# A Review Paper on \"Graphene Field Emission for Electron Microscopy\"
## 3 Graphene-Based Cold Field Emitters
### Graphene Film Emitters  
To achieve successful electron emission from graphene, it is crucial to orientate thin graphene edges or control the dimension of graphene protrusions on some supporting substrate, in order to sufficiently enhance the applied electric field to produce quantum tunneling. Until now, many emerging technologies have become available for fabricating vertically aligned graphene nanosheets (VG) or graphene protrusions. The plasma-enhanced chemical vapor deposition (PECVD) is the most widely used way to synthesize large-scale VG graphene films [56], which offers the advantages of a lower substrate temperature and higher growth selectivity (Figure 3A). [PERSON] et al. presented a pulsed laser deposition (PLD) method [57] to directly fabricate graphene thin film onto metal nano-sized Spindt tips by using the transformation of solid carbon deposited from a pulsed laser system at low temperature. The main drawback of these two deposition methods is the difficulty of controlling the graphene film ordering and density without incurring problems such as the screening effect [58; 59].  
Figure 2: Well-developed nanowire, nanotube, and nanocone point-cathode cold field emitters. (**A**) Illustration showing the electron beam deposition process to fix a LaB\({}_{6}\) nanowire onto a W needle; (**B**) SEM image showing the finished LaB\({}_{6}\) NW emitter and the complete emitter assembly used for field-emission SEM, scale bar, 10 \(\upmu\)m. Reproduced with permission from [16], Copyright Springer Nature, 2015; (**C**) Transmission electron microscopy (TEM) image of a very thin and short nanotube sample. Reproduced with permission from [10], Copyright Elsevier, 2003; (**D**) View of part of the starting carbon cone nanotip (CCnT) object; (**E**) Resulting new tip after welding the carbon microfiber to the tungsten tip. The inset displays the whole set-up inside the dual beam. Reproduced with permission from [52], Copyright Elsevier, 2012.  
[PERSON] et al. applied the electrophoretic deposition (EPD) technique [60] to fabricate a large area of homogeneous single-layer graphene films, and this processing technique is more economic and versatile compared to PECVD (Figure 3B). Screen printing technology is a sophisticated technology widely used in large-scale applications coating on various substrates. A screen-printed graphene cathode was fabricated by [PERSON] et al. [61] using high-yield graphene prepared by a modified Hummers method and a hydrazine hydrate reduction process (Figure 3C). Another efficient way to produce uniform nanometer-scale graphene protrusions is through transferring graphene onto some sharp supporting substrates (Figure 3D) [24; 25; 62]. However, the exfoliated graphene by using either screen printing technologies, or electrophoretic deposition, or simple transfer have been found to lead to many defects to the as-grown graphene, that degrades its physical properties [63; 64; 65]. For graphene film emitters, high current densities, around 23 mAcm\({}^{-2}\), have been reported with good stability over a period of several hours [60]. Current density here, is calculated by dividing the total measured emission current by the average area on the substrate occupied by emitter sites. Although not suitable for electron microscopy applications, graphene film emitters are very useful for many non-focused electron beam applications, such as flat panel display [28; 66], X-ray sources [67; 68; 69], etc.