Zengquan Xue

Optical properties of the ZnO nanotubes synthesized via vapor phase growth

A large quantity of nanosized ZnO tubular structures was prepared using a very simple thermal evaporation of mixed Zn–ZnO powders under a wet oxidation condition. The ZnO nanotubes have a hollow core with crystalline wall of 8–20 nm in thickness. Optical properties of ZnO nanotubes were studied at room temperature. Raman peaks arising from the ZnO nanotubes were analyzed, which correspond well to that of the bulk ZnO sample. The photoluminescence measurements of ZnO nanotubes revealed an intensive UV peak at 377 nm corresponding to the free exciton emission, and a broad peak at about 500 nm arising from defect-related emission.

An accurate and efficient self-consistent approach for calculating electron transport through molecular electronic devices: including the corrections of electrodes

A self-consistent ab initio approach for calculating electron transport through molecular electronic devices is developed. It is based on density functional theory (DFT) calculations and the Greens function technique employing a finite basis of local orbitals. The device is rigorously separated into the extended molecule region and the electrode region. In the DFT part calculating the Hamiltonian matrix of the extended molecule from its density matrix, the electrostatic correction induced by electrodes and the exchange?correlation correction due to the spatial diffuseness of localized basis functions are included. Our approach is efficient and accurate, with a controllable error to deal with such open systems. A one-dimensional infinite gold monatomic chain, whose electronic structure can be known from conventional DFT calculations with periodic boundary conditions (PBCs), is employed to validate the accuracy of our approach. With both corrections, our result for the gold chain at equilibrium is in excellent agreement with the PBC DFT result. We find that, for the gold chain, the exchange?correlation correction is more significant than the electrostatic correction.

Solid–liquid–solid (SLS) growth of coaxial nanocables: silicon carbide sheathed with silicon oxide

Abstract Coaxial silicon carbide–silicon oxide nanocables on silicon substrates were synthesized from the ternary system of Si–Ni–C at 950°C under Ar/H 2 atmosphere. The nanocables consist of a hexagonal crystalline SiC core and a surface layer of amorphous silicon oxide, which have an average diameter of ∼50 nm and a length of several tens of microns. The microstructure and composition of the nanocables were characterized using high-resolution transmission electron microscope (HREM), and electron energy loss spectroscopy (EELS), and the growth mechanism of the nanocables was explained under the framework of a solid–liquid–solid (SLS) mechanism.

Positive electron affinity of fullerenes: Its effect and origin

The universal variation pattern of the total energy of various fullerenes including single-walled carbon nanotubes with respect to their extra charge is revealed by the density-functional-theory calculations. The parabolic energy-charge curve with its lowest energy value corresponding to a negatively charged fullerene indicates that these carbon materials have positive electron affinity and are not in the most stable state. The positive electron affinity seems to originate from the pi-electrons and is found to be related to the aggregation property of fullerenes.

Filling of single-walled carbon nanotubes with silver

We showed, by high-resolution transmission electron microscopy and spatially resolved electron energy-loss spectroscopy, that silver atoms have been introduced into single-walled carbon nanotubes (SWCNTs) produced by the are-discharge method with solution phase chemistry by capillary filling, and a high efficiency of filling may be realized. Both partially filled and completely filled SWCNTs were observed, and the metal inside the tube may be regarded as one-dimensional silver nanowires with high length-to-diameter ratios, The length of filling ranged from several to tens of nanometers, Under strong electron beam irradiation the filling metal condensed to a particle, while the silver-containing SWCNT shrank and disintegrated.

Quantum chemistry study on the open end of single-walled carbon nanotubes

Abstract Geometrical and electronic structures of open-ended single-walled carbon nanotubes (SWCNTs) are calculated using density functional theory (DFT) with hybrid functional (B3LYP) approximation. Due to different distances between carbon atoms along the edge, reconstruction occurs at the open end of the (4,4) armchair SWCNT, i.e., triple bonds are formed in the carbon atom pairs at the mouth; however, for the (6,0) zigzag SWCNT, electrons in dangling bonds still remain at ‘no-bonding’ states. The ionization potential (IP) of both (4,4) and (6,0) SWCNTs is increased by their negative intrinsic dipole moments, and localized electronic states existed at both of their open ends.

First-principles calculation on the conductance of a single 1,4-diisocyanatobenzene molecule with single-walled carbon nanotubes as the electrodes

The conductance of a single 1,4-diisocyanatobenzene molecule sandwiched between two single-walled carbon nanotube (SWCNT) electrodes are studied using a fully self-consistent ab initio approach which combines nonequilibrium Greens function formalism with density functional theory calculations. Several metallic zigzag and armchair SWCNTs with different diameters are used as electrodes; dangling bonds at their open ends are terminated with hydrogen atoms. Within the energy range of a few eV of the Fermi energy, all the SWCNT electrodes couple strongly only with the frontier molecular orbitals that are related to nonlocal pi bonds. Although the chirality of SWCNT electrodes has significant influences on this coupling and thus the molecular conductance, the diameter of electrodes, the distance, and the torsion angle between electrodes have only minor influences on the conductance, showing the advantage of using SWCNTs as the electrodes for molecular electronic devices.

Atomically resolved field emission patterns of single-walled carbon nanotubes

The electron emission and structural properties of single-walled carbon nanotubes (SWCNTs) were investigated by using field emission microscopy (FEM). The transmission electron microscopy micrograph confirmed the existence of an SWCNT bundle on the W tip. Under appropriate experimental conditions, an FEM image with an elliptic ring-like structure composed of separated bright dots was obtained, a reasonable interpretation of it is that it was produced from the open end for a zigzag (16,0) SWCNT protruding from the SWCNT bundle, each bright dot corresponding to a single atom at the open end. And, if true, this means that the FEM demonstrated 0.2nm resolution, which was theoretically possible for the assumed geometry. The calculated value of the magnification of the pattern was also consistent with the measured value if the value of the compression factor beta was set at 1.76.

Study of a novel C60−2,6‐bis(2,2‐bicyanovinyl)pyridine complex thin film

A novel complex thin film of 2,6‐bis(2,2‐bicyanovinyl)pyridine (BDCP) and C60 has been fabricated by vacuum coevaporation of BDCP and C60 from two different evaporation sources. The C60‐BDCP thin films have shown totally different optical and electronic properties from the films of both the BDCP and C60. Stable and reproducible electric bistable properties have been observed in sandwichlike device Ag/C60‐BDCP/Ag. The films are characterized by several methods including high‐resolution scanning electron microscopy, x‐ray diffraction, UV‐visible absorption and infrared transmission spectroscopy.

Local oxidation of titanium thin films using an atomic force microscope under static and pulsed voltages

Scanning probe microscope tip-induced local oxidation is a promising tool for the fabrication of nanometre-scale structures and devices. In this study, oxide line patterns were fabricated on the surface of a titanium thin film using a conductive atomic force microscope (AFM). Geometrical characters of the oxide line patterns and their dependence on the exposure parameters in fabricating, i.e. the applied voltage amplitude and duration, ambient humidity, AFM set point value, and the mode of applied voltage, are investigated. The dependence of the oxide width on the applied voltage duration was found to have two distinct growth rates and a two-stage growth model was proposed to account for it. Application of pulsed voltages was proved to be an efficient method for suppressing the growth of oxide width by repeatedly breaking the directional transport of OH− ions in the process of oxidation. A line-width of 8 nm was achieved with an optimized pulsed voltage. Based on the experimental results, optimal controlling of exposure parameters to improve the fabricating resolution and reliability are discussed.