Unveiling Zn incorporation in CuInS$_2$ quantum dots: X-ray and optical analysis of doping effects, structural modifications and surface passivation

TL;DR

This study employs X-ray absorption spectroscopy and optical techniques to analyze zinc incorporation in CuInS₂ quantum dots, revealing how Zn affects surface passivation, structural modifications, and optical properties for improved luminescent performance.

Contribution

The paper introduces a novel element-specific X-ray spectroscopy approach to quantify and characterize Zn incorporation in CuInS₂ QDs, advancing understanding of doping effects and surface passivation.

Findings

Zn forms a surface shell passivating the QDs

Zn can substitute or interstitially incorporate into the lattice

Passivation improves photoluminescence efficiency

Abstract

Quantum dots (QDs) exhibit unique properties arising from their reduced size and quantum confinement effects, including exceptionally bright and tunable photoluminescence. Among these, CuInS$_{2}$ QDs have gained significant attention owing to their remarkable broadband emission, making them highly desirable for various optoelectronic applications requiring efficient luminescent nanomaterials. However, maximizing radiative recombination in CuInS$_{2}$ QDs often necessitates minimizing intragap trap states. A common approach involves the introduction of Zn during the synthesis, which typically promotes the formation of a ZnS shell that passivates the QD surface. Despite its importance, the characterization and quantification of Zn incorporation using conventional techniques, such as optical spectroscopy or electron microscopy, remains challenging. In this study, we utilized X-ray absorption spectroscopy (XAS), in both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectral ranges, to investigate Zn incorporation into CuInS$_{2}$ QDs with element-specific precision. This approach allowed us to detect the formation of a ZnS surface shell and to resolve the spatial distribution of Zn atoms within the QD lattice, distinguishing between Zn as a substituent, or as an interstitial defect. Additionally, we explored the optical and dynamical properties of CuInS$_{2}$ QDs using time-resolved optical spectroscopies, particularly in the presence of electron and hole acceptors. These results provide deeper insights into the role and effectiveness of the Zn-induced passivating layer, paving the way for optimizing QD performance in photoluminescence applications.