Liquid-like cationic sub-lattice in copper selenide clusters

Abstract

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.

Lithiation of Copper Selenide Nanocrystals

“…140 8 8C. [8] But, we recently found, [9] in line with findings in other nanostructures, [10][11][12][13][14][15][16] that Cu 2 Se nanocrystals (NCs) in the few-nm size range exhibit room-temperature SI behavior.A saresult, nanostructured Cu 2 Se has potential as asolid electrolyte for Li + batteries.As af urther advance toward this goal, we show that Li can be incorporated in SI Cu 2 Se NCs.High mole fractions of Li can be incorporated via substitution of Cu in the NCs,eventually resulting in the formation of lithium selenide (Li 2 Se) NCs. Such insight may be useful in the future for designing nanostructured SI electrolytes for Li + transport.…”

“…Structure of Li-doped NCs.P owder X-ray diffraction (pXRD) was employed to determine the lattice structure of the NCs (Figure 2and Figures S3-S8) at four stages of the Liexchange reaction. From pXRD,t he initial Cu 2Ày Se NCs are observed to possess the defect anti-fluorite structure known for bulk Cu 2 Se [32,33] as well as for NCs [9] of 6nmdiameter. The structure consists of an fcc Se sub-lattice and aCusub-lattice with ca.…”

“…6nmC u 2 Se NCs at room temperature,w ith stirring. We chose this size of Cu 2 Se NCs,because it is small enough for the SI phase to be stable near ambient temperature, [9] while being large enough for facile crystallographic characterization. In the cation exchange transformation, Cu + in the NCs is substituted by the added Li + ,while the Se 2À anionic sub-lattice is expected to remain untransformed.…”

“…In the cation exchange transformation, Cu + in the NCs is substituted by the added Li + ,while the Se 2À anionic sub-lattice is expected to remain untransformed. [9,20,28,29] Thecation exchange of Cu + by Li + was performed in steps by controlled addition of aliquots of the Li + precursor,u ntil no further replacement of Cu + by Li + was possible.Ateach stage in the titration, the NCs were characterized in terms of their structural properties,c omposition, and spectroscopic attributes.T his allowed us to follow the progress of Li-doping and track the evolution of structure and properties of the NC lattice in the course of lithiation.…”

“…0.2. Thef ormation of Cu vacancies is accompanied by the emergence of aw ell-defined localized surface plasmon resonance (LSPR) band [9,19,29,31] in the near-infrared (NIR) region of the spectrum (Figure 1a). We monitored this LSPR band as the cation exchange of the Cu 2Ày Se NCs with the Li + salt was performed step-wise.W ith increasing degree of Liexchange,t he LSPR band red-shifted and decreased in absorbance.U pon approaching the saturation degree of Liexchange,t he LSPR band was fully extinguished;o nly af eatureless spectrum with an absorption onset in the ca.…”