case is a positively biased Schottky diode, and directly in touch with the Au tip at the 10 pW/um. By choosing a NW array of size it produces a sudden increase in the output ginning of the forced displacement. Because of 10 um x 10 um, the power generated may be electric current. The current is the result of A. the conducting channel at the compressed side enough to drive a single NW/NB/nanotube driven flow of electrons from the semicon- of the Nw, the polar charges produced by the based device(26-28). If we can find a way to ductor ZnO NW to the metal tip. The flow of displaced Zn" and o ions caused by the pz induce the resonance of a Nw array and output the free electrons from the loop through the effect are immediately neutralized by external the Pz-converted power generated in each cy NW to the tip will neutralize the ionic charges free charges as soon as they are produced by cle of the vibration, a significantly strong power distributed in the volume of the Nw and thus the deformation. Therefore, no accumulative source may be possible for self-powering nano will reduce the magnitudes of the potential potential profile is formed at the M-s interface, devices. Furthermore, if the energy produced by and v. Thus, V starts to drop and reaches and no measurable output voltage is detected acoustic waves, ultrasonic waves, or hydraulic zero after all of the ionic charges in the Nw in the experimental setup we have designed pressure/force could be harvested, electricity are fully neutralized. This mechanism explains (Fig. IC) could be generated by means of Zno NW ar why the discharge curve in Fig. 2E is nearly We also measured the Pz voltage output for rays grown on solid substrates or even symmetric. According to the model, the dis- the same samples of aligned ZnO NWs in the flexible polymer films(29). The principle and charge occurs when the nw is bent nearly to AFM tapping mode. In this case, the deforma- the nanogenerator demonstrated could be the its maximum deflection, so VL should have a tion occurred longitudinally and there was no basis for new self-powering nanotechnology small offset in reference to the corresponding side displacement. The Nw was vertically com- that harvests electricity from the environment topography peak along the direction of tip scan, pressed, so the voltage created at the top of for applications such as implantable biomed- as is observed experimentally in Fig 2D the NW was negative V. so long as the base ical devices, wireless sensors, and portable elec- We next consider the case of a Zno NW with electrode was grounded(Fig 4A). There was tronics(29). a small Au particle at the top as a result of Vls no voltage drop across the width of the Nw. the AFm tip displacing the Nw, the tip is di- positively biased Schottky diode formed and 1.XFDuan, YHuang, RAgarwal,CMLieber, Nature rectly in contact with the Zno Nw but not thethe electrons could freely flow across the 421.241(2003) Au particle(Fig. 3G) because of its small size interface. Electrons flowed as the deformatio 2. G. F. Zheng, F Patolsky, Y. Cui, W. U. Wang, C. M. Lieber, Nat Biotechnol. 23, 1294(2005). and hemispherical shape(Fig. 1B and fig. SI urred, and there was no accumulation of net 3. x D Bai, P X Gao, Z L Wang, E. G. Wang, Appl. Phys. a process similar to that described in Fig. 3E charge in the Nw volume. This type of slowl Let82.4806(2003) occurs, and no output voltage will be observed." leaked"current produces no detectable signal When the tip is in contact with the Au particle, Therefore, for the ZnO NWs whose AFM im- 5. M.H. Huang et al, Ad Mater. 13, 113(200 the tip is integrated with the particle as one metal age in tapping mode is shown in Fig. 4B, (2001) contact, and at the local interface between the output voltage V, was detected beyond the noi 7. X Y. Kong, Z L Wang, Nano Lett 3, 1625(2003 Au particle and the negatively charged, com- level(Fig. 4C). B. X.Y. Kong, Y. Ding, R Yang, Z L Wang, Science 303, pressed side of the ZnO NW(s), a forward- We now estimate the possibility of powering biased Schottky diode is formed; thus, the nanodevices with the Nw-based power genera- 9. W. L. Hughes, Z L Wang, Am. Chem. Soc. 126, 6703 current flows from the tip through the Au tor. From table Sl, the Pz energy output by one 10 P X Gao et al, Science 309, 1700(2005) article to the interface region with negative Nw in one discharge event is -0.05 f, and the 11. X D Wang, C). Summers, Z L Wang, Nano Lett. 3, 423 PZ voltage (s)(Fig. 3H). A discharge process output voltage on the load is -8 mV For a Nw similar to that shown in Fig. 3F occurs, and a of typical resonance frequency -10 MHz, the A D Wang et al, 1.Am. Chem Soc. 127, 7920(2005) output power of the nw would be 0.5 pw.If 13.J. H. Song, X. D. Wang, E. Riedo, Z. L. Wang, Nano lett. arp voltage output is produced. In the case of a Nw with a large Au particle the density of NWs per unit area on the sub- 14. A piezoelectric material can be approximately charac that fully covers its top(Fig. 31), the metal tip is strate is 20/um2, the output power density is erized by a capacitor and a resistor. The capacitor olume, and the resistor represents its inner resistance. 15. See supporting material on Science On line cally coupled discharging 16. From our recent measurements of Zno nanowires, the resistivity is from 10-2 to 10 ohm-cm (17) depending c process of aligned piezo the contacts at the electrodes and the concentration of electric ZnO NWs observed oxygen vacancies. For a NW of length 0. 2 um and diameter in tapping mode. (A) Ex 40 nm. the resistance is between 16 kilohms and 16 perimental setup.(B and megohms, which is much smaller than the applied extemal C) Topography image( B) load of 500 megohms. Here we ignored the resistance nd corresponding output produced by the Zno film at the bottom of the nanowires large and covers the entire area of oltage image (O) of the the substrate; thus, the inner resistance is dominated by NWs. The tapping force was 5 nN, tapping frequen- o 17.I. H. He, C.S. Lao, L ]. Chen, D. Davidovic, Z. L. Wang. cy 68 KHz, and tapping l.Am.chem.Soc.127,16376(2005) speed 6 um/s. The voltage 18. W. L. Hughes, Z. L Wang, Appl. Phys. Left. 86, 043106 output contains no infor- 19. M. H. Zhao, Z. L Wang, S.X. Mao, Nano Left 4, 587 mation but noise, proving (2004) the physical mechanism 20. The wurtzite-structured Zno can be described as a emonstrated in Fig. 3 ong the c axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (000)°pol es. The Zn-terminated surface is at the growth front(positive c-axis direction) because of its 21. Z L Wang, x.Y. Kong. ].M 185502(2003) www.sciencemag.orgscIencEVol31214ApriL2006 245case is a positively biased Schottky diode, and it produces a sudden increase in the output electric current. The current is the result of DV￾driven flow of electrons from the semicon￾ductor ZnO NW to the metal tip. The flow of the free electrons from the loop through the NW to the tip will neutralize the ionic charges distributed in the volume of the NW and thus will reduce the magnitudes of the potential Vs – and Vs þ. Thus, VL starts to drop and reaches zero after all of the ionic charges in the NW are fully neutralized. This mechanism explains why the discharge curve in Fig. 2E is nearly symmetric. According to the model, the dis￾charge occurs when the NW is bent nearly to its maximum deflection, so VL should have a small offset in reference to the corresponding topography peak along the direction of tip scan, as is observed experimentally in Fig. 2D. We next consider the case of a ZnO NW with a small Au particle at the top as a result of VLS growth (Fig. 3, G and H). In the first step of the AFM tip displacing the NW, the tip is di￾rectly in contact with the ZnO NW but not the Au particle (Fig. 3G) because of its small size and hemispherical shape (Fig. 1B and fig. S1). A process similar to that described in Fig. 3E occurs, and no output voltage will be observed. When the tip is in contact with the Au particle, the tip is integrated with the particle as one metal contact, and at the local interface between the Au particle and the negatively charged, com￾pressed side of the ZnO NW (Vs – ), a forward￾biased Schottky diode is formed; thus, the current flows from the tip through the Au particle to the interface region with negative PZ voltage (Vs – ) (Fig. 3H). A discharge process similar to that shown in Fig. 3F occurs, and a sharp voltage output is produced. In the case of a NW with a large Au particle that fully covers its top (Fig. 3I), the metal tip is directly in touch with the Au tip at the be￾ginning of the forced displacement. Because of the conducting channel at the compressed side of the NW, the polar charges produced by the displaced Zn2þ and O2– ions caused by the PZ effect are immediately neutralized by external free charges as soon as they are produced by the deformation. Therefore, no accumulative potential profile is formed at the M-S interface, and no measurable output voltage is detected in the experimental setup we have designed (Fig. 1C). We also measured the PZ voltage output for the same samples of aligned ZnO NWs in the AFM tapping mode. In this case, the deforma￾tion occurred longitudinally and there was no side displacement. The NW was vertically com￾pressed, so the voltage created at the top of the NW was negative Vs – so long as the base electrode was grounded (Fig. 4A). There was no voltage drop across the width of the NW. Thus, across the metal tip–ZnO interface, a positively biased Schottky diode formed and the electrons could freely flow across the interface. Electrons flowed as the deformation occurred, and there was no accumulation of net charge in the NW volume. This type of slowly Bleaked[ current produces no detectable signal. Therefore, for the ZnO NWs whose AFM im￾age in tapping mode is shown in Fig. 4B, no output voltage VL was detected beyond the noise level (Fig. 4C). We now estimate the possibility of powering nanodevices with the NW-based power genera￾tor. From table S1, the PZ energy output by one NW in one discharge event is È0.05 fJ, and the output voltage on the load is È8 mV. For a NW of typical resonance frequency È10 MHz, the output power of the NW would be È0.5 pW. If the density of NWs per unit area on the sub￾strate is 20/mm2, the output power density is È10 pW/mm2. By choosing a NW array of size 10 mm
10 mm, the power generated may be enough to drive a single NW/NB/nanotube￾based device (26–28). If we can find a way to induce the resonance of a NW array and output the PZ-converted power generated in each cy￾cle of the vibration, a significantly strong power source may be possible for self-powering nano￾devices. Furthermore, if the energy produced by acoustic waves, ultrasonic waves, or hydraulic pressure/force could be harvested, electricity could be generated by means of ZnO NW ar￾rays grown on solid substrates or even on flexible polymer films (29). The principle and the nanogenerator demonstrated could be the basis for new self-powering nanotechnology that harvests electricity from the environment for applications such as implantable biomed￾ical devices, wireless sensors, and portable elec￾tronics (29). References and Notes 1. X. F. Duan, Y. Huang, R. Agarwal, C. M. Lieber, Nature 421, 241 (2003). 2. G. F. Zheng, F. Patolsky, Y. Cui, W. U. Wang, C. M. Lieber, Nat. Biotechnol. 23, 1294 (2005). 3. X. D. Bai, P. X. Gao, Z. L. Wang, E. G. Wang, Appl. Phys. Lett. 82, 4806 (2003). 4. J. Zhou, N. S. Xu, Z. L. Wang, unpublished data. 5. M. H. Huang et al., Adv. Mater. 13, 113 (2001). 6. Z. W. Pan, Z. R. Dai, Z. L. Wang, Science 291, 1947 (2001). 7. X. Y. Kong, Z. L. Wang, Nano Lett. 3, 1625 (2003). 8. X. Y. Kong, Y. Ding, R. Yang, Z. L. Wang, Science 303, 1348 (2004). 9. W. L. Hughes, Z. L. Wang, J. Am. Chem. Soc. 126, 6703 (2004). 10. P. X. Gao et al., Science 309, 1700 (2005). 11. X. D. Wang, C. J. Summers, Z. L. Wang, Nano Lett. 3, 423 (2004). 12. X. D. Wang et al., J. Am. Chem. Soc. 127, 7920 (2005). 13. J. H. Song, X. D. Wang, E. Riedo, Z. L. Wang, Nano Lett. 5, 1954 (2005). 14. A piezoelectric material can be approximately charac￾terized by a capacitor and a resistor. The capacitor represents the piezoelectric charges accumulated in the volume, and the resistor represents its inner resistance. 15. See supporting material on Science Online. 16. From our recent measurements of ZnO nanowires, the resistivity is from 10j2 to 10 ohmIcm (17) depending on the contacts at the electrodes and the concentration of oxygen vacancies. For a NW of length 0.2 mm 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. 17. J. H. He, C. S. Lao, L. J. Chen, D. Davidovic, Z. L. Wang, J. Am. Chem. Soc. 127, 16376 (2005). 18. W. L. Hughes, Z. L. Wang, Appl. Phys. Lett. 86, 043106 (2005). 19. M. H. Zhao, Z. L. Wang, S. X. Mao, Nano Lett. 4, 587 (2004). 20. The wurtzite-structured ZnO can be described as a number of alternating planes composed of tetrahedrally coordinated O2– and Zn2þ ions stacked alternatively along the c axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged ð0001Þ-O polar surfaces. The Zn-terminated surface is at the growth front (positive c-axis direction) because of its higher catalytic activity (19). 21. Z. L. Wang, X. Y. Kong, J. M. Zuo, Phys. Rev. Lett. 91, 185502 (2003). Fig. 4. Electromechani￾cally coupled discharging process of aligned piezo￾electric ZnO NWs observed in tapping mode. (A) Ex￾perimental setup. (B and C) Topography image (B) and corresponding output voltage image (C) of the NWs. The tapping force was 5 nN, tapping frequen￾cy 68 KHz, and tapping speed 6 mm/s. The voltage output contains no infor￾mation but noise, proving the physical mechanism demonstrated in Fig. 3. REPORTS www.sciencemag.org SCIENCE VOL 312 14 APRIL 2006 245