# Field Emission from Carbon Nanostructures
## 2 Field Emission from Carbon Nanostructures
### Emission from Graphene and Graphitic Materials  
The most stable allotrope of carbon is three-dimensional graphite, known since 1789, although until 2004 it was impossible to isolate a single graphite layer. Then, [PERSON] and [PERSON] [149] succeeded in producing a monolayer graphene by mechanical exfoliation from highly ordered pyrolytic graphite. Graphene is the natural building block of any graphitic material. Since its discovery it has been exploited for back-gated field effect transistors [149; 150; 151; 152], hybrid photodiode [153; 154], and several other nano-electronics applications [155; 156]. Several studies on graphene films demonstrated that the FE from graphene edges or pleats occurs at low applied fields [157; 158; 159]. A graphene layer exfoliated or directly grown on a substrate is not the most suitable for FE, because the planar geometry suppresses field enhancement [160]. Despite this, [PERSON] et al. [38] demonstrated that emitted current up to 1 \(\upmu\)A can occur from the flat part of graphene flakes, likely favored by graphene wrinkles, under applied fields of \(\sim\)600 V/\(\upmu\)m and with significant stability over a period of several hours. Field emission from graphene is challenging because the deposition methods typically lead to sheets that lay flat on the substrate surface, which limits the field enhancement. Wet transferred suspended graphene fabricated on interdigitated electrodes [161] has been demonstrated as a high-quality emitter. It has been shown that the suspended graphene has better emission properties (from the whole surface) compared to the graphene commonly exfoliated on flat substrates. However, fields as high as 170 V/\(\upmu\)m are necessary to extract electrons from the graphene layer and to fully exploit the FE capabilities of graphene it is convenient to arrange graphene flakes perpendicular to the substrate to favor the emission from the sharp edges. It has been reported [162] that dispersion of graphene in polystyrene and successive deposition of composite allow graphene sheets to be somewhat aligned. In particular, it has been demonstrated that the orientation of graphene sheets can be modified from randomly oriented to laterally oriented by tuning the spin coating speed. This method realizes graphene/polymer composite thin films with oriented graphene sheets at certain angles on a Si substrate. Such orientation causes an important field enhancement factor up to 1.2 \(\times\) 10\({}^{3}\) allowing FE at turn-on field as low as 4 V/\(\upmu\)m. By applying the same method on Si microtips, it has been demonstrated that FE can be turned on at lower fields, \(\sim\)2.3 V/\(\upmu\)m [163]. A different technique, based on microwave plasma enhanced CVD, has been also applied to realize vertically aligned graphene sheets on Ti or Si substrates [164]. Remarkably, a lower turn-on field of about 1 V/\(\upmu\)m was obtained for few layers of graphene grown on titanium. The main drawback of this method remains the difficulty of controlling graphene sheet density with related problems arising due to the screening effect in high density systems. Exfoliated graphene has also been investigated for FE applications by using either screen printing techniques [158], or electrophoretic deposition [165] reporting turn-on field up to 5.2 V/\(\upmu\)m. A comparative study on the FE performance of graphene oxide (initially exfoliated by modified Hummers method), reduced either by hydrazine or by microwave assistance, has been reported [166] with the result of improved turn-on field in case of microwave-reduced graphene (\(E_{\text{on}}\) = 0.39 V/\(\upmu\)m vs. \(E_{\text{on}}\) = 0.94 V/\(\upmu\)m). This experimental observation has been explained by the presence of larger graphene sheets, and the microwave reduction not breaking the large sheets into small pieces during the process, contrary to what happens with hydrazine reduction. Consequently, larger sheets favor natural rolls and tips because of the higher surface tension, with clear benefit to FE performances. [PERSON] et al.