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.

Figure 1

Transient luminescence spectra, at different time delays, of $4\text{−}\mathrm{nm}$-diameter CdSe colloidal dots, along with the steady-state luminescence spectrum.

Figure 2

Photoluminescence of $3\text{−}\mathrm{nm}$-diam CdSe dots, $\sim 1\mathrm{ps}$ after excitation, at different pump-laser fluences, along with the steady-state spectrum.

Figure 3

Time-resolved luminescence profiles at different energies for a $3\text{−}\mathrm{nm}$ CdSe excited at a fluence of $2.4\mathrm{mJ}∕{\mathrm{cm}}^2$. The inset shows the time profiles at early times.

Figure 4

Luminescence spectrum of a $3\text{−}\mathrm{nm}$ dot, at time delays of $1\mathrm{ps}$ (dotted line) and $160\mathrm{ps}$ (solid line), excited at a fluence of $2.4\mathrm{mJ}∕{\mathrm{cm}}^2$. The symbols represent the energy-dependent pre-exponential factors associated with the decay times of band $A$ ($\sim 5\mathrm{ps}$, squares), band $B$ ($\sim 25\mathrm{ps}$, triangles), band $C$ ($\sim 10\mathrm{ns}$, diamonds), band $D$ ($3–4\mathrm{ps}$, inverted triangles). Note that the pre-exponential factors of the long-lived ns component fit very well the luminescence spectrum at $160\mathrm{ps}$. The inset compares the time-zero spectrum with the sum of the preexponential factors (points), including the rising component. The band at $\sim 2.64\mathrm{eV}$ in the inset is due to the Raman line of the solvent.