Fig. 3: Biexcitons in type-(I + II) QDs.

a, Pump-intensity-dependent PL spectra for a single type-(I + II) QD recorded using cw 2.54 eV excitation. The spectrum with the lowest pump intensity is decomposed into two bands corresponding to direct (X_d) and indirect (X_i) excitons. All the panels in this figure belong to the same type-(I + II) QD sample as that shown in Fig. 2. b, Single-dot measurements of biexciton PL decay (blue circles). These data were obtained using the Hanbury Brown-Twiss (HBT) experiment, which allows the identification of excitation cycles with two emitted photons and the derivation of the biexciton lifetime from the delay of the first photon relative to the pump pulse (inset). The red line represents a biexponential fit, which yields time constants of 13.5 ns and 83 ns for the fast (τ_f,XX) and slow (τ_s,XX) relaxation components, respectively. c, Biexciton lifetime as a function of effective QD volume for different QD types: plain CdSe core-only QDs^21,38,39,40 (black), core/alloy/shell (C/A/S) CdSe/CdSe_0.5S_0.5/CdS QDs¹ (green), CdSe/Cd_xZn_1−xSe/ZnSe_0.5S_0.5 cg-QDs^7,17 (blue) and type-(I + II) QDs from the present study (red) with CdSe core radii of 2.3 nm (square) and 2.6 nm (circle). The effective volumes were calculated from the size of the sphere that confines charge carriers with a probability of 99% (ref. ⁴¹). d, Single-dot measurements of the second-order intensity correlation (g⁽²⁾) function without photon discrimination (top, green line) and with photon discrimination (bottom, gold line) performed by selecting (‘gating’) photons with a time delay of more than 200 ns time relative to the pump pulse. The non-gated g⁽²⁾(0) averaged over 26 QDs, is 0.76 (derived from the measurements without photon discrimination). This value gives the ratio of the quantum yields of the biexciton and single-exciton emission. The fact that the gated g⁽²⁾(0) is close to zero confirms that the measurements were performed on a single QD, and not on a cluster containing several QDs.

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