Electron Emission from Pentagons on a Carbon Nanotube Tip Revealed by Field Emission Microscopy
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Electron Emission from Pentagons on a Carbon Nanotube Tip Revealed by Field Emission Microscopy Koichi Hata, Akihiro Takakura and Yahachi Saito Department of Electrical and Electronic Engineering, Mie University, Kamihama-cho, Tsu-city 514-8507, Japan ABSTRACT Field emission of electrons from multiwall carbon nanotubes (MWCNTs) has been investigated by field emission microscopy (FEM) in an ultra-high vacuum chamber. An MWCNT whose tip is capped by curved graphite layers gives a FEM pattern consisting of 6 bright pentagons when the surface of the nanotube tip is clean. Even in the ultra-high vacuum with a base pressure of about 10-10 Torr, residual gas molecules, attracted by polarization forces, adsorb on the nanotube tips. The adsorbed molecules reside preferentially on the pentagonal sites, giving bright spots in the FEM pattern. A flash heating the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers the original clean surfaces. The adsorption and desorption of gas molecules are responsible for stepwise increases and decreases in the emission current, respectively.
INTRODUCTION Carbon nanotubes possess unique geometrical, electrical, mechanical and chemical properties that make them excellent field emitters. In fact, field emission display (FED) devices with nanotube cold cathodes have been experimentally manufactured, and their performance high enough for practical use has been demonstrated [1-3]. However, there still remains several difficulties (say, emission uniformity, reduction of driving voltage etc.) to be solved before nanotube-based FEDs appears in a commercial market. In a viewpoint of fundamental researches, on the other hand, field emission from carbon nanotubes contains interesting and important physical phenomena. Carbon nanotubes are composed of graphenes (hexagonal network sheets) rolled into seamless hollow cylinders with their ends being capped. At least six pentagons exist at one end of a nanotube as shown in Fig. 1. The portions where the pentagons are located extrude like vertices of a polyhedron, while regions other than pentagons are flat, as observed in Fig. 1. Recently, we observed pentagons existing on a multiwall carbon nanotube (MWCNT) tip by field emission microscopy (FEM) in ultra-high vacuum, and revealed that electron emission preferentially occurs through pentagons when the nanotube surfaces are clean [4]. Using single-wall carbon nanotubes (SWCNTs), Dean et al. reported the effect of adsorbed molecules on the electron emission [5]. Here, we report on FEM observation of adsorption and desorption process of residual gas molecules on clean MWCNT surfaces. It is found that gas molecules adsorb selectively on pentagon sites, and that the emission current abruptly increases and decreases upon the adsorption and desorption of a molecule, respectively.
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EXPERIMENTAL
The FEM study was carried out in an ultra-high vacuum chamber with a 6 10-10 Torr base pressure. Carbon nanotubes used in this study were MWCNTs produced by carbon arc disc
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