LETTERS NATURE NANOTECHNOLOGY DOL: 10.1038/NNANO2009304 GND Vbias (v) Figure 5 I Topologically defined nanoelectronic devices. a, I-V data recorded from a kinked p-n silicon nanowire device Inset: SEM image of the device structure. Scale bar, 2 um. b, EFM image of a p-n diode reverse-biased at 5 V. The AFM tip voltage was modulated by 3 V at the ca frequency. The signal brightness is proportional to the nanowire device surface potential, and shows an abrupt drop around the kink position. The dashed hes mark the nanowire position. Scale bar, 2 um. c, d, AFM and SGM images of one nt-kink-nt-kink-(n-nt) dopant modulated double-kinked silicon nanowire structure Scale bar in c, 2 um. The SGM images were recorded with a Vin of 10V(i) and -10V(ID), respectively, and Vsd of 1 V. The dark and bright regions correspond to reduced and enhanced conductance, respectively. the black dashed lines mark the nanowire position. synthesis of uniform n-type, 80-nm kinked silicon nanowires, the flow rates of SiHg, meast were carried out with a Digital Instruments Nanoscope Illa respectively, and the total pressure 40 torr and purge duration 10-15 s; the minimum surface potential maps and SGM con tips(Nanosensors, PPP-NCHPt).The EFM PH, and H, were 1-2, 2-10 and 60 standard cubic centimetres per minute, MultiMode AFM and metal-coated pressure during the purge cycle was x3 x 10- torr. The dopant feed-in ratios lift heights of 40 and 20 nm, respectively. In the surface potential measurements, the (silicon: boron/phosphorus) in kinked p-n silicon nanowires were 500: 1 for both p-n diode was reverse-biased at 5 V and the tip voltage was modulated by 3 V at the p-and n-type segments. In nt-kink-nt-kink-(n-nt)dopant modulated silicon resonance frequency. In SGM measurements, the tip functions as a local gate V n*-and n-type segments, respectively, and the n-type segment was grown for 30s +10V, and the conductance versus position provides a measure of local anowires, the silicon-phosphorus feed-in ratios were 200: l and 10,000:1 for Germanium nanowires were synthesized at 270-290.. 40 torr, with (GeHa, 10% in H2)and H, as the reactant and carrier gas, respectively. Cds Received 2 June 2009; accepted 15 September 2009 with nanowire growth using gold nanocluster-catalysed VLS method at published online 18 October 2009 The purge cycle used to form kinks in the germanium and Cds Refere Gudiksen, M. S, Lauhon, LJ, Wang, J, Smith, D. C. Lieber, C M. Growth of Structure characterization. Zeiss Ultra55/Supra55VP field-emission SEMs and a nanowire superlattice structures for nanoscale photonics and electronics. Nature JEOL 2010 field-emission TEM were carry out SEM and TEN 415,617-620(2002 espectively. For sample preparation, kinked nanowires 2. Bjork, M. T et al. One-dimensional heterostructures in semiconductor cohol and dispersed onto heavily doped silicon substrate whiskers. Appl. Phys. Left. 80, 1058-1060(2002). 200-nm nitride; resistivity, 1-10 n2 cm, Nova Electronic Mater M.S. Wang, D. L. grids (Ted Pella). anowire heterostructures. Nature 420, 57-61(2002 4. Qian, F, Li, Y, Gradecak, S, Wang, D L, Barrelet, c Device n and me ent. Devices were fabricated on silicon substrates itride-based nanowire radial hete tures for nanophotonics Nano Lett. 4 (Nova Electronic Materials, n-type 0.005 Q cm)with 100-nm thermal oxide and 5. followed by titanium/palladium(1.5nm/100 nm)contact deposition in a thermal Pathrc, Zhong,, z. H.&Lieber, C.MEncoding electronic properties by waporator Current-voltage(I-V) data 1304-130702005) iconductor parameter analyser(Model 4156C)with contacts to devices made 6. Algra, R. E et al. Twinning superlattices in indium phosphide nanowires. Nafure a probe station(Desert Cryogenics, Model TTP4) EFM and SGM 456,369-372(2008 828 NatureNanotEchnOlogYIVol4IDecemBer2009Iwww.nature.com/naturenanotechnologysynthesis of uniform n-type, 80-nm kinked silicon nanowires, the flow rates of SiH4, PH3 and H2 were 1–2, 2–10 and 60 standard cubic centimetres per minute, respectively, and the total pressure 40 torr and purge duration 10–15 s; the minimum pressure during the purge cycle was 3 1023 torr. The dopant feed-in ratios (silicon:boron/phosphorus) in kinked p–n silicon nanowires were 500:1 for both p- and n-type segments. In nþ–kink–nþ–kink–(n–nþ) dopant modulated silicon nanowires, the silicon–phosphorus feed-in ratios were 200:1 and 10,000:1 for nþ- and n-type segments, respectively, and the n-type segment was grown for 30 s. Germanium nanowires were synthesized at 270–2908C, 40 torr, with germane (GeH4, 10% in H2) and H2 as the reactant and carrier gas, respectively. CdS nanowires were grown in a three-zone furnace by evaporating CdS power at 650–720 8C, with nanowire growth using gold nanocluster-catalysed VLS method at 550–500 8C. The purge cycle used to form kinks in the germanium and CdS nanowires was typically 15 s. Structure characterization. Zeiss Ultra55/Supra55VP field-emission SEMs and a JEOL 2010 field-emission TEM were used to carry out SEM and TEM analyses, respectively. For sample preparation, kinked nanowires were gently sonicated in isopropyl alcohol and dispersed onto heavily doped silicon substrates (100-nm oxide/200-nm nitride; resistivity, 1–10 V cm, Nova Electronic Materials) or lacey carbon grids (Ted Pella). Device fabrication and measurement. Devices were fabricated on silicon substrates (Nova Electronic Materials, n-type 0.005 V cm) with 100-nm thermal oxide and 200-nm SiN at the surface. Devices were defined by electron-beam lithography followed by titanium/palladium (1.5 nm/100 nm) contact deposition in a thermal evaporator. Current–voltage (I–V) data were recorded using an Agilent semiconductor parameter analyser (Model 4156C) with contacts to devices made using a probe station (Desert Cryogenics, Model TTP4). EFM and SGM measurements were carried out with a Digital Instruments Nanoscope IIIa MultiMode AFM and metal-coated tips (Nanosensors, PPP-NCHPt). The EFM surface potential maps and SGM conductance maps were acquired in lift mode with lift heights of 40 and 20 nm, respectively. In the surface potential measurements, the p–n diode was reverse-biased at 5 V and the tip voltage was modulated by 3 V at the resonance frequency. In SGM measurements, the tip functions as a local gate Vtip ¼ +10 V, and the conductance versus position provides a measure of local accumulation or depletion of carriers in the device. Received 2 June 2009; accepted 15 September 2009; published online 18 October 2009 References 1. Gudiksen, M. S., Lauhon, L. J., Wang, J., Smith, D. C. & Lieber, C. M. Growth of nanowire superlattice structures for nanoscale photonics and electronics. Nature 415, 617–620 (2002). 2. Bjork, M. T. et al. One-dimensional heterostructures in semiconductor nanowhiskers. Appl. Phys. Lett. 80, 1058–1060 (2002). 3. Lauhon, L. J., Gudiksen, M. S., Wang, D. L. & Lieber, C. M. Epitaxial core–shell and core–multishell nanowire heterostructures. Nature 420, 57–61 (2002). 4. Qian, F., Li, Y., Gradecˇak, S., Wang, D. L., Barrelet, C. J. & Lieber, C. M. Gallium nitride-based nanowire radial heterostructures for nanophotonics. Nano Lett. 4, 1975–1979 (2004). 5. Yang, C., Zhong, Z. H. & Lieber, C. M. Encoding electronic properties by synthesis of axial modulation-doped silicon nanowires. Science 310, 1304–1307 (2005). 6. Algra, R. E. et al. Twinning superlattices in indium phosphide nanowires. Nature 456, 369–372 (2008). c n+ n+ n n+ b GND 5 V p n a −4 −2 0 2 0 20 40 60 80 Vbias (V) I II d Figure 5 | Topologically defined nanoelectronic devices. a, I–V data recorded from a kinked p–n silicon nanowire device. Inset: SEM image of the device structure. Scale bar, 2 mm. b, EFM image of a p–n diode reverse-biased at 5 V. The AFM tip voltage was modulated by 3 V at the cantilever-tip resonance frequency. The signal brightness is proportional to the nanowire device surface potential, and shows an abrupt drop around the kink position. The dashed lines mark the nanowire position. Scale bar, 2 mm. c,d, AFM and SGM images of one nþ2kink2nþ2kink2(n2nþ) dopant modulated double-kinked silicon nanowire structure. Scale bar in c, 2 mm. The SGM images were recorded with a Vtip of 10 V (I) and 210 V (II), respectively, and Vsd of 1 V. The dark and bright regions correspond to reduced and enhanced conductance, respectively. The black dashed lines mark the nanowire position. LETTERS NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2009.304 828 NATURE NANOTECHNOLOGY | VOL 4 | DECEMBER 2009 | www.nature.com/naturenanotechnology