Band edge evolution of transparent Zn M2III O4 (MIII=Co, Rh, Ir) spinels

ZnMIII 2 O4 (MIII = Co, Rh, Ir) spinels have been recently identified as promising p-type semiconductors for transparent electronics. However, discrepancies exist in the literature regarding their fundamental optoelectronic properties. In this paper, the electronic structures of these spinels are directly investigated using soft/hard x-ray photoelectron and x-ray absorption spectroscopies in conjunction with density functional theory calculations. In contrast to previous results, ZnCo2O4 is found to have a small electronic band gap with forbidden optical transitions between the true band edges, allowing for both bipolar doping and high optical transparency. Furthermore, increased d-d splitting combined with a concomitant lowering of Zn s/p conduction states is found to result in a ZnCo2O4 (ZCO) < ZnRh2O4 (ZRO) ≈ ZnIr2O4 (ZIO) band gap trend, finally resolving long-standing discrepancies in the literature

Transition Metal Migration Can Facilitate Ionic Diffusion in Defect Garnet-Based Intercalation Electrodes

The importance of metal migration during multielectron redox activity has been characterized, revealing a competing demand to satisfy bonding requirements and local strains in structures upon alkali intercalation. The local structural evolution required to accommodate intercalation in Y2(MoO4)3 and Al2(MoO4)3 has been contrasted by operando characterization methods, including X-ray absorption spectroscopy and diffraction, along with nuclear magnetic resonance measurements. Computational modeling further rationalized behavioral differences. The local structure of Y2(MoO4)3 was maintained upon lithiation, while the structure of Al2(MoO4)3 underwent substantial local atomic rearrangements as the more ionic character of the bonds in Al2(MoO4)3 allowed Al to mix off its starting octahedral position to accommodate strain during cycling. However, this mixing was prevented in the more covalent Y2(MoO4)3, which accommodated strain through rotational motion of polyhedral subunits. Knowing that an increased ionic character can facilitate the diffusion of redox-inactive metals when cycling multielectron electrodes offers a powerful design principle when identifying next-generation intercalation hosts

Direct observation of delithiation as the origin of analog memristance in LixNbO2

The discovery of analog LixNbO2 memristors revealed a promising new memristive mechanism wherein the diffusion of Li+ rather than O2- ions enables precise control of the resistive states. However, directly correlating lithium concentration with changes to the electronic structure in active layers remains a challenge and is required to truly understand the underlying physics. Chemically delithiated single crystals of LiNbO2 present a model system for correlating lithium variation with spectroscopic signatures from operando soft x-ray spectroscopy studies of device active layers. Using electronic structure modeling of the x-ray spectroscopy of LixNbO2 single crystals, we demonstrate that the intrinsic memristive behavior in LixNbO2 active layers results from field-induced degenerate p-type doping. We show that electrical operation of LixNbO2-based memristors is viable even at marginal Li deficiency and that the analog memristive switching occurs well before the system is fully metallic. This study serves as a benchmark for material synthesis and characterization of future LixNbO2-based memristor devices and suggests that valence change switching is a scalable alternative that circumvents the electroforming typically required for filamentary-based memristors

Nature of the band gap of In2O3 revealed by first-principles calculations and x-ray spectroscopy

Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for In2O3 is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit a small dispersion and the conduction band minimum is positioned at Gamma. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV

Electrochemical Oxidative Fluorination of an Oxide Perovskite

We report on the electrochemical fluorination of the A-site vacant perovskite ReO3 using high-temperature solid-state cells as well as room-temperature liquid electrolytes. Using galvanostatic oxidation and electrochemical impedance spectroscopy, we find that ReO3 can be oxidized by approximately 0.5 equiv of electrons when in contact with fluoride-rich electrolytes. Results from our density functional theory calculations clearly rule out the most intuitive mechanism for charge compensation, whereby F-ions would simply insert onto the A-site of the perovskite structure. Operando X-ray diffraction, neutron total scattering measurements, X-ray spectroscopy, and solid-state 19F NMR with magic-angle spinning were, therefore, used to explore the mechanism by which fluoride ions react with the ReO3 electrode during oxidation. Taken together, our results indicate that a complex structural transformation occurs following fluorination to stabilize the resulting material. While we find that this process of fluorinating ReO3 appears to be only partially reversible, this work demonstrates a practical electrolyte and cell design that can be used to evaluate the mobility of small anions like fluoride that is robust at room temperature and opens new opportunities for exploring the electrochemical fluorination of many new materials

Correlated Polyhedral Rotations in the Absence of Polarons during Electrochemical Insertion of Lithium in ReO₃

Understanding the structural transformations that materials undergo during (de)insertion of Li ions is crucial for designing high-performance intercalation hosts as these deformations can lead to significant capacity fade. Herein, we present a study of the metallic defect perovskite ReO₃ to determine whether these distortions are driven by polaronic charge transport (i.e., the electrons and ions moving through the lattice in a coupled way) due to the semiconducting nature of most oxide hosts. Employing numerous techniques, including electrochemical probes, operando X-ray diffraction, X-ray photoelectron spectroscopy, and density functional theory calculations, we find that the cubic structure of ReO₃ experiences multiple phase changes involving the correlated twisting of rigid octahedral subunits upon lithiation. This results in exceptionally poor long-term cyclability due to large strains upon lithiation, even though metallic character is maintained throughout. This suggests that phase transformations during alkali ion intercalation are the result of local strains in the lattice and not exclusively due to polaron migration

Coexistence of the topological state and a two-dimensional electron gas on the surface of Bi2Se3

Topological insulators are a recently discovered class of materials with fascinating properties: While the inside of the solid is insulating, fundamental symmetry considerations require the surfaces to be metallic. The metallic surface states show an unconventional spin texture, electron dynamics and stability. Recently, surfaces with only a single Dirac cone dispersion have received particular attention. These are predicted to play host to a number of novel physical phenomena such as Majorana fermions, magnetic monopoles and unconventional superconductivity. Such effects will mostly occur when the topological surface state lies in close proximity to a magnetic or electric field, a (superconducting) metal, or if the material is in a confined geometry. Here we show that a band bending near to the surface of the topological insulator Bi$_2$Se$_3$ gives rise to the formation of a two-dimensional electron gas (2DEG). The 2DEG, renowned from semiconductor surfaces and interfaces where it forms the basis of the integer and fractional quantum Hall effects, two-dimensional superconductivity, and a plethora of practical applications, coexists with the topological surface state in Bi$_2$Se$_3$. This leads to the unique situation where a topological and a non-topological, easily tunable and potentially superconducting, metallic state are confined to the same region of space.Comment: 12 pages, 3 figure

Electronic and transport properties of Li-doped NiO epitaxial thin films

NiO is a p-type wide bandgap semiconductor of use in various electronic devices ranging from solar cells to transparent transistors. Understanding and improving its optical and transport properties have been of considerable interest. In this work, we have investigated the effect of Li doping on the electronic, optical and transport properties of NiO epitaxial thin films grown by pulsed laser deposition. We show that Li doping significantly increases the p-type conductivity of NiO, but all the films have relatively low room-temperature mobilities (<0.05 cm2 V−1 s−1). The conduction mechanism is better described by small-polaron hoping model in the temperature range of 200 K < T < 330 K, and variable range hopping at T < 200 K. A combination of X-ray photoemission and O K-edge X-ray absorption spectroscopic investigations reveal that the Fermi level gradually shifts toward the valence band maximum (VBM) and a new hole state develops with Li doping. Both the VBM and hole states are composed of primarily Zhang-Rice bound states, which accounts for the small polaron character (low mobility) of hole conduction. Our work provides guidelines for the search for p-type oxide materials and device optimization

Electronic and transport properties of Li-doped NiO epitaxial thin films (vol 6, pg 2275, 2018)

Correction for ‘Electronic and transport properties of Li-doped NiO epitaxial thin films’ by J. Y. Zhang et al., J. Mater. Chem. C, 2018, 6, 2275–2282.</p