Single-Walled Carbon Nanotubes of Controlled Diameter and Bundle Size and Their Field Emission Properties Liang Zhang, Leandro Balzano, and Daniel E. Resasco* School of Chemical, Biochemical, and Materials Engineering, UniVersity of Oklahoma, Norman, Oklahoma 73019 ReceiVed: March 2, 2005; In Final Form: June 8, 2005 Field emission studies were conducted on as-produced CoMoCAT single-walled carbon nanotube/silica composites with controlled nanotube diameter and bundle size. It has been observed that the as-produced nanotube material does not need to be separated from the high-surface area catalyst to be an effective electron emitter. By adjusting the catalytic synthesis conditions, single-walled carbon nanotubes (SWNT) of different diameters and bundle sizes were synthesized. A detailed characterization involving Raman spectroscopy, optical absorption (vis-NIR), SEM, and TEM was conducted to identify the nanotube species present in the different samples. The synthesis reaction temperature was found to affect the nanotube diameter and bundle size in opposite ways; that is, as the synthesis temperature increased the nanotube average diameter became larger, but the bundle size became smaller. A gradual and consistent reduction in the emission onset field was observed as the synthesis temperature increased. It is suggested that the bundle size, more than the nanotube diameter or chirality, determines the field emission characteristics of these composites. This is a clear demonstration that field emission characteristics of SWNT can be controlled by the nanotube synthesis conditions. Introduction During the past few years, single-walled carbon nanotubes have generated much interest because of their unique structure and properties. It is widely accepted that one of their potential applications might be in field emission devices. 1-4 Among the numerous studies dedicated to nanotube field emitters, great attention has been paid to optimizing the techniques for practical and reliable device fabrication. 5-14 In more fundamental studies, the mechanisms of field-induced electron emission, as well as the relationship between the nanotube structural parameters and emission properties, have been the focus of many publi- cations. 1,12,15-18 What makes SWNTs good candidates for field emitters is the combination of their natural geometry, chemical stability, and electrical characteristics. They could be used as the electron source in a whole range of devices, including flat panel displays, light elements, e-beam sources for lithography, and so forth. Among these applications, field emission displays (FEDs) have attracted significant attention as they could become one of the first commercial products using nanotubes. In 1999, Choi et al. 19 built the first SWNT-based FED at Samsung. Carbon nanotube-based FEDs are characterized by superior display performances such as fast response time, wide viewing angles, wide operation temperatures, cathode ray tube (CRT) like colors, ultraslim features, low-cost, and low-power con- sumption. FED technology is one of the most promising approaches for direct view displays larger than 60-in. diagonal. 20 Research efforts are currently devoted to the in-situ growth of vertically aligned nanotubes over a large area of glass substrates at low temperatures. However, while MWNTs can be produced at relatively low temperatures, high synthesis temperatures are required to produce single-walled carbon nanotubes. In the latter case, the use of nanotubes produced separately and later deposited on the cathode by techniques such as the screen- printing method might be required. In the present contribution, we report a comparative study of the field emission characteristics of CoMoCAT SWNTs still embedded in the silica-supported catalyst used in their synthesis. Researchers at Applied Nanotech Inc have recently shown that deposition of a mixture of nanotubes and dielectric nanoparticles leads to much improved emission characteristics. 21 This devel- opment makes a perfect combination with the high-quality single-walled carbon nanotubes (SWNTs) produced by the CoMoCAT catalytic method that we have developed. 22-29 The CoMoCAT product is particularly suitable for this application because it may be used in its as-prepared form, without elimination of the catalyst, given that its main component is silica (SiO 2 ), in the form of dielectric submicron particles. In the as-produced CoMoCAT product, SWNTs with controlled diameter distribution remain well dispersed and imbedded in the silica support in the form of bundles of different sizes, which can in turn be controlled by adjusting the synthesis parameters. It is important to compare the field emission (FE) parameters obtained from nanotubes with controlled structure and morphol- ogy and to compare these experimental values with available theoretical predictions 18,30-33 since such a direct comparison has not been previously done. Experimental Section Preparation of Materials. The SWNT used in this study were synthesized by the CoMoCAT method, described in previous publications. 22,23,28,29,34 Briefly, the silica-supported Co-Mo catalyst used in this method was prepared using cobalt nitrate and ammonium heptamolybdate as precursors in an aqueous impregnation method. The total metal loading was 2 * Author to whom correspondence should be addressed. Phone: 405- 325-4370; fax: 405-325-5813; e-mail: resasco@ou.edu. 14375 J. Phys. Chem. B 2005, 109, 14375-14381 10.1021/jp0510488 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/13/2005