TRAPPING A SINGLE NANOPARTICLE BETWEEN TWO ELECTRODES has been controllably achieved for the first time, enabling researchers to deposit individual nanoparticles onto surfaces and offering possibilities such as single-nanoparticle switches. Researchers (Cees Dekker, Delft University of Technology, dekker@qt.tn.tudelft.nl) construct a circuit containing two platinum electrodes separated by as little as 4 nm--a gap that the researchers believe to be a world record. To trap nm-scale molecules or clusters, they immerse the electrodes in a solution containing the nanoparticles. Applying a voltage to the electrodes polarizes each particle and attracts a particle to the gap between the electrodes. Once a particle bridges the gap, current flows through the circuit, and a resistor then sharply reduces the electric field, discouraging any additional nanoparticles from entering the gap. In principle, this electrostatic-trapping technique can work for any polarizable nanoparticle; it has been demonstrated for nanometer-scale clusters of palladium (Pd) atoms, carbon nanotubes, and a 5 nm-long chain of thiophene (a conducting polymer). The researchers have also studied the properties of single electrons as they cross a Pd nanocluster between the electrodes. (A. Bezryadin et al., Applied Physics Letters, 1 September; images and more info available at Physics News Graphics)