Highly luminescent silica-coated cd-based quantum dots with enhanced stability

Abstract

In this work, facile synthesis of bright CdS/CdSe/CdS@$SiO_2$ nanoparticles with 72% of quantum yields (QYs) retaining ca. 80% of the original QYs is presented. The main innovative point is the utilization of the highly luminescent CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) as silica coating seeds. The significance of inorganic semiconductor shell passivation and structure design of quantum dots (QDs) for obtaining bright QD@$SiO_2$ is demonstrated by applying silica encapsulation via reverse micro-emulsion method to three kinds of QDs with different structure: CdSe core and 2 nm CdS shell (CdSe/CdS-thin)

CdSe core and 6 nm CdS shell (CdSe/CdS-thick) and CdS core, CdSe intermediate shell and 5 nm CdS outer shell (CdS/CdSe/CdS-SQW). Inevitably lower photoluminescence quantum yield (PL QY) than pristine QDs Due to formation of surface defects.However, the retaining ratio of pristine QY is different in the three silica coated samples

CdSe/CdS-thin/$SiO_2$ shows the lowest retain ratio (36%) while the retaining ratio of pristine PL QY in CdSe/CdS-thick/$SiO_2$ (84%) and SQW/$SiO_2$ (80%) appears to be. Thick outermost CdS shell isolates the excitons from the defects at surface, making PL QY relatively insensitive to silica encapsulation. The bright $SiO_2$-coated SQW sample shows robustness against harsh conditions, such as acid etching and thermal annealing. The high luminescence and long-term stability highlights the potential of using the SQW/SiO2 nanoparticles in bio-labelling or display applications.