The needle, made by Moh'd Rezeq in the group lead by Professor Robert Wolkow at the University of Alberta and the National Institute for Nanotechnology, was initially much blunter. The needle was exposed to a pure nitrogen atmosphere that made it gradually thinner and thinner.
Tungsten is highly chemically reactive and nitrogen is used to roughen the tungsten surface. But at the tip, where the electric field created by applying a voltage to the tungsten is at its maximum, N2 molecules are driven away. This process reaches an equilibrium condition in which the point is very sharp.
Furthermore, when N2 is present near the tip this helps to stabilize the tungsten against further chemical degradation. Wolkow says that although a narrower tip will be useful in the construction of high power microscopes yhe real benefit of the sharp tungsten tips will be as superb electron emitters.