•Controllable preparation of 1-D and dendritic ZnO nanowires was achieved.•A unified growth mechanism was proposed for the different ZnO nanostructures.•Enhanced and stable field emission was achieved from dendritic ZnO nanowires.Large-scale one-dimensional (1-D) and dendritic ZnO nanowires were synthesized on Si substrates using a catalyst-free thermal oxidation technique. By adjusting the stress of the starting Zn thin film for the growth using an Ar ion bombardment process, controllable syntheses of samples composed of only one or a mixture of these two morphologies were achieved. Different nucleation mechanisms were proposed by detailed observations of nucleation and growth process of 1-D and dendritic ZnO nanowires. The strain-induced mass diffusion model and self-catalytic gas-liquid-solid model were used to explain the growth of 1-D and dendritic ZnO nanowires, respectively. In addition, the field-emission characteristics of ZnO nanowires were measured, which showed that dendritic ZnO nanowires have stable emission current.Download high-res image (273KB)Download full-size image

Zinc oxide (ZnO) nanowires are prepared for application in large area gated field emitter arrays (FEAs). By oxidizing Al-coated Zn films, the population density of the ZnO nanowires was tuned precisely by varying the thickness of the Al film. The nanowire density decreased linearly as the thickness of the Al film increased. Optimal field emission properties with a turn-on field of 6.21 V μm–1 and current fluctuations less than 1% are obtained. This can be explained by the minimized screening effect and good electrical conductivity of the back-contact layer. The mechanism responsible for the linear variation in the nanowire density is investigated in detail. Addressable FEAs using the optimal ZnO nanowire cathodes were fabricated and applied in a display device. Good gate-controlled characteristics and the display of video images are realized. The results indicate that ZnO nanowires could be applied in large area FEAs.Keywords: electrical conductivity; field emitter arrays; screening effect; thermal oxidation; zinc oxide nanowires;

•A vacuum-sealed double-sided radiating flat-panel X-ray source device using ZnO nanowire field emitters.•Good field emission properties from ZnO nanowires and similar X-ray radiation from both sides of the device.•Potential applications of the device in compact and portable X-ray radiography and irradiation systems.A double-sided radiating flat-panel X-ray source device has been developed. The device has a diode configuration composed of an anode target of a tungsten thin film and field-emitter arrays of ZnO nanowires prepared on glass substrates. The X-ray radiation comes from both transmission and reflection of X-rays induced by electron bombardment. When operated at 24 kV with an anode current of 3.16 mA, radiation dose rates of ∼200 mGy/min and ∼210 mGy/min were recorded positioning at 4 cm away from the front and back sides of the vacuum-sealed flat-panel X-ray source, respectively. Clear X-ray images with high contrast were taken by the radiation from both sides of the device. The reported device has great potential in making compact X-ray radiography and irradiation systems.

•Comb-shape-gate structure ZnO nanowire field emitter arrays were fabricated.•The back contact affects the emission uniformity of ZnO nanowires.•Addressing capability of the gated ZnO nanowire field emitter arrays was verified.•Display device using ZnO nanowire field emitter arrays was achieved.Addressable field emitter arrays (FEAs) have important applications in vacuum electronic devices. However, it is important to integrate nanowire emitters into a gated structure without influencing the device structure and maintain the excellent field emission properties of nanowire emitters in the FEAs after the fabrication process. In this study, gate-structure ZnO nanowire FEAs were fabricated by a microfabrication process. The structure combines a planar gate and an under-gate, which is compatible with the preparation of ZnO nanowire emitters. The effect of electrode materials on the field emission properties of ZnO nanowires was studied using a diode structure, and it was found that ZnO nanowire pads on indium-tin-oxide (ITO) electrode showed better field emission performance compared with chromium (Cr) electrode. In addition, effective emission current modulation by the gate voltage was achieved and the addressing capability was demonstrated by integrating the ZnO nanowire FEAs in a vacuum-encapsulated field emission display. The reported technique could be a promising route to achieve large area addressable FEAs.

•Enhanced field emission was found in nitrogen functionalization of FLG nanowalls.•Work function shows remarkable reduce of 0.23 eV after nitrogen functionalization.•Nitrogen in FLG edge is proposed to be amine terminal configuration.•The underlying mechanism about the nonlinear F–N plots was proposed.The effects of nitrogen functionalization on the field emission properties of vertically aligned few-layer graphene (FLG) nanowall thin films are investigated. Lowered work function and enhanced electron emission property are observed after ammonia plasma treatment. The turn-on field (defined at 10 μAcm−2) is found to be decreased from 6.5 to 4.6 Vμm−1. Additionally, more stable emission with fluctuation less than 5% is obtained after surface N-functionalization. The electron configuration of nitrogen in FLG edge is proposed to be amine (C–NH2) terminal configuration. Nitrogen doping could significantly increases the π density states near the Fermi level. The maximum upward shift of EF is determined to be ∼0.23 eV by low-energy XPS measurements, which mainly accounts for the lower turn-on electric field. Meanwhile, nitrogen functionalization would minimize surface desorption and accounts for more uniform work function distribution. Finally, the nonlinear characteristics of Fowler–Nordheim plots could be understood by thermionic-field emission process.

Large-scale WO3 nanowire patterns have been successfully fabricated on a 3.5 inch glass substrate at atmospheric pressure by a simple no-catalyst method. The nanowires were observed to have a mean length of about 40 μm, and their aspect ratio reaches 200. The nanowires were proven to be single crystalline WO3 with a monoclinic structure. It is found that the growth region and growth density of the WO3 nanowires differ with the interval or the width of the W stripes. By combination of designing a series of experiments and analyzing the growth kinetics theory, a novel self-supported vapor–solid (SVS) mechanism is proposed to be responsible for the formation of WO3 nanowires. Field emission (FE) measurements show that the WO3 nanowire patterns have excellent FE performance, which have a low turn-on field of 2.9 V μm−1 and good field emission uniformity of over 85%. Moreover, this SVS method may provide a helpful reference on low-temperature and no-catalyst growth of other metal oxide nanostructure arrays at atmospheric pressure.

Excellent field emission is observed from vertically aligned W20O58 nanowires that are prepared by thermal evaporation. The effects of atomic oxygen (AO) exposure on its surface chemistry, gasochromism, and field emission properties have been investigated. Morphology and structural characterization indicates little change was observed in the nanowire after a high dose of AO exposure. It is worth noting that a stable and high emission current could be retained, with a little increase of turn-on electric field (defined at 10 μA cm−2). Work function measurement indicates that the Fermi level has a distinctive decrease of ∼0.29 eV after AO exposure. The W valence state shows a clear transition from +5 to +6, corresponding to AO exposure induced color bleaching process. The origin of the work function increase is ascribed to the strengthened surface dipole, which is the main reason that accounts for the observed field emission characteristics.

Understanding the origin crystal nucleation and driving force is critical for the synthesis of one-dimensional nanomaterials with controllable size, morphology, and crystal structure. The growth behavior of ZnO nanowires prepared by thermal oxidation of zinc film is studied. It is found that the grown nanowires have a bicrystalline structure and growth direction significantly different from the commonly observed [0001] direction. On the basis of detailed high resolution morphology and structural analysis, we propose the origin of the initial growth site, as well as the driving force for formation of the bicrystalline structure of aligned ZnO nanowires. The initial zinc film plays an important role in the growth of nanowire. The edge-enhanced oxidation effect of zinc grain initiates the ZnO nanowire nucleation. The strain within the ZnO layer drives and stimulates the nanowire growth. The present study provides insight into the growth mechanism of ZnO nanowires grown from thermal oxidation of zinc film. Field emission measurement results show that the prepared ZnO nanowires have excellent field emission properties. Uniform emission can be obtained and the turn-on field is 7.8 V/μm. The results indicate that the method is advantageous for large-scale synthesis of ZnO nanowires for field emission applications.

Vertically aligned few-layer graphene is prepared using microwave plasma enhanced chemical vapor deposition. The vertically aligned few-layer graphene is irradiated by X-ray of different doses from synchrotron radiation. The main structure of vertically aligned few-layer graphene is found to remain unchanged after X-ray irradiation both in high and low vacuum. X-ray photoelectron spectroscopy results reveal that the oxygen content in pristine and irradiated few-layer graphene remain unchanged after X-ray irradiation in high vacuum. When irradiated in low vacuum, the oxygen content in few-layer graphene increases with X-ray dose. For the vertically aligned few-layer graphene irradiated in low vacuum, the work function first increases and then slightly decreases with further increase irradiation dose from 9.0 × 1014 to 3.6 × 1015 phs/cm2. The field electron emission turn-on field is observed to increase from 4.0 to 9.1 MV/m after X-ray irradiation with the dose of 3.6 × 1015 phs/cm2. The mechanism of the changes in work function and field emission performance of vertically aligned few-layer graphene after X-ray irradiation in low vacuum are discussed.

The field emission properties of the screen-printed carbon nanotube (CNT) composite cathode have close relationship with its microstructure. In this study, carbon nanotube composite cold cathode with ZnO nano-particles as binding material was prepared using screen-printing method. Electric field cycles were used to post-treat the carbon nanotube composite cold cathode. During the process of electric field cycle treatment, obvious heat-induced damages were observed from the cathode. Scanning electron microscope and transmission electron microscope were employed to analyze the morphology and microstructure of the cathode. The possible mechanisms responsible for damages were discussed.

A carbon nanotube (CNT) composite cold cathode was studied for field emission display application. The CNT composite cold cathode was composed of CNTs and silicon dioxide binder. Field emission from CNT composite cold cathode with different CNT contents was studied. It was found that with increase in CNT contents, the threshold field decreased. The conductance of the composite cathode was measured and with increasing CNT content, there was a critical CNT content where the conductance increased several orders of magnitude. Plasma etching using SF6 as the etchant was adopted to treat the cathode. Improvement in emission uniformity was achieved. It was also found that after post-treatment the threshold field of the cathode decreased. The morphology of the etched cathode was analyzed and the improvement of uniformity and lowering of the threshold field was attributed to the exposure of CNTs after etching.

High density ZnO nanowire arrays were successfully synthesized without catalyst by direct oxidation of zinc substrate in air below 400 °C, lower than the melting point of zinc metal. The as-grown ZnO nanowires are single crystalline with a Wurzite structure extending in <110> direction. The diameters of the ZnO nanowires range from 20 to 150 nm and the lengths from several to over ten micrometers. Room temperature photoluminescence measurements reveal an intense ultraviolet emission at 397 nm. The present work highlights the promise of the low temperature, direct oxidation process in the high-yield synthesis of high quality semiconductor nanowire arrays for nano-devices.