±RS ammonia(NH,)as Ga, In and N sources, respectively, and nickel netic energy and the ratio of electric field intensity overlapped with nanoclusters as core gro catalysts. GaN cores vn in hydrogen the active region, respectively at 950C and 700 torr for 4, 800s using TMG(22u mol min )and NHy (67 mmol min). Under this condition, most GaN ere of 100-200nm Received 4 March 2008: accepted 15 July 2008: published 17 August 2008 ze and 20-40 um length. Subsequently, the growth conditions were altered to favour homogeneous MQW shell deposition onto the nanowire surface versus 1. Lieber. C M. Wang. Z. L Functional nanowires. Mater Res So. BalL 32,99-104(2007 NH, flow(290 mmol min-)in nitrogen at 400 torr. An In GaN quantum well as deposited at 600-800.C for 20-30s using TMG (5.3 umol min )an 3. Xiang. et al. Ge/Si nanowire heterostructures as high-performance field-effect transistors. Natre dium (6.5 umol min). The temperature was varied to achieve he desired In% as higher deposition temperature leads to less efficient In 4. Xiang, k, Vidan, A, Tinkham, M, corporation. A gan quantum barrier was grown at 830 C for 10-360s 5. Hu.Y. ef aL A Ge/S heterostructure based double quantum dot with integrated charge using TMG(12.3 1). A purge time of 60s was appl hnl2.622-625(2007) depositions. This quantum well/quantum barrier growth unit was alternated for 6. Hochbaum, A.L. et al. Rough silicon nanowires as high performance thermoelectric materials. Natur 3, 13, 26 cycles, respectively, to achieve 3, 13 and 26 MQW nanowire structures. 7. Mao, SS Nanolasers: lasing from nanoscale quantum wires. Imt. J. Nanmotechnol 1, 42-85(2004). ELECTRON MICROSCOPY 115 al Room-temperature ultraviolet nanowire .Scie For conventional TEM studies, nanowires were first sonicated from growth bstrate and then dispersed onto Cu/lacey-carbon TEM grids For cross-section 11. Gradecak, S, Qian, E.Li, Y. Park, H -G&Lieber, C.M. GaN nanowire Lasers with low lasing ransfer from the growth substrate 2, and embedded into an Eponate-Araldite 12 Duan, X Hu PpL Phys. Lett. 87,173111 TEM studies, nanowires were aligned on Thermanox plastic slips by direc C. M. Sin epoxy resin. Embedded samples were cut perpendicular to the nanowire axis 421,241-245(2003) into 50-200-nm-thick slices using a diamond ultramicrotome knife and then l4 Zapien, 1. et al. Room-temperature single nanoribbon lasers. ApPL Phys. Left. 84, 1189-1191(2004). Liu,.K et al. Wavelength-controlled lasing in Zn, Cd-S single-crystal nanoribbe transferred onto TEM grids. STEM images were obtained using a field-emission TEM/STEM (JEOL 2010F)operated at 200kV High-resolution bright-field 5. Liu, Y.K. Zapien, 1.A Shan, Y. Y Tang, H. Lee, S TEM imaging was carried out using a JEOL 4000EX TEM operated at 400 kV. 16 Asda, M. Min dimensional quantum-box EDS elemental mapping was conducted using a VG HB603 STEM. The experimental CBED patterns were taken along the(lion) zone axis of 17 Colder Corzine, s.w. Diode lasers and Photonic Integmted Circuits(wiley. a 216-nm-thick cross-sectional nanowire sample 30 from the nanowire growth direction using a JEOL2100 TEM operating at 200 kv. By underfocusing and Northrup, 1. E& Neugebauer, I Strong aft erfocusing the condenser lens, we verified that no 180 rotation is present am epitaxy and metalorganic chemic 一 以sItt85 between the bright-field image and the corresponding diffraction pattern. The Bm单时甲m ies of CBED patterns was calculated with thicknesses ranging from 20 Z the gan (1100)zone axis with 50 zero-order reflections and zero defocus 21. Wu, I Walukiewicz, w. Band gaps of InN and group Ill nitride alloys. Superlatt. Microstruct. 34. 280 nm with 2 nm steps and the pattern best matching the experimental data 22. Qian, F Gradecak, S, Li, Y wen, C-Y. Lieber, C M. Core/multishell manowire heterostructures as was selected. Zone-axis CBED Patterns present significant differences in(0001) nd (o001)diffraction discs that enable unambiguous diffraction labelling and 23. Nakamura, . Pearton S. Fasol, G. The Blue Laser Diode. The Camplete Story(Springer, polarity determination. OPTICAL MEASUREMENTS Room-temperature optical studies of individual MQW nanowire structure 27. Tawara, T, Gotch, H, Akasaka, I. Kobayashi, N. Saitoh, T. Low-threshold lasing of In GaN vertical-cavity surface emitting lasers with dielectric distributed Bragg reflectors. AppL. Phys. Lett.83, a Q-switched Nd: YVO. laser(266 nm, 35 kHz repetition rate and 7ns 2& Kawakami, Yet ad In inhomogeneity and emission characteristics of In GaN. I. Phys. Condens Matter pulse duration The excitation laser was focused by a microscope objective(numerical aperture 0.65)to a 30 um spot onto 29. Yablonski, G P et al. Luminescence and lasing inIn GaN/GaN multiple quantum well dispersed onto an oxidized Si(600 nm thermal SiO2)substrate ures grown at dif 30. Taka Photoluminescence data were recorded using a 300 mm spectrometer 150lines mm- grating) and a liquid-nitrogen-cooled CCD(charge-coupled 31. Yablonski, G P et aL. Multiple quantum well In GaN/Ga N blue optically pumped lasers operating in device)detector; images were obtained with the same system by replacing the 32. Jave,, A Nam, S, Friedman, R. S, Yan, H. Lieber, CM. Layer-by-layer assembly of nanowires three-dimensional, multifunctional electronics. Nano. Lett. 7. 773-777(2007) FDTD CALCULATION SupplementaryInformationaccompaniesthispaperonwww.naturecom/naturematerials. he full-vector time-dependent Maxwells equations were solved using the layers)boundary conditions. The computation grid size is 12.5 nm. A core/shell The authors thank c L Barrelet and Yn. Wu for helpful discussions, P, Stadelmann for providing with triangular cross-section was introduced in the calculation EDS elemental mapping measurements. This work was supported by the Air Force domain with size of 1.5um x 1.5 um x 12 um. The same structural paramete of Scientific Research(CM.L and the Department of Energy Basic Energy Saier of the nanowire as those obtained from TEM analysis in the manuscript were DE-HGO2-07ER46394(ZLW. ed in our simulation. Only the shorter nanowire length of 10 um was Author contrbutions to save computation time. Average refractive indices of the core and the Y.J.D. synthesized the nanowire structures. YL.an of the nanowire were 2.537 and 2.625, respectively. To excite resonant modes in the nanowire cavity, temporally Gaussian dipole sources with the lasing ng studies. All authors contributed to the design of the experiments wavelength are introduced at several arbitrary positions of the nanowire ar and data analysis. EQ and C M L wrote the paper and all authors contributed to manuscript revisions. then the simulation stores the time evolution of the electromagnetic fields. The Author information simulation results are Fourier transformed to determine the resonant frequency. Reprints and permission information is Q and T of each excited mode are computed from the temporal att naturematerialsIVol7iSeptEmbeR2008Iwww.nature.com/naturematerials 2008 Macmillan Publishers Limited. All rights reservedLETTERS and ammonia (NH3) as Ga, In and N sources, respectively, and nickel nanoclusters as core growth catalysts. GaN cores were grown in hydrogen at 950 ◦C and 700 torr for 4,800 s using TMG (22 µ mol min−1 ) and NH3 (67 mmol min−1 ). Under this condition, most GaN cores were of 100–200 nm size and 20–40 µm length. Subsequently, the growth conditions were altered to favour homogeneous MQW shell deposition onto the nanowire surface versus axial addition at the catalyst site. MQW shell growth was carried out in constant NH3 flow (290 mmol min−1 ) in nitrogen at 400 torr. An InGaN quantum well was deposited at 600–800 ◦C for 20–30 s using TMG (5.3 µmol min−1 ) and trimethylindium (6.5 µmol min−1 ). The temperature was varied to achieve the desired In% as higher deposition temperature leads to less efficient In incorporation. A GaN quantum barrier was grown at 830 ◦C for 10–360 s using TMG (12.3 µmol min−1 ). A purge time of 60 s was applied between layer depositions. This quantum well/quantum barrier growth unit was alternated for 3, 13, 26 cycles, respectively, to achieve 3, 13 and 26 MQW nanowire structures. ELECTRON MICROSCOPY For conventional TEM studies, nanowires were first sonicated from growth substrate and then dispersed onto Cu/lacey-carbon TEM grids. For cross-section TEM studies, nanowires were aligned on Thermanox plastic slips by direct transfer from the growth substrate32, and embedded into an Eponate–Araldite epoxy resin. Embedded samples were cut perpendicular to the nanowire axis into 50–200-nm-thick slices using a diamond ultramicrotome knife and then transferred onto TEM grids. STEM images were obtained using a field-emission TEM/STEM (JEOL 2010F) operated at 200 kV. High-resolution bright-field TEM imaging was carried out using a JEOL 4000EX TEM operated at 400 kV. EDS elemental mapping was conducted using a VG HB603 STEM. The experimental CBED patterns were taken along the h1100 ¯ i zone axis of a 216-nm-thick cross-sectional nanowire sample 30◦ from the nanowire growth direction using a JEOL2100 TEM operating at 200 kV. By underfocusing and overfocusing the condenser lens, we verified that no 180◦ rotation is present between the bright-field image and the corresponding diffraction pattern. The numerical CBED simulations were carried out using the JEMS software for the GaN h1100 ¯ i zone axis with 50 zero-order reflections and zero defocus. A series of CBED patterns was calculated with thicknesses ranging from 200 to 280 nm with 2 nm steps and the pattern best matching the experimental data was selected. Zone-axis CBED patterns present significant differences in (0001) and (0001) di ¯ ffraction discs that enable unambiguous diffraction labelling and polarity determination. OPTICAL MEASUREMENTS Room-temperature optical studies of individual MQW nanowire structures were carried out using home-built far-field epifluorescence apparatus using a Q-switched Nd:YVO4 laser (266 nm, 35 kHz repetition rate and 7 ns pulse duration) as the excitation source. The excitation laser was focused by a microscope objective (numerical aperture 0.65) to a 30 µm spot onto nanowires dispersed onto an oxidized Si (600 nm thermal SiO2) substrate. Photoluminescence data were recorded using a 300 mm spectrometer (150 lines mm−1 grating) and a liquid-nitrogen-cooled CCD (charge-coupled device) detector; images were obtained with the same system by replacing the grating with a mirror. FDTD CALCULATION The full-vector time-dependent Maxwell’s equations were solved using the 3D-FDTD method on a computational grid with absorbing (perfectly matched layers) boundary conditions. The computation grid size is 12.5 nm. A core/shell nanowire with triangular cross-section was introduced in the calculation domain with size of 1.5 µm×1.5 µm×12 µm. The same structural parameters of the nanowire as those obtained from TEM analysis in the manuscript were used in our simulation. Only the shorter nanowire length of 10 µm was assumed to save computation time. Average refractive indices of the core and the shell of the nanowire were 2.537 and 2.625, respectively. To excite resonant modes in the nanowire cavity, temporally Gaussian dipole sources with the lasing wavelength are introduced at several arbitrary positions of the nanowire and then the simulation stores the time evolution of the electromagnetic fields. The simulation results are Fourier transformed to determine the resonant frequency. Q and Γ of each excited mode are computed from the temporal attenuation of electromagnetic energy and the ratio of electric field intensity overlapped with the active region, respectively. Received 4 March 2008; accepted 15 July 2008; published 17 August 2008. References 1. Lieber, C. M. & Wang, Z. L. Functional nanowires. Mater. Res. Soc. Bull. 32, 99–104 (2007). 2. Li, Y., Qian, F., Xiang, J. & Lieber, C. M. Nanowire electronic and optoelectronic devices. Mater. Today 9, 18–27 (2006). 3. Xiang, J. et al. Ge/Si nanowire heterostructures as high-performance field-effect transistors. Nature 441, 489–493 (2006). 4. 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Comparison between a graded and step-index optical cavity in InGaN MQW laser diodes. Semicond. Sci. Technol. 20, 290–295 (2005). 26. Chuang, S. L. Physics of Optoelectronic Devices (Wiley, New York, 1995). 27. Tawara, T., Gotch, H., Akasaka, T., Kobayashi, N. & Saitoh, T. Low-threshold lasing of InGaN vertical-cavity surface-emitting lasers with dielectric distributed Bragg reflectors. Appl. Phys. Lett. 83, 830–832 (2003). 28. Kawakami, Y. et al. In inhomogeneity and emission characteristics of InGaN. J. Phys. Condens. Matter 13, 6993–7010 (2001). 29. Yablonskii, G. P. et al. Luminescence and lasing in InGaN/GaN multiple quantum well heterostructures grown at different temperatures. Appl. Phys. Lett. 85, 5158–5160 (2004). 30. Takahashi, K., Yoshikawa, A. & Sandhu, A. Wide Bandgap Semiconductors: Fundamental Properties and Modern Photonic and Electronic Devices (Springer, New York, 2007). 31. Yablonskii, G. P. et al. Multiple quantum well InGaN/GaN blue optically pumped lasers operating in the spectral range of 450–470 nm. Phys. Status Solidi A 188, 79–82 (2001). 32. 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). Supplementary Information accompanies this paper on www.nature.com/naturematerials. Acknowledgements The authors thank C. J. Barrelet and Y. N. Wu for helpful discussions, P. Stadelmann for providing JEMS simulation software, R. Schalek for help with ultramicrotomy and A. J. Garratt-Reed for assistance in EDS elemental mapping measurements. This work was supported by the Air Force Office of Scientific Research (C.M.L.) and the Department of Energy Basic Energy Sciences, DE-FG02-07ER46394, (Z.L.W.). Author contributions F.Q., Y.L. and Y.J.D. synthesized the nanowire structures. Y.L. and Y.D. carried out TEM characterization, S.G. carried out CBED studies and analysis, F.Q. carried out optical measurements and H.-G.P. carried out modelling studies. All authors contributed to the design of the experiments and data analysis. F.Q. and C.M.L. wrote the paper and all authors contributed to manuscript revisions. Author information Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions. Correspondence and requests for materials should be addressed to Z.L.W. or C.M.L. 706 nature materials VOL 7 SEPTEMBER 2008 www.nature.com/naturematerials © 2008 Macmillan Publishers Limited. All rights reserved