# Field Emission from Carbon Nanostructures
## 2 Field Emission from Carbon Nanostructures
### Emission from Individual Nanotubes  
The characterization of field emission properties of a single CNT is very important. Typically, electron sources are realized with several emitters and their performances are the result of statistical averaging. To fully understand the physical mechanisms of electron emission, several studies have been devoted to quantifying the performance of a single CNT as an emitter [13; 108; 109; 110; 111]. The experiments on individual tubes allows a more accurate characterization of work function, turn-on voltages, current densities, and field enhancement factor as well as monitoring structural modifications [112] and failure mechanisms [102]. Consequently, measurements on individual tubes provide valuable information towards a fundamental understanding of the emission mechanism. Typically, multi-walled CNTs are employed for higher conductance and robustness. The importance of studying the FE behavior of individual tubes is also strictly related to their technological applicability as nano-sized electron sources in high resolution e-beam techniques [113]. I-V characterization of the FE properties of individual multi-walled CNT has often been performed operating with a large cathode-anode separation (several millimeters) to overcome the uncertainty on the cathode-anode separation as well as to obtain a configuration in which the field enhancement factor is only slightly sensitive to the distance. On the other hand, relevant FE applications typically deal with cathode-anode separation below 15 \(\upmu\)m to obtain higher fields with lower voltages. At such a distances, the main FE parameters, the field enhancement factor and the turn-on voltage, are very sensitive to distance variations [114]. Field emission properties of a single CNT have been characterized by several groups, reporting emitted currents as high as several tens of microamperes, implementing different measurement setups. A summary of more relevant results available in the literature are collected in Table 1, listing the different parameters, such as CNT dimensions, anode-cathode separation distance, and the extracted FE parameters. [PERSON] et al. [115] reported the FE characteristics for individual multi-walled CNT inside a SEM by using a sharp anode for precise measurements in a range of cathode-anode distance from 0.06 to 5 \(\upmu\)m. They reported a dependence of the field enhancement factor (up to 460) on such distance in agreement with the Edgcombe and Valdre model [74]. The lowest turn-on voltage was estimated 47 V. Their results also confirmed that only CNTs with the highest field enhancement factor contribute to the emitted current when performing large area measurements. The electrical properties of individual multi-walled carbon nanotubes grown by plasma enhanced CVD has been also reported [116]. The I-V characteristics have been measured by means of scanning anode field emission system on a vertical array of CNTs separated by 25 \(\upmu\)m spacing (allowing to probe the emission from individual tubes). It has been found that field enhancement factor is strongly dependent on the geometry (height/radius) of CNT. To avoid the effects of adsorbates, current annealing (in the micro-ampere range) was performed to favor evaporation of adsorbates/residuals from emitter surface. The experimental data correctly followed F-N theory allowing to extract a field enhancement factor of 208 for 10 \(\upmu\)m anode-cathode separation.