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Field Emission of NoPo HiPCO Nanotubes

Emission of Electrons from Carbon Nanotubes was a project carried out by students of BMS College of Engineering, Electronics Engineering led by Sujay, Sanjana, Suhas and Shriram. The unique ability of Nanotubes to emit electrons was studied as part of this project. 

Carbon nanotubes (CNTs) are hollow cylinders of carbon atoms. Their appearance is that of rolled tubes of graphite, such that their walls are hexagonal carbon rings, and they are often formed in large bundles. The ends of CNTs are domed structures of six-membered rings, capped by a five-membered ring. There are two types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs), which differ in the arrangement of their graphene cylinders. SWNTs have only one single layer of graphene cylinders; while MWNTs have many layers. Carbon Nanotubes were discovered and first characterized in 1991 by Iijima of NEC laboratories. Shortly after the discovery of multiwall carbon nanotubes, single wall carbon nanotubes were synthesized. Because the microscopic structure of single wall carbon nanotubes is closely related to that of graphene, the tubes are usually labeled in terms of the graphene lattice vectors.

Field emission involves the extraction of electrons from a solid by tunneling through the surface potential barrier. It involves discharge of electrons from the surface of a material subjected to a strong electric field. In the absence of a strong electric field, an electron must acquire a certain minimum energy, called the work function, to escape through the surface of a given material, which acts as a barrier to electron passage. If the material is subjected to a strong electric field, the work function is so lowered that some electrons will have sufficient energy to leak through the surface barrier. The resulting current of electrons through the surface of a material under the influence of a strong electric field is called field emission.

Both Single walled and Multi walled CNTs offer promising features for electron emission. A lot of research has been happening on field emission using MWCNT. So far MWCNTs are proved to be emitting electrons at lower energies. However, it is found that field emission using MWCNTs is inconsistent and hence they are not reliable. Research suggests that SWCNT are consistent and hence more reliable and efficient field emission can be obtained using them.

Carbon nanotubes (CNTs) are widely used as field emission electron emitters for X-ray tubes, field emission displays and high-resolution electron beam instruments because of their excellent electron emission property, chemical inertness, and high electrical and thermal conductivity. In spite of these superior characteristics, practical applications of CNT field emitters to devices particularly requiring high-voltage operation are limited due to unstable 2 electron emission properties of the CNT emitters. Electron beam current emitted from CNT emitters can be fluctuated or degraded because CNTs are damaged by the back bombardment of ions produced from the residual gas or CNTs are detached from a substrate. If a very high current (300 nA per single CNT) flows through a CNT, adhesion between the CNT and the substrate becomes weak due to resistive heating and accordingly the CNT can be peeled off from the substrate or a strong electric field exerts electrostatic force on CNTs, leading to the detachment of the CNTs. Weak adhesion of CNTS to a substrate deteriorates the removal of CNTs. In addition, if CNT emitters are opera ted at a high voltage or at a high electric field, electrical arcing (or vacuum breakdown) can occur. Arching can be initiated by the removed CNTs, impurities on the CNTs or substrates protrusion of CNTs, low operating vacuum, and a very high electric field. Since arcing is accompanying with a very high current flow and it can produce a plasma channel near the emitter, CNTS are seriously damaged or sometimes CNTs are almost completely removed from the substrate by the arcing events. Detachment of CNTs from a substrate is an irreversible catastrophic phenomenon for a device operation. In addition to the detachment of CNTs, arching induces a sudden voltage drop and thus device operation is stopped. Therefore, for a stable operation of a device using CNT emitters, arcing should be prevented. Particularly, CNT emitters on small metal tips (diameter<1 mm) are necessary for miniature X-ray tubes and micro-focus X-ray tubes. Small metal tips produce much higher electric field than flat substrates at the same applied voltage due to their sharp geometry. As a consequence, CNT emitters on small metal tips can suffer from much serious and frequent arcing, and hence, stable operation of the CNT emitters against arcing is a big issue. In this project we have carried out substantial research on making field emitters using SWCNT and characterizing their performance under suitable conditions.

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