NATURE Vol 449 18 October 20 LETTERS semiconductor nanowire- and carbon-nanotube-based nanoelectro- 2. Le nic elements, given that these elements require power as low as a few Report of the Basic Energy Sciences Workshop on Solar Energy Utilization, US anowatts"-.Recent work addressing this key issue has involved the use of piezoelectric ZnO nanowires for mechanical-to-electrical con- 3. Gratzel, M Photoelectrochemical cells. Nature 414, 338-344(2001) version,although the direct current(d. c )power developed by this 4.Huynh, W.U. Dittmer, J1&Alivisatos, A P Hybrid nanorod-polymer solar cell nanogenerator., 1-4 fw per nanowire, is at present less than is ence295,2425-2427(2002) needed to drive nanoelectronic devices. Silicon nanowire photovol- 5. Law, M. Greene LIE Johnson, .C, Saykally R, Yang, P Nanowire dye. taic elements can produce 50-200 pW per nanowire at I-sun illu- 6. Baxter, J.B. Aydil, E S Nanowire-based dye-sensitized solar cells. Appl. Phys. mination, and thus could function as nanoscale power supplies fo ett86,053114(2005) nanoelectronics by either increasing the Marti, A. Nozik, A J. Solar cells based on quantum dots: several coupled elements. For example, a single silicon nanowire ultiple exciton generation and intermediate bands. MRS Bull. 32, 236-241 photovoltaic device, operating under 8-sun illumination (P 1.86nW, 1=4.8%)was used to drive a silicon nanowire pH sensor ayes, B M, Atwater, H. A. Lewis, N S Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells. J. Appl. Phys. 97. without additional power(Fig. 4a). Measurements of the voltage 114302(2005) drop across the p-type silicon nanowire sensor(powered solely by 9. Zhang Y, Wang, L W& Mascarenhas, A Quantum coaxial cables. Nano Lett. 7. the silicon nanowire photovoltaic element) as a function of time 1264-1269(2007) (Fig. 4a)show reversible increase (or decrease)in voltage as the 10. Gust, D, Moore, T. A. Moore, A. L Mimicking photosynthetic solar energy lution pH is decreased (or increased)that are consistent with the ll. Klauk, H Zschieschang U, Pflaum, I& Halik, M Ultralow-power organic expected changes in resistance of the silicon nanowire with surface complementary circuits. Nature 445, 745-748(2007) charge2. In addition, we note that the photovoltaic(under constant 12. Browne, W.R. Feringa, B L Making molecular machines work.Nature 8-sun illumination) and sensor devices both exhibited excellent lanotechnol. 1, 25-35(2006) stability over the approximately two-hour time of experiments 13. Avouris, P& Chen, J Nanotube electronics and optoelectronics. Mater. Today 9 Last, the core/shell silicon nanowire photovoltaic devices were 14. Wagner, R.S.& Ellis, W.C. Vapor-liquid-solid mechanism of single crystal interconnected in series and in parallel to demonstrate scaling of growth. Appl. Phys. Lett. 4, 89(1964 he output characteristics and to drive larger loads. F-Vdata recorded 15. Zheng, G.F. Lu, W, Jin, S& Lieber, C.M. Synthesis and fabrication of high- from two illuminated silicon nanowire ance n-type silicon nanowire transistors. Adv Mater. 16, 1890-1893 eral important features. First, the individual elements exhibit very 16. Hayden, o. Agarwal. R. Lieber. C M Nanoscale avalanche photodiodes for similar behaviour, highlighting the good reproducibility of our core/ shell nanowire devices. Second, interconnection of the two elements in series and parallel yields Vo and Ise values, respectively, that are 17. Karpov, VG, Cooray, M.L.C.& Shvydka, D Physics of ultrathin photovoltaics. approximately the sum of two, as expected. Notably, we have used l. Phys. Lett89,163518(2006 terconnected silicon nanowire photovoltaic elements as the sole 18. Shah, A V et al. Thin-film silicon solar cell technology. Prog. Photovolt Res Appl 12.113-142(2004) power supply driving a nanowire-based AND logic gate(Fig 4 19. Luque, A& Hegedus, S. Handbook of Photovoltaic Science and Engineering(Wiley, where Ve and the voltage inputs 1 and 2 Vil(HIGH)and Viz (HIGH)are provided by two nanowire photovoltaic devices in series 20. Javey, A, Nam, S. Friedman, R. Yan, H. Lieber, CM. Layer-by-layer assembly at 2-sun illumination(HIGH is the input state and Vil (HIGH) Viz(HIGH)are close to Voc of the PV devices. )A summary of the 773-777(2007) el, P. Physics of Solar Cells, From Principles to New Concepts(Wiley-VCH, input/output results(right inset, Fig. 4c) shows correct AND logic. This work thus demonstrates the potential for self-powered nano- 22. Green, M. A General temperature dependence of solar cell performanc wire-based logic circuits and, more generally, the possibility of self- and implications for device modeling. Prog. Photovolt Res. App powered functional nanoelectronic systems through, for example, 23. Aberle, A. G. Surface passivation of crystalline silicon solar cells: a review. Prog. Photovolt Res. Appl. 8, 4 lements with nanoelectronic, photonic and biological sensing devices. 24. Green, MA, Emery, K, King, D L, Hishikawa, Y. Warta, W Solar cell efficiency rsion 29). Photovolt Res. Appl. 15, 35-40(200 METHODS SUMMARY 25. Cui, Y, Wei, QQ, Park, H. K. Lieber, C M. Nanowire nanoser Single-crystalline silicon nanowire p-cores were synthesized by means of a sensitive and selective detection of biological and chemical speci nanocluster-catalysed Vis method" and then chemical vapour deposition 26. Huang, Y et al. Logic gates and computation from assembled nanowire building growth to inhibit axial elongation of the silicon nanowire After transistors. Science 294, 1317-1320(2001) growth, SiO2 was deposited conformally by means of plasma-enhanced chemical 28. Wang,ZL& vapour deposition(PECVD). Standard electron beam lithography, silicon we chemical etching(KOH etchant)and thermal evaporation were used to make 29. Wang, X, Song, J, Liu, J& Wang, Z L Direct-current nanogenerator driven by coaxial nanowire devices, with selective contacts on the p-core and n-shell.A ultrasonic waves. Science 316, 102-105 (2007) standard solar simulator(150 W, Newport Stratford) with an AM 1. 5G filter supplementary Information is linked to the online version of the paper at asusedtocharacterizethephotovoltaicdeviceresponsewheretheaveragewww.nature.com/nature. tensity was calibrated using a power meter. For multiple-sun illumination, an aspheric lens was placed between the light source and nanowire devices. All elec- Acknowledgements We thank D W.Pang, D.C.Bell, H.G.Park,HS.Choe, HYan and For self-powered pH sensing and aND logic gate experiments, a computer. support from the MITRE Corporation and the Air Force office of Scientific ontrolled analogue-to-digital converter(6030E, National Instruments)was used to record the voltage drop or voltage output of the silicon nanowire devices. Author Contributions CML, B T X.Z. and TJ. K designed the experiments. B.T. X.Z., T.J. K, Y F N Y and G.Y. performed experiments and analyses. C.M. L B T. Full Methods and any associated references are available in the online on of X.Z. and T.J. K wrote the paper. All authors discussed the results and commented n the manuscript. Received 15 May; accepted 7 August 2007. Author Information Reprints and permissions information is available at www.nature.com/reprints.Correspondenceandrequestsformaterialsshouldbe 1. Lewis, N S Toward cost-effective solar energy use. Science 315, 798-801(2007). add to CM.L(cmlacmliri 8s9 E2007 Nature Publishing Groupsemiconductor nanowire- and carbon-nanotube-based nanoelectro￾nic elements, given that these elements require power as low as a few nanowatts25–27. Recent work addressing this key issue has involved the use of piezoelectric ZnO nanowires for mechanical-to-electrical con￾version, although the direct current (d.c.) power developed by this nanogenerator28,29, 1–4 fW per nanowire, is at present less than is needed to drive nanoelectronic devices. Silicon nanowire photovol￾taic elements can produce 50–200 pW per nanowire at 1-sun illu￾mination, and thus could function as nanoscale power supplies for nanoelectronics by either increasing the light intensity or using several coupled elements. For example, a single silicon nanowire photovoltaic device, operating under 8-sun illumination (Pmax 5 1.86 nW, g 5 4.8%) was used to drive a silicon nanowire pH sensor25 without additional power (Fig. 4a). Measurements of the voltage drop across the p-type silicon nanowire sensor (powered solely by the silicon nanowire photovoltaic element) as a function of time (Fig. 4a) show reversible increase (or decrease) in voltage as the solution pH is decreased (or increased) that are consistent with the expected changes in resistance of the silicon nanowire with surface charge25. In addition, we note that the photovoltaic (under constant 8-sun illumination) and sensor devices both exhibited excellent stability over the approximately two-hour time of experiments. Last, the core/shell silicon nanowire photovoltaic devices were interconnected in series and in parallel to demonstrate scaling of the output characteristics and to drive larger loads. I–V data recorded from two illuminated silicon nanowire elements (Fig. 4b) show sev￾eral important features. First, the individual elements exhibit very similar behaviour, highlighting the good reproducibility of our core/ shell nanowire devices. Second, interconnection of the two elements in series and parallel yields Voc and Isc values, respectively, that are approximately the sum of two, as expected. Notably, we have used interconnected silicon nanowire photovoltaic elements as the sole power supply driving a nanowire-based AND logic gate (Fig. 4c), where Vc and the voltage inputs 1 and 2 Vi1 (HIGH) and Vi2 (HIGH) are provided by two nanowire photovoltaic devices in series at 2-sun illumination. (HIGH is the input state and Vi1 (HIGH) and Vi2 (HIGH) are close to Voc of the PV devices.) A summary of the input/output results (right inset, Fig. 4c) shows correct AND logic. This work thus demonstrates the potential for self-powered nano￾wire-based logic circuits and, more generally, the possibility of self￾powered functional nanoelectronic systems through, for example, the integration of multiple stacked silicon nanowire photovoltaic elements with nanoelectronic, photonic and biological sensing devices. METHODS SUMMARY Single-crystalline silicon nanowire p-cores were synthesized by means of a nanocluster-catalysed VLS method14,15, and then chemical vapour deposition was used to deposit i- and n-type nanocrystalline silicon shells; shell growth was carried out at higher temperature and lower pressure than those used in core growth to inhibit axial elongation of the silicon nanowire core. After nanowire growth, SiO2 was deposited conformally by means of plasma-enhanced chemical vapour deposition (PECVD). Standard electron beam lithography, silicon wet chemical etching (KOH etchant) and thermal evaporation were used to make coaxial nanowire devices, with selective contacts on the p-core and n-shell. A standard solar simulator (150 W, Newport Stratford) with an AM 1.5G filter was used to characterize the photovoltaic device response, where the average intensity was calibrated using a power meter. For multiple-sun illumination, an aspheric lens was placed between the light source and nanowire devices. All elec￾trical measurements were made with a probe station (TTP-4, Desert Cryogenics). For self-powered pH sensing and AND logic gate experiments, a computer￾controlled analogue-to-digital converter (6030E, National Instruments) was used to record the voltage drop or voltage output of the silicon nanowire devices. Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature. Received 15 May; accepted 7 August 2007. 1. Lewis, N. S. Toward cost-effective solar energy use. Science 315, 798–801 (2007). 2. Lewis, N. S. & Crabtree, G. (eds) Basic Research Needs for Solar Energy Utilization. (Report of the Basic Energy Sciences Workshop on Solar Energy Utilization, US Department of Energy, Washington DC, 2005); ,http://www.er.doe.gov/bes/ reports/abstracts.html#SEU. (18–21 April, 2005). 3. Gratzel, M. Photoelectrochemical cells. Nature 414, 338–344 (2001). 4. Huynh, W. U., Dittmer, J. J. & Alivisatos, A. P. Hybrid nanorod-polymer solar cells. Science 295, 2425–2427 (2002). 5. Law, M., Greene, L. E., Johnson, J. C., Saykally, R. & Yang, P. Nanowire dye￾sensitized solar cells. Nature Mater. 4, 455–459 (2005). 6. Baxter, J. B. & Aydil, E. S. Nanowire-based dye-sensitized solar cells. Appl. Phys. Lett. 86, 053114 (2005). 7. Luque, A., Marti, A. & Nozik, A. J. Solar cells based on quantum dots: multiple exciton generation and intermediate bands. MRS Bull. 32, 236–241 (2007). 8. Kayes, B. M., Atwater, H. A. & Lewis, N. S. Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells. J. Appl. Phys. 97, 114302 (2005). 9. Zhang, Y., Wang, L. W. & Mascarenhas, A. Quantum coaxial cables. Nano Lett. 7, 1264–1269 (2007). 10. Gust, D., Moore, T. A. & Moore, A. L. Mimicking photosynthetic solar energy transduction. Acc. Chem. Res. 34, 40–48 (2001). 11. Klauk, H., Zschieschang, U., Pflaum, J. & Halik, M. Ultralow-power organic complementary circuits. Nature 445, 745–748 (2007). 12. Browne, W. R. & Feringa, B. L. Making molecular machines work. Nature Nanotechnol. 1, 25–35 (2006). 13. Avouris, P. & Chen, J. Nanotube electronics and optoelectronics. Mater. Today 9, 46–54 (2006). 14. Wagner, R. S. & Ellis, W. C. Vapor–liquid–solid mechanism of single crystal growth. Appl. Phys. Lett. 4, 89 (1964). 15. Zheng, G. F., Lu, W., Jin, S. & Lieber, C. M. Synthesis and fabrication of high￾performance n-type silicon nanowire transistors. Adv. Mater. 16, 1890–1893 (2004). 16. Hayden, O., Agarwal, R. & Lieber, C. M. Nanoscale avalanche photodiodes for highly sensitive and spatially resolved photon detection. Nature Mater. 5, 352–356 (2006). 17. Karpov, V. G., Cooray, M. L. C. & Shvydka, D. Physics of ultrathin photovoltaics. Appl. Phys. Lett. 89, 163518 (2006). 18. Shah, A. V. et al. Thin-film silicon solar cell technology. Prog. Photovolt. Res. Appl. 12, 113–142 (2004). 19. Luque, A. & Hegedus, S. Handbook of Photovoltaic Science and Engineering (Wiley, Chichester, 2003). 20. Javey, A., Nam, S., Friedman, R. S., Yan, H. & Lieber, C. M. Layer-by-layer assembly of nanowires for three-dimensional, multifunctional electronics. Nano Lett. 7, 773–777 (2007). 21. Wu¨rfel, P. Physics of Solar Cells, From Principles to New Concepts (Wiley-VCH, Weinheim, 2005). 22. Green, M. A. General temperature dependence of solar cell performance and implications for device modeling. Prog. Photovolt. Res. Appl. 11, 333–340 (2003). 23. Aberle, A. G. Surface passivation of crystalline silicon solar cells: a review. Prog. Photovolt. Res. Appl. 8, 473–487 (2000). 24. Green, M. A., Emery, K., King, D. L., Hishikawa, Y. & Warta, W. Solar cell efficiency tables (version 29). Photovolt. Res. Appl. 15, 35–40 (2007). 25. Cui, Y., Wei, Q. Q., Park, H. K. & Lieber, C. M. Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species. Science 293, 1289–1292 (2001). 26. Huang, Y. et al. Logic gates and computation from assembled nanowire building blocks. Science 294, 1313–1317 (2001). 27. Bachtold, A., Hadley, P., Nakanishi, T. & Dekker, C. Logic circuits with carbon nanotube transistors. Science 294, 1317–1320 (2001). 28. Wang, Z. L. & Song, J. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 312, 242–246 (2006). 29. Wang, X., Song, J., Liu, J. & Wang, Z. L. Direct-current nanogenerator driven by ultrasonic waves. Science 316, 102–105 (2007). Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Acknowledgements We thank D. W. Pang, D. C. Bell, H. G. Park, H. S. Choe, H. Yan and P. Xie for help with experiment and data analysis. C.M.L. acknowledges support from the MITRE Corporation and the Air Force Office of Scientific Research, and T.J.K. acknowledges an NSF graduate fellowship. Author Contributions C.M.L., B.T., X.Z. and T.J.K. designed the experiments. B.T., X.Z., T.J.K., Y.F., N.Y. and G.Y. performed experiments and analyses. C.M.L., B.T., X.Z. and T.J.K. wrote the paper. All authors discussed the results and commented on the manuscript. Author Information Reprints and permissions information is available at www.nature.com/reprints. Correspondence and requests for materials should be addressed to C.M.L. (cml@cmliris.harvard.edu). NATURE| Vol 449|18 October 2007 LETTERS 889 ©2007 NaturePublishingGroup