Taisheng Wang

Low-field electron emission from tetrapod-like ZnO nanostructures synthesized by rapid evaporation

Tetrapod-like zinc oxide (ZnO) nanostructures can be synthesized by rapid heating metal zinc pellet at 900 °C under air ambient. Catalyst, vacuum, and carrying gas are not necessary, and more importantly, it is a volcanic process for rapid and mass production. The turn-on field of the synthesized tetrapod-like ZnO nanostructures is found to be as low as 1.6 V/μm at the current density of 1 μA/cm2. These ZnO nanostructures are technologically useful for vacuum electron devices because they can be easily and economically synthesized and deposited on large substrates.

Electronic transport through individual ZnO nanowires

Electronic transport through individual ZnO nanowires has been investigated. The current increases linearly with the bias and the conductance jumps upon ultraviolet illumination. The increase rate upon the illumination is much faster than the decrease rate as the light is off. The decrease rate under vacuum is slower than that in air. These phenomena are related to the surface oxygen species and further confirmed by in situ current–voltage measurements as a function of oxygen pressure at room temperature. Also, the conductance increases greatly as the temperature is raised. These results demonstrate that the surface oxygen species dominate the transport process through individual ZnO nanowires, which indicates their potential application to room temperature gas sensors.

Single-crystalline tin-doped indium oxide whiskers: Synthesis and characterization

An in situ doping approach was adopted to synthesize single-crystalline Sn-doped In2O3 [indium tin oxide (ITO)] nanowhiskers. Similar synthesis strategy could be used in the production of various doped metal oxide nanowhiskers. The growth mode of ITO nanowhiskers was discussed based on a self-catalytic vapor–liquid–solid growth mechanism. At room temperature, a photoluminescence peak at 510nm was observed, which was likely related to the surface defects of ITO nanowhiskers. In air ambient, the resistivity of individual ITO whisker was measured to be 0.40Ωcm, which was much higher than that of ITO films. A possible explanation for the high resistivity was proposed based on the influence of surface oxygen absorption.

Memory and negative photoconductivity effects of Ge nanocrystals embedded in ZrO2/Al2O3 gate dielectrics

Metal–insulator–semiconductor (MIS) structures containing Ge nanocrystals embedded in high permittivity dielectrics (ZrO2/Al2O3) are fabricated by electron-beam evaporation method. Capacitance–voltage (C–V) and I–V characteristics of the fabricated MIS structures are investigated in the dark and under illumination. Charge storing and negative photoconductivity effects of the Ge nanocrystals are experimentally demonstrated by the hysteresis in the C–V curves and the decrease of the current under illumination at a given voltage, respectively.

Numerical study of the meta-nanopyramid array as efficient solar energy absorber

Conventional dielectric moth eye structure is well known to be antireflective, but cannot work well in the whole solar spectrum. In addition, it cannot be used as a light absorber. However, in some cases, light absorbing and harvesting are important for energy conversion from light to heat or electricity. Here, we propose a metamaterial-based nanopyramid array which shows near 100% absorbing property in the entire solar spectrum (i.e. 0.2-2.5 m). In addition, the high absorption performance of meta-nanopyramid array retains very well at a wide receiving angle with polarization-independent. Thus, it can dramatically improve the efficiency of the solar light absorbing. The efficient light absorbing property can be explained in terms of the synergetic effects of slow light mode, surface plasmon polariton resonance and magnetic polariton resonance. 2013 Optical Society of America.

Synthesis and optical properties of semiconducting beta-FeSi2 nanocrystals

Vacuum electron-beam coevaporation of Fe and Si followed by annealing in N2 ambient is used to synthesize beta iron disilicide (β-FeSi2) nanocrystals. The reason for β-FeSi2 nanocrystal formation is discussed based on the principle of minimization of the interface energy. X-ray diffraction studies suggest that β-phase FeSi2 nanostructures turn into the α phase when the annealing temperature is 1000 °C. The optical transmission measurements indicate that the β-FeSi2 nanocrystals have a direct-band structure with the energy gap in the range of 0.84–0.88 eV. A sharp photoluminescence peak of the β-FeSi2 nanocrystals located at 1.5 μm is measured at 77 K.

Synthesis of large-area germanium cone-arrays for application in electron field emission

High-vacuum electron-beam evaporation was used to synthesize germanium (Ge) cone-arrays on N+-type (100) Si. The surface morphology of Ti nanocrystals’ catalyst and Ge cone-arrays was investigated by scanning electron microscopy and atomic force microscopy. The lattice constant of the Ge cones determined from x-ray diffraction was nearly identical to that of bulk Ge. The mechanism of Ge cone-arrays’ formation was investigated. The electron field emission properties of the as-prepared Ge cone-arrays were studied based on current–voltage measurements and the Fowler–Nordheim equation.

Biomimetic compound eye with a high numerical aperture and anti-reflective nanostructures on curved surfaces

Biomimetic compound eyes with a high numerical aperture on a curved surface were successfully fabricated by intelligent integration of traditional top-down and bottom-up micro- and nanofabrication methods together. In addition, the new hybrid micro- and nanofabrication method allows us to fabricate the antireflective nanostructures on each ommatidium to increase its vision sensitivity by improving the light transmission. The fabricated compound eye was optically characterized and was shown to have a numerical aperture of 0.77 for each ommatidium. Furthermore, it is shown that the transmission of the compound eye can be improved by 2.3% for the wavelength of 632.8 nm and a clearer image can be formed by the fabricated compound eye with antireflective nanostructures compared with that without antireflective nanostructures. In addition, the developed hybrid manufacturing method can be adapted to the fabrication of other complex micro- and nanodevices for photonics or other research areas.

Meta-microwindmill structure with multiple absorption peaks for the detection of ketamine and amphetamine type stimulants in terahertz domain

We report a metamaterial based microwindmill array with a periodic arrangement that can be used in terahertz detector as an absorbing layer. It is found that this structure can absorb terahertz waves efficiently with an average absorptivity of 95% at multiple frequencies of 1.516, 2.205, 2.424 and 2.565 THz, which are absorption peaks of four kinds of drugs. The efficient absorbing property of meta-microwindmill on terahertz wave can be explained in terms of the synergetic effects of localized surface plasmon resonant effect and slow light mode. Moreover, the effect of the error of the structural parameters on the absorption efficiency is carefully analyzed in detail to guide the fabrication

Fabrication of diffractive optical elements on 3-D curved surfaces by capillary force lithography.

We demonstrate the fabrication of diffractive optical elements (DOEs) on 3-Dimensional curved surfaces by capillary force lithography (CFL). Curved gratings with a period of 20mum and 820nm have been successfully fabricated in polymer on concave surfaces by CFL. The experiment results indicate that the capillary force lithography is an effective method to replicate DOEs on curved surfaces with a very high fidelity and a relatively fast speed. In addition, we found that the growth rate of the polymer in the sub-microfabrication is much faster and the step height is much closer to the master than that in the microfabrication for CFL, which makes CFL more attractive in the fabrication of DOEs with a sub-microscale or even nanoscale feature size than a microscale feature size.