# Field Emission from Carbon Nanostructures
## 2 Field Emission from Carbon Nanostructures
### Emission from CNT Films
Absorbates on the emitter apex can cause slight modifications of the work-function or of the cap geometry, creating large variations in the emitted current. Such fluctuations disappear almost completely when operating at higher currents. The presence of noble gases as well as H\({}_{2}\) usually provokes an increase in the current fluctuations, but the emission stability can be restored by pumping down the vacuum level. For instance, no degradation was reported on single nanotubes after exposure to Ar and H\({}_{2}\). On the other hand, exposure to oxygen and water causes an irreversible decrease of the emitted current, probably due to reactive sputter etching [103]. A vacuum level of the order of 10\({}^{-9}\) mbar is suitable for an emission stability of a few percent. Higher pressure also allows FE but with reduced stability. In such a case, it would be useful to reduce the partial pressure of oxygen and water. Because field emission is strongly influenced by the surface condition of the emitters, it is important to prepare clean emitter surfaces, in order to lower the current fluctuations. Further improvements can be achieved by introducing a ballast resistance in series with the extraction voltage [104]. [PERSON] et al. [105] have investigated the stabilization of the FE current by using a ballast resistance in the range from 1 to 1500 M\(\Omega\) at an emission current of 1 to 100 \(\upmu\)A. They demonstrated excellent stability where the ratio (\(\Delta\)I/I) of the current fluctuation with respect the absolute current was as small as 0.0018, using 10 M\(\Omega\) ballast resistance on an average current of about 80 \(\upmu\)A. A different approach to use CNTs in realizing an effective emission cathode (for a flat-panel lighting device) with stable FE as well as low current fluctuations, examines the possibility of properly dispersing CNTs uniformly in a liquid media, thus favoring an extremely homogeneous deposition process. [PERSON] et al. [106] successfully produced field emission lighting elements (based on single-walled nanotubes) showing stable electron emission, good luminance, and low power consumption. They chose a low viscosity solvent containing highly crystalline single-walled nanotubes dispersed in a tin-doped indium oxide (ITO) precursor solution (conductive matrix). Also, a non-ionic surfactant was used to favor the dispersion of CNTs. The field emission cathode was obtained by sputtering ITO coating film with single-walled CNTs on glass substrate. Protrusions of the CNTs was obtained by manual scratching. The FE characterization evidenced high electron emission homogeneity, high brightness efficiency (over 70 \(\mathrm{\SIUnitSymbolMicro m}/\mathrm{W}\)) and long emission life-time (estimated above 5000 \(\mathrm{\SIUnitSymbol h}\)). Current-fluctuation mechanisms of field emitters using metallic single-walled carbon nanotubes with high crystallinity have also been recently discussed [107]. This study is related to the possibility of using field emitters as a cathode electrode in a cathodoluminescence device. The main drawback is due to the use of field emitters that radiate rays whose intensity fluctuates at low frequency, due to FE current fluctuation. Consequently, an important task is having a stable FE output. The authors succeeded in realizing and characterizing cathodoluminescence devices with homogeneous dispersion of metallic single-walled CNTs with high crystallinity. They also developed a theoretical model to study the power spectrum of the current fluctuation considering an inelastic-electron-tunneling model with a Wentzel-Kramers-Brillouin approximation.