Ab initio Study of Metal Atoms on SWNT Surface
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Ab initio Study of Metal Atoms on SWNT Surface Shu Peng and Kyeongjae Cho Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA ABSTRACT Interactions of metal atoms (Al, Ti) with semiconducting single walled carbon nanotube (SWNT) are investigated using first-principles pseudopotential calculations. Six different adsorption configurations for aluminum and titanium atoms are studied. Comparison of the energetics of these metal atoms on (8,0) SWNT surface shows significant differences in binding energy and diffusion barrier. These differences give an insight to explain why most of metal atoms (such as Al) form discrete particles on nanotube while continuous nanowires are obtained by using titanium in the experiment.
INTRODUCTION Carbon nanotubes [1] have induced great research interests due to their unusual physical, chemical and mechanical properties [2-5]. These unique properties make them an ideal candidate for the building blocks of molecular scale machines and nanoelectronic devices [6-9]. One of the proposed applications is to use carbon nanotubes as templates to obtain a variety of nanowire materials [10]. Both experimental and theoretical studies of metal-nanotube systems are necessary to achieve metal nanowires using nanotubes [11, 12]. Recently, it has been reported that metal nanowires can be formed by coating various metals on suspended single walled carbon naotubes (SWNT) using electron beam evaporation [13,14]. Many types of metal atoms (such as Au, Al, Fe) form discrete isolated particles on nanotubes while Ti coating on the suspended tubes are continuous. Other metals can also form continuous nanowires by depositing metal atoms (Au, Al, Fe, etc) on the first coated buffer layer (or adhesion layer) of Ti nanotube nanowire. The experiments show that different metal atoms have different binding characters on the nanotube surface that correspond to different macroscopic coating phenomena. These experimental results suggest that structural and chemical characteristics of metal-tube systems are sensitive to the identity of metal atoms and motivate a detailed theoretical study to investigate the interactions between nanotubes and various metal atoms. Although the interactions between deposited metal atoms with graphite (or graphene sheet) and C60 have been extensively studied [15,16], a detailed theoretical account for the interactions between various metals and nanotubes is currently lacking. The interactions of metal atoms with SWNTs offer another challenging problem since the characteristics of these interactions would be quite different from graphene sheet and C60. This is because SWNTs are quasi-one dimensional, and this confined geometry makes the chemistry of SWNTs significantly different from those of two dimensional graphene sheet and finite molecule C60. Nanotubes are different from graphite or graphene sheet because nanotubes have curved geometry that induces rehybridization of carbon bonding orbitals (non-planar sp2 bonding configuration) [17,18]. These different bond
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