Cold cathodes for applications in poor vacuum and low pressure air environments: Carbon nanotubes versus ZnO nanoneedles A.-J. Cheng a , D. Wang b , H.W. Seo b , C. Liu a , M. Park b , Y. Tzeng a, * a Alabama Microelectronics Science and Technology Center, Department of Electrical and Computer Engineering, Auburn University, Auburn, Alabama 36849, USA b Department of Physics, Auburn University, Alabama 36849, USA Available online 2 March 2006 Abstract Effects of gas pressure on the electron field emission (FE) properties of zinc oxide (ZnO) nanoneedles and carbon nanotubes (CNTs) were investigated. The FE properties for ZnO nanoneedles almost fully recovered after being subjected to FE tests in poor vacuum and low pressure gas environments and then characterized again in better vacuum around 2 Â 10 À 6 Torr. In the contrast, the FE properties for CNTs did not recover after being subjected to FE tests in poor vacuum and low pressure gas environments. Reversibility and sensitivity of the FE of ZnO and CNTs to air pressures were studied for potential applications to field emission display (FED) and vacuum microelectronic devices. The pressure-dependant, time-dependant, and pressure –time-dependant field emission behaviors of ZnO nanoneedles and CNTs will be compared and discussed. D 2006 Published by Elsevier B.V. Keywords: Electron field emission; Zinc oxide; Carbon nanotubes; Pressure effect 1. Introduction One dimensional (1-D) nanostructured materials, such as ZnO, GaN [1], AlN [2], and CNTs [3], have high aspect ratios and have been considered to be among the best cold cathode materials for applications to field emission (FE) electron sources. 1-D nanostructured field emitters have advantages over thermionic electron emitters in terms of the power efficiency and their physical sizes. Nanostructured materials have thus stimulated a great deal of research and development with regard to their mechanical, thermal, and electrical properties. For FED applications, a number of carbon-based materials, for instance, diamond-like carbon (DLC) [4], diamond, and carbon nanotubes, have been studied for many years due to their high aspect ratio, mechanical stability, low work function and high electrical conductivity. Carbon nanotubes are considered to be the most promising candidate for field emission display (FED) applications because they are small in diameters and large in length leading to a very large field enhancement at the tips of the nanotubes. The field enhancement allows high electron field emission current density at low electrical fields. A relatively simple fabrication process using screen printing of CNTs paste or chemical vapor deposition of CNTs on glass substrates have been reported to have superior properties as compared to molybdenum (Mo) micro-tip arrays [5,6]. Field emission for single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes exhibit a permanent decrease in FE current density and an increase in the turn-on electrical field in oxygen environments [7]. This degradation phenomenon greatly impacts CNTs application in FED under poor vacuum or low-pressure gas filled environments. ZnO, is an n-type semiconductor with a wide band gap energy of 3.37 eV and a large exciton binding energy of 60 meV. It has also been reported to be a valuable candidate for field emission applications in high vacuum [8]. ZnO nanos- tructured materials have been synthesized by several methods, such as metalorganic vapor-phase epitaxy [10], infrared irradiation [11], thermal evaporation [12–14], and thermal decomposition [15]. Thermal evaporation is among the most popular methods for synthesizing ZnO nanostructures. The more important processing parameters include environments, substrate temperature, gas pressure, type of carrier gas, substrate, the thermal evaporation time, and the selection of 0925-9635/$ - see front matter D 2006 Published by Elsevier B.V. doi:10.1016/j.diamond.2005.08.024 * Corresponding author. Tel.: +1 334 844 1869; fax: +1 334 844 1809. E-mail address: tzengyo@auburn.edu (Y. Tzeng). Diamond & Related Materials 15 (2006) 426 – 432 www.elsevier.com/locate/diamond