Supporting Information

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Supporting Information

New Insights into the Multiexciton Dynamics in Phase-Pure

Thick-Shell CdSe/CdS Quantum Dots

Lei Zhang, †_{Hao Li,} ‡_{Chen Liao,} †_{Hongyu Yang,} †_{Ruilin Xu,} † _{Xiaoshun Jiang,} *, ‡

Min Xiao, ‡_{Changgui Lu,} †_{Yiping Cui,} †_{and Jiayu Zhang}*, †

†_{Advanced Photonics Center, Southeast University, Nanjing 210096, Jiangsu, China} ‡_{National Laboratory of Solid State Microstructures, College of Engineering and}

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Synthesis of Phase-Pure Wurtzite CdSe/CdS Core/Shell QDs

Chemicals: Cadmium oxide (CdO, ≥99.99%), sulfer powder (S, 99.98%), octadecane (OD, 99%), oleylamine (OAm, 70%) ， oleic acid (OA, 90%), tri-n-octylphosphine (TOP, 97%) were purchased from Sigma-Aldrich. Selenium powder (Se, 99.999%), octadecylphosphonic acid (ODPA, 97%), tri-n-octylphosphine Oxide (TOPO, 98%) and 1-octadecene (ODE, 90%) were ordered from Alfa Aesar. All Chemicals were used as received without further purification.

Synthesis Process: Briefly, CdSe/CdS core/shell QDs were synthesized in two steps. First, WZ CdSe cores were synthesized at 380 °C following the established hightemperature procedure.1_{Then, the CdS shells were epitaxially grown on the}

surface of the CdSe cores by SILAR method.2_{Cd and S-precursors were prepared}

separately as a 0.1 M solution of Cd-oleate and S powder dissolved in 1-octadecene (ODE), respectively. The injection amounts of Cd and S-precursors required for each CdS monolayer (ML) was determined by the volume increment of every shell and QD’s concentration.

The above CdSe cores (300 nmol, dispersed in hexane solution), oleylamine (OAm, 6 mL) and octadecane (OD, 15 g) were mixed into a 150 mL three-neck flask and degassed under Ar flow at room temperature for 1 h and 120 °C for 30 min. After removal of the hexane, water, and oxygen, the temperature was further heated to 240 °C under an argon atmosphere, where the shell growth was performed. And then, Cd and S-precursors were alternately injected into the reaction flask. For the growth of 1–5 MLs CdS shell, each monolayer grown temperature increases 20 °C until up to 310 °C, and each injection with a period of 20 min. For the growth of 6–20 MLs CdS shell, the reaction temperature was kept at 310 °C, and the period of each injection was extended to 30 min in cycles of cadmium and sulfer deposition. After finishing precursor infusion, the reaction mixture was annealed at 310 °C for 1 hour and then was cooled to room temperature by an ice-water bath. The final WZ CdSe/CdS QDs were purified with acetone and methanol about 2–3 times, subsequently redispersed in hexaneto form a long-term stable solution for measurement.

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Figure S1. (a) PL QYs as a function of CdS shell thickness. PL intensity of ensemble CdSe/CdS QDs with 5 MLs (b) and 15 MLs (c) of CdS shell, measured under lower laser power density P and fitted with the function ∝ 1 – e−δP_,3_{where δ is a fitting}

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Figure S2. The emission spectral of CdSe/CdS QDs (11MLs) in time range of 0−1 ns

(a) and 1−50 ns (b), measured with the increase of pump intensity. The results show

that the emission intensity is increased with pump intensity, which is in direct proportion to the emission quantum yield, indicating that the fast decay component is mainly dominate by a robust radiative recombination process rather than nonradiative decay process.

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Figure S3. The relationships between emission intensity (Ie) and power density (Ip) of

CdSe/CdS core/shell QDs (11 MLs) in hexane solution. Figure 3a shows the PL saturation curve of CdSe/CdS QDs (11 MLs) under the lower pump levels (< 10 mW cm-2_{), which corresponds to the lower power intensity as shown here (Figure S3).}

However, with the increase of power intensity (> 10 mW cm-2_{), the emission intensity}

does not keep the theoretical relationship ∝ 1 – e−jσ_{, indicating that the multiexciton}

states would influence the emission properties of QDs. The emission intensity would have a nonlinear increase, the red solid line shows the fitting curve by the form: Ie∝

Ip2.17,which is similar to the peculiarity of excitons superradiance.4 When the power

intensity further increased (> 37 mW cm-2_{), the saturation trend of emission intensity}

began to emerge, which may be induced by nonradiative losses of multiexcitons at higher pump intensity. Importantly, the process of emission intensity changing with power intensity is almost accorded with the <N> as a function of pump intensity for QDs (11 MLs) shown in Figure 3c, indicating that the saturation phenomenon of emission intensity at higher pump intensities is associated with the saturation behavior of <N>. 0 10 20 30 40 50 60 I_e∝ I_p2.17 Emis sio n intensity (a.u.) Power density (mW cm-2) J

duoxiangshi (User) Fit of Sheet1 J

Model duoxiangsh_{i (User)} Equation A+B*x^C Reduced Chi-Sqr 27468.159 01 Adj. R-Squ 0.997 J A J B J C

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Figure S4. The fitting of PL decay trace using tri-exponential function and eq 5, respectively. The fitting results indicate that the fitting by eq 5 is comparable to that with tri-exponential function. The adjusted R2_{value is about 0.99,}_{indicating very high}

fidelity of the fitting procedure.

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WGMs Cavity Free Spectral Range (FSR) for the Microdisk Lasers

The free spectral range of WGM microcavity can be derived from the following equation5as:

FSR = 𝜆

2 2𝑛𝑒𝑓𝑓𝜋𝑅

Where λ is the emission wavelength, neff is the effective refractive index, and R is the

radius of the WGM microcavity. Using a radius of 40 μm, we can calculate a FSR of 1.07 nm (simulation neff = 1.5623).

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References

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[2] Liao, C.; Xu, R.; Xu, Y.; Zhang, C.; Xiao, M.; Zhang, L.; Lu, C.; Cui, Y.; Zhang, J. Ultralow-Threshold Single-Mode Lasing from Phase-Pure CdSe/CdS Core/Shell Quantum Dots. J. Phys. Chem. Lett. 2016, 7, 4968−4976.

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