A two-dimensional type I superionic conductor.

Superionic conductors possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I superionic conductors (for example, AgI, Ag2Se and so on) are defined by a first-order transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a two-dimensional type I superionic conductor, α-KAg3Se2, by scattering techniques and complementary simulations. Quasi-elastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag+ ions are confined to subnanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that compose the immobile charge-balancing layers. Our work thus extends the known classes of superionic conductors and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.

The Subchalcogenides Ir2In8Q (Q = S, Se, Te): Dirac Semimetal Candidates with Re-entrant Structural Modulation.

Subchalcogenides are uncommon compounds where the metal atoms are in unusually low formal oxidation states. They bridge the gap between intermetallics and semiconductors and can have unexpected structures and properties because of the exotic nature of their chemical bonding as they contain both metal-metal and metal-main group (e.g., halide, chalcogenide) interactions. Finding new members of this class of materials presents synthetic challenges as attempts to make them often result in phase separation into binary compounds. We overcome this difficulty by utilizing indium as a metal flux to synthesize large (millimeter scale) single crystals of novel subchalcogenide materials. Herein, we report two new compounds Ir2In8Q (Q = Se, Te) and compare their structural and electrical properties to the previously reported Ir2In8S analogue. Ir2In8Se and Ir2In8Te crystallize in the P42/mnm space group and are isostructural to Ir2In8S, but also have commensurately modulated (with q vectors q = 1/6a* + 1/6b* and q = 1/10a* + 1/10b* for Ir2In8Se and Ir2In8Te, respectively) low-temperature phase transitions, where the chalcogenide anions in the channels experience a distortion in the form of In-Q bond alternation along the ab plane. Both compounds display re-entrant structural behavior, where the supercells appear on cooling but revert to the original subcell below 100 K, suggesting competing structural and electronic interactions dictate the overall structure. Notably, these materials are topological semimetal candidates with symmetry-protected Dirac crossings near the Fermi level and exhibit high electron mobilities (∼1500 cm2 V-1 s-1 at 1.8 K) and moderate carrier concentrations (∼1020 cm-3) from charge transport measurements. This work highlights metal flux as a synthetic route to high quality single crystals of novel intermetallic subchalcogenides with Dirac semimetal behavior.

A Natural 2D Heterostructure [Pb3.1Sb0.9S4][Au xTe2- x] with Large Transverse Nonsaturating Negative Magnetoresistance and High Electron Mobility.

We report the two-dimensional (2D) natural heterostructure [Pb3.1Sb0.9S4][Au xTe2- x] ( x = 0.52-0.36) which shows anomalous, transverse nonsaturating negative magnetoresistance (MR). For x = 0.52, the material has a commensurately modulated structure with alternating [Pb3.1Sb0.9S4] rocksalt layers and atomically thin [Au xTe2- x] sheets, as determined by single-crystal X-ray diffraction using a (3 + 1)-dimensional space group; for other x compositions, the modulated structure is absent and the Au and Te atoms are disordered. The transport properties in this system at low temperature (<100 K) are dominated by an unusual 2D hopping mechanism, while at room temperature a high carrier mobility of ∼1352 cm2 V-1 s-1 is obtained ( x = 0.36). The confined electrons within the [Au xTe2- x] layers are also exposed to interlayer coupling with the insulating [Pb3.1Sb0.9S4] layers, and as a result, the properties of the heterostructures emerge not only from the constituent layers but also the interactions between them. Furthermore, the various Au and Te coordination patterns found in the [Au xTe2- x] sheets as a function of x further contribute to a unique electronic structure that leads to the anomalous nonsaturating negative MR with different field dependent behaviors. First-principles calculations indicate that the [Au xTe2- x] sheets are responsible for the unusual electrical transport properties in this 2D system.

A New Three-Dimensional Subsulfide Ir2In8S with Dirac Semimetal Behavior.

Dirac and Weyl semimetals host exotic quasiparticles with unconventional transport properties, such as high magnetoresistance and carrier mobility. Recent years have witnessed a huge number of newly predicted topological semimetals from existing databases; however, experimental verification often lags behind such predictions. Common reasons are synthetic difficulties or the stability of predicted phases. Here, we report the synthesis of the type-II Dirac semimetal Ir2In8S, an air-stable compound with a new structure type. This material has two Dirac crossings in its electronic structure along the Γ-Z direction of the Brillouin zone. We further show that Ir2In8S has a high electron carrier mobility of ∼10 000 cm2/(V s) at 1.8 K and a large, nonsaturating transverse magnetoresistance of ∼6000% at 3.34 K in a 14 T applied field. Shubnikov de-Haas oscillations reveal several small Fermi pockets and the possibility of a nontrivial Berry phase. With its facile crystal growth, novel structure type, and striking electronic structure, Ir2In8S introduces a new material system to study topological semimetals and enable advances in the field of topological materials.

High Hole Mobility and Nonsaturating Giant Magnetoresistance in the New 2D Metal NaCu4Se4 Synthesized by a Unique Pathway.

The new compound NaCu4Se4 forms by the reaction of CuO and Cu in a molten sodium polyselenide flux, with the existence of CuO being unexpectedly critical to its synthesis. It adopts a layered hexagonal structure (space group P63/ mmc with cell parameters a = 3.9931(6) Å and c = 25.167(5) Å), consisting of infinite two-dimensional [Cu4Se4]- slabs separated by Na+ cations. X-ray photoelectron spectroscopy suggests that NaCu4Se4 is mixed-valent with the formula (Na+)(Cu+)4(Se2-)(Se-)(Se2)2-. NaCu4Se4 is a p-type metal with a carrier density of ∼1021 cm-3 and a high hole mobility of ∼808 cm2 V-1 s-1 at 2 K based on electronic transport measurements. First-principles calculations suggest the density of states around the Fermi level are composed of Cu-d and Se-p orbitals. At 2 K, a very large transverse magnetoresistance of ∼1400% was observed, with a nonsaturating, linear dependence on field up to 9 T. Our results indicate that the use of metal oxide chemical precursors can open reaction paths to new low-dimensional compounds.

KCu7P3: A Two-Dimensional Noncentrosymmetric Metallic Pnictide.

We report a 2D material, KCu7P3, with a noncentrosymmetric structure (trigonal space group P31m, a = 6.9637(2) Å, c = 24.1338 (10) Å), which forms both from a molten potassium polyphosphide flux and from the elements. This phase consists of infinite [Cu7P3]- layers with hexagonal P sheets separated by K+ ions. The structure of the layers is unique but related to both Cu3P and the CaCu4P2 structure-types. Single-crystal refinement reveals extensive disorder within the Cu3P-like slabs. KCu7P3 is paramagnetic and exhibits a room temperature resistivity of ∼335 µΩ cm with a metal-like temperature dependence. The metallic character is supported by density functional theory electronic structure calculations. Hall and Seebeck effect measurements yield p-type behavior with a hole mobility of ∼15 cm2 V-1 s-1 at 300 K and a carrier concentration on the order of 1021 cm-3. KCu7P3 is chemically stable in ambient conditions, as well as in aqueous neutral and acidic solutions.

Ag2Se to KAg3Se2: Suppressing Order-Disorder Transitions via Reduced Dimensionality.

We report an order-disorder phase transition in the 2D semiconductor KAg3Se2, which is a dimensionally reduced derivative of 3D Ag2Se. At ∼695 K, the room temperature ß-phase (CsAg3S2 structure type, monoclinic space group C2/ m) transforms to the high temperature α-phase (new structure type, hexagonal space group R3̅ m, a = 4.5638(5) Å, c = 25.4109(6) Å), as revealed by in situ temperature-dependent X-ray diffraction. Significant Ag+ ion disorder accompanies the phase transition, which resembles the low temperature (∼400 K) superionic transition in the 3D parent compound. Ultralow thermal conductivity of ∼0.4 W m-1 K-1 was measured in the "ordered" ß-phase, suggesting anharmonic Ag motion efficiently impedes phonon transport even without extensive disordering. The optical and electronic properties of ß-KAg3Se2 are modified as expected in the context of the dimensional reduction framework. UV-vis spectroscopy shows an optical band gap of ∼1 eV that is indirect in nature as confirmed by electronic structure calculations. Electronic transport measurements on ß-KAg3Se2 yielded n-type behavior with a high electron mobility of ∼400 cm2 V-1 s-1 at 300 K due to a highly disperse conduction band. Our results thus imply that dimensional reduction may be used as a design strategy to frustrate order-disorder phenomena while retaining desirable electronic and thermal properties.

Bandgap engineering of Fe2O3 with Cr - application to photoelectrochemical oxidation.

Hematite is a promising material for photoelectrochemical (PEC) water splitting. While it has a low bandgap of ∼2.1 eV it is still larger than the optimal value of ∼1.8 eV. Previous work on epitaxial films has shown that Cr-doping leads to a shift of the bandgap as measured optically, but more importantly, also as measured by photoconductivity - to a value as low as 1.6 eV. We extend this work to polycrystalline films and attempt to use Cr-doping to lower the photon energy for which photocurrent can be generated. Our polycrystalline films show strong agreement with epitaxial films with regards to optical measurements of the direct and indirect bandgap. Furthermore, we find that Cr-doped polycrystalline films show photoconductivity at notably lower photon energies than undoped films, consistent with epitaxial results. However, when using Cr-doped films for photoelectrochemistry we find little to no shift of the photocurrent onset. We outline a number of proposals for why this could be the case, with a focus on the possibility of the existence of separate O 2p and Cr 3d states that would impact PEC but not PC behaviour.

A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals.

Doped BiVO4 is a promising photoelectrochemical water splitting anode, whose activity is hampered by poor charge transport. Here we use a set of X-ray spectroscopic methods to probe the origin and nature of localized electron states in W:BiVO4. Furthermore, using the polarized nature of the X-rays, we probe variations in the electronic structure along the crystal axes. In this manner, we reveal aspects of the electronic structure related to electron localization and observations consistent with conductivity anisotropy between the ab-plane and c-axis. We verify that tungsten substitutes as W6+ for V5+ in BiVO4. This is shown to result in the presence of inter-band gap states related to electrons at V4+ sites of e symmetry. The energetic position of the states in the band gap suggest that they are highly localized and may act as recombination centres. Polarization dependent X-ray absorption spectra reveal anisotropy in the electronic structure between the ab-plane and c-axis. Results show the superior hybridization between V 3d and O 2p states, higher V wavefunction overlap and broader conduction bands in the ab-plane than in the c-axis. These insights into the electronic structure are discussed in the context of existing experimental and theoretical reports regarding charge transport in BiVO4.

Illuminating Polysulfide Distribution in Lithium Sulfur Batteries; Tracking Polysulfide Shuttle Using Operando Optical Fluorescence Microscopy.

High-energy-density lithium sulfur (Li-S) batteries suffer heavily from the polysulfide shuttle effect, a result of the dissolution and transport of intermediate polysulfides from the cathode, into the electrolyte, and onto the anode, leading to rapid cell degradation. If this primary mechanism of cell failure is to be overcome, the distribution, dynamics, and degree of polysulfide transport must first be understood in depth. In this work, operando optical fluorescence microscope imaging of optically accessible Li-S cells is shown to enable real-time qualitative visualization of the spatial distribution of lithium polysulfides, both within the electrolyte and porous cathode. Quantitative determinations of spatial concentration are also possible at a low enough concentration. The distribution throughout cycling is monitored, including direct observation of polysulfide shuttling to the anode and consequent dendrite formation. This was enabled through the optimization of a selective fluorescent dye, verified to fluoresce proportionally with concentration of polysulfides within Li-S cells. This ability to directly and conveniently track the spatial distribution of soluble polysulfide intermediates in Li-S battery electrolytes, while the cell operates, has the potential to have a widespread impact across the field, for example, by enabling the influence of a variety of polysulfide mitigation strategies to be assessed and optimized, including in this work the LiNO3 additive.

Combined charge carrier transport and photoelectrochemical characterization of BiVO4 single crystals: intrinsic behavior of a complex metal oxide.

Bismuth vanadate (BiVO4) is a promising photoelectrode material for the oxidation of water, but fundamental studies of this material are lacking. To address this, we report electrical and photoelectrochemical (PEC) properties of BiVO4 single crystals (undoped, 0.6% Mo, and 0.3% W:BiVO4) grown using the floating zone technique. We demonstrate that a small polaron hopping conduction mechanism dominates from 250 to 400 K, undergoing a transition to a variable-range hopping mechanism at lower temperatures. An anisotropy ratio of ~3 was observed along the c axis, attributed to the layered structure of BiVO4. Measurements of the ac field Hall effect yielded an electron mobility of ~0.2 cm(2) V(-1) s(-1) for Mo and W:BiVO4 at 300 K. By application of the Gärtner model, a hole diffusion length of ~100 nm was estimated. As a result of low carrier mobility, attempts to measure the dc Hall effect were unsuccessful. Analyses of the Raman spectra showed that Mo and W substituted for V and acted as donor impurities. Mott-Schottky analysis of electrodes with the (001) face exposed yielded a flat band potential of 0.03-0.08 V versus the reversible H2 electrode, while incident photon conversion efficiency tests showed that the dark coloration of the doped single crystals did not result in additional photocurrent. Comparison of these intrinsic properties to those of other metal oxides for PEC applications gives valuable insight into this material as a photoanode.

High-pressure synthesis, structure, and photoluminescence of a new KSbO3-type bismuth germanate Bi3Ge3O10.5.

A new Bi(3)Ge(3)O(10.5) compound has been synthesized under high pressure, P = 7 GPa, and 700 °C. Instead of the pyrochlore that is normally stabilized under high pressure, the Bi(3)Ge(3)O(10.5) crystallizes in a KSbO(3)-ype crystal structure. The crystal structure has been refined by the Rietveld method from synchrotron X-ray diffraction data. Moreover, we have also characterized the Bi(3)Ge(3)O(10.5) by X-ray photoelectron spectroscopy, photoluminescence, and specific heat.

Silicon-Based Solid-State Batteries: Electrochemistry and Mechanics to Guide Design and Operation.

Solid-state batteries (SSBs) are promising alternatives to the incumbent lithium-ion technology; however, they face a unique set of challenges that must be overcome to enable their widespread adoption. These challenges include solid-solid interfaces that are highly resistive, with slow kinetics, and a tendency to form interfacial voids causing diminished cycle life due to fracture and delamination. This modeling study probes the evolution of stresses at the solid electrolyte (SE) solid-solid interfaces, by linking the chemical and mechanical material properties to their electrochemical response, which can be used as a guide to optimize the design and manufacture of silicon (Si) based SSBs. A thin-film solid-state battery consisting of an amorphous Si negative electrode (NE) is studied, which exerts compressive stress on the SE, caused by the lithiation-induced expansion of the Si. By using a 2D chemo-mechanical model, continuum scale simulations are used to probe the effect of applied pressure and C-rate on the stress-strain response of the cell and their impacts on the overall cell capacity. A complex concentration gradient is generated within the Si electrode due to slow diffusion of Li through Si, which leads to localized strains. To reduce the interfacial stress and strain at 100% SOC, operation at moderate C-rates with low applied pressure is desirable. Alternatively, the mechanical properties of the SE could be tailored to optimize cell performance. To reduce Si stress, a SE with a moderate Young's modulus similar to that of lithium phosphorous oxynitride (∼77 GPa) with a low yield strength comparable to sulfides (∼0.67 GPa) should be selected. However, if the reduction in SE stress is of greater concern, then a compliant Young's modulus (∼29 GPa) with a moderate yield strength (1-3 GPa) should be targeted. This study emphasizes the need for SE material selection and the consideration of other cell components in order to optimize the performance of thin film solid-state batteries.

Engineering Solution-Processed Non-Crystalline Solid Electrolytes for Li Metal Batteries.

Non-crystalline Li-ion solid electrolytes (SEs), such as lithium phosphorus oxynitride, can uniquely enable high-rate solid-state battery operation over thousands of cycles in thin film form. However, they are typically produced by expensive and low throughput vacuum deposition, limiting their wide application and study. Here, we report non-crystalline SEs of composition Li-Al-P-O (LAPO) with ionic conductivities > 10-7 S cm-1 at room temperature made by spin coating from aqueous solutions and subsequent annealing in air. Homogenous, dense, flat layers can be synthesized with submicrometer thickness at temperatures as low as 230 °C. Control of the composition is shown to significantly affect the ionic conductivity, with increased Li and decreased P content being optimal, while higher annealing temperatures result in decreased ionic conductivity. Activation energy analysis reveals a Li-ion hopping barrier of ≈0.4 eV. Additionally, these SEs exhibit low room temperature electronic conductivity (< 10-11 S cm-1) and a moderate Young's modulus of ≈54 GPa, which may be beneficial in preventing Li dendrite formation. In contact with Li metal, LAPO is found to form a stable but high impedance passivation layer comprised of Al metal, Li-P, and Li-O species. These findings should be of value when engineering non-crystalline SEs for Li-metal batteries with high energy and power densities.

Incorporation of Mo and W into nanostructured BiVO4 films for efficient photoelectrochemical water oxidation.

Porous, nanostructured BiVO4 films are incorporated with Mo and W by simultaneous evaporation of Bi, V, Mo, and W in vacuum followed by oxidation in air. Synthesis parameters such as the Bi : V : Mo : W atomic ratio and deposition angle are adjusted to optimize the films for photoelectrochemical (PEC) water oxidation. Films synthesized with a Bi : V : Mo : W atomic ratio of 46 : 46 : 6 : 2 (6% Mo, 2% W) demonstrate the best PEC performance with photocurrent densities 10 times higher than for pure BiVO4 and greater than previously reported for Mo and W containing BiVO4. The films consist of a directional, nanocolumnar layer beneath an irregular surface structure. Backside illumination utilizes light scattering off the irregular surface structure resulting in 30-45% higher photocurrent densities than for frontside illumination. To improve the kinetics for water oxidation Pt is photo-deposited onto the surface of the 6% Mo, 2% W BiVO4 films as an electrocatalyst. These films achieve quantum efficiencies of 37% at 1.1 V vs. RHE and 50% at 1.6 V vs. RHE for 450 nm light.

Ultralow Thermal Conductivity, Multiband Electronic Structure and High Thermoelectric Figure of Merit in TlCuSe.

The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m-1 K-1 ), a high power factor (11.6 µW cm-1 K-2 ), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe4 tetrahedra and long TlSe bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl+ lone-pair electrons, boosts phonon-phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl+ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Γ-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

Lattice Softening Significantly Reduces Thermal Conductivity and Leads to High Thermoelectric Efficiency.

The influence of micro/nanostructure on thermal conductivity is a topic of great scientific interest, particularly to thermoelectrics. The current understanding is that structural defects decrease thermal conductivity through phonon scattering where the phonon dispersion and speed of sound are assumed to remain constant. Experimental work on a PbTe model system is presented, which shows that the speed of sound linearly decreases with increased internal strain. This softening of the materials lattice completely accounts for the reduction in lattice thermal conductivity, without the introduction of additional phonon scattering mechanisms. Additionally, it is shown that a major contribution to the improvement in the thermoelectric figure of merit (zT > 2) of high-efficiency Na-doped PbTe can be attributed to lattice softening. While inhomogeneous internal strain fields are known to introduce phonon scattering centers, this study demonstrates that internal strain can modify phonon propagation speed as well. This presents new avenues to control lattice thermal conductivity, beyond phonon scattering. In practice, many engineering materials will exhibit both softening and scattering effects, as is shown in silicon. This work shines new light on studies of thermal conductivity in fields of energy materials, microelectronics, and nanoscale heat transfer.

Unravelling Small-Polaron Transport in Metal Oxide Photoelectrodes.

Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10(-4) and 10(-2) cm(2) V(-1) s(-1), respectively. Future directions to build a full picture of charge transport in this family of materials are discussed.

Microwell arrays reveal cellular heterogeneity during the clonal expansion of transformed human cells.

We developed micromolded microwell arrays to study the proliferation and senescence of single cells. Microwell arrays were designed to be compatible with conventional cell culture protocols to simplify cell loading, cell culture, and imaging. We demonstrated the utility of these arrays by measuring the proliferation and senescence of isogenic cells which expressed or had been depleted of the human Werner syndrome protein. Our results allowed us to reveal cell-to-cell heterogeneity in proliferation in WRN+ and WRN-depleted fibroblasts during clonal growth.