Number 227 (Story #1), May 22, 1995 by Phillip F. Schewe and Ben Stein ULTRANARROW LUMINESCENCE LINES FROM SINGLE QUANTUM DOTS. In general, the miniaturization of electronic devices not only saves space. If carried far enough it can also bring into play useful quantum phenomena (which lead to greater efficiency and control in the switching of currents). In a quantum well, for example, electrons are confined between two thin semiconductor layers in a two-dimensional zone. The electrons trapped there do not possess a continuum of energies but only a set of discrete energies. If one plotted the density of allowed electron states versus electron energy, the graph would be not a smooth curve but a series of steps. Nanofabrication techniques can be used to restrict further the electron's freedom; in a quantum wire electrons are restricted to one dimension and the density-of-states-vs-energy plot becomes more spiky than for the quantum well. Finally, when electrons are confined in all three dimensions, in a structure called a quantum dot, the allowable energies are completely quantized: that is, the spectrum of electron states corresponds to a set of specific energies. Nanometer-size dots can be produced and have been studied for several years. It has been difficult, however, to measure the discrete nature of electron energies in dots because often they are studied in ensembles whose properties, averaged over many dissimilar dots, are not precisely the same as for any one dot. Now a Berlin-Halle-St. Petersburg collaboration has observed luminescence (excited by a Ne-He laser) from single dots. The light from a 12-nm InAs dot appears, as expected, as ultranarrow lines, monoenergetic to less than 0.15 milli-electron-volts. The researchers claim that this represents the first direct evidence of the discrete nature of electron states in single nm-scale quantum dots. (M. Grundmannn et al., Physical Review Letters, 15 May.)