Temperature dependence of the photoluminescence properties of colloidal $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ core/shell quantum dots embedded in a polystyrene matrix

We report on the temperature dependence of the photoluminescence (PL) spectrum and of the PL relaxation dynamics for colloidal $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ core/shell quantum dots (QDs) embedded in an inert polystyrene matrix. We demonstrate that the confinement energy in the QDs is independent of the temperature. The coupling with both acoustic and optical phonons is also studied. Quantum confinement results in a strong increase of the exciton–acoustic-phonon coupling constant, up to $71\mu \mathrm{eV}∕\mathrm{K}$, and in a reduced exciton–longitudinal-optical (LO)-phonon coupling constant, down to $21\mathrm{meV}$, with respect to bulk CdSe. In addition, we demonstrate that the main nonradiative process that limits the quantum efficiency of the QD at room temperature is the thermal escape from the dot assisted by scattering with four LO phonons. Thermally activated trapping in surface states is also observed at low temperature, with an activation energy of about $15\mathrm{meV}$.

Figure 1

PL spectra as a function of the temperature; the spectra are vertically translated for clarity. Inset: PL spectrum at $T=45\mathrm{K}$ (continuous line) and the three best-fit Gaussians (dot-dashed lines).

Figure 2

PL peak energy (full dots) and FWHM (empty squares) as a function of the temperature. The lines are the best-fit curves as discussed in the text.

Figure 3

Integrated emission intensity as a function of the sample temperature. The dashed line is the best-fit curve with the model described in the text.

Figure 4

Absorption spectrum of the $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ core shell quantum dots in chloroform solution. The gray lines are the best-fit curves of the first six excited states and the total fit. Inset: Comparison between the absorption spectrum of the core/shell and the core samples.

Figure 5

Relaxation dynamics as a function of the sample temperature. The gray lines are the best-fit curves.

Figure 6

PL decay time as a function of $1∕k_{B}T$. The dotted line is the best-fit curve with the model described in the text.