doi:10.1038/ nature06181 nature METHODS I-V data analysis. The saturation current, Io, together with the diode ideality Nanowire synthesis p-i-n coaxial silicon nanowires were prepared using 100- factor N was extrapolated from the dark k-v curve using the ideal diode gold nanoclusters as catalysts, silaneSiH,)as the silicon reactant, diboron equate (B2H6, 100 p.P. m in H2)as the p-type dopant, phosphine(PH,, 1,000 p-p.min H2) as the n-type dopant, and hydrogen(H2) as the carrier gas. For the p-core In(D=NT V+In( nanowire growth, the flow rates of SiHa, B2H and Hz were 1, 10 and 60 standard where g is the electronic charge and k is the Boltzmann constant.The respectively. For the i-shell deposition, the flow rates of SiH and H2 were 0.15 coaxial silicon nanowire were 2.23+0.13 and 4.70+0.77, respectively. Under and 60 standard cubic centimetres per minute, respectively, and 0.75 standard cubic centimetres per minute of PH, was added during the subsequent n-shell illumination, the ideal diode equation can b expressed in terms of Isc and Vocas: deposition. The growth temperatures for core and shells were 440C and 650C, respectively; the total pressures were 40 torr and 25 torr, respectively. The p-core NkT growth lasted 3 h, and the deposition of i- and n- shells took I h and 0.5 h, where fits to In(Lsc)versus Voc (for example, Fig 3d) are used to determine Nand respectively. Following the growth, the nanowire growth substrate was cleaned lo from the slope and intercept, respectively by oxygen plasma and the SiOz hard mask(30-60-nm thick) was deposited Silicon nanowire photovoltaic-powered nanowire sensor and logic devices. Device fabrication. All p-i-n devices were fabricated on heavily doped silicon (Fig. 4a) were fabricated as described sd cthanol/; 0(9596/596), and a single <0.005 2 cm, Nova Electronic Materials). To fix the nanowires, chro- polydimethylsiloxane microfluidic channel was used to deliver different pH pads were patterned by electron beam lithography(EBL) and deposited solutions during the experiments The silicon nanowire sensor resistance on the SiOz hard mask by thermal evaporation. The second EBL step (10-30MQ2)was chosen to allow for operation in the high-power working defined an etching window to expose the p-core in selected regions. The SiOz at regime of the photovoltaic device in which the output voltage ranges from the exposed nanowire region was first etched away using buffered HF with one-third to one-half of Vo but the output current is relatively constant.The the e-beam resist as an etching mask, and the underlying shells of the silicon p-contact of the silicon nanowire photovoltaic device was connected to one end nanowire were further removed by KOH etching(70C, 45s). In the last step, of the sensor device, whereas the n-contact and the other end of the sensor device titanium/palladium contacts (3 nm/500 nm thick) at the p-core n-shell of were grounded, and a computer-controlled analogue-to-digital converter dividual silicon nanowires were patterned by EBL and deposited by thermal (6030E, National Instruments)was used to record the voltage drop across the evaporation. No annealing was required to ensure ohmic contact formation. silicon nanowire sensor. The self-powered AND gate( Fig. 4c)was made entire Sample illumination. A standard solar simulator(150W, Newport Stratford) from nanowires, in which p-i-n coaxial silicon nanowires were configured as the with an AM 1. 5G filter was used in our experiments, in which the average inten- two diodes and a CdSe nanowire was used as the resistor. The large resistance of sity was calibrated using a lens was placed between t the Case nanowire and reverse-biased p-i-n diodes yielded V and Vi (HIGH) t source and the nanowire photovoltaic values close to the V (0.53v) of the photovoltaic device( two p-i-n coaxial device. It should be noted that the exact light intensity incident on the nanowire silicon nanowire elements in series). Voltage outputs for all logic gate devices cannot be measured exactly because its physical dimensions(around 300 nm in were recorded using a computer-controlled analogue-to-digital converter. diameter and 3-22 um in length)are orders of magnitude smaller than those of the pin hole of a power meter(millimetre range ). Nevertheless, the intensity 30. Patolsky F, Zheng, G F& Lieber, C M Fabrication of silicon nanowire devices for should be fairly close to l-sun according to the light intensity uniformity gua- ranted by the solar simulator vendor. Nature protocols11711-1724;doi:10.1038/ prot2006227(2006) E2007 Nature Publishing GroupMETHODS Nanowire synthesis. p-i-n coaxial silicon nanowires were prepared using 100- nm gold nanoclusters as catalysts, silane (SiH4) as the silicon reactant, diboron (B2H6, 100 p.p.m. in H2) as the p-type dopant, phosphine (PH3, 1,000 p.p.m. in H2) as the n-type dopant, and hydrogen (H2) as the carrier gas. For the p-core nanowire growth, the flow rates of SiH4, B2H6 and H2 were 1, 10 and 60 standard (converted to standard temperature and pressure) cubic centimetres per minute, respectively. For the i-shell deposition, the flow rates of SiH4 and H2 were 0.15 and 60 standard cubic centimetres per minute, respectively, and 0.75 standard cubic centimetres per minute of PH3 was added during the subsequent n-shell deposition. The growth temperatures for core and shells were 440 uC and 650 uC, respectively; the total pressures were 40 torr and 25 torr, respectively. The p-core growth lasted 3 h, and the deposition of i- and n- shells took 1 h and 0.5 h, respectively. Following the growth, the nanowire growth substrate was cleaned by oxygen plasma and the SiO2 hard mask (30–60-nm thick) was deposited conformally onto the silicon nanowire surface by means of PECVD. Device fabrication. All p-i-n devices were fabricated on heavily doped silicon substrates with 100 nm thermal oxide and 200 nm silicon nitride (n-type, res￾istivity ,0.005 V cm, Nova Electronic Materials). To fix the nanowires, chro￾mium pads were patterned by electron beam lithography (EBL) and deposited directly on the SiO2 hard mask by thermal evaporation. The second EBL step defined an etching window to expose the p-core in selected regions. The SiO2 at the exposed nanowire region was first etched away using buffered HF with the e-beam resist as an etching mask, and the underlying shells of the silicon nanowire were further removed by KOH etching (70 uC, 45 s). In the last step, titanium/palladium contacts (3 nm/500 nm thick) at the p-core and n-shell of individual silicon nanowires were patterned by EBL and deposited by thermal evaporation. No annealing was required to ensure ohmic contact formation. Sample illumination. A standard solar simulator (150 W, Newport Stratford) with an AM 1.5G filter was used in our experiments, in which the average inten￾sity was calibrated using a power meter. For multiple-sun illumination, an aspheric lens was placed between the light source and the nanowire photovoltaic device. It should be noted that the exact light intensity incident on the nanowire cannot be measured exactly because its physical dimensions (around 300 nm in diameter and 3–22 mm in length) are orders of magnitude smaller than those of the pin hole of a power meter (millimetre range). Nevertheless, the intensity should be fairly close to 1-sun according to the light intensity uniformity gua￾ranteed by the solar simulator vendor. I–V data analysis. The saturation current, I0, together with the diode ideality factor N was extrapolated from the dark I–V curve using the ideal diode equation: ln(I)~ q NkT Vzln(I0) where q is the electronic charge and k is the Boltzmann constant. The average 6 1s ideality factor values obtained from the analysis of p-i-n and p-n coaxial silicon nanowire were 2.23 6 0.13 and 4.70 6 0.77, respectively. Under illumination, the ideal diode equation can be expressed in terms of Isc and Voc as: ln(Isc)~ q NkT Voczln(I0) where fits to ln(Isc) versus Voc (for example, Fig. 3d) are used to determine N and I0 from the slope and intercept, respectively. Silicon nanowire photovoltaic-powered nanowire sensor and logic devices. p-type silicon nanowire (diameter, 20 nm; Si:B 5 16,000:1) sensor devices (Fig. 4a) were fabricated as described elsewhere30. The sensor devices were modi￾fied with aminopropyltriethoxysilane in ethanol/H2O (95%/5%), and a single polydimethylsiloxane microfluidic channel was used to deliver different pH solutions during the experiments30. The silicon nanowire sensor resistance (10–30 MV) was chosen to allow for operation in the high-power working regime of the photovoltaic device in which the output voltage ranges from one-third to one-half of Voc but the output current is relatively constant. The p-contact of the silicon nanowire photovoltaic device was connected to one end of the sensor device, whereas the n-contact and the other end of the sensor device were grounded, and a computer-controlled analogue-to-digital converter (6030E, National Instruments) was used to record the voltage drop across the silicon nanowire sensor. The self-powered AND gate (Fig. 4c) was made entirely from nanowires, in which p-i-n coaxial silicon nanowires were configured as the two diodes and a CdSe nanowire was used as the resistor. The large resistance of the CdSe nanowire and reverse-biased p-i-n diodes yielded Vc and Vi (HIGH) values close to the Voc (0.53 V) of the photovoltaic device (two p-i-n coaxial silicon nanowire elements in series). Voltage outputs for all logic gate devices were recorded using a computer-controlled analogue-to-digital converter. 30. Patolsky, F., Zheng, G. F. & Lieber, C. M. Fabrication of silicon nanowire devices for ultrasensitive, label-free, real-time detection of biological and chemical species. Nature Protocols 1, 1711–1724; doi:10.1038/nprot.2006.227 (2006). doi:10.1038/nature06181 ©2007 NaturePublishingGroup