Yubao Li

ZnO nanoneedles with tip surface perturbations: Excellent field emitters

ZnO nanoneedles were prepared on a silicon wafer through a chemical vapor deposition. The diameters of the needle tips were in a range of 20–50 nm. High-resolution transmission electron microscopy revealed that the nanoneedles were single crystals growing along the [001] direction and exhibiting multiple tip surface perturbations, just 1–3 nm in dimension. Field-emission measurements on the prepared nanostructures showed fairly low turn-on and threshold fields of 2.5 and 4.0 V/μm, respectively. The nanosize perturbations on the nanoneedle tips are assumed to cause such excellent field-emission performance.

ZNO NANOBELTS GROWN ON SI SUBSTRATE

Using infrared irradiation to heat an industrial brass (Cu–Zn alloy) disk in moderate vacuum, ZnO nanobelts were directly prepared on a Si substrate. The nanobelts had a single-crystal hexagonal structure and grew along the [0001] direction. The nanobelts had two distinct widths along their entire length. Photoluminescence measurement showed that the nanobelts had an intensive near-band ultraviolet emission at 379 nm. Large-area growth and high quality indicate that the prepared ZnO nanobelts have potential application in optoelectronic devices.

Field emission from MoO3 nanobelts

Single-crystalline MoO3 nanobelts having an orthorhombic structure were prepared on a Si wafer via heating a Mo foil in air. The nanobelts were 50–300 nm wide and tens of nanometers thick. The nanobelt lengths lie in the [001] direction. Field-emission measurements showed that the threshold field decreased with the anode–sample separation increasing. Typically, a threshold field of 12.9 V/μm was determined at a spacing of 80 μm. The nanobelts exhibited a sharp increase in emission current density near the threshold field and, thus, reached a high current density at a relatively low field. Emission from both sharp corners and edges of the nanobelts is assumed to contribute to the high emission current. The high-current emission paired with high stability indicates that the prepared MoO3 nanobelt films are excellent field emitters.

Two-dimensional micrometer-sized single-crystalline ZnO thin nanosheets

Two-dimensional micrometer-sized single-crystalline ZnO thin nanosheets were achieved on a large scale, in which Zn thin nanosheets (precursor) were first grown via a thermal decomposition and reduction of the starting ZnS powder, and then converted to the ZnO nanosheets via a simple oxidation process. The ZnO nanosheets, growing along [100] or [010] direction and enclosed by ± (001) facets, have lateral dimensions up to several hundreds of microns, and thicknesses of 30–70 nm. Both room-temperature cathodoluminescence and photoluminescence measurements reveal that the present ZnO nanosheets have visible emission bands ranging from the green to red.

MoS2 nanoflowers and their field-emission properties

Elegant three-dimensional MoS2 nanoflowers were uniformly formed via heating a MoO2 thin film in a vapor sulfur atmosphere. Tens to hundreds of petals were self-assembled within a single nanoflower. Each petal, 100–300 nm wide and only several nanometers thick, exhibited a hexagonal structure. The number of petal layers gradually decreased towards the edges, resulting in uniquely thin edges, typically less than 3 nm. The MoS2 nanoflowers appeared to be excellent field emitters displaying a current density of 0.01 and 10 mA/cm2 at macroscopic fields of 4.5–5.5 and 7.6–8.6 V/μm, respectively; the electron field emission was consistent with the Fowler–Nordheim theory.

Synthesis, structure, and photoluminescence of very thin and wide alpha silicon nitride (α-Si3N4) single-crystalline nanobelts

Large quantities of very thin and wide single-crystal alpha silicon nitride (α-Si3N4) nanobelts were synthesized by a vapor-solid thermal reaction between ammonia and silicon monoxide (SiO) without using any added catalyst. Scanning electron microscopy, high-resolution electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction were used to characterize the formed nanobelts. The single-crystal α-Si3N4 nanobelts are about 800–1200 nm in width, 20–35 nm in thickness and about several tens to several hundreds of micrometers in length. The nanobelts are perfect in structure. The nanobelts grow along [011] and [100] direction. Intense visible photoluminescence (PL) occurring on the wide and thin nanobelts over a broad spectrum ranging from 420 to 750 nm was observed. The visible PL emission is related to the inherently imperfect Si and N dangling bonds in the α-Si3N4 structure.

Ga-filled single-crystalline MgO nanotube: Wide-temperature range nanothermometer

A highly effective one-step approach was developed to synthesize single-crystalline MgO nanotubes and in situ fill nanotubes with Ga. The axes of nanotubes are in the [100] direction of cubic MgO. The prepared nanotube exhibits a square-like cross section both for its interior and exterior. The liquid metal-assisted route is suggested to be a general way to prepare oxide nanotubes. Linear thermal expansion behavior recorded for liquid gallium column confined in the MgO nanotube makes possible creation of a wide-temperature range nanothermometer with superior mechanical properties and environmental structural stability.

Crystallization behavior of the amorphous carbon nanotubes prepared by the CVD method

We report the crystallization behavior of the amorphous carbon nanotubes at high temperature in this paper. Transmission electron microscopy and X-ray diffraction were performed to characterize the structures of the carbon nanotubes. The results reveal that the microstructure of the as-grown carbon nanotubes, prepared by the floating catalyst method, is roughly amorphous. The as-grown carbon nanotubes were annealed at high temperatures, and the crystallization behavior of the amorphous carbon nanotubes was investigated systematically. Because the carbon nanotubes have finite dimensions and tube-like shape, their crystallization behavior is completely different from the bulk amorphous carbons. The results reveal that the graphene layers in the annealed carbon nanotubes will form into a uniform two-dimensional turbostratic stack, a configuration of carbon nanotubes with the lower Gibbs free energy. A thermodynamic model is presented to explain the crystallization behavior of the amorphous carbon nanotube.

Controllable growth of single wall carbon nanotubes by pyrolizing acetylene on the floating iron catalysts

Single wall carbon nanotubes (SWNTs) without amorphous carbon coating were prepared by thermally decomposing acetylene (C2H2) at the temperature range 750–1200 °C in a floating iron catalyst system. The C2H2 partial pressure was controlled to make a carbon supply limiting growth of SWNTs. The higher reaction temperature above 1100 °C seemed not to favor the SWNT production due to the quick thermal decomposition of C2H2.

SiO2-sheathed InS nanowires and SiO2 nanotubes

InS nanowires uniformly sheathed with amorphous SiO2 were synthesized via a physical vapor deposition process. InS nanowires were 20–100 nm in diameter, and the SiO2 sheaths were 5–20 nm in thickness. Single-crystalline InS cores displayed orthorhombic structure and their longitudinal directions were preferentially aligned in the [100] orientation. Pure SiO2 nanotubes of typically round cross sections were also obtained by removing InS cores from the prepared nanocables via thermal evaporation. Photoluminescence measurements on these SiO2 nanotubes demonstrated strong visible-light emission peaked at 570 nm.