Single Zinc Oxide Nanowires Research Articles

Fabricating AC/DC nanogenerators based on single ZnO nanowires by using a nanomanipulator in a scanning electron microscope

Single zinc oxide nanowires (ZnO NWs) are promising for nanogenerators because of their excellent semiconducting and piezoelectric properties, but characterizing the latter efficiently is challenging. As reported here, an electrical breakdown strategy was used to construct single ZnO NWs with a specific length. With the high operability of a nanomanipulator in a scanning electron microscope, ZnO-NW-based two-probe and three-probe structures were constructed for fabricating AC/DC nanogenerators, respectively. For a ZnO NW, an AC output of between −15.31 mV and 5.82 mV was achieved, while for a DC nanogenerator, an output of ∼24.3 mV was realized. Also, the three-probe structure’s output method was changed to verify the distribution of piezoelectric charges when a single ZnO NW is bent by a probe, and DC outputs of different amplitudes were achieved. This study provides a low-cost, highly convenient, and operational method for studying the AC/DC output characteristics of single NWs, which is beneficial for the further development of nanogenerators.

Tailoring nanowire lasing modes via coupling to metal gratings

Tailoring the emission of plasmonic nanowire-based lasers represents one of the major challenges in the field of nanoplasmonics, given the envisaged integration of such devices into on-chip all-optical circuits. In this study, we proposed a mode selection scheme based on distributed feedback, achieved via the external coupling of single zinc oxide nanowires to an aluminum grating, which enabled a quasi-single mode lasing action. The nano-manipulation of a single nanowire allowed for a reliable comparison of lasing emission characteristics in both planar (i.e., a nanowire on the metallic substrate) and on-grating configurations. We found that, by varying the orientation of the nanowire on the grating, only when the nano-cavity was perpendicular to the ridge direction, an additional peak emerged in the emission spectrum on the low-energy side of the gain envelope. As a consequence of the fulfillment of the Bragg condition, such a peak was attributed to a hybrid mode dominating the mode competition. Simulation results showed that the hybrid mode could be efficiently waveguided along the nanowire cavity and supported by localized plasmon polaritons building up at the raised features (“fences”) on top of metal grating ridges. Moreover, the hybrid mode was found to experience an extra reflectance of nearly 50% across the grating periods in addition to that provided by nanowire end facets.

Non-linear nanoscale piezoresponse of single ZnO nanowires affected by piezotronic effect

Zinc oxide (ZnO) nanowires (NWs) as semiconductor piezoelectric nanostructures have emerged as material of interest for applications in energy harvesting, photonics, sensing, biomedical science, actuators or spintronics. The expression for the piezoelectric properties in semiconductor materials is concealed by the screening effect of the available carriers and the piezotronic effect, leading to complex nanoscale piezoresponse signals. Here, we have developed a metal–semiconductor–metal model to simulate the piezoresponse of single ZnO NWs, demonstrating that the apparent non-linearity in the piezoelectric coefficient arises from the asymmetry created by the forward and reversed biased Schottky barriers at the semiconductor–metal junctions. By directly measuring the experimental I–V characteristics of ZnO NWs with conductive atomic force microscope together with the piezoelectric vertical coefficient by piezoresponse force microscopy, and comparing them with the numerical calculations for our model, effective piezoelectric coefficients in the range d 33eff ∼ 8.6 pm V−1–12.3 pm V−1 have been extracted for ZnO NWs. We have further demonstrated via simulations the dependence between the effective piezoelectric coefficient d 33eff and the geometry and physical dimensions of the NW (radius to length ratio), revealing that the higher d 33eff is obtained for thin and long NWs due to the tensor nature proportionality between electric fields and deformation in NW geometries. Moreover, the non-linearity of the piezoresponse also leads to multiharmonic electromechanical response observed at the second and higher harmonics that indeed is not restricted to piezoelectric semiconductor materials but can be generalized to any type of asymmetric voltage drops on a piezoelectric structure as well as leaky wide band-gap semiconductor ferroelectrics.

Mitigating the photocurrent persistence of single ZnO nanowires for low noise photodetection applications

In this work, we investigate the optoelectronic properties of zinc oxide (ZnO) nanowires, which are good candidates for applications based on integrated optics. Single ZnO nanowire photodetectors were fabricated with ohmic contacts. By taking current transient measurements in different atmospheres (oxygen, air, vacuum and argon), and at various temperatures, we point out the importance of surface effects on the electrical behaviour. Results confirm that oxygen chemisorption is responsible for the existence of a high photoconductive gain in these devices, and for the first time a two step process in the photocurrent rise transient is reported. A maximum gain of G = 7.8 × 107 is achieved. However, under certain conditions, the persistence of the photocurrent can last up to several hours and as such may prevent the device from operating at useful rates. From a knowledge of the photocurrent response mechanisms, we establish a method to restore the photodetector to its initial state, with very low dark current, by applying an appropriate gate voltage sequence. This advances the state of the art for these detectors towards commercial applications.

Hydrothermally Synthesized Ultrathin Zinc Oxide Nanowires Based Field-Effect Transistors

In this experiment, ultrathin zinc oxide (ZnO) nanowires-based field-effect transistors (FETs) were fabricated. The ZnO seed layer was deposited using RF sputtering on top of a silicon dioxide dielectric layer. After the deposition of Ti and Pt films on the ZnO seed layer, ZnO nanowires were laterally grown from the ZnO seed layer by hydrothermal method. We have shown that the ultrathin single crystal ZnO nanowires could connect the source and the drain electrodes under an appropriate precursor concentration. Besides these nanowires grew along the surface of the dielectric with good attachment. ZnO nanowire-based FET thus fabricated have a threshold voltage of around -2.04 V, a field-effect mobility of ~ 64.28 cm2 V-1 s-1, a sub-threshold swing of 4.006 V/dec, and a current on/off ratio as high as 1.01 x 104. Compared to the common ZnO nanowire-based FET, the close attachment of these nanowires to the gate dielectric decreases the distance between the gate electrode and the nanowire channels, thus enhancing the switching control of the fabricated FET.

Plasma enhanced multistate storage capability of single ZnO nanowire based memory

Multiple-state storage (MSS) is common for resistive random access memory, but the effects of plasma treatment on the MSS and the switching properties have been scarcely investigated. We have demonstrated a stable four-state storage capability of single zinc oxide nanowire (ZnO NW) treated by argon plasma. The electrical switching is attributed to the electron trapping and detrapping from the oxygen vacancies (Vos). The MSS relates to the electrical-thermal induced distribution of the Vos which determines electron transport behavior to show different resistance states. Additionally, programming (set and reset) voltages decrease with plasma treatment due to the thickness modulation of the interface barrier.

Ohmic contact on single ZnO nanowires grown by MOCVD

AbstractWe present work on the fabrication and the characterization of ohmic contacts on single zinc oxide nanowires grown by metalorganic chemical vapour deposition. We demonstrate pre‐positionning of single nanowires before electronic‐beam lithography for 2 point metallic contacts. We characterize single nanowires by microphotoluminescence and by current‐voltage measurements. I–V results present a linear curve down to 1 mV with resistivities from 0.23 to 2.4 Ω ċ cm are measured, consistent with previous observation on ZnO nanowires. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single ZnO nanowire–PZT optothermal field effect transistors

A new type of pyroelectric field effect transistor based on a composite consisting of single zinc oxide nanowire and lead zirconate titanate (ZnO NW–PZT) has been developed. Under infrared (IR) laser illumination, the transconductance of the ZnO NW can be modulated by optothermal gating. The drain current can be increased or decreased by IR illumination depending on the polarization orientation of the Pb(Zr0.3Ti0.7)O3 (PZT) substrate. Furthermore, by combining the photocurrent behavior in the UV range and the optothermal gating effect in the IR range, the wide spectrum of response of current by light offers a variety of opportunities for nanoscale optoelectronic devices.

Room temperature ultra-sensitive resistive humidity sensor based on single zinc oxide nanowire

A highly sensitive relative humidity (RH) sensor is fabricated by a low-cost and scalable dielectrophoresis method that places a single crystalline zinc oxide nanowire (ZnO NWs) between pre-patterned electrodes in a freestanding device structure. The resistive device demonstrates an exponential sensitivity of more than 5 decades to changes from dry air to 60% RH at room temperature, indebted to a subthreshold carrier modulation due to the high surface-to-volume-ratio and completely exposed NW surface that facilitates surface physisorption of water molecules. Transient and temperature-dependence measurements signify a reproducible sensing performance and an Arrhenius behavior with activation energy of 0.6eV, expected for these metal oxide semiconductors. These results demonstrate the potential of such nanoscale devices for accurate monitoring of RH in different applications, including fuel cells and automotive sensors, smart clothing, smart food packaging and sensor networks.

Synthesis and Nano-Processing of ZnO Nano-Crystals for Controlled Laser Action

ABSTRACTLasing characteristics of a single Zinc oxide (ZnO) nanosheet and a single ZnO nanowire were investigated by an ultraviolet light excitation. ZnO nanocrystals were synthesized by chemical vapor deposition (CVD) method, and those ZnO nanocrystals were excited by a third-harmonic Q-switched Nd:YAG laser beam (355 nm, 5 ns). The emission spectra from a single ZnO nanocrystal was collected by an objective lens with a magnification factor of 100 or 50, coupled with a spectrometer with a light fiber. The area observed by the spectrometer is about 10 μm in diameter, and therefore the emission spectra from a single ZnO nanocrystal can be observed. The emission spectra showed the obvious lasing characteristics having mode structure and a threshold for lasing. The lasing threshold power density of a ZnO nanosheet and a ZnO nanowire were measured to be about 60 kW/cm2 and 150 kW/cm2, respectively. ZnO nanosheet can be a superior laser medium due to the lower threshold for lasing compared to the threshold of the ZnO nanowire. However, since the lasing spectra had mode structure, a single-longitudinal mode lasing would be required for a practical application. The single longitudinal mode lasing can be realized by a nanomachining of a grating on the ZnO nanocrystal surface due to distributed bragg reflector (DBR) laser. The minimum DBR pitch was estimated to be about 81 nm, which can be machined by focused-ion beam (FIB) focused up to 7 nm at minimum, and therefore, we demonstrated the nanomachining on a single ZnO nanowire. However, the single-longitudinal mode lasing was not observed so far, and thus optimization of experimental conditions such as the DBR pitch, ion dose amount and increasing the number of repetition of DBR would be required.

Light–matter interaction and polarization of single ZnO nanowire lasers

The optical properties of single zinc oxide (ZnO) nanolasers are investigated. ZnO nanowires with different diameters and lengths are prepared by chemical vapor transport. The diameter plays an important role in the stimulated emission process in nanowires. The spectral shift and spacing of Fabry-Pérot-type modes imply a strong light-matter interaction in the lasing nanowires, which is explained by the exciton-polariton model. The polarization of the electric field in the lasing nanowires is perpendicular to the long axis of the nanowire and parallel to the substrate plane. The coexistence of the transverse modes is distinguished by decomposing the peak shape and the degree of polarization. In addition to the transverse mode of the lasing with the polarization parallel to the substrate plane, the lasing mode with the polarization perpendicular to the substrate plane is observed.

Hybrid AlGaN/GaN-ZnO-Nanowire Gas Sensors

The potential of AIGaN/GaN heterostructures integrated with zinc oxide (ZnO) nanowires for gas sensing applications is demonstrated. Single crystal ZnO nanowires, serving as sensing probes, were selectively grown between two ohmic electrodes of AIGaN/GaN two dimensional electron gas heterostructures by thermal oxidation of sputtered zinc films in air. Electron diffraction and transmission electron microscopy showed the ZnO-nanowires to be crystalline structures oriented in the [001] direction. The fabricated structures were used to detect ethanol, acetone and methanol in a nitrogen background. The results indicate that the hybrid AIGaN/GaN-ZnO nanowires gas sensors are operable over a broad range of temperatures and could potentially be integrated with devices for wireless environmental monitoring.

ZnO nanowire-based all-optical switch with Reset-Set flip-flop function

An all-optical switch with Reset-Set (RS) flip-flop function has been developed by attaching a derivative of spiropyran on the surface of zinc oxide (ZnO) Nanowire. Using UV/visible irradiation and the fluorescence of spiropyran-modified ZnO nanowire as inputs—set/reset and output, RS flip-flop function can be performed on a single ZnO nanowire or a nanowire array. The configuration of the current all-optical switch represents a potential for developing small-sized all-optical devices, which could be further exploited at higher level of integration.

Annealing effects on the cathodoluminescence properties of individual ZnO nanowire

Annealing effects on cathodoluminescence of single zinc oxide nanowire are investigated. It is found that ultraviolet emission is gradually decreased and further disappears with temperatures in all the atmospheres. Deep-level emission is blue-shift with temperature in both argon and vacuum, compared with fresh nanowire. Oxygen vacancies are the main defects for the blue-shift in the oxygen-deficient condition such as argon and vacuum. However, deep-level emission is red-shift in oxygen atmosphere with temperature. It is thought that complex defects zinc vacancies and oxygen interstitials should be responsible for the interesting red-shift of deep-level emission.

Nitric oxide gas sensing at room temperature by functionalized single zinc oxide nanowire

We report on room temperature detection of nitric oxide (NO) gas at low-ppm level by Cr nanoparticle decorated single zinc oxide nanowire (ZNW) sensor. In this sensor device Cr nanoparticles, act as catalyst to transform NO into NO2 and results in change in current value through ZNW, corresponding to NO gas concentration. The sensor exhibits average sensitivity of ∼46% for 10 ppm of NO gas. The lowest detection limit of the sensor is ∼1.5 ppm for NO gas. Sensor shows selectivity towards NO gas during a cross-sensitivity test with N2, CO and CO2 gases. Recovery of the sensor is achieved by ultraviolet light induced desorption of gases from ZNW surface.

Field emission microscopy study of zinc oxide nanowires on tungsten tip

This article focuses on the field emission from a small number of zinc oxide (ZnO) nanowires. ZnO nanowires were grown directly on W tips and their field emission was studied in a traditional field emission microscope (FEM). The FEM pattern of a single ZnO nanowire was ring shaped, indicating that most of the emission from the (0001) facets of the ZnO nanowires came from their circumferences. Scanning electron microscopy observation, measurement of dependence of the emission current on the applied cathode voltage (I-V behavior), and acquirement of FEM patterns were performed before and after heat treatment of the samples, respectively. The heat treatment was found to play either a favorable or an unfavorable role. It could lead to a stable emission or damage of the emitters, depending on the heating conditions.

Zinc oxide nanowire electromechanical oscillator

This paper reports a novel nanoelectromechanical oscillator by using a single semiconducting zinc oxide (ZnO) nanowire that is suspended across on two micromachined metal electrodes. The oscillator is electrostatic-driven into a motion that is self-detected based on a field effect transistor (FET) principle and a lock-in detection method. A continuum electromechanical model is established to realize a theoretical analysis on the oscillator. The primary experimental measurements on the devices are performed. The experimental results show that the oscillator is with a resonant frequency of tens of MHz and two resonance peaks of the current response at double frequency points are observed. The current response of the oscillator can be adjusted through tuning AC and DC gate and source–drain voltages. Compared with other reported nanoelectromechanical oscillators, our oscillator based on a single ZnO nanowire is provided with an enhanced electromechanical performance. The merits significantly enhance the device's practicability. The device can be potentially applied for actuators and sensors.

Ultrafast Upconversion Probing of Lasing Dynamics in Single ZnO Nanowire Lasers

The ultrafast lasing dynamics of single zinc oxide nanowires are studied by time-resolved upconversion of the lasing emission as a function of the ultraviolet excitation intensity. Induction times for stimulated emission in individual nanowires are observed to be 1−5 ps or 3−15 roundtrips for nominal 20 μm long nanowires, depending on the 267 nm excitation intensity. In separate experiments, transient absorption profiles are also obtained for time-delayed 800 and 400 nm pulses to elucidate the carrier dynamics, such as rapid decay (2−3 ps) of the electron−hole plasma (EHP) states at high carrier densities during lasing in the nanowire and the slower exciton decay component (15−60 ps) at lower excitation densities. The dynamics of the lasing wavelength dependence on carrier density is also studied and related to the band gap renormalization in the EHP regime.

Recombination Dynamics of Surface-Related Excitonic States in Single ZnO Nanowires

With decreasing diameter the influence of surface-related effects becomes increasingly important for an understanding of the optical properties of semiconductor nanowires. We present time integrated and time resolved photoluminescence studies of single zincoxide nanowires with different diameters. We analyze the changes in the optical spectra for wires with different surface-to-volume ratios, present optical spectra of single wires at different excitation densities, and study the time-resolved dynamics of the surface related and donor-bound exciton related emission lines for a single nanowire at low temperatures.

Pulsed-laser deposition and characterization of ZnO nanowires with regular lateral arrangement

We report on the high-pressure pulsed-laser deposition growth of periodic arrays of free-standing single zinc oxide nanowires with uniform hexagonal arrangement and cross-section with thickness of less than 100 nm. In order to achieve the wire alignment, we prepared an ordered array of catalytic gold seed particles by a nanosphere lithography mask transfer technique using monodisperse spherical polystyrol nanoparticles. These templates were investigated by scanning electron microscopy and atomic force microscopy prior to nanowire growth. X-ray diffraction revealed the epitaxial relationships between the nanostructures and the a-plane sapphire substrate and excellent crystal quality. The optical properties of the ZnO nanowire arrays were measured by cathodoluminescence.