Gradiently alloyed ZnxCd1-xS colloidal photoluminescent quantum dots synthesized via a noninjection one-pot approach

摘要

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)center dot 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 degrees C in a reaction flask consisting of the source compounds, together with stearic acid (SA), 2,2'-dithiobisbenzothiazole (MBTS), and 1d-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 ZnxCd1-xS 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 ZnxCd1-xS 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.