ENHANCEMENT OF ELECTRON-HOLE EXCHANGE INTERACTION IN CDSE NANOCRYSTALS - A QUANTUM CONFINEMENT EFFECT

Photoluminescence of CdSe nanocrystals in a glass matrix was investiga ted at low temperature with size-selective excitation. The emission sp ectrum consists of a line a few meV below the excitation laser energy (denoted the F line) and a two-phonon replica superimposed on a broadb and. The energy difference between the excitation energy and the F-lin e position increases with decreasing nanocrystal size. From the analys is of the time behavior of the luminescence and the degree of linear p olarization, we attribute the F line to the recombination of the optic ally forbidden A exciton. Radiation recombination is made possible thr ough a phonon-assisted virtual transition to the confined B-exciton st ate. The experimental degree of linear polarization is in good agreeme nt with the theoretical calculations. The value of the electron-hole e xchange energy obtained from the energy separation between the excitat ion energy and the F line is much larger than the bulk value and reach es 24 meV in 30-Angstrom-diam. nanocrystals. The size dependence of th e exchange energy is in good agreement with the theoretical prediction in the limit of small nanocrystals.