SINGLE-LAYER CARBON NANOTUBES CAN BE EITHER SEMICONDUCTORS OR METALS, two independent teams have conclusively demonstrated for the first time. The most recently discovered form of pure carbon, carbon nanotubes are rolled-up sheets of carbon hexagons. Shortly after nanotubes were discovered in 1991, theorists predicted that the carbon nanotubes were either metals or semiconductors, depending on the tube diameter and the "helicity," which describes the value of the corkscrew-like angle with which the flat carbon sheets can be wrapped. Using scanning tunneling microscopes (STMs), a Dutch-US team (Cees Dekker, Delft University of Technology, dekker@qt.tn.tudelft.nl ) and a Harvard group (Charles Lieber, cml@cml.iris.harvard.edu) have confirmed this idea, by relating, for the first time, atom-scale images of the nanotubes to STM measurements of the "electron density of states" which describes the relative populations of electrons at various energy levels in the nanotube. One surprising finding: the STM images revealed nanotubes with a wide range of helicities, suggesting that the tubes do not wrap themselves at preferential angles. (Nature, 1 January 1998.)Images at Physics News Graphics website.)