Factors effecting field emission from multiwalled carbon nanotubes

Abstract

Carbon nanotubes (CNT) have emerged to be one of the most versatile of materials ever discovered. The small dimensions, high electrical conductivity and strength along with other physical and electrical properties make them a unique material with a wide range of promising applications. One such use is that of CNTs as electron beam sources. A typical CNT has a diameter of only a few nanometers but can be hundreds of microns long. Applying a voltage across the length of such an object results in field emission of electrons from one end of the tube. This effect is due to intense electric field enhancement that occurs at the ultra sharp tip of the nanotube. A viable field emission electron beam source can be fabricated from CNTs. The primary goal of this work is to study the effects of various factors that influence field emission from multiwalled CNTs. For the set of factors that was chosen for investigation, a suitable field emission testing system was designed and assembled. Temperature of the CNTs was observed to have a considerable effect on the field emission from CNTs. Current saturation is observed at high temperatures. These findings can prove to be critical if the field emission device is operating in conditions of high temperature. The effects of variation in ambient pressure and changes in the background gas species are also studied. The field emission device characteristic is found to be very sensitive to the ambient gas pressure and more so when the gas species used was helium. Among Ar, He and N2, it is observed that He is the most suitable for field emission based device applications. It has been experimentally proven that aligned CNTs are far superior to random CNTs in terms of field emission characteristics. Effect of different substrate materials on field emission has also been examined. It has been found that metallic substrates like stainless steel show promise of better performance. CNT growth conditions have also been shown to influence their field emission property. Young’s interference fringes found on the copper anode surface after field emission have been reported here. Emitter and anode degradation as a result of field emission have been discussed as part of this wok. However it is important to note that CNTs are relatively more robust and less prone to degradation when compared to many other conventional field emitters. These results can be applied to find a set of optimal parameters that could be used for any field emission device design in order to get maximum field emitted current density at low operating voltages.