Publication date: 21st July 2025
Zero-dimensional copper halides Cs₃Cu₂X₅ (X = Cl, Br, I) are promising materials for optoelectronic applications due to their high photoluminescence efficiency, stability, and large Stokes shifts [1]. In this work, we use density functional theory to uncover the chemical bonding origin of the Stokes shift in these materials.
Upon excitation, the [Cu₂X₅]³⁻ cluster undergoes strong local distortions, including shortened Cu–Cu and Cu–X bonds. These structural changes are driven by the formation of a self-trapped exciton, where a hole localizes on Cu(d) orbitals [2-3]. Analysis of the electronic structure and -pCOHP reveals reduced antibonding interactions and enhanced bonding character in the excited state, stabilizing the distorted geometry.
Our results establish a direct link between orbital-specific hole localization, bonding rearrangement, and the resulting Stokes shift. This provides a fundamental understanding of the excitation mechanism in Cs₃Cu₂X₅ and offers design principles for tuning optical properties in 0D copper halides.
Z.W. and S.T. acknowledge funding from Vidi (project no. VI.Vid.213.091) from the Dutch Research Council (NWO).
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