Yuhua Yang

Correlation between resistance and field emission performance of individual ZnO one-dimensional nanostructures.

Both electrical and field emission measurements were carried out to study the correlation between resistance and field emission performance of individual one-dimensional (1D) ZnO nanostructures. Three types of 1D ZnO nanostructures were investigated (i.e., agave-like shape, pencil-like shape, and hierarchical structure) and were prepared by thermal chemical vapor transport and condensation without using any catalyst. The 1D ZnO nanostructures have obvious differences in resistance and thus conductivity from type to type. In addition, in the same type of 1D ZnO nanostructure, each individual emitter may also have variation in resistance and thus in conductivity. The field emission performance of the ZnO emitters was found to be strongly correlated with the resistance of each individual ZnO nanostructure: (i) a ZnO emitter with low resistance will have better emission; (ii) a high resistance region in a ZnO nanostructure is liable to the initiation of a vacuum breakdown event. The results indicate that, besides the uniformity in the geometrical structure, the uniformity in conductivity of the emitters in an array should be ensured, in order to meet the requirement of device application.

Great blue-shift of luminescence of ZnO nanoparticle array constructed from ZnO quantum dots

ZnO nanoparticle array has been fabricated on the Si substrate by a simple thermal chemical vapor transport and condensation without any metal catalysts. This ZnO nanoparticles array is constructed from ZnO quantum dots (QDs), and half-embedded in the amorphous silicon oxide layer on the surface of the Si substrate. The cathodoluminescence measurements showed that there is a pronounced blue-shift of luminescence comparable to those of the bulk counterpart, which is suggested to originate from ZnO QDs with small size where the quantum confinement effect can work well. The fabrication mechanism of the ZnO nanoparticle array constructed from ZnO QDs was proposed, in which the immiscible-like interaction between ZnO nuclei and Si surface play a key role in the ZnO QDs cluster formation. These investigations showed the fabricated nanostructure has potential applications in ultraviolet emitters.

Phase stability of diamond nanocrystals upon pulsed-laser-induced liquid-solid interfacial reaction: Experiments and ab initio calculations

To clarify different phase formations of diamond nanocrystals upon pulsed-laser-induced liquid-solid interfacial reaction (PLIIR), we preformed the first-principles investigation of the total energy of cubic and hexagonal diamond. Interestingly, by a comparison of the structure stabilities of cubic and hexagonal diamond, we found that both cubic and hexagonal structures could form, simultaneously or one by one, upon those processes like as far-from equilibrium growth processes, such as PLIIR, due to the relatively small different of total energies and similar crystalline structures. Eventually, these calculations were in good agreement with the experimental results.

One-dimensional nanostructures grown inside carbon nanotubes upon vapor deposition: A growth kinetic approach

Aiming at controlling the growth of one-dimensional nanostructures inside carbon nanotubes, a growth kinetic approach was performed, with respect to the effect of nanosize induced additional pressure on growing kinetics, to theoretically elucidate the growth of one-dimensional nanostructures inside carbon nanotubes upon vapor deposition. Our analysis showed that the growth rate of one-dimensional nanostructures would go much higher once nuclei formed inside carbon nanotubes, due to the effect of surface tension induced by the nanosize curvature of carbon nanotubes. The results based on the proposed model are in good agreement with experimental data for nanowires grown inside carbon nanotubes upon vapor deposition.

General Strategy for Nanoscopic Light Source Fabrication

IO N A general tendency in the development of nanotechnology is the miniaturization of electronic and photonic devices. Small-scale light emitters, such as nanolasers, are an important part of integrated nanophotonic devices. Thus, they have been given much research attention. [ 1 ] Currently, there are two independent techniques for the fabrication of nanolasers. One is based on lasing semiconductor nanowires where the nanowire acts as both the emitter and the cavity resonator. [ 2 ] Another is based on a lasing plasmon-assisted semiconductor nanowire, in which the light originates from the nanowire. In this confi guration, the optical modes are confi ned to the metal/semiconductor interface region, which acts as cavity resonator. [ 3 ] Based on the waveguide geometry at the nanometer scale, [ 4,5 ] the large discontinuity of the electric fi eld at the high-index-contrast interfaces can confi ne and enhance the intensity of the light fi eld in the low-index material when light is guided by total internal refl ection. Accordingly, we propose a facile and effective route for the fabrication of small-scale light emitters, which could serve as nanoscopic light sources. The luminescence emission from the nanocolumn is confi ned in the hollow region when a small hole is drilled in a luminescent semiconductor nanocolumn. Meanwhile, the size of the luminescent region of the nanocolumn is greatly reduced from the total nanocolumn to the small hole. Therefore, we can fabricate a smaller light source than would be possible using the nanocolumn alone via this unique strategy. In this paper, using the hollow ZnO nanocolumn as an example, we establish a theoretical model to address the basic physics involved in transport in a hollow luminescent semiconductor nanocolumn. Moreover, we demonstrate that by using such a nanostructure, the luminescence emission originating from the hollow ZnO nanocolumn can be confi ned to a low-index region (hollow region). This intensity is much higher than that which can be achieved at the edge of the ZnO nanocolumn. We then perform experiments to confi rm the validity of these theoretical results by measuring the cathodoluminescence (CL) spectra and images of an individual hollow hexagonal ZnO nanocolumn using scanning electron microscopy (SEM). Finally, computer simulations based on the fi nite element method corroborate our theoretical results and experimental fi ndings.

Nanothermodynamic analysis of the low-threshold-pressure-synthesized cubic boron nitride in supercritical-fluid systems

A thermodynamic model in nanoscale was developed to elucidate the nucleation of the cubic boron nitrides (c-BN) synthesized in the high-pressure and high-temperature (HPHT) supercritical-fluid systems under the conditions of the low-threshold-pressure (from 2.0±0.1 to 3.0 GPa) and low-temperature (1300–1500 K). Notably, taking the nanosize-induced interior pressure into account, the nucleation of c-BN could be driven to the new stable region from the metastable region in the general accepted equilibrium phase (P,T) diagram of BN proposed by Corrigan and Bundy, upon HPHT supercritical-fluid systems. The threshold pressure of the formation of c-BN was calculated based on our model, and these results are in excellent agreement with the experiment.

Diffuse reflection inside a hexagonal nanocavity

Geometrical diffuse reflection is a common optical phenomenon that occurs when a reflecting surface has roughness of order of hundreds of micrometres. Light rays thus reflect uniformly in all directions with each ray obeying Snells law. Of interest is knowing what happens when light reflects off surfaces with roughness of nanometres. Here, by introducing nanoscaled roughness on the hexagonal faces of ZnO nanocavities, we observe luminescent profiles with flowery patterns, replacing the usual whispering gallery modes. The unique profile for these nanocavities is attributed to wave diffuse reflection, which occurs when the features on the reflecting surfaces are typically nanometre-sized. Light with wavelengths of similar scale “sees” these nano-perturbations, and undergoes scattering rather than geometrical diffuse reflection. These findings could benefit the fields of nanoscale topography and nanoscopic uniform lighting by using wave diffuse reflection.

Preparation and characterization of the [CaCu(bet)4](ClO4)4 thin films

Details are given of an experimental study on the deposition process of CaCuB (B = betaine) thin films and their optical and electrical properties. It is demonstrated that such films may be prepared using thermal evaporation technique. The films were characterized by using Fourier transform infrared spectroscopy (FTIR), electron probe microanalyses (EPM) and scanning electron microscope (SEM). Similar infrared spectra were obtained for both the films and the powder samples. Results of EPM show that each element of the chemical compound was distributed evenly in the evaporated films. In addition, electrical conductivity, optical absorption and fluorescent spectra of the film were measured. The results reveal that the evaporated compound is a wide band gap insulator. Significant fluorescence was detected and the spectrum with peak at 409 nm was recorded.