Zn Nanowires Research Articles

A simple fabrication method of randomly oriented polycrystalline zinc oxide nanowires and their application to gas sensing

Randomly oriented polycrystalline ZnO nanowires with a mean diameter of 100–150 nm have been successfully synthesized on SiO 2/Si substrates through the thermal oxidation of sputtered Zn nanowires in dry air at 400 °C. Structural characterization by x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that each ZnO nanowire consisted of a chain of ZnO nanocrystallites. From gas sensing measurements for CO, H 2 and NO gases, the polycrystalline ZnO nanowires showed a highly sensitive and fast response to both reducing and oxidizing gases in dry air at relatively low concentrations and operating temperatures, indicating potential applications of polycrystalline ZnO nanowires as sensing materials for gas sensors.

Stabilization of superconductivity by magnetic field in out-of-equilibrium nanowires

A systematic study has been carried out on the previously reported "magnetic-field-induced superconductivity" of Zn nanowires. By varying parameters such as magnetic field orientation and wire length, the results provide evidence that the phenomenon is a nonequilibrium effect associated with the boundary electrodes. They also suggest there are two length scales involved, the superconducting coherence length and quasiparticle relaxation length. As wire lengths approach either of these length scales, the effect weakens. We demonstrate that it is appropriate to consider the effect to be a stabilization of superconductivity, that has been suppressed by an applied current.

Zinc and Zinc Oxide Nanowires Grown on PEDOT:PSS/SiO2 Conductive Polymer Thin Films by Vapor Phase Transport Deposition

ABSTRACTEasily available spin coating and vapor phase transport deposition techniques offer tremendous possibilities in fabricating inorganic/organic heterojunctions. We present here preliminary results from different thin film growth mechanisms including: i) PEDOT:PSS/Zn-nanowires/SiO2, ii) Zn-nanowires/PEDOT:PSS/SiO2, and iii) ZnO-nanowires/PEDOT:PSS/SiO2. The preliminary scanning electron microscopy and energy dispersive spectroscopy results show that Zn nanowires bond better to the non-annealed PEDOT:PSS thin films. It was also found that ZnO nanowires grow homogeneously on annealed PEDOT:PSS surfaces with colloidal Au nanoparticles as bonding reaction catalysts.

PHONON DRAG, CARRIER DIFFUSIVE THERMOELECTRIC POWER AND SEMICONDUCTING RESISTIVITY BEHAVIOR OF Zn NANOWIRES

In this paper, we undertake a quantitative analysis of observed temperature-dependent thermoelectric power (S) of 4 nm Zn /Vycor composite nanowires by developing a model Hamiltonian that incorporates scattering of acoustic phonons with impurities, grain boundaries, charge carriers and phonons. Mott expression is used to determine the carrier diffusive thermoelectric power [Formula: see text]. The [Formula: see text] shows linear temperature dependence and the computed [Formula: see text] when subtracted from the experimental data is interpreted as phonon drag thermoelectric power [Formula: see text]. The model Hamiltonian within the relaxation time approximation sets the limitations of the scattering of acoustic phonons with impurities, grain boundaries, charge carriers and phonons for thermoelectric power in the nanowires. It is shown that for acoustic phonons the scattering and transport cross sections are proportional to fourth power of the phonon in the Rayleigh regime. The resultant thermoelectric powers is an artefact of various operating scattering mechanisms and are computed for the first time to our knowledge for Zn nanowires consistent with the experimentally reported behavior. The semiconducting nature of resistivity is discussed with small polaron conduction (SPC) model which consistently retraces the temperature-dependent resistivity behavior of Zn /Vycor composite.

The growth of ordered ZnAl 2O 4 nanostructures using AAO as a reactive template

The use of anodic aluminum oxide (AAO) as a reactive template in the fabrication of ordered arrays of spinel ZnAl 2O 4 nanostructures is demonstrated. This involves the growth of monocrystalline Zn nanowires into an AAO template using a pulse dc electrodeposition technique followed by a heat treatment in air at 800 °C. The formation of ZnAl 2O 4 nanotubes in the solid-state reaction is driven by the Kirkendall effect. In addition, the formation of such ZnAl 2O 4 nanostructure arrays requires that the molar ratio of ZnO to Al 2O 3 be less than unity. This corresponds to a ratio of pore radius to half inter-pore separation of 0.63 which serves as a guide to the initial template geometry required for the formation of an array of discrete nanotubes.

Single crystalline zinc structures synthesized spontaneously in solution

We report a method for the synthesis of Zn structures with a variety of morphologies through a spontaneous solution phase reaction without application of an electropotential under ambient conditions. We also describe, in detail, the influence of several experimental conditions on the morphology and microstructure of the Zn structures. The morphology of the Zn structures transitioned from hexagonal disks to dendritic forms as the reaction temperature was increased due to an increase in the driving force for Zn structure formation (over a temperature range from room temperature, 22 °C, to 95 °C). The size of the Zn structures was controlled by adjusting the concentration of zinc acetate dihydrate added to the reaction solution. When the concentration of zinc salt was reduced, smaller Zn structures were synthesized. Control experiments revealed that the aluminium layer, ammonia additions, and ammonia concentration were key factors for the formation of Zn metal structures. A zinc ion complex, Zn(NH3)42+, played a critical role in the formation of the Zn structures. Zn nanowires were synthesized in the presence of a 1.5 μm thick Al layer substrate. Based on these observations, we propose possible formation mechanisms for the spontaneous assembly of the various Zn structures.

A Novel Synthesis of Nanostructured ZnO via Thermal Oxidation of Zn Nanowires Obtained by a Green Route

ZnO nanowires were synthesised in a green and novel, two-step process: (1) the production of Zn nanowires by carbo-thermal reduction of a mixture of ZnO/biopitch (Eucalyptus sp. tar pitch) at 900°C for 1 h in a quartz tube placed in an electric furnace in a N2 atmosphere and (2) the oxidation of the as-prepared Zn nanowires in air at 300°C for 3 h and 6 h and at 400°C for 3 h. The structural properties and phase compositions of the oxidised samples were studied by X-ray diffraction (XRD), and the morphologies were investigated by scanning electron microscopy (SEM). The XRD results demonstrated the formation of ZnO phase, as the main product. The oxidised products exhibited good crystallinity. Maximal conversion of the Zn nanowires into ZnO nanowires (99%) resulted from oxidation of the sample for 3 h in air at 300°C. The formation of ZnO was also confirmed by Fourier transform infrared (FTIR) spectroscopy.

Preparation by Electrodeposition and Characterization of ZnO Nanowire Arrays

Ordered zinc (Zn) nanowire arrays embedded in anodic aluminum oxide (AAO) templates were prepared by an effective electrodeposition method. Oxygen was used to oxidize the electrodeposited zinc nanowire arrays in the AAO templates. By thermal treatment in an oxygen atmosphere at 800 °C for 2 h, the deposited Zn was completely oxidized. The microstructures and optical properties of the synthesized zinc oxide (ZnO) nanowire arrays were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and photoluminescence (PL) spectrum analytic apparatus. We found that large scale polycrystalline ZnO nanowire arrays were uniformly assembled in the nanochannels of the AAO template. The nanowires have a very high aspect ratio of about 1000:1 with the length equaling the template thickness and a diameter of about 80 nm. PL measurements of the ZnO/AAO assembly showed a strong green emission at 504 nm, which was attributed to the oxygen vacancy defects of the ZnO nanowires. These results can be used in further studies of the structural and functional properties of electroluminescence devices based on the ZnO/AAO assembly.

The photoluminescence properties of TiO 2-sheathed ZnSe nanowires

ZnSe nanowires have been synthesized by thermal evaporation of ZnSe powders on gold-coated Al 2O 3(0 0 0 1) substrates and then sheathed with TiO 2 by sputtering. Our results show that sheathing Zn nanowires with thin TiO 2 layers can significantly enhance the photoluminescence (PL) emission intensity. XPS analysis results suggest that the PL enhancement is attributed to increases in the concentrations of deep levels such as oxygen and titanium interstitials as well as the density of interface states. The PL emission of ZnSe nanowires is also enhanced by thermal annealing. Annealing in an argon atmosphere is more efficient in enhancing the PL emission than annealing in an oxygen atmosphere.

Interpretation of thermoelectric power behaviour of Zinc nanowire composites: Phonon-scattering mechanism

We develop a theoretical model for quantitative analysis of temperature-dependent thermoelectric power ( S) of Zn nanowires. In doing so, we first use the Mott expression to compute the electron diffusive thermoelectric power ( S c diff. ) using Fermi energy as electron-free parameter, S c diff. shows linear temperature dependence. Further, the S c diff. contribution is subtracted from the experimental data and the difference ( S experimental – S c dif ) is characterized as phonon drag thermoelectric power ( S ph drag ) which is obtained within the relaxation time approximation where the thermoelectric power is limited by the scattering of phonons with impurities, grain boundaries, charge careers and phonons in the nanowires. The S ph drag shows anomalous temperature-dependent behaviour, which is an artifact of various operating scattering mechanisms. The observed anomalies are well accounted in terms of interaction among the phonons–impurity, phonon-grain boundaries, phonon–electron and the umklapp scattering. It is also shown that for phonons the scattering and transport cross-sections are proportional to ω 4 in the Rayleigh regime where ω is the frequency of the phonons. Numerical analysis of thermoelectric power from the present model shows similar results as those revealed from experiments.

Nanowire Transformation by Size-Dependent Cation Exchange Reactions

The unique properties of nanostructured materials enable their transformation into complex, kinetically controlled morphologies that cannot be obtained during their growth. Solution-phase cation-exchange reactions can transform sub-10 nm nanocrystals/nanorods into varying compositions and superlattice structures; however, because of their small size, cation-exchange reaction rates are extremely fast, which limits control over the transformed products and possibilities for obtaining new morphologies. Nanowires are routinely synthesized via gas-phase reactions with 5-200 nm diameters, and their large aspect ratios allow them to be electrically addressed individually. To realize their full potential, it is desirable to develop techniques that can transform nanowires into tunable but precisely controlled morphologies, especially in the gas-phase, to be compatible with nanowire growth schemes. We report transformation of single-crystalline cadmium sulfide nanowires into composition-controlled Zn(x)Cd((1-x))S nanowires, core-shell heterostructures, metal-semiconductor superlattices (Zn-Zn(x)Cd((1-x))S), single-crystalline ZnS nanotubes, and eventually metallic Zn nanowires by utilizing size-dependent cation-exchange reaction along with temperature and gas-phase reactant delivery control. This versatile synthetic ability to transform nanowires offers new opportunities to study size-dependent phenomena at the nanoscale and tune their chemical/physical properties to design reconfigurable circuits.

Magnetic-Field-Induced Superconducting State in Zn Nanowires Driven in the Normal State by an Electric Current

Four-terminal resistance measurements have been carried out on Zn nanowires formed using electron-beam lithography. When driven resistive by current, these wires reenter the superconducting state upon application of small magnetic fields. The data are qualitatively different from those of previous experiments on superconducting nanowires, which revealed either negative magnetoresistance near T_{c} or high-magnetic-field-enhanced critical currents.

Effect of oxygen partial pressure on the growth of zinc micro and nanostructures

Zinc micro and nanostructures were synthesized in vacuum by condensing evaporated zinc on Si substrate at different gas pressures. The morphology of the grown Zn structures was found to be dependent on the oxygen partial pressure. Depending on oxygen partial pressure it varied from two-dimensional microdisks to one-dimensional nanowire. The morphology and structural properties of the grown micro and nanostructures were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Transmission electron microscopy (TEM) studies on the grown Zn nanowires have shown that they exhibit core/shell-like structures, where a thin ZnO layer forms the shell. A possible growth mechanism behind the formation of different micro and nanostructures has been proposed. In addition, we have synthesized ZnO nanocanal-like structures by annealing Zn nanowires in vacuum at 350 °C for 30 min.

Fabrication and magnetic properties of Ni–Zn nanowire arrays

Ni–Zn nanowire arrays, with diameters of approximately 60 nm and lengths of around 40 μm, were fabricated by electrodeposition in porous anodic aluminum oxide templates at different electric potentials. X-ray diffraction observations demonstrated that the isolated nanowires had polycrystalline structure and that their phases changed with the deposition potential. The amount of deposited zinc in the nanowires increased with the deposition potential, whereas the amount of nickel decreased. Magnetic measurements showed that there was a gradual change of magnetism from isotropic to anistropic with increasing potential amplitude and that the coercivity reached a maximum value in the nanowire deposited at −1.35 V.

Self-sacrificed template method for the synthesis of ZnO-tubular nanostructures:reaction kinetics and pathways

Using Zn nanowires as a self-sacrificed template, hierarchical tubes constructed by zinc oxide (ZnO) nanoflakes and ZnO nanotubes have been successfully fabricated by two different thermal-oxidation modes. The products were characterized by the X-ray powder diffraction, transmission electron microscopy and field-emission scanning electron microscopy. The experimental results show that the formation processes of ZnO nanostructures are sensitive to the growth temperature, which is lower or higher the melting point of Zn (419 °C). ZnO nanoflake tubes and ZnO nanotubes can be controlled through the variation of the heat-treatment process of Zn nanowires and their growth pathway can be described by two types of growth mechanism, in terms of Kirkendall effect and the sublimation of the Zn cores, respectively. Our method provides an easy and convenient way to prepare metal oxides tubular nanostructures with different morphologies through self-sacrificed template method via adjusting the heat-treatment process.

Zinc nanowires synthesized on a large scale by a simple carbothermal process

Metallic Zn nanowires have been synthesized by a new carbothermal reduction route in which ZnO and Eucalyptus sp. tar pitch were used as source materials. This simple practical procedure was capable of producing Zn nanowires in large quantities without reoxidation. This process was carried out in inert atmosphere, without vacuum or catalyst, at temperatures (800–900 °C) lower than those required in the carbothermal reduction of ZnO with graphite. A comparative study was performed using graphite that is traditionally used as a ZnO reducer, under the same experimental conditions, however, no reaction was observed. The new process involves the pyrolysis of biopitch to obtain a highly reactive coke and the reduction of ZnO with the release of Zn (v) for the growth of Zn (s) nanowires. The resulting Zn nanowires were characterized by X-ray diffractometry, energy-dispersive spectroscopy and scanning electron microscopy. Differential thermal analysis and thermogravimetric analysis coupled with infrared analysis techniques were used in an effort to understand the underlying mechanism and establish the best ratio biopitch/ZnO to be used. This paper presents the characterization of the as-synthesized nanowires and discusses the main reactions involved in their production.

Morphology alterations during postsynthesis oxidation of Zn nanowires

Zinc metal nanowires have been oxidized at temperatures of 200–500 °C in ambient air in order to obtain ZnO nanowires. X-ray diffraction investigations reveal a complete transformation from Zn to ZnO at a temperature of ≥300 °C within 30 min. Temperature-induced grain growth attains saturation at about 350 °C. Scanning electron microscopy studies show a pronounced morphology change from smooth, unidirectional to rough, bursted nanorods at a temperature of >250 °C. The corresponding transformation mechanism is discussed.

SYNTHESIS OF ZnO NANOTUBE ARRAYS BY ANNEALING Zn NANOWIRE ARRAYS IN ANODIC ALUMINA MEMBRANE

Uniform and aligned ZnO nanotube arrays have been synthesized by annealing Zn nanowire arrays in anodic aluminum oxide (AAO) membrane. In our method, Zn nanowire arrays were fabricated by electrochemical deposition technique based on ordered nanoporous AAO, and then a heat-treatment method was used to convert Zn nanowire arrays to ZnO nanotube arrays. The ZnO arrays were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and absorption spectra. The results show that the polycrystalline ZnO nanotubes have a diameter of about 45 nm. This method can also be used to fabricate other metal oxide nanotube arrays.

High-performance, fully transparent, and flexible zinc-doped indium oxide nanowire transistors

We report the fabrication of fully transparent and flexible nanowire transistors by combining a high-quality In2O3:Zn nanowire channel, a SiNx high-κ dielectric, and conducting Sn-doped In2O3 electrodes on a polyethylene terephthalate substrate. The devices show excellent operating characteristics with high carrier mobilities up to 631 cm2 V−1 s−1, a drain-source current on/off modulation ratio ∼1×106, a high on-state current ∼1×10−5 A, a small subthreshold gate voltage swing of 120 mV decade−1, and a near zero threshold voltage. The devices further show high reproducibility and stable performance under bending condition. The high-performance nanowire transistors would enable application opportunities in flexible and transparent electronics.

Localized phase-slip centers in proximity-induced long superconducting nanowires

Current-voltage (I-V) curves have been measured on 60-μm-long Zn nanowires in contact with bulk film electrodes of Sn or Pb. The Zn nanowires become superconducting at the transition temperatures of the respectively electrodes, which are well above the transition temperature of bulk Zn. At temperatures well above the transition temperature of bulk Zn but below the transition temperatures of the electrodes, the I-V curves display a reproducible steplike pattern characteristic of the establishment of multiple localized phase-slip centers. The typical quasiparticle diffusion length associated with a phase-slip center is estimated to be 5–10μm. These results demonstrate that the observed proximity effect occurs along the entire length of a nanowire, rather than only at the nanowire-electrode interfaces.