Chun Cheng

Enhanced photocatalytic performance of TiO2-ZnO hybrid nanostructures

We studied the photocatalytic properties of rational designed TiO2-ZnO hybrid nanostructures, which were fabricated by the site-specific deposition of amorphous TiO2 on the tips of ZnO nanorods. Compared with the pure components of ZnO nanorods and amorphous TiO2 nanoparticles, these TiO2-ZnO hybrid nanostructures demonstrated a higher catalytic activity. The strong green emission quenching observed from photoluminescence of TiO2-ZnO hybrid nanostructures implied an enhanced charge transfer/separation process resulting from the novel type II heterostructures with fine interfaces. The catalytic performance of annealing products with different TiO2 phase varied with the annealing temperatures. This is attributed to the combinational changes in Eg of the TiO2 phase, the specific surface area and the quantity of surface hydroxyl groups.

Comprehensive study of the metal-insulator transition in pulsed laser deposited epitaxial VO2 thin films

In this paper, we present a comprehensive, correlative study of the structural, transport, optical and thermoelectric properties of high-quality VO2 thin films across its metal-insulator phase transition. Detailed x-ray diffraction study shows that its textured polycrystalline along [010]M1, with in-plane lattice orienting along three equivalent crystallographic directions. Across the metal-insulator transition, the conductivity increases by more than 3 orders of magnitude with a value of 3.8 × 103 S/cm in the metallic phase. This increase is almost entirely accounted for by a change in electron density, while the electron mobility changes only slightly between the two phases, yet shows strong domain boundary scattering when the two phases coexist. Electron effective mass was determined to be ∼65m0 in the insulating phase. From the optical and infrared reflection spectra in the metallic phase, we obtained the plasma edge of VO2, from which the electron effective mass was determined to be ∼23m0. The bandg...

Giant-Amplitude, High-Work Density Microactuators with Phase Transition Activated Nanolayer Bimorphs

Various mechanisms are currently exploited to transduce a wide range of stimulating sources into mechanical motion. At the microscale, simultaneously high amplitude, high work output, and high speed in actuation are hindered by limitations of these actuation mechanisms. Here we demonstrate a set of microactuators fabricated by a simple microfabrication process, showing simultaneously high performance by these metrics, operated on the structural phase transition in vanadium dioxide responding to diverse stimuli of heat, electric current, and light. In both ambient and aqueous conditions, the actuators bend with exceedingly high displacement-to-length ratios up to 1 in the sub-100 μm length scale, work densities over 0.63 J/cm(3), and at frequencies up to 6 kHz. The functionalities of actuation can be further enriched with integrated designs of planar as well as three-dimensional geometries. Combining the superior performance, high durability, diversity in responsive stimuli, versatile working environments, and microscale manufacturability, these actuators offer potential applications in microelectromechanical systems, microfluidics, robotics, drug delivery, and artificial muscles.

Ultra-long, free-standing, single-crystalline vanadium dioxide micro/nanowires grown by simple thermal evaporation

Recently, it was discovered that single-crystalline VO2 nanostructures exhibit unique, single-domain metal-insulator phase transition. They enable a wide range of device applications as well as discoveries of oxide physics beyond those can be achieved with VO2 bulk or thin films. Previous syntheses of these nanostructures are limited in density, aspect ratio, single-crystallinity, or by substrate clamping. Here we break these limitations and synthesize ultra-long, ultra-dense, and free-standing VO2 micro/nanowires using a simple vapor transport method. These are achieved by enhancing the VO2 nucleation and growth rates using rough-surface quartz as the substrate and V2O5 powder as the evaporation source.

High-Quality ZnO Nanowire Arrays Directly Fabricated from Photoresists

We report a simple and effective method for fabricating and patterning high-quality ZnO nanowire arrays using carbonized photoresists to control the nucleation site, density, and growth direction of the nanowires. The ZnO nanowires fabricated using this method show excellent alignment, crystal quality, and optical properties that are independent of the substrates. The carbonized photoresists provide perfect nucleation sites for the growth of aligned ZnO nanowires and they also perfectly connect to the nanowires to form ideal electrodes that can be used in many applications of ZnO nanomaterials.

Axially engineered metal-insulator phase transition by graded doping VO2 nanowires.

The abrupt first-order metal-insulator phase transition in single-crystal vanadium dioxide nanowires (NWs) is engineered to be a gradual transition by axially grading the doping level of tungsten. We also demonstrate the potential of these NWs for thermal sensing and actuation applications. At room temperature, the graded-doped NWs show metal phase on the tips and insulator phase near the center of the NW, and the metal phase grows progressively toward the center when the temperature rises. As such, each individual NW acts as a microthermometer that can be simply read out with an optical microscope. The NW resistance decreases gradually with the temperature rise, eventually reaching 2 orders of magnitude drop, in stark contrast to the abrupt resistance change in undoped VO2 wires. This novel phase transition yields an extremely high temperature coefficient of resistivity ~10%/K, simultaneously with a very low resistivity down to 0.001 Ω·cm, making these NWs promising infrared sensing materials for uncooled microbolometers. Lastly, they form bimorph thermal actuators that bend with an unusually high curvature, ~900 m(-1)·K(-1) over a wide temperature range (35-80 °C), significantly broadening the response temperature range of previous VO2 bimorph actuators. Given that the phase transition responds to a diverse range of stimuli-heat, electric current, strain, focused light, and electric field-the graded-doped NWs may find wide applications in thermo-opto-electro-mechanical sensing and energy conversion.

Optimizing nanosheet-based ZnO hierarchical structure through ultrasonic-assisted precipitation for remarkable photovoltaic enhancement in quasi-solid dye-sensitized solar cells

For ZnO hierarchical structures composed of interlaced nanosheets, it has been proved that they are more favorable for electron transportation in the photoanodes of ZnO-based dye-sensitized solar cells (DSCs). Here, we introduce ultrasonic-assisted precipitation for fabricating novel nanosheet-based ZnO hierarchical flowers (HFs) in aqueous solution. With the powerful ultrasound irradiation, these nanosheets on the HFs are not only interlaced and monocrystalline, but also axially oriented, porous and ultrathin. Furthermore, broad channels enclosed by adjacent nanosheets can deeply extend into the inner parts of the HFs. Structural improvements reveal that the specific area of the novel HFs as well as their performances on light-capturing and electron transport have been largely improved compared with those prepared through direct precipitation. Remarkably, when assembled into quasi-solid DSCs, ZnO HF photoanodes show a high conversion efficiency up to 6.19% (under AM 1.5, 100 mW cm−2 illumination), the highest record of quasi-solid ZnO-based DSCs up to now.

Heat transfer across the interface between nanoscale solids and gas.

When solid materials and devices scale down in size, heat transfer from the active region to the gas environment becomes increasingly significant. We show that the heat transfer coefficient across the solid-gas interface behaves very differently when the size of the solid is reduced to the nanoscale, such as that of a single nanowire. Unlike for macroscopic solids, the coefficient is strongly pressure dependent above ∼10 Torr, and at lower pressures it is much higher than predictions of the kinetic gas theory. The heat transfer coefficient was measured between a single, free-standing VO(2) nanowire and surrounding air using laser thermography, where the temperature distribution along the VO(2) nanowire was determined by imaging its domain structure of metal-insulator phase transition. The one-dimensional domain structure along the nanowire results from the balance between heat generation by the focused laser and heat dissipation to the substrate as well as to the surrounding gas, and thus serves as a nanoscale power-meter and thermometer. We quantified the heat loss rate across the nanowire-air interface, and found that it dominates over all other heat dissipation channels for small-diameter nanowires near ambient pressure. As the heat transfer across the solid-gas interface is nearly independent of the chemical identity of the solid, the results reveal a general scaling relationship for gaseous heat dissipation from nanostructures of all solid materials, which is applicable to nanoscale electronic and thermal devices exposed to gaseous environments.

Dense Electron System from Gate-Controlled Surface Metal–Insulator Transition

Two-dimensional electron systems offer enormous opportunities for science discoveries and technological innovations. Here we report a dense electron system on the surface of single-crystal vanadium dioxide nanobeam via electrolyte gating. The overall conductance of the nanobeam increases by nearly 100 times at a gate voltage of 3 V. A series of experiments were carried out which rule out electrochemical reaction, impurity doping, and oxygen vacancy diffusion as the dominant mechanism for the conductance modulation. A surface insulator-to-metal transition is electrostatically triggered, thereby collapsing the bandgap and unleashing an extremely high density of free electrons from the original valence band within a depth self-limited by the energetics of the system. The dense surface electron system can be reversibly tuned by the gating electric field, which provides direct evidence of the electron correlation driving mechanism of the phase transition in VO(2). It also offers a new material platform for implementing Mott transistor and novel sensors and investigating low-dimensional correlated electron behavior.

Universal low-temperature Ohmic contacts for quantum transport in transition metal dichalcogenides

Low carrier mobility and high electrical contact resistance are two major obstacles prohibiting explorations of quantum transport in TMDCs. Here, we demonstrate an effective method to establish low-temperature Ohmic contacts in boron nitride encapsulated TMDC devices based on selective etching and conventional electron-beam evaporation of metal electrodes. This method works for most extensively studied TMDCs in recent years, including MoS2, MoSe2, WSe2, WS2, and 2H-MoTe2. Low electrical contact resistance is achieved at 2 K. All of the few-layer TMDC devices studied show excellent performance with remarkably improved field-effect mobilities ranging from 2300 cm2/V s to 16000 cm2/V s, as verified by the high carrier mobilities extracted from Hall effect measurements. Moreover, both high-mobility n-type and p-type TMDC channels can be realized by simply using appropriate contact metals. Prominent Shubnikov-de Haas oscillations have been observed and investigated in these high-quality TMDC devices.