No abstract

No abstract

Rectifying diodes of single nanobelt/nanowire-based devices have been fabricated by aligning single ZnO nanobelts/nanowires across paired Au electrodes using dielectrophoresis. A current of 0.5 µA at 1.5 V forward bias has been received, and the diode can bear an applied voltage of up to 10 V. The ideality factor of the diode is ∼3, and the on-to-off current ratio is as high as 2000. The detailed IV characteristics of the Schottky diodes have been investigated at low temperatures. The formation of the Schottky diodes is suggested due to the asymmetric contacts formed in the dielectrophoresis aligning process.Zinc oxide is an important optical and optoelectronic material. Recently, utilizing its unique crystal structure and the three major fastest growth directions, various singlecrystal/crystalline nanostructures of ZnO have been synthesized, such as nanobelts, 1 nanorings, 2 and nanohelices. 3 From the abundance of the surface morphologies, ZnO offers the most diverse nanostructure of any material known today. With a large direct band gap of 3.37 eV, together with its piezoelectricity and pyroelectricity, ZnO is most attractive for applications as a field-effect transistor (FET) 4 or sensor 5 and in optical electronics. 6 Extensive research on the electronic properties of various one-dimensional nanostructures has been performed. [7][8][9][10] To apply ZnO nanostructures on various electronic devices, it is important for one to understand its transport properties and its interaction with metal contacts. In this letter, we investigated the contact of a single ZnO nanobelt with gold electrodes. After investigating the transport properties of over 60 single nanobelt-based circuits, we found a spontaneous formation of a Au/ZnO nanobelt Schottky diode in 80% of the samples when nanobelt sizes are well controlled. This effect is likely due to the nonsymmetric contacts at the two ends of the nanobelt.The ZnO nanobelts to be used for fabricating the FET devices were synthesized through a solid-vapor process in a high-temperature horizontal furnace system. 1 The Au electrode patterns were defined with photolithography on a SiO 2 substrate. The electrodes consisted of two 3-µm-wide fingers pointed head to head at a distance of 4 µm. These two fingers are connected to two 500 × 500 µm 2 contacting pads for probe contacts. The as-synthesized nanobelt samples were placed in ethanol and ultrasonicated for 15 min to disperse the bundles into individual nanobelts. A single nanobelt is "placed" across the prefabricated electrodes using the dielectrophoresis technique. 11 After applying a droplet of the nanobelt suspension onto the electrodes, the electrodes were connected to a 5 V and 1 MHz AC signal, which was chosen for optimizing the alignment of a single nanobelt. This signal generated an alternating electrostatic force on the nanobelts in the solution. Under the electrical polarization force, the nanobelts were deposited on the electrodes. By precisely controlling the concentration of the nanobelt in the solution, ...

Wurtzite-structured ZnO has versatile properties that are important for applications in electronics, optoelectronics, photovoltaics, and sensors. 1 Recently, one-dimensional (1D) nanostructures of ZnO, such as nanowires (NWs), nanorods (NRs), nanobelts, and nanotubes, are attracting much interest. 2 It is highly desired to grow 1D nanomaterials that have not only controlled shapes and crystal structure but also designed electrical and optical properties for applications as sensors, field-emitters, p-n diodes, and the diluted magnetic semiconductors (DMS) for spintronics. 3 A key requirement for many of these applications is the doping of ZnO with various elements for enhancing and controlling its electrical and optical performance. 3c,g,4 In this paper, large-scale Ni-doped ZnO NW arrays are prepared for the first time using a metal vapor vacuum arc (MEVVA) ion source doping technique. 5 By measuring the transport property of a single NW across two electrodes, the electrical conductivity of the doped ZnO NW has been increased for 30 times after doped to a dose of 2 × 10 17 cm -2 . The photoluminescence (PL) spectrum of the doped ZnO NWs has a red shift, suggesting possible doping induced band edge bending. The doped ZnO NW arrays could be the basis for building integrated nanoscale transistors, sensors, and photodetectors.In the vapor-liquid-solid (VLS) growth, 6 a nominally 2-nmthick Au thin film was deposited onto (11-20) Al 2 O 3 substrate and served as the catalyst for the VLS growth. The experimental apparatus has been described in a previous study. 7 A mixture of commercial ZnO and graphite powders in a ratio of Zn:C ) 4:1 was placed in an alumina boat, which was heated to a peak temperature of 1100°C. The Al 2 O 3 substrate was placed at a temperature zone of ∼800°C for collecting ZnO nanostructures. After the tube had been evacuated to a pressure of 1 × 10 -3 Torr, the samples were heated to 1100°C and held at 1100°C for 60 min with a carrier gas of Ar + O 2 flowing through the tube.Without any treatment, the well-aligned ZnO NWs were directly doped by MEVVA with Ni ion at a dose of 2 × 10 17 cm -2 and an incident angle of 5°with the vertical NWs (see Figure 1a), at an extraction voltage of 100 kV and at 200°C. The doped samples were then treated by thermal annealing at 950°C for 2 h in O 2 ambient for eliminating the doping-induced defects.After the annealing process, the substrate-bound NWs were mechanically scrapped off and sonicated in ethanol and deposited on a grid for transmission electron microscope (TEM) characterization. To perform electrical measurements, the NW devices were fabricated by electron beam lithography with lift-off technique. The PL properties of the synthesized NWs were studied at room temperature using a He-Cd laser in the spectral range of 350-800 nm with an excitation wavelength of 325 nm. Figure 1b is the well-aligned, Ni-doped ZnO NWs, showing fairly uniform morphology. After the doping process and post-doping annealing, the distribution of Ni dopants and structure of th...