Single-crystalline ZnO Nanowires Research Articles

Electrical transport properties of ZnO nanofibers

ZnO nanofibers were synthesized by the electrospinning method. Their electrical transport properties were investigated by fabricating field effect transistors based on single ZnO nanofiber. The electron concentration (n) and the field-effect mobility (μe) in the ZnO nanofibers are not so sensitive to the calcination atmosphere; O2 and N2. Importantly, n (μe) in the ZnO nanofibers is bigger (much smaller) than those in single-crystalline ZnO nanowires. The existence of nanosized gains in the ZnO nanofibers is likely to be responsible for such big differences in n and μe. Huge grain boundaries created by nanograins seem to play the role of a source providing electrons and a barrier for electrons to flow as well. To enhance the μe, the calcination condition to obtain ZnO nanofibers with larger grains needs to be applied.

Gold Nanoparticles Embedded Single Crystalline ZnO (Au NPs@ZnO) Nanowire Arrays for Plasmonic Enhanced Dye-sensitized Solar Cells (DSSCs)

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Growth of High Quality ZnO Nanowires on Graphene

We report growth of the ZnO nanowires on graphene/SiO2/Si substrates using a chemical vapor deposition method. The length of nanowires varies from 1 microm to 10 microm with increasing the growth time from 30 min to 90 min. X-ray diffraction and high-resolution transmission electron microscopy investigations predict the high structural quality of the c-axis grown single crystalline ZnO nanowires. Temperature dependent photoluminescence spectra from the nanowires reveal excellent optical quality and excitonic behavior in the single crystalline ZnO nanowires. A well-resolved free exciton emission at 3.375 eV, indicates high crystalline quality nanowires and a strong PL peak at 3.370 eV is assigned to neutral-donor bound excitons (D0X).

Giant optical anisotropy of oblique-aligned ZnO nanowire arrays

A combined method of modified oblique-angle deposition and hydrothermal growth was adopted to grow an optically anisotropic nanomaterial based on single crystalline ZnO nanowire arrays (NWAs) with highly oblique angles (75°-85°), exhibiting giant in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of natural quartz. The strong optical anisotropy in emission due to the optical confinement was observed. The oblique-aligned NWAs not only allow important technological applications in passive photonic components but also benefit the development of the optoelectronic devices in polarized light sensing and emission.

Facilitating ZnO nanostructure growths by making seeds for self-catalytic reactions

Long and straight single-crystalline ZnO nanowires were successfully synthesized on ZnCl2-coated Zn foils in oxygen environment by using simple thermal annealing processes. With relatively low reaction temperatures (410 and 700°C), nanowires whose lengths and diameters are up to ∼50μm and 10–100nm were obtained. We found that ZnO seeds created from ZnCl2 played an important role in facilitating the ZnO nanowire growth via self-catalytic reactions. Systematic studies by altering critical synthesis factors that determine shape, length, diameter, and density of the nanowires were performed in order to unveil the growth mechanisms. We also compared the nanowires synthesized from Zn foils with tetrapod ZnO nanostructures synthesized from Zn powders at various temperatures.

Field-Emission Characteristics of Zinc Oxide Nanowires Using Low-Temperature Supercritical Carbon Dioxide Fluid Method

Single-crystalline ZnO nanowires (NWs) were directly grown on Zn/glass substrates by using a low-temperature (i.e., 40 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C) supercritical carbon dioxide (SCCO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) fluid method. The optical, physical, and field-emission (FE) characteristics of the SCCO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -synthesized ZnO nanostructures are systematically investigated. ZnO NWs exhibited the low turn-on field of 3.16 V/μm at a current density of 10 μA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a low threshold field of 4.38 V/μm at a current density of 1 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The current fluctuation of ZnO emitters was less than 8% at 5.3 V/μm in 12 h. The excellent FE properties of SCCO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -synthesized ZnO emitters at low temperature make them a superior candidate for FE-based display devices.

Synthesis of Porous Single Crystalline ZnO Nanowires and the Derivation of Surface Free Energy from Equilibrium Nanopore

Single crystalline ZnO nanowires were grown using standard hydrothermal growth process and subsequently closed nanopores were obtained by post-annealing in various conditions. The pore size, shape and spatial distribution are studied with various temperatures, time and gas (Ar and H2) mixture combinations during the annealing. The annealing treatment with Ar only at elevated temperatures seems to produce more thermodynamically stable closed pores. From the thermodynamically equilibrium pores with average size of 12 nm, the surface free energy of ZnO{10−13} is determined as ∼0.99 Jm−2 experimentally by “inverse Wulff's construction”.

Effect of growth time on morphology and photovoltaic properties of ZnO nanowire array films

Single-crystalline ZnO nanowire arrays with different aspect ratios and nanowire densities were prepared by the hydrothermal growing method using polyethyleneimine (PEI) as a surfactant. PEI can only hinder the lateral growth of the ZnO nanowires, which is observed by high resolution transmission electron microscopy (HRTEM) analysis. Dye-sensitized solar cells were assembled by the ZnO nanowire arrays with different thicknesses, which can be controlled by the growing time and characterized using photocurrent-voltage measurements. Their photocurrent densities and energy allover conversion efficiencies increased with increasing ZnO nanowire lengths. Short-circuit current density of 4.31 mA·cm−2 and allover energy conversion efficiency of 0.87% were achieved with 12.9-μm-long ZnO nanowire arrays.

Top-Gate ZnO Nanowire Transistors and Integrated Circuits with Ultrathin Self-Assembled Monolayer Gate Dielectric

A novel approach for the fabrication of transistors and circuits based on individual single-crystalline ZnO nanowires synthesized by a low-temperature hydrothermal method is reported. The gate dielectric of these transistors is a self-assembled monolayer that has a thickness of 2 nm and efficiently isolates the ZnO nanowire from the top-gate electrodes. Inverters fabricated on a single ZnO nanowire operate with frequencies up to 1 MHz. Compared with metal-semiconductor field-effect transistors, in which the isolation of the gate electrode from the carrier channel relies solely on the depletion layer in the semiconductor, the self-assembled monolayer dielectric leads to a reduction of the gate current by more than 3 orders of magnitude.

Position-controlled vertical arrays of single-crystalline ZnO nanowires on periodically polarity inverted templates

Position controlled single crystalline ZnO nanowires are grown on periodically polarity-inverted (PPI) ZnO heterostructures without catalyst. PPI ZnO templates were fabricated by using selective growth method of polarity with Cr-compound intermediate layers by plasma assisted molecular beam epitaxy. In order to control the position, we used the in-plane two-dimensionally discrete PPI ZnO templates. The lateral polarity inversion in PPI template was confirmed by piezo response microscopy. After syntheses of ZnO nanowires, vertically aligned ZnO nanowires were grown only onto the Zn-polar regions without the O-polar regions. The results clearly show that the position control of ZnO nanowires is possible using the PPI ZnO templates.

Electrically pumped near-ultraviolet lasing from ZnO/MgO core/shell nanowires

Electrically pumped near-ultraviolet lasing was achieved in a metal/insulator/semiconductor laser diode based on ZnO/MgO core/shell nanowires. The nanowire diode shows higher emission intensity at relatively low operating current density compared with the planar device. The improved efficiency is attributed to enhanced exciton oscillator strength and superior carrier transport properties of single-crystalline ZnO nanowires, and effective surface passivation by MgO coating. Random laser action was confirmed by the calculation of quality factor and the real-time changes of lasing spectra. The results reveal that the MgO coating serves as electron blocking, hole supplying and surface passivation layer for the nanowire heterostructure.

Quantifying the defect-dominated size effect of fracture strain in single crystalline ZnO nanowires

The diameter (D) dependence of fracture strains in [0001]-oriented single crystalline ZnO nanowires (NWs) with D ranging from 18 to 114 nm is experimentally revealed via in situ uniaxial tension and is well understood based on an analytical model developed by combining molecular dynamics simulations with fracture mechanics theories. We show that the scattered fracture strains are dominated by the effective quantities of atomic vacancies, and their lower bound follows a power-form scaling law, resembling the Griffith-type behavior of single critical defects with diameter-dependent sizes, when D is larger than a critical DC. In addition, theoretical strength is expected in NWs with D &amp;lt; DC. Our studies provide a simple, but basic, understanding for the size effect of strengths in single crystalline NWs.

Size-controlled growth of ZnO nanowires by catalyst-free high-pressure pulsed laser deposition and their optical properties

Single crystalline ZnO nanowires were fabricated on Si (100) substrates by catalyst-free high-pressure pulsed laser deposition. It is found that the nanowires start to form when the substrate temperature and growth pressure exceed the critical values of 700 oC and 700 Pa, and their size strongly depends on these growth conditions. That is, the aspect ratio of the nanowires decreases with increasing temperature or decreasing pressure. Such a size dependence on growth conditions was discussed in terms of surface migration and scattering of ablated atoms. Room-temperature photoluminescence spectrum of ZnO nanowires shows a dominant near-band-edge emission peak at 3.28 eV and a visible emission band centered at 2.39 eV. Temperature-dependent photoluminescence studies reveal that the former consists of the acceptor-bound exciton and free exciton emissions; while the latter varies in intensity with the aspect ratio of the nanowires and is attributed to the surface-mediated deep level emission.

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.

Defect-dominated diameter dependence of fracture strength in single-crystalline ZnO nanowires:In situexperiments

Diameter ($D$) dependence of fracture strength in [0001]-oriented single-crystalline ZnO nanowires (NWs) with $D$ ranging from 18 to 114 nm is experimentally revealed via in situ uniaxial tension. The lower bound of the scattered strengths increases with decreasing $D$, following a modified power law, and critical defects that effectively dominate the strengths are attributed to the diameter-dependent quantities of discrete point defects in NWs, based on in situ cathodoluminescence spectra. As a result, the size-effect mechanism of fracture strength is well understood, accounting for a simple and basic case of single-crystalline NWs.

Nonvolatile resistive switching in single crystalline ZnO nanowires

We demonstrate nonvolatile resistive switching in single crystalline ZnO nanowires with high ON/OFF ratios and low threshold voltages. Unlike the mechanism of continuous metal filament formation along grain boundaries in polycrystalline films, the resistive switching in single crystalline ZnO nanowires is speculated to be induced by the formation of a metal island chain on the nanowire surface. Resistive memories based on bottom-up semiconductor nanowires hold potential for next generation ultra-dense nonvolatile memories.

ZnO-Nanowires/PANI Inorganic/Organic Heterostructure Light-Emitting Diode

In this paper, we report a flexible inorganic/organic heterostructure light-emitting diode, in which inorganic ZnO nanowires are the optically active components and organic polyaniline (PANI) is the hole-transporting layer. The fabrication of the hybrid LED is as follows, the ordered single-crystalline ZnO nanowires were uniformly distributed on flexible polyethylene terephthalate (PET)-based indium-tin-oxide-coated substrates by our polymer-assisted growth method, and proper materials were chosen as electrode and carrier. In this construction, an array of ZnO nanowires grown on PET substrate is successfully embedded in a polyaniline thin film. The performance of the hybrid device of organic-inorganic hetero-junction of ITO/(ZnO nanowires-PANI) for LED application in the blue and UV ranges are investigated, and tunable electroluminescence has been demonstrated by contacting the upper tips of ZnO nanowires and the PET substrate. The effect of surface capping with polyvinyl alcohol (PANI) on the photocarrier relaxation of the aqueous chemically grown ZnO nanowires has been investigated. The photoluminescence spectrum shows an enhanced ultraviolet emission and reduced defect-related emission in the capped ZnO NWs compared to bare ZnO. The results of our study may offer a fundamental understanding in the field of inorganic/organic heterostructure light-emitting diode, which may be useful for potential applications of hybrid ZnO nanowires with conductive polymers.

Single-Crystalline Zinc Oxide Nanowires as Photoanode Material for Dye-Sensitized Solar Cells

This study reports the use of single-crystalline and well-aligned ZnO nanowires as photoanode material for dye-sensitized solar cells. The ZnO nanowires are grown on fluorine-doped tin oxide coated glass substrates without catalysts by thermal evaporation. In spite of low roughness factors of around 25 for the nanowire photoanodes, the fabricated solar cells yield power conversion efficiencies of around 1.3% under AM 1.5G (100 mW cm-2) illumination. Moreover, fill factors of around 0.5 have been achieved and are relatively high when compared with reported values from ZnO nanowire photoanodes. The results reveal the advantage of using single-crystalline nanowires as photoanode material and provide clues for the advancement of nanowire based dye-sensitized solar cells.

High sensitivity of a ZnO nanowire-based ammonia gas sensor with Pt nano-particles

We report the growth of high-density single crystalline ZnO nanowires on a patterned ZnO:Ga/SiO 2/Si template, the adsorption of Pt nano-particles on the nanowire surface, and the fabrication of a ZnO nanowire-based NH 3 gas sensor. It was found that the sensor responses were 22.5% and 36% for the nanowires without and with Pt adsorption when the chamber was injected with 1000 ppm of NH 3 gas at 300 °C. With Pt adsorption, it was found that the measured sensitivities were around 16%, 22%, 26% and 36% when the concentration of the injected NH 3 gas was 100, 200, 500 and 1000 ppm, respectively.

Single‐Crystalline ZnO Nanowire Bundles: Synthesis, Mechanism and Their Application in Dielectric Composites

Single-crystalline ZnO nanowire bundles have been synthesized in large-scale by an improved microemulsion method in the presence of excessive hydrate hydrazine and dodecyl benzene sulfonic acid sodium salt (DBS) in xylene. The product is characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The result shows that the bundle is composed by many oriented single-crystalline ZnO nanowires with a length of about 1 mum and a diameter of about 20-30 nm. The influence of DBS, hydrazine and the reaction time on the morphology of final product and the formation mechanism of such nanostructures were discussed; the application in the dielectric composites is also studied.