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Self-Powered Systems Chemie properties of materials were previously unavailable. These new possibilities mostly involve changes induced by dimen- sion confinement in the electronic band structures as well as enhanced phonon scattering due to the increased size of interfaces and surfaces.[93] The ability to synthesize nano- structured materials with well-defined dimensionalities makes the quantum confinement of electrons in space possible, which further increases the electronic density of states (DOS) near the Fermi level and hence enhances the thermoelectric power factor. Owing to the increased surface- to-volume ratio and thus the amount of surface/interface states, phonon scattering can be significantly enhanced at surfaces and interfaces in nanomaterials without sacrificing electronic conduction, which has been considered the main contributing factor in the substantial enhancement of TE conversion in some applications.[91c,94] Semiconductor NWs have been investigated as promising TE materials, although the accuracy of ZT measurement in these experiments remains doubtful. One practical challenge in characterizing the TE properties of low-dimensional nanostructures lies in the increasing surface scattering of electrons, which renders these measurements sample-depen- dent.[95] Among these low-dimensional TE materials, silicon NWs with small diameters exhibit interesting and promising TE properties, with reported a ZT value of 0.6 at room temperature[92b] and a ZT value approaching those of commercial devices at 200K. However, the fabrication techniques for these nanostructures cannot be readily incor- porated with commercial devices and scaled up. Uniformity in TE conversion and other properties of the as-fabricated nanomaterials remain another practical issue which needs proper attention before these nanomaterials can be imple- mented in any practical applications for TE energy harvest- ing. TE generators have been reliably providing power in remote terrestrial and extraterrestrial applications during the past decades.[79b] By integrating material design with advanced fabrication techniques, TE systems can not only harness waste heat from engines at the macroscale to address environmental issues globally, but can also serve as one of the sources for powering small electronic systems in applications such as MNS-based health monitoring by directly converting dissipated heat from the human body into electricity.[80b, 96] 3.4. Nanotechnology-Enabled Piezoelectric Mechanical-Energy Harvesting Vibration-based mechanical energy is ubiquitous in the environment and more accessible than solar and thermal energy. Mechanical vibrations with frequencies spanning a broad spectrum, from a few hertz to several kilohertz, exist abundantly in the ambient, with available energy density ranging from a few hundred microwatts to milliwatts per cubic centimeter.[97] This energy can potentially facilitate the continuous and adaptable operation of sensors as well as electronic devices and systems, especially under circumstan- ces in which other energy sources, such as solar or thermal energy, are not readily available.[7a,98] Several methods have been developed for the conversion of mechanical energy into electricity through the use of electromagnetic induction, static-electricity generation, and piezoelectric materials.[99] The harvesting of mechanical energy by piezoelectric materi- als in particular has received enormous attention owing to the ability of these materials to convert mechanical energy into electricity directly and the feasibility of this approach for integrated applications. Traditionally, lead zirconate titanate, or PZT, has been the material used most for mechanical- energy harvesting.[100] Nevertheless, the extremely brittle nature of PZT ceramic and the incorporation of lead create issues such as the reliability, durability, and safety of this material for long-term sustainable operation and hinder its application. Recently, piezoelectric ZnO NWs have been demon- strated to show promise in the harvesting of mechanical energy at the micro-/nanoscale in various nanogenerator (NG) configurations.[101] They exhibited potential as a sustain- able, efficient, and environmentally friendly power source for self-powered MNSs. Notably, piezoelectric NWs excel their bulk counterparts in terms of their enhanced piezoelectric effect, superior mechanical properties, and extreme sensitiv- ity to vibrations of ultrasmall magnitude.[97b, 102] The kernel of electricity generation under external strain in piezoelectric NWs is the absence of central symmetry in the crystal structure and the existence of piezoelectric potential, or piezopotential, which is also the fundamental concept in the emerging field of piezotronics.[15, 103] Theoretical calculations for ZnO NWs in a clamped–free configuration with an external force applied at the free end showed that piezopo- tential can be induced on the side surfaces of NWs owing to distortion of the crystal lattice; the calculated distribution of the electric field across the NW is shown in the inset in Figure 6.[104] Theoretical investigations for piezoelectric nanomaterials in various morphologies and configurations have also been carried out subsequently to facilitate fundamental under- standing and the design of related devices.[105] In a typical configuration of doubly clamped NWs on flexible substrates, the calculated distribution of piezopotential was shown to be along the c axis of the ZnO NW and along the straining direction, with values up to even the hundred-volt range if the substantially decreased carrier concentration was taken into account.[105b] This significant result has directed subsequent advances in the field of NGs (inset in Figure 6). However, previous research suggested that carrier density in ZnO NWs plays an important role in the NG output performance.[106] The experimentally observed output voltage is much lower than the theoretically calculated piezopotential of the mate- rial if the material has been doped, as the positive side of the piezopotential induced by mechanical deformation is partially screened by free electrons, which occur dominantly in n-type ZnO NWs. The negative side of the piezopotential is preserved as long as the donor concentration is not too high, as in the case of unintentionally doped as-grown n-type ZnO. Further theoretical calculations predict that the piezo- potential is reduced by almost a factor of 10 at the positive side relative to the value at the negative side when the n-type doping level reaches 1 1017 cm3. It can therefore be Angew. Chem. Int. Ed. 2012, 51, 2 – 24 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org &&&& These are not the final page numbers! 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