# A Review Paper on \"Graphene Field Emission for Electron Microscopy\"
## 2 Development of a Single-Tip Cathode Cold Field Emission Source
### The State-Of-The-Art Electron Source Used in Electron Microscope  
The state-of-the-art electron sources used for commercial high-resolution electron microscopes (such as Nion UltraSTEM 100 manufactured in the Nion company, Kirkland, WA, USA [43] and JEOL ARM200F Aberration-Corrected S/TEM manufactured in the JEOL USA, Inc., Peabody, MA, USA [44]) are made from a single crystal W field emitter (CFE) oriented in the <310> direction with a work function of 4.5 eV [4]. Electron emission from the cathode occurs when the field strength at the cathode tip typically exceeds 4 V/nm, allowing electrons to escape from its surface by quantum tunnelling [4]. As electrons are accelerated through the gun unit, the wide-angle ones are filtered out by the first and second anode plate holes, and they exit the gun unit in the form of a rotationally symmetric electron beam which appears emanate from a single point on the axis (virtual source). The relatively small virtual source size of cold field emission guns (a few nanometers in diameter) is one of the main reasons for them having the highest brightness of all electron sources used in focused electron beam instruments. Another advantage is that they have a relatively low energy spread (0.2-0.3 eV), and it is these types of properties that make cold field emission sources the best type of electron source for forming nano-sized electron probes with high current intensity. Figure 1A,B show typical SEM images of the W<310> and W<111> CFE sources, respectively. As observed in the SEM images, the emitters have a spherical apex with a radius of around 130 nm at the end of the cathode-tip. For a single crystal W field emitter, the emitter surface is required to be etched in a <310> or <111> plane at the emitter apex, due to their lower work function values, and this can be achieved by using a zone melting technique [45]. The source reduced brightness (\(B_{\text{r}}\)) versus the full width at half maximum (FWHM(E)) values for the W<310> and W<111> sources are plotted in Figure 1C. For the state-of-the-art W<310> field emitter, a reduced brightness value of 4 \(\times\) 10\({}^{9}\) Am\({}^{-2}\)sr\({}^{-1}\)V\({}^{-1}\) has been reported, and the measured energy spread ranges from 0.33 eV to 0.48 eV [4]. The relatively large values of \(B_{\text{r}}\) mainly come from the small values of virtual source size \(d_{\text{v}}\) (-3 nm) and the large values of angular current density (-100 \(\mu\)A/sr). Figure 1: Scanning electron microscopy (SEM) photos of the W<310> (**A**) and W<111> (**B**) cold field emission (CFE) sources; (**C**) The source reduced brightness \(B_{\text{r}}\) versus the corresponding Full width at half maximum (FWHM(E)) values for the two CFE sources. Reproduced with permission from [4], Copyright Elsevier, 2009. Although the state-of-the-art single crystal W field emitters have desirable electron optical performances, they suffer from some well-known practical engineering problems. One difficulty comes from the presence of the oxide layer on the metallic tungsten surface and subsequently affects the stability of the electron emission, requiring the use of UHV levels in the gun unit (<10\({}^{-10}\) Torr). It has been shown the oxide layer has a significant impact on the tunneling current stability since the thin oxide layer acting as a n-type semiconductor with \(\phi\) equal to 5.59-5.7 eV involves noise mechanisms typical for semiconductor devices [46]. Another difficulty comes from residual gas molecules around the emitting surface which ionize positively under the impact of high emission current. The ionized ions are then attracted back striking the emitting surface under the strong local electric field, causing large current fluctuations and reducing the lifetime. Thus, regular thermal heating of the tip is needed to reset the current fluctuations [15]. Moreover, the single crystal W field emitters are made very sharp in order for the local electric field strength to be strong enough for quantum tunneling.