Atomic structure of CdS magic-size clusters by X-ray absorption spectroscopy

TL;DR

This study uses X-ray absorption spectroscopy to analyze the atomic structure of CdS magic-size clusters, revealing their composition, symmetry, and isomeric differences despite their ultra-small size.

Contribution

It demonstrates the application of EXAFS and XANES techniques to determine the atomic structure and isomerism of CdS magic-size clusters, which are challenging to characterize.

Findings

Both clusters have approximately 2:1 Cd:S ratio.

Clusters show a significant deviation from tetrahedral geometry.

Core structures are quasi-isomers with different atomic arrangements.

Abstract

Magic-size clusters are ultra-small colloidal semiconductor systems that are intensively studied due to their monodisperse nature and sharp UV-vis absorption peak compared with regular quantum dots. However, the small size of such clusters (<2 nm), and the large surface-to-bulk ratio significantly limit characterisation techniques that can be utilised. Here we demonstrate how a combination of EXAFS and XANES can be used to obtain information about sample stoichiometry and cluster symmetry. Investigating two types of clusters that show sharp UV-vis absorption peaks at 311 nm and 322 nm, we found that both samples possess approximately 2:1 Cd:S ratio and have similar nearest-neighbour structural arrangements. However, both samples demonstrate a significant departure from the tetrahedral structural arrangement, with an average bond angle determined to be around 106.1 degree showing a bi-fold bond angle distribution. Our results suggest that both samples are quazi-isomers. Their core structure has identical chemical composition but a different atomic arrangement with distinct bond angle distributions.