Zinc oxide, an important semiconducting and piezoelectric material, has three key characteristics. First, it is a semiconductor, with a direct bandgap of 3.37 eV and a large excitation binding energy (60 meV), and exhibits near‐UV emission and transparent conductivity. Secondly, due to its non‐centrosymmetric symmetry, it is piezoelectric, which is a key phenomenon in building electro‐mechanical coupled sensors and transducers. Finally, ZnO is bio‐safe and bio‐compatible, and can be used for biomedical applications without coating. With these unique advantages, ZnO is one of the most important nanomaterials for integration with microsystems and biotechnology. Structurally, due to the three types of fastest growth directions—<0001>, <01$ \bar 1 $0>, and <2$ \bar 1 $$ \bar 1 $0>—as well as the ±(0001) polar surfaces, a diverse group of ZnO nanostructures have been grown in our laboratory. These include nanocombs, nanosaws, nanosprings, nanorings, nanobows, and nanopropellers. This article reviews our recent progress in the synthesis and characterization of polar‐surface‐induced ZnO nanostructures, their growth mechanisms, and possible applications as sensors, transducers, and resonators. It is suggested that ZnO could be the next most important nanomaterial after carbon nanotubes.

The mechanical resonance of a single ZnO nanobelt, induced by an alternative electric field, was studied by in situ transmission electron microscopy. Due to the rectangular cross section of the nanobelt, two fundamental resonance modes have been observed corresponding to two orthogonal transverse vibration directions, showing the versatile applications of nanobelts as nanocantilevers and nanoresonators. The bending modulus of the ZnO nanobelts was measured to be ∼52 GPa and the damping time constant of the resonance in a vacuum of 5×10−8 Torr was ∼1.2 ms and quality factor Q=500.

Solution‐phase synthesis of single‐crystal ZnO disks and rings was achieved in high yield at low temperature (70–90 °C) by using an anionic surfactant as a template. The reaction can be controlled by means of the growth temperature and the molar ratio of reagents to favor formation of disks or rings. A growth mechanism is proposed on the basis of structural information provided by SEM and TEM.

We report the synthesis and characterization of nanowire−nanoribbon junction arrays of ZnO, which were grown by thermal evaporation of the mixture of ZnO and SnO2 powders at 1300 °C through a vapor−liquid−solid process. The Sn particles produced by the reduction of SnO2 act as the catalyst; the structure is formed due to a fast growth of ZnO nanowires along [0001] and the subsequent “epitaxial” radial growth of the ZnO nanoribbons along the six 〈011̄0〉 directions around the nanowire. The “liana” shape nanostructure could be a candidate for fabricating ultrahigh sensitive sensors.

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, ...

Well-aligned ZnO nanorods with identical crystallographic orientation have been synthesized using a vapor transport deposition process. Orientation-ordered nanorods grow normal to the c planes of the as-deposited micrometer-sized ZnO rods on a polycrystalline Al 2 O 3 substrate, and each nanorod is along [0001] and enclosed by {21 h1 h0} facet surfaces. The nanorods remain in an identical crystal orientation with a homoepitaxial orientation relationship with the microrod. During the synthesis, reduced Sn from SnO 2 powder added to the source materials functions as a catalyst, guiding the orientation-aligned growth of ZnO nanorods. By controlling the growth time at high temperature, uniform lengths of aligned nanorods have been received. This work demonstrates that metallic Sn could be a good candidate for catalyzing the growth of 1D nanostructures.

Nanostructured ZnS nanocables and nanotubes (see Figure) with a rectangular cross section have been synthesized by chemical reaction using as‐synthesized ZnO nanobelts as a template. A small blue shift is observed for the ZnO–ZnS cable structures, suggesting a small quantum confinement effect. The template‐assisted method is demonstrated to be a unique technique for producing nanostructures with controlled morphology.

Zinc oxide ͑ZnO͒ nanobelts synthesized by thermal evaporation have been ion implanted with 30 keV Mn ϩ ions. Both transmission electron microscopy and photoluminescence investigations show highly defective material directly after the implantation process. Upon annealing to 800°C, the implanted Mn remains in the ZnO nanobelts and the matrix recovers both in structure and luminescence. The produced high-quality ZnO:Mn nanobelts are potentially useful for spintronics.