Fig. 2: Structural and optical characteristics of type-(I + II) QDs.
From: Colloidal quantum dots enable tunable liquid-state lasers
a, Energy levels of single exciton (blue) and biexciton (red) states in type-(I + II) QDs. To realize a stable hybrid XiXd biexciton, three conditions must be simultaneously met: Δd,i > kBT, Δdd,id > kBT and Δid,ii < kBT. b, Transmission electron microscopy image of type-(I + II) QDs with r = 2.6 nm, l = 1.7 nm, h = 2.2 nm and d = 0.3 nm. Scale bar, 20 nm. c, Series of absorption and PL spectra (dashed and solid lines, respectively) recorded at different stages of the synthesis of type-(I + II) CdSe QDs shown in b. From top to bottom: CdSe cores, CdSe/ZnSe QDs, CdSe/ZnSe/CdS QDs and CdSe/ZnSe/CdS/ZnS QDs. The PL spectrum of the final type-(I + II) QDs (red) can be presented as a sum of two Gaussian profiles centred at 2.02 eV and 1.95 eV. These bands correspond to the direct (blue) and indirect (gold) transitions, respectively. All the PL spectra were acquired using continuous-wave (cw) 2.76 eV excitation. d, Measured spectrally integrated PL dynamics of type-(I + II) QDs (grey circles) together with modelling (red line) performed using a three-level scheme shown in the inset. The best agreement between the simulation and experiment was obtained for the following parameters: τd = 44 ns, τi = 735 ns, τd-i = 85 ns and τi-d = 8.8 μs. e, Spectrally resolved PL dynamics of type-(I + II) QDs measured at 1.95 eV (yellow) and 2.02 eV (blue). The faster component is more pronounced in the trace recorded at 2.02 eV, that is, near the centre of the direct exciton band.
