Yonhua Tzeng

Similarities in the 'cold' electron emission characteristics of diamond coated molybdenum electrodes and polished bulk graphite surfaces

Details are given of an experimental study of field emission characteristics of diamond-coated Mo electrodes: in particular, a transparent anode imaging technique was used to monitor the spatial distribution of the individual emission centres. This study has revealed the important fact that substantial emission can be obtained at fields as low as 5 MV m-1. In order to investigate the physical nature of the emission process, a comparative study has been made of emission obtained from a diamond-coated electrode and a bulk carbon graphite electrode. Significantly, it was found that both the graphite-rich diamond film and the diamond-rich graphite electrode shared a similar high-emissivity characteristic, with a high surface density of emission sites. It has also been noted that CVD diamond films have two important properties that are favourable to low-field cold electron emission, namely their negative electron affinity and the presence of graphite inclusions.

Lasing in whispering gallery mode in ZnO nanonails

We report on the observation of ultraviolet lasing in optically pumped ZnO nanonails synthesized by thermal chemical vapor deposition method. The lasing threshold was found to be 17MWcm−2. Very sharp emission peaks (full width at half maximum of 0.08nm) were observed in the emission spectrum, indicating a high Q factor of the cavity formed by the hexagonally shaped nanonail head. The analysis of the lasing spectra strongly suggests the whispering gallery mode lasing from a hexagonally shaped head of the single ZnO nanonail.

Thermal chemical vapor deposition growth of zinc oxide nanostructures for dye-sensitized solar cell fabrication

Dye-sensitized solar cells (DSSCs) were fabricated using a quasialigned one-dimensional zinc oxide (ZnO) nanostructure. The ZnO nanostructures were grown on indium tin oxide (ITO) coated glass substrate via a thermal chemical vapor deposition (CVD). It has been considered that thermal CVD is not suitable for the growth of ZnO nanostructure on ITO/glass due to the high processing temperature. However, we have demonstrated that a densely populated ZnO nanostructure can be prepared on ITO/glass substrate by a double-source double-tube CVD process. The power conversion efficiency of our device is 0.6%.

Effects of postgrowth annealing treatment on the photoluminescence of zinc oxide nanorods

Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence.

The origins of ferromagnetism in Co-doped ZnO single crystalline films: From bound magnetic polaron to free carrier-mediated exchange interaction

High-quality Co-doped ZnO single crystalline films with a wide range of carrier concentration and good reproducibility have been grown by molecular beam epitaxy. After the systematic studies of the magnetic and transport properties of the films, we suggest that there are two distinct ferromagnetic mechanisms in different conductivity regimes. In the insulating regime, carriers tend to be localized, favoring the formation of bound magnetic polarons, which leads to ferromagnetism. In the metallic regime, however, most carriers are weakly localized and the free carrier-mediated exchange is dominant. Our experimental observations are well consistent with the recent theoretical description of magnetism in Co-doped ZnO and helpful for understanding the ferromagnetic mechanism in oxide-based diluted magnetic semiconductors.

High performance silver ohmic contacts to YBa2Cu3O6+x superconductors

Silver‐YBa2Cu3O6+x superconductor ohmic contacts with specific contact resistance on the order of 10−8 Ω cm2 have been demonstrated. Alloying the contacts in O2 at 500 °C enhances the contact conductivity by two orders of magnitude.

Solid particle impact of CVD diamond films

Abstract Studies of the dynamic impact and static indentation of chemically vapour deposited (CVD) diamond films are described. Polished and unpolished CVD diamond films of thickness ∼ 6 to 15 μm, deposited on both silicon nitride and silicon substrates, were investigated. In the dynamic impact experiments, sand particles with impactvelocities of 34 m s−1 and 59 m s−1 were used. Three stages of impact damage were identified: (i) formation of ring cracks; (ii) partial debonding and penetration of the film; and (iii) full debonding and film removal. The first stage is similar to that occuring with natural diamond. However, the last stage, i.e. the material loss mechanism for CVD films, is mainly caused by delamination of the film. Examination of fracture surfaces using scanning electron microscopy showed that transgranular fracture of the diamond films was quite common despite the polycrystalline nature of the films This suggests that bonding strength between diamond grains strong as that within the grains. Finally, static indentation with a tungsten carbide ball of similar size to the sand particles was performed to determine the fracture strength of polished CVD diamond film and to make a comparison with the impact experiments. The fracture tensile strength for CVD diamond films was found to be ∼2.8 GPa, which is much lower than the reported value of 8.6–12 GPa for natural diamonds.

Raman analysis of longitudinal optical phonon-plasmon coupled modes of aligned ZnO nanorods

The electronic properties of vertically aligned ZnO nanorods have been investigated using micro-Raman spectroscopy. The concentration and mobility of the charge carriers were determined via Raman line shape analysis using longitudinal-optical-phonon-plasmon coupled mode. The local laser heating and the stress effects have been considered when analyzing the Raman spectra. The mobility and carrier concentration of the aligned ZnO nanorods are 84.8cm2∕Vs and 3.8×1017cm−3, respectively. As a comparison, the mobility and carrier concentration of the undoped bulk ZnO were also obtained from the Raman line shape analysis. The mobility of the aligned ZnO nanorods is about 20% lower than that of the undoped bulk ZnO, which can be attributed to enhanced surface scattering due to the reduction in dimension.

Growth of diamond films on silicon from an oxygen‐acetylene flame

Clear diamond films have been deposited on silicon substrates by a tilted oxygen‐acetylene flame operating at one atmosphere pressure in air. High quality diamond films are formed by well‐defined diamond crystallites in the octahedral and cubooctahedral shapes as well as of intermediate forms. Raman spectra of the flame deposited diamond display a peak very close to that for natural diamond with little or no broad band corresponding to graphitic bonding as well as a peak corresponding to the underlying silicon substrate. This shows that the diamond is transparent to visible light and that there are little or no interfacial materials other than silicon and diamond. Using an O2 :C2 H2 volume ratio around 0.98 high quality diamond films thicker than 10 μm have been grown on silicon in less than 30 min by an oxygen‐acetylene flame that is aimed at the silicon substrate at about 70° with respect to the direction normal to the silicon surface.

Monte Carlo technique for simulating the evolution of an assembly of particles increasing in number

Abstract We describe an algorithm for simulating the evolution of an assembly of particles which is increasing in number. It is based on the use of the Monte Carlo method and a renormalization procedure which allows the mapping of the growing assembly into another consisting of fewer particles. The results of test calculations are presented and error analysis of the results are included.