Kai-Ying Wang

Synthesis and photovoltaic effect of vertically aligned ZnO/ZnS core/shell nanowire arrays

A vertically aligned ZnO/ZnS core/shell nanowire array with type II band alignment was directly synthesized on an indium-tin-oxide glass substrate and the photovoltaic effect of the nanowire array was investigated. The epitaxial relationship, wurtzite (0001) matching zinc-blende (111), was observed in the ZnO/ZnS nano-heterostructure. ZnS coating is found to quench the photoluminescence of ZnO nanowires but enhance the photocurrent with faster response in the photovoltaic device, indicating improvement in charge separation and collection in the type II core/shell nanowire.

Magnetic properties of nanocrystalline Fe{sub 3}O{sub 4} films

Nanocrystalline magnetite Fe3O4 films of about 180 nm thick have been deposited on Si(100) substrates by pulsed laser deposition. Zero-field-cooled magnetization shows clearly the Verwey transition near 120 K by an abrupt change, which is absent from the field-cooled magnetization. This is correlated to its hysteresis curves where the loops remain open until a high field of 2 T. The magnetization does not saturate in field 2 orders of magnitude higher than its coercive field. Such behaviors may result from the existence of antiphase domains. Antiphase boundaries inside the grains are clearly observed with transmission electron microscopy. Negative magnetoresistance of about 12% has been observed near 120 K in a field of 9 T.

Characterization of the natural barriers of intergranular tunnel junctions: Cr2O3 surface layers on CrO2 nanoparticles

Cold-pressed powder compacts of CrO2 show large negative magnetoresistance (MR) due to intergranular tunneling. Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature. In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). A very thin native oxide layer with a thickness of 1–3 nm on the surface of CrO2 powders has been observed. The composition and crystal structure of this surface layer has been determined to be Cr2O3 by XPS and XRD. The dense and uniform Cr2O3 surface layers play an ideal role of tunnel barriers in the CrO2 powder compacts.

Magnetic properties of cobalt and cobalt-platinum alloy nanoparticles synthesized via microemulsion technique

Metallic cobalt and cobalt-platinum alloys of various nanometer sizes have been synthesized via the microemulsion technique and their magnetic properties have been characterized. Preparation of cobalt and cobalt-platinum alloy nanoparticles was achieved by reducing aqueous metallic salts confined in the polar regions of the reverse micelle of cetyltrimethyl bromide (CTAB) with sodium borohydride. These particles are further coated with gold by reducing aqueous gold salts with borohydride. The dc susceptibility data of 15 nm gold coated Co, CoPt and CoPt/sub 3/ particles exhibit a blocking temperature of 4 K, 80 K and 106 K and coercivity of 20 Oe, 300 Oe, and 415 Oe at 10 K, respectively. Annealing these samples at 400/spl deg/C further enhanced their magnetic properties. The two cobalt-platinum alloys have been synthesized and characterized by x-ray powder diffraction and transmission electron microscopy.

Current-controlled channel switching and magnetoresistance in an Fe3C island film supported on a Si substrate

A film of magnetic Fe3C islands separated by nanochannels of graphite was prepared with pulsed laser deposition on a Si substrate with a native SiO2 surface. When the temperature is increased above 250 K the resistance suddenly drops because electron conduction switches from the film to the Si inversion layer underneath. The film shows a negative magnetoresistance. The inversion layer exhibits a large positive magnetoresistance. The transition to the low resistance channel can be reversed by applying a large measuring current, making possible current-controlled switching between two types of electron magnetotransport at room temperature.

Colossal magnetoresistance and magnetic phase transitions in bulk Nd0.7Ba0.3MnO3

Perovskite-type Nd0.7Bao0.3MnO3 has been synthesized and its magnetic and magnetotransport properties studied in the temperature range from 5 K to 300 K. X-ray diffraction indicates that the crystal-symmetry is orthorhombic with a=0.5508(1) nm, b=0.5496(2) nm, and c=0.7764(3) nm. There exist two magnetic phase transitions. It first undergoes a paramaganetic-to-ferromagnetic phase transition at Curie temperature, TC=115 K, and then partial ordering of Nd magnetic moments develops at TNd=40 K as temperature is further decreased. Resistivity measurements indicate corresponding insulator-to-metal transition near TC and a reentrant metal-to-insulator transition near 40 K. Colossal magnetoresistance exists over a wide temperature range, MR=[R(H)-R(0)]/R(0) is over 90% below 120 K. The maximum MR (98%) appears at 95 K, close to the paramagnetic-to-ferromagnetic phase transition.

Magnetotransport and micro-x-ray absorption near-edge structure studies of glass-coated Fe-Ni-Cu microwires

Magnetotransport and micro-x-ray absorption near-edge structure (micro-XANES) studies of glass-coated 20Fe–20Ni–60Cu microwires have been carried out on both as-cast and heat-treated samples. The micro-XANES spectroscopy data were collected at the K edges of Cu, Ni, and Fe with the x-ray microprobe beamline at the Center for Advanced Microstructures and Devices. Comparison of the Fe K-edge spectra from the microwires to standard spectra reveals that the Fe atoms in the as-cast sample are in a face-centered cubic (fcc) configuration and they remain in the fcc phase throughout the annealing processes. Giant magnetoresistance (GMR) has been observed in the microwires and it reaches ∼6.5% at 300 K in a field of 9 T. The MR decreases to ∼2.5% as the annealing temperature increases to 500 °C. The loss of GMR upon annealing is attributed to the growth of Fe–Ni rich magnetic particles. Significant reduction in the number of extremely small particles is obtained after annealing at 500 °C, which shows MR characteri...

TEM studies of nanostructures

Transmission electron microscope (TEM) is an invaluable tool for the characterization of one-dimensional (1D) nanostructures, providing information on morphology, crystal structure, and chemical composition through imaging, diffraction, and spectroscopic analysis. This chapter illustrates the capabilities of TEM techniques in structure analysis with a number of 1D semiconducting nanostructures. Polarity determination and planar defects imaging by TEM will be discussed in ZnO nanostructures. Most emphasis will be given to the comprehensive TEM analysis of nanowire superlattice and heterostructured nanowires, including axially heterostructured nanowires, radial core-shell nanowires, and surface decorated nanowires.