# Field Emission from Carbon Nanostructures
## 2 Field Emission from Carbon Nanostructures
### Field Emission from Diamond and Related Structures  
Despite their insulating properties and the expensive cost of both natural and high-pressure synthesis, diamond structures have been identified, since the 1990s, as ideal coating materials for field emission devices due to their negative surface electron affinity, high thermal conductivity and chemical inertness [121; 122; 123]. Currently both carbon diamond films (CDF), grown by chemical vapor deposition, or diamond-like carbon films (DLC), produced by ion-beam sputtering deposition of carbon ions, are in use [124]. CDFs can grow on substrates heated at high temperature as poly or single crystalline diamonds with a relatively high surface roughness. Conversely, DLCs have reduced surface roughness and can be obtained on substrates at low temperatures, but generally are characterized by poor thermal stability. Growth conditions have been found to be an important parameter for the field emission properties of these materials, with varying experimental results [125]. Indeed, an extensive experimental study [126] has reported the comparison of FE data obtained on microcrystalline diamond films deposited by hot-filament CVD method and undoped DLC films deposited using radio-frequency CVD technique, showing that surface morphology and emission properties critically depends on the fabrication technique. When imperfect fine-grained diamond films (with the relative high presence of sp\({}^{2}\) carbon phase) are examined, the field emission properties are clearly enhanced (with turn-on fields of about 50 V/\(\upmu\)m) with respect to the case of high quality coarse-grained diamond films (characterized by a large amount of the sp\({}^{3}\) phase and a turn-on field of about 140 V/\(\upmu\)m). To explain the mechanism of field emission from CVD diamond films, various models have been reported, focusing principally on the negative surface electron affinity [127], on the presence of grain boundaries [128; 129], or on the presence of small protuberances [129; 130]. The electron affinity (i.e., the energy difference between the vacuum level and the conduction band minimum in a semiconductor) is typically few eV (positive electron affinity) and it represents the potential barriers that an electron (raised from the valence band to the minimum of conduction band by an excitation) encounters to leave the sample towards the vacuum. On the other hand, in the case of negative electron affinity, the electron at the bottom of the conduction band is free to leave the surface of the material. It has been suggested [67] that negative electron affinity alone cannot completely explain the electron emission from diamond and DLC films. It is now accepted that a pure diamond surface has a small positive electron affinity (\(\sim\)+0.38 eV) [131] that can become negative in the presence of defects or hydrogen terminations. A schematic of the energy band diagram for a clean diamond surface (with positive electron affinity) and for hydrogenated diamond surface (with negative electron affinity) is shown in Figure 5.  
Low field emission from DLC films has been the subject of several theoretical studies, considering the role of hot electron injection [132], dielectric breakdown [133], tunneling [134], tip emission [135], and defects [136]. It has been shown that low field emission is systematically measured when carbon materials have sp\({}^{2}\) bonds. Graphitic phases, often present in CVD diamond films [137] and in diamond/sp\({}^{2}\)-bonded carbon nanocomposites [138], are a crucial ingredient for field emission. [PERSON] et al. [139] analyzed the FE properties for different diamond/sp\({}^{2}\)-bonded carbon nanostructures including CVD diamond films. Threshold fields as low as 1 to 3 V/\(\upmu\)m have been found for samples having large amount of sp\({}^{2}\)-bonded carbon phase. On the contrary, poor emission is obtained for the dominant diamond phase.