Brightness and coherence of electron beam emitted from a multi-wall carbon nanotube

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1. A capped multiwall carbon nanotube with a clean surface gives filed emission patterns consisting of six pentagonal rings corresponding to pentagons located at the tip. To evaluate the optical properties of a single pentagon as an electron source, the I-V characteristics and angular current densities for a single clean pentagon have been measured, by probe-hole-type field emission microscopy. Reduced brightness estimated from the angular current density and the geometrical size of the pentagon reached 〜5.6×109 Am-2sr-1 V-1 at an emission current of 53nA. This value is one order of magnitude or more higher than that of an individual multiwall carbon nanotube field emitter reported by Jonge et al.2. A capped multiwall carbon nanotube with clean surface gives field emission patterns consisting six pentagonal rings which correspond to pentagons located at the tip end. One or a few bright streaks are also observed at the boundaries of neighboring pentagons. The spacing of streaks is inversely proportional to the square root of the accelerating voltage. Namely, the spacing changes with the wave length of emitted electrons according to Young's interference equation. The visibility of streaks increased with the accelerating voltage, which can be explained successfully in terms of a concept of a virtual source size. These experimental results suggest that the streaks are no more than Young's interference fringes for which the adjacent pentagons behave as double slits.3. The direct observation method of electromagnetic micro-fields with an electron biprism is developed. This interference method is simpler and more effective than the three-wave interference one.