X-ray absorption spectroscopy insights on the structure anisotropy and charge transfer in Chevrel Phase chalcogenides.

The electronic structure and local coordination of binary (Mo6T8) and ternary Chevrel Phases (MxMo6T8) are investigated for a range of metal intercalant and chalcogen compositions. We evaluate differences in the Mo L3-edge and K-edge X-ray absorption near edge structure across the suite of chalcogenides MxMo6T8 (M = Cu, Ni, x = 1-2, T = S, Se, Te), quantifying the effect of compositional and structural modification on electronic structure. Furthermore, we highlight the expansion, contraction, and anisotropy of Mo6 clusters within these Chevrel Phase frameworks through extended X-ray absorption fine structure analysis. Our results show that metal-to-cluster charge transfer upon intercalation is dominated by the chalcogen acceptors, evidenced by significant changes in their respective X-ray absorption spectra in comparison to relatively unaffected Mo cations. These results explain the effects of metal intercalation on the electronic and local structure of Chevrel Phases across various chalcogen compositions, and aid in rationalizing electron distribution within the structure.

Operando XAFS on Hydrated Calcium Vanadium Bronze Cathode for Aqueous Zn-ion Storage.

Multivalent ion storage and aqueous electrochemical systems continue to build interest for energy application. The Zn-ion system with 2 electron transfer and an ideal metal anode is a strong candidate but is still at the early stage of development. Using both in situ near-edge (XANES) and X-ray absorption fine structure spectroscopy, EXAFS, a nanostructured cathode material, Cax V2 O5 -H2 O (CVO), was probed at the V-K absorption edge. This operando study reveals the local electronic and geometric structure changes for CVO during galvanostatic cycling as the active material in an aqueous Zn-ion cell. The XANES data provides a fine resolution to track the evolution of the vanadium oxidative state and near-neighbor coordination sphere showing subtle shifts and delocalized charge. The Zn-ion influence on the V-K absorption edge is visualized using a difference technique called Δµ. Coupled with theoretical calculations and modelling, the extended region extracted local bonding information further confirms excellent electronic and structural reversibility of this vanadium oxide bronze in an aqueous Zn-ion electrochemical cell.

Imaging local electronic corrugations and doped regions in graphene.

Electronic structure heterogeneities are ubiquitous in two-dimensional graphene and profoundly impact the transport properties of this material. Here we show the mapping of discrete electronic domains within a single graphene sheet using scanning transmission X-ray microscopy in conjunction with ab initio density functional theory calculations. Scanning transmission X-ray microscopy imaging provides a wealth of detail regarding the extent to which the unoccupied levels of graphene are modified by corrugation, doping and adventitious impurities, as a result of synthesis and processing. Local electronic corrugations, visualized as distortions of the π*cloud, have been imaged alongside inhomogeneously doped regions characterized by distinctive spectral signatures of altered unoccupied density of states. The combination of density functional theory calculations, scanning transmission X-ray microscopy imaging, and in situ near-edge X-ray absorption fine structure spectroscopy experiments also provide resolution of a longstanding debate in the literature regarding the spectral assignments of pre-edge and interlayer states.

Colossal above-room-temperature metal-insulator switching of a Wadsley-type tunnel bronze.

We show evidence of the manifestation of enormous metal-insulator switching ranging up to almost six orders of magnitude in individual nanowires of ß'-Cu(x)V(2)O(5). The magnitude and temperature of the phase transition is in strong contrast to data reported over several decades for other Wadsley-type mixed valence tunnel bronze structures.

Synthesis, spectroscopic characterization, and observation of massive metal-insulator transitions in nanowires of a nonstoichiometric vanadium oxide bronze.

Metal-insulator transitions in strongly correlated materials, induced by varying either temperature or dopant concentration, remain a topic of enduring interest in solid-state chemistry and physics owing to their fundamental importance in answering longstanding questions regarding correlation effects. We note here the unprecedented observation of a four-orders-of-magnitude metal-insulator transition in single nanowires of delta-K(x)V(2)O(5), when temperature is varied, which thus represents a rare new addition to the pantheon of materials exhibiting pronounced metal-insulator transitions in proximity to room temperature.

Synthesis, structural characterization, and electronic structure of single-crystalline Cu(x)V2O5 nanowires.

Single-crystalline copper vanadium oxide nanowires beta'-Cu(x)V(2)O(5) (x approximately 0.60) have been synthesized by the hydrothermal reduction of bulk CuV(2)O(6) using small-molecule aliphatic alcohols as reducing agents. The prepared copper vanadium bronze nanowires are metallic in nature and exhibit aspect ratios as high as 300. The recent discovery of superconductivity and charge disproportionation in bulk beta'-Cu(x)V(2)O(5) has led to renewed interest in these one-dimensional metallic systems. Scaling these systems to nanoscale dimensions offers the potential for further tunability of electronic transport and Li-ion intercalation kinetics. A combination of spectroscopic and electrical measurement methods has been used to provide evidence for the metallic nature and the presence of room-temperature charge disproportionation in these nanowires.