# A Review Paper on \"Graphene Field Emission for Electron Microscopy\"
## 3 Graphene-Based Cold Field Emitters
### Graphene Point-Cathode Field Emitters  
Until now, graphene point-cathode field emitters for electron microscopy applications have been rarely studied due to the difficulties inherent in controlling graphene morphology. [PERSON] et al. [70] reported an idea of overlaying an ultra-thin (\(\sim\)1 nm) graphene flake on to a blunted tungsten probe by van der Waals forces, as shown in Figure 4A, which would make it applicable to single-column electron beam microscopy applications. However, this loosely hanging ultra-thin freestanding graphene flake is not suitable for  
Figure 3: SEM images of microfabricated graphene film emitters. (**A**) Top view SEM micrograph of as grown few-layer graphene (FLG) by microwave plasma-enhanced chemical vapor deposition (PECVD). Reproduced with permission from [56], Copyright AIP, 2008; (**B**) Graphene films prepared by electrophoretic deposition. Reproduced with permission from [60], Copyright Wiley, 2009; (**C**) Screen-printed graphene films. Reproduced with permission from [61], Copyright IOP, 2009; (**D**) Hybrid structures of graphene stretched on patterned and size-controllable Si nanowires. Reproduced with permission from [25], Copyright Springer Nature, 2015. ITO: indium tin oxide; PMMA: poly(methyl methacrylate). practical applications, since the graphene can easily be damaged or detached from its supporting probe. More importantly, the emitting surface is not circular, it spans a few micrometers along one direction, and this means that it cannot subsequently be focused down to a single nanometer electron spot. For most single-tip point field emitters, the emission current is surface sensitive and the adsorption of any gas molecules will increase the surface work function [15; 72]. A high emission current under excessive applied voltage ionizes many gas molecules around the emission surface and eventually triggers a vacuum arc. Therefore, the practical use of cold field emitters in electron microscopes typically requires cathode flashing at regular intervals to remove the residual contaminant molecules. [PERSON] et al. [73] reported that CFE electron sources can be cleaned by in situ Joule heating during the emission process in HV conditions, which is well known as a \"conditioning\" process in many cold field emission experiments [74; 75; 71]. For example, during the first and second cycle of voltage ramping up from the graphene flake overlaid on the tungsten probe (Figure 4A), the current-applied voltage characteristic presents a typical conditioning process, which is observed to deviate from the conventional FN plot. The third cycle of electron emission shows a well fitted FN plot with an emission current up to 6 \(\mu\)A. Similar work has been done by other researchers studying the field emission characteristics from reduced graphene oxide (Figure 4B) and carbon nanowalls, and they have obtained high emission current, around several tens of microamperes [71; 76]. Recently, the research group at the National University of Singapore, led by [PERSON], have presented the development of a completely new type of cold field emission electron source (Figure 5A), a Graphene-Ni point cathode for electron microscopy systems [77]. They present a way of overcoming the morphological problems associated with micro-fabricating single-tip graphene emitters, by coating it on to a sharpen wire metal wire tip. In doing so, they have overcome many of the practical difficulties associated with conventional metallic tungsten CFE sources, such as misalignment, stringent vacuum requirements, and current instabilities. They reported stable field emission from a point cold field emission source that can operate in High Vacuum (HV), as opposed to stringent UHV conditions, and uses relatively large cathode-tip sizes (microns), an advantage created by their very low electric field strength requirements (0.5-1.5 V/nm) and low work function (\(\sim\)1.1 eV). Figure 4: Images of graphene edge field emitters and field emission characteristics.