Effect of Diameter on Electron Field Emission of Carbon Nanotube Bundles
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Effect of Diameter on Electron Field Emission of Carbon Nanotube Bundles Devon McClain, Mason DeRoss, Noel Tavan, Jun Jiao Department of Physics, Portland State University, Portland, OR 97201 Coralee M. McCarter, Robert F. Richards, Sinisa Mesarovic, Cecilia D. Richards, David F. Bahr Mechanical and Materials Engineering, Washington State University; Pullman WA 99163, USA ABSTRACT Arrays of multi-walled carbon nanotunbe (CNT) bundles were fabricated on silicon [100] substrate with iron-nitrate sol-gel catalyst patterned via standard photolithographic techniques. Nanotube bundles with diameters ranging from 400µm to 15µm were grown in a chemical vapor deposition reactor and electrically characterized using a scanning-anode probe apparatus. Results showed a relatively low number of graphitic layers in individual nanotubes and a definite increase in field emission performance with decreasing bundle diameter. A 400µm wide matt of CNTs yielded a turn-on field of 6.7 V/µm and field enhancement of 602 while 15µm bundles performed significantly better with turn-on fields of 1.6 V/µm and field enhancement factors of 2425. The overall trend strongly suggests that the field emission character of CNT based aggregate structures such as those presented here is proportional to their aspect ratio. INTRODUCTION It has long been known that carbon nanotubes have exceptional electron field emission properties. However, it has also been observed that the emission capability of a single nanotube is inhibited by the presence of nearby tubes through the electrostatic screening effect. Much work has recently gone into optimizing the packing density of CNT growth to achieve stable, long-lived current densities [1-2]. Most of these investigations have focused on arrays of single tubes which are often difficult to fabricate and not particularly robust when high emission current is desired. Other groups have shown that arrays of CNT bundles may be an attractive alternative with greater resiliency but comparable field emission performance [3-4]. However, the emission character of individual bundles has not been adequately addressed. Toward that end, we present here a simple method to fabricate CNT bundles of different sizes and characterize the effect of diameter on their field emission performance. EXPERIMENTAL METHOD Catalyst Patterning In order to fabricate CNT bundle arrays, a chromium photo mask was designed to define growth sites. The array design consisted of 320 individual bundles with diameters of 120 µm, 60 µm, 30 µm, and 15µm. A 400 µm wide strip was included to simulate a bundle with infinite diameter. A 200 µm center-to-center distance was maintained throughout the pattern. The substrate was prepared by the following procedures. A silicon wafer (100) was etched for 30 seconds in Buffered Oxide Etch (BOE) to remove the native silicon oxide. An iron nitrate (1.5M, 15mL) sol gel catalyst was spun onto the wafer at 3000 rpm for 30 seconds and dried. Photoresist was then
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spun on at 3000 r
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