Field evaporation behaviour for carbon nanotube thin-film

Abstract

In recent years, the carbon nanotube (CNT) emitters used for ion sources or gas sensors have been investigated, and the progress of several approaches such as field ionization and field desorption sources has been reported. However, a major concern for these applications is possible loss of CNTs caused by field evaporation, which can shorten the lifetimes of CNT-based emitters used for high electric field ion sources. So in CNT-based field emitter technology, emitter lifetime and degradation will be key parameters to be controlled. However, up to now only very few investigations in this direction have been conducted. The reason for this might lie in the fact that one often considers that the threshold value of field evaporation for a kind of material ( 40 V/nm) is much higher than the field of ionization or desorption ( 10 V/nm) according to the metal material characteristics (such as the threshold values of field evaporation for tungsten and molybdenum are 54 V/nm and 45 V/nm, respectively). In this work, the carbon nanotube thin-film (the density of CNTs is about 2.5108/cm2) is fabricated by screen-printing method, and the field evaporation behavior of CNT thin-film is studied experimentally in an ultrahigh vacuum system typically operating at a pressure of lower than 10-9 Torr after a 4-hour bake-out at ~200℃. Unlike the vertically aligned CNT array having higher electric field around the edge of the array because of the shielding effect, the printed CNT thin-film has more uniform distribution of electric field and is very easy to relize the mass production. The results show that the prepared CNT thin-film has quite obvious field evaporation behavior (some contaminants have deposited on the surface of grid after field evaporation, and energy-dispersive X-ray spectroscopy elemental mapping result of the grid indicates that the contaminants consist mainly of carbon elements), with turn-on field in a range of 10.0-12.6 V/nm, ion current could reach up to hundreds of pA. Meanwhile, the results with scanning electron microscope analysis and field electron emission measurement indicate that the CNT distribution turns into more non-uniform distribution after field evaporation; even some CNTs are directly dragged away from the substrate by the strong field. The field evaporation of CNT thin-film also leads to field electron emission onset voltage increasing from 240 V to 300 V, field enhancement factor decreasing from 8300 to 4200, and threshold field of field evaporation rising from 10.0 V/nm to 12.6 V/nm. However, the repeatability of sample treated by the field evaporation brings about an improvement to a certain extent. It could be understood in this way: upon applying a positive voltage, the most protruding parts, which have the strongest emissive capability, are evaporated first, which leads to the declined field enhancement factor; the parts of CNTs which have relatively weak emissive capability are not evaporated. So the uniformity of electric field is improved through reducing the difference in field enhancement factor rather than surface morphology between carbon nanotubes. The field evaporation of CNT thin-film is also a process which improves the uniformity of electric field. Therefore, the stability and repeatability of the field electron emission for carbon nanotube thin-film are improved naturally.