2D planar field emission devices based on individual ZnO nanowires

2D planar field emission devices based on individual ZnO nanowires were achieved on Si/SiO₂ substrate via a standard e-beam lithography method. The anode, cathode and ZnO nanowires were on the same substrate; so the electron field emission is changed to 2D. Using e-beam lithography, the emitter (cathode) to anode distance could be precisely controlled. Real time, in situ observation of the planar field emission was realized in a scanning electron microscope. For individual ZnO nanowires, an onset voltage of 200 V was obtained at 1 nA. This innovative approach provides a viable and practical methodology to directly implement into the integrated field emission electrical devices for achieving “on-chip” fabrication.     

Field emission characteristics study for ZnO/Ag and ZnO/CNTs composites produced by DC electrophoresis

A simple controllable method is reported for the coating of ZnO nanowires with Ag nanoparticles and ZnO/carbon nanotubes (CNTs) composite. It has been achieved through DC electrophoresis AgNO₃ electrolyte and CNTs in the presence of isopropanol dispersion of ZnO nanowires. In the present work, the influence of Ag nanoparticles and CNTs on the field emission properties of the composite materials is studied. The results of this research demonstrate a remarkable enhancement of field emission current of ZnO nanowires in case of CNTs mixture and Ag nanoparticles coating.     

Enhanced near-band-edge emission and field emission properties from plasma treated ZnO nanowires

Near-band-edge emission in photoluminescence of ZnO nanowires was found to be significantly improved after plasma treatment. The ratio of ultraviolet emission peak intensity before and after plasma treatment is as high as 3.5. Field emission properties were considerably enhanced after plasma treatment as well. Current emission density has been increased two orders of magnitude under the same electric field. Passivation of surface states and surface morphology change were found to be responsible for such an effective improvement. Our results suggest that the plasma treatment method is effective in enhancing both the near-band-edge emission in photoluminescence and field electron emission performance from ZnO nanowires.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.     

Non-catalytic growth of high aspect-ratio ZnO nanowires by thermal evaporation

High-density and high aspect-ratio ZnO nanowires were grown on Si(100) substrates by the thermal evaporation of metallic zinc powder without the use of metal catalysts or additives. The as-grown nanowires had diameters in the range of 60-100 nm with lengths 5-15 μm. Detailed structural characterization indicated that the obtained nanowires are single-crystalline with a perfect hexagonal facet and surfaces. The room temperature PL spectrum exhibited strong UV emission, affirming that the as-grown products have good optical properties. The possible growth mechanism for the formation of hexagonal-faceted and perfect surface ZnO nanowires is also discussed.     

Atomic layer deposition coating of ZnO shell for GaN-ZnO core-sheath heteronanowires

We have synthesized GaN-core/ZnO-shell nanowires and investigated effects of the ZnO coating. The X-ray diffraction pattern showed that as-synthesized samples are composed of GaN and ZnO. Transmission electron microscopy indicated that the deposited ZnO shell layer is poly-crystalline. The photoluminescence (PL) spectrum of GaN has been changed by the ZnO coating, where emission bands centered at roughly 1.9 eV, 2.5 eV, and 3.3 eV were newly added to the emissions from core GaN nanowires. We found that overall PL intensity has been significantly increased by coating the ZnO shell layers.     

Photoluminescence of ZnO nanowires grown on sapphire (1 1 2¯ 0) substrates

ZnO nanowires were fabricated on c-plane (0 0 0 1), a-plane (1 1 2¯ 0) sapphire, and boron doped p-type (1 0 0) Si substrates in vacuum furnace by simple physical vapor deposition. Room temperature photoluminescence spectra of the nanowires show the near band-edge emission and the deep-level green light emission. The ZnO nanowires formed on sapphire (1 1 2¯ 0) substrates exhibited enhancement on optical properties and better crystalline structures than those of nanowires grown on other substrates. The formation mechanism and the effect of substrate direction on structural and optical properties of the nanowires are discussed.     

Non-volatile memory characteristics of polyimide layers embedded with ZnO nanowires

We fabricated 8 × 8 cross-bar array-type organic non-volatile memory devices of polyimide (PI) layers embedded with ZnO nanowires. The ZnO nanowires were synthesized by chemical vapor deposition and deposited into the PI layers by a solution coating process. The devices of PI layer without ZnO nanowires showed an insulating characteristic without exhibiting any memory behavior. The ZnO nanowires acted as carrier trapping sites in the insulating PI layers for our memory devices. The organic memory devices exhibited write-once-read-many-times-type non-volatile memory characteristics with an excellent ON/OFF switching ratio over 10⁶, good uniformity in cumulative probability, and stability without serious degradation over 10⁴ s.     

Synthesis and surface modification of ZnO:Cu nanoparticles by silica and PMMA

ZnO nanoparticles doped with Cu were synthesized by solid state reaction using different precursor routes and varying growth environment. Average crystallite size varied from 40 to 100 nm depending upon synthesis temperature, lower temperature favouring smaller particle size. Scanning electron microscope (SEM) images showed that particles synthesized at 250 °C were in the shape of nanorods but those synthesized at 900 °C had spherical shape. Luminescence emission showed marked dependence on the growth conditions varying from ultraviolet (UV) emission to green emission. For making the luminescent nanoparticles bio-compatible, a bioinorganic interface on ZnO:Cu nanoparticles was created by coating them with inert silica. Surface modification of ZnO:Cu was also done with lipophilic polymethylmethacrylate (PMMA). ZnO:Cu nanoparticles showed hexagonal wurtzite structure and the coating of silica was confirmed with the presence of two extra peaks due to silica in the XRD spectra. Thermogravimetric analysis (TGA) and FTIR spectroscopy indicated that PMMA molecules were adsorbed on the surface of ZnO:Cu nanoparticles. SEM images revealed that PMMA adsorption improved the dispersibilty of ZnO:Cu nanoparticles.     

Fabrication of ZnO nanowires through thermal heating of metallic Zn by solar energy

ZnO nanowires were synthesized in a short time of a few seconds through a simple thermal evaporation of Zn powder using solar energy under air atmosphere. The Zn powder was heated by focusing sunlight on the Zn powder employing a magnifying lens. This strategy heated Zn to its evaporation temperature resulting in its oxidation in air. This procedure formed ZnO nanowires of ∼10 nm diameter and ∼2 μm length. As only Zn powder without any catalysts was used as the source material, it is suggested that the growth of the nanowires occurs through a vapor-solid mechanism. The cathodoluminescence (CL) spectrum from such ZnO nanowires showed strong ultraviolet emission indicating their highly crystalline quality besides good optical properties.     

Growth,structure, and cathodoluminescence of Dy-doped ZnO nanowires

Dy-doped ZnO nanowires have been prepared using high-temperature and high-pressure pulsed-laser deposition. The morphology, structure, and composition of the as-prepared nanostructures are characterized by field emission scanning electron microscopy, X-ray diffraction, Raman scattering spectrometry, X-ray photoelectron spectrometry, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The alloying droplets are located at the top of the as-prepared Dy-doped ZnO nanowires, which means that the growth of the Dy-doped ZnO nanowires is a typical vapor-liquid-solid process. The luminescence properties of Dy-doped ZnO nanowires are characterized by cathodoluminescence spectra and photoluminescence spectra at low temperature (8 K). Two peaks at 481 and 583 nm, respectively, are identified to be from the doped Dy³⁺ ions in the CL spectra of Dy-doped ZnO nanowires.     

Influence of metallic coatings on the photoluminescence properties of ZnO nanowires

We report on low‐temperature photoluminescence studies of ZnO nanowires coated with thin metallic films. For all analyzed metals (Al, In, Au, Ni, Cu), we find an increased relative intensity of the green deep‐level emission. This is accompanied by a significant reduction of the relative intensity of the surface exciton band. The observed effects are most likely related to the formation of metal induced gap states in the surface region of the ZnO nanowires. A model for the band structure in the surface region of the metal‐coated nanowires is proposed that successfully explains the changes in the photoluminescence spectra after the coating process. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W

Nanocrystalline ZnO thin films have been deposited on rhenium and tungsten pointed and flat substrates by pulsed laser deposition method. An emission current of 1 nA with an onset voltage of 120 V was observed repeatedly and maximum current density ∼1.3 A/cm² and 9.3 mA/cm² has been drawn from ZnO/Re and ZnO/W pointed emitters at an applied voltage of 12.8 and 14 kV, respectively. In case of planar emitters (ZnO deposited on flat substrates), the onset field required to draw 1 nA emission current is observed to be 0.87 and 1.2 V/μm for ZnO/Re and ZnO/W planar emitters, respectively. The Fowler–Nordheim plots of both the emitters show nonlinear behaviour, typical for a semiconducting field emitter. The field enhancement factor β is estimated to be ∼2.15×10⁵ cm⁻¹ and 2.16×10⁵ cm⁻¹ for pointed and 3.2×10⁴ and 1.74×10⁴ for planar ZnO/Re and ZnO/W emitters, respectively. The high value of β factor suggests that the emission is from the nanometric features of the emitter surface. The emission current–time plots exhibit good stability of emission current over a period of more than three hours. The post field emission surface morphology studies show no significant deterioration of the emitter surface indicating that the ZnO thin film has a very strong adherence to both the substrates and exhibits a remarkable structural stability against high-field-induced mechanical stresses and ion bombardment. The results reveal that PLD offers unprecedented advantages in fabricating the ZnO field emitters for practical applications in field-emission-based electron sources.     

Synthesis of Na-doped ZnO nanowires and their photocatalytic properties

Na-doped ZnO nanowires with an average diameter of ∼40 nm have been fabricated by a thermal decomposition route at temperature around 400 °C. Their properties have been investigated using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), UV-visible spectroscopy, Raman spectra, and photoluminescence (PL) spectroscopy. Room temperature photoluminescence (RT-PL) showed that the as-synthesized ZnO samples exhibited strong visible emission with a major peak at 420 nm. Furthermore, intensity of the visible emission increased and then decreased with increase in Na concentration. The improvement of visible emission at 420 nm in the Na-doped ZnO samples should be a result of the surface defects increased by doping of Na in zinc oxide. In addition, photocatalytic studies indicated that these nanomaterials showed good photocatalytic performance for organic pollutants in water.     

Influence of Sb on a controlled-growth of aligned ZnO nanowires in nanoparticle-assisted pulsed-laser deposition

Single-crystalline ZnO nanowires on a sapphire substrate have been synthesized by a nanoparticle-assisted pulsed-laser deposition (NAPLD) using a pure and Sb₂O₃ doped ZnO target. Low density and vertically well-aligned ZnO nanowires were grown on hexagonal cone-shape ZnO cores by introduction of a ZnO buffer layer. More than 90% of the ZnO cores of the Sb-induced ZnO nanowires are formed in the same size of 400 nm. The ZnO nanowires consist of single-crystalline wurtzite ZnO crystal and grow along [0001] direction. The room-temperature photoluminescence spectrum exhibited a strong ultraviolet emission at around 380 nm and a relatively low broad band emission in the visible region, indicating a low concentration of structural defect in the nanowires. Sb can be used as one of the effective additives to control the morphology and alignment of ZnO nanowires synthesized by NAPLD.     

Ag/ZnO纳米结构的荧光增强效应

利用化学合成方法制备了Ag纳米线和ZnO量子点。对这两种纳米结构的表面形貌、晶体结构和光学性质分别进行了研究。结果表明：Ag纳米线和ZnO量子点均为单晶结构,平均直径分别为160 nm和5 nm左右。将Ag纳米线混入ZnO量子点可以使其紫外荧光显著增强,其中位于345 nm和383 nm 的荧光分别增强30倍和12倍。这与Ag纳米线和ZnO量子点混合体系的局域表面等离子体共振耦合吸收峰位相一致,说明该体系存在两种共振耦合模式。该研究结果为将来开发ZnO基纳米发光器件提供了一条新的途径。     

Ammonia sensing characteristics of ZnO nanowires studied by quartz crystal microbalance

The ZnO nanowires have been prepared and studied as the sensing element for the detection of ammonia. The ZnO nanowires were first synthesized by evaporating high purity zinc pellets at 900 °C and then distributed onto the electrode surfaces of quartz crystals at room temperatures. Gas sensitive properties of ZnO nanowires layer were studied in terms of the quartz crystal microbalance (QCM) at room temperature. It is found that the obtained response of the sensors varied with the thickness of the ZnO nanowires layer. ZnO nanowires showed high sensitivity to ammonia in the range of 40-1000 ppm. The response time of the sensor was as fast as ∼5 s at any concentration (40-1000 ppm) of ammonia gas. The ZnO nanowires-coated sensors have a good frequency stability and reproducibility. All results demonstrated that the ZnO nanowire was a potential gas sensing material for practical use.     

聚苯胺/ZnO纳米线薄膜材料作为发光层的柔性光电器件 及其发光机理

综合氧化锌纳米线(ZnO NWs)的光学活性与聚苯胺(PANI)的空穴传输特性,设计并制备了一种聚合物/ZnO纳米线电致发光材料,并对其发光特性进行了研究。通过高分子络合软模板法,将有序的单晶ZnO NWs均匀生长在覆有铟锡氧化物(ITO)涂层的柔性聚乙烯对苯二甲酸乙二醇酯(PET)衬底上并嵌入PANI薄膜,获得了电致发光薄膜材料和有机/无机异质结实验器件ITO/(ZnO NWs-PANI)。有机/无机异质结器件电致发光可调,在相对低的开启电压下呈现室温蓝紫外发光,并且ZnO NWs表面覆盖PANI增加了蓝紫外发光的强度和稳定性;而无PANI的ZnO NWs阵列具有450 nm处的缺陷发射峰,这可能是电子从扩展态锌间隙Zn_i到价带的跃迁引起的。这些结果表明,基于PANI/ZnO纳米线的复合材料在柔性光电器件方面的应用极具潜力。     

Synthesis of ZnO nanowires by pulsed laser deposition in furnace

ZnO nanowires were fabricated on Au coated (0 0 0 1) sapphire substrates by using a pulsed Nd:YAG laser with a ZnO target in furnace. ZnO nanowires have various sizes and shapes with a different substrate position inside a furnace. The length and the diameter of these ZnO nanowires were around 3-4 μm and 120-200 nm, respectively, confirmed by scanning electron microscopy (SEM). The diameter control of the nanowires was achieved by varying the position of substrates. The ultraviolet emission of nanowires from the near band-edge emission (NBE) was observed at room temperature. The formation mechanism and the effect of different position of substrates on the structural and optical properties of ZnO nanowires are discussed.     

Tribological characteristics of ZnO nanowires investigated by atomic force microscope

Zinc oxide (ZnO) nanowires have attracted great interest in nanodevices. In this work, the tribological characteristics of vertically grown ZnO nanowires obtained by metalorganic chemical vapor deposition were investigated by using an atomic force microscope (AFM). The ZnO nanowires were slid against flattened silicon and diamond-coated AFM probes under 50–150 nN normal force while monitoring the frictional force. The wear of the ZnO nanowires was observed by a scanning electron microscope and quantified based on Archard’s wear law. Also, the wear debris accumulated on the silicon probe was analyzed by using a transmission electron microscope (TEM). The results showed that the wear of ZnO nanowires slid against the silicon probe was extremely small. However, when the ZnO nanowires were slid against the diamond-coated probe, the wear coefficients ranged from 0.006 to 0.162, which correspond to the range of severe wear at the macroscale. It was also shown that the friction coefficient decreased from 0.30 to 0.25 as the sliding cycles increased. From TEM observation, it was found that the ZnO wear debris was mainly amorphous in structure. Also, crystalline ZnO nanoparticles were observed among the wear debris. PACS  07.79.Lh; 46.55.+d; 81.07.Bc