letters to nature dopant profiles. Current (n versus voltage(VsD)measurements Photoluminescence measurements ges were obtained using a ntra-nanowire p-n junction(Fig. 4a). To establish that the current epifluorescence microscope. Excitation light(488 nm)was focuse sed by an obiective d (NA= rectification was due to intra-nanowire p-n junctions, we char- 0. 7)to a-30-um-diameter spot at -1.0kwcm-"on the quartz substrate with dispersed acterized the local nanowire potential and gate response by electro- nanowires N O he resulting photoluminescence was focused and imaged on a liquid-nitrogen-cooled static force microscopy(EFM) and scanned gate microscopy charge coupled device. The emission polarization was measured using a Glan-Thompson (SGM), respectively. An EFM image of a typical p-n junction in polarizer. reverse bias showed that the entire voltage drop occurs at the p-n junction itself( Fig. 4b); EFM measurements showed no potential Electrical measurements Irop at the contact regions under forward or reverse bias(not Nanowires dis rates(600-nm shown), ruling out the contact/nanowire interface as the source of and electrical contacts were defined using electron beam lithography. Ti/Au contacts were rectification in the I-VsD behaviour. In addition, SGM images recorded with the nanowire device in forward bias and the scanned GelAu or Ni/In/Au and the second (p-type)was made using Zn/Au. The contacts were tip-gate positive( Fig. 4c)show enhanced conduction to the right of annealed at 300-350"C following deposition. the junction, indicating an n-type region, and reduced conduction to the left of the junction, indicating depletion of a p-type region. Scanned probe microscopy The abrupt change in majority carrier type coincides with the A Dig ocation of the intra- nanowire junction determined by efm, and nus further confirms that the diode behaviour results from well with a lift I面m如mah平r controlled dopant modulation. We believe that the direct and controlled growth of nanowire p-n Received 24 December 2001; accepted 1l January junctions represents an advance over previous work. 102. The ability 1. Lieber, C M. The incredible shrinking circuit. Sci. Am. 285, 58-64(2001) to synthesize modulation-doped nanowire superlattices opens up 2. Cui, Y.& Lieber, C M. Functional nanoscale electronic devices assembled using silicon nanowire aiding blocks. Science 291, 851-853(2001). chemical detectors to bipolar transistors and highly integrated 3. weaken dt and pated tt ion foan ing e indium phophide n movies saLome 29, h09-14357 logic gates for nanoelectronics. Moreover, the direct growth of (2001) lodulation-doped nanowires eliminates the lithographic steps 4. Cui, Y- Wei Q, Q- Park, H K. Lieber, C.M. Nanow sors for highly sensitive and selective used to create doped nanotube p-n junctions., and thus facilitates 5. Duan, X E, Huang, Y Cui, Wang l. E. Lieber. C.M. Indium p the bottom-up assembly of complex functional structures when blocks for nanoscale dectronic and optoelectronic devices. Nature 409, 66-69(2001). combined with recent advances in the directed en masse organiza- 6. Huang Y et al Logic gates and computation from assembled nanowire building blocks. Science 294. tion of nanowire structures.9 Last, the use of single InP nanowire P-n Junctions as nanoscale 7. Bachtold, A, Hadley. P, Nakanishi, T& Dekker, C. Logic circuits with carbon nanotube transistors. LEDs has been investigated(Fig. 4d). I-VsD measurements of InP 8. Derycke, V, Martel, R. Appenzeller. L &Avouris, P. Carbon nanotube inter-and intramolecular logic nanowire p-n structures exhibit rectification similar to that gates. Nano Lett. 1, 453-456(200 described above for the silicon nanowire p-n junctions. In forward 9. Huang Y Duan, X. E, Wei, Q.Q. &Licher, C.M. Directed assembly of one-dimensional bias, individual InP nanowire devices exhibit light emission from 10 Zhe hou, C. W Kong, L Yenilmez, E. Dai. H. Modulated p-n junctions that is both highly polarized and blue-shifted due to nanotubes. Science 290, the one-dimensional structure and radial quantum confineme 1. Gudiksen, M. S& Lieber, C M. Diameter-selective synthesis of semiconductor nanowires. J.Am. respectively( Fig. 4e). The efficiency of these intra-nanowire LED 1. Cui, Y, Lauhon, L I, Gudiksen, M. S, Wang, J.E. Lieber. C M. Diameter-controlled synthesis of is-0.1%, although it can be increased substantially. By defining a single-crystal silicon nanowires. Appl Phys. Lem. 78, 2214-2216(2001 quantum dot heterostructure within a p-n diode during nanowire 13. Gudiksen, M S, Wang. I.E. Lieber. C. M Synthetic control of the diameter and length of single ynthesis, it should be possible to engineer an electrically driven hoton source with well-defined polarization. Such a nano- 14. Wagner, R.S. in Whisker Techndogy 47-119( wire device could be extremely useful in quantum cryptography, B EBP..& 15. Morales, A M. &Liebe, C.M.A. information processing. More generally, we believe that the pre- 16. Duan, x.E. Lieber, C M. General is of compound semiconductor nanowires Ad Mater. 12. sent results open up many opportunities in nanoscale photonics 17. Cui, Y. uan. X E. Hu, L T. Licher, C.M. Doping and electrical transpor in silicon nanowires. and electronics, ranging from the relatively simple nanoscale emit- hys.Chm.B1045213-5216(2000 ters and complementary logic, which can be obtained from single 18. Nicewarner-Pena S.R. ef al. Submicrometer metallic barcodes. Science 294, 137-141(2001). nanowire p-n junctions, to complex periodic superlattices that may 19. Chow, E et al Three-dimensional control of light in a two-dimens ional photonic crystal slab Niature enable applications such as nanowire injection lasers and engi- 20. Huang, M H at al room-temperature ultraviolet nanolasers. cience 292, 1897-1899 peered one-dimensional electron waveguides Methods 22. Hu, L, Ouyang, M, Yang P.& Lieber, C. M. Controlled growth and electrical properties of Nanowire synthesis rbon nanotubes and silicon nanowires Nature 399, 48-51(1999). ser-assisted catalytic growth(GaAs, GaP and 23. Bennett, C.H. Divincema, D.P.c and computation. Nature 404, 247-255 InP)or chemical vapour deposition(Si), using Au nanoclusters to direct the growth.Au nto oxidized silicon substrates and then placed in th th. solid Acknowledgements GaAs, GaP and InP were ablated using either a pulsed ArF excimer or Nd: YAG lasers, Research and Defense Advanced Projects Research Agency. n)in helium(18 cm STP per minute) as dopants. The furnace was evacuated before Competing interests statement The resulting nanowires were sonicated briefly in ethanol and deposited onto copper The authors declare that they have no competing financial interests. resolution TEM images and EDS spectra fron n a JEOL 2010F microscope. The elemental mapping of Correspondence and requests for materials should be addressed to C.M.L. single junction was conducted on a VG HB603 STEM. (e-mail: cml@cmliris. harvard. edu) 620 A@2002 Macmillan Magazines Ltd NATURE VOL 4157FEBRUARY 200letters to nature 620 NATURE | VOL 415 | 7 FEBRUARY 2002 | www.nature.com dopant pro®les. Current (I) versus voltage (VSD) measurements showed rectifying behaviour consistent with the presence of an intra-nanowire p±n junction (Fig. 4a). To establish that the current recti®cation was due to intra-nanowire p±n junctions, we char￾acterized the local nanowire potential and gate response by electro￾static force microscopy (EFM) and scanned gate microscopy (SGM), respectively21. An EFM image of a typical p±n junction in reverse bias showed that the entire voltage drop occurs at the p±n junction itself (Fig. 4b); EFM measurements showed no potential drop at the contact regions under forward or reverse bias (not shown), ruling out the contact/nanowire interface as the source of recti®cation in the I±VSD behaviour. In addition, SGM images recorded with the nanowire device in forward bias and the scanned tip±gate positive (Fig. 4c) show enhanced conduction to the right of the junction, indicating an n-type region, and reduced conduction to the left of the junction, indicating depletion of a p-type region. The abrupt change in majority carrier type coincides with the location of the intra-nanowire junction determined by EFM, and thus further con®rms that the diode behaviour results from well controlled dopant modulation. We believe that the direct and controlled growth of nanowire p±n junctions represents an advance over previous work8,10,22. The ability to synthesize modulation-doped nanowire superlattices opens up new opportunities, ranging from ultra-sensitive biological and chemical detectors to bipolar transistors and highly integrated logic gates for nanoelectronics. Moreover, the direct growth of modulation-doped nanowires eliminates the lithographic steps used to create doped nanotube p±n junctions8,10, and thus facilitates the bottom-up assembly of complex functional structures when combined with recent advances in the directed en masse organiza￾tion of nanowire structures5,9. Last, the use of single InP nanowire p±n junctions as nanoscale LEDs has been investigated (Fig. 4d). I±VSD measurements of InP nanowire p±n structures exhibit recti®cation similar to that described above for the silicon nanowire p±n junctions. In forward bias, individual InP nanowire devices exhibit light emission from p±n junctions that is both highly polarized and blue-shifted due to the one-dimensional structure and radial quantum con®nement, respectively (Fig. 4e)3 . The ef®ciency of these intra-nanowire LEDs is ,0.1%, although it can be increased substantially. By de®ning a quantum dot heterostructure within a p±n diode during nanowire synthesis, it should be possible to engineer an electrically driven single-photon source with well-de®ned polarization. Such a nano￾wire device could be extremely useful in quantum cryptography and information processing23. More generally, we believe that the pre￾sent results open up many opportunities in nanoscale photonics and electronics, ranging from the relatively simple nanoscale emit￾ters and complementary logic, which can be obtained from single nanowire p±n junctions, to complex periodic superlattices that may enable applications such as nanowire injection lasers and `engi￾neered' one-dimensional electron waveguides. M Methods Nanowire synthesis Nanowires were synthesized either via laser-assisted catalytic growth (GaAs, GaP and InP) or chemical vapour deposition (Si), using Au nanoclusters to direct the growth. Au nanoclusters were deposited onto oxidized silicon substrates and then placed in the reactor furnace. For nanowires produced by laser-assisted catalytic growth, solid targets of GaAs, GaP and InP were ablated using either a pulsed ArF excimer or Nd:YAG lasers, and growth was carried out at 700±850 8C in an argon ¯ow of 100 cm3 STP per minute at 100 torr. A pause of ,45 s in the ablation was made between each layer in a given superlattice. Silicon nanowires were grown by chemical vapour deposition at 450 8C using silane (3 cm3 STP per minute) and either 100 p.p.m. diborane (p) or phosphine (n) in helium (18 cm3 STP per minute) as dopants. The furnace was evacuated before switching dopants. The resulting nanowires were sonicated brie¯y in ethanol and deposited onto copper grids for TEM analysis. The high-resolution TEM images and EDS spectra from nanowire superlattices were collected on a JEOL 2010F microscope. The elemental mapping of the single junction was conducted on a VG HB603 STEM. Photoluminescence measurements Single-nanowire photoluminescence images were obtained using a purpose-built, far-®eld epi¯uorescence microscope. Excitation light (488 nm) was focused by an objective (NA = 0.7) to a ,30-mm-diameter spot at ,1.0 kW cm-2 on the quartz substrate with dispersed nanowires. A l/2 wave plate was used to rotate the polarization of the excitation light, and the resulting photoluminescence was focused and imaged on a liquid-nitrogen-cooled charge coupled device. The emission polarization was measured using a Glan±Thompson polarizer. Electrical measurements Nanowires dispersed in ethanol were deposited onto silicon substrates (600-nm oxide), and electrical contacts were de®ned using electron beam lithography. Ti/Au contacts were used for Si nanowires, and were annealed at 400 8C following deposition. InP LED contacts were fabricated by a two-step process in which the ®rst contact (n-type) was made using Ge/Au or Ni/In/Au and the second (p-type) was made using Zn/Au. The contacts were annealed at 300±350 8C following deposition. Scanned probe microscopy A Digital Instruments Nanoscope III with extender module was used for the EFM and SGM measurements. Force modulation etched silicon probe (FESP) tips coated with 5 nm Cr/45 nm Au were used for imaging. For EFM, the nanoscope was operated in lift mode with a lift height of 60 nm and a scan rate of 0.5 Hz. Received 24 December 2001; accepted 11 January 2002. 1. Lieber, C. M. The incredible shrinking circuit. Sci. Am. 285, 58±64 (2001). 2. Cui, Y. & Lieber, C. M. Functional nanoscale electronic devices assembled using silicon nanowire building blocks. Science 291, 851±853 (2001). 3. Wang, J. F., Gudiksen, M. S., Duan, X. F., Cui, Y. & Lieber, C. M. Highly polarized photolumi￾nescence and photodetection from single indium phosphide nanowires. Science 293, 1455±1457 (2001). 4. 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). 5. Duan, X. F., Huang, Y., Cui, Y., Wang, J. F. & Lieber, C. M. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 409, 66±69 (2001). 6. Huang, Y. et al. Logic gates and computation from assembled nanowire building blocks. Science 294, 1313±1317 (2001). 7. Bachtold, A., Hadley, P., Nakanishi, T. & Dekker, C. Logic circuits with carbon nanotube transistors. Science 294, 1317±1320 (2001). 8. Derycke, V., Martel, R., Appenzeller, J. & Avouris, P. Carbon nanotube inter- and intramolecular logic gates. Nano Lett. 1, 453±456 (2001). 9. Huang, Y., Duan, X. F., Wei, Q. Q. & Lieber, C. M. Directed assembly of one-dimensional nanostructures into functional networks. Science 291, 630±633 (2001). 10. Zhou, C. W., Kong, J., Yenilmez, E. & Dai, H. J. Modulated chemical doping of individual carbon nanotubes. Science 290, 1552±1555 (2000). 11. Gudiksen, M. S. & Lieber, C. M. Diameter-selective synthesis of semiconductor nanowires. J. Am. Chem. Soc. 122, 8801±8802 (2000). 12. Cui, Y., Lauhon, L. J., Gudiksen, M. S., Wang, J. F. & Lieber, C. M. Diameter-controlled synthesis of single-crystal silicon nanowires. Appl. Phys. Lett. 78, 2214±2216 (2001). 13. Gudiksen, M. S., Wang, J. F. & Lieber, C. M. Synthetic control of the diameter and length of single crystal semiconductor nanowires. J. Phys. Chem. B 105, 4062±4064 (2001). 14. Wagner, R. S. in Whisker Technology 47±119 (Wiley-Interscience, New York, 1970). 15. Morales, A. M. & Lieber, C. M. A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 279, 208±211 (1998). 16. Duan, X. F. & Lieber, C. M. General synthesis of compound semiconductor nanowires. Adv. Mater. 12, 298±302 (2000). 17. Cui, Y., Duan, X. F., Hu, J. T. & Lieber, C. M. Doping and electrical transport in silicon nanowires. J. Phys. Chem. B 104, 5213±5216 (2000). 18. Nicewarner-Pena, S. R. et al. Submicrometer metallic barcodes. Science 294, 137±141 (2001). 19. Chow, E. et al. Three-dimensional control of light in a two-dimensional photonic crystal slab. Nature 407, 983±986 (2000). 20. Huang, M. H. et al. Room-temperature ultraviolet nanowire nanolasers. Science 292, 1897±1899 (2001). 21. Bachtold, A. et al. Scanned probe microscopy of electronic transport in carbon nanotubes. Phys. Rev. Lett. 84, 6082±6085 (2000). 22. Hu, J., Ouyang, M., Yang, P. & Lieber, C. M. Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires. Nature 399, 48±51 (1999). 23. Bennett, C. H. & DiVincenzo, D. P. Quantum information and computation. Nature 404, 247±255 (2000). Acknowledgements We thank X. Duan for discussions, and W. MoberlyChan and A. J. Garratt-Reed for assistance with TEM imaging and analysis. M.S.G. thanks the NSF for predoctoral fellowship support. C.M.L. acknowledges support of this work by the Of®ce of Naval Research and Defense Advanced Projects Research Agency. Competing interests statement The authors declare that they have no competing ®nancial interests. Correspondence and requests for materials should be addressed to C.M.L. (e-mail: cml@cmliris.harvard.edu). © 2002 Macmillan Magazines Ltd