Surface Oriented Self-Assembly of Carbon Nanotubes
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Surface Oriented Self-Assembly of Carbon Nanotubes Kousik Sivakumar and Balaji Panchapakesan* Delaware MEMS and Nanotechnology Laboratory Department of Electrical Engineering, University of Delaware, Newark, Delaware 19716, U.S.A. ABSTRACT In this paper, we demonstrate the self assembled growth of nanotubes along the surface of (100), (110) and (111) silicon wafers using thermal CVD. Iron nanoparticles, 10 nm in diameter, were used as the catalyst. Carbon nanotubes were grown in a methane atmosphere at 1000oC. SEM and AFM characterization revealed single wall carbon nanotubes, about 10 nm in diameter and up to 10 µm in length, growing along the direction of the silicon wafer. The mechanism of growth of nanotubes is similar to that of molecular epitaxy which occurs due to the lattice matching of the silicon and iron crystal lattices forming self aligned silicides at high temperature which help orient the nanotubes. This process may enable the integration of nanotubes with CMOS processing technology. INTRODUCTION Nanotechnology encompasses the creation and utilization of materials, devices and systems at the nano-meter length scales, i.e. at the level of atoms, molecules and supra-molecular structures. One of the key aspects in nanotechnology research is the construction of atomically controlled periodic systems with reduced dimensionalities. One dimensional nanomaterials are expected to play a key role in future nanotechnology research and also can be model systems for demonstration of quantum size effects. One such material that has achieved considerable attention since it was discovered by Ijima [1] in 1991 is single wall carbon nanotubes. Single wall carbon nanotubes have grown to be interesting materials in recent years due to their unique electronic [7-12] and superior mechanical properties [13-16]. The progress achieved in the growth of carbon nanotubes over the years has been staggering. From an accidental discovery to controlled, vertically aligned growth [2-6] and electrical field assisted growth [17], the science involved in the growth of these materials has reached new heights. Nanotubes have been grown using various methods such as chemical vapor deposition [18-21], arc-discharge [22-24], pyrolysis [25]. These have largely helped in studying their unique structure and properties. They can be semiconducting or metallic depending on their tube diameter and chirality [12] thus offering possibilities to create semiconductor-semiconductor and metallic-semiconductor junctions, useful in electronic and sensor devices. One of the main barriers for CMOS integration of carbon nanotube is the random growth of carbon nanotubes on the surface of various substrates. Nanotubes have usually been grown in bundles and with no control of their chirality thereby growing both semiconducting and metallic nanotubes at the same time. These then have to be separated into individual nanotubes and then segregated into metallic and semiconducting ones using processes such as alternating current dielectrophoresis [27, 28]. E
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