SCANNING GATE MICROSCOPY Scanned probe microscopes not only provide images of surface atoms, they also allow one to move atoms and to study the spectroscopy (the quantum energy levels) of those surface atoms (or molecules or metallic clusters). Concerning the latter, physicists at the Delft University of Technology (in the Netherlands) can better assay the energy levels of target particles at a surface by positioning a second probe right next to the main probe in a standard scanning tunneling microscope (STM) setup, giving it a tong-like appearance (see figure at Physics news Graphics).

The second probe acts much like a gate in a transistor: by shifting energy levels of the target particle it allows or disallows the passage of the tunneling current. In the reported experiment, the so-called Coulomb blockade (the difficulty of yet another electron to join many other electrons already on a tiny electrode) for single-electron tunneling in a 20 nm gold cluster was controlled using the gate electrode. (Gurevich et al., Applied Physics Letters, 17 January /pnu/2000/; Select Article.)