Bright Gradient-Alloyed CdSe_xS_1–x Quantum Dots Exhibiting Cyan-Blue Emission

Highly emissive alloyed CdSeS quantum dots (QDs) with a gradient structure exhibiting photoluminescence (PL) peaking at 490 nm and an absolute quantum yield (QY) of 79% (in toluene with excitation wavelength of 430 nm) were designed and synthesized. The cyan-blue emitters were synthesized at 180 °C in 1-octadecene (ODE) with cadmium oleate (Cd(OA) 2 ), tri- n -octylphosphine selenide (TOPSe), and tri- n -octylphosphine sulfide (TOPS) as the Cd, Se, and S precursors, respectively; importantly, a commercial secondary phosphine, diphenyl phosphine (DPP or HPPh 2 ), was used as a beneficial additive. Also, our high Cd/(Se + S) feed molar ratio aids in shifting the equilibrium of the chalcogenide exchange, TOPE + HPPh 2 ⇔ TOP + E═PPh 2 H, to the right. Density functional theory (DFT) calculations suggest that the formation of Se═PPh 2 H proceeds faster than that of S═PPh 2 H, which supports our high S/Se feed molar ratio used to synthesize the bright gradient-alloyed CdSeS QDs. Compositional and structural characterization was carried out using powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and solid-state nuclear magnetic resonance spectroscopy (NMR). Particularly, our solid-state 113 Cd NMR analysis reveals that the highly emissive CdSeS QDs consist of a three-domain structure with a Se-rich inner core region, a Se/S equivalent middle region, and a S-rich outer region. The present study highlights the importance on the use of secondary phosphines together with high cation/anion feed molar ratios in the rational design and synthesis of high-quality metal chalcogenide QDs at relatively low temperature but with high yield and reproducibility.