Fabrication of Freestanding Carbon Nanotube Arrays in Large Scale
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Fabrication of Freestanding Carbon Nanotube Arrays in Large Scale Z.P. Huang, J. Moser, M. Sennett*, H. Gibson*, M.J. Naughton, J.G. Wen, and Z.F. Ren, Dept. of Physics, Boston College, Chestnut Hill, MA 02467, USA Material Science Team, US Army Soldier Biological & Chemical Command, Research, Natick Soldier Center, One Kansas Street, Natick, Massachusetts, USA
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ABSTRACT We have successfully fabricated many freestanding carbon nanotube arrays on silicon substrates. Two sizes of nickel dot arrays have been made by E-beam lithography. It has been found that the size of the carbon nanotubes is closely related to the size of the dot. Compared with our previous report on diameters of about 300 –400 nm, much thinner carbon nanotubes of 100 –150 nm have been made. With even smaller dots, it is expected that even thinner nanotubes of a few tens of nanometers could be made. The nanotube height is controlled by the growth time and nanotube uniformity has been greatly improved by introduction of a two-phase process of nucleation and growth. INTRODUCTION Field emission is considered a most promising application for carbon nanotubes simply because of their unique mechanical and electrical properties. In addition, they have a sharp tip and very high aspect ratio. Recently, extensive investigations have been done [19] . For field emission applications, flat panel displays (FED) require large-scale nanotube arrays with required separation to prevent the electric field shielding effect. Uniform and well-distributed nanotubes are absolutely required for high FED performances. For these reasons, it is important to explore large-scale production of nanotube arrays with uniform size and distribution. In previous research [10], we investigated growth of single freestanding multi-wall carbon nanotubes on sub-micron Ni dots. The diameters of freestanding nanotubes ranged from 200 to 300 nm for different samples. However, the height of the nanotubes in those arrays varied from 1 to 5 µm, which will result in unsatisfactory FED performance in practical applications. In this report, we present the improvement of nanotube height uniformity and reduction of nanotube diameters by nickel dot’s size. EXPERIMENT As in our previous research, nickel dot patterns with 10-15 nm thickness were fabricated on a boron doped (100) silicon substrate by metal evaporation and electron beam lithography. 15 × 14 and 50 × 49 nickel dots were evenly scattered within the arrays of 28 × 25 and 100 × 93 µm2 respectively. The dot size is estimated between 50 and 100 nm in accordance with the diameter of electron beam. The patterned substrate was loaded into a plasma enhanced hot filament CVD system described previously[11] to grow freestanding nanotubes. The growth process is performed in an atmosphere of acetylene and ammonia A13.22.1
(40:160 SCCM) under 10 Torr pressure. The substrate temperature was maintained below 660 °C. The height variation of nanotubes can result from the non-uniformity of catalyst dots. Our recent experiments indicate that nickel thickne
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