The Effects of Crystallinity and Catalyst Dynamics on Boron Carbide Nanospring Formation
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The Effects of Crystallinity and Catalyst Dynamics on Boron Carbide Nanospring Formation D. N. McIlroy, D. Zhang, Y. Kranov, H. Han, A. Alkhateeb, and M. Grant Norton1 Department of Physics, Engineering and Physics Bldg., University of Idaho, Moscow, ID, 83844-0903, U.S.A. 1 School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920, U.S.A. ABSTRACT The formation of helical nanowires—nanosprings—of boron carbide have been observed and a growth mechanism, based on the work of adhesion of the metal catalyst and the tip of the nanowire, developed. The model demonstrates that the asymmetry necessary for helical growth is introduced when the following conditions are met: (1) The radius of the droplet is larger than the radius of the nanowire, and (2) The center of mass of the metal droplet is displaced laterally from the central axis of the nanowire. Furthermore, this model indicates that only amorphous nanowires will exhibit this unique form of growth and that in monocrystalline nanowires it is the crystal structure that inhibits helical growth. High-resolution transmission electron microscopy and electron diffraction has been used to compare the structure of both amorphous and crystalline nanowires. INTRODUCTION A revolution is on the horizon for optics and electronic devices as we currently know them. It is being spurred on by breakthroughs in the development of nanoscaled materials. Over the past decade it has been demonstrated that nanoscale materials exhibit novel optical [1-4] and electronic [5-7] properties attributable to quantum confinement. This, in turn, has accelerated efforts to develop new nanoscale materials, as well as new techniques for their synthesis. A major area of research on nanoscale materials has focused on the development of solid nanowires because of their potential use in nanoelectronics, nanomechanics, and flat panel displays [8-11]. Critical to the implementation of nanotubes and nanowires into high tech commercial applications will be the ability to develop techniques for their self-assembly. The ability to control the selfassembly of nanotubes and nanowires, however, may be on the horizon. The synthesis of nanotubes [7,12,13] and nanowires [14-17] on surfaces can be promoted through the introduction of a metallic catalyst. This growth mechanism is known as the vapor-liquid-solid (VLS) growth mode, first described by Wagner and Ellis [18]. Briefly, in VLS growth a liquid droplet of a metal or a eutectic alloy resides on a substrate. The droplet absorbs the necessary components for nanowire growth from the surrounding vapor. Once the concentration of the element, or elements, in the droplet reaches a level of supersaturation, the excess material is secreted at the liquid/solid interface. As more material is secreted to this interface the droplet is lifted off the surface concomitantly with wire formation beneath the droplet. Wire growth is sustained by
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maintaining a steady rate of delivery of source material to the droplet. It is cl
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