Large-area, quasialigned nanotips of tungsten oxide have been grown by a two-step high-temperature, catalyst-free, physical evaporation deposition process. The tungsten oxide nanotips are single crystalline with growth direction of ͓010͔. The tungsten oxide nanotips exhibit excellent field-emission properties with a low threshold field ͑for an emission current density of 10 mA/ cm 2 ͒ ϳ4.37 MV/ m and uniform emission from the entire arrays, as well as high time stability. These results make tungsten oxide nanotip arrays a competitive candidate for field-emission displays.

We report a physical mechanism responsible for initiating a vacuum breakdown process of a single carbon nanotube (CNT) during field emission. A quasidynamic method has been developed to simulate the breakdown process and calculate the critical field, critical emission current density and critical temperature beyond which thermal runaway occurs before the CNT temperature reaches its melting point. This model is in good agreement with experiments carried out with a single CNT on a silicon microtip.

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H 2 sensors able to operate at room temperature are very important for safe detection of H 2 leakage. We report the large electrical response to H 2 of Pt-coated WO 3 (Pt-WO 3 ) nanowire films without the need of using an external heater. More important, hydrogen sensing processes have been investigated under various conditions, including in air, vacuum filled with pure gas, and a mixture of H 2 and other gases. This is carried out with both electrical and optical methods. The evidence shows that hydrogen will inject into the nanowire and create oxygen vacancy, in addition to reducing adsorbed oxygen at the surface, as is often recognized. It is experimentally demonstrated that the increase of electrical conductivity resulting from the reaction with hydrogen is hampered by coadsorption of O 2 , while N 2 has no such effect. A model has been developed to combine these new findings to give a clearer understanding of the mechanism responsible for H 2 sensing behavior.

A technique involving a combination of using self-assembled nanomask and anisotropic plasma etching is developed for fabricating vertically aligned single-crystalline Si nanowires (SiNWs). The SiNWs are shown to have excellent field emission performance with the turn-on field as low as 0.8MV∕m and the threshold field being 5.0MV∕m. In addition, an emission current density of 442mA∕cm2 can be obtained at an applied field of ∼14MV∕m. The technique is easily employed to fabricate arrays of SiNW-based field emission microtriodes. Mechanisms are proposed to explain the formation of the SiNWs and the observed field emission properties.