Yaguang Wei

Self-powered nanowire devices

The harvesting of mechanical energy from ambient sources could power electrical devices without the need for batteries. However, although the efficiency and durability of harvesting materials such as piezoelectric nanowires have steadily improved, the voltage and power produced by a single nanowire are insufficient for real devices. The integration of large numbers of nanowire energy harvesters into a single power source is therefore necessary, requiring alignment of the nanowires as well as synchronization of their charging and discharging processes. Here, we demonstrate the vertical and lateral integration of ZnO nanowires into arrays that are capable of producing sufficient power to operate real devices. A lateral integration of 700 rows of ZnO nanowires produces a peak voltage of 1.26 V at a low strain of 0.19%, which is potentially sufficient to recharge an AA battery. In a separate device, a vertical integration of three layers of ZnO nanowire arrays produces a peak power density of 2.7 mW cm(-3). We use the vertically integrated nanogenerator to power a nanowire pH sensor and a nanowire UV sensor, thus demonstrating a self-powered system composed entirely of nanowires.

Patterned growth of vertically aligned ZnO nanowire arrays on inorganic substrates at low temperature without catalyst.

We report an approach for growing aligned ZnO nanowire arrays with a high degree control over size, orientation, dimensionality, uniformity, and possibly shape. Our method combines e-beam lithography and a low temperature hydrothermal method to achieve patterned and aligned growth of ZnO NWs at <100degreesC on general inorganic substrates, such as Si and GaN, without using catalyst. This approach opens up the possibility of applying ZnO nanowires as sensor arrays, piezoelectric antenna arrays, two-dimensional photonic crystals, IC interconnects, and nanogenerators.

Optical Fiber/Nanowire Hybrid Structures for Efficient Three-Dimensional Dye-Sensitized Solar Cells†

Renewable and green energy are the technological drivers of the future economy. Solar cells (SCs) are one of the most important sustainable energy technologies that have the potential to meet the world s energy demands. Among the various approaches to SCs, the performance of dyesensitized solar cells (DSSCs) is largely influenced by the surface area of adsorbed light-harvesting molecules. Traditional DSSCs utilize a nanoparticle film for enhancing the SC conversion efficiency. Photons absorbed by the dye monolayer create excitons that are rapidly split at the surface of the nanoparticles. After splitting, electrons are injected into the nanoparticles and holes move towards the opposite electrode by means of a redox species in an electrolyte. The surface area of the nanoparticle film and the effectiveness of charge collection by the electrodes determine the photovoltaic efficiency of the cell. The latter property has been improved by using aligned ZnO nanowire (NW) arrays, which provide direct electrical pathways for rapid collection of carriers generated throughout the device, and a full-sun efficiency of 1.5% has been demonstrated. However, the design is still based on a two-dimensional (2D) planar substrate, which has a relatively low surface area that limits the dye loading capacity and restricts mobility and adaptability for remote operation. Moreover, the increasing surface area is limited by the requirement that the electron transport distance d remains significantly smaller than the electron diffusion length Ln in order to minimize recombination of electrons with holes or other species. For wire-based SCs, in which light is illuminated perpendicular to the wire, the shadow effect from the entangled wire shaped electrode may limit the enhancement in power efficiency. We report herein an innovative hybrid structure that integrates optical fibers and nanowire (NW) arrays as threedimensional (3D) dye-sensitized solar cells (DSSCs) that have a significantly enhanced energy conversion efficiency. The ZnO NWs grow normal to the optical fiber surface and enhance the surface area for the interaction of light with dye molecules. The light illuminates the fiber from one end along the axial direction, and its internal reflection within the fiber creates multiple opportunities for energy conversion at the interfaces. In comparison to the case of light illumination normal to the fiber axis from outside the device (2D case), the internal axial illumination enhances the energy conversion efficiency of a rectangular fiber-based hybrid structure by a factor of up to six for the same device. Furthermore, the absolute full-sun efficiency (AM 1.5 illumination, 100 mWcm ) is increased to 3.3%, which is 120% higher than the highest value reported for ZnO NWs grown on a flat substrate surface and 47% higher than that of ZnO NWs coated with a TiO2 film. This research demonstrates a new approach from 2D to 3D solar cells with advantages of high efficiency, expanded mobility, surface adaptability, and concealed/remote operation capability. The DSSC hybrid structure is an integrates optical fibers and ZnO NWs grown by a chemical approach on the fiber surfaces. The design principle is shown in Figure 1. The main structure consists of a bundle of quartz fibers arranged such

Wafer-Scale High-Throughput Ordered Growth of Vertically Aligned ZnO Nanowire Arrays

This article presents an effective approach for patterned growth of vertically aligned ZnO nanowire (NW) arrays with high throughput and low cost at wafer scale without using cleanroom technology. Periodic hole patterns are generated using laser interference lithography on substrates coated with the photoresist SU-8. ZnO NWs are selectively grown through the holes via a low-temperature hydrothermal method without using a catalyst and with a superior control over orientation, location/density, and as-synthesized morphology. The development of textured ZnO seed layers for replacing single crystalline GaN and ZnO substrates extends the large-scale fabrication of vertically aligned ZnO NW arrays on substrates of other materials, such as polymers, Si, and glass. This combined approach demonstrates a novel method of manufacturing large-scale patterned one-dimensional nanostructures on various substrates for applications in energy harvesting, sensing, optoelectronics, and electronic devices.

Strain-Gated Piezotronic Logic Nanodevices

www.MaterialsViews.com C O M Strain-Gated Piezotronic Logic Nanodevices M U N By Wenzhuo Wu , Yaguang Wei , and Zhong Lin Wang * IC A IO N A self-powered [ 1 ] autonomous intelligent nanoscale system should consist of ultrasensitive nanowire (NW) based sensors, [ 2–5 ] integrated high-performance memory and logic computing components for data storage and processing as well as decision making, [ 6–12 ] and an energy scavenging unit for sustainable, self-suffi cient, and independent operation. [ 1 , 13–20 ] The existing semiconductor NW logic devices are based on electrically-gated fi eld-effect transistors, which function as both the drivers and the active loads of the logic units by adjusting the conducting channel width. [ 22 , 23 ] Moreover, the currently existing logic units are “static” and are almost completely triggered or agitated by electric signals, while the “dynamic” movable mechanical actuation is carried out by another unit possibly made of different materials. Here, we present the fi rst piezoelectric triggered mechanicalelectronic logic operation using the piezotronic effect, through which the integrated mechanical electrical coupled and controlled logic computation is achieved using only ZnO NWs. By utilizing the piezoelectric potential created in a ZnO NW under externally applied deformation, strain-gated transistors (SGTs) have been fabricated, using which universal logic components such as inverters, NAND, NOR, XOR gates have been demonstrated for performing piezotronic logic calculations, which have the potential to be integrated with the NEMS technology for achieving advanced and complex functional actions in applications of vital importance in portable electronics, medical sciences and defense technology, such as in nanorobotics for sensing and actuating, in microfl uidics [ 24 ] for controlling the circuitry of the fl uid fl ow, in other micro/nano-systems for intelligent control and action. ZnO is unique because of its coupled piezoelectric and semiconductor properties, which is the piezotronic effect dealing with the piezoelectric potential (piezopotential) tuned/gated charge carrier transport process in a semiconductor material. [ 25–27 ] The piezopotential created inside a ZnO NW under strain can be effectively used as a gate voltage, which has been applied for fabricating a range of piezotronic nanodevices, [ 26 , 28 ]

Integrated multilayer nanogenerator fabricated using paired nanotip-to-nanowire brushes.

We present a new approach to a nanogenerator (NG) that is composed of integrated, paired nanobrushes made of pyramid-shaped metal-coated ZnO nanotip (NTP) arrays and hexagonal-prism-shaped ZnO nanowire (NW) arrays, which were synthesized using a chemical approach at <100 degrees C on the two surfaces of a common substrate, respectively. The operation of the NGs relies on mechanical deflection/bending of the NWs, in which resonance of NWs is not required to activate the NG. This largely expands the application of the NGs from low frequency (approximately the hertz range) to a relatively high frequency (approximately the megahertz range) for effectively harvesting mechanical energies in our living environment. With one piece of such a structure stacked in close proximity over another to form a layer-by-layer matched brush architecture, direct current is generated by exciting the architecture using ultrasonic waves. A four-layer integrated NG is demonstrated to generate an output power density of 0.11 microW/cm(2) at 62 mV. The layer-by-layer assembly provides a feasible technology for building three-dimensional NGs for applications where force or pressure variations are available, such as a shoe pad, an underskin layer for airplanes, and next to a vibration source such as a car engine or tire.

Planar Waveguide-Nanowire Integrated Three-Dimensional Dye-Sensitized Solar Cells

We present a new approach to fabricate three-dimensional (3D) dye-sensitized solar cells (DSSCs) by integrating planar optical waveguide and nanowires (NWs). The ZnO NWs are grown normally to the quartz slide. The 3D cell is constructed by alternatively stacking a slide and a planar electrode. The slide serves as a planar waveguide for light propagation. The 3D structure effectively increases the light absorbing surface area due to internal multiple reflections without increasing electron path length to the collecting electrode, resulting in a significant improvement in energy conversion efficiency by a factor of 5.8 on average compared to the planar illumination case. Our approach demonstrates a new methodology for building large scale and high-efficient 3D solar cells that can be expanded to organic- and inorganic-based solar cells.

Patterned Growth of Horizontal ZnO Nanowire Arrays

We report an approach to fabricating patterned horizontal ZnO nanowire arrays with a high degree of control over their dimensionality, orientation, and uniformity. Our method combines electron beam lithography and a low temperature hydrothermal decomposition. This approach opens up possibilities to fabricate ZnO NW array based strain and force sensors, two-dimensional photonic crystals, integrated circuit interconnects, and alternative current nanogenerators.

Phosphorus Doped Zn1-xMgxO Nanowire Arrays

We demonstrate the growth of phosphorus doped Zn(1-x)Mg(x)O nanowire (NW) using pulsed laser deposition. For the first time, p-type Zn(0.92)Mg(0.08)O:P NWs are likely obtained in reference to atomic force microscopy based piezoelectric output measurements, X-ray photoelectron spectroscopy, and the transport property between the NWs and a n-type ZnO film. A shallow acceptor level of approximately 140 meV is identified by temperature-dependent photoluminescence. A piezoelectric output of 60 mV on average has been received using the doped NWs. Besides a control on NW aspect ratio and density, band gap engineering has also been achieved by alloying with Mg to a content of x = 0.23. The alloyed NWs with controllable conductivity type have potential application in high-efficiency all-ZnO NWs based LED, high-output ZnO nanogenerator, and other optical or electrical devices.

Spin-Polarized Electron Transport through Nanometer-Scale Al Grains

We investigate spin-polarized electron tunneling through ensembles of nanometer-scale Al grains embedded between two Co reservoirs at 4.2 K, and observe tunneling-magnetoresistance (TMR) and effects from spin precession in the perpendicular applied magnetic field (the Hanle effect). The spin-coherence time