Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers

Carbon nanotube (NT) layers grown by CVD on Ni foil demonstrated low voltage characteristics of field electron emission: the value of the field amplification coefficient beta was in the range 1000-4000. The influence of electric field (Eav), emission current (IFE ) and exposure time on the configuration of conical-layer carbon nanotubes grown by CVD on the edge of a Ni foil has been investigated. TEM profile imaging revealed a significant concentration of NTs close to the edge surface, whereas on the NTs layers' outer surfaces single, non-oriented NTs with open ends free of catalytic particles, were observed. After sufficient electric field application many NTs become oriented towards the anode, but one or two of them were always a few microns more extended. In-situ SEM investigation showed that below Eav = 3.2 - 3.9 V/mum, emission was achieved at the expense of originally existing free NTs ends. Configuration changes began at the higher applied fields. On the observed foil edge length (14.6 - 17.8 mum) and the edge thickness 200 mum one or two NTs extended towards the anode and probably become main emitters. On further increasing the field to Eav= 5.7 - 8 V/mum and at IFE=2times10-5 A these tubes disappeared (or essentially shortened). At Eav = 8 V/mum and higher, and at an exposure time of up to 40 min, several tens of extended NTs appeared with one or two extended significantly beyond the others. This NT configuration pattern is probably connected with electrostatic screening between the NTs. It is suggested that in the range of Eav and IFE that was investigated, a limited number of NTs were emitting and these nanotubes were constantly changing as Eav, IFE and exposure time increase