Rusen Yang

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.

Flexible High-Output Nanogenerator Based on Lateral ZnO Nanowire Array

We report here a simple and effective approach, named scalable sweeping-printing-method, for fabricating flexible high-output nanogenerator (HONG) that can effectively harvesting mechanical energy for driving a small commercial electronic component. The technique consists of two main steps. In the first step, the vertically aligned ZnO nanowires (NWs) are transferred to a receiving substrate to form horizontally aligned arrays. Then, parallel stripe type of electrodes are deposited to connect all of the NWs together. Using a single layer of HONG structure, an open-circuit voltage of up to 2.03 V and a peak output power density of approximately 11 mW/cm(3) have been achieved. The generated electric energy was effectively stored by utilizing capacitors, and it was successfully used to light up a commercial light-emitting diode (LED), which is a landmark progress toward building self-powered devices by harvesting energy from the environment. This research opens up the path for practical applications of nanowire-based piezoelectric nanogeneragtors for self-powered nanosystems.

Flexible Piezotronic Strain Sensor

Strain sensors based on individual ZnO piezoelectric fine-wires (PFWs; nanowires, microwires) have been fabricated by a simple, reliable, and cost-effective technique. The electromechanical sensor device consists of a single electrically connected PFW that is placed on the outer surface of a flexible polystyrene (PS) substrate and bonded at its two ends. The entire device is fully packaged by a polydimethylsiloxane (PDMS) thin layer. The PFW has Schottky contacts at its two ends but with distinctly different barrier heights. The I- V characteristic is highly sensitive to strain mainly due to the change in Schottky barrier height (SBH), which scales linear with strain. The change in SBH is suggested owing to the strain induced band structure change and piezoelectric effect. The experimental data can be well-described by the thermionic emission-diffusion model. A gauge factor of as high as 1250 has been demonstrated, which is 25% higher than the best gauge factor demonstrated for carbon nanotubes. The strain sensor developed here has applications in strain and stress measurements in cell biology, biomedical sciences, MEMS devices, structure monitoring, and more.

Converting Biomechanical Energy into Electricity by a Muscle-Movement-Driven Nanogenerator

A living species has numerous sources of mechanical energy, such as muscle stretching, arm/leg swings, walking/running, heart beats, and blood flow. We demonstrate a piezoelectric nanowire based nanogenerator that converts biomechanical energy, such as the movement of a human finger and the body motion of a live hamster (Campbells dwarf), into electricity. A single wire generator (SWG) consists of a flexible substrate with a ZnO nanowire affixed laterally at its two ends on the substrate surface. Muscle stretching results in the back and forth stretching of the substrate and the nanowire. The piezoelectric potential created inside the wire leads to the flow of electrons in the external circuit. The output voltage has been increased by integrating multiple SWGs. A series connection of four SWGs produced an output voltage of up to approximately 0.1-0.15 V. The success of energy harvesting from a tapping finger and a running hamster reveals the potential of using the nanogenerators for scavenging low-frequency energy from regular and irregular biomotion.

Growth and field-emission property of tungsten oxide nanotip arrays

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 [010]. The tungsten oxide nanotips exhibit excellent field-emission properties with a low threshold field (for an emission current density of 10mA∕cm2) ∼4.37MV∕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.

Ordered Nanowire Array Blue/Near-UV Light Emitting Diodes

[∗] S. Xu , C. Xu , Y. Liu , Y. F. Hu , R. S. Yang , Q. Yang , Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology Atlanta, Georgia, 30332–0245 (USA) E-mail: zlwang@gatech.edu J. H. Ryou , H. J. Kim , Z. Lochner , S. Choi , Prof. R. Dupuis School of Electrical and Computer Engineering Georgia Institute of Technology Atlanta, Georgia, 30332–0245 (USA) ZnO-based light emitting diodes (LEDs) have been considered as a potential candidate for the next generation of blue/ near-UV light sources, [ 1 ] due to a direct wide bandgap energy of 3.37 eV, a large exciton binding energy of 60 meV at room temperature, and several other manufacturing advantages of ZnO. [ 2 ] While the pursuit of stable and reproducible p-ZnO is still undergoing, [ 3,4 ] heterojunctions of n-ZnO and p-GaN are employed as an alternative approach in this regard by considering the similar crystallographic and electronic properties of ZnO and GaN. [ 5–7 ] Compared with the thin fi lm/thin fi lm LEDs, [ 5,6 , 8 ] which may suffer from the total internal refl ection, n-ZnO nanowire/p-GaN thin fi lm heterostructures are utilized in order to increase the extraction effi ciency of the LEDs by virtue of the waveguiding properties of the nanowires. [ 9–11 ] But in all of these cases, the n-ZnO nanowires are randomly distributed on the substrate, which largely limits their applications in high performance optoelectronic devices. Here in this work, we demonstrate the capability of controlling the spatial distribution of the blue/near-UV LEDs composed of position controlled arrays of n-ZnO nanowires on a p-GaN thin fi lm substrate. The device was fabricated by a conjunction of low temperature wet chemical methods and electron beam lithography (EBL). The EBL could be replaced by other more convenient patterning techniques, such as photolithography and nanosphere lithography, rendering our technique low cost and capable of scaling up easily. Under forward bias, each single nanowire is a light emitter. By Gaussian deconvolution of the emission spectrum, the origins of the blue/nearUV emission are assigned particularly to three distinct electronhole recombination processes. By virtue of the nanowire/thin fi lm heterostructures, these LEDs give an external quantum effi ciency of 2.5%. This approach has great potential applications in high resolution electronic display, optical interconnect, and high density data storage. The design of the LED is shown in Figure 1a . Ordered ZnO nanowire arrays were grown on p-GaN (Figure 1 b–d), [ 12–14 ]

Muscle‐Driven In Vivo Nanogenerator

2010 WILEY-VCH Verlag Gmb Harvesting energy from the environment is crucial for the independent, wireless, and sustainable operation of nanodevices. This is a key requirement for building self-powered nanosystems. The living environment of nanodevices is diverse ranging from natural to in vitro and in vivo. Mechanical energy is one of the most abundant and popular energies in the environment, which can range from wind energy to mechanical vibration, sonic/ ultrasonic waves, noise, fluidics, biomotions, muscle stretching, and more. Harvesting energy using piezoelectric materials has been demonstrated some time ago, but these structures are rather large and a tiny physical motion, such as the contraction of a blood vessel is not strong enough to drive the generator. More importantly, the demonstrated cantilever-based microelectromechanical systems (MEMS) work only under a specific driving resonance frequency that is determined by the cantilever. In a real biological system, the mechanical disturbance has a large frequency range and the mechanical vibration is time dependent. We have demonstrated a few approaches for harvesting mechanical energy from several different sources, one of which was based on an alternating-current (AC) nanogenerator using a two-ends-bonded piezoelectric nanowire (NW). The NW is laterally bonded on a flexible substrate, and the physical deformation of the NW is directly driven by the shape change of the substrate that is induced by external dynamic mechanical sources. When a ZnO NW is subject to a periodic mechanical stretching and releasing, the mechanical–electric coupling effect of the NW, combined with the gate effect of the Schottky contact at the interface, results in a alternating flow of the charge in the external circuit. The single-wire generator (SWG) acts as a ‘‘charging pump’’ that drives the electronmotion in accordance to the mechanical deformation of the NW. Recently, we have applied the AC generator to harvest mechanical energy from body movement under in vitro conditions. However, the applications of the nanogenerators under in vivo and in vitro environments are distinct. Some crucial problems need to be addressed before using these devices in the human body, such as biocompatibility and toxicity. To directly interface nanowires with cells, our studies have indicated that ZnO nanowires can be safely used for in vivo applications and they are biodegradable. In this Communication, in vivo biomechanical-energy harvesting using an AC nanogenerator has been achieved for the first time. We demonstrate the implanting of the nanogenerator in a live rat to harvest energy generated by its breath and heartbeat. This study shows the potential of applying nanogenerators for the scavenging of low-frequency dynamic muscle energy created by very small-scale physical motion for the possible driving of in vivo nanodevices. The fabrication process of a SWG was presented in detail in our previous publication. The piezoelectric ZnO NW was grown using a physical-vapor deposition process and had a diameter of 100–800 nm and a length of 100–500mm. The two ends of the NW were tightly fixed to the surface of a flexible polyimide substrate by applying silver paste and two lead wires, isolated from the environment, were connected to the ends. Because of the presence of bio-fluids under the in vivo working condition, the entire device was covered with a flexible polymer to isolate it from the surrounding medium and to improve its robustness. The short-circuit current (Isc) and open-circuit voltage (Voc) were measured to examine the performance of the SWG. All of the measurements were performed in a wellgrounded and screened environment and the noise level was carefully minimized. The output Voc and Isc of the SWGare typically less than 50mV and 500 pA, respectively, in most cases. Therefore, careful experiments have to be conducted to rule out possible artifacts introduced by factors such as themeasurement system, change in capacitance of the nanowire and the electric circuit, and/or the coupling of the SWGwith the measurement system. A series of testing criteria has been established before to identify the true signal generated by the SWG. An effective SWG must exhibit Schottky behavior at one end before and after measurement (Fig. 1c). The output voltage and current of a SWG should meet the switching polarity test. For easy notation and reference, we define the side of a SWGwith Schottky contact as the positive side.When the positive and negative probes of the measurement system are connected to the positive and negative sides of the SWG, respectively, the configuration is described as being a forward connection. The configuration with the two probes switched over is defined as a reverse connection. Both configurations were tested. The magnitude of the signal from different connecting configurations may differ because of the influence of a small bias current in themeasurement system. So the magnitude of the true signal is an average of those under forward and reverse connections. The first group of experiments was based on the conversion of the mechanical deformation related to the periodic expansion and contraction of the diaphragm of a rat into electricity (Fig. 1a). Adult rats (Hsd: Sprague Dawley SD, male, 200–224g) were used for the experiment. Our procedure in handling the animals followed National Institutes of Health (NIH) policies and guidelines, university policies, and Association for Assessment and Accreditation of Laboratory Animal Care (AAALAC)

Temperature dependence of the Raman spectra of single-wall carbon nanotubes

Raman spectra of single-wall carbon nanotubes (SWCNTs) were measured at different temperatures by varying the incident laser power. The elevated temperature of the SWCNTs and multiwall carbon nanotubes (MWCNTs) is confirmed to be due to the presence of impurities, defects, and disorder. The temperature coefficient of the frequency of the C–C stretching mode E2g (GM) and that of the radial breathing mode in the SWCNT were determined to be ∼−0.038 and ∼−0.013 cm−1/K, respectively. It is found that the temperature coefficient of the GM in the SWCNT is larger than that of the MWCNT, highly oriented pyrolytic graphite, and the graphite. This is attributed to the structural characteristic of the SWCNT—a single tubular carbon sheet with smaller diameter.

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.

Enhanced ferroelectric-nanocrystal-based hybrid photocatalysis by ultrasonic-wave-generated piezophototronic effect.

An electric field built inside a crystal was proposed to enhance photoinduced carrier separation for improving photocatalytic property of semiconductor photocatalysts. However, a static built-in electric field can easily be saturated by the free carriers due to electrostatic screening, and the enhancement of photocatalysis, thus, is halted. To overcome this problem, here, we propose sonophotocatalysis based on a new hybrid photocatalyst, which combines ferroelectric nanocrystals (BaTiO3) and semiconductor nanoparticles (Ag2O) to form an Ag2O-BaTiO3 hybrid photocatalyst. Under periodic ultrasonic excitation, a spontaneous polarization potential of BaTiO3 nanocrystals in responding to ultrasonic wave can act as alternating built-in electric field to separate photoinduced carriers incessantly, which can significantly enhance the photocatalytic activity and cyclic performance of the Ag2O-BaTiO3 hybrid structure. The piezoelectric effect combined with photoelectric conversion realizes an ultrasonic-wave-driven piezophototronic process in the hybrid photocatalyst, which is the fundamental of sonophotocatalysis.