Electron source, CNTs

One-dimensional conducting polymer nanomaterials have been utilized as the field emission electron sources for flat panel displays [365-367]. Conducting polymer nanotubes or nanowires were mostly prepared by the electrochemical polymerization within the cylindrical pores of alumina membranes, and the field emission characteristics were evaluated. As a typical example, a field emission cell was composed of PEDOT nanowire (conductivity, 3.4 x 10 S cm ) tips (cathode) and ITO (anode). The turn-on field of PEDOT nanowire was 3.5-4.0 jiAcm at 10V jim , and the current density increased up to 100 xAcm at 4.5 V jim . The field enhancement factor of the PEDOT nanowire tips was 1200 and this value was comparable to that of CNT. PPy nanowire and PANI nanotube was also prepared using nanoporous template, and their field emission characteristics were investigated [365]. PPy nanowire and PANI nanotubes showed the turn-on fields of 3.5-4.0 and 5.0 jjlA cm at 6 and 8 V im . These studies offered a great feasibility of conducting polymers as the building blocks for all-polymer field emission displays. 

Table 5.2 Typical Operating Parameters of Various Industry Pervading Electron Sources (Therm.[ionic], Schot.[tky], and C.C. [Cold cathode]) Compared with CNT Field Emitters... 

Figure of merit polar plot. CNT-based field emitters are seen to outperform all of the state-of-the-art electron sources across most metrics. 

CNT electron sources were thought to have their largest potential market in flat-panel field-emission displays (FEDs) [320,321], In 2011 the flat-panel display market had an estimated net worth in excess of 135 billion—one of the largest global markets in human history. Figure 5.26(g) illustrates a simple CNT-based display pixel, where the CNTs are vertically deposited on a matrix of electrodes in a vacuum housing. 

CNTs are also valuable as field emitters because they have a small virtual source size [30], a high brightness, and a small positive temperature coefficient of resistance [31]. The latter means that they can run hot under high emission currents, but not go into thermal runaway. Emission from nanotubes can be visualized by electron holography in a TEM [32],... 

The other interesting material for electronics is carbon nanotubes. We have shown the application of individual single-walled carbon nanotubes for field effect transistors (FETs) [3]. Carbon nanotubes (CNT) or CNT bundles can be placed between two carbon electrodes playing the role of source and drain. The gate electrode can be made of thin metal stripe under the dielectric film in the region between source and drain. 

Fig. 12 An electronic device based on a single rolled-up sheet of carbon atoms. (From Ref. () In the figure, a CNT (red about 1 nm in diameter) bridges two closely spaced (400 nm apart) platinum electrodes (labeled source and drain) atop a silicon surface coated with an insulating silicon oxide layer. Applying an electric field to the silicon (via a gate electrode, not shown) turns on and off the fiow of current across the nanotube, by controlling the movement of charge carriers onto the nanotube. (View this art in color at www.dekker.com.)...

Since their discovery in 1991, carbon nanotubes (CNTs) have been a constant source of scientific inspiration [307]. Among the most intriguing properties of CNTs is the electric field enhanced electron emission from the nanotubes [308]. Initial studies combining CNTs and conjugated polymers concentrated on the diode properties in a CNT-polymer heterojimction [309]. Romero et al. demonstrated fight-sensitive photodetectors in combination with a PPV derivative. The authors showed that hole injection from the CNT electrode proved to be much more efficient than using an ITO electrode, and related this phenomenon to the enhancement of the local electric field at the tip of the nanotubes [309]. 

In summary, depending on the nature of sources and conditions, the radiation effects of the CNTs in a relatively short period of time were very different. 1) The effective surface oxidation was observed for the samples exposed to the proton for 15 min and UV-ozone environment in 60 min, and the oxidative functional groups produced by proton and UV-ozone were essentially similar to those produced by acid-based treatment.[31] 2) The electron radiation could remove the carbon impurities, when the operating conditions are carefully optimized. Nevertheless, these radiation methods described above were performed in a form of a dried CNT powder, thus did not produce any harmful chemical byproducts. 

The proposed CNT-transistor can fulfill the ITRS requirements even with these reduced performance values. If we take into account, to scale the transistor not only by width, but by the used area, as shown in Fig. 5, we can imagine a 2-dimensional vertical array of individual nanotubes, creating a very promising device. If we compare this CNT-device with the silicon world, we have to keep in mind that the silicon device needs area for source and drain contacts and is not stackable. Whereas our proposed CNT-transistor incorporates already source and drain contacts and is stackable. So, with this concept, we can create real 3-dimensional electronics. 

CNTs find applications in the areas such as micro electronics, field emission displays. X-ray sources and gas sensors. Single waUed and multi-waUed CNTs can be grown using high pressure arcs, laser ablation and chemical vapour deposition. 

The anode resistivity is attributed from electrode resistivity and the resistivity associated with the electron transfer from exoelectrogen to the anode (it can also be seen as the contact resistance. Resistance of biofilm is also incorporated into the electron transfer resistivity). For conductive anode, such as gold, CNT, carbon, etc., the electrode resistivity is negligible. Consequently the main source for the anode resistivity comes from the electron transfer from exoelectrogen to the anode. 

For all-printed thin film transistors (TFT), various organic and inorganic metal electrode materials, such as conductive polymer, carbon nanotube (CNT), organic metal compound, or metal nano-particles, have been used as gate and source/drain electrodes [6-11] in a combination with inkjet- and laser-based printing methods. One of the immediate applications for all-printed TFT would be flexible or rugged display backplane and disposable radio frequency identification (RFID) tags. In addition, printed metal electrodes and TFT have also been used to fabricate passive circuit components, power transmission sheets and sensors for ambient electronics and electronic skin [12-13]. 

The following section reviews the field emission process and various field emission applications in which CNTs have found particular recognition, including displays, microwave and X-ray sources, parallel electron beam lithography systems, gas ionization sensors, and interstellar propulsion. 

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