# Field Emission from Carbon Nanostructures
## 2 Field Emission from Carbon Nanostructures
### Emission from Graphene and Graphitic Materials
[167] demonstrated an improved FE (higher current density and lower turn-on field) with respect to planar graphene when introducing morphological disorder (such as edges, discontinuity and ripples) in thermal-CVD graphene due to the poor adhesion on Si (100) substrates. A different approach, concerning growth of graphene on CNT or nanostructures, has been also intensively investigated. [PERSON] et al. [168, 169] reported excellent FE from few-layer graphene (FLG)-carbon nanotube hybrids synthesized using radio frequency hydrogen plasma sputtering deposition. They obtained few-layer graphene sheets sparsely distributed on top of CNTs with about 2-10 layer sharp edges (see Figure 6). Such a unique structure offers advantages from both the high aspect ratio of CNTs as well as from the sharp edges of flakes, resulting in a turn-on field below 1 V/\(\upmu\)m and a field enhancement factor of about 4000. Several studies have examined the transfer of graphene on different structures in order to realize high performance emitting cathodes. Deposition of graphene oxide (exfoliated from graphite and made into solution drops) on Ni nano-tips has been reported [170], demonstrating that the sheets supported on nanometer-scale sharp Ni nano-tip arrays behave as excellent field emitters, not depending on the alignment of the sheets with the electric field, with a turn-on field as low as 0.5 V/\(\upmu\)m. [PERSON] et al. [30]  
Figure 6: **(a,b)** SEM image of few-layer graphene–CNT hybrids (the inset is a TEM image showing a hybrid structure between a CNT and a FLG); **(c)** Schematic of a two-step few-layer graphene growth model: nucleation and 2D growth; **(d)** FE properties of the as-grown CNT arrays and the FLG–CNT hybrids (the corresponding F–N plots in the inset is). Image adapted with permission from [168], Copyright Elsevier, 2012.  
also demonstrated that the FE characteristics of InP nanocrystals were enhanced by placing a graphene monolayer on the nanocrystal array. It has been reported that graphene can be used to realize hybrid structured Si-nanowires/graphene for FE purposes [171]. Such configuration allows the creation of uniform protrusions in graphene, starting from well-aligned silicon nanowires of different diameters (from 400 to 600 nm). It has been demonstrated that large-scale CVD-graphene transferred onto the ordered array, significantly improve the FE performance with respect to nanowires alone, due to the formation of regular undulations of the graphene layer. The reported values of the turn-on field were in the range 2.0 to 2.6 V/\(\upmu\)m depending on the nanowire diameter, while the estimated enhancement factor was from 4000 to 6500. The hybrid structure realized with nanowires having a 600 nm diameter, exhibit the lowest turn-on field and highest \(\beta\) values, because it realizes more contact points and protrusions, favoring electron transfer. Indeed, the protrusions are a crucial element responsible for the field emission properties of the hybrid structure. Graphene samples produced by the arc-discharge method in a hydrogen atmosphere have been characterized as field emitters either as undoped and as nitrogen- and boron-doped samples [172]. Graphene sheets have been vertically oriented in order by means of electrophoretic deposition on Si substrate to enhance the FE properties. To achieve a current density of 10 \(\upmu\)A/cm\({}^{2}\) a turn-on field of 0.7, 0.8, and 0.6 V/\(\upmu\)m for undoped, boron-doped, and nitrogen-doped graphene, respectively, have been reported. Such values are remarkably lower than the values reported for graphene samples [158; 165; 173] and sub-nanometer graphite sheets [174]. The FE behavior is explained in terms of the nanometric features of graphene and the resonance tunneling phenomenon. Finally, the lowest turn-on field for N-doped graphene was identified as the result of the up-shift of the Fermi level of graphene due to N-doping [175].