Spectral and dynamical characterization of multiexcitons in colloidal CdSe semiconductor quantum dots

Femtosecond photoluminescence (PL) up-conversion studies in polychromatic mode and laser-power-dependence studies of the PL were carried out on colloidal solutions of CdSe quantum dots with diameters ranging from $2.4\text{to}4\mathrm{nm}$. The main features of the spectra consist, in addition to the band-gap luminescence, of three bands, one $120–130\mathrm{meV}$ and another $30–40\mathrm{meV}$ at low energy to the band-gap luminescence, while the third lies $220–260\mathrm{meV}$ higher in energy. The first band is attributed to a positively charged biexciton and the second to a neutral biexciton, while the blue band is attributed to a neutral triexciton. These assignments are based on the energetics, the lifetimes, and the pump-power dependence of the various bands. The energetics and lifetimes are in good agreement with the theoretical studies present in literature.