Field emission, using CNTs

Fig. 12. (a) Schematic illustration of the setup for field emission experiment using a CNT film, (b) TEM picture of a tip of an MWCNT where electrons are emitted, (c) Illustration of the electron emission by applied electric field [38]. 

Because of their diverse structure, one-third of the tubes are expected to possess metallic character and the remaining two-thirds to behave as semiconductors [2, 3]. CNTs represent potential candidates to be used in field emission [4-6] and nanoelectrical devices [6-10], components of electrochemical energy [11, 12] and hydrogen storage systems [13, 14] and as components in composite materials [15-17]. They represent the ultimate carbon fiber, exhibiting exceptional mechanical properties [18-21] by being up to 100 times stronger than steel [22]. 

A crucial problem connected to carbon nanotube synthesis on supported catalysts on an industrial scale is the purification step required to remove the support and possibly the catalyst from the final material. To avoid this costly operation, the use of CNT- or CNF-supported catalysts to produce CNTs or CNFs has been investigated. Although most catalytic systems are based on nickel supported on CNFs (see Table 9.4), the use of MWCNTs [305,306] or SWCNTs [307] as supports has also been reported. Nickel, iron [304,308-310], and bimetallic Fe-Mo [305] and Ni-Pd [295] catalysts have been used. Compared to the starting CNTs or CNFs, the hybrid materials produced present higher specific surface area [297,308] or improved field emission characteristics [309]. 

Electronically, the carbon nanotubes can have either metallic or semiconducting properties depending upon their chirality and diameter. The electronic gap is also controllable within a range of 0-l eV. This gives rise to possible metal-semiconductor or semiconductor-semiconductor junctions for use in nanoelectronic devices. The possibility of integrating carbon nanotubes into logic circuits was demonstrated in 2001. Another property of CNTs, the ability of electron field emission, has reached technological relevance. 

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. 

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. 

Contraiy to popular belief, carbon nanotubes (CNTs) were not discovered by lijima in 1991 [1], but rather by Radushkevich and Lukyanovich in 1952 [2,3], who published clear transmission electron micrographs of 50nm diameter mbes made of carbon. Unfortunately, due to the pohlical tensions of the time this work wait largely unnoticed. In fact, with the discovery that CNTs were responsible for the high strength of Damascus steel from the seventeenth century [4], it became clear that thdr advantageous mechanical properties have been employed for some time. Today their novel properties have found use in a number of innovative electronic devices. This chapta details some of the more prominent appUcations based on their excellent field emission behavior. 

Synthesis of CNT over oxides supports by Catalytic Chemical Vapor Deposition (CCVD) is one of the most important techniques for mass production of non-aligned CNT. It could be useful for the production of composite materials, field emission sources, fuel cells, supercapacitors among others technological applications. The CCVD method consists on the decomposition of a gas or a liquid precursor, which supplies carbon to the surface of the catalytic particles (e.g. Fe) in a tube furnace at temperatures around 900 °C. This technique is scalable for mass production at lower temperatures and could be adapted for continuous production. 

Areas for commercial exploitation of electronic CNT nanotechnology include backlighting, field emission displays together with various films using nanomaterials. 

We illustrate CNT-polymer interface morphology using a CNT/PS and a CNT/ epoxy system." Rod specimen of CNT/PS composite with 1 mm diameter was fabricated using an extrusion process, with a CNT content of about 1 wt.%. Tensile failure surfaces of the CNT/PS composite rod were examined under a field emission seanning electronic microscope (FESEM) and transmission electron microscope (TEM). CNT/epoxy (EPON SU-8 photo resist) thin film with 0.1 wt.% CNT and 5.8/xm in thickness was fabricated by spin-coating mixture of CNT and epoxy on to a silicon wafer. The fracture surface of CNT/ epoxy specimens was examined under FESEM and TEM after shaft-loaded test (inset of Fig. 13.7). 

In order to measure the field emission characteristics, the Cgo/CNT composite, which was produced by the drawing process, was mounted on a copper grid using an adhesive and a silver paste and was used as an electron emission source (Fig. 14.21). Six readings of the emission current (I) in a sample for the applied voltage (V) were measured in a high vacuum chamber with a base pressure of approximately 6.5 x 10 ° Pa. The distance between the electrodes was fixed at 200 /im using a mica spacer. 

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