Photon-correlation Fourier spectroscopy of the trion fluorescence in thick-shell CdSe/CdS nanocrystals

The emission spectrum of the trion state in very thick shell CdSe/CdS nanocrystals is characterized at 4 K by photon correlation Fourier spectroscopy. A value of $50 \mu \mathrm{eV}$ for the width of the zero phonon line is measured. The absence of blinking and the high photostability of these emitters offer the possibility to investigate the dynamics of the emission spectrum at a time scale as short as 250 ns. We show that the high value of the linewidth $\left(50 \mu \mathrm{eV}\right)$ is not due to spectral diffusion induced by the close environment of the emitter at time scales larger than 250 ns. The broadening is attributed to the additional third carrier when compared to the monoexcitonic state.

Figure 1

(a) PL spectrum of a single NC at 4 K (acquisition time of 10 s, 600 grooves/mm grating). The zero phonon line (ZPL), the respective contributions of the CdSe and CdS (LO) phonon replica are indicated. For clarity, the LO phonon sideband is enhanced by a factor 3. Inset: photoluminescence decay of the fluorescence intensity. The red line is a monoexponential decay corresponding to a lifetime of 6 ns. (b) PL spectrum with a 1800 grooves/mm grating (acquisition time of 1 s) showing a sharp ZPL together with a broadened acoustic phonon tail. The red line is a fit with two Gaussians having a full width at half maximum (FWHM) of $150 \mu \mathrm{eV}$ and 1.8 meV. Note that the reference of the energy is 1.9211 eV.

Figure 2

Normalized value of $\sqrt{2G_n(t,\tau )}$ measured by PCFS for $\tau =1$ ms as a function of the time path difference (in semilog scale). The normalization is made with the value of $\sqrt{2G_n(t,\tau )}$ at zero delay. The red line corresponds to an exponential decay of 0.9 ps (1.5 meV).

Figure 3

Normalized value of $\sqrt{2G_n(t,\tau )}$ as a function of the temporal difference path for $\tau =10 \mu \mathrm{s}$ (in semilog scale). The normalization is made with the value of $\sqrt{2G_n(t,\tau )}$ at zero delay. The red line represents a linear fit corresponding to the delays $d_t$ above 3 ps and corresponds to a linewidth of $46 \mu \mathrm{eV}$.

Figure 4

(a) Normalized value of $\sqrt{2G_n(t,\tau )}$ as a function of the delay $\tau$ for $d_t=0.12$ ps $(•)$ and $d_t=13$ ps $\left(▴\right)$. $\sqrt{2G_n(t,\tau )}$ is normalized to the value of $\sqrt{2G_n(t,\tau )}$ measured at zero delay in Fig. 3.

Figure 5

(a) Evolution of ${\tilde{G}}_{ab}(t,\tau )/\overline{I^2\left(t\right)}$ as a function of $\tau$ at $d_t=67\mathrm{ps}$ for ${\mathrm{NC}}_1$. (b) Evolution of ${\tilde{G}}_{ab}(t,\tau )/\overline{I^2\left(t\right)}$ as a function of $\tau$ at $d_t=46\mathrm{ps}$ for ${\mathrm{NC}}_2$.