Near-Unity Quantum Yields of Biexciton Emission from $\mathrm{CdSe}/\mathrm{CdS}$ Nanocrystals Measured Using Single-Particle Spectroscopy

Biexciton photoluminescence (PL) quantum yields ($Q_{2\mathrm{X}}$) of individual $\mathrm{CdSe}/\mathrm{CdS}$ core-shell nanocrystal quantum dots with various shell thicknesses are derived from independent PL saturation and two-photon correlation measurements. We observe a near-unity $Q_{2\mathrm{X}}$ for some nanocrystals with an ultrathick 19-monolayer shell. High $Q_{2\mathrm{X}}$’s are, however, not universal and vary widely among nominally identical nanocrystals indicating a significant dependence of $Q_{2\mathrm{X}}$ upon subtle structural differences. Interestingly, our measurements indicate that high $Q_{2\mathrm{X}}$’s are not required to achieve complete suppression of PL intensity fluctuations in individual nanocrystals.

Figure 1

(a) Pump-power dependence of the PL intensity for four single NQDs. Solid lines are the fits to the “PL-saturation” model which yield $Q_{2\mathrm{X}}\approx 0.14$ and 0.56 for 16- and 19-ML shell NQDs, respectively. Inset: NQD absorption cross sections plotted as a function of shell thickness. Pump-power-dependent PL spectra of (b) 16- and (c) 19-ML shell GNQDs showing the emergence of higher-energy emission bands (450–575 nm) at large $⟨N⟩$.

Figure 2

(a). The $g^{(2)}$ measurements for a single 19-ML shell GNQD at different pump intensities. The zero-delay signal still persists at $⟨N⟩<1$. (b) Total PL ($\Delta$) and time-integrated areas of $g_0^{(2)}$ (□) and $g_s^{(2)}$(○) plotted as a function of $⟨N⟩$. (c) Plot of $g_0^{(2)}/g_s^{(2)}$ vs $⟨N⟩$ approaches 0.18 (solid line) as $⟨N⟩\to 0$, in agreement with $Q_{2\mathrm{X}}=0.22\pm 0.07$ obtained from the PL-saturation analysis [dashed line in (b)].

Figure 3

(a)–(c) $g^{(2)}$ traces measured at $⟨N⟩=0.46$, 0.34, and 1 for NQD 1, 2, and 3, respectively. NQD1 and NQD2 have the shell thickness of 19 MLs and NQD3 has a 16-ML-thick shell. (d) The plot of $g_0^{(2)}/g_s^{(2)}$ vs $⟨N⟩$ gives $g_0^{(2)}/g_s^{(2)}$ of 0.60 (NQD1), 0.32 (NQD2), and 0.03 (NQD3) as the $⟨N⟩\to 0$. (e) The PL intensity saturation analysis for the same set of the NQDs yields $Q_{2\mathrm{X}}$ of $0.55\pm 0.07$, $0.28\pm 0.05$, and $<0.05$, respectively.

Figure 4

(a) A plot of the $Q_{2\mathrm{X}}$ vs $Q_{2\mathrm{X}}/Q_{1\mathrm{X}}$ for 13 different single GNQDs. Solid, dashed, dotted, and dash-dotted traces represent the simulations of $Q_{2\mathrm{X}}$ for the “PL-saturation” model for $Q_{1\mathrm{X}}$ of 1.0, 0.8, 0.5, and 0.3, respectively. (b) $Q_{2\mathrm{X}}$ obtained from the “PL-saturation” model for $>50$ individual NQDs (open symbols); average $Q_{2\mathrm{X}}$ for a given shell thickness are shown by solid circles.