H. Y. Yang

Origin of room temperature ferromagnetism in ZnO:Cu films

Copper-doped ZnO (ZnO:Cu) films were prepared on silicon substrates by filtered cathodic vacuum arc technique at room temperature using a Zn target containing 5at.% of Cu. Room temperature ferromagnetism was observed in the ZnO:Cu films with saturation magnetization of 0.037μB∕Cu atom. The origin of the ferromagnetism in ZnO:Cu was mainly due to Cu ions substituted into the ZnO lattice. X-ray diffraction, x-ray photoelectron spectroscopy, and transmission electron microscopy revealed that no ferromagnetic-related secondary phase could be detected in ZnO:Cu.

Magnetic anisotropy in the ferromagnetic Cu-doped ZnO nanoneedles

Copper-doped ZnO (ZnO:Cu) nanoneedles exhibiting room-temperature ferromagnetism were fabricated by an ion beam technique using Cu plate and ZnO film. A saturated magnetization moment of 0.698emu∕cm3 was found in the nanoneedles when a field of 10kOe was applied perpendicular to the substrate, which was 15% larger than the field applied parallel to the substrate. The magnetic ordering of the nanoneedles was enhanced significantly to 0.968emu∕cm3 after annealing of 400°C for 20min. However, the magnetic anisotropy at high field is vanished but an “easy plane” ferromagnetism becomes apparent at low field region. The possible mechanisms of the magnetic ordering and anisotropy in the ZnO:Cu nanoneedles are discussed.

Exciton radiative lifetime in ZnO nanorods fabricated by vapor phase transport method

The exciton radiative lifetime in ZnO nanorods is studied. It is found that the exciton radiative lifetime increases with temperature as T2. Furthermore, the spectral linewidth of the photoluminescence of the ZnO nanorods also increases with temperature as T2, suggesting a linear dependence of exciton radiative lifetime on the spectral linewidth. The physics behind is that the oscillator strength of excitons at k=0 is shared equally among all the states within the spectral linewidth and the coherence extension of an exciton decreases with temperature due to the scattering by phonons, defects, or impurities.

Enhancement of ultraviolet lasing from Ag-coated highly disordered ZnO films by surface-plasmon resonance

Large improvement in random lasing action at ultraviolet wavelength has been achieved from highly disordered ZnO films with Ag coating. The lasing threshold can be reduced by two times and slope efficiency can be increased by 5.5 times. The improvement is due to the presence of Ag coating, which enhances the surface coupling of lasing emission from the ZnO films by surface-plasmon resonance and reduces the scattering loss experienced by the random cavity modes. Furthermore, the enhancement of lasing efficiency is dependent on the Ag coating’s surface roughness, which can be controlled through the surface morphology of ZnO films.Large improvement in random lasing action at ultraviolet wavelength has been achieved from highly disordered ZnO films with Ag coating. The lasing threshold can be reduced by two times and slope efficiency can be increased by 5.5 times. The improvement is due to the presence of Ag coating, which enhances the surface coupling of lasing emission from the ZnO films by surface-plasmon resonance and reduces the scattering loss experienced by the random cavity modes. Furthermore, the enhancement of lasing efficiency is dependent on the Ag coating’s surface roughness, which can be controlled through the surface morphology of ZnO films.

Directional and controllable edge-emitting ZnO ultraviolet random laser diodes

Room-temperature ultraviolet random lasing action is demonstrated from a p-GaN/annealed i-ZnO:Al(3%)/n-ZnO:Al(5%) buried heterojunction diode with a 2u2002μm rib waveguide. Excellent electrical-to-optical conversion efficiency is achieved by strong electrical and optical confinement of a buried heterojunction rib waveguide structure. Hence, emission intensity (threshold current) can be enhanced (reduced) by ∼9 times (∼40%). Directional emission as well as controllability on the number of the random lasing modes can also be achieved.

Laser action in ZnO nanoneedles selectively grown on silicon and plastic substrates

An ion-beam technique has been employed to fabricate nanoscale needlelike structures in ZnO thin films on silicon and plastic substrates at room temperature. The ZnO nanoneedles showed a single-crystalline wurtzite structure, the stem of which was around 100nm in diameter. The sharp tips of the nanoneedles exhibited an apex angle of 20° as measured by transmission electron microscopy. Room-temperature ultraviolet random lasing action was observed in the ZnO nanoneedle arrays under 355nm optical excitation.

X-ray generation using carbon-nanofiber-based flexible field emitters

Carbon nanofibers were grown on flexible polyimide substrates using an ion-beam sputtering technique. Field emission measurement showed a fairly low threshold voltage of 1.5V∕μm with a current density of 1μA∕cm2. The field enhancement factor was determined to be 4400. The emitter showed resilience when exploited as a high voltage electron source for x-ray generation. The x-ray generated by the flexible emitter is capable of delivering fine images of biological samples with superior sharpness, resolution, and contrast.

Low-loss and directional output ZnO thin-film ridge waveguide random lasers with MgO capped layer

Room-temperature ultraviolet lasing characteristics of ZnO thin-film ridge-waveguide random lasers with MgO capped layer fabricated on n-type (100) Si substrate are reported. It is demonstrated that highly directional emission from the facets of the random lasers can be achieved. Reduction of scattering loss inside the random cavities can also be obtained. In addition, the improvement in the efficiency of the lasing characteristics of the random lasers by optical feedback is studied.

Field emission from zinc oxide nanoneedles on plastic substrates

The electron field emission from zinc oxide (ZnO) nanoneedles on flexible plastic substrates is reported. ZnO thin films were first deposited on plastic substrates at 200 °C using a filtered cathodic vacuum arc technique; the films were then bombarded by Ar+ ion. After ion beam irradiation, high-density ZnO nanoneedle arrays were selectively formed on the thin films. The average diameter and length of the ZnO nanoneedles is around 100 and 700 nm respectively. Field emission measurement showed a fairly low threshold voltage of 4.1 V µm−1 with a current density of 1 µA cm−2. The emission current density can be as high as 1 mA cm−2 at 9.6 V µm−1. The result establishes a method of fabricating a flexible field emitter, which should find practical applications in vacuum electronic devices.

Deposition of hydrogenated diamond-like carbon films under the impact of energetic hydrocarbon ions

In this article we studied the influence of bombardment energy of hydrocarbon ions on the properties of hydrogenated diamond-like carbon (DLC) films using x-ray reflectivity, Raman spectroscopy, and Fourier-transform infrared. The DLC films were prepared with an electron cyclotron resonance system using H2 and CH4 gases and the ion energy was tunable through a rf-induced dc bias voltage. It was observed that the surface roughness is increased and C–H bonded hydrogen concentration is decreased with increased ion energy, whereas the mass density, hardness, and sp3/sp2 ratio exhibited optimum values. A thin SiC layer was found to form between the DLC films and silicon substrates. Two proposed carbon deposition mechanisms, i.e., the shallow implantation (subplantation) model and the adsorbed layer model, are examined based on the results obtained in this study. Our results indicate that ion bombardment energy is a critical factor in determining the film properties and the ion subplantation could be an importa...