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Nanopiezotronics - a pathway to self-powering nanodevices

(Nanowerk Spotlight) Piezoelectricity is a coupling between a material's mechanical and electrical behavior. When a piezoelectric material is squeezed, twisted, or bent, electric charges collect on its surfaces. Conversely, when a piezoelectric material is subjected to a voltage drop, it mechanically deforms. Many crystalline materials exhibit piezoelectric behavior and when such a crystal is mechanically deformed, the positive- and negative-charge centers are displaced with respect to each other. So while the overall crystal remains electrically neutral, the difference in charge center displacements results in an electric polarization within the crystal. Electric polarization resulting from mechanical deformation is perceived as piezoelectricity. This phenomenon was discovered by the brothers Pierre and Jacques Curie in 1880 and the word is derived from the Greek piezein, which means to squeeze or press.
The piezoelectric effect finds useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalance, and ultra fine focusing of optical assemblies. For instance, types of piezoelectric motor include the well-known traveling-wave motor used for auto-focus in reflex cameras. A new research field, nanopiezotronics refers to generation of electrical energy at the nanometer scale via mechanical stress to the nanopiezotronic device. For example, bending of a zinc oxide nanowire transforms that mechanical energy into electrical energy. This new approach has the potential of converting biological mechanical energy, acoustic/ultrasonic vibration energy, and biofluid hydraulic energy into electricity, demonstrating a new pathway for self-powering of wireless nanodevices and nanosystems.
The term "nanopiezotronics" was coined by Professor Zhong Lin Wang at Georgia Tech to describe the coupled piezoelectric and semiconducting property of nanowires and nanobelts for designing and fabricating novel electronic devices such as nanotransistors and nanodiodes. These devices could provide the fundamental building blocks that would allow the creation of a new area of nanoelectronics.
The nanopiezotronic mechanism takes advantage of the fundamental property of nanowires or nanobelts made from piezoelectric materials: bending the structures creates a charge separation – positive on one side and negative on the other. The connection between bending and charge creation has also been used to create nanogenerators that produce measurable electrical currents when an array of zinc oxide nanowires is bent and then released.
Wang explained to Nanowerk how nanopiezotronics, based on nanowires and nanobelts as the fundamental building blocks, have unique advantages that will help make integrated nanosystems possible:
  • "The nanowire-based nanogenerators can be subjected to extremely large deformation, so they can be used for flexible electronics as a flexible/foldable power source.
  • The large degree of deformation that can be withstood by the nanowires is likely to result in a larger volume density of power output.
  • The material used – wurtzite zinc oxide (ZnO) – is a biocompatible and biosafe material; it has great potential as an implantable power source within the human body.
  • The flexibility of the polymer substrate used for growing zinc oxide nanowires and nanobelts makes it feasible to accommodate the flexibility of human muscles so that the mechanical energy (body movement, muscle stretching) in the human body can be used to generate electricity.
  • ZnO nanowires and nanobelts nanogenerators can directly produce current as a result of their enhanced conductivity with the presence of oxygen vacancies.
  • Zinc oxide is an environmentally 'green' material. The phenomena that was demonstrated for ZnO can also be applied to other wurtzite-structured materials, such as GaN and ZnS."
  • In comparison to conventional electronic components, the nanopiezotronic devices operate much differently and exhibit unique characteristics.
    In conventional field-effect transistors, for instance, an electrical potential – called the gate voltage – is applied to create an electrical field that controls the flow of current between the device’s source and its drain. In the nanopiezotronic transistors developed by Wang and his research team, the current flow is controlled by changing the conductance of the nanostructure by bending it between the source and drain electrodes. The bending produces a “gate” potential across the nanowire, and the resulting conductance is directly related to the degree of bending applied.
    "The effect is to reduce the width of the channel to carry the current, so you can have a 10-fold difference in the conductivity before and after the bending," Wang explained.
    Wang and his group anticipate nanopiezotronics to have a wide range of applications in electro- mechanical coupled sensors and devices, nanoscale energy conversion for self-powered nanosystems, and harvesting/recycling of energy from environment. So far, they have been able to demonstrate field-effect transistors, diodes, sensors and current-producing nanogenerators that operate by bending zinc oxide nanowires and nanobelts.
    The physical principle of nanopiezotronics
    The physical principle of nanopiezotronics
    a–d) The principle of the piezoelectric nanogenerator: a) schematic definition of a nanobelt (NB) and the coordination system; b) longitudinal strain εz distribution in the NB after being deflected by an atomic force microscope (AFM) tip from the side; c) the corresponding longitudinal piezoelectric induced electric field Ez distribution in the NB; d) potential distribution in the NB as a result of the piezoelectric effect; e) metal–semiconductor Schottky contact between the AFM tip and the semiconductor ZnO NB at reverse bias, which is responsible for separating and preserving the piezoelectric charges; f) metal–semiconductor Schottky contact between the AFM tip and the semiconductor ZnO NB at forward bias, which is responsible for releasing the accumulated piezoelectric charges. g) Schematics of conventional field-effect transistors (FETs) using a single nanowire/nanobelt, with gate, source, and drain. h) The principle of the piezoelectric field-effect transistor (PE-FET), in which the piezoelectric potential across the nanowire created by the bending force F replaces the gate in a conventional FET. The contacts at both ends are Ohmic. i) The principle of the piezoelectric gated diode, in which one end is fixed and enclosed by a metal electrode, and the other end is bent by a moving metal tip. Both have Ohmic contact with ZnO. The piezoelectric potential V+p at the tensile surface acts like the p–n junction in a conventional diode. (Reprinted with permission from Wiley)
    "The physical principle for creating and separating the piezoelectric charges in the nanobelt is the first half of a nanogenerator for converting mechanical energy into electricity" explains Wang. "Then, the preserving and discharging is the second half of the nanogenerator, which is a coupling process of piezoelectric and semiconducting properties."
    It is important to find various approaches that are feasible for harvesting energy and recycling energy from the environment to self-power a nanosystem so that it can operate wirelessly, remotely, and independently with a sustainable energy supply. It is also important to develop zero-power sensors that respond to a change in the environment.
    "The principle demonstrated for the piezoelectric nanogenerator could be the foundation for self-powered nanosystems" says Wang. "It also has the potential to harvest/recycle energy from the environment and/or recycle energy that is wasted, such as the energy when walking." He gives two examples:
    Piezotronic nanosensors can measure nano-Newton forces by examining the shape of the structure under pressure. Implantable sensors based on the principle could continuously measure blood pressure inside the body and relay the information wirelessly to an external device similar to a watch. The device could be powered by a nanogenerator harvesting energy from blood flow.
    Other nanosensors can detect very low levels of specific compounds by measuring the current change created when molecules of the target are adsorbed to the nanostructure’s surface.
    "Utilizing this kind of device, you could potentially sense a single molecule because the surface area-to-volume ratio is so high," Wang said.
    Wang's article – "Nanopiezotronics" – has been published in the February 27, 2007 online issue of Advanced Materials. In addition to Wang, the research team included J.H. Song, X.D. Wang, P.X. Gao, J.H. He, J. Zhou, N.S. Xu, L.J. Chen and J. Liu from Georgia Tech, the National Tsing Hua University in Taiwan and Sun Yat-Sen University in PR China.
    Last year we ran a Nanowerk Spotlight on Wang's research on nanowires: "3-dimensional nanowire networks make for ultrasensitive and highly selective gas sensors".
    By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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