Gradiently Alloyed Zn_xCd_(1-x)S 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)2·2H2O),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_xCd_(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_xCd_(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.