Thermally activated electron emission from nano-tips of amorphous diamond and carbon nano-tubes

Abstract

The sp 2 character of graphite can carry electricity like a metal and the sp 3 character of diamond can emit electrons in vacuum like an insulator. In this research, we have studied two ways of combining the electrical conductance and electron emission properties in carbon materials. In one case, the two characters are mingled atomistically to form a rather uniform mixture of amorphous diamond. Alternatively, graphite basal planes are wrapped around to form nano-tubes that exhibit a slight diamond character. Both amorphous diamond and carbon nano-tubes (CNTs) contain emission tips of nanometer sizes. When they are connected to a negative bias, they can emit electrons in vacuum toward an anode at very low turn-on field. However, when the cathode material is heated up, the responses of electron emission in vacuum are dramatically different between the two types of carbon materials. At a temperature of 300 °C, amorphous diamond can emit 13 times more electrons than it did at room temperature; but CNTs show no response to thermal agitation. The thermally sensitive emission of amorphous diamond indicated that electrons climbed up an energy ladder to reach the vacuum level for efficient emission. The energy ladder contained minute but discrete energy levels that were created by distorting the tetrahedral bonds of carbon atoms to different degrees. On the other hand, the CNTs are substantially graphitic so the energy gap between their conduction band and valence band overlap and are continuous in energy. In this case, electrons could not acquire energy to reach the vacuum energy unless the temperature could be sufficiently high. Hence, it confirms that CNTs emit electrons primarily by enhancing the applied field on nano-tips. This is in contrast to amorphous diamond that emits electrons by combining graphitic carbon atoms and diamond-like carbon atoms of various states.