Electron beam assisted deposition of sharp needles from a decomposed gas mixture of methane and hydrogen

Abstract

Very sharp and very thin needles, typically 0.5 μm thick and 10 μm long, were deposited with the aid of electron beam assisted decomposition of diluted 1% methane gas in hydrogen. A conventional SEM was modified to be able to introduce several torr of the gas mixture in the specimen chamber. The typical pressure of the gas during the decomposition was about 1 torr, and the back pressure of the specimen chamber was 10 −5 torr. By irradiating the 0.1 mm-thick tungsten wire substrate with a 4 kV electron beam by stopping the scanning action, a needle-shaped decomposed material was deposited. The material of this needle has not been identified explicitly. However, the material is probably an electrical semiconductor or insulator, because the electron beam was deflected by charging, while a SEM picture was taken. Considering the composition of the introduced gas mixture, we speculate that the needle is diamond-like carbon. Field emission characteristics also show that the needle is semiconducting or insulating in nature. For example, the field emission current is much smaller than the widely accepted value. The work function was determined from the Fowler-Nordheim plot to be 5.8 eV. This value of the work function is much larger than the widely accepted value for carbon, 4.7 eV.