Electron emission from carbon fibre tips

Abstract

7 μm diameter carbon fibres were electrolytically etched in a 0.1M sodium hydroxide solution to produce micropoint emitters. The behaviour of these tips was tested in a retarding potential spectrometer under a variety of vacuum conditions. At a medium vacuum of ∼ 10 −3 mbar, the emission of electrons was very noisy and the lifetime of the source was relatively short. At pressures of < 10 −6 mbar, it was found that the performance of the cathodes, including their lifetime, is largely independent of the vacuum pressure. The emission characteristics of carbon fibre tips were recorded. Electron microscopy techniques employed in this investigation were used to study the tip profile. They also showed that the inherent instability associated with carbon fibre emitters resulted from an explosive phenomenon which was encountered during the initial switch-on of the emission current from freshly prepared cathodes. This process led to the destruction of these cathodes. Pre-conditioning treatment of the tip overcame this problem and produced a large reduction in the noise fluctuation of the total emission current. This enabled the recording of energy distributions of field emitted electrons. The spectra obtained demonstrated narrow halfwidths (FWHM) and an energy shift compared with the Fermi level of tungsten. By increasing the applied electric field on the tip, the spectral shift was found to increase. The emission pattern consisted of a random distribution of sub-emission centres, which switch “on” and “off” randomly with time. The Fowler-Nordheim current-voltage plots of these carbon fibre tips showed that the work function was insensitive to the vacuum pressure.