The total electron charge density (shown in green) of a quantum dot of gallium arsenide, containing just 465 atoms. (Image: Lin-Wang Wang)
Here green shows the change in charge response when a single-electron perturbation is introduced into bulk gallium arsenide (left) and into a 465-atom quantum dot of the same material near its surface (right): except where the dots surface intervenes, the responses of the two systems are very similar. (Image: Lin-Wang Wang)
Quantum dots, tiny crystals consisting of a few hundred to a few thousand atoms, sparkle with promise for uses ranging from tagging proteins in living cells to foiling counterfeiters to enabling quantum computers. The optics and electronics of these semiconductor nanocrystals are dramatically different from the same materials in bulk. But it turns out that one of the most important electronic properties of quantum dots has been misunderstood for over a decade.
Theorists at the Department of Energy’s Lawrence Berkeley National Laboratory have shown that a quantum dot’s dielectric function (a term indicating how charge responds to an electric field) does not depend on its band gap, as researchers long believed. On the contrary, the dielectric function of a quantum dot, measured on the microscopic scale, is virtually the same as that of the bulk material -- except near the dot’s surface. "One of the interesting things about quantum dots is that their band gaps are much larger than the same material in bulk. At the same time their overall dielectric constants are much smaller," says Lin-Wang Wang of Berkeley Lab’s Computational Research Division. "Therefore it was natural to assume that the size of the band gap in a quantum dot is what determines its overall dielectric constant."
Recently French researchers led by Christophe Delerue of the Institut Supérieur d’Electronique du Nord raised doubts about this assumed relationship, however, basing their argument on approximate calculations. To test the questions posed by the French group, Wang and postdoctoral fellow Xavier Cartoixà performed, for the first time, ab initio ("from first principles") microscopic studies of the dielectric function in quantum dots. To do so they used PEtot, a quantum-mechanical electronic-structure program developed by Wang, on the Seaborg supercomputer at the Department of Energy’s National Energy Research Scientific Computing Center (NERSC), based at Berkeley Lab.
Paul Preuss | EurekAlert!
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