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Super-ionic solids, which exhibit ion mobilities as high as those in liquids or molten salts, have been employed as solid-state electrolytes in batteries, improved thermoelectrics and fast-ion conductors in super-capacitors and fuel cells. Fast-ion transport in many of these solids is supported by a disordered, ‘liquid-like' sub-lattice of cations mobile within a rigid anionic sub-lattice, often achieved at high temperatures or pressures via a phase transition. Here we show that ultrasmall clusters of copper selenide exhibit a disordered cationic sub-lattice under ambient conditions unlike larger nanocrystals, where Cu+ ions and vacancies form an ordered super-structure similar to the bulk solid. The clusters exhibit an unusual cationic sub-lattice arrangement wherein octahedral sites, which serve as bridges for cation migration, are stabilized by compressive strain. The room-temperature liquid-like nature of the Cu+ sub-lattice combined with the actively tunable plasmonic properties of the Cu2Se clusters make them suitable as fast electro-optic switches.

Co-operativity is a remarkable phenomenon mostly seen in biology, where initial reaction events significantly alter the propensity of subsequent reaction events, giving rise to a nonlinear tightly regulated synergistic response. Here we have found unique evidence of atomic level co-operativity in an inorganic material. A thousand-atom nanocrystal (NC) of the inorganic solid cadmium selenide exhibits strong positive co-operativity in its reaction with copper ions. A NC doped with a few copper impurities becomes highly prone to be doped even further, driving an abrupt transition of the entire NC to the copper selenide phase, as manifested by a strongly sigmoidal response in optical spectroscopy and electron diffraction measurements. The examples presented here suggest that cooperative phenomena may have an important role in the solid state, especially in the nucleation of new chemical phases, crystal growth, and other materials' transformations.

We review the discovery of localized surface plasmon resonances (LSPRs) in doped semiconductor quantum dots (QDs), an advance that has extended nanoplasmonics to materials beyond the classic gamut of noble metals. The initial demonstrations of near‐infrared LSPRs in QDs of heavily self‐doped copper chalcogenides and conducting metal oxides are setting the broad stage for this new field. We describe the key properties of QD LSPRs. Although the essential physics of plasmon resonances are similar to that in metal nanoparticles, the attributes of QD LSPRs represent a paradigm shift from metal nanoplasmonics. Carrier doping of quantum dots allows access to tunable LSPRs in the wide frequency range from the THz to the near‐infrared. Such composition or carrier density tunability is unique to semiconductor quantum dots and not achievable in metal nanoparticles. Most strikingly, semiconductor quantum dots allow plasmon resonances to be dynamically tuned or switched by active control of carriers. Semiconducting quantum dots thus represent the ideal building blocks for active plasmonics. A number of potential applications are discussed, including the use of plasmonic quantum dots as ultrasmall labels for biomedicine and electrochromic materials, the utility of LSPRs for probing nanoscale charge dynamics in semiconductors, and the exploitation of strong coupling between photons and excitons. Further advances in this field necessitate efforts toward generalizing plasmonic phenomena to a wider range of semiconductors, developing strategies for achieving controlled levels of doping and stabilizing them, investigating the spectroscopy of these systems on a fundamental level, and exploring their integration into optoelectronic devices.

No abstract

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 Cu 2 Se 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.

In analogy with benzene-derived molecular structures, we construct plasmonic metamolecules by attaching Au nanospheres to specific sites on a hexagonal Au nanoplate. We employ a ligand exchange strategy that allows regioselective control of nanosphere attachment and study resulting structures using correlated electron microscopy/optical spectroscopy at the single-metamolecule level. We find that plasmonic coupling within the resulting assembly is strongly dependent on the structure of the metamolecule, in particular the site of attachment of the nanosphere(s). We also uncover a synergy in the polarizing effect of multiple nanospheres attached to the nanoplate. Regioselective control of plasmonic properties demonstrated here enables the design of novel structure-dependent electromagnetic modes and applications in three-dimensional spatial nanosensors.

The tripodal heptadentate Schiff base trensal3– ligand allowed the synthesis and characterization of stable uranyl(v) complexes presenting UO2+···K+ or UO2+···Fe2+ cation–cation interactions. The presence of Fe2+ bound to the uranyl(v) oxygen leads to increased stability with respect to proton induced disproportionation and to an increased range of stability of the uranyl(v) species with respect both to oxidation and reduction reactions.