Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays

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A piezoelectric material can be approximately characterized by a capacitor and a resistor. The capacitor represents the piezoelectric charges accumulated in the volume and the resistor represents its inner resistance.

See supporting material on Science Online.

From our recent measurements of ZnO nanowires the resistivity is from 10 –2 to 10 ohm·cm ( 17 ) depending on the contacts at the electrodes and the concentration of oxygen vacancies. For a NW of length 0.2 μm and diameter 40 nm the resistance is between 16 kilohms and 16 megohms which is much smaller than the applied external load of 500 megohms. Here we ignored the resistance produced by the ZnO film at the bottom of the nanowires because it is very large and covers the entire area of the substrate; thus the inner resistance is dominated by the NW.

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The wurtzite-structured ZnO can be described as a number of alternating planes composed of tetrahedrally coordinated O 2– and Zn 2+ ions stacked alternatively along the c axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged \batchmode \documentclass[fleqn 10pt legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \((000{\bar{1}})\mathrm{-}\mathrm{O}\) \end{document} polar surfaces. The Zn-terminated surface is at the growth front (positive c -axis direction) because of its higher catalytic activity ( 19 ).

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A simple calculation indicates that the magnitudes of \batchmode \documentclass[fleqn 10pt legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(V_{\mathrm{s}}^{+}\) \end{document} and \batchmode \documentclass[fleqn 10pt legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(V_{\mathrm{s}}^{-}\) \end{document} are on the order of a few tens to hundreds of volts. In practice if we consider the polarization and dielectric screening in the calculation the local potential is much smaller than the numbers given by the equations here. An accurate calculation of the potential distribution as a result of the ionic charges introduced by the PZ effect and the surface charges caused by boundaries must be solved numerically and self-consistently. In our analysis a correct magnitude and sign of the potential is sufficient for illustrating the physical model.

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U.S. patent pending.

Supported by NSF grant DMR 9733160 the NASA Vehicle Systems Program and Department of Defense Research and Engineering and the Defense Advanced Research Projects Agency. We thank X. Wang W. L. Hughes J. Zhou and J. Liu for their help.