Xudong Shan

Electronic and Mechanical Coupling in Bent ZnO Nanowires

A red shift of the exciton of ZnO nanowires is efficiently produced by bending strain, as demonstrated by a low-temperature (81 K) cathodoluminescence (CL) study of ZnO nanowires bent into L- or S-shapes. The figure shows a nanowire (Fig. a) with the positions of CL measurements marked. The corresponding CL spectra-revealing a peak shift and broadening in the region of the bend-are shown in Figure b.

Effects of annealing on the ferromagnetism and photoluminescence of Cu-doped ZnO nanowires

Room temperature ferromagnetic Cu-doped ZnO nanowires have been synthesized using the chemical vapor deposition method. By combining structural characterizations and comparative annealing experiments, it has been found that both extrinsic (CuO nanoparticles) and intrinsic (Zn(1-x)Cu(x)O nanowires) sources are responsible for the observed ferromagnetic ordering of the as-grown samples. As regards the former, annealing in Zn vapor led to a dramatic decrease of the ferromagnetism. For the latter, a reversible switching of the ferromagnetism was observed with sequential annealings in Zn vapor and oxygen ambience respectively, which agreed well with previous reports for Cu-doped ZnO films. In addition, we have for the first time observed low temperature photoluminescence changed with magnetic properties upon annealing in different conditions, which revealed the crucial role played by interstitial zinc in directly mediating high T(c) ferromagnetism and indirectly modulating the Cu-related structured green emission via different charge transfer transitions.

Study of the structural damage in the "0001… GaN epilayer processed by laser lift-off techniques

The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaN surface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.

Microanalyses of the reverse-bias leakage current increase in the laser lift off GaN-based light emitting diodes

Microanalyses of the reverse-bias leakage current increase in the laser lift off (LLO) GaN-based light emitting diodes were performed. It was found that the amount of dislocations did not obviously change after LLO process in our experiments. The conductive-atomic force microscopy images and transmission electron microscope (TEM) images results revealed that almost all screw dislocations became to be related with leakage current and the current intensity increased over 100 times after the LLO process; however, only nanopipes corresponded to leakage current in the sample without the laser irradiation. Scanning TEM images indicated microstructure changes induced by LLO process. Amount of point defects around dislocations might be responsible for the increase in leakage current by providing more levels for tunneling.

Solution Grown Se/Te Nanowires: Nucleation, Evolution, and The Role of Triganol Te seeds

We have studied the nucleation and growth of Se–Te nanowires (NWs), with different morphologies, grown by a chemical solution process. Through systematic characterization of the Se–Te NW morphology as a function of the Te nanocrystallines (NCs) precursor, the relative ratio between Se and Te, and the growth time, a number of significant insights into Se–Te NW growth by chemical solution processes have been developed. Specifically, we have found that: (i) the growth of Se–Te NWs can be initiated from either long or short triganol Te nanorods, (ii) the frequency of proximal interactions between nanorod tips and the competition between Se and Te at the end of short Te nanorods results in V-shaped structures of Se–Te NWs, the ratio between Se and Te having great effect on the morphology of Se–Te NWs, (iii) by using long Te nanorods as seeds, Se–Te NWs with straight morphology were obtained. Many of these findings on Se–Te NW growth can be further generalized and provide very useful information for the rational synthesis of group VI based semiconductor NW compounds.

MgB2 Superconducting Whiskers Synthesized by Using the Hybrid Physical-Chemical Vapor Deposition

In this work, MgB(2) whiskers were fabricated on a copper substrate by using the hybrid physical-chemical vapor deposition, which was one of the most effective ways to make high quality pure MgB(2) films, with the possible growth mechanism discussed. The whiskers are hexagonal and conelike and grow along the [0001] direction with a single-crystal structure. The onset transition temperature is approximately 39 K, which is among the best in the published nanostructure MgB(2) papers. Fabrication of nanoscale MgB(2) whiskers provides the fundamental understanding of the effect of dimensionality and size on superconductivity.

Non-exponential photoluminescence decay dynamics of localized carriers in disordered InGaN/GaN quantum wells: the role of localization length

In this article, we report a combined experimental and theoretical study on the luminescence dynamics of localized carriers in disordered InGaN/GaN quantum wells. The luminescence intensity of localized carriers is found to exhibit an unusual non-exponential decay. Adopting a new model taking the radiative recombination and phonon-assisted hopping transition between different localized states into account, which was recently developed by Rubel et al., the non-exponential decay behavior of the carriers can be quantitatively interpreted. Combining with precise structure characterization, the theoretical simulations show that the localization length of localized carriers is a key parameter governing their luminescence decay dynamics.

In situ study of epitaxial growth of ZnO nanowires at the junctions of nanowall networks on zinc particles

The authors show the real-time growth of ZnO nanowalls and nanowires on zinc particles via in situ observation in an environmental scanning electron microscopy. It was observed that a ZnO polycrystalline film is first deposited on zinc particles. The nanowires started to grow when the nanowalls had just formed and they grew epitaxially on the junctions of the nanowalls. The nanowalls and the nanowires grew together until the source of zinc was exhausted. The vapor-solid mechanism is deemed to be the growth mechanism as it quantitatively accounts for the growth speed of the nanowalls and nanowires observed in the experiment. Cathodoluminescence reveals that the growth at low zinc concentration leads to blue emission from defects, which may be zinc vacancies.