Carbon Nanotube Modification Using BaF 2 Vapor in Ultra-High Vacuum Environment
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Carbon Nanotube Modification Using BaF2 Vapor in Ultra-High Vacuum Environment Francisco Santiago, Victor H. Gehman Jr., Karen Long, and Kevin A. Boulais Electromagnetic and Solid State Technologies Division Dahlgren Division, Naval Surface Warfare Center, Dahlgren, VA 22448, U.S.A ABSTRACT Carbon nanotubes have attracted significant attention in the scientific community due to their unique properties and potential applications. One of the most promising applications is a carbon-nanotube transistor. The motivation of this work is to find ways to connect carbon nanotubes directly to silicon using Ba as a chemical link. We studied chemical interactions between carbon nanotubes and BaF2 vapors using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Surfaces of silicon wafers were chemically modified to allow the epitaxial growth of BaF2 using molecular beam epitaxy (MBE). Samples containing 2D single crystal islands of BaF2 were covered with carbon nanotubes with an average coverage of 10 nanotubes per um2. The samples were transferred to an outgasing station inside the MBE system and heated to 900oC for two hours in a pressure of 10-9 mbar. XPS C1s data before and after heat show a major change in the nature of the carbon nanotube electronic states. In addition XPS shows formation of a Ba-C “carbide like” bond and no presence of fluorine. AFM images of the same region taken before and after heat exposure show remarkable changes in the surface morphology of the carbon-nanotube wall. INTRODUCTION Carbon nanotubes have attracted significant attention in the scientific community due to their unique properties and potential applications. Carbon nanotubes can be metallic or semiconducting depending on the chiral properties and diameter. One of the most promising applications is a carbon-nanotube transistor [1]. In addition there is interest in developing ways to grow oriented carbon nanotubes [2]. The motivation of this work is to find ways to connect carbon nanotubes with known semiconductor surfaces using chemical bonds. This approach may open the possibility of coupling electronic states of carbon nanotubes with the electronic states of the semiconductor and be able to explore new electronic and optical phenomena. In the past, we have been successful in growing epitaxial films of barium fluoride (BaF2) on both silicon and GaAs. [3-6] During the process there is an intermediate layer of elemental barium bonded to the surface. This layer helps reduce the lattice mismatch between BaF2 and the semiconductor. We wanted to explore the possibility of creating a bond between carbon, barium and silicon via thermal activation by exposing carbon nanotubes to hot BaF2 vapor under ultrahigh vacuum from epitaxial film islands deposited on silicon. The barium 6s electronic level has a binding energy of 5.21eV compared with the carbon 2p of 11.264 eV. In addition Hamwi et. al. [7] reported that hot carbon nanotubes are chemically attacked by fluorine gas. BaF2 is chemically very active at temperatures abo
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