Band energy control of molybdenum oxide by surface hydration

EPSRC (Grants EP/M009580/1, EP/J017361/1, EP/I01330X/1, and EP/I028641/1), the Royal Society, and the European Research Council. The work benefited from the University of Bath's High Performance Computing Facility, and access to the HECToR supercomputer through membership of the UKs HPC Materials Chemistry Consortium, which is funded by EPSRC (Grant No. EP/F067496) and the UltraFOx grant

N incorporation and associated localized vibrational modes in GaSb

We present results of electronic structure calculations on the N-related localized vibrational modes in the dilute nitride alloy GaSb1−xNx. By calculating the formation energies of various possible N incorporation modes in the alloy, we determine the most favorable N configurations, and we calculate their vibrational mode frequencies using density functional theory under the generalized gradient approximation to electron exchange and correlation, including the effects of the relativistic spin-orbit interactions. For a single N impurity, we find substitution on an Sb site, NSb, to be most favorable, and for a two-N-atom complex, we find the N-N split interstitial on an Sb site to be most favorable. For these defects, as well as, for comparison, defects comprising two N atoms on neighboring Sb sites and a N-Sb split interstitial on an Sb site, we find well-localized vibration modes (LVMs), which should be experimentally observable. The frequency of the triply degenerate LVM associated with NSb is determined to be 427.6 cm−1. Our results serve as a guide to future experimental studies to elucidate the incorporation of small concentrations of N in GaSb, which is known to lead to a reduction of the band gap and opens the possibility of using the material for long-wavelength applications

Antiferromagnetism at T > 500 K in the Layered Hexagonal Ruthenate SrRu2O6

We report an experimental and computational study of magnetic and electronic properties of the layered Ru(V) oxide SrRu2O6 (hexagonal, P-3 1m), which shows antiferromagnetic order with a N\'eel temperature of 563(2) K, among the highest for 4d oxides. Magnetic order occurs both within edge-shared octahedral sheets and between layers and is accompanied by anisotropic thermal expansivity that implies strong magnetoelastic coupling of Ru(V) centers. Electrical transport measurements using focused ion beam induced deposited contacts on a micron-scale crystallite as a function of temperature show p-type semiconductivity. The calculated electronic structure using hybrid density functional theory successfully accounts for the experimentally observed magnetic and electronic structure and Monte Carlo simulations reveals how strong intralayer as well as weaker interlayer interactions are a defining feature of the high temperature magnetic order in the material.Comment: Physical Review B 2015 accepted for publicatio

Structural and magnetic characterization of the complete delafossite solid solution (CuAlO2){1-x}(CuCrO2){x}

We have prepared the complete delafossite solid solution series between diamagnetic CuAlO2 and the t2g^3 frustrated antiferromagnet CuCrO2. The evolution with composition x in CuAl(1-x)Cr(x)O2 of the crystal structure and magnetic properties has been studied and is reported here. The room-temperature unit cell parameters follow the Vegard law and increase with x as expected. The effective moment is equal to the Cr^3+ spin-only S = 3/2 value throughout the entire solid solution. Theta is negative, indicating that the dominant interactions are antiferromagnetic, and its magnitude increases with Cr substitution. For dilute Cr compositions, J_BB was estimated by mean-field theory to be 2.0 meV. Despite the sizable Theta, long-range antiferromagnetic order does not develop until very large x, and is preceeded by glassy behavior. Data presented here, and that on dilute Al-substitution from Okuda et al., suggest that the reduction in magnetic frustration due to the presence of non-magnetic Al does not have as dominant an effect on magnetism as chemical disorder and dilution of the magnetic exchange. For all samples, the 5 K isothermal magnetization does not saturate in fields up to 5 T and minimal hysteresis is observed. The presence of antiferromagnetic interactions is clearly evident in the sub-Brillouin behavior with a reduced magnetization per Cr atom. An inspection of the scaled Curie plot reveals that significant short-range antiferromagnetic interactions occur in CuCrO2 above its Neel temperature, consistent with its magnetic frustration. Uncompensated short-range interactions are present in the Al-substituted samples and are likely a result of chemical disorder

The key position: influence of staple location on constrained peptide conformation and binding

First published online 29 Sep 2016Constrained α-helical peptides are showing potential as biological probes and therapeutic agents that target protein-protein interactions. However, the factors that determine the optimal constraint locations are still largely unknown. Using the β-integrin/talin protein interaction as a model system, we examine the effect of constraint location on helical conformation, as well as binding affinity, using circular dichroism and NMR spectroscopy. Stapling increased the overall helical content of each integrin-based peptide tested. However, NMR analysis revealed that different regions within the peptide are stabilised, depending on constraint location, and that these differences correlate with the changes observed in talin binding mode and affinity. In addition, we show that examination of the atomic structure of the parent peptide provides insight into the appropriate placement of helical constraints.Kelly L. Keeling, Okki Cho, Denis B. Scanlon, Grant W. Booker, Andrew D. Abell and Kate L. Wegene

Growth and properties of GaSbBi alloys

Molecular-beam epitaxy has been used to grow GaSb 1− x Bi x alloys with x up to 0.05. The Bi content, lattice expansion, and film thickness were determined by Rutherford backscattering and x-ray diffraction, which also indicate high crystallinity and that >98% of the Bi atoms are substitutional. The observed Bi-induced lattice dilation is consistent with density functional theory calculations. Optical absorption measurements and valence band anticrossing modeling indicate that the room temperature band gap varies from 720 meV for GaSb to 540 meV for GaSb 0.95Bi0.05, corresponding to a reduction of 36 meV/%Bi or 210 meV per 0.01 Å change in lattice constant

Valence band modification of Cr2O3 by Ni-doping: creating a high figure of merit p-type TCO

p-Type transparent conductors and semiconductors still suffer from remarkably low performance compared to their more widespread n-type counterparts, despite extensive investigation into their development. In this contribution, we present a comparative study on the defect chemistry of potential p-type transparent conducting oxides Mg-doped and Ni-doped Cr 2 O 3 . Conductivities as high as 28 S cm -1 were achieved by Ni-doping. By benchmarking crystallography and spectroscopy characterization against density functional theory calculations, we show that the incorporation of Ni into Cr 2 O 3 contributes to the composition of the valence band, making the formed holes more delocalized, while Mg states do not interact with the valence band in Mg-doped Cr 2 O 3 . Furthermore, it is experimentally proven that Ni has a higher solubility in Cr 2 O 3 than Mg, at least in the highly non-thermodynamic deposition conditions used for these experiments, which directly translates into a higher acceptor concentration. The combination of these two effects means that Ni is a more effective acceptor in Cr 2 O 3 than Mg and explains the improved conductivity observed for the former

Band gap reduction in InNxSb1-x alloys: Optical absorption, k . P modeling, and density functional theory

Using infrared absorption, the room temperature band gap of InSb is found to reduce from 174 (7.1 μm) to 85 meV (14.6 μm) upon incorporation of up to 1.13% N, a reduction of ∼79 meV/%N. The experimentally observed band gap reduction in molecular-beam epitaxial InNSb thin films is reproduced by a five band k ⋅· P band anticrossing model incorporating a nitrogen level, EN, 0.75 eV above the valence band maximum of the host InSb and an interaction coupling matrix element between the host conduction band and the N level of β = 1.80 eV. This observation is consistent with the presented results from hybrid density functional theory

Deep vs shallow nature of oxygen vacancies and consequent n -type carrier concentrations in transparent conducting oxides

The source of n -type conductivity in undoped transparent conducting oxides has been a topic of debate for several decades. The point defect of most interest in this respect is the oxygen vacancy, but there are many conflicting reports on the shallow versus deep nature of its related electronic states. Here, using a hybrid quantum mechanical/molecular mechanical embedded cluster approach, we have computed formation and ionization energies of oxygen vacancies in three representative transparent conducting oxides: In 2 O 3 , SnO 2 , and ZnO. We find that, in all three systems, oxygen vacancies form well-localized, compact donors. We demonstrate, however, that such compactness does not preclude the possibility of these states being shallow in nature, by considering the energetic balance between the vacancy binding electrons that are in localized orbitals or in effective-mass-like diffuse orbitals. Our results show that, thermodynamically, oxygen vacancies in bulk In 2 O 3 introduce states above the conduction band minimum that contribute significantly to the observed conductivity properties of undoped samples. For ZnO and SnO 2 , the states are deep, and our calculated ionization energies agree well with thermochemical and optical experiments. Our computed equilibrium defect and carrier concentrations, however, demonstrate that these deep states may nevertheless lead to significant intrinsic n -type conductivity under reducing conditions at elevated temperatures. Our study indicates the importance of oxygen vacancies in relation to intrinsic carrier concentrations not only in In 2 O 3 , but also in SnO 2 and ZnO

Narrow-band anisotropic electronic structure of ReS2

We have used angle resolved photoemission spectroscopy to investigate the band structure of ReS2, a transition-metal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of the structural distortion and spin-orbit coupling. We further image how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zig-zag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone centre along kz. These experiments are in good agreement with our density-functional theory calculations, shedding new light on the bulk electronic structure of ReS2, and how it can be expected to evolve when thinned to a single layer.PostprintPeer reviewe