Diameter-dependent ultra-high thermoelectric performance of ZnO nanowires

Zinc oxide(ZnO)shows great potential in electronics,but its large intrinsic thermal conductivity limits its thermoelectric applications.In this work,we explore the significant carrier transport capacity and diameter-dependent thermoelectric characteristics of wurtzite-ZnO(0001)nanowires based on first-principles and molecular dynamics simulations.Under the synergistic effect of band degeneracy and weak phonon-electron scattering,P-type(ZnO)_(73) nanowires achieve an ultrahigh power factor above 1500μW·cm^(-1)·K^(-2)over a wide temperature range.The lattice thermal conductivity and carrier transport properties of ZnO nanowires exhibit a strong diameter size dependence.When the ZnO nanowire diameter exceeds 12.72A,the carrier transport properties increase significantly,while the thermal conductivity shows a slight increase with the diameter size,resulting in a ZT value of up to 6.4 at 700 K for P-type(ZnO)_(73).For the first time,the size effect is also illustrated by introducing two geometrical configurations of the ZnO nanowires.This work theoretically depicts the size optimization strategy for the thermoelectric conversion of ZnO nanowires.

Pick-up strategies for and electrical characterization of ZnO nanowires with a SEM-based nanomanipulator

Because of their unique mechanical and electrical properties,zinc oxide(ZnO)nanowires are used widely in microscopic and nanoscopic devices and structures,but characterizing them remains challenging.In this paper,two pick-up strategies are proposed for characterizing the electrical properties of ZnO nanowires using SEM equipped with a nanomanipulator.To pick up nanowires efficiently,direct sampling is compared with electrification fusing,and experiments show that direct sampling is more stable while electrification fusing is more efficient.ZnO nanowires have cut-off properties,and good Schottky contact with the tungsten probes was established.In piezoelectric experiments,the maximum piezoelectric voltage generated by an individual ZnO nanowire was 0.07 V,and its impedance decreased with increasing input signal frequency until it became stable.This work offers a technical reference for the pick-up and construction of nanomaterials and nanogeneration technology.

ZnO nanowires based degradable high-performance photodetectors for eco-friendly green electronics

Disposable devices designed for single and/or multiple reliable measurements over a short duration have attracted considerable interest recently. However, these devices often use non-recyclable and non-biodegradable materials and wasteful fabrication methods. Herein, we present ZnO nanowires(NWs) based degradable high-performance UV photodetectors(PDs) on flexible chitosan substrate. Systematic investigations reveal the presented device exhibits excellent photo response, including high responsivity(55 A/W), superior specific detectivity(4×10^(14) jones), and the highest gain(8.5×10~(10)) among the reported state of the art biodegradable PDs. Further, the presented PDs display excellent mechanical flexibility under wide range of bending conditions and thermal stability in the measured temperature range(5–50 ℃).The biodegradability studies performed on the device, in both deionized(DI) water(pH≈6) and PBS solution(pH=7.4),show fast degradability in DI water(20 mins) as compared to PBS(48 h). These results show the potential the presented approach holds for green and cost-effective fabrication of wearable, and disposable sensing systems with reduced adverse environmental impact.

First-principles study of the electronic and optical properties of ZnO nanowires

The geometric, energetic, electronic structures and optical properties of ZnO nanowires （NWs） with hexagonal cross sections are investigated by using the first-principles calculation of plane wave ultra-soft pseudo-potential technology based on the density functional theory （DFT）. The calculated results reveal that the initial Zn-O double layers merge into single layers after structural relaxations, the band gap and binding energies decrease with the increase of the ZnO nanowire size. Those properties show great dimension and size dependence. It is also found that the dielectric functions of ZnO NWs have different peaks with respect to light polarization, and the peaks of ZnO NWs exhibit a significant blueshift in comparison with those of bulk ZnO. Our results gives some reference to the thorough understanding of optical properties of ZnO, and also enables more precise monitoring and controlling during the growth of ZnO materials to be possible.

Flexible electrically pumped random lasing from ZnO nanowires based on metal–insulator–semiconductor structure

Flexible electrically pumped random laser（RL） based on ZnO nanowires is demonstrated for the first time to our knowledge. The ZnO nanowires each with a length of 5 μm and an average diameter of 180 nm are synthesized on flexible substrate（ITO/PET） by a simple hydrothermal method. No obvious visible defect-related-emission band is observed in the photoluminescence（PL） spectrum, indicating that the ZnO nanowires grown on the flexible ITO/PET substrate have few defects. In order to achieve electrically pumped random lasing with a lower threshold, the metal–insulator–semiconductor（MIS） structure of Au/SiO2/ZnO on ITO/PET substrate is fabricated by low temperature process. With sufficient forward bias, the as-fabricated flexible device exhibits random lasing, and a low threshold current of ～ 11.5 m A and high luminous intensity are obtained from the ZnO-based random laser. It is believed that this work offers a case study for developing the flexible electrically pumped random lasing from ZnO nanowires.

Effects of Annealing on the Structural and Photoluminescent Properties of Ag-Doped ZnO Nanowires Prepared by Ion Implantation

ZnO nanowires deposited on Si substrates were prepared by thermal evaporation of a mixture of ZnO and carbon powder. Ag ions with an energy of 63 keV and a dose of 5×1015 ions/cm-2 were implanted into the as-prepared ZnO nanowires. After ion implantation, the Ag-implanted ZnO nanowires were annealed in air at different temperatures from 600℃ to 1000℃. Effects of ion implantation and thermal annealing on the structural and photoluminescent （PL） properties of the ZnO nanowires were investigated by transmission electron microscopy （TEM）, selected area energy dispersive X-ray spectroscopy （SAEDX）, X-ray diffraction （XRD）, and fluorescence spectrophotometry. TEM, HR-TEM, and SAEDX analyses demonstrated that efficient doping of Ag was achieved by ion implantation and the subsequent annealing process. XRD patterns revealed that the hexagonal wurtzite structure of ZnO nanowires was maintained after ion implantation. Photoluminescent emissions of ZnO nanowires were decreased significantly by Ag implantation but could be recovered by thermal annealing. The mechanism of the influence of ion implantation and annealing on the PL intensity was assessed.

Surface-Induced Enhancement of Piezoelectricity in ZnO Nanowires

Piezoelectric nanowires are promising building blocks in various micro-electromechanical systems. Using firstprinciples calculations, we systematically investigate the influence of surface and volume changes on piezoelectric coefficients in [001]-oriented ZnO nanowires and hollow nanowires. We find that the increased non-axial ion displacements under strain near the {100} surface cause a notable enhancement in piezoelectric coefficients for these nanowires. Furthermore, by introducing the obtained surface modifications, we break through the limitation of simulation size and obtain the piezoelectric coefficients at the experimental size. Our findings are of importance to expand simulations and guide experimental explorations.

Reduction of defect-induced ferromagnetic stability in passivated ZnO nanowires

First-principles calculations are performed to study the electronic structures and magnetic properties of ZnO nanowires（NM）. Our results indicate that the single Zn defect can induce large local magnetic moment（～ 2μB） in the ZnO NWs, regardless of the surface modification. Interestingly, we find that local magnetic defects have strong spin interaction, and favor room-temperature ferromagnetism in bared ZnO NW. On the other hand, although H passivation does not destroy the local magnetic moment of Zn vacancy, it does greatly reduce the spin interaction between magnetic defects. Therefore, our results indicate that H passivation should be avoided in the process of experiments to maintain the room-temperature ferromagnetism.

Optical processes in the formation of stimulated emission from ZnO nanowires

This paper studies power dependent photoluminescence spectra, the stimulated emission occurring at ultraviolet （UV） band instead of the green emission band of ZnO nanowires, which are prepared with a chemical reduction method. The dynamics of the UV emission and green emission is given to demonstrate the reason of stimulated emission occurring at UV band but not the green emission band under high excitation, which indicates that the slow decay rate of trap state makes it easy to be fully filled and saturated, while the fast decay rate of near-band-edge exciton state makes the UV emission dominate the radiative recombination under high excitation. The UV emission, as well as the corresponding stimulated emission, occurs in competition with the green deep-trap emission. In addition, when pump fluence further increases, the multiple lasing modes appear. The dependence of these lasing modes on the pump fluence is first discussed. This diagram should be helpful to understand and design the optical nanodevices of ZnO nanowires.

Mechanical and field-emission properties of individual ZnO nanowires studied in situ by transmission electron microscopy

The mechanical and field-emission properties of individual ZnO nanowires,grown by a solid-vapour phase thermal sublimation process,were studied in situ by transmission electron microscopy(TEM)using a home-made TEM specimen holder.The mechanical resonance is electrically induced by applying an oscillating voltage,and in situ imaging has been achieved simultaneously.The mechanical results indicate that the elastic bending modulus of individual ZnO nanowires were measured to be～58 GPa.A nanobalance was buil...

Ferromagnetism of V-doped ZnO nanowires

The geometry structures,electronic structures,and magnetic properties of Zn46V2O48 nanowires are studied by density functional theory（DFT） calculations.We find that the ferromagnetic（FM） coupling is more stable for six configurations of Zn46V2O48 nanowires,and is mediated by neighboring O as evidenced from the strong hybridization of V 3d and O 2p states,exhibiting strong spin polarization.The spin polarization is found to be 100% in the Zn46V2O48 nanowires,which confirms that it is a half-metallic ferromagnet and very suitable for the injection of the spin carriers,which shows that Zn46V2O48 nanowire is one of the ideal materials to realize spin electronic devices.At the same time,the magnetic coupling mechanisms of Zn46V2O48 nanowires are analyzed with V 3d and O 2p orbitals and their magnetic moments mainly come from the contributions of the unpaired electrons of V 3d orbitals.The above results provide a theoretical basis for the preparation of 3d transition metal-doped ZnO nanowire materials.

Influence of Co Content on Raman and Photoluminescence Spectra of Co Doped ZnO Nanowires

Co doped ZnO nanowires with different Co contents have been fabricated by a chemical vapor deposition method. X-ray diffraction results show that all the samples are of single phase and crystallize in wurtzite ZnO structure. The lattice parameter a increases with increasing Co content, while the parameter c has no obvious change with increasing Co. Raman spectra show that the nonpolar E2（High） mode becomes broad and weak with the doping of Co, which indicates that the incorporation of Co causes structural disorder in the crystalline columnar ZnO lattice. The photolurninescence spectra exhibit that the position of the ultraviolet emission shifts to short wavelength and the intensity decreases with increasing Co. The green emission is affected by two contrary factors. It is increased by the introduced defects, but suppressed by the interaction between Co doping and native defects and the later affects it more significantly.

Elaboration of ZnO nanowires by solution based method,characterization and solar cell applications

ZnO nanowires（NWs）layers have been synthesized using a two-step chemical solution method on ITO glass substrates coated with ZnO seeds at different immersing times.The structures,morphology and optical properties of the synthesized ZnO NWs have been investigated.The prepared ZnO NWs have an obvious polycrystalline hexangular wurtzite structure and are preferentially oriented along the c-axis（002）.FESEM micrographs showed that the prepared ZnO NWs are close to being vertically grown and more densely at higher immersing times.Poly[2-methoxy-5（2-′-ethyl-hexyloxy）-1,4-phenylenevinylene],MEH-PPV,was used as an active layer to prepare three samples of MEH-PPV/ZnO solar cell based on ZnO NWs that were prepared at different immersing times.A maximum power conversion efficiency of 0.812%was achieved for MEH-PPV/ZnO solar cell prepared at a higher immersing time.The improved efficiency may be attributed to the enhancement of both open-circuit voltage and fill factor.

Mechanical Properties of ZnO Nanowires Under Different Loading Modes

A systematic experimental and theoretical investigation of the elastic and failure properties of ZnO nanowires (NWs) under different loading modes has been carried out. In situ scanning electron microscopy (SEM) tension and buckling tests on single ZnO NWs along the polar direction [0001] were conducted. Both tensile modulus (from tension) and bending modulus (from buckling) were found to increase as the NW diameter decreased from 80 to 20 nm. The bending modulus increased more rapidly than the tensile modulus, which demonstrates that the elasticity size effects in ZnO NWs are mainly due to surface stiffening. Two models based on continuum mechanics were able to fit the experimental data very well. The tension experiments showed that fracture strain and strength of ZnO NWs increased as the NW diameter decreased. The excellent resilience of ZnO NWs is advantageous for their applications in nanoscale actuation, sensing, and energy conversion.

Modifying optical properties of ZnO nanowires via strain-gradient

We conduct systematical cathodolumiuescence study on red-shift of near-band-edge emission energy in elastic bent ZnO nanowires with diameters within the exciton diffusion length （- 200 nm） in liquid nitrogen temperature （81 K）. By charactering the emission spectra of the nanowires with different; local curvatures, we find a linear relationship between strain-gradient and the red-shift of near-band-edge emission photon energy, an elastic strain-gradient effect in semiconductor similar to the famous flexoelectric effect in liquid crystals. Our results provide a new route to understand the inhomogeneous strain effect on the energy bands and optical properties of semiconductors and should be useful for designing advanced nano-optoelectronic devices.

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.

Slow-rise and fast-drop current feature of ultraviolet response spectra for ZnO-nanowire film modulated by water molecules

This study describes the fabrication of ZnO-nanowire films by electro-chemical anodization of Zn foil. The ZnO films are characterized by field emission scanning electron microscopy, X-ray diffraction patterns, and transmission electron microscopy, respectively. The ultraviolet （UV） photo-response properties of the surface-contacted ZnO film are studied through the current evolution processes under different relative humidities. Unlike the usually observed current spectra of the ZnO films, the drop time is shorter than the rise time. The photo-conductivity gain G and the response time T are both increased with the increase of the applied bias. The photo-conductivity gain G is lowered with the increase of the environmental humidity, while the response time τ- is increased. These results can be explained by considering three different surface processes： 1） the electron-hole （e-p） pair generation by the UV light illumination, 2） the following surface O2 species desorption, and 3） the photo-catalytic hydrolysis of water molecules adsorbed on the ZnO surface. The slow-rise and fast-drop current feature is suggested to originate from the sponge-like structure of the ZnO nanowires.

Emerging of Ag particles on ZnO nanowire arrays for blue-ray hologram storage

Noble-metal/metal-oxide-semiconductor nanostructures as an important material platform have been applied in massive data storage. ZnO exhibits excellent optical modulation ability. However, plasmon induced charge separation effect in Ag/ZnO systems is very weak due to the low chemical activity on surface of the oxide. Herein, we prepare ZnO nanowire arrays via the hydrothermal method, and measure their absorption spectra, photoluminescence spectra and electron paramagnetic resonance, proving the existence of oxygen defects in ZnO. Accordingly, an idea of “electron reverse transfer” is proposed such that blue-ray(403.4 nm) induces reduction of Ag^(+) ions through the excitation of ZnO. Rod-like and spherical silver nanoparticles emerge on the surface and in the gap of ZnO nanowire arrays, respectively, after the visible light stimulus. It is found that nanowire density, oxygen defects and surface roughness are dependent on hydrothermal time. The optimized diffraction efficiency of 0.08% is obtained for reconstructing hologram in the nanocomposite film. This work provides a bright way for construction of ZnO-based optoelectronic integrated devices.

Enhancing UV photosensitivity of a ZnO UV nanosensor using electrical stimulation at megahertz frequency

We report a novel technique to enhance the ultraviolet （UV） photosensitivity of a ZnO nanosensor with ZnO nanowires bridged on micromachined metallic electrodes. The experimental results reveal that the photoconductivity and the time response of the ZnO nanowire sensor with either Schottky or Ohmic contacts are significantly improved by electrifying the nanowire sensors using an alternating current at the frequency of megahertz. An integrated UV sensor incorporating ZnO nanowires with a constant current mode driving circuit is developed, which demonstrates promising sensitivity and time response to UV illumination with a low power consumption.

Physical model for the exotic ultraviolet photo-conductivity of ZnO nanowire films

Employing a simple and efficient method of electro-chemical anodization, ZnO nanowire films are fabricated on Zn foil, and an ultraviolet （UV） sensor prototype is formed for investigating the electronic transport through back-to-back double junctions. The UV （365 nm） responses of surface-contacted ZnO film are provided by I-V measurement, along with the current evolution process by on/off of UV illumination. In this paper, the back-to-back metal-seconductor-metal （M-S-M） model is used to explain the electronic transport of a ZnO nanowire film based structure. A thermionic-field electron emission mechanism is employed to fit and explain the as-observed UV sensitive electronic transport properties of ZnO film with surface-modulation by oxygen and water molecular coverage.