Small cars are in vogue, thanks to rising fuel costs and environmental concerns. But the Fiats and Smart Cars of the world have nothing on the new four-wheeler developed by European researchers.

Netherlands-based researchers Tibor Kudernac of Twente University and Nopporn Ruangsupapichat of the University of Groningen and their colleagues have engineered a single molecule with four electric "wheels," depicted here in an artist's conception. By applying less than a volt with the tip of a scanning tunneling microscope (STM), the researchers could spin the four wheels and scoot the molecular vehicle six nanometers across a metallic surface in a more or less straight line. (An STM image of the nanovehicle appears in the inset.) The researchers announced their achievement in the November 10 issue of Nature. (Scientific American is part of Nature Publishing Group.)

Molecular vehicles have been constructed before, but past designs have been passive rollers, the researchers report. The new molecule, in contrast, drives by a kind of paddlewheel motion as electrons from the STM tip excite the four wheels, inducing a cycle of structural changes that results in rotary motion. But each wheel turns in one direction only, so forward motion is possible only if all four wheels are oriented in the right way. With other wheel orientations the molecule moves randomly or not at all, Kudernac, Ruangsupapichat and their colleagues found. They predict that their work could form the basis of more complex nanotech systems that require movement on demand.