Giant exciton oscillator strength and radiatively limited dephasing in two-dimensional platelets

We measured the intrinsic ground-state exciton dephasing and population dynamics in colloidal quasi-two-dimensional (2D) CdSe nanoplatelets at low temperature (5–50 K) using transient resonant four-wave mixing in heterodyne detection. Our results indicate that below 20 K the exciton dephasing is lifetime limited, with the exciton population lifetime being as fast as 1 ps. This is consistent with an exciton lifetime given by a fast radiative decay due to the large in-plane coherence area of the exciton center-of-mass motion in these quasi-2D systems compared to spherical nanocrystals. The radiative rate in such 2D platelet systems can be controlled by the platelet area over orders of magnitude.

Figure 1

Linear optical properties of the investigated CdSe NPL ensemble. Absorption and photoluminescence spectra at 295 K (dashed-dotted lines), and absorbtion at 20 K with a fit (solid lines). The spectra of the laser pulses used in the FWM experiment are also shown, labeled according to their center photon energy. Inset: Transmission electron microscope (TEM) image of the NPLs. Scale bar: 50 nm.

Figure 2

Time-integrated (TI) FWM field amplitude vs ${\tau }_{12}$ at ${\tau }_{23}=1$ ps. The lines are fits to the data. (a) At different temperatures, as indicated, for a center energy of 2.552 eV. The inset shows a sketch of the pulse sequence and the directional selection. (b) For different center energies, as indicated (spectra given in Fig. 1), at a temperature of 20 K. The inset shows the resulting linewidths $2\hslash {\gamma }_{1,2}$.

Figure 3

FWM field amplitude vs ${\tau }_{23}$ at fixed ${\tau }_{12}$. Dashed lines are fits to the data. (a) For $T=12.5$ K for different values of ${\tau }_{12}$, as indicated. (b) For ${\tau }_{12}=0$ ps for different temperatures, as indicated. The data are offset for clarity, as indicated by the bars.

Figure 4

Homogeneous linewidths $2\hslash {\gamma }_{1,2,3}$, lifetime-limited linewidth $\hslash {\Gamma }_1$, and zero-phonon-line weight $Z$ vs temperature. The line is a fit to the data for ${\gamma }_2$. The inset shows the measured ${\gamma }_2$ at $T=5$ K as a function of the NPL area.