J.Am.Chem.Soc.2001,l23,3165-3166 Direct Observation of vapor-Liquid-Solid Nanowire growth Yiying wu and Peidong y ang+ Materials Science Divisio lawrence Berkeley National Laboratory Department of Chemistry University of California, Berkeley, California 94720 Received December 20. 2000 Nanotubes and semiconductor nanowires are of fundamental mportance to the study of size- and dimensionality-dependent Figure 1. In situ TEM images recorded during the process of nanowire chemical and physical phenomena. 12 How to rationally synthesize growth.(a) Au nanoclusters in solid state at 500C; (b)alloying initiates these l-dimensional nanostructures has been a major challenge, at 800C, at this stage Au exists in mostly solid state; (c)liquid Au/Ge although several strategies have been pursued recently 3-16For loy, (d) the nucleation of Ge nanocrystal on the alloy surface; (e)G example, carbon nanotubes have been prepared via condensation nanocrystal elongates with further Ge cond of hot carbon plasmas in the presence of certain metals, although forms (f).(g)Several other examples of Ge nanowire nucleation,(h, i) the real growth mechanism has been elusive. 3-5 Recently, TEM images showing two nucleation events on single alloy droplet semiconductor nanowires with different compositions have been successfully synthesized using either vapor6-12 or solution-based methodologies. 3-16 One key feature of these syntheses is the romotion of anisotropic crystal growth using metal nanoparticles s catalysts. The growth mechanism has been extrapolated from the vapor-liquid-solid (VLS)mechanism which was proposed oxide-assisted growth mechanism has also been proposed Direct evidence for the nanowire growth mechanism. however is still lacking except for the fact that these nanowires generally have alloy droplets on their tips. Hence, a better understanding of the nanowire growth process in the ohase is necessar to pin down the growth mechanism and to be able to rationally control their compositions, sizes, crystal structures, and growth directions. Herein we report the first real-time observation of semiconductor nanowire growth in an in situ high-temperature ransmission electron microscope(TEM), which unambiguously demonstrates the validity of the VLS growth mechanism at (1)Hu, J; Odom, T. W: Lieber, C. M. Acc Chem. Res. 1999, 32, 435- Figure 2.(a) Schematic illustration of vapor-liquid-solid nanot Prokes, S M. Wang, K. L. Mater. Res. Bull. 1999, 24 growth mechanism including three stages( n) alloying, (n) nucleation nd(lin) axial growth. The three stages are projected onto the conventional Hafner. J. H D. W: Kotula, P G: Carter, C. B. We Au-Ge binary phase diagram(b)to show the compositional and phase evolution during the nanowire growth process Cassell, A. M; Raymakers, J. A; Kong, J; Dai, H. J.J. Phys.Chem. )Bethune, D. S; Kiang, C. H, Devries, M. S; Gorman, G: Savoy, R. nanometer scale. 21 Three well-defined stages have been clearly Vazquez, J; Beyers, R Nature 1993, 363, 605- identified during the process: metal alloying, crystal nucleation, A. M: Liebe and axial growth. On the basis of this mechanism study, selective 7)Duan, X. F: Lieber, C. M. Adu Mater. 2000, 12, 298-302 D. P: Lee. C.S.: Bello growth of Si nanowires with different diameters has been demonstrated using monodispersed gold nanoclusters as catalysts )Gudiksen, M. S, Lieber. C m.Chem.Soc.2000,l22,8801 In situ observation of wire nucleation/growth at nanometer scale 8802. was conducted within a high-temperature transmission electron Tang, CC: Fan,SS Dang, H. Y: Li, P: Liu, Y. M. Appl. Phys. microscope(JEOL CX200). A small amount of micrometer-sized Le.2000,77 Ge particles were dispersed on TEM grids together with solution made monodispersed Au nanoclusters. The gold clusters have C3Btrotl Hencke ag k. Mo. 1791-S1193 vian, A. M. Gibbons average sizes of 20.2+ 3. 1 nm. Although pure Ge has negligible vapor pressure up to 900C, we found that a thin layer of carbon 堂减, unga oy d e omo Gelc entepfaciaon interaction 24 TOns Rop r 15) Heath, J.R.; LeGoues, F K. A Chem. Phys. Lett.1993,208,263 prolonged heating of these carbon-coated Ge particles in a vacuum (16) Holmes, J D; Johnson, K. P, Doty, R C. Korgel, B. A. Science 2000.287,1471-1473 diameters of 150 um and identified alloying, whisker nucleation and growth (17)Whisker Technology, Levitt, A. P, Ed ;, Wiley-Interscience, New York, .2)Colloidal Gold: Principles, Methods and Applications, Hayat, M.A 13:5:mm:2 Wu,Y; Yang, P. Appl. Phys. Lett. 2000, 77, 43-45 10.102lja0059084CCC:S20.00 n Web c 001 American Chemical Society 03/13/2001Direct Observation of Vapor-Liquid-Solid Nanowire Growth Yiying Wu and Peidong Yang* Materials Science DiVision Lawrence Berkeley National Laboratory Department of Chemistry UniVersity of California, Berkeley, California 94720 ReceiVed December 20, 2000 Nanotubes and semiconductor nanowires are of fundamental importance to the study of size- and dimensionality-dependent chemical and physical phenomena.1,2 How to rationally synthesize these 1-dimensional nanostructures has been a major challenge, although several strategies have been pursued recently.3-16 For example, carbon nanotubes have been prepared via condensation of hot carbon plasmas in the presence of certain metals, although the real growth mechanism has been elusive.3-5 Recently, semiconductor nanowires with different compositions have been successfully synthesized using either vapor6-12 or solution-based methodologies.13-16 One key feature of these syntheses is the promotion of anisotropic crystal growth using metal nanoparticles as catalysts. The growth mechanism has been extrapolated from the vapor-liquid-solid (VLS) mechanism which was proposed in the 1960s-1970s for large whisker growth,17-19 although an oxide-assisted growth mechanism has also been proposed.2,20 Direct evidence for the nanowire growth mechanism, however, is still lacking except for the fact that these nanowires generally have alloy droplets on their tips. Hence, a better understanding of the nanowire growth process in the vapor phase is necessary to pin down the growth mechanism and to be able to rationally control their compositions, sizes, crystal structures, and growth directions. Herein we report the first real-time observation of semiconductor nanowire growth in an in situ high-temperature transmission electron microscope (TEM), which unambiguously demonstrates the validity of the VLS growth mechanism at nanometer scale.21 Three well-defined stages have been clearly identified during the process: metal alloying, crystal nucleation, and axial growth. On the basis of this mechanism study, selective growth of Si nanowires with different diameters has been demonstrated using monodispersed gold nanoclusters as catalysts. In situ observation of wire nucleation/growth at nanometer scale was conducted within a high-temperature transmission electron microscope (JEOL CX200). A small amount of micrometer-sized Ge particles were dispersed on TEM grids together with solution￾made monodispersed Au nanoclusters.22 The gold clusters have average sizes of 20.2 ( 3.1 nm. Although pure Ge has negligible vapor pressure up to 900 °C, we found that a thin layer of carbon coating could promote Ge evaporation within the microscope, presumably due to Ge/C interfacial interaction.23,24 In fact, prolonged heating of these carbon-coated Ge particles in a vacuum (1) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435- 445. (2) Prokes, S. M.; Wang, K. L. Mater. Res. Bull. 1999, 24, 13-36. (3) Colbert, D. T.; Zhang, J.; Mcclure, S. M.; Nikolaev, P.; Cheng, Z.; Hafner, J. H.; Owens, D. W.; Kotula, P. G.; Carter, C. B.; Weaver, J. H.; Rinzler, A. G.; Smalley, R. E. Science 1994, 266, 1218-1222. (4) Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. J. Phys. Chem. B 1999, 103, 6484-6492. (5) Bethune, D. S.; Kiang, C. H.; Devries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605-607. (6) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208-210. (7) Duan, X. F.; Lieber, C. M. AdV. Mater. 2000, 12, 298-302. (8) Zhang, Y. F.; Tang, Y. H.; Wang, N.; Yu, D. P.; Lee, C. S.; Bello, I.; Lee, S. T. Appl. Phys. Lett. 1998, 72, 1835-1837. (9) Gudiksen, M. S.; Lieber. C. M. J. Am. Chem. Soc. 2000, 122, 8801- 8802. (10) Wu, Y.; Yang, P. Chem. Mater. 2000, 12, 605-607. (11) Tang, C. C.; Fan, S. S.; Dang, H. Y.; Li, P.; Liu, Y. M. Appl. Phys. Lett. 2000, 77, 1961-1963. (12) Wang, Z. L.; Dai, Z. R.; Gao, R. P.; Bai, Z. G.; Gole, J. L. Appl. Phys. Lett. 2000, 77, 3349-3351. (13) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791-1793. (14) Trentler, T. J.; Goel, S. C.; Hickman, K. M.; Viano, A. M.; Chiang, M. Y.; Beatty, A. M.; Gibbons, P. C.; Buhro, W. E. J. Am. Chem. Soc. 1997, 119, 2172-2181. (15) Heath, J. R.; LeGoues, F. K. A Chem. Phys. Lett. 1993, 208, 263- 268. (16) Holmes, J. D.; Johnson, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471-1473. (17) Whisker Technology; Levitt, A. P., Ed.; Wiley-Interscience, New York, 1970. (18) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89-91. (19) Bootsma, G. A.; Gassen, H. J. J. Crystal Growth 1971, 10, 223-227. (20) Wang, N.; Tang, Y. H.; Zhang, Y. F.; Lee, C. S.; Lee, S. T. Phys. ReV. B 1998, 58, R16024-16026. (21) Previously, Wagner has examined the initial growth of whiskers with diameters of 150 µm and identified alloying, whisker nucleation and growth processes (see ref 17). (22) Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.; Academic Press: New York, 1989. (23) Lisiecki, I.; Sack-Kongehl, H., Weiss, K., Urban, J., Pileni, M. P. Langmuir 2000, 24, 8802-8808. (24) Wu, Y.; Yang, P. Appl. Phys. Lett. 2000, 77, 43-45. Figure 1. In situ TEM images recorded during the process of nanowire growth. (a) Au nanoclusters in solid state at 500 °C; (b) alloying initiates at 800 °C, at this stage Au exists in mostly solid state; (c) liquid Au/Ge alloy; (d) the nucleation of Ge nanocrystal on the alloy surface; (e) Ge nanocrystal elongates with further Ge condensation and eventually a wire forms (f). (g) Several other examples of Ge nanowire nucleation, (h,i) TEM images showing two nucleation events on single alloy droplet. Figure 2. (a) Schematic illustration of vapor-liquid-solid nanowire growth mechanism including three stages (I) alloying, (II) nucleation, and (III) axial growth. The three stages are projected onto the conventional Au-Ge binary phase diagram (b) to show the compositional and phase evolution during the nanowire growth process. J. Am. Chem. Soc. 2001, 123, 3165-3166 3165 10.1021/ja0059084 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/13/2001