CHARGE SEPARATION EFFECTS ON THE RATE OF NONRADIATIVE RELAXATION PROCESSES IN QUANTUM DOTS QUANTUM-WELL HETERONANOSTRUCTURES

Using time-resolved optical hole (oh)-burning techniques with femtosec ond lasers, the time dependence of the spectral diffusion of the oh is examined for both the CdS quantum dot (QD) and the CdS/HgS/CdS quantu m dot quantum well (QDQW) nanoparticles. It is found that the nonradia tive relaxation of the optical hole is at least 3 orders of magnitude slower in the QDQW than in the QD system. Analysis of the second deriv ative of the broad transient bleach spectrum of the QDQW system in the 1.6-2.5 eV energy region at 50 fs delay time is found to have a minim um at 2.1 eV, corresponding to a minimum in the radiative probability. Around this energy, the rise and decay times of the transient bleach in the spectrum an found to change greatly. These results suggest that spectral diffusion in the QDQW is a result of relaxation from high- t o low-energy exciton states, involving an intervening dark state at an energy of similar to 2.0 eV. The energies of the maxima and minimum o f the second-derivative curve are found to be in good agreement with r ecent theoretical calculations by Jaskolski and Bryant(1) of the energ ies of the radiative and dark charge-separated state, respectively. In the latter, the hole is in the CdS clad and the electron is in the Hg S well. The slow nonradiative relaxation processes involving this stat e are expected to be slow owing to the large change in the charge carr ier effective masses as they cross from the CdS clad to the HgS well.