# A Review Paper on \"Graphene Field Emission for Electron Microscopy\"
## 2 Development of a Single-Tip Cathode Cold Field Emission Source
### Nanoscale Point-Cathode Emitters  
Nanostructures in the form of one-dimensional (1D) nanowires, nanotubes, and nanocones have attracted much attention in past decades and have been demonstrated for many potential applications. In particular, their use as electron field emitters exhibit excellent geometric, electrical and mechanical properties, as compared to conventional metallic cold field emitters [47; 48; 49; 50]. However, there are also significant technical challenges that have yet to be resolved.  
Generally, preparation of an individual nanoscopic CFE electron emitter is an extremely labor-intensive delicate process. To mount a single nanowire or nanotube onto a supporting tip, the most common method is to use microscopic manipulators under the observation of an electron microscope. Figure 2A,C show a single LaB\({}_{6}\) nanowire [16] and a single carbon nanotube (CNT) [10] mounted on a sharpened W tip, respectively. The ends of the emitter were deposited with platinum (or carbon) by electron beam induced deposition (EBID) in order to make good electrical contact and the final emitter structure is fitted into an electron gun unit as shown in Figure 2B, and its inset. Figure 2D,E show the mounting procedure for a carbon cone nanotip (CCnT) [14], where a CCnT was welded onto the apex of the supporting tip by using a gas injecting system. The whole process has a very high probability of incurring misalignment and damage.  
These nanoscale point-cathode emitters also have the limitation that they need to be operated in an UHV level, in the 10\({}^{-11}\) to 10\({}^{-10}\) Torr range. For a LaB\({}_{6}\) point-cathode emitter with a diameter of nearly 60 nm, a reduced brightness (\(B_{\rm r}\)) value of 2.77 \(\times\) 10\({}^{11}\) Am\({}^{-2}\)sr\({}^{-1}\)V\({}^{-1}\) and an energy spread value of 0.367 eV have been reported in a vacuum with a base pressure of 10\({}^{-10}\) Torr. The short-term stability profile of the LaB\({}_{6}\) point-cathode is 0.32% over 60 s, and the emission current deviates from the mean within 5% throughout a week-long test. For the CNT and CCnT point-cathode emitters with diameters of around 10 nm, the source electron optics parameters were reported to be 8.2 \(\times\) 10\({}^{9}\) Am\({}^{-2}\)sr\({}^{-1}\)V\({}^{-1}\), 0.48 eV, and 1.6 \(\times\) 10\({}^{9}\) Am\({}^{-2}\)sr\({}^{-1}\)V\({}^{-1}\), 0.32 eV respectively. The vacuum pressure requirements are even more stringent, typically around 10\({}^{-11}\) Torr. It was found that there exists some unrecoverable structure damage to the CNT during electron emission under the high vacuum (HV) conditions of 9 \(\times\) 10\({}^{-8}\) Torr, and this leads to emission reduction or failure [51]. In addition to its inherent UHV requirements, the CNT emitter is reported to have relatively large fluctuations in emission current, e.g., up to 24% for an emission current of about 2 uA over 6 hours with a sampling rate of 3 \(\times\) 10\({}^{-3}\) Hz.  
These nanoscale CFE electron sources have not as yet been successfully integrated in any commercial electron microscopes due to the aforementioned significant technical challenges, and studies of nanowire, nanotube, and nanocone based cold field emitters have remained at the research level. Meanwhile, other developments in CFE electron emitter research have come from the use of materials such as graphene.