Structure Of ZnO Nanowires Research Articles

Contact welding study of carbon nanotube with ZnO nanowire

Study of proton beam induced welding of multiwall carbon nanotubes (MWCNTs) with ZnO nanowires (NWs) has been carried out by proton (H +) beam irradiation. The samples were irradiated by 70-keV proton (H +) ion beams at different substrate temperatures. The irradiation-induced defects in CNTs and ZnO NWs were greatly reduced at elevated temperature. The crystalline structure of ZnO NWs and MWCNTs were found to remain stable after the irradiation at 700 K. As a preparation step, a coupling of two parallel ZnO NWs with irradiation has also been demonstrated. The welding mechanisms of MWCNTs and ZnO NWs were also been suggested. These two welding processes between same and distinct nanostructures to form homo- and hetero-junctions have provided an opportunity to mass produce interconnecting one-dimensional structures used for the manufacturing of future nanowire-based electronic circuits and devices.

Simple ZnO Nanowires Patterned Growth by Microcontact Printing for High Performance Field Emission Device

We present a novel and simple method for the patterned growth of ZnO nanowires (NWs) that combines (1) the direct patterning of ZnO nanoparticle (NP) seeds via microcontact printing and (2) subsequent low-temperature hydrothermal growth. The ZnO NPs can be patterned as seed layers for ZnO NW growth on various substrates including flexible polymer films. The NW geometry and configuration can be controlled by varying the printing conditions (time and pressure) and the hydrothermal reaction time. The “needleleaf-like” sharp-tipped ZnO NWs with a radially grown structure were examined at the pattern edges. To assess the possibility of high-performance electronic applications of the patterned ZnO NWs, their field emission characteristics were examined by fabricating a high-performance field emission device with a patterned ZnO NW array. The remarkable enhancement of the field emission properties is attributed to the minimized field emission screening that results from the radial ZnO NW structures and micropatt...

ZnO Nanowires, Nanotubes, and Complex Hierarchical Structures Obtained by Electrochemical Deposition

Increasing the aspect ratio of ZnO nanostructures is one possible strategy to improve their thermoelectric properties. ZnO nanostructures with one-dimensional (1D) and three-dimensional (3D) morphologies were obtained using electrochemical deposition. Adjusting various deposition parameters made it possible to obtain arrays of vertically aligned ZnO nanowires (NWs) with controlled dimensions, density, and electrical properties. The concentrations of zinc or chloride ions in the solution were found to be key parameters. ZnO NWs were transformed into ZnO nanotubes (NTs), with an increased aspect ratio compared with the NWs, by selectively dissolving the core of the ZnO NWs in a concentrated KCl solution. The aspect ratio was strongly increased when the ZnO NWs were hierarchically organized in a 3D morphology. The synthesis of thin films composed of ordered hollow urchin-like ZnO NW structures was performed by combining the electrochemical deposition and polystyrene sphere templating methods. The electronic properties of the urchin-like ZnO structures were investigated by means of photoluminescence and transmission measurements.

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.

Electronic structure and magnetism of Cr-doped ZnO nanowires

According to the spin-polarized density functional theory, we study the electronic structures, the magnetic and the optical properties of Cr-doped ZnO nanowires. The calculated results show ferromagnetic coupling for Cr atoms substitution for Zn atoms in ZnO nanowires along the [0001] direction, and the antiferromagnetic coupling with Cr-doped in ZnO nanowires along the [1010] and [0110] directions. The results reveal that the magnetic coupling state near the Fermi level gives rise to such a spin splitting phenomenon near the Fermi level, which indicates that Cr 3d and O 2p orbitals have intense hybrid effects. In addition, the spin electronic density results indicate that system magnetic moments are generated mainly by the unpaired 3d electrons of Cr atoms and are also related to the electron configuration. Moreover, the results of optical properties show that the obvious absorption peaks are observed in the far ultraviolet and the near ultraviolet regions and there is a red shift phenomenon in the ultraviolet region. These results indicate that the Cr-doped ZnO nanowires could be a promising dilute magnetic semiconductor material.

Electronic Structure and Magnetism of Ni-Doped ZnO Nanowires

Based on spin-polarized density functional theory we studied the electronic structures, magnetic and optical properties of Ni-doped ZnO nanowires. The magnetic results show that three magnetic coupling states are present: ferromagnetic (FM), antiferromagnetic (AFM), and paramagnetic (PM) states for the six kinds of Ni-doped configurations. The calculated energy results indicate that antiferromagnetic coupling is more stable when Ni atoms substitute for Zn atoms in the ZnO nanowires on the outside surface along the (0001) direction. AFM coupling has a metallic nature. The FM results from the density of states show that the spin polarization phenomenon appears near the Fermi level and causes strong hybridization between Ni 3d and O 2p. Moreover, the magnetic moments mainly originate from the unpaired electrons of the Ni 3d orbitals and the electrons of the O 2p orbitals contribute a little to the magnetic moments. The coupling of FM has a half-metal nature. In addition, the optical properties indicate that the absorption peaks show a significant red shift and good emission in the far UV band while a blue shift is apparent for the near UV band (380 nm). These results indicate that the Ni-doped ZnO nanowires are promising magneto-optical electronic materials and they can be used for nanoscale spintronics device materials.

PH-Dependent Toxicity of High Aspect Ratio ZnO Nanowires in Macrophages Due to Intracellular Dissolution

High-aspect ratio ZnO nanowires have become one of the most promising products in the nanosciences within the past few years with a multitude of applications at the interface of optics and electronics. The interaction of zinc with cells and organisms is complex, with both deficiency and excess causing severe effects. The emerging significance of zinc for many cellular processes makes it imperative to investigate the biological safety of ZnO nanowires in order to guarantee their safe economic exploitation. In this study, ZnO nanowires were found to be toxic to human monocyte macrophages (HMMs) at similar concentrations as ZnCl(2). Confocal microscopy on live cells confirmed a rise in intracellular Zn(2+) concentrations prior to cell death. In vitro, ZnO nanowires dissolved very rapidly in a simulated body fluid of lysosomal pH, whereas they were comparatively stable at extracellular pH. Bright-field transmission electron microscopy (TEM) showed a rapid macrophage uptake of ZnO nanowire aggregates by phagocytosis. Nanowire dissolution occurred within membrane-bound compartments, triggered by the acidic pH of the lysosomes. ZnO nanowire dissolution was confirmed by scanning electron microscopy/energy-dispersive X-ray spectrometry. Deposition of electron-dense material throughout the ZnO nanowire structures observed by TEM could indicate adsorption of cellular components onto the wires or localized zinc-induced protein precipitation. Our study demonstrates that ZnO nanowire toxicity in HMMs is due to pH-triggered, intracellular release of ionic Zn(2+) rather than the high-aspect nature of the wires. Cell death had features of necrosis as well as apoptosis, with mitochondria displaying severe structural changes. The implications of these findings for the application of ZnO nanowires are discussed.

First‐principles calculations of atomic and electronic properties of ZnO nanostructures

AbstractWe employ density‐functional theory within the generalized‐gradient approximation to investigate the formation energies and atomic and electronic structure of ZnO nanowires and nanotubes (NTs). We find that relaxations on the facets of the bare wires are very similar to those in nonpolar $(10{\bar {1}}0)$ surfaces and play an important role in stabilizing the nanowires. All bare wires are found to be semiconducting, with band gaps larger than that in bulk ZnO. We further investigated hydrogen and water adsorption on ZnO nanowires. We find that the electronic structure of ZnO nanowires can be tuned by hydrogen adsorption and that adsorption of water leads to dissociation of a half‐monolayer. Next, the stability of ZnO NTs has been investigated. We show that multiwall NTs are more stable than single‐walled tubes. Finally, point defects in ZnO NTs have been investigated using spin‐polarized calculations. All calculations were shown to introduce defect levels in the band gap, thus changing the electronic structure of the NTs drastically.

Electrical field induced direct-to-indirect bandgap transition in ZnO nanowires

As a wide bandgap (3.37 eV) compound semiconductor with the exciton binding energy of 60 meV, ZnO nanowires have attracted considerable interest because of great potential applications in micro-optoelectronics and nano-optoelectronics. However, ZnO nanowires as basic units of microelectronic and optoelectronic nanodevices usually work in the circumstance of the electrical field. Therefore, the influence of electrical field on the electronic structure of ZnO nanowires partly determines the performance of nanodevices. For this issue, we inhere have studied the effects of the applied electrical field on the electronic structure of ZnO nanowires using the density functional theory. It was found that the applied electrical field cannot only effectively modulate the bandgap of ZnO nanowires but also leads to the bandgap transition from direct to indirect. Meanwhile, the above effects of electrical field depend on the size and shape of ZnO nanowires. The physical understanding of the electrical field effects of the energy band structure of ZnO nanowires is attributed to the bond length changes in lattice and the charge transfer between Zn and O atoms. Thus, these valuable investigations are important for designing the microelectronic and optoelectronic nanodevices based on ZnO nanowires.

High-resolution TEM characterization of ZnO core-shell nanowires for dye-sensitized solar cells

Recently ZnO nanowire films have been used in very promising and inexpensive dye-sensitized solar cells (DSSC). It was found that the performance of the devices can be enhanced by functionalising the nanowires with a thin metal oxide coating. This nm-scale shell is believed to tailor the electronic structure of the nanowire, and help the absorption of the dye. Core-shell ZnO nanowire structures are synthesised at low temperature (below 120°C) by consecutive hydrothermal growth steps. Different materials are investigated for the coating, including Mg, Al, Cs and Zr oxides. High resolution TEM is used to characterise the quality of both the nanowire core and the shell, and to monitor the thickness and the degree of crystallisation of the oxide coating. The interface between the nanowire core and the outer shell is investigated in order to understand the adhesion of the coating, and give valuable feedback for the synthesis process. Nanowire films are packaged into dye-sensitised solar cell prototypes; samples coated with ZrO2 and MgO show the largest enhancement in the photocurrent and open-circuit voltage and look very promising for further improvement.

Synthesis of ZnO compound nanostructures via a chemical route for photovoltaic applications

A facile, low-temperature, and low-cost chemical route has been developed to prepare ZnO nanowire and nanosphere compound structures. The morphology, structure, and composition of the yielded products have been examined by field-emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction measurements. We have systematically investigated the optical properties of the ZnO nanostructures by micro-Raman, photoluminescence, and transmission spectroscopy. The results demonstrate that the yielded ZnO nanostructures possess good optical quality with high light absorption. We have further successfully employed the obtained ZnO compound nanostructures in dye-sensitized solar cells. The light-to-electricity conversion results show that the compound nanostructure exhibits a significant enhancement of short-circuit current density due to the increased surface area and light scattering in the compound nanostructures. The present chemical route provides a simple way to synthesize various compound nanostructures with high surface area for nanodevice applications.

ZnO Nanowires Synthesized by Vapor Phase Transport Deposition on Transparent Oxide Substrates.

Zinc oxide nanowires have been synthesized without using metal catalyst seed layers on fluorine-doped tin oxide (FTO) substrates by a modified vapor phase transport deposition process using a double-tube reactor. The unique reactor configuration creates a Zn-rich vapor environment that facilitates formation and growth of zinc oxide nanoparticles and wires (20–80 nm in diameter, up to 6 μm in length, density <40 nm apart) at substrate temperatures down to 300°C. Electron microscopy and other characterization techniques show nanowires with distinct morphologies when grown under different conditions. The effect of reaction parameters including reaction time, temperature, and carrier gas flow rate on the size, morphology, crystalline structure, and density of ZnO nanowires has been investigated. The nanowires grown by this method have a diameter, length, and density appropriate for use in fabricating hybrid polymer/metal oxide nanostructure solar cells. For example, it is preferable to have nanowires no more than 40 nm apart to minimize exciton recombination in polymer solar cells.

Uniaxial strain modulated band gap of ZnO nanostructures

Influence of the uniaxial strain on the electronic structures of ZnO nanowires (NWs) and nanotubes (NTs) has been studied using the density functional theory. It was found that the uniaxial compress strain can lead to a band gap transition from direct to indirect band for both ZnO NWs and NTs. Under the same strain, the phase transition of the thick NW is easier than that of the thin one, and the phase transition of NWs seems easier than that of NTs. These results suggested the uniaxial strain can be used to tune the band structures of ZnO nanostructures, which may be help for design of ZnO-based nanodevices.

Tuning Electronic Structures of ZnO Nanowires by Surface Functionalization: A First-Principles Study

Using first-principles calculations, we systematically investigated electronic structures of ZnO nanowires of various sizes modified with different surface coverages of H, F, Cl, NH2, and NH3. We found that the 50% H(O), 50% F(Zn); 100% F; 50% H(O), 50% Cl(Zn); and 50% NH3(Zn) passivations are energetically very favorable compared with 100% H and 100% Cl passivations. The surface chemistry involved presents a strong effect on the band structure of ZnO nanowires as significant as that of quantum confinement. The 100% F passivation of the surfaces of ZnO nanowires leads to a decrease in the band gap, whereas the 100% H, 50% H(O), 50% F(Zn); 50% H(O), 50% Cl(Zn); 100% Cl; 50% H(O), 50% NH2(Zn); and 50% NH3(Zn) passivations increase the band gap when compared with the bare wires. Like the size effect, the surface passivation is an additional option to engineer electronic properties of ZnO nanowires.

Growth, structure, and cathodeluminescence of Eu-doped ZnO nanowires prepared by high-temperature and high-pressure pulsed-laser deposition

Eu-doped ZnO nanowires have been prepared by the high-temperature and high-pressure pulsed-laser deposition, and the structure and cathodeluminescence (CL) of the as-prepared nanostructures were characterized. It was found that the alloying catalyst droplets are located at the top of the as-prepared Eu-doped ZnO nanowires, meaning that the Eu-doped ZnO nanowires growth is a typical vapor-liquid-solid process. X-ray photoelectron spectra of samples provided the experimental evidence of the Eu-doping in ZnO nanowires. Two peaks near 611 and 755 nm, respectively, are identified to be from the doped Eu in the CL spectra of samples.

Effect of growth temperature on the ZnO nanowires prepared by thermal heating of Zn powders

ZnO nanowires have been synthesized by heating Zn powders under nitrogen (N 2) gas atmosphere. The influence of the growth temperature on the morphology, structure, and photoluminescence (PL) properties of ZnO nanowires has been investigated. At the higher-temperature growth process, thinner nanowires have been obtained. Interestingly, it is observed that the variation of growth temperature has significantly affected the photoluminescence spectra of the ZnO nanowires, showing an enhancement in the relative intensity of the green to UV emission bands with the increase of the growth temperature. In addition, the oxygen sensing properties of the as-synthesized ZnO nanowires were tested.

Self-assembled tin-doped ZnO nanowire and nanoplate structures grown by thermal treatment of ZnS powder

Sintering of a ZnS–SnO 2 mixture under argon flow leads to the growth of columnar nanoplate arrays as well as arrays of nanowires, nanorods and nanoplates with six-fold symmetry. The six-fold nanoplate structures correspond to a more advanced stage of growth than the nanowire structures. Cathodoluminescence (CL) in the scanning electron microscope (SEM) shows that the structures contain Sn, but the amount of this element is normally under the detection limit of X-ray energy-dispersive spectroscopy (EDS). The formation of branches in the hierarchical structures depends on the presence of Sn and on defects in the mixture powder.

Modification of structure and optical property of ZnO nanowires by Ga ion beam

AbstractBased on the focused ion beam (FIB) technology, we have prepared ZnO nanowires containing periodic nano-sized structures by an ultra thin Ga ion beam. ZnO nanowires can keep a good crystal quality after Ga ion bombardment. The cathodoluminescence (CL) spectroscopy study of the Ga-doped ZnO nanowires at low temperatures shows that the Ga doping effect can largely suppress the green emission that may mainly originate from the defects on the surfaces of ZnO nanowires.

Field emission properties of ZnO nanowires synthesized by thermal decomposition process

Vertical ZnO nanowires were uniformly fabricated on a tin-doped indium oxide (ITO) glass substrate by low-temperature thermal decomposition. The morphology and structure of ZnO nanowires were analysed by scanning electron microscopy, x-ray diffraction (XRD) and transmission electron microscopy (TEM). The diameter and length of the ZnO nanowires were about 30–50 nm and 5 µm, respectively. The XRD and TEM investigations show that the ZnO nanowires were single crystals with growth direction along the c-axis of the crystal plane. The turn-on field of the ZnO nanowires grown at 300 °C was found to be about 3.5 V µm−1 at a current density of 10 µA cm−2. The emission current density of the ZnO nanowires grown at 300 °C reached 1 mA cm−2 at an applied field of 7.0 V µm−1.

Structure and optical properties of ZnO nanowires fabricated by pulsed laser deposition on GaN/Si(111) films with the use of Au and NiO catalysts

The growth of ZnO nanowires on a Si(111) substrate with a GaN buffer layer by pulsed laser deposition at a high argon pressure and with the use of Au and NiO catalysts has been investigated. Application of the low-temperature NiO catalyst makes it possible to reduce the optimal growth temperature from 900°C (in the case of the Au catalyst) to T = 570°C and grow ZnO nanowires 20 nm in diameter, which are highly-oriented in the direction of the c lattice axis and completely free of dislocations.