Surface Engineering in Alloyed CdSe/CdSexCdS1–x/CdS Core-Shell Colloidal Quantum Dots for Enhanced Optoelectronic Applications

Abstract

The optical properties of nanocrystal (NC) colloidal quantum dots (QDs) largely depend on their size and shape. These properties can be easily tuned by temperature and the concentration of ligands during their synthesis to generate QD particles with different optical features. However, the enormous complexity of these QD systems limits the understanding of the critical impact of passivating ligands and surface defects on their optical properties. In the present study, we systematically investigated the effect of different strategies on the optical properties of alloyed CdSe/CdSexCdS1−x/CdS core shell (CS) QDs capped with several ligands, including trioctylphosphine (TOP) and hexadecylamine (HDA). The CdSe covered with TOP ligands were produced using the hot injection method, whereas CdSe covered with HDA ligands were produced by the exchange reaction method from as-synthesized samples. The CdSe/CdSexCdS1−x/CdS QDs samples were prepared from a simple chemical route that involved an increasing concentration of thioglycerol to grow the CdS shell on the top of the as-precipitated CdSe core with different ligands in a controlled manner. Two emission peaks (at approximately 595 and 635 nm) were observed for three different surface coverages beyond the exciton recombination. These emissions were mainly attributed to the surface localized state in all samples and the charge carrier transfer between the exciton and surface states. Our findings revealed an increase in the photoluminescence (PL) intensity with increasing temperature for the alloyed CdSe/CdSexCdS1−x/CdS CS QDs. The findings also revealed a continuous red-shift in the optical absorption peak, as a function of ligand concentration. This suggests a strong electronic coupling between the surface localized states and delocalized excitonic alloyed CdSe/CdSexCdS1−x/CdS CS QDs. However, such colloidal nanocrystals (NCs) need to be further investigated to gain an in-depth understanding of their nanoscale behavior as well as explore their huge potential for several emerging technological applications.