Yunhua Huang

Scanning Probe Study on the Piezotronic Effect in ZnO Nanomaterials and Nanodevices

ZnO nanomaterials with their unique semiconducting and piezoelectric coupled properties have become promising materials for applications in piezotronic devices including nanogenerators, piezoelectric field effect transistors, and diodes. This article will mainly introduce the research progress on piezotronic properties of ZnO nanomaterials investigated by scanning probe microscopy (SPM) and ZnO-based prototype piezotronic nanodevices built in virtue of SPM, including piezoelectric field effect transistors, piezoelectric diodes, and strain sensors. Additionally, nanodamage and nanofailure of ZnO materials and their relevant piezotronic nanodevices will be critically discussed in their safe service in future nanoelectromechanical system (NEMS) engineering.

In Situ Transmission Electron Microscopy Investigation on Fatigue Behavior of Single ZnO Wires under High-Cycle Strain

The fatigue behavior of ZnO nanowires (NWs) and microwires was systematically investigated with in situ transmission electron microscopy electromechanical resonance method. The elastic modulus and mechanical quality factors of ZnO wires were obtained. No damage or failure was found in the intact ZnO wires after resonance for about 10(8)-10(9) cycles, while the damaged ZnO NW under electron beam (e-beam) irradiation fractured after resonance for seconds. The research results will provide a useful guide for designing, fabricating, and optimizing electromechanical nanodevices based on ZnO nanomaterials, as well as future applications.

Functional nanogenerators as vibration sensors enhanced by piezotronic effects

ZnO nanomaterials have been shown to have novel applications in optoelectronics, energy harvesting and piezotronics, due to their coupled semiconducting and piezoelectric properties. Here a functional nanogenerator (FNG) based on ZnO nanowire arrays has been fabricated, which can be employed to detect vibration in both self-powered (SP) and external-powered (EP) modes. In SP mode, the vibration responses of the FNG can be measured through converting mechanical energy directly into an electrical signal. The FNG shows consistent alternating current responses (relative error < 0.37%) at regular frequencies from 1 to 15 Hz. In EP mode, the current responses of FNG are significantly enhanced via the piezotronic effect. Under a forward bias of 3 V, the sensor presented a sensitivity of 3700% and an accurate measurement (relative error < 0.91%) of vibration frequencies in the range 0.05–15 Hz. The results show that this type of functional nanogenerator sensor can detect vibration in both SP and EP modes according to the demands of the applications.

Doping and defects in the formation of single-crystal ZnO nanodisks

High purity growth of polar surface dominated ZnO nanodisks was fabricated by introducing In ions in the raw material by thermal evaporation process without a catalyst. The nature of the sharp-contrast lines in the disks was investigated. The results suggested that the existence of sharp-contrast lines is due to the local segregation of In. Defects were initiated by segregation of the doping element of indium, which reduced the surface energy of ZnO (0001) leading to the fastest growth of the nanodisks along ⟨011¯0⟩. The preferred growth along ⟨011¯0⟩ is considered to maximize the effect of the piezoelectricity.

Improvement of room temperature tensile properties for Fe3Al-based alloys by thermomechanical and annealing processes

Abstract The effects of thermomechanical and annealing processes on room temperature tensile properties of Fe 3 Al-based alloys have been investigated. It is found that the thermomechanical process affects the room temperature tensile properties of Fe 3 Al-based alloys greatly. It is very important to control the hot deformation process so that a fine grain microstructure can be obtained before the final warm-rolling process. A higher room temperature ductility combined with a higher yield stress can be obtained by refining grains of Fe 3 Al-based alloys and obtaining the B2 ordered structure with a suitable surface state, such as oil film, Al 2 O 3 film and an aluminium-poor region near the surface. The intrinsic factors, such as microstructure and crystal structure, and extrinsic factors, such as surface condition, must be considered when the sensitivity to environmental embrittlement is investigated. An advanced thermomechanical process, named controlled thermomechanical process, to further improve room temperature tensile properties has been developed. It consists of hot forging an ingot to 20 mm at 1000–1200°C, hot rolling it to 10 mm at 850–1000°C, warm rolling to 5 mm thickness at 500–680°C, then annealing at 800°C for 1 h, then warm rolling to 2 mm thickness at 500–680°C.

Electrical breakdown of ZnO nanowires in metal-semiconductor-metal structure

We investigated the stability of ZnO nanowires in a metal-semiconductor-metal structure by applying a longitudinal electric field inside a scanning electron microscope equipped with manipulators. The electrical transport was well simulated by the thermionic-field-emission model and the failure of single crystalline ZnO nanowires was directly observed when the applied electric field reached the break point, an electric field intensity of ∼106 V/m. The recrystallization of ZnO nanowires from single crystalline to polycrystalline pearl-like structure in the failure process was also investigated. Experimental results indicated that the failure is attributed to a joint effect of high electric field and Joule heating.

Controllable synthesis of well-dispersed and uniform-sized single crystalline zinc hydroxystannate nanocubes

Single crystalline zinc hydroxystannate (ZnSn(OH)6) nanocubes have been synthesized on α-{Cu,Sn} copper foils by a low-temperature hydrothermal method. The obtained ZnSn(OH)6 nanocubes have the cubic perovskite crystalline structure and uniform arrangement on the substrate. The possible growth process of the ZnSn(OH)6 crystalline is proposed. The well-dispersed distribution and uniform size of the ZnSn(OH)6 cubes are successfully achieved thanks to the substrate which plays an important role to control the nucleation site and provide Sn4+ ions. The size of the cubes can be adjusted from nanometre to micrometre scale by prolonging the reaction time. After the secondary nucleation, a novel morphology of mother–daughter double cubes was formed. The decomposition of ZnSn(OH)6 cubes annealed at high temperature is also investigated, and interesting hollow-shaped morphology changes are discovered. This method will be useful for the controllable fabrication of nanomaterials and potential applications in nanoscale devices.

Explosive field emission and plasma expansion of carbon nanotube cathodes

High intensity electron emission cathodes based on carbon nanotube films have been successfully fabricated. An investigation of the explosive field emission properties of the carbon nanotube cathode in a double-pulse mode was presented and a high emission current density of 309A∕cm2 was obtained. The time-and-space resolution of the electron-beam flow from the cathode was investigated. The formation of the cathode plasma layer was proven and the plasma expanded at a velocity of ∼7.8cm∕μs toward the anode. The formation of cathode plasma has no preferential position and the local enhancement of electron beams is random.

Synthesis, Structure and Properties of Sn-doped ZnO Nanobelts

Abstract Single crystalline Sn-doped ZnO nanobelts were successfully synthesized by the carbon thermal reduction deposition process without using any catalyst. XRD investigation confirmed that the products were of the wurtzite structure of ZnO. SEM, EDS, and TEM analyses showed that the Sn-doped ZnO nanostructures contained a belt-like morphology with Sn doping content about 1.9%, and the growth direction of nanobelts was along the [0001] direction. A weak UV emission peak at around 398.4 nm and the strong green emission peak at around 494.8 nm were observed at room temperature. The luminescence mechanism of the Sn/ZnO nanobelts was also discussed. These nanobelts were promising building blocks for the fabrication of nanoscale optoelectronic devices.

Growth mechanism and optical properties of ZnS nanotetrapods

ZnS nanotetrapods have been synthesized through the direct reaction of Zn with S via a thermal evaporation method in Ar at 800 °C without catalyst. Studies on the microstructures give experimental evidence for the growth mechanism of ZnS nanotetrapods. A zinc blende (ZB) ZnS core nucleates in the first stage during the formation of ZnS nanotetrapods; wurtzite twins based on the ZB core then form and lead to the assembly of ZnS nanotetrapod structures. Room-temperature photoluminescence properties were also examined, showing a strong green emission centered at 495.2 nm.