Unveiling Zn Incorporation in CuInS2 Quantum Dots: X‑ray and Optical Analysis of Doping Effects, Structural Modifications, and Surface Passivation

TL;DR

This paper explores how zinc is incorporated into CuInS2 quantum dots and how it affects their structure and optical properties.

Contribution

The study introduces X-ray absorption spectroscopy to precisely characterize Zn incorporation and its effects on CuInS2 quantum dots.

Findings

X-ray absorption spectroscopy detected a ZnS surface shell and estimated its thickness.

Zn was identified as either a substituent or interstitial defect in the shell.

Hole transfer in CuInS2 quantum dots is sensitive to shell thickness.

Abstract

CuInS2 quantum dots (QDs) have gained significant
attention
owing to their remarkable broadband emission, making them desirable
for various optoelectronic applications requiring efficient luminescent
nanomaterials. However, maximizing radiative recombination in CuInS2 QDs 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, in both X-ray absorption
near-edge structure and extended X-ray absorption fine structure
spectral ranges, to investigate Zn incorporation into CuInS2 QDs, probing at the Zn, S, and Cu K-edges. This approach allowed
us to detect the formation of a ZnS surface shell, tentatively quantifying
its thickness, and to distinguish between Zn as a substituent at the
shell or as an interstitial defect. Additionally, we explored the
dynamical properties of CuInS2 QDs using time-resolved
optical spectroscopies, particularly in the presence of electron and
hole acceptors (benzoquinone and phenothiazine), observing that hole
transfer is highly sensitive to shell thickness. These results provide
deeper insights into the Zn-induced shell.

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