Intrinsically\nSlow Cooling of Hot Electrons in CdSe\nNanocrystals Compared to CdS

The utilization of excited charge carriers in semiconductor\nnanocrystals\n(NCs) for optoelectronic technologies has been a long-standing goal\nin the field of nanoscience. Experimental efforts to extend the lifetime\nof excited carriers have therefore been a principal focus. To understand\nthe limits of these lifetimes, in this work, we theoretically study\nthe time scales of pure electron relaxation in negatively charged\nNCs composed of two prototypical materials: CdSe and CdS. We find\nthat hot electrons in CdSe have lifetimes that are 5 to 6 orders of\nmagnitude longer than in CdS when the relaxation is governed only\nby the intrinsic properties of the materials. Although these two materials\nare known to have somewhat different electronic structure, we elucidate\nhow this enormous difference in lifetimes arises from relatively small\nquantitative differences in electronic energy gaps and phonon frequencies,\nas well as the crucial role of Fröhlich-type electron–phonon\ncouplings.