. We suggest that the Mn doping of GaP nanowires would form a dilute magnetic semiconductor.

Short-period superlattice structure of Sn-doped In(2)O(3)(ZnO)(4) and In(2)O(3)(ZnO)(5) nanowires.

Two longitudinal superlattice structures of In(2)O(3)(ZnO)(4) and In(2)O(3)(ZnO)(5) nanowires were exclusively produced by a thermal evaporation method. The diameter is periodically modulated in the range of 50-90 nm. The nanowires consist of one In-O layer and five (or six) layered Zn-O slabs stacked alternately perpendicular to the long axis, with a modulation period of 1.65 (or 1.9) nm. These superlattice nanowires were doped with 6-8% Sn. The X-ray diffraction pattern reveals the structural defects of wurtzite ZnO crystals due to the In/Sn incorporation. The high-resolution X-ray photoelectron spectrum suggests that In and Sn withdraw the electrons from Zn and enhance the number of dangling-bond O 2p states, resulting in the reduction of the band gap. Photoluminescence and cathodoluminescence exhibit the peak shift of near band edge emission to the lower energy and the enhancement of green emission as the In/Sn content increases.

Synthesis of blue-light-emitting ZnGa2O4 nanowires using chemical vapor deposition.

A high-density array of vertically aligned ZnGa(2)O(4) nanowires has been synthesized on Si substrates via CVD of ZnO-Ga at 1000 degrees C consisting of a single-crystalline cubic spinel structure grown in a [111] direction and exhibiting strong photoluminescence and cathodoluminescence in the blue wavelength region.

Fabrication of SiC-C coaxial nanocables: thickness control of C outer layers.

Aligned SiC-C coaxial nanocables were synthesized via the direct growth of SiC nanowires from silicon substrates and subsequent carbon deposition using pyrolysis of methane; the average diameter of the SiC nanowire cores is 20 nm; the thickness of the C outer layers is controlled in the range 3-50 nm; the degree of crystalline perfection of the graphitic sheets increases with the thickness.