Morphology dependent field emission of acid-spun carbon nanotube fibers S B Fairchild 1 , J Boeckl 1 , T C Back 2 , J B Ferguson 1 , H Koerner 1 , P T Murray 2 , B Maruyama 1 , M A Lange 3 , M M Cahay 5 , N Behabtu 6 , C C Young 6 , M Pasquali 6 , N P Lockwood 4 , K L Averett 1 , G Gruen 3 and D E Tsentalovich 6 1 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, USA 2 Research Institute, University of Dayton, Dayton, OH 45469, USA 3 TechFlow Scientific, Albuquerque, NM 87110, USA 4 Directed Energy Directorate, Air Force Research Laboratory, Kirtland Air Force Base, NM 87117, USA 5 Spintronics and Vacuum Nanoelectronics Laboratory, University of Cincinnati, Cincinnati, OH 45221, USA 6 Department of Chemical and Biomolecular Engineering and Department of Chemistry, The Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, TX 77005, USA E-mail: steven.fairchild@us.af.mil Received 8 September 2014, revised 19 January 2015 Accepted for publication 20 January 2015 Published 19 February 2015 Abstract Acid spun carbon nanotube (CNT) fibers were investigated for their field emission properties and performance was determined to be dependent on fiber morphology. The fibers were fabricated by wet-spinning of pre-made CNTs. Fiber morphology was controlled by a fabrication method and processing conditions, as well as purity, size, and type of the CNT starting material. The internal fiber structure consisted of CNT fibrils held together by van der Waals forces. Alignment and packing density of the CNTs affects the fiber’s electrical and thermal conductivity. Fibers with similar diameters and differing morphology were compared, and those composed of the most densely packed and well aligned CNTs were the best field emitters as exhibited by a lower turn- on voltage and a larger field enhancement factor. Fibers with higher electrical and thermal conductivity demonstrated higher maximum current before failure and longer lifetimes. A stable emission current at 3 mA was obtained for 10 h at a field strength of <1 V μm -1 . This stable high current operation makes these CNT fibers excellent candidates for use as low voltage electron sources for vacuum electronic devices. Keywords: carbon nanotube, fiber, field emission (Some figures may appear in colour only in the online journal) Introduction Nanostructured carbon materials have demonstrated con- siderable promise for use as electron sources in vacuum electronic devices, and their field emission (FE) properties have been investigated for well over a decade. Various forms have been extensively studied, including carbon nanotubes (CNTs) [1–10], carbon nanosheets and nanopearls [11–15], and nanodiamond [16–18]. The applications are many, including x-ray sources [19–21], TWT amplifiers [22–24], electron guns [25] and cathodes for ion propulsion [26]. Towards the goal of achieving high currents, FE cathodes have been fabricated from vertically aligned CNT arrays [27– 29], as well as patterned arrays to reduce the effect of electric field screening [30, 31]. This approach has successfully pro- duced currents as high as 10 mA [32]. However a complex combination of e-beam lithography and patterned catalytic CNT growth is required to make such a device. A simpler approach to making devices that achieve milliamp level FE currents is by creating FE cathodes from Nanotechnology Nanotechnology 26 (2015) 105706 (9pp) doi:10.1088/0957-4484/26/10/105706 0957-4484/15/105706+09$33.00 © 2015 IOP Publishing Ltd Printed in the UK 1