Femtosecond time-resolved electron-hole dynamics and radiative transitions in the double-layer quantum well of theCdS/(HgS)2/CdSquantum-dot–quantum-well nanoparticle

Abstract

The femtosecond time-resolved exciton dynamics of the ${\mathrm{C}\mathrm{d}\mathrm{S}/\left(\mathrm{HgS}\right)\mathrm{}}_2/\mathrm{C}\mathrm{d}\mathrm{S}$ quantum-dot–quantum-well system (QDQW), which contains a double-layer HgS QW, was investigated and compared to the dynamics of the QDQW system with a single-layer HgS QW. The femtosecond hole-burning technique allowed us to resolve the energy of the different optically allowed excitonic states involved in the ultrafast relaxation pathway. The experimentally obtained exciton energies were in excellent agreement with the previously theoretically predicted values. The femtosecond time-resolved pump-probe measurements reveal a fast relaxation component of ∼5 ps at wavelengths ⩽700 nm. At longer wavelengths, a slow decay component is found, which increases in decay time with increasing wavelength. The fast decay component (5 ps) was attributed to an energy relaxation process of the two ${1P}_e{-1P}_h$ exciton states, whereas the slow one was assigned to the decay of the dim ${1S}_e{-1S}_{3/2}$ state. The inhomogeneously broadened absorption band and the wide distribution of decay times in the low-energy region give strong evidence for a broad inhomogeneous energy distribution of the lowest energetic ${1S}_e{-1S}_{3/2}$ dim state. This is discussed in terms of the morphological structure of the quantum well.