Number 118 (Story #3), March 12, 1993 by Phillip F. Schewe and Ben Stein ARTIFICIAL ATOMS are manmade, essentially zero-dimensional systems---usually involving specially tailored nanometer semiconductor structures---in which the presence or movement of single electrons can be important. One prominent example is the quantum dot, a pointlike quantum-well structure that can be fashioned as a tiny stump on a substrate by selectively etching away surrounding material or as a pointlike isolated region inside a semiconductor sandwich by pinching off a small volume of the material with electric fields from overlying metal electrodes. In such a system, as in atoms, quantum mechanics dictates that particles confined in a small enough space (roughly 10 nm for electrons in semiconductors) can assume only discrete energies. If quantum dots are artificial atoms, then a planar array of a million dots constitutes a sort of artificial lattice. Scientists have created such arrays but have not yet been able to control crystalline uniformity and the placement of electrodes sufficiently for studying the energy band structure in this system as one does for a "real" crystal. In addition to spatial-confinement effects, charge-quantization effects can also influence the behavior of dots. That is, dots can be made to accept only a single electron or just a few, and their coming and going can be monitored; as the voltage is turned up, new electrons are able to overcome the efforts of the electrons already on the dot to exclude newcomers through an electrostatic "Coulomb blockade." This dependence on individual electrons may make possible a range of new devices. (Physics Today, Jan. 1993; Scientific American, Jan. 1993; Science News, 20 Feb. 1993; The Economist, 27 Feb.)