Ion Exchange Transformation of Magic-Sized Clusters

Ultrasmall semiconductor clusters are exciting materials because of their molecularly precise structures and their unique optical spectra. “Magic-sized” CdSe clusters are transformed into their Cu₂Se counterparts by means of ion exchange. We leverage the molecularly precise structure and high sensitivity of these clusters to investigate the mechanism of cation exchange. We optically identify a metastable intermediate in the solid-state transformation. Isolation and characterization of this intermediate provide insight into the dynamic structural rearrangement of the cationic sublattice in the course of cation exchange and the role of ligand passivation. Such understanding of the dynamics of ion exchange at the solid–liquid interface could help engineer improved materials for solid-state electrolytes and energy storage devices

A Non-Natural Wurtzite Polymorph of HgSe: A Potential 3D Topological Insulator

This article demonstrates the power of topotactic synthesis coupled with density functional theory (DFT) for accessing and exploring new phases of matter. Naturally occurring HgSe is a semimetal with a zero gap. Unlike this natural zincblende form of HgSe, our DFT investigations predict that wurtzite HgSe has both an inverted band structure and a band gap, making it a 3D topological insulator (TI). Calculated band structures of Hg_<i>x</i>Cd_1–<i>x</i>Se alloys containing strongly relativistic Hg and weakly relativistic Cd show that band gap opening is a consequence of symmetry breaking resulting from a combination of crystal anisotropy and the scalar relativistic effect of Hg electrons. The relativistic contribution of Hg is significant enough in alloys with <i>x</i> ≥ 0.33 for achieving 3D TI behavior at room temperature. We experimentally realize the non-natural wurtzite form by topotactic ion exchange of wurtzite CdSe nanocrystals (NCs), which yields alloy NCs in the range <i>x</i> = 0–0.54 whose measured band gaps follow the predicted trend. We introduce crystal anisotropy as a new handle for expanding the classes of TI materials and also shed light on electronic principles in nanocrystalline alloys containing relativistic metals. NCs of this new wurtzite phase can become platforms for discovery of rich topological states and properties

Prioritizing Mentorship as Scientific Leaders

Prioritizing Mentorship as Scientific Leader