Determination of the localization times of electrons and holes in the HgS well in a CdS/HgS/CdS quantum dot–quantum well nanoparticle

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Abstract
The femtosecond time-resolved electron-hole dynamics of the CdS/HgS/CdS quantum dot–quantum well system (QDQW) was investigated as a function of excitation energy. In the transient absorption spectra four bleach bands and a stimulated emission signal in the visible spectral range between 450 and 780 nm were resolved. By using an IR probe pulse at 4.7 μm a transient induced absorption due to intraband transitions was found. The decay and rise times of these signals were measured when the CdS core or the HgS well of the nanoparticles was excited by the pump pulse. After excitation within the HgS well the transient signals rise within the resolution of our pump pulse, while after core excitation slower rise times were measured. From the 1.5 ps rise time of the stimulated emission originating from the HgS well and the intraband hole IR absorption (150 fs) after excitation into the CdS core, the electron localization time (transfer time from the core to the well) is found to be 1.5 ps while that of the hole is ∼150 fs. This large difference in the observed dynamics of the electron and hole in crossing the CdS/HgS interface is discussed.