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upon the linear piezoelectric coefficient and the strain tensor, a more pronounced piezotronic effect can be obtained by using • Materials that are capable of sustaining large strains without failure and • Making use of certain piezoelectric materials that have pronounced piezoelectric coefficients and attractive semiconductor functionality. The first case results in a more rugged piezoelectric component capable of enduring substantial strain. A design where peripheral, robust, strained piezoelectric films sandwich an active semiconductor heterojunction located within a neutral strain axis is a conceivable architecture for enhancing piezotronic performance. A key challenge facing the second case is the low conductivity of the piezoelectric materials in conjunction with their pronounced piezoelectric coefficients. In devices whose functionality depends on the transport of charge carriers, this impediment cannot be overstated. A peripheral approach, where the piezoelectric material itself does not take part in the active heterojunction, may alleviate the problem. Alternatively, GaN and many other III–V wurtzite materials are the core semiconductor components in solar cells, lasers, LEDs, and PEC cells. These materials also exhibit appreciable piezoelectric effect, and thus make good candidates for using the piezotronic effect to regulate their functionalities. In general, piezotronics brings new knowledge to classic semiconductor theories, where semiconductor band theory and the behaviors of electrons and holes are interpreted with additional contributions from Ppz. Piezotronics also introduces a new concept to the classic piezoelectric electromechanical coupling effect by addressing the contributions from free-charges, junction materials, surface and interface properties, and external illumination. New science obtained from the coupling between crystal structure, mechanical strain and electronic properties opens a new route toward designing, operating, and enhancing electronic, optoelectronic, photovoltaic, and even catalytic materials and systems. Piezotronics will find a significant role in the operational principles of flexible devices, MEMs, sensors, human-CMOS interfacing, and energy conversion and storage systems. Acknowledgements The author thanks Zhong Lin Wang at Georgia Institute of Technology for his pioneering work on piezotronics, as well as J. Shi and M. Starr for their contributions to the work in this article. The author gratefully acknowledges the financial support of DARPA under Grant No. N66001-11-1-4139 and the National Science Foundation under Grant No. CMMI-1148919. Figure 6. (a) H2 concentrations measured as a function of oscillating time of the piezoelectric beam in deionized water with a frequency of 10 Hz (triangles) and 20 Hz (diamonds). A silicon cantilever with identical configuration was used as a control (circles). Inset is a photo of the piezocatalysis system. (b) Proposed mechanism of piezocatalysis at the piezoelectric–water interface. About the author Xudong Wang is an assistant professor in the Department of Materials Science and Engineering at the University of Wisconsin-Madison. Contact: xudong@engr.wisc.edu. References 1Z.L. Wang, “Nanopiezotronics,” Adv. Mater., 19 6 889–92 (2007). 2Z.L. Wang, “Piezopotential gated nanowire devices: Piezotronics and piezo-phototronics,” Nano Today, 5 6 540–52 (2010). 3J. Shi, M.B. Starr, and X. Wang, “Band structure engineering at heterojunction interfaces via the piezotronic effect,” Adv. Mater., 24 34 4683–91 (2012). 4J. Shi, M.B. Starr, H. Xiang, Y. Hara, M.A. Anderson, J.-H. Seo, Z. Ma, and X.D. Wang, “Interface engineering by piezoelectric potential in ZnO-based photoelectrochemical anode,” Nano Lett., 11 12 5587–93 (2011). 5X.D. Wang, J. Zhou, J.H. Song, J. Liu, N.S. Xu, and Z.L. Wang, “Piezoelectric field effect transistor and nanoforce sensor based on a single ZnO nanowire,” Nano Lett., 6 12 2768–72 (2006). 6Z.L. Wang, Piezotronics and Piezo-Phototronics. Springer, New York 2012. 7J. Zhou, P. Fei, Y.D. Gu, W.J. Mai, Y.F. Gao, R. Yang, G. Bao, and Z.L. Wang, “Piezoelectric-potential-control led polarity-reversible Schottky diodes and switches of ZnO wires,” Nano Lett., 8 11 3973–77 (2008). 8J. Zhou, Y.D. Gu, P. Fei, W.J. Mai, Y.F. Gao, R.S. Yang, G. Bao, and Z.L. Wang, “Flexible piezotronic strain sensor,” Nano Lett., 8 9 3035–40 (2008). 9W. Wu, X. Wen, and Z.L. Wang, “Taxel-addressable matrix of vertical-nanowire piezotronic transistors for active and adaptive tactile imaging,” Science, 340 6135 952–57 (2013). 10W. Wu, Y. Wei, and Z.L. Wang, “Strain-gated piezotronic logic nanodevices,” Adv. Mater., 22 42 4711–15 (2010). 11Q. Yang, W. Wang, S. Xu, and Z.L. Wang, “Enhancing light emission of ZnO microwire-based diodes by piezophototronic effect,” Nano Lett., 11 9 4012–17 (2011). 12S.-H. Park and S.-L. Chuang, “Piezoelectric effects on electrical and optical properties of wurtzite GaN/AlGaN quantum well lasers,” Appl. Phys. Lett., 72 24 3103–105 (1998). 13J. Shi, P. Zhao, and X. Wang, “Piezoelectric-polarizationenhanced photovoltaic performance in depleted-heterojunction quantum-dot solar cells,” Adv. Mater., 25 6 916–21 (2013). 14Y. Yang, W. Guo, Y. Zhang, Y. Ding, X. Wang, and Z.L. Wang, “Piezotronic effect on the output voltage of P3HT/ ZnO micro/nanowire heterojunction solar cells,” Nano Lett., 11 11 4812–17 (2011). 15J.S. Huang, Y.B. Yuan, T.J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, and Y. Yang, “Efficiency enhancement in organic solar cells with ferroelectric polymers,” Nat. Mater., 10 4 296–302 (2011). 16M.B. Starr, J. Shi, and X. Wang, “Piezopotential-driven redox reactions at the surface of piezoelectric materials,” Angew. Chem. Int. Ed., 51 24 5962–66 (2012). 17X.Y. Xue, S.H. Wang, W.X. Guo, Y. Zhang, and Z.L. Wang, “Hybridizing energy conversion and storage in a mechanical-to-electrochemical process for self-charging power cell,” Nano Lett., 12 9 5048–54 (2012). 18G.S. Rohrer, N.V. Burbure, and P.A. Salvador, “Photochemical reactivity of titania films on BaTiO3 substrates: Origin of spatial selectivity,” Chem. Mater., 22 21 5823–30 (2010). n American Ceramic Society Bulletin, Vol. 92, No. 6 | www.ceramics.org 23 (Adapted from Ref. 16.)


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