# Field Emission from Carbon Nanostructures
## 2 Field Emission from Carbon Nanostructures
### Emission from CNT Films
Indeed, it has been shown that when characterizing large emitting areas by parallel plate technique, emission from a CNT film typically happens from isolated spots [90], i.e., the emission phenomenon is mostly due to a limited subset of emitters. Therefore, in order to obtain accurate analysis of FE performance from CNT films, it is essential to characterize emitting areas as small as possible. It has been proposed that FE experiments on vertically aligned MWCNTs (Figure 4) could be performed by scanning probe microscopy (STM/AFM), with the probe working as an anode [12; 32]. This technique enables accurate characterization of the local field emission behavior, with only a limited number of emitters contributing to the total current. Indeed, the high spatial resolution provided by the piezo-driven positioner used for STM/AFM microscopy technique allows precise control of the tip-sample separation in the submicron region. By taking into account the small curvature radius (below 50 nm) of the probe tip, it has been estimated that the emitted current comes only from a limited circular area, of radius \(r\ \sim\ d\) when working at separation distances of the order of 1 \(\upmu\)m [12]. Moreover, for a correct analysis of the FE \(I\)-\(V\) characteristics, it is necessary to introduce a further amplification factor, the tip correction factor \(k_{eff}\), due to the tip shaped anode. Accordingly, the slope \(m\) of the F-N plot, \(\ln\left(\frac{I}{V^{2}}\right)\) vs. \(1/V\), can be reasonably approximated as \(m=k_{eff}\)-\(b\cdot\phi^{3/2}\cdot d/\beta\). An example of \(I\)-\(V\) characteristics measured using a Pt-Ti coated cantilever probe as anode and fixing a cathode-anode separation of 1 \(\upmu\)m and a pressure of 2 \(\times\) 10\({}^{-8}\) mbar is shown in Figure 4. Electrical conditioning is obtained during the first sweep and it has been explained in terms of a few protruding longer tubes (degraded/destroyed by raising the current density during the first sweep) and/or the effect of desorption of adsorbates. Moreover, it has been demonstrated that the limited area of emission favors very stable FE current, with fluctuations below 10% for more than 72 h also at intensities as high as 10\({}^{-8}\)-10\({}^{-5}\) A. Understanding the physical mechanisms responsible for the emission stability from CNT cathodes, has been widely investigated [99; 100; 101; 102], identifying as key factors either the presence of residual gases  
Figure 4: (**a**) SEM image (lateral view) of vertically aligned MWCNTs, grown by catalytic chemical vapor deposition on a silicon substrate with Ni catalyst. (**b**) Scanning Probe Microscopy imaging of 1 \(\upmu\)m \(\times\) 1 \(\upmu\)m area of the film surface. Lower plot corresponds to the profile of solid line represented on the image. (**c**) Field emission current versus voltage measured at 1 \(\upmu\)m cathode–anode separation and pressure of 2 \(\times\) 10\({}^{-8}\) mbar. After a first sweep from 0 to 200 V the \(I\)–\(V\) characteristic remains stable for successive sweeps. Adapted with permission from [32], Copyright Elsevier, 2009.  
and the resistive heating of emitters and the mechanical actions due to high electric fields. Both resistive heating and mechanical actions are related to high fields and high currents and they can be consequently reduced by properly realizing the operational conditions of the FE device. On the contrary, controlling the effects due to a degraded vacuum level, i.e., due to the presence of residual gases, is a much more complicated task. Indeed, residual gases are first of all always present in sealed vacuum devices. Control of the vacuum level between anode and cathode is a crucial parameter, because a degraded vacuum can provoke ionization of the residual gas by the emitted electrons as well as poisoning of the emitting surface, reducing the overall field emission performance. Small variations in the electric field and the work-function may have large effects on the fluctuations of the emitted current due to the exponential behavior of FE.