Synthesis of Si/semiconductor and Si/Sio2/semiconductor branched NWs. Si or The Au branches were modified in two steps. First, the devices were reacted posited Au-NPs were dispersed on SiOz surface of heavily with a 10 mg/mL solution of DMSO( Sigma-Aldrich) for approximately 4 h, followed by extensive rinsing with DMSO. Anti-PSA (Abl, clone ER-PR8, Neo- as phase growth system to prepare branched semiconductor NWs. Ge Markers)was then coupled to the succinimidyl(NHS)-terminated Au branches 15 min with the flow of 10 sce urfaces by reaction of 10-20 ug/mL antibody in a pH 8.4, 10 mM phosphate GeHa(10%), 10 sccm PHa(1,000 ppm in H2), and 200 scam H2 as described buffer solution for a period of 2-4 h Unreacted NHS groups were subse previously (41). The growth of other Ill-V and ll-VI branches was achieved quently passivated by reaction with ethanolamine under similar conditions. y thermal evaporation and vapor transport method (42).Powders with psa and BSa protein samples in 1 uM phosphate buffer solution(pH, 7. 4) the same composition were put into the center of the quartz tube, which were flowed under a flow rate of 0.30-0.60 mL/h through the microfluidic approximately 400-600C 30 scam of Hz was used as the carrier gas, an channel while monitoring the branch nanowire device properties as de. pressure was kept at 40 scribed in detail elsewhere (37) devices were fabricated on Sioz surface of g< multiple-branch input Stress Field Simulation Stress field simulations were carried out using finite ele in Si/GaAs branched structure. we took the axis of gaas branch and si lithography(43)followed by thermal of metals. Ti/Pd(5/50 nm) backbone as(111)and (211), respectively, and the following material con- ntacts were used for both Si and Ge NWs: Ti/Al/Pd/Au(20/80/20/30 n tants are used: modulus of elasticity, Gu(GaAs)=1.18 x 10Pa, C12(GaAs) ontacts were used for other II-V and I-VI semiconductor NWs. Current- 0.538 x 1011 Pa, aA=0.594×1011Pa,c1is=1.662×101Pa,cns)= oltage (-v) data were recorded using an Agilent semiconductor parameter 664x 10 Pa, Ca4(si)=0.798x 10 Pa; lattice constant, asa)=0.543 nm halyzer(Model 4156o) with contacts to devices made using a probe station a(GaAs)=0.565 nm; backbone to branch width ratio, 2: 1 esert Cryogenics, Model TTP4) EL from branched NW structures was char- luminescence ment(44). Arrays of AcKNOWLEDGMENTS We thank profs. R.Gordon and /Au-NP Nw devices were defined by photolithography (37). Ti/Pd Drs. H. Yan, Y. Dong, J. Huang, Y. Wu, B deposited by thermal evaporation and then discussions and constructive comments on passivated by subsequent deposition of 50-nm thick Si3 N4 coating(37). The edges support of this work by the Air Fo completed device chip was subject to Au-branch growth as described above. and a National Security Science and Engine on the man ffice of so search Faculty Fellow award. 1. Hu J, Odom T, Lieber CM(1999)Chemistry and physics in one dimension: Synthesis 23. Dick KA, et al. (2006)Position-controlled interconnected InAs nanowire networks. Nanoscale science and technology. Building a big future from small 24. Suyatin DB, et al. (2008)Electrical properties of self-assembled branched InAs Lett8:11001104. 3. LiY, Qian F, Xiang J, Lieber CM(2006)Nanowire electronic and op 25. Gautam UK, Fang x Bando Y, Zhan J, Golberg D (200 branched ZnS nanotube-In nanowire Semiconductor nanowires and nanotubes. Annu 26. Meng G, et al. (2009)A ge 5. Thelander C et al. (2006)Nanowire- based one-dimensional electronics. Mater Today nanotube and nanotube/nanowire/nanotube heterojunctions with branched topol- 6. Wang ZL (2004)Functional oxide nanobelts: Materials, properties 27. Chen B, et al. (2010 their connections with gold nanowires in both linear and branched topologies. ACS Nano 4: 7105-7112 7. Gudiksen MS, Lauhon U, Wang I, Smith DC, Lieber CM(2002) Growth Jun K, Jacobson JM(2010)Programmable growth of branched silicon nanowires using 8. Wu Y, Fan R, Yang P (2002) Block-by-block growth of single-crysta 29.Jo C Dujardin E, Davis SA Murphy C, Mann S(2002)Growth and form of gold 9. Bjork MT, et al. (2002)One-dimensional heterostructures in semiconductor nanowhis- 12:1765-177 10. Lauhon L, Gudiksen MS, Wang D, Lieber CM (2002) Epitaxial core-shell and core- nthesis of crystalline Science 279: 208- 31 11. Tian B, Xie P, Kempa T3, Bell DC, Lieber CM(2009)Single crystalline kinked semicon- 32. Hausmann DM, Kim, iconductor Devices (Wiley, New York) 12. Dick KA, et al. (2004) sis of branched 'nanotrees' by controlled seeding of multi- Mater14:43504358. 33. Mzhari B, Morkoc H (1993)Surface recombinationin GaAs PN junction diode. J/ 13. Wang onal growth of branched and Lett4871-874 34. Huang Y, Duan X Lieber CM(2005) Nanowires for integrated multicolor Self-assembled nanowire-nanoribbon junction 5. Huang Y, et al. (2001)Logic gates and computation from assembled nanowin 15. Yan HQ. He RR, Pham J, Yang pD (2003)Morphogenesis of one-dimensional Zno no-and microcrystals. Adv Mater 15: 402-405 36. Cui Y, Wei Q, Park H, Lieber CM(2001)N 16. Zhou ], et al. (2005) Three-dimensional tungsten oxide nar selective detection of biological and chemical species. Science 293: 1289-1292. 37. Zheng G, Patolsky F, Cui Y, Wang wU, Lieber CM(2005) 17. Bierman M, Lau YKA, Kvit AV, Schmitt AL Jin S(2008)Dislocation-driven Science320:1060-1063. ation of chiral branched nanowires by the Eshelby Twist. 39. Wh ed growth and structures of molecular-scale silicon nano- R Buhro WE (2007) Solution-b no-and heterobranched semiconductor nanowires. J Am Chem Soc 41. Greytak AB, Lauhon U, Gudiksen MS, Liebe es of complementary germanium nanowire field-effect transistors. App! Phys Lett 20. Jung Y, Ko DK, Agarwal R (2007) Synthesis and structural characterization of single- 4:41764178. heterostructures as high electron 21. Zhou al.(2008)Controllable fabrication of high-quality 6-fold symmetry- mobility devices". Nano Lett 7: 3214-3218. ranched Cds nanostructures with ZnS nanowires as templates. J Phys Chem c 43. Cui Y, Zhong z, Wang D, 22 Milliron D), et aL. (2004)Colloidal nanocrystal heterostructures with linear and 44. Qian F, Gradecak S,ui Y, Wen Y, Lieber CM(2005)Corel multishell nanowire hetero- branched topology. Nature 430: 190-195 structures as multicolor, high-efficiency light-emitting diodes. Nano Lett 5:-2287-2291 12216iwww.pnas.org/cgi/doi/10.1073/pnas.1108584108 Jiang et alSynthesis of Si∕semiconductor and Si∕SiO2∕semiconductor branched NWs. Si or Si∕SiO2 NWs with deposited Au-NPs were dispersed on SiO2 surface of heavily Si substrates as above and then immediately placed into the appropriate NW gas phase growth system to prepare branched semiconductor NWs. Ge branches were grown at 290 °C, 200 torr for 15 min, with the flow of 10 sccm GeH4 (10%), 10 sccm PH3 (1,000 ppm in H2), and 200 sccm H2 as described previously (41). The growth of other III–V and II–VI branches was achieved by thermal evaporation and vapor transport method (42). Powders with the same composition were put into the center of the quartz tube, which was heated to 650–780 °C, while the branch growth temperature was approximately 400–600 °C. 30 sccm of H2 was used as the carrier gas, and pressure was kept at 40 torr. Device Fabrication and Measurement. Single- and multiple-branch input devices were fabricated on SiO2 surface of Si substrates (50-nm thermal oxide, n-type 0.005 Ω-cm, Nova Electronic Materials) using electron beam lithography (43) followed by thermal evaporation of metals. Ti∕Pd (5∕50 nm) contacts were used for both Si and Ge NWs; Ti∕Al∕Pd∕Au (20∕80∕20∕30 nm) contacts were used for other III–V and II–VI semiconductor NWs. Current– voltage (I–V) data were recorded using an Agilent semiconductor parameter analyzer (Model 4156C) with contacts to devices made using a probe station (Desert Cryogenics, Model TTP4). EL from branched NW structures was char￾acterized with a homebuilt microluminescence instrument (44). Arrays of Si∕Au-NP NW devices were defined by photolithography (37). Ti∕Pd (5∕50 nm) metal contacts were deposited by thermal evaporation and then passivated by subsequent deposition of 50-nm thick Si3N4 coating (37). The completed device chip was subject to Au-branch growth as described above. The Au branches were modified in two steps. First, the devices were reacted with a 10 mg∕mL solution of DMSO (Sigma-Aldrich) for approximately 4 h, followed by extensive rinsing with DMSO. Anti-PSA (AbI, clone ER-PR8, Neo￾Markers) was then coupled to the succinimidyl(NHS)-terminated Au branches surfaces by reaction of 10–20 μg∕mL antibody in a pH 8.4, 10 mM phosphate buffer solution for a period of 2–4 h. Unreacted NHS groups were subse￾quently passivated by reaction with ethanolamine under similar conditions. PSA and BSA protein samples in 1 μM phosphate buffer solution (pH, 7.4) were flowed under a flow rate of 0.30–0.60 mL∕h through the microfluidic channel while monitoring the branch nanowire device properties as de￾scribed in detail elsewhere (37). Stress Field Simulation. Stress field simulations were carried out using finite element method (ABAQUS software, version, 6.5-1). To simulate the stress in Si/GaAs branched structure, we took the axis of GaAs branch and Si backbone as h111i and h211i, respectively, and the following material con￾stants are used: modulus of elasticity, c11ðGaAsÞ ¼ 1.18 × 1011 Pa, c12ðGaAsÞ ¼ 0.538 × 1011 Pa, c44ðGaAsÞ ¼ 0.594 × 1011 Pa, c11ðSiÞ ¼ 1.662 × 1011 Pa, c12ðSiÞ ¼ 0.664 × 1011 Pa, c44ðSiÞ ¼ 0.798 × 1011 Pa; lattice constant, aðSiÞ ¼ 0.543 nm, aðGaAsÞ ¼ 0.565 nm; backbone to branch width ratio, 2∶1. ACKNOWLEDGMENTS. We thank Profs. R. Gordon and F. Spaepen and Drs. H. Yan, Y. Dong, J. Huang, Y. Wu, B. Timko, and Y. Fang for helpful discussions and constructive comments on the manuscript. C.M.L. acknowl￾edges support of this work by the Air Force Office of Scientific Research and a National Security Science and Engineering Faculty Fellow award. 1. Hu J, Odom T, Lieber CM (1999) Chemistry and physics in one dimension: Synthesis and properties of nanowires and nanotubes. Acc Chem Res 6:435–445. 2. 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Qian F, Gradecak S, Li Y, Wen Y, Lieber CM (2005) Core/multishell nanowire hetero￾structures as multicolor, high-efficiency light-emitting diodes. Nano Lett 5:2287–2291. 12216 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1108584108 Jiang et al