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The metal-insulator transition of NbO2 is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed NbOx that may be inherently different to crystalline NbO2. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial NbO2 thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at ∼810°C. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline NbO2 as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap.

Ni-rich layered oxide cathodes can deliver higher energy density batteries, but uncertainties remain over their charge compensation mechanisms and the degradation processes that limit cycle life. Trapped molecular O 2 has been identified within LiNiO 2 at high states of charge, as seen for Li-rich cathodes where excess capacity is associated with reversible oxygen redox. Here we show that bulk redox in LiNiO 2 occurs by Ni-O rehybridization, lowering the electron density on O sites, but importantly without the involvement of molecular O 2 . Instead, trapped O 2 is related to degradation at surfaces in contact with the electrolyte, and is accompanied by Ni reduction. O 2 is removed on discharge, but excess Ni 2+ persists forming a reduced surface layer, associated with impeded Li transport. This implicates the instability of delithiated LiNiO 2 in contact with the electrolyte in surface degradation through O 2 formation and Ni reduction, highlighting the importance of surface stabilisation strategies in suppressing LiNiO 2 degradation., Competing Interests: The authors declare that there are no conflicts of interest., (This journal is © The Royal Society of Chemistry.)

A comprehensive study of bulk molybdenum dichalcogenides is presented with the use of soft and hard X-ray photoelectron (SXPS and HAXPES) spectroscopy combined with hybrid density functional theory (DFT). The main core levels of MoS 2 , MoSe 2 , and MoTe 2 are explored. Laboratory-based X-ray photoelectron spectroscopy (XPS) is used to determine the ionization potential (IP) values of the MoX 2 series as 5.86, 5.40, and 5.00 eV for MoSe 2 , MoSe 2 , and MoTe 2 , respectively, enabling the band alignment of the series to be established. Finally, the valence band measurements are compared with the calculated density of states which shows the role of p-d hybridization in these materials. Down the group, an increase in the p-d hybridization from the sulfide to the telluride is observed, explained by the configuration energy of the chalcogen p orbitals becoming closer to that of the valence Mo 4d orbitals. This pushes the valence band maximum closer to the vacuum level, explaining the decreasing IP down the series. High-resolution SXPS and HAXPES core-level spectra address the shortcomings of the XPS analysis in the literature. Furthermore, the experimentally determined band alignment can be used to inform future device work., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Germanium selenide (GeSe) bulk crystals, thin films and solar cells are investigated with a focus on acceptor-doping with silver (Ag) and the use of an Sb 2 Se 3 interfacial layer. The Ag-doping of GeSe occurred by a stoichiometric melt growth technique that created Ag-doped GeSe bulk crystals. A combination of capacitance voltage measurements, synchrotron radiation photoemission spectroscopy and surface space-charge calculations indicates that Ag-doping increases the hole density from 5.2 × 10 15 cm -3 to 1.9 × 10 16 cm -3 . The melt-grown material is used as the source for thermally evaporated GeSe films within solar cells. The cell structure with the highest efficiency of 0.260% is FTO/CdS/Sb 2 Se 3 /undoped-GeSe/Au, compared with solar cells without the Sb 2 Se 3 interfacial layer or with the Ag-doped GeSe.

Ga 2 O 3 and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga 2 O 3 offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ-Ga 2 O 3 presents a particular challenge across synthesis, characterization, and theory due to its inherent disorder and resulting complex structure-electronic-structure relationship. Here, density functional theory is used in combination with a machine-learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ-Ga 2 O 3 . The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by photoluminescence excitation spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step toward exploring how their electronic structure can be understood in terms of local coordination and overall structure., (© 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Shell-isolated nanoparticles (SHINs) with a 37 nm gold core and an 11 nm tin dioxide (SnO 2 ) coating exhibited long-life Raman enhancement for 3 months and a wide pH stability of pH 2-13 in comparison with conventional SiO 2 -coated SHINs. Herein, Au-SnO 2 is demonstrated as a more durable SHIN for use in the technique Shell-Isolated Nanoparticles for Enhanced Raman Spectroscopy (SHINERS)., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Transparent conducting oxides (TCOs) are ubiquitous in modern consumer electronics. SnO 2 is an earth abundant, cheaper alternative to In 2 O 3 as a TCO. However, its performance in terms of mobilities and conductivities lags behind that of In 2 O 3 . On the basis of the recent discovery of mobility and conductivity enhancements in In 2 O 3 from resonant dopants, we use a combination of state-of-the-art hybrid density functional theory calculations, high resolution photoelectron spectroscopy, and semiconductor statistics modeling to understand what is the optimal dopant to maximize performance of SnO 2 -based TCOs. We demonstrate that Ta is the optimal dopant for high performance SnO 2 , as it is a resonant dopant which is readily incorporated into SnO 2 with the Ta 5d states sitting ∼1.4 eV above the conduction band minimum. Experimentally, the band edge electron effective mass of Ta doped SnO 2 was shown to be 0.23 m 0 , compared to 0.29 m 0 seen with conventional Sb doping, explaining its ability to yield higher mobilities and conductivities., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Making new van der Waals materials with electronic or magnetic functionality is a chemical design challenge for the development of two-dimensional nanoelectronic and energy conversion devices. We present the synthesis and properties of the van der Waals material Bi 4 O 4 SeCl 2 , which is a 1:1 superlattice of the structural units present in the van der Waals insulator BiOCl and the three-dimensionally connected semiconductor Bi 2 O 2 Se. The presence of three anions gives the new structure both the bridging selenide anion sites that connect pairs of Bi 2 O 2 layers in Bi 2 O 2 Se and the terminal chloride sites that produce the van der Waals gap in BiOCl. This retains the electronic properties of Bi 2 O 2 Se while reducing the dimensionality of the bonding network connecting the Bi 2 O 2 Se units to allow exfoliation of Bi 4 O 4 SeCl 2 to 1.4 nm height. The superlattice structure is stabilized by the configurational entropy of anion disorder across the terminal and bridging sites. The reduction in connective dimensionality with retention of electronic functionality stems from the expanded anion compositional diversity.

Mixed ionic-electronic conductors (MIECs) that display high oxide ion conductivity (σ o ) and electronic conductivity (σ e ) constitute an important family of electrocatalysts for a variety of applications including fuel cells and oxygen separation membranes. Often MIECs exhibit sufficient σ e but inadequate σ o . It has been a long-standing challenge to develop MIECs with both high σ o and stability under device operation conditions. For example, the well-known perovskite oxide Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3- δ (BSCF) exhibits exceptional σ o and electrocatalytic activity. The reactivity of BSCF with CO 2 , however, limits its use in practical applications. Here, the perovskite oxide Bi 0.15 Sr 0.85 Co 0.8 Fe 0.2 O 3- δ (BiSCF) is shown to exhibit not only exceptional bulk transport properties, with a σ o among the highest for known MIECs, but also high CO 2 tolerance. When used as an oxygen separation membrane, BiSCF displays high oxygen permeability comparable to that of BSCF and much higher stability under CO 2 . The combination of high oxide transport properties and CO 2 tolerance in a single-phase MIEC gives BiSCF a significant advantage over existing MIECs for practical applications., (© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)