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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 ∼107cm−1 and an electron mobility of ∼17cm2∕Vs. 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.

This article provides a comprehensive review of the current research activities that focus on the ZnO nanostructure materials and their physical property characterizations. It begins with the synthetic methods that have been exploited to grow ZnO nanostructures. A range of remarkable characteristics are then presented, organized into sections describing the mechanical, electrical, optical, magnetic, and chemical sensing properties. These studies constitute the basis for developing versatile applications of ZnO nanostructures.

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ZnO nanowire field effect transistors were implemented as highly sensitive chemical sensors for detection of NO2 and NH3 at room temperature. Due to a Debye screening length comparable to the nanowire diameter, the electric field applied over the back gate electrode was found to significantly affect the sensitivity as it modulates the carrier concentration. A strong negative field was utilized to refresh the sensors by an electrodesorption mechanism. In addition, different chemisorbed species could be distinguished from the “refresh” threshold voltage and the temporal response of the conductance. These results demonstrated a refreshable field effect sensor with a potential gas identification function.

ZnO nanowires with high crystalline and optical properties are characterized, showing strong effect of the surface defect states. In order to optimize the performance of devices based on these nanowires, a series of complementary metal-oxide semiconductor compatible surface passivation procedures is employed. Electrical transport measurements demonstrate significantly reduced subthreshold swing, high on/off ratio, and unprecedented field effect mobility.

A chemical vapor deposition (CVD) process modified with vapor trapping method has been used to synthesize n-type ZnO nanowires with high carrier concentration without incorporating impurity dopants. With this method, a spatial variation of synthesis condition was created and the donors were directly introduced into the nanowires during the synthesis process. Electron microscopy and electrical transport studies show that nanowires having distinct morphologies and electrical properties were obtained at different locations in the CVD system. The vapor trapping method elucidates the effect of synthesis conditions, and provides an approach to control nanowire growth for tailorable device applications.

First-principles calculations were performed to study the stabilities, electronic structures, and chemical activities of various CuO surfaces for the understanding of the gas-induced change of conductance of CuO nanowires. It was found that CuO(111) and CuO(1 j 11) have the lowest surface energies under ambient conditions and hence should be the most preferential facets of CuO nanowires. While band gaps of these surfaces are narrower than that of bulk CuO, they maintain the semiconductor feature. Adsorption of oxidizing gas such as O 2 or NO 2 on the CuO(111) and CuO(1 j 11) surfaces induces metallic behavior, and molecules gain electrons from the substrates. These two effects result in an increase of hole density and hence enhance the surface conductivity of CuO nanowires as observed in our experiments. On the contrary, adsorption of H 2 O molecules on CuO(111) not only widens the band gap but also donates electrons to the surface, which leads to reduction of surface conductivity. In addition, we found that CuO(111) is potentially an efficient catalyst for CO oxidation through the Mars-van Krevelen mechanism. † Part of the "D. Wayne Goodman Festschrift".