# Field Emission from Carbon Nanostructures
## 2 Field Emission from Carbon Nanostructures
### Emission from CNT Films  
Field emission devices based on CNTs can deal either with individual CNTs (working as a point electron emitter) or with CNT ensembles realizing (quasi) planar emission cathodes by producing CNT films, patterned arrays, buckypapers, etc. Point emitters are mostly suitable for focused emission in high-resolution applications, while planar cathodes are the preferred choice for devices working with large emission current (for instance, X-ray sources or flat panel displays). Consequently, a large number of experimental and theoretical studies have been devoted to the characterization of CNT films. FE from CNT films is reviewed in this section, while the next section is dedicated to emission from individual CNTs. In 1995, the seminal paper by [PERSON] et al. [63] demonstrated the fabrication of a continuous nanotube film emitter by using a nanopore alumina filter to draw MWCNTs in a colloidal suspension and transferring the obtained film on a teflon coated metal surface [79]. They demonstrated field emission from MWCNTs with current densities as high as 100 mA/cm\({}^{2}\). Alternatively, suspension can be sprayed on heated substrate [80] to obtain a CNT film. A different approach to obtaining CNT films is to disperse CNTs in a non-conducting epoxy [81]. Other methods reported for the fabrication of CNT film include electrophoresis [82], vacuum filtration [59] and screen-printing [83]. Patterned CNT films can be obtained using post deposition techniques [70; 84; 85] or direct growth by chemical vapor deposition on catalyst patterns. Actually, field emission properties have been reported to be excellent for almost all types of nanotubes, with threshold fields as low as 1 V/\(\upmu\)m, recording current densities up to a few A/cm\({}^{2}\) for applied fields below 10 V/\(\upmu\)m. A relevant parameter when dealing with films is the emitter density; although CNT density is typically \(\sim 10^{8}\) cm\({}^{-2}\), the number of effective emitters for randomly oriented films is generally lower, from \(\sim 10^{3}\) emitters/cm\({}^{2}\)[86; 87; 88; 79] to \(\sim~{}10^{7}\) emitters/cm\({}^{2}\)[89]. In order to provide FE performances comparable to conventional molybdenum (Mo) field emitters in realizing flat panel display, CNT films need to have high uniformity with a density of emitters \(\sim 10^{6}\) emitters/cm\({}^{2}\) and a current density capability exceeding 80 \(\upmu\)A/cm\({}^{2}\)[90]. The use of Chemical Vapor Deposition (CVD) techniques to directly grow CNT films on a given substrate is widely applied due to the easy production scalability, for either randomly or vertically aligned tubes. For randomly oriented films, the FE properties are similar to coated CNT films. The main purpose of CNT films is represented by their application as large area electron sources as well as field emission displays [91; 92]. On the other hand, patterned films can be obtained by means of lithographic techniques to opportunely shape the catalyst deposition. The further advantage of patterning consistsof the opportunity to further reduce pixel size (typically an intrinsic limitation when producing large area sources with un-patterned films). Consequently, films of vertically aligned CNTs have the largest potential for technological applications due to relatively easy production and their high emission capability [93; 94; 95]. Since the nineties, a wide variety of CNT films, vertically or randomly oriented, different for density, substrate, single- or multi-walled tubes, have been deeply investigated as field emission devices [96; 97; 98]. Unfortunately, the extreme inhomogeneity arising from the composition and morphology of CNT films, even on small areas, caused a wide variation in the reported experimental data regarding the basic characterization parameters, such as the field enhancement factor, and the turn-on field.