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The cold field emission gun (C-FEG) is the brightest electron source available, which also exhibits the smallest energy spread [1]. This technology has been greatly improved over the years concerning the electron optics and the vacuum, but the same cathode materials are still in use [2]. We have recently developed a new C-FEG source using a carbon cone nanotip (CCnT)[3] mounted on a standard tungsten cathode using a focused ion beam (FIB)[4]. This source exhibits very good spatial coherence properties, which could be useful for electron interferometry applications [5, 6]. Using the FIB-mounting procedure, we have inserted a CCnT inside a biasing transmission electron microscope (TEM) sample holder incorporating a nanomanipulator (Nanofactory Instruments), in order to approach the CCnT towards an Au-anode plate (Fig. 1). We then ramped up the voltage between the nanotip and the anode from 0 to 100 V until the electric field around the tip was strong enough to allow the electrons to tunnel through the barrier and a field emission current could be recorded.

When compared with the macroscopic electric field, the local field around the tip is enhanced, which is described by the field enhancement factor. This factor has been calculated from the current as a function of the applied voltage using the theory described by the Fowler-Nordheim (FN) equations [7]. The theory originally expressed the field emission current between two bulk metal plates but has also proven to be valid for carbon-based emitters [8]. In addition to measuring the field emission current, off-axis electron holography was performed to measure the electric field (Fig. 2 (b …