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Chemically reversible isomerization of inorganic clusters

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Science  15 Feb 2019:
Vol. 363, Issue 6428, pp. 731-735
DOI: 10.1126/science.aau9464

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Cluster isomerization

Structural rearrangements at the atomic scale can range from isomerization of small molecules to solid-solid phase transformations of crystals. Williamson et al. show that magic-size cadmium sulfide (CdS) crystalline clusters, which are about 2 nanometers in diameter and expose a large fraction of surface atoms capped by bidentate oleate ligands, undergo a reversible isomerization. The initial α-Cd37S20 phase, which has a wurtzite-like crystal structure, isomerizes to β-Cd37S20, which has a zinc blende–like structure upon exposure to methanol, and then transforms back under vacuum. This transition is driven by distortion of the ligand shell and shifts the excitonic energy gap of the clusters.

Science, this issue p. 731

Abstract

Structural transformations in molecules and solids have generally been studied in isolation, whereas intermediate systems have eluded characterization. We show that a pair of cadmium sulfide (CdS) cluster isomers provides an advantageous experimental platform to study isomerization in well-defined, atomically precise systems. The clusters coherently interconvert over an ~1–electron volt energy barrier with a 140–milli–electron volt shift in their excitonic energy gaps. There is a diffusionless, displacive reconfiguration of the inorganic core (solid-solid transformation) with first order (isomerization-like) transformation kinetics. Driven by a distortion of the ligand-binding motifs, the presence of hydroxyl species changes the surface energy via physisorption, which determines “phase” stability in this system. This reaction possesses essential characteristics of both solid-solid transformations and molecular isomerizations and bridges these disparate length scales.

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