Gradiently Alloyed ZnxCd₁₋xS Colloidal Photoluminescent Quantum Dots Synthesized via a Noninjection One-Pot Approach

High-quality colloidal photoluminescent (PL) ZnCdS quantum dots (QDs) with gradient distribution of components, consisting of Cd-rich inner cores and Zn-rich outer shells, were synthesized via a noninjection one-pot approach. This newly developed synthetic approach uses zinc stearate (Zn(St) 2 ), cadmium acetate dihydrate (Cd(OAc) 2 ·2H 2 O), and elemental sulfur as Zn, Cd, and S source compounds, respectively. The growth of the cubic-structured QDs was carried out at 240 °C in a reaction flask consisting of the source compounds, together with stearic acid (SA), 2,2‘-dithiobisbenzothiazole (MBTS), and 1-octadecene (ODE); all of these chemicals were loaded at room temperature. The temporal evolution of the optical properties of the growing QDs, including absorption and photoemission, was monitored in detail; the evolution monitored indicates that the growth kinetics, the composition, and the distribution of the Zn and Cd in the resulting ternary Zn x Cd 1- x S QDs are sensitive to feed Zn−Cd−S molar ratios. Detailed investigation on the feed Zn−Cd−S molar ratio affecting the QD growth and optical property was performed, together with detailed structural characterization by solid-state NMR including cross polarization (CP) and dipolar dephased cross polarization (DDCP); also, the resulting QDs were characterized by XPS, TEM, and XRD. Accordingly, the synthesis-structure−property relationship was explored systematically. Tuning the feed Zn−Cd−S stoichiometry, high-quality Zn x Cd 1- x S QDs exhibiting sharp excitonic absorptions, narrow PL emissions with full width at half-maximum (fwhm) on the order of 19−21 nm, and high PL quantum yield (QY, up to 23%), could be fabricated with relatively low Zn and S feed amount, such as 1Zn−1Cd−1S feed molar ratio. In general, our noninjection approach features great synthetic reproducibility and is easy to scale up, with a plausible alloying mechanism of the formation of a Cd-rich core in the early growth stage, followed by a gradual formation of Zn-rich outer layers with increasing zinc content toward the surface.