Quantum dots stack into a 3-D array.

Tiny islands of a semiconductor material can spontaneously arrange themselves into a three-dimensional lattice, says Gunther Springholz of the Johannes Kepler University of Linz in Austria. The islands, known as quantum dots (SN: 4/11/98, p. 236), adopt a pattern similar to the atomic structure of many materials.

To make the lattice, Springholz and his colleagues lay down alternating layers of lead telluride sheets and arrays of pyramid-shaped dots of lead selenide. By controlling the thickness of the lead telluride spacer layers, they can control the three-dimensional arrangement that the dots will assume, which in turn influences their collective optical and electronic properties.

Spacer layers between 30 and 60 nanometers thick appear to produce the best results, Springholz says. If the spacers are too thin, the dots on individual layers arrange themselves randomly. If the layers are too thick, however, the dots don't align with those above and below.