The origin of the almost unique combination of optical transparency and the ability to bipolar dope tin monoxide is explained using a combination of soft and hard Xray photoemission spectroscopy, O K-edge X-ray emission and absorption spectroscopy, and density functional theory calculations incorporating van der Waals corrections. We reveal that the origin of the high hole mobility, bipolar ability, and transparency is a result of (i) significant Sn 5s character at the valence band maximum (due to O 2p−Sn 5s antibonding character associated with the lone pair distortion), (ii) the combination of a small indirect band gap of ∼0.7 eV (Γ−M) and a much larger direct band gap of 2.6−2.7 eV, and (iii) the location of both band edges with respect to the vacuum level. This work supports Sn 2+ -based oxides as a paradigm for nextgeneration transparent semiconducting oxides.

calculations revealed that altering the degree of cation disorder from none to maximal disorder using the special quasirandom structure (SQS) approach can change the fundamental band gap from 2.09 to 1.12 eV. The possibility of realizing such a variation experimentally was supported by the observation of the cation-disorder-induced wurtzite phase in molecular-beam epitaxy-grown thin fi lms as opposed to the theoretically more stable orthorhombic phase. [ 3 ] The cation-ordered orthorhombic structure is a superstructure of wurtzite structure, with similar tetrahedral bonding, as described in ref. [ 7 ] .Here, the observed absorption edges as a function of free electron density for the differently grown fi lms are consistent with the room temperature fundamental band gap varying from 1.0 to 2.0 eV as the cation ordering increases. Moreover, the conduction and valence band density of states (DOS) have been calculated using hybrid DFT for the cation-ordered orthorhombic and cation-disordered SQS pseudowurtzite phases. Comparison of these DOS with photoemission data from single crystal ZnSnN 2 fi lms grown under different conditions indicates that samples with different degrees of cation ordering have been synthesised, opening up a new method for tuning the band gap of ZnSnN 2 fi lms.The growth conditions for molecular-beam epitaxy (MBE) of the single crystal ZnSnN 2 thin fi lms are summarized in Table 1 . The substrate temperature, the Zn:Sn fl ux ratio and the N 2 pressure were adjusted to vary the degree of cation disorder in the ZnSnN 2 in order to experimentally test the theoretical predictions. X-ray diffraction indicates that all the samples have the wurtzite structure rather than the fully ordered orthorhombic structure (and also that Zn-nitride and Sn-nitride phases are below the detection limit); specifi cally, the peak at 22° which is characteristic of the orthorhombic structure is absent for all fi lms. [ 3 ] Thus, XRD characterization alone is insuffi cient to exclude the possibility of a signifi cant variation of the degree of cation disorder within the samples grown under the range of conditions employed. Consequently, evidence of cation disorder induced variations in the band gap and band structure has been sought and found in the optical, transport and photoemission properties of the ZnSnN 2 fi lms. It is important to note here that in the nonequilibrium growth technique used in this study (plasma-assisted MBE), substrate temperature and fl ux ratio have a profound infl uence on adatom migration length, and hence how atoms incorporate into an epitaxially driven structure. Therefore, higher growth temperature is likely to produce a more ordered fi lm than a lower growth temperature.

The pronounced capacity fade in Ni-rich layered oxide lithium ion battery cathodes observed when cycling above 4.1 V (versus Li/Li+) is associated with a rise in impedance, which is thought to be due to either bulk structural fatigue or surface reactions with the electrolyte (or combination of both). Here, we examine the surface reactions at electrochemically stressed Li1–xNi0.8Co0.15Al0.05O2 binder-free powder electrodes with a combination of electrochemical impedance spectroscopy, spatially resolving electron microscopy, and spatially averaging X-ray spectroscopy techniques. We circumvent issues associated with cycling by holding our electrodes at high states of charge (4.1 V, 4.5 V, and 4.75 V) for extended periods and correlate charge-transfer impedance rises observed at high voltages with surface modifications retained in the discharged state (2.7 V). The surface modifications involve significant cation migration (and disorder) along with Ni and Co reduction, and can occur even in the absence of significant Li2CO3 and LiF. These data provide evidence that surface oxygen loss at the highest levels of Li+ extraction is driving the rise in impedance.

We have directly measured the band gap renormalization associated with the Moss-Burstein shift in the perovskite transparent conducting oxide (TCO), La-doped BaSnO 3 , using hard x-ray photoelectron spectroscopy. We determine that the band gap renormalization is almost entirely associated with the evolution of the conduction band. Our experimental results are supported by hybrid density functional theory supercell calculations. We determine that unlike conventional TCOs where interactions with the dopant orbitals are important, the band gap renormalization in La-BaSnO 3 is driven purely by electrostatic interactions.

We have combined hard X-ray photoelectron spectroscopy with angular dependent O K-edge and V L-edge X-ray absorption spectroscopy to study the electronic structure of metallic and insulating end point phases in 4.1 nm thick (14 units cells along the c-axis of VO2) films on TiO2(001) substrates, each displaying an abrupt MIT centered at ~300 K with width <20 K and a resistance change of ΔR/R > 10(3). The dimensions, quality of the films, and stoichiometry were confirmed by a combination of scanning transmission electron microscopy with electron energy loss spectroscopy, X-ray spectroscopy, and resistivity measurements. The measured end point phases agree with their bulk counterparts. This clearly shows that, apart from the strain induced change in transition temperature, the underlying mechanism of the MIT for technologically relevant dimensions must be the same as the bulk for this orientation.

We report direct evidence of conduction band filling in 3% La-doped BaSnO3 using hard x-ray photoelectron spectroscopy. Direct comparisons with hybrid density functional theory calculations support a 3.2 eV indirect band gap. The use of hybrid DFT is verified by excellent agreement between our photoelectron spectra and O K-edge x-ray emission and absorption spectra. Our experimental and computational results demonstrate that the conduction band is primarily of Sn 5s orbital character with little O 2p contribution, which is a prerequisite for designing a perovskite-based transparent conducting oxide.

In this work, we demonstrate the stable cycling of more than one Li in solid-state-synthesized ε-LiVOPO4 over more than 20 cycles for the first time. Using a combination of density functional theory (DFT) calculations, X-ray pair distribution function (PDF) analysis and X-ray absorption near edge structure (XANES) measurements, we present a comprehensive analysis of the thermodynamics, kinetics, and structural evolution of ε-Li x VOPO4 over the entire lithiation range. We identify two intermediate phases at x = 1.5 and 1.75 in the low-voltage regime using DFT calculations, and the computed and electrochemical voltage profiles are in excellent agreement. Operando PDF and EXAFS techniques show a reversible hysteretic change in the short (<2 Å) VO bond lengths coupled with an irreversible extension of the long VO bond (>2.4 Å) during low-voltage cycling. Hydrogen intercalation from electrolyte decomposition is a possible explanation for the ∼2.4 Å VO bond and its irreversible extension. Finally, we show that ε-Li x VOPO4 is likely a pseudo-1D ionic diffuser with low electronic conductivity using DFT calculations, which suggests that nanosizing and carbon coating is necessary to achieve good electrochemical performance in this material.

Tensile strain along the cR-axis in epitaxial VO2 films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surfacesensitive x-ray spectroscopy to distinguish from the TiO2 substrate and identify the phases of VO2 as a function of temperature in epitaxial VO2/TiO2 thin films with well defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the stabilization of the M2 phase in epitaxial films results from nearly an order of magnitude more strain than expected. Our results reveal that the elongation of the cR axis is insufficient for describing the transition pathway of VO2 epitaxial films and that a strain induced increase of electron correlation effects must be considered.