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 03/13/2001
Direct 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 solutionmade 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
3166 J. Am. Chem. Soc.. Vol. 123. No. 13. 200/ Communications to the editor results in their complete evaporation at 900C. Thus, these Ge articles could generate sufficient vapor to condense onto the chboring Au clusters. During the experiments, the sample stage was heated resistively to 800-900C. The real-time evolution of the Au nanoparticle morphology was monitored in bright field igure la-f shows a sequence of TEM images during the growth of a Ge nanowire in situ. This real-time observation of the nanowire growth directly mirrors the proposed VLS mecha- nism in Figure 2a. We have examined over 50 individual Au clusters during the in situ catalytic nanowire growth. In general b three stages(I-Ill) could be clearly identified. (): Alloying process (Figure la-c). Au clusters remain in the solid state up to our maximum experimental temperature 900 oC if there is no Ge vapor condensation. This is confirmed by selected area electron diffraction on the pure Au clusters. With increasing amount of Ge vapor condensation and dissolution, Ge and Au form an alloy and liquefy. The volume of the alloy droplets increases, and the elemental contrast decreases(due to solution tem dilution of the heavy metal Au with the lighter element Ge) while nge.(b)TEM the alloy composition crosses sequentially, from left to right, a biphasic region(solid Au and Au/Ge liquid alloy) and a single- an isothermal line in the Au-Ge phase diagram(Figure 2b) the nanowires are larger than the sizes of the initial clusters by (I): Nucleation(Figure Id, e). Once the composition of the several nanometers due to the Au/Ge alloying process. This siz alloy crosses the second liquidus line, it enters another biphasic correlation clearly points out a simple approach to selectively grow nanowires of different diameters using monodispersed clusters region(Aw/Ge alloy and Ge crystal). This is where nanowire of different sizes as catalysts. 6 We have successfully utilized nucleation starts. Knowing the alloy volume change, we estimate that the nucleation generally occurs at Ge weight percentage of this strategy to grow uniform Si nanowires in a chemical vapor 50-60%. This value differs from the composition calculated from deposition(CVD)system. For example, uniform nanowires of he equilibrium phase diagram which indicates the first precipita 20.6±3.2,246±4.0,293±4.5,and60.7±62 nm in diameter tion of Ge crystal should occur at 40% Ge(weight)and 800oC (Wires) were grown using Au clusters with sizes of 15.3+ 2. 4, This difference indicates that the nucleation indeed occurs in 20.1±3.1,256±4.1,524±5.3nm(Dses), respectively2 supersaturated alloy liquid Figure 3a shows a field emission scanning electron microscope Interestingly, we have also occasionally observed that two Ge (FESEM) image of uniform, long, and flexible nanowires grown using 15 nm Au clusters as catalysts. For direct comparison, liquid/solid interfaces(Figure Ih, i). 26 The finite volume of the deposited the nanowires and nanoclusters on the same TEM grids alloy liquid, on the order of 10-17 cm-3, apparently limits the Figure 3b indicates an obvious correlation between the starting number of possible heterogeneous nucleation events, unlike those cluster size and produced nanowire size. These Si nanowires are croscopic systems where tens or hundreds of whiskers can be highly crystalline and generally have [Ill growth direction observed on single alloy droplet. 17,I9 (Figure 3a inset) (Ill). Axial growth(Figure Id-f. Once the Ge nanocrystal The direct observation of nanowire growth unambiguous nucleates at the liquid/solid interface, further condensation/ confirms the validity of vapor-liquid-solid crystal growth dissolution of Ge vapor into the system will increase the amount mechanism at the nanometer scale and should allow us to of Ge crystal precipitation from the alloy. This can be readily rationally control the nanowire growth which is critical for their existing solid/liquid interface, primarily due to the fact wise potential implementation into the nanoscale electronic and opto- nergy will be involved with the crystal step growth as compared ith secondary nucleation events in a finite volume. Consequentl knowledgment. This work was supported by a Camille and Henry condary nucleation events are efficiently suppressed, and ne aculty Award, Research Corporation, and the University new solid liquid inte face will be created. T he existing interface thanks the 3m company for an untenured faculty award and thanks dr E. Stach and D. C. Nelson for help with the TEM studies. We thank the on the nanowire tips. Their compositions were analyzed with National Center for Electron Microscopy for the use of their facilitie nergy-dispersive X-ray spectroscopy(EDAX), and it was found JA0059084 that the weight percentage of Ge matches qualitatively well with he estimated alloy composition at which first Ge nanocrystal 28)The as-made Au clusters nucleates Ge nanowire growth processes (figure 1g), we found that there To zhao Dtr a reng, s. a e: Pine, D. cChmelka, B F. White:sides G.m! is a linear correlation between the sizes of Au nanoclusters and the diameters of the Ge nanowires. Generally, the diameters of mple was then calcined at 600 he substrate was then used to grow Si nanowires in a home-built CVd system Au and Ge, the composition Briefly, SiCI/H2 gas precursors with flow rate of 400 sccm were introduced bserved volume change during the alloying takes 1.5 min. The nanowires were examined by FESEM (EOL FSM6430) (26) For 3 out of 50 examined Au clusters, we observed double nucleation and TEM 29)A linear correlation can be extrapolated as Duire(nm)=5.0+ Dc (27) D. J. App. Phys
results in their complete evaporation at 900 °C. Thus, these Ge particles could generate sufficient vapor to condense onto the neighboring Au clusters. During the experiments, the sample stage was heated resistively to 800-900 °C. The real-time evolution of the Au nanoparticle morphology was monitored in bright field mode. Figure 1a-f shows a sequence of TEM images during the growth of a Ge nanowire in situ. This real-time observation of the nanowire growth directly mirrors the proposed VLS mechanism in Figure 2a. We have examined over 50 individual Au clusters during the in situ catalytic nanowire growth. In general, three stages (I-III) could be clearly identified. (I): Alloying process (Figure 1a-c). Au clusters remain in the solid state up to our maximum experimental temperature 900 °C if there is no Ge vapor condensation. This is confirmed by selected area electron diffraction on the pure Au clusters. With increasing amount of Ge vapor condensation and dissolution, Ge and Au form an alloy and liquefy. The volume of the alloy droplets increases, and the elemental contrast decreases (due to dilution of the heavy metal Au with the lighter element Ge) while the alloy composition crosses sequentially,25 from left to right, a biphasic region (solid Au and Au/Ge liquid alloy) and a singlephase region (liquid). This alloying process can be depicted as an isothermal line in the Au-Ge phase diagram (Figure 2b). (II): Nucleation (Figure 1d,e). Once the composition of the alloy crosses the second liquidus line, it enters another biphasic region (Au/Ge alloy and Ge crystal). This is where nanowire nucleation starts. Knowing the alloy volume change, we estimate that the nucleation generally occurs at Ge weight percentage of 50-60%. This value differs from the composition calculated from the equilibrium phase diagram which indicates the first precipitation of Ge crystal should occur at 40% Ge (weight) and 800 °C. This difference indicates that the nucleation indeed occurs in a supersaturated alloy liquid. Interestingly, we have also occasionally observed that two Ge nanocrystals precipitate from single alloy droplets and create two liquid/solid interfaces (Figure 1h,i).26 The finite volume of the alloy liquid, on the order of 10-17 cm-3, apparently limits the number of possible heterogeneous nucleation events,27 unlike those microscopic systems where tens or hundreds of whiskers can be observed on single alloy droplet.17,19 (III). Axial growth (Figure 1d-f). Once the Ge nanocrystal nucleates at the liquid/solid interface, further condensation/ dissolution of Ge vapor into the system will increase the amount of Ge crystal precipitation from the alloy. This can be readily accounted for, using the famous lever rule of phase diagram. The incoming Ge species prefer to diffuse to and condense at the existing solid/liquid interface, primarily due to the fact that less energy will be involved with the crystal step growth as compared with secondary nucleation events in a finite volume. Consequently, secondary nucleation events are efficiently suppressed, and no new solid/liquid interface will be created. The existing interface will then be pushed forward (or backward) to form a nanowire (Figures 1f, 2b). After the system cools, the alloy droplets solidify on the nanowire tips. Their compositions were analyzed with energy-dispersive X-ray spectroscopy (EDAX), and it was found that the weight percentage of Ge matches qualitatively well with the estimated alloy composition at which first Ge nanocrystal nucleates. In addition, by surveying over 50 Au nanocluster-catalyzed Ge nanowire growth processes (Figure 1g), we found that there is a linear correlation between the sizes of Au nanoclusters and the diameters of the Ge nanowires. Generally, the diameters of the nanowires are larger than the sizes of the initial clusters by several nanometers due to the Au/Ge alloying process. This size correlation clearly points out a simple approach to selectively grow nanowires of different diameters using monodispersed clusters of different sizes as catalysts.9,16 We have successfully utilized this strategy to grow uniform Si nanowires in a chemical vapor deposition (CVD) system.28 For example, uniform nanowires of 20.6 ( 3.2, 24.6 ( 4.0, 29.3 (4.5, and 60.7 ( 6.2 nm in diameter (Dwires) were grown using Au clusters with sizes of 15.3 ( 2.4, 20.1 ( 3.1, 25.6 ( 4.1, 52.4 (5.3 nm (Dclusters), respectively.29 Figure 3a shows a field emission scanning electron microscope (FESEM) image of uniform, long, and flexible nanowires grown using 15 nm Au clusters as catalysts. For direct comparison, we deposited the nanowires and nanoclusters on the same TEM grids. Figure 3b indicates an obvious correlation between the starting cluster size and produced nanowire size. These Si nanowires are highly crystalline and generally have [111] growth direction (Figure 3a inset). The direct observation of nanowire growth unambiguously confirms the validity of vapor-liquid-solid crystal growth mechanism at the nanometer scale and should allow us to rationally control the nanowire growth which is critical for their potential implementation into the nanoscale electronic and optoelectronic devices. Acknowledgment. This work was supported by a Camille and Henry Dreyfus New Faculty Award, Research Corporation, and the University of California, Berkeley. P.Y. is an Alfred P. Sloan Research Fellow. P.Y. thanks the 3M company for an untenured faculty award and thanks Dr. E. Stach and D. C. Nelson for help with the TEM studies. We thank the National Center for Electron Microscopy for the use of their facilities. JA0059084 (25) Using the densities for solid and liquid Au and Ge, the composition is estimated on the basis of the observed volume change during the alloying process, assuming the droplet is spherical. (26) For 3 out of 50 examined Au clusters, we observed double nucleation events. No triple-nucleation is observed for these clusters. (27) Turnbull, D. J. Appl. Phys. 1950, 21, 1022-1028. (28) The as-made Au clusters21 were dispersed in a precursor solution for synthesizing mesoporous silica. The precursor solution consists of (molar ratio) 0.01 poly(ethyleneoxide)-b-poly(propyleneoxide)-b-poly(ethyleneoxide) (EO20- PO70EO20):1 tetraethoxysilane:40 ethanol:0.02 HCl:8 H2O (Yang, P. D.; Deng, T.; Zhao, D. Y.; Feng, P. Y.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1999, 282, 2244-2246). A thin film was deposited on a Si wafer using spin coating. The sample was then calcined at 600 °C for 5 h, resulting in a thin mesoporous film embedded with uniform Au clusters. The substrate was then used to grow Si nanowires in a home-built CVD system. Briefly, SiCl4/H2 gas precursors with flow rate of 400 sccm were introduced into a tube reactor at reaction temperature of 965 °C. The growth generally takes 1.5 min. The nanowires were examined by FESEM (JEOL FSM6430) and TEM (Philips CM 200). (29) A linear correlation can be extrapolated as Dwire (nm) ) 5.0 + Dclusters- (nm). Figure 3. (a) FESEM image of Si nanowires grown on Au clusters embedded mesoporous silica thin film. Inset shows high-resolution TEM image of individual Si nanowire with the [111] lattice fringe. (b) TEM image of the nanowires and the Au clusters on the same copper grids. 3166 J. Am. Chem. Soc., Vol. 123, No. 13, 2001 Communications to the Editor