Discovery of Isomerization Intermediates in CdS Magic-Size Clusters

Organized assemblies of nanoparticles could benefit from dynamic tunable properties, such as controlled, reversible shifts in the electronic structure. Cadmium chalcogenide magic-size clusters (MSCs) have recently been found to exhibit the remarkable ability to undergo chemically-induced, reversible isomeric transformations between discrete states, with an 140 meV change in bandgap. This diffusionless reconfiguration of the inorganic core follows first order kinetics driven by distortions in ligand binding motifs. The isomerization of these ~1.5 nm CdS clusters bridges small molecule isomerization and large-scale solid-solid transformation, representing a paradigm shift in our understanding of inorganic materials. Our previous study demonstrated that methanol modifies the atomic and electronic structure of the native α-phase (324 nm) MSC to the β-phase (313 nm). Recently we have identified new intermediate states with smaller bandgaps, and larger overall shifts. We report the discovery of new intermediate phases in the isomerization process and a new functional group that facilitates the transformation. Amide organic functional groups not only facilitate the α-phase to β-phase isomerization, but also to evolve three other unique excitonic features. Pair distribution function (PDF) analysis, correlates these features into three intermediate MSC isomers: the β_o-phase, β₁-phase, and β₂-phase. All three of these phases resemble the final β-phase, but have variations in the fit parameters. The β₂-phase shows a nearly exact atomic correspondence with the β-phase despite a significant band gap difference (up to 583 meV). The β_o-phase and β₁-phase are red-shifted from the initial phase, but these bandgaps then then blue shift to reach the final β-phase. FTIR studies combined with density functional theory calculations reveal that the multifunctional nature of amides forms an amphoteric surface binding motif. This motif promotes a change from chelating to bridging binding modes. Kinetic studies indicate a thermodynamic stability trend of α-phase < β_o-phase < β-phases. This finding is surprising as the α-phase, although natively synthesized, is the least thermodynamically stable. These isomerization reactions provide an ideal testbed for investigating fundamental concepts in tunability for nanomaterial assemblies.