Y. G. Wang

High-efficiency visible photoluminescence from amorphous silicon nanoparticles embedded in silicon nitride

Confinement of silicon nanoparticles in silicon nitride instead of an oxide matrix might materially facilitate its potential applications as a light-emitting component in optoelectronics. We report in this letter the production of high-density (up to 4.0×1012/cm2 from micrographs) silicon nanoparticles in SiNx thin films by chemical vapor deposition on cold substrates. Strong room-temperature photoluminescence was observed in the whole visible light range from the deposits that were postannealed at 500 °C for 2 min. The Si-in-SiNx films provide a significantly more effective photoluminescence than Si-in-SiOx fabricated with similar processing parameters: for blue light, the external quantum efficiency is over three times as large. The present results demonstrate that the nanostructured Si-in-SiNx system can be a very competitive candidate for the development of tunable high-efficiency light-emitting devices.

Self-assembly and magnetic properties of cobalt nanoparticles

Two- and three-dimensional superlattices of passivated cobalt nanoparticles were formed by a self-assembly technique. The size and stabilization of the cobalt nanoparticles are controlled by using the combination of oleic acid and triphenylphosphine. The cobalt nanoparticles are stable for at least 90 days without oxidation at room temperature under ambient conditions. The magnetic properties of the cobalt nanoparticles in different forms are compared, which provides helpful information on the magnetostatic interaction of the nanoparticles.

Highly efficient visible and infrared β‐BaB2O4 optical parametric oscillator with pump reflection

We report the successful operation of a highly efficient visible and near‐infrared optical parametric oscillator (OPO) that uses β‐BaB2O4 as the nonlinear medium and is pumped at 354.7 nm. An energy conversion efficiency of 41% corresponding to a quantum conversion efficiency as high as 57% has been achieved in a 10‐mm‐long crystal. Pump depletion is estimated to be 70% or so. This OPO can generate an average output power to 507 mW at 490 nm and can be continuously tuned from 415 to 2411 nm. The reverse frequency conversion process has also been observed in the OPO. The reason for its high efficiency is discussed.

High-density aligned carbon nanotubes with uniform diameters

A new method was found to synthesize large-area (7 x 15 mm(2)), high-density (higher than 10(9) cm(-2)), aligned carbon nanotubes (CNTs) with uniform diameters on a silica wafer. Ferrocene/melamine mixtures were pyrolyzed through a three-step process in an Ar atmosphere in a single-stage furnace. The structure and composition of the CNTs were investigated by scanning electron microscopy, transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and electron energy-loss spectroscopy (EELS). It was found that these nanotubes have uniform outer diameters of about 22 nm and varying lengths from 10 to 40 mum. High-resolution TEM images showed that CNT is composed of graphite-like layers arranged in a stacked-cup-like structure. XPS results showed that the layer covering the tops of the aligned CNTs consists of carbon and iron. The EELS spectrum showed that these tubes are pure carbon.

Ohmic contact junction of carbon nanotubes fabricated by in situ electron beam deposition

We present experimental evidence of in situ fabrication of multi-walled carbon nanotube junctions via electron beam induced deposition. The tip-to-tip interconnection of the nanotubes involves the alignment of two nanotubes via a piezodriven nanomanipulator and nano-welding by electron beam deposition. Hydrocarbon contamination from the pump oil vapour of the vacuum system of the TEM chamber was used as the solder; this is superior to the already available metallic solders because its composition is identical to the carbon nanotube. The hydrocarbon deposition, with perfect wettability, on the nanotubes establishes strong mechanical binding between the two nanotubes to form an integrated structure. Consequently, the nanotubes cross-linked by the hydrocarbon solder produce good electrical and mechanical connections. The joint dimension was determined by the size of the electron beam, which results in a sound junction with well-defined geometry and the smallest junction size obtained so far. In situ electric measurement showed a linear current-voltage property for the multi-walled nanotube junction.

Visible optical parametric oscillation in LiB3O5

The successful operation of a visible optical parametric oscillator (OPO) using a LiB3O5 crystal as the nonlinear element is reported. An energy conversion efficiency as high as 22% for a signal wave has been obtained in a 15.9‐mm‐long crystal. This OPO can generate visible output energy of 2.7 mJ and be continuously tuned from 435 to 1922 nm.

Abnormal temperature dependence of conductance of single Cd-doped ZnO nanowires

Positive temperature coefficient of resistance is observed on single Cd-doped ZnO nanowires. The current along the nanowire increases linearly with the bias and saturates at large biases. The conductance is greatly enhanced either by ultraviolet illumination or infrared illumination. However, the conductance decreases with increasing temperature, in contrast to the reported temperature behavior either for ZnO nanostructures or for CdO nanoneedles. The increase of the conductance under illumination is related to surface effect and the decrease with increasing temperature to bulk effect. These results show that Cd doping does not change surface effect but affects bulk effect. Such a bulk effect could be used to realize on-chip temperature-independent varistors.

Determination of near coincident site lattice orientations in MgO/Cu composite

Orientation relations between MgO precipitates and Cu matrix have been characterized by electron diffraction. Four orientation relations were newly found to be coincident with Σ41 [110], Σ13 [111], Σ29 [100] and Σ35 [112] near coincidence-site orientations. The possible dislocation network for these orientation relations was analyzed using O-lattice theory. The size of MgO precipitates with these orientations are in a range of 0.5–1 micron. Frequently appearance of these special orientation relations implies that they may be the favorable orientations for precipitation and coarsening of MgO particles to some extent.

Fully oriented decagonal quasicrystalline thin films on crystalline substrates

Fully oriented AlCuCo decagonal quasicrystalline films on crystalline substrates were successfully prepared by multitarget magnetron sputtering followed by postannealing. The width of the rocking curve competes with that of a single crystal. To explain the result, we suggest that the full orientation of the films originates from the fact that only one atomic flat plane of the structure is available to match the flat surface of the substrate, which gives a minimum interface free energy.

Cu/SiO2-x nanowires with compositional modulation structure grown via thermal evaporation

One-dimensional compositional modulation has been achieved in Cu/SiO2−x nanowires prepared at the substrate temperature of 1000 °C by thermal evaporation of a cuprous oxide and silicon mono-oxide mixture. The synthesized nanowires consist of the Cu spheres uniformly piled up along the longitudinal direction of the nanowires to form a modulation structure with an average period of about 140 nm. This periodicity could be adjusted by changing the CuO concentration in the source materials mixture.