ENHANCEMENT OF EXCITON EXCHANGE INTERACTION BY QUANTUM CONFINEMENT INCDSE NANOCRYSTALS

We have investigated the degree of linear polarization and time behavi our of photoluminescence of very small size CdSe nanocrystals under si ze-selective excitation. At low temperature we observe a luminescence line Stokes-shifted by a few meV and its phonon replica The degree of linear polarization is positive at short times then becomes negative a nd remains constant and equal to -15%. The following model is proposed . The exciton state is split by electron-hole exchange interaction int o two states with the optically forbidden state at lower energy. Laser light creates excitons in the upper energy state. The luminescence li ne arises from the lower energy state where excitons have relaxed afte r a few tens of ps. Recombination is made possible through a phonon as sisted virtual transition to the confined B-exciton state. As a result the polarization degree is negative at long times. The value of the e xchange energy obtained from the luminescence Stokes shift (8 meV in 4 0A-diameter nanocrystals) is much larger than the bulk value (0.1 meV) . The enhancement of exchange interaction by quantum confinement and t he value of the degree of polarization are in good agreement with theo retical estimates.