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Nanostructures are the building blocks of future nanodevices and thus methods for fabricating nanostructures of various materials in various forms are fundamentally important. Among those nanostructures ZnO has received much attention over the past few years due to the wide range of research by many different groups focused on different novel nanostructures with different properties. Although ZnO nanowires have been intensively studied, there are only a few methods that showed promising characteristics for practical applications. Without much effort, it can be grown in many different nanostructure forms, thus allowing various novel devices to be achieved. In this study, we intend to review those methods that enable nanostructure growth to be more controllable and feasible for applications. The methods for fabricating ZnO nanostructures are introduced in the first part. In the second part, the application of those nanostructures are mentioned and explained. Finally, the future realization of nanodevices is discussed. [ABSTRACT FROM AUTHOR]

The role of Chemical Vapor Deposition's (CVD) temperature and time duration on the mass transport properties and repercussions of these properties on the crystalline growth of ZnO nanowires inside Anodic Aluminum Oxide (AAO) nanotubes (aspect ratio = 1500) have been investigated. CVD assisted growth of nanowires in AAO nanochannels was carried out for 27 min at deposition temperature of 813 K (A1) and for 180 min at 903 K (A2) under identical gas flow rates (Ar: 23 sccm and O2: 0.45 sccm) under a chamber pressure of 4.45 torr. Electron diffraction patterns (SAED) of the ZnO diffused inside A1 nanotube (exposed in Focused Ion Beam milled cross-sections) showed a ringed pattern, signifying polycrystalline morphology at the tip as well as the channel bottom. For A2, a single crystal formation at least in the tip region was confirmed by the SAED pattern while the corresponding High Resolution TEM image (HRTEM) revealed a lattice spacing of 0.24 nm corresponding to the (1 0 1) growth direction of zincite (JCPDS-36-1451). Chemical composition of the nanowires, reflected the inherent stoichiometry of preferentially nucleated ZnO in the grooved orifices of individual nanopores. Data points of normalised values of the Zn concentration (TEM-EDS line scan) versus penetration depth (~ 0.6 μm), were empirically fitted with solutions of modified Fickian diffusion equation in 1-D. The Diffusion coefficient values of the order of 0.06 nm2 s-1 and 0.22 nm2 s-1 were obtained for ZnO flow inside nanochannels of A1 and A2, respectively. Observed variations, in diffusivity and relative density, have been explained qualitatively by considering the transport of ZnO inside the nanochannels as arising from superposition of the temperature dependent contributions of the convectional viscous creep and surface diffusion components. [ABSTRACT FROM AUTHOR]

Abstract: We have developed a range of single and multilayer transparent conducting coatings consisting of three to five alternating layers of Al-doped ZnO (AZO) and metals using E-beam evaporation method. The prepared optimized coatings show excellent optical and electrical properties, improved thermal and long-term stability. Optimum thickness of metal and AZO layers was determined for high optical transmittance and good electrical conductivity. X-ray diffraction, spectrophotometer, atomic-force microscopy, scanning electron microscopy and four-point probe were used to explore the possible changes in electrical and optical properties. It was found that the multilayer coatings consisting of Al-doped ZnO and Ag metal show satisfactory properties of low resistance of 5Ω/sq, high transmittance of 90% and thermal stability up to 500°C. [Copyright &y& Elsevier]

Transparent conducting ZnO films were heavily doped with Al (ZnO/Al) by simultaneous r.f. magnetron sputtering of ZnO and d.c. magnetron sputtering of Al. TEM image showed the coexistence of an amorphous Al complex and polycrystalline ZnO. A strong (002) preferential orientation in the ZnO/Al film was caused by the presence of ZnO. According to XRD and XPS analysis, most Al atoms were not in the ZnO crystalline and most oxygen atoms were attributed to the formation of Na zeolite structure. The Na zeolite could result in the neutral impurity scattering and cause low mobility. Electrical conductivity was primarily due to the contribution of Zn, Al interstitial atoms and oxygen vacancies. Little contribution was due to Al donors (AlZn⋅). The resistivity decreased rapidly with the increase of substrate temperature to a minimum of 6.12×10-2 Ω cm at 150 °C, and then slightly increased. The conductivity of films was closely related to the crystallinity, which was dependent on the substrate temperature. The excellent transmission in the visible region decreased substantially at short wavelength near the ultraviolet range for films prepared at various substrate temperatures. [Copyright &y& Elsevier]

In this study, the ZnO films were deposited to different thicknesses by r.f. magnetron sputtering. X-Ray diffraction and pole-figure analysis were used to study the crystallinity and crystal orientation. The results showed that ZnO films deposited to a thickness below 500 nm were polycrystalline with a c-axis preferential orientation. However, ZnO films, 500 or 600 nm in thickness, exhibited good self-texture. The optical properties of ZnO films did not depend significantly on the crystallographic orientation or degree of texturing. They were mainly affected by the grain size and carrier concentration. [Copyright &y& Elsevier]

ZnO thin films were prepared by r.f. magnetron sputtering from a zinc oxide target. The effects of r.f. power and substrate temperature on the properties of ZnO films were studied. The composition of ZnO films was analyzed by X-ray photoemission spectroscopy. The Zn atomic composition in the films was higher than the O atomic composition at any r.f. power and growth temperature conditions. This behavior might be explained by the preferential sputtering of zinc atoms rather than oxygen atoms in the ceramic target. In this study, ZnO films with epitaxial grain growth had a preferred (0 0 2) orientation. The crystallographic orientation and the piezoelectric properties of ZnO films were influenced by the sputtering parameters. A good condition exists for making the c-axis oriented films in the range of deposition rate and substrate temperature under the experimental conditions in this study. ZnO films showed different surface morphologies and surface roughness under different deposition conditions. ZnO films were transparent in the visible region, but had obvious absorption in the UV region. There were no obvious changes in the optical properties of ZnO films by varying the r.f. power and substrate temperature in this study. The phase velocity in this study was relatively higher at the optimal r.f. power and substrate temperature. [Copyright &y& Elsevier]

AZO/Cu/AZO multilayer thin films produced under different annealing conditions are studied in this paper, to examine the effects of atmosphere and annealing temperature on their optical and electrical properties. The multilayer thin films are prepared by simultaneous RF magnetron sputtering (for AZO) and DC magnetron sputtering (for Cu). The thin films were annealed in a vacuum or an atmosphere of oxygen at temperatures ranging from 100 to 400 °C in steps of 100 °C for 3 min. High-quality multilayer films (at Cu layer thickness of 15 nm) with resistivity of 1.99×10 −5 Ω-cm and maximum optical transmittance of 76.23% were obtained at 400 °C annealing temperature in a vacuum. These results show the films to be good candidates for use as high quality electrodes in various displays applications. [ABSTRACT FROM AUTHOR]

We report a significant improvement of the piezoelectric coefficient of the ZnO thin film (12.4 p.m./V) by yttrium (Y)-doping (49.6 p.m./V @1.6 at % Y), demonstrating further to enhance piezoelectric nanogenerator (PENG) performance, generating output voltage (0.012 V) and output current (45 nA) more than the undoped ZnO thin film by 6 and 1.8 times, respectively. Y doping (1.6 at %) led to the formation of permanent dipoles as ferroelectric phase in ZnO evidenced by a clear, distinct hysteresis loop with remnant polarization of 0.0423 μC/cm2 and a coercive field of 0.142 V/cm. All the films were grown by radio frequency magnetron co-sputtering with the maximum Y doping concentration up to 3.5 at%. The films maintain highly oriented along the c-axis (0002) at low Y concentrations (∼1.6 at%), but become more randomly oriented with the emergence of both the (10 1 ‾ 1) and (11 2 ‾ 0) peaks at high Y concentrations (>1.6 at%). The morphology varies from granular microstructure with smaller grain size at low Y doping to larger elongated crystals at the Y > 1.3 at%. Concurrently, cross-sectional images revealed the conversion of columnar nanorod like microstructure into film like (thickness: 650–750 nm) with increased Y doping concentration. [ABSTRACT FROM AUTHOR]

We report prominent specificity on acetone gas sensing at room temperature in p-type Cu doped ZnO nanostructures synthesized by chemical vapor deposition. [Display omitted] • Cu doped p-type ZnO nanostructures as efficient room temperature acetone sensor. • Substitution of Cu+1 at Zn+2 lattice sites in ZnO converts n-type ZnO to p-type. • Cu doping concentration is varied from 0.8 at %– 3.7 at. %. • Conversion of n-type to p-type ZnO is achieved at 1.1 at. % of Cu in ZnO. • Room temperature acetone gas sensing mechanism is discussed in detail. We demonstrate prominent specificity on acetone gas sensing at room temperature (~25 °C) enabled solely by rendering ZnO nanostructures p-type via Cu doping of 0.8–3.7 at%, synthesized by chemical vapor deposition at 575 °C. Structural and chemical bonding analysis confirmed the successful substitution of Cu+1 for Zn+2 lattice sites leading to the conversion of n-type ZnO to p-type. Besides, Cu doping caused significant change in the morphology of the resulting ZnO nanostructures from highly aligned ZnO nanowires to randomly-aligned nanobelts, nanoribbons, and needle-like nanowires with length of 15–20 μm. The electrical measurement based on field-effect transistors comprising of individual nanowires revealed the transition of n- to p-type at a doping concentration of 1.1 at. %. The same devices were then exposed to acetone to monitor the gas sensing properties through chemiresistive responses as gas sensors. Both the un-doped and the Cu-doped n-type ZnO nanowires did not respond toward acetone exposure at room temperature. Exclusively, only the p-type Cu-doped ZnO nanostructures (>1.1 at% Cu) exhibited striking responses toward the exposure of acetone at even 1 ppm with supreme specificity over various gas species (acetone, ammonia, oxygen and alcohols) at room temperature. These results promise the unique implementation of the devices into wearable gadgets for early diagnosis of diabetes. [ABSTRACT FROM AUTHOR]

Abstract: A layer of silver was deposited onto the surface of glass substrates, coated with AZO (Al-doped ZnO), to form Ag/AZO film structures, using e-beam evaporation techniques. The electrical and optical properties of AZO, Ag and Ag/AZO film structures were studied. The deposition of Ag layer on the surface of AZO films resulted in lowering the effective electrical resistivity with a slight reduction of their optical transmittance. Ag (11nm)/AZO (25nm) film structure, with an accuracy of ±0.5nm for the thickness shows a sheet resistance as low as 5.6±0.5Ω/sq and a transmittance of about 66±2%. A coating consisting of AZO (25nm)/Ag (11nm)/AZO (25nm) trilayer structure, exhibits a resistance of 7.7±0.5Ω/sq and a high transmittance of 85±2%. The coatings have satisfactory properties of low resistance, high transmittance and highest figure of merit for application in optoelectronics devices including flat displays, thin films transistors and solar cells as transparent conductive electrodes. [Copyright &y& Elsevier]

Abstract: Transparent conductive ZnO/Ag/ZnO multilayer electrodes having much lower electrical resistance than the widely used transparent electrodes were prepared by simultaneous RF magnetron sputtering of ZnO and DC magnetron sputtering of Ag. An Ag film with different thickness was used as intermediate metallic layers. The optimum thickness of Ag thin films was determined to be 6nm for high optical transmittance and good electrical conductivity. With about 20–25nm thick ZnO films, the multilayer showed high optical transmittance in the visible range of the spectrum and had color neutrality. The electrical and optical properties of the multilayers were changed mainly by Ag film properties. A high quality transparent electrode, having sheet resistance as low as 3ohm/sq and high transmittance of 90% at 580nm, was obtained and could be reproduced by controlling the preparation parameter properly. The above property is suitable as transparent electrode for dye sensitized solar cells (DSSC). [Copyright &y& Elsevier]

We demonstrate room temperature (25 °C) and low temperature (≦100 °C) NH 3 gas sensing properties of Cd-doped ZnO nanorods (NRs) synthesized by a low temperature (90 °C) hydrothermal method. Although, the Cd concentration in the growth solution was varied over a large range from 0 to 80 mol %, the maximum estimated Cd doping concentration in ZnO NRs was 0.5 at % (80 mol %). Structural analysis confirmed the successful doping of Cd in ZnO and microstructure investigation revealed the presence of single nanorod and flower like nanorods in the same ensemble whose dimensions reduced with the Cd doping concentration. Photoluminescence and Raman spectra analyses confirmed the increase of defect concentrations in ZnO NRs by Cd doping thereby enhancing the overall gas sensing. In response to NH 3 , the nanostructure sensors exhibited a gradual increase in the sensitivity with the Cd doping concentration where the 0.5 at % Cd-doped ZnO NRs showed the highest sensitivity with an enhancement of ∼9% (at 60 ppm NH 3) as compared to the un-doped ZnO. The sensitivity continued to rise with the increase of temperature from 110% (25 °C), 120% (50 °C), 170% (75 °C) to 243% (100 °C) indicating a gigantic enhancement of ∼133% at 100 °C from 25 °C. The Cd-doped ZnO NRs also revealed superior specificity of detecting NH 3 over NO 2 , and H 2 S even at room temperature. • Room temperature (RT-25 °C) and low temperature (≦100 °C) NH 3 gas sensing properties of Cd-doped ZnO nanorods (NRs). • Cd doped ZnO NRs showed enhanced RT response (∼10%) as compared to undoped ZnO. • The Cd-doped ZnO NRs revealed superior specificity of detecting NH 3 over NO 2 , and H 2 S even at room temperature. • Cd doping led to increased defect concentration that might be benefitting the overall RT NH 3 gas sensing property. • A detailed investigation about structure, microstructure, elemental composition, and optical property was carried out. [ABSTRACT FROM AUTHOR]

In this work, two kinds of one-dimensional ZnO nanowires (NWs) and nanotubes (NTs) were synthesized by using electrochemical deposition with the aid of a high aspect ratio anodic alumina oxide (AAO) template. ZnO NWs and NTs were characterized by using X-ray diffraction, field emission scanning microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. An AAO template was fabricated by two-step anodization in 0.3 M oxalic acid with a voltage of 80 V. The thickness and channel diameter of the AAO template were about 150 μm and 120–150 nm, respectively. The morphologies of the ZnO nanostructures synthesized under 20 vol.% H 2 O 2 with various electrolyte concentrations of 0.1 M and 0.5 M ZnSO 4 , were NTs and NWs, respectively. Both NTs and NWs were uniform in size, which corresponded to the sizes of AAO pores. The thickness of the NTs walls can be controlled based on the deposition time and current density. The crystallinity of the ZnO NTs and NWs annealing in the air were restricted by AAO pore. The growth of the ZnO NTs and NWs was caused by heterogeneous nucleation, and different growth rates through the wall of the AAO will result in different nanostructures, with the growth of the NTs being slower than that of the NWs. [ABSTRACT FROM AUTHOR]

Abstract: In this work, indium (In) doped ZnO nanowires are grown on a Si substrate by chemical vapor deposition (CVD), at a relatively low temperature of 550°C. The effects of In concentration on the morphology, microstructure, luminescence and electrical properties of ZnO nanowires are investigated. The diameters and lengths of these nanowires are in the ranges of 70–311nm and 10–15μm, respectively. These nanowires are single crystals growing in the [0001] direction. The maximum solubility of In in ZnO is estimated to be 3.47at.%. Photoluminescence (PL) spectra reveal a red shift in the ultraviolet emission and intensity enhancement in the green emission with increasing indium doping concentration. Besides, carrier concentration, mobility and resistivity of the nanowires with different doping concentrations are determined based on single-nanowire field effect transistors (FET). [Copyright &y& Elsevier]

Abstract: An Al-doped ZnO (AZO) seed layer is prepared on the back side of a porous alumina membrane (PAM) substrate by spin coating followed by annealing in a vacuum at 400°C. Zinc oxide in ordered arrays mediated by a high aspect ratio and an ordered pore array of AZO/PAM is synthesized. The ZnO nanowire array is prepared via a 3-electrode electrochemical deposition process using ZnSO4 and H2O2 solutions at a potential of −1V (versus saturated calomel electrode) and temperatures of 65 and 80°C. The microstructure and chemical composition of the AZO seed layer and ZnO/AZO/PAM nanowire arrays are characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS). Results indicate that the ZnO/AZO/PAM nanowire arrays were assembled in the nanochannel of the porous alumina template with diameters of 110–140nm. The crystallinity of the ZnO nanowires depends on the AZO seed layer during the annealing process. The nucleation and growth process of ZnO/AZO/PAM nanowires are interpreted by the seed-layer-assisted growth mechanism. [Copyright &y& Elsevier]

Abstract: We report our novel result regarding the observation of unusual but clean ferromagnetic (FM) signature at room temperature (RT) in high pure (99.999%) ZnO bulk ceramics processed by slow step sintering schedule (SSSS). All the sintered samples showed ferromagnetic signature at room temperature. However, sample sintered at 850°C showed more prominent M–H loop and saturation magnetization with respect to samples sintered at 500°C and 1300°C. Observation of FM behavior in sintered ceramics may be attributed to the presence of defects like cation (Zn)/anion (O) vacancies and interstitials created during the processing of sample. It is anticipated that SSSS promotes enhanced physical bulk as well as surface densification and at the same time confined the outgassing of free oxygen and Zn from the bulk which are acting as suitable defect sites inside the sintered grains and grain boundary junctions. Our micro-Raman and X-ray photoelectron studies revealed the existence of these defects which are believed to be the origin of the unconventional ferromagnetism (FM) of ZnO bulk ceramic at room temperature. [ABSTRACT FROM AUTHOR]

We investigate the piezoelectric coefficient (d 33) of Mg x Zn 1−x O and Mg x Zn 1−x O/ZnO films with the variation of the Mg concentration. The films are grown on Si (111) substrate using MgO and ZnO as targets by radio frequency magnetron sputtering. The thickness of all the films is fixed at around 500 nm for both Mg x Zn 1−x O and Mg x Zn 1−x O/ZnO films. All the films have high crystallinity with preferred orientation along c-axis and that shows columnar microstructures. Besides, the substitution of magnesium ions at zinc sites is confirmed by X-ray diffraction patterns, X-ray photoelectron spectroscopy and UV–Visible spectroscopy. The d 33 values reach 41.7 pm/V and 47.5 pm/V for Mg x Zn 1−x O and Mg x Zn 1−x O/ZnO (x = 0.30) films which is around 3.4 and 3.8 times larger than pure ZnO films (12.4 pm/V). The films may be considered as promising candidate for piezoelectric nanogenerators and piezotronic transistors. • Highly (0002) oriented MgxZn1-xO and MgxZn1-xO/ZnO films with various Mg contents. • Proofs of substitution of Mg atoms at Zn sites. • Maximum d33 of MgxZn1-xO films are 3.4 times larger than pure ZnO films. • Maximum d33 of MgxZn1-xO/ZnO films are 3.8 times larger than pure ZnO films. • The d33 enhancement may be attributed to alloying effect and strain effect. [ABSTRACT FROM AUTHOR]