Enhanced Emission from the Bright Exciton and Locating the Dark Exciton in Strained CdSe/Cd_xZn_1–xSe QDs

Colloidal CdSe quantum dots (QDs) designed with a high degree of asymmetric internal strain have recently been shown to possess a number of desirable optical properties including subthermal room-temperature line widths, suppressed spectral diffusion, and high photoluminescence (PL) quantum yields. It remains an open question, however, whether they are well-suited for applications requiring emission of identical single photons. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that, in asymmetrically strained CdSe QDs, over six times more light is emitted directly by the bright exciton. We measure the low-temperature PL dynamics and the polarization-resolved fluorescence line narrowing spectra from ensembles of these strained QDs. Our spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. These results are therefore encouraging for the prospects of chemically synthesized colloidal QDs as emitters of single indistinguishable photons.