Crystal Structure

As a II-VI compound semiconductor with wide band gap (3.37 eV), zinc oxide (ZnO) has been broadly used in optical applications, for example, optical waveguide, transparent conducting layer, surface acoustic wave device. Recently one-dimensional ZnO nanostructures, such as nanowire, nanoneedle, and nanobelt, are attracting more and more attention. Their optical, mechanical, field emission and magnetic properties have been studied. ZnO nanowire (NW) is expected to be one of the candidates for nanoscale ultra-violet (UV) lasing, light emitting and photodetection. However, electrical transport properties through ZnO NWs have not yet been broadly investigated. Here we report the transport studies on field effect transistors (FET) made of individual ZnO NWs synthesized via chemical vapor deposition (CVD) method. The effects of oxygen adsorption on NW's electrical behavior are investigated and the oxygen sensing property is characterized. The results indicate that ZnO NWs can serve as potential building blocks for nanoscale electronic and sensing devices.

Single crystal ZnO nanowires doped with indium are synthesized via the laser-assisted chemical vapor deposition method. The conductivity of the nanowires is measured at low temperatures in magnetic fields with directions both perpendicular and parallel to the wire axes. A quantitative fit of our data is obtained, consistent with the theory of a quasi-one-dimensional metallic system with quantum corrections due to weak localization and electron−electron interactions. The anisotropy of the magneto-conductivity agrees with theory. The two quantum corrections are of approximately equal magnitude with respective temperature dependences of T−1/3and T−1/2. The alternative model of quasi-two-dimensional surface conductivity is excluded by the absence of oscillations in the magneto-conductivity in parallel magnetic fields.

 

Optical Properties

To clarify the size effect in semiconductor nanowires with decreasing diameters but not yet reaching the quantum confinement region, single crystalline zinc oxide nanowires with diameters around 10nm are synthesized. Electrical transport measurements of these thin nanowires show significant increase in conductivity accompanied by diminished gate modulation and reduced mobility. This phenomenon is a result of the enrichment of surface states owing to the increased surface-to-volume ratio. The enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and contributes to the “anomalous” blueshift. This study shows that surface states play a dominant role in the electrical and optical properties of quasi-one dimensional materials.

Single crystal ZnO nanowires are synthesized and configured as field-effect transistors. Photoluminescence and photoconductivity measurements show defect-related deep electronic states giving rise to green-red emission and absorption. Photocurrent temporal response shows that current decay time is significantly prolonged in vacuum due to a slower oxygen chemisorption process. The photoconductivity of ZnO nanowires is strongly polarization dependent. Collectively, these results demonstrate that ZnO nanowire is a remarkable optoelectronic material for nanoscale device applications.

Transport Properties

Single-crystal ZnO nanowires are synthesized using a vapor trapping chemical vapor deposition method and configured as field-effect transistors. Electrical transport studies show n-type semiconducting behavior with a carrier concentration of ,~10E7 cm−1 and an electron mobility of ,~17 cm2 /V s. The contact Schottky barrier between the Au/Ni electrode and nanowire is determined from the temperature dependence of the conductance. Thermionic emission is found to dominate the transport mechanism. The effect of oxygen adsorption on electron transport through the nanowires is investigated. The sensitivity to oxygen is demonstrated to be higher with smaller radii nanowires. Moreover, the oxygen detection sensitivity can be modulated by the gate voltage. These results indicate that ZnO holds high potential for nanoscale sensing applications.

Thin ZnO nanowires with diameters of less than 50 nm are configured as field effect transistors and studied for their transport mechanisms at different temperatures under UV illumination and gate modulation. The conductivity exhibits two regimes: at T>50 K, thermally activated transport dominates with activation energy around 30–60 meV attributed to the shallow donor states and at T<50 K, three dimensional variable range hopping reveals in the conduction. In addition, UV irradiation leads to a metal-to-insulator transition at 210 K. Furthermore, electrostatic gating results in a band bending giving rise to a change in the activation energy.

Gallium oxide (Ga2O3), a trivalent metal oxide semiconductor, exhibits a wide band gap (4.9eV) and electrically insulating behavior in normal circumstances. Its remarkable thermal and chemical stability renders it suitable for many potential applications. Ga2O3 thin film has been extensively studied and implemented as high temperature oxygen sensor, insulating barrier in spin tunneling junction, and UV-transparent conductive oxide (TCO). In recent years, low dimensional structures have become the rising stars that draw intensive attention because of their unique properties resulted from the high aspect ratio. Quasi-one-dimensional structures of Ga2O3, such as nanowires and nanobelts, have been synthesized and characterized. However, the electrical transport studies on Ga2O3 nanowires demonstrate poor conductivity at room temperature, making it impractical for device applications. With the aim to enhance the charge carrier density, this report describes the synthesis and a p-type doping method. The electrical and optical properties of Ga2O3 nanowires are characterized.

 

Quasi-one-dimensional (Q1D) materials, such as nanotubes and nanowires, are considered as highly promising nanoscale building blocks for integrated electronic and photonic circuits.1,2 In this regard, the control of electron (n-type) and hole (p-type) doping in these nanostructures is of paramount importance.3 The doping approach is usually implemented by incorporating impurity elements during the synthesis procedures.4 In our work, a-Fe2O3 nanobelts are configured as field effect transistors (FETs), followed by a controlled in-situ doping method using zinc (Zn) as the impurity element to achieve p- or enhanced n- type semiconducting property. Carrier concentrations and mobilities are obtained from electrical transport studies. Furthermore, the mechanism of p- and n-type doping using only one impurity element is discussed.

 

 

 

Development of novel magnetic materials has played an important role in modern-day science and technology. Ferromagnetic nanowires are good candidates for studying one dimensional electron spin dynamics, magnetic domain wall dynamics, and their interaction properties. Technologically, vertical magnetic nanowire arrays embedded in insulating matrix may serve as high density patterned perpendicular magnetic medium with superior signal-to-noise ratio and reduced transition position jitter. On the other hand, magnetic nanotubes with very small wall thickness are predicted to reveal quantum-mechanical effects, such as spin-dependent electron transport with quantization around the tube circumference. Hereby, we present the synthesis of Co nanowires and nanotubes using template-assisted method, and show a systematic characterization of these nanostructures.

Co nanotube arrays with high filling rate and homogeneous growth have been successfully fabricated by a direct electrodeposition process with low current density. The as-grown nanotubes are predominately of hcp single crystalline structure with the magnetocrystalline easy axis perpendicular to the tube axis. MFM imaging shows a weak magnetic signal and SQUID measurement reveals sheared hysteresis responses for field applied along the tube axis. Combined with theoretical modeling taking into account the shape demagnetization, crystal anisotropy, magnetic exchange, and external magnetic interaction energies, our measurements confirm that the magnetization curls circumferentially around the tube in order to minimize the total magnetic energy for small external fields. Lastly, these magnetic nanotubes with little hysteresis hold promising applications. For example, they may be employed in spintronic devices with reduced hysteresis loss, in magnetic shielding to inhibit field penetration into the center of the tube, and in biomedical applications such as drug delivery and biomolecular separation.