Guorui Wang

Structural, electrical and optical properties of yttrium-doped ZnO thin films prepared by sol–gel method

Yttrium-doped ZnO thin films were deposited on silica glass substrates by the sol–gel method. The structural, electrical and optical properties of yttrium-doped ZnO thin films were investigated systematically and in detail. All the thin films have a preferred (0 0 2) orientation. When compared with the electrical resistivity values of films without annealing treatment, the values of films annealed in the reducing atmosphere were decreased by about three orders of magnitude. The lowest electrical resistivity value was 6.75 × 10−3 Ω cm, which was obtained in the 0.5 at% yttrium-doped ZnO thin film annealed in nitrogen with 5% hydrogen at 500 °C. In room-temperature photoluminescence (PL) spectra, two PL emission peaks are found in the pure ZnO thin film; one is the near-band-edge (NBE) emission at 3.22 eV and the other is a green emission at about 2.38 eV. Nevertheless, the green emission is not found in the PL of the yttrium-doped ZnO thin films. The low-temperature PL spectrum of the undoped ZnO thin film at 83 K is split into well-resolved free and bound excition emission peaks in the ultraviolet region, but the NBE emission of the 5 at% yttrium-doped ZnO thin film at 83 K has only one broad emission peak.

Synthesis of ZnS nanocrystals with controllable structure and morphology and their photoluminescence property

The controllable synthesis of ZnS nanocrystals with desirable morphology and correlative structure has been carried out via the solvothermal method by simply changing the molar ratio of the reactants. The hexagonal-shaped ZnS nanosheets with a zinc-blende structure were synthesized in one step for the first time. ZnS nanorods with wurtzite structure and large ratio of length to diameter were also fabricated. We found that phase transformation is easily induced and there is a strong correlation between the morphology and structure of the ZnS nanocrystals by changing the ratio of the reactants. The photoluminescence spectra of the ZnS nanosheets and nanorods exhibit different emission bands. ZnS nanosheets show a strong emission at 534 nm while the nanorods have two emissions located at 520 and 578 nm.

Synthesis and high pressure induced amorphization of C60 nanosheets

C60 nanosheets with thicknesses in the nanometer range were synthesized by a simple method. Compared to bulk C60, the lattice of the nanosheets is expanded by about 0.4%. In situ Raman spectroscopy and energy-dispersive x-ray diffraction under high pressures have been employed to study the structure of the nanosheets. The studies indicate that the bulk modulus of the C60 nanosheets is significantly larger than that of bulk C60. The C60 cages in nanosheets can persist at pressures over 30GPa, 3GPa higher than for bulk C60. These results suggest that C60 crystals in even small size will be a potential candidate of superhard materials.

Structural properties and photoluminescence of ZnO nanowalls prepared by two-step growth with oxygen-plasma-assisted molecular beam epitaxy

Low dimensional (nanowall) ZnO structures were prepared by a two-step growth method with oxygen-plasma-assisted molecular beam epitaxy, where the as-grown film was first engraved on a porous template using the oxygen plasma and then the ZnO nanowalls were grown on the template. The resonance Raman spectra showed the surface mode. A morphology model was proposed on the basis of the scanning electron microscopy patterns and this mode. The room and low temperature photoluminescence showed that the nanowalls had intense ultraviolet emission properties, which benefited from the low dimensional structure with few defects.

Sp2 clustering-induced improvement of resistive switching uniformity in Cu/amorphous carbon/Pt electrochemical metallization memory

We studied the influence of sp2 clustering on resistive switching uniformity in Cu/amorphous carbon/Pt electrochemical metallization memory. Herein, a simple method was utilized to adjust the degree of sp2 clustering by changing compliance currents (CCs). The small sp2 clusters within the a-C layer were found to condense into larger clusters with increasing CCs because of the CC-dependent Joule heating effect. Raman spectra experimentally verified the increase of sp2 cluster diameter with increasing CCs. Importantly, a switching uniformity improvement can be obtained by increasing the degree of sp2 clustering. The enhanced local electric field around sp2 clusters can account for the reduction of the Cu conductive filament randomness and corresponding improvement of memory performance.

Brush-controlled oriented growth of TCNQ microwire arrays for field-effect transistors

We demonstrate a novel solution-based assembly method using a writing brush to realize the controllable fabrication of highly-oriented and large-scale TCNQ single-crystal microwire arrays. The arrays can not only be grown on conventional rigid substrates, such as Si, Si/SiO2 and low-cost glass, but also on nonconventional substrates, which include flexible polyethylene terephthalate (PET), curved glass hemispheres and commercially available plastic contact lenses. Their morphology is optimized by tuning solution concentration, substrate temperature, brush type, inclination angles and pressure of the brush. The length of the microwire arrays can extend to the millimeter level, and their preferential orientation is perpendicular to the lengthwise direction of the brush hair. The coverage area of microwire arrays with a consistent orientation can reach 1.5 × 2.0 mm2 and the success ratio is as high as 93%. Based on these microwire arrays, devices on different substrates, including rigid Si/SiO2 and flexible PET, can be easily realized in one step. The anisotropic transport of TCNQ crystals is studied with respect to the concentration controlled morphology. All these results illustrate the broad application prospects of this facile writing-brush method in the growth of large-scale, high-quality organic micro/nanowires for integration into flexible organic semiconductor devices and circuits.

Individual single-crystal nanowires as electrodes for organic single-crystal nanodevices

Conductive, transparent, and flexible SnO2:Sb single-crystal nanowires are shown as electrodes for F16CuPc single-crystal nanowire devices on flexible plastic, which includes anisotropic-transport OFETs, electrode-movable OFETs, and p–n junction photovoltaic devices. The SnO2:Sb nanowires provide a good energy level match and excellent soft contact with F16CuPc nanowires, leading to multifaceted applications of the SnO2:Sb nanowire in nanowire electronics and optoelectronics, as well as high device performance. Combined with their good size compatibility, these results show that the conductive SnO2:Sb single-crystal nanowire opens a window into the fundamental understanding of the intrinsic properties of highly ordered organic semiconductors, optimization and miniaturization of organic nanocircuits, and development of new-generation flexible organic nanodevices.

Gate-modulated transport properties and mechanism for nanowire cross junction based on SnO2 semiconductor

The transport properties and mechanism of the three-terminal field-effect nanowire cross junction have been systematically investigated. An interesting phenomenon, such as applied voltage bias on nanowire cross junction makes the ON/OFF current ratio of the transistor improved by over 2 orders of magnitude, has been observed. Different from the two-terminal nanowire cross junctions, the cross junction induced potential barrier in three-terminal counterparts is found to be capable to prevent the current of the top semiconductor nanowire from injecting into the bottom nanowire at off state, while to make the current of the top semiconductor nanowire contribute to the current of the bottom nanowire at on state, resulting in the current switch between on state and off state by the gate voltage modulation.

Organic Single-Crystal Nanowire Transistor Fabricated by Glass Fiber Mask Method

The commercialized, low-cost, and lightweight glass fiber microwires have been shown as a new mask to simplify the fabrication of organic single-crystal nanowire transistors based on copper phthalocyanine (CuPc). The glass fiber is light weight so that the van der Waals force is capable of resisting the effect of gravity when the substrate is flipped over. In this case, the glass fiber mask need not be artificially fixed onto the substrate during metal deposition for electrode fabrication, so the device fabrication process can be simplified. With such a facile method, multiple organic single-crystal nanowire transistors can be fabricated at the same time, and the high mobility up to 0.7 cm2V-1s-1 has been realized for CuPc nanowire transistors. Good stability has been also demonstrated. This method effectively improves the efficiency of devices preparation, showing the potential in largescale fabrication of organic nanowire transistors. Meanwhile, it is favorable for the fundamental electrical studies of 1-D organic nanomaterials.

Correction: Individual single-crystal nanowires as electrodes for organic single-crystal nanodevices

Correction for ‘Individual single-crystal nanowires as electrodes for organic single-crystal nanodevices’ by Guorui Wang et al., J. Mater. Chem. C, 2015, DOI: 10.1039/c5tc01920f.