Paper NS-ThP9
Ring-shaped Emission Patterns from Carbon Nanotube Films

Thursday, October 18, 2007, 5:30 pm, Room 4C

Field emission patterns in the shape of perfect rings were observed on the luminescent screen of a specially designed triode cell with parallel electrodes. This cell is designed for investigation of 25.5 mm diameter flat samples. During the routine I-U measurements of broad-area flat cathodes, consisting of silicon or molybdenum substrates coated with carbon nanotubes (CNTs), rings appeared sporadically as relatively stable images among other field emission patterns like lobes and daisies. Their projected size was a few mm and was thus big enough to allow providing the analysis of their details. The origin of the electrons projected onto the screen is presumably a uniformly emitting cap atop of a long single wall CNT. This is consistent with models which predict high enhancement of the electric field and a ring-shaped pattern mainly as a result of imaging on a flat screen. The macroscopic value of the electric field at which rings were recorded was between 0.7 and 2.5 V/µm. The picture brightness analysis allows estimating the current corresponding to a particular ring and its brightness profile. The current of an individual ring was in the order of 2 - 4 μA. An outstanding finding of this study is a much greater angular width of the emitted beam for most of the rings compared to those that had been observed or predicted by reported calculations. The true mechanism that causes the difference between the model and our experiment is probably related to the atomic scale phenomena not included in their calculations.