Whisker Technology

Abstract

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 vapor 6-12 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. 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. 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. 3165