E. G. Wang

Dual-mode mechanical resonance of individual ZnO nanobelts

The mechanical resonance of a single ZnO nanobelt, induced by an alternative electric field, was studied by in situ transmission electron microscopy. Due to the rectangular cross section of the nanobelt, two fundamental resonance modes have been observed corresponding to two orthogonal transverse vibration directions, showing the versatile applications of nanobelts as nanocantilevers and nanoresonators. The bending modulus of the ZnO nanobelts was measured to be ∼52 GPa and the damping time constant of the resonance in a vacuum of 5×10−8 Torr was ∼1.2 ms and quality factor Q=500.

Vertically aligned boron nitride nanosheets: chemical vapor synthesis, ultraviolet light emission, and superhydrophobicity.

Boron nitride (BN) is a promising semiconductor with a wide band gap ( approximately 6 eV). Here, we report the synthesis of vertically aligned BN nanosheets (BNNSs) on silicon substrates by microwave plasma chemical vapor deposition from a gas mixture of BF(3)-N(2)-H(2). The size, shape, thickness, density, and alignment of the BNNSs were well-controlled by appropriately changing the growth conditions. With changing the gas flow rates of BF(3) and H(2) as well as their ratio, the BNNSs evolve from three-dimensional with branches to two-dimensional with smooth surface and their thickness changes from 20 to below 5 nm. The growth of the BNNSs rather than uniform granular films is attributed to the particular chemical properties of the gas system, mainly the strong etching effect of fluorine. The alignment of the BNNSs is possibly induced by the electrical field generated in plasma sheath. Strong UV light emission with a broad band ranging from 200 to 400 nm and superhydrophobicity with contact angles over 150 degrees were obtained for the vertically aligned BNNSs. The present BNNSs possess the properties complementary to carbon nanosheets such as intrinsically semiconducting, high temperature stability, and high chemical inertness and may find applications in ultraviolet nanoelectronics, catalyst supports, electron field emission, and self-cleaning coatings, etc., especially those working at high temperature and in harsh environments.

An Anisotropic Etching Effect in the Graphene Basal Plane

A highly controllable, dry, anisotropic etching technique for graphene sheets has been achieved using hydrogen plasma etching. Zigzag edge formation was achieved by starting the etching at edges and defects and depends strongly on crystallographic orientation of the graphene. This dry, anisotropic etching approach combined with the standard lithographic technique is ideal for scalable graphene tailoring because the etching rates can be precisely controlled and the quality of the graphene can be preserved.

First Principles Calculation of Anomalous Hall Conductivity in Ferromagnetic bcc Fe

We perform a first principles calculation of the anomalous Hall effect in ferromagnetic bcc Fe. Our theory identifies an intrinsic contribution to the anomalous Hall conductivity and relates it to the k-space Berry phase of occupied Bloch states. This dc conductivity has the same origin as the well-known magneto-optical effect, and our result accounts for experimental measurement on Fe crystals with no adjustable parameters.

Polymerized carbon nanobells and their field-emission properties

Aligned nitrogen-containing carbon nanofibers consisting of polymerized “nanobells” have been grown on a large scale using microwave plasma-assisted chemical-vapor deposition with a mixture of methane and nitrogen. A greater part of the fiber surface consists of open ends of the graphitic sheets. A side-emission mechanism is proposed. A low-threshold field of 1.0 V/μm and a high-emission current density of 200 mA/cm2 for an applied field of 5–6 V/μm were achieved, implying that the materials have a high potential for future application as electron field emitters, especially in flat-panel displays.

Nanocrystalline Tungsten Oxide Thin Film: Preparation, Microstructure, and Photochromic Behavior

A nanocrystalline tungsten oxide photochromic thin film was prepared by colloid chemistry method. The microstructure, phase transition involved in the solution process, photochromic behavior, and mechanism of the film were investigated by means of transmission electron microscope, x-ray diffraction, ultraviolet-visible absorption spectra, and x-ray photoelectron spectra. It was found that the particle size and crystallinity of the thin film could be easily controlled by adjusting the concentration of oxalic acid in the colloid solution of tungsten oxide hydrate. With the increase of the oxalic acid concentration, the size of nanoparticles in the film decreased sharply, and meanwhile, a blue shift of the absorption peaks caused by the quantum size effect was observed accordingly. With the increase of the pH in the solution, tungsten oxide hydrate was gradually transformed into an oxided 12-tungstate with Keggin structure, which led to the change of photochromic property of the films. The photochromism of the film is believed to be due to the electron transfer between the different valence states of tungsten ions located in adjacent sites.

Field emission of individual carbon nanotube with in situ tip image and real work function

The field emission properties of individual multiwalled carbon nanotubes have been measured simultaneously in correlation to the emitter images and their real work functions at tips by the in situ transmission electron microscopy method. The field emission of a single nanotube still follows the Fowler-Nordheim law. The field enhancement factor has been determined by the real work function rather than a given constant. In situ imaging and measurement show that the work function at the nanotube tip depends strongly on its structure and surface condition. This study provides an approach of direct linking field emission with the in situ emitter structure and the real work function at the emitter tip.

Research on carbon nitrides

The status of research on carbon nitride compounds is reviewed including exciting recent results. On the basis of the early theoretical predictions by Cohen and co-workers in 1985-1990, covalent carbon nitrides should be as hard as diamond. The high hardness, high wear resistance and low coefficient of friction make it attractive for industrial operation. The various empirical and nb initio theoretical results are summarized. Attention is paid to him synthesis techniques and crystal characterizations, such as chemical, structural, electrical, optical, thermal and mechanical properties. New carbon nitride structures and selected-phase growth are also described. Finally, some major outstanding problems facing carbon nitride researchers are identified, and some important problems, which should be addressed in the near future, are suggested

Aqueous Noncovalent Functionalization and Controlled Near-Surface Carbon Doping of Multiwalled Boron Nitride Nanotubes

Noncovalent functionalization of boron nitride nanotubes (BNNTs) in aqueous solution was achieved by means of pi-stacking of an anionic perylene derivative, through which carboxylate-functionalized BNNTs were prepared for the first time. Starting from the functionalized nanotubes, an innovative methodology was designed and demonstrated for the controlled near-surface carbon doping of BNNTs. As a result of such delicate doping, novel B-C-N/BN coaxial nanotubes have been fabricated, and their p-type semiconducting behaviors were elucidated through gate-dependent transport measurements.

Synthesis and field-emission behavior of highly oriented boron carbonitride nanofibers

Large-area highly oriented boron carbonitride (BCN) nanofibers with various compositions were synthesized directly on polished polycrystalline nickel substrates from a gas mixture of N2, H2, CH4, and B2H6 by bias-assisted hot-filament chemical-vapor deposition. The morphology of BCN nanofibers was examined by scanning electron microscopy, the nanofiber structure was studied by high-resolution transmission electron microscopy, and the chemical composition of individual nanofibers was determined by electron energy-loss spectroscopy. Field-emission behavior of the BCN nanofibers was characterized and a high emission current density of about 20–80 mA/cm2 at a low electric field of 5–6 V/μm implies a promising application as field-emission sources.