# Field Emission from Carbon Nanostructures
## 2 Field Emission from Carbon Nanostructures
### Field Emission from Diamond and Related Structures
Importantly, it has also been demonstrated that the threshold field can be reduced when growing CVD diamond with several structural defects [140]. Correlation between the threshold field and the width of the diamond Raman peak (used to quantify the defect density) was reported and explained considering that the various types of defects in the diamond structure can create additional energy bands within the diamond band gap, contributing to FE at low fields. Doped diamonds have been also widely investigated [134; 141]. By comparing B-, Li-, P-, and N-doped diamonds and different treatments it has been demonstrated that one of the lowest reported field \(\sim\)0.2 V/\(\upmu\)m, can be obtained for N-doped diamond samples after a complex surface  
Figure 5: (**a**) Schematic of the energy band diagram for a clean diamond surface with a positive electron affinity. (**b**) Schematic of the energy band diagram of a hydrogenated diamond surface with negative electron affinity. (**c**) Possible atomic arrangement of dipole layer on the hydrogenated diamond surface. The mentioned ”surfaces” refer to the single crystalline type IIb diamond. Image reproduced with permission from [67], Copyright Elsevier, 2005.  
treatment with O\({}_{2}\) plasma, coated with Cs, heated, and exposed to O\({}_{2}\). The FE improvement has been explained by taking into account that the diamond surface is oxygen terminated, due to the action of O\({}_{2}\) plasma, and consequently reacts with Cs to form an oxygen-stable diamond-O-Cs surface, which lowers the work function of diamond. It has been recognized that diamond-based electron emitters with negative electron affinity are suitable for the realization of vacuum power switches [142]. High current density and uniformity represent the most relevant feature of the switch, and many conventional field emitters are not suitable for such a purpose. On the contrary, hydrogen-terminated diamond is the best material to be exploited due to its negative electron affinity [143; 144; 145; 146]. Experiments of total photoelectron yield spectroscopy [142] have clearly demonstrated that PN and PIN junctions are an effective method of using negative electron affinity for emitting devices [143; 147]. Moreover, [PERSON] and [PERSON] have demonstrated that electron emission from [111] PN junction diodes can be realized, obtaining negative electron affinity through surface hydrogen-termination [148]. They also achieved high electron emission efficiency (close to 10%) and 1 \(\upmu\)A emission current. [PERSON] et al. [142] fabricated a diamond [111] P-I-N diode with an N\({}^{+}\) top layer and thick high-quality (insulating) I-layer. Field emission experiments were performed using a W-needle as an anode (curvature radius of 25 \(\upmu\)m) placed 100 \(\upmu\)m above the diode. Field emission was not observed before the hydrogenation process; such a result demonstrated that electron emission was a result of the negative electron affinity of the diamond surface. Accordingly, a 10 kV vacuum power switch using diamond P-I-N electron emitter has been developed and characterized [142] as showing a breakthrough power transmission efficiency of 73% at 9.8 kV during operation at room temperature. Such a result was explained as a combination of the negative electron affinity of the diamond surface, and a high level of electron injection with a hopping conduction-type in the N\({}^{+}\) layer. Importantly, these findings open a path for the development of vacuum power switches operating at 100 kV with an efficiency beyond 99.9%.