The Sharpest Object YetThe Sharpest object yet made is a
tungsten needle tapering down to about the thickness
of single atom. The needle, made by postdoc Moh'd Rezeq in the group of Robert Wolkow at the University of Alberta and the National Institute for Nanotechnology, starts out much blunter. Exposed to a pure nitrogen atmosphere, however, a rapid slimming begins. To start with the
tungsten is chemically very reactive and the nitrogen roughens 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. (For a single picture, go to Physics News Graphics; for a movie showing the evaporation process all the way down to a single atom at the tip, see the) Furthermore, what N2 is present near the tip helps to stabilize the tungsten against further chemical degradation. Indeed, the resultant needle is stable up to temperatures of 900 degrees Celsius even after 24 hours of exposure to air. The probe tips used in scanning tunneling microscopes (STMs), even though they produce atomic-
resolution pictures of atoms sitting on the top layer of a solid material, are not themselves atomically thin. Rather their radius of curvature at the bottom is typically 10 nm or more. Wolkow (rwolkow@ualberta.ca) says that although a narrower tip will be useful in the construction of STM arrays (you can pack more tips into a small area; and a wide array might even permit movies of atomic motions) the spatial resolution won't improve thereby. The real benefit of the sharp tungsten tips, he believes, will be as superb electron emitters. Being so slender, they would emit electrons in a bright, narrow, stable stream.