# A Review Paper on \"Graphene Field Emission for Electron Microscopy\"
## 1 Introduction  
The Dyke group proposed a variety of applications of a cold field emission (CFE) source and formed the first company to produce CFE based commercial products in the early 1960s [1]. The successful application of CFE cathodes to electron microscopy, including both scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), was achieved by a group led by [PERSON] et al. in the late 1960s [2; 3]. For over 40 years, state-of-the-art metal tip CFE sources used for commercial high-resolution STEM consist of a single crystal tungsten sharpened wire, etched in a specific plane (such as the \(<\)310\(>\) plane) to generate the lowest work function, and its tip radius is in the 100-200 nm range. However, they suffer from many well-known practical challenges, such as an inherent ultrahigh vacuum (UHV) condition requirement (\(<\)10\({}^{-9}\) Torr) and relatively large current instabilities (\(\sim\)40% fall-off within the first hour), which have impeded their wide-ranging use [4]. As a result, most of the latest generation electron microscopes use the more stable and reliable Schottky electron source, which has a lower brightness (\(\sim\)10\({}^{8}\) Am\({}^{-2}\)sr\({}^{-1}\)V\({}^{-1}\)) and higher energy spread (\(\sim\)0.5 eV) [5; 6; 7; 8]. Although CFE sources have the desirable characteristics of a low energy spread (\(\sim\)0.3 eV) and high brightness (\(\sim\)10\({}^{9}\) Am\({}^{-2}\)sr\({}^{-1}\)V\({}^{-1}\)), their lifetimes need to be improved (\(>\)1 year) and their current stabilities need to be reduced (\(<\)1%) [4; 5; 9]. Nanostructures in the form of nanotubes [10; 11; 12; 13], nanotips [14], and nanowires [15; 16] have the potential to be used as cathodes in CFE sources. Their large field enhancement factors stem from nanometer-scale emission sites which can, in some cases, produce electron beams with brightness values, one order of magnitude higher than the state-of-the-art single crystal tungsten cathode tips. Despite their promising electron optics performance, they have not been successfully integrated into any commercial electron microscopes, and this is largely due to the same technological difficulties faced by single crystal tungsten cathode-tips (stringent vacuum, low lifetime and high beam current fluctuations). However, in addition, they require the delicate transfer/mounting process of an individual emitter being attached tothe tip of a sharpened supporting metal wire, and this is very difficult to do without incurring significant misalignment or damage. Since the discovery of two-dimensional (2D) graphene in 2004, graphene has attracted much attention as a potential candidate for CFE source emitters, due to its high aspect ratio (the lateral size to the thickness) [17; 18; 19] and excellent thermal, mechanical, and electrical properties [20; 21; 22; 23]. Excellent field emission from graphene has been demonstrated from a wide variety of different microfabricated structures, and most of them consist of densely packed nano-emitters that produce a large array of defocused overlapping electron beams, and therefore cannot be subsequently defocused down to a single nanometer electron probe [24; 25; 26], making them unsuitable for electron microscopy/lithography applications. This paper primarily focuses on reviewing developments in the field emission of graphene-based emitters for electron microscopy/lithography applications. In the first part of the paper, it will summarize basic field emission theory in relation to single crystal W field emitters (CFE) oriented in the \(<\)310\(>\) or \(<\)111\(>\) directions and other well-developed one-dimensional (1D) nanostructure CFE electron emitters. In the second part, different types of graphene field emitters will be introduced, including graphene film emitters, graphene point emitters and graphene ring emitters.