Dipole-dipole interaction effect on the optical response of quantum dot ensembles

The effect of concentration on the optical absorption and photoluminescence (PL) spectra was studied for ensembles of chemically prepared semiconductor quantum dots (QDs). We found experimentally that, when the QD concentration increases, the excitonic absorption peaks slightly shift to the lower-energy side and broaden (both effects are of the order of 1 nm). The absorption spectra can be successfully modelled using the Maxwell-Garnett formalism and scale linearly with the QD concentration, without any significant change of the shape. This indicates a rather weak dipole-dipole interaction between the dots polarised by the electromagnetic wave in the visible spectral range. At the same time, we observed a sizable shift of the band-edge PL peak with the increase of the QD concentration. This effect is explained by assuming that, although weak, the dipole-dipole interaction is still sufficient to provide a mechanism for the energy transfer from smaller to larger dots within the size distribution. (C) 2003 Elsevier B.V. All rights reserved.