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Philip A. E.
Murgatroyd
,
Matthew J.
Smiles
,
Christopher N.
Savory
,
Thomas P.
Shalvey
,
Jack E. N.
Swallow
,
Nicole
Fleck
,
Craig M.
Robertson
,
Frank
Jaeckel
,
Jonathan
Alaria
,
Jonathan D.
Major
,
David O.
Scanlon
,
Tim D.
Veal
Abstract: The van der Waals material GeSe is a potential solar absorber, but its optoelectronic properties are not yet fully understood. Here, through a combined theoretical and experimental approach, the optoelectronic and structural properties of GeSe are determined. A fundamental absorption onset of 1.30 eV is found at room temperature, close to the optimum value according to the Shockley-Queisser detailed balance limit, in contrast to previous reports of an indirect fundamental transition of 1.10 eV. The measured absorption spectra and first-principles joint density of states are mutually consistent, both exhibiting an additional distinct onset $\sim$0.3~eV above the fundamental absorption edge. The band gap values obtained from first-principles calculations converge, as the level of theory and corresponding computational cost increases, to 1.33 eV from the quasiparticle self-consistent GW method, including the solution to the Bethe-Salpeter equation. This agrees with the 0~K value determined from temperature-dependent optical absorption measurements. Relaxed structures based on hybrid functionals reveal a direct fundamental transition in contrast to previous reports. The optoelectronic properties of GeSe are resolved with the system described as a direct semiconductor with a 1.30 eV room temperature band gap. The high level of agreement between experiment and theory encourages the application of this computational methodology to other van der Waals materials.
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Mar 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Patrick
Hirschle
,
Christian
Hirschle
,
Konstantin
Böll
,
Markus
Döblinger
,
Miriam
Höhn
,
Joshua M.
Tuffnell
,
Christopher W.
Ashling
,
David
Keen
,
Thomas D.
Bennett
,
Joachim O.
Rädler
,
Ernst
Wagner
,
Michael
Peller
,
Ulrich
Lächelt
,
Stefan
Wuttke
Diamond Proposal Number(s):
[21260]
Abstract: Iron(III) fumarate materials are well suited for biomedical applications as they feature biocompatible building blocks, porosity, chemical functionalizability, and magnetic resonance imaging (MRI) activity. The synthesis of these materials however is difficult to control, and it has been challenging to produce monodisperse particle sizes and morphologies that are required in medical use. Here, we report the optimization of iron(III) fumarate nano- and microparticle synthesis by surfactant-free methods, including room temperature, solvothermal, microwave, and microfluidic conditions. Four variants of iron(III) fumarate with distinct morphologies were isolated and are characterized in detail. Structural characterization shows that all iron(III) fumarate variants exhibit the metal–organic framework (MOF) structure of MIL-88A. Nanoparticles with a diameter of 50 nm were produced, which contain crystalline areas not exceeding 5 nm. Solvent-dependent swelling of the crystalline particles was monitored using in situ X-ray diffraction. Cytotoxicity experiments showed that all iron(III) fumarate variants feature adequate biotolerability and no distinct interference with cellular metabolism at low concentrations. Magnetic resonance relaxivity studies using clinical MRI equipment, on the other hand, proved that the MRI contrast characteristics depend on particle size and morphology. All in all, this study demonstrates the possibility of tuning the morphological appearance of iron(III) fumarate particles and illustrates the importance of optimizing synthesis conditions for the development of new biomedical materials.
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Mar 2020
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I09-Surface and Interface Structural Analysis
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Theodore D. C.
Hobson
,
Laurie J
Phillips
,
Oliver S
Hutter
,
Huw
Shiel
,
Jack E. N.
Swallow
,
Christopher N.
Savory
,
Pabitra K
Nayak
,
Silvia
Mariotti
,
Bhaskar
Das
,
Leon
Bowen
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Mark A
Farnworth
,
Guillaume
Zoppi
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Henry J.
Snaith
,
Chris
Leighton
,
David O.
Scanlon
,
Vinod R.
Dhanak
,
Ken
Durose
,
Tim D.
Veal
,
Jonathan D
Major
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The carrier type of Sb2Se3 was evaluated for both thin films and bulk crystals via a range of complementary techniques. X-ray photoelectron spectroscopy (XPS), hot-probe, hall effect and surface photo-voltage spectroscopy showed material synthesized from Sb2Se3 granulate mate-rial to be n-type with chlorine identified as an unintentional n-type dopant via secondary ion mass spectrometry analysis. The validity of chlorine as a dopant was con-firmed by synthesis of intrinsic crystals from metallic precursors and subsequent n-type doping by the addition of MgCl2. Chlorine was also shown to be a substitutional n-type shallow dopant by density functional theory calculations. TiO2/Sb2Se3 n-n isotype heterojunction solar cells of 7.3% efficiency based are demonstrated with band alignment analyzed via XPS.
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Mar 2020
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E02-JEM ARM 300CF
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Diamond Proposal Number(s):
[21980]
Abstract: Advances in the production of two-dimensional (2D) materials such as graphene and MoS2 during the past two decades have spurred the search for other van der Waals materials with distinct functional properties. However, reducing the dimensionality of bulk van der Waals materials can lead to structural rearrangement and chemical degradation, especially in the presence of air. These challenges have slowed the progress of the discovery and analysis of chemically diverse 2D materials. Here, we provide a case study on the shear exfoliation of a class of wide band gap van der Waals materials termed II–VI layered hybrids (II–VI LHs) and show how reducing their dimension influences their structural and chemical stabilities. ZnSe(butylamine) and ZnSe(octylamine) are exfoliated, yielding shear-thinned material whose resistance toward degradation via oxidation is studied in depth by a variety of macro- and microscopic characterization techniques. Mechanical energy input, solvent–ligand interaction, and exposure to ambient conditions all play important roles in the stability of these materials. Our findings suggest that moderately coordinating alkylamine layers stabilize 2D materials that would otherwise degrade during exfoliation and exposure to air.
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Mar 2020
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Abstract: The Li-stuffed garnets LixM2M3′O12 are promising Li-ion solid electrolytes with potential use in solid-state batteries. One strategy for optimizing ionic conductivities in these materials is to tune lithium stoichiometries through aliovalent doping, which is often assumed to produce proportionate numbers of charge-compensating Li vacancies. The native defect chemistry of the Li-stuffed garnets and their response to doping, however, are not well understood, and it is unknown to what degree a simple vacancy-compensation model is valid. Here, we report hybrid density functional theory calculations of a broad range of native defects in the prototypical Li garnet Li7La3Zr2O12. We calculate equilibrium defect concentrations as a function of synthesis conditions and model the response of these defect populations to extrinsic doping. We predict a rich defect chemistry that includes Li and O vacancies and interstitials, and significant numbers of cation-antisite defects. Under reducing conditions, O vacancies act as color centers by trapping electrons. We find that supervalent (donor) doping does not produce charge compensating Li vacancies under all synthesis conditions; under Li-rich/Zr-poor conditions the dominant compensating defects are LiZr antisites, and Li stoichiometries strongly deviate from those predicted by simple “vacancy compensation” models.
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Feb 2020
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I09-Surface and Interface Structural Analysis
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Benjamin A. D.
Williamson
,
Thomas J.
Featherstone
,
Sanjayan S.
Sathasivam
,
Jack E. N.
Swallow
,
Huw
Shiel
,
Leanne A. H.
Jones
,
Matthew J
Smiles
,
Anna
Regoutz
,
Tien-lin
Lee
,
Xueming
Xia
,
Christopher
Blackman
,
Pardeep K.
Thakur
,
Claire J.
Carmalt
,
Ivan P.
Parkin
,
Tim D.
Veal
,
David O.
Scanlon
Diamond Proposal Number(s):
[18195, 21431]
Abstract: Transparent conducting oxides (TCOs) are ubiquitous in modern consumer electronics. SnO2 is an earth abundant, cheaper alternative to In2O3 as a TCO however, its performance in terms of electrical properties lags behind that of In2O3. Based on the recent discovery of mobility and conductivity enhancements in In2O3 from resonant dopants, we use a combination of state-of-the-art hybrid density functional theory calculations, high resolution photoelectron spectroscopy and semiconductor statistics modelling to understand what the optimal dopant is to maximise performance of SnO2-based TCOs. We demonstrate that Ta is the optimal dopant for high performance SnO2, as it is a resonant dopant which is readily incorporated into SnO2 with the Ta 5d states sitting ca. 1.4 eV above the conduction band minimum. Experimentally, the electron effective mass of Ta doped SnO2 was shown to be 0.23m0, compared to 0.29m0 seen with conventional Sb doping, explaining its ability to yield higher mobilities and conductivities.
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Feb 2020
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I11-High Resolution Powder Diffraction
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Mathias
Jorgensen
,
Patrick T.
Shea
,
Anton W.
Tomich
,
Joel B.
Varley
,
Marnik
Bercx
,
Sergio
Lovera
,
Radovan
Cerny
,
Wei
Zhou
,
Terrence J.
Udovic
,
Vincent
Lavallo
,
Torben
Jensen
,
Brandon C.
Wood
,
Vitalie
Stavila
Abstract: Solid-state ion conductors based on closo-polyborate anions combine high ionic conductivity with a rich array of tunable properties. Cation mobility in these systems is intimately related to the strength of the interaction with the neighboring anionic network and the energy for reorganizing the coordination polyhedra. Here, we explore such factors in solid electrolytes with two anions of the weakest coordinating ability, [HCB11H5Cl6]– and [HCB11H5Br6]–, and a total of eleven polymorphs are identified for their lithium and sodium salts. Our approach combines ab initio molecular dynamics, synchrotron X-ray powder diffraction, differential scanning calorimetry, and AC impedance measurements to investigate their structures, phase-transition behavior, anion orientational mobilities, and ionic conductivities. We find that M(HCB11H5X6) (M = Li, Na, X = Cl, Br) compounds exhibit order-disorder polymorphic transitions between 203 and 305 °C, and display Li and Na superionic conductivity in the disordered state. Through detailed analysis, we illustrate how cation disordering in these compounds originates from a competitive interplay among the lattice symmetry, the anion reorientational mobility, the geometric and electronic asymmetry of the anion, and the polarizability of the halogen atoms. These factors are compared to other closo-polyborate-based ion conductors to suggest guidelines for optimizing the cation-anion interaction for fast ion mobility. This study expands the known solid-state, poly(carba)borate-based materials capable of liquid-like ionic conductivities, unravels the mechanisms responsible for fast ion transport, and provides insights into the development of practical superionic solid electrolytes.
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Jan 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Timothee
Stassin
,
Ivo
Stassen
,
Joao
Marreiros
,
Alexander John
Cruz
,
Rhea
Verbeke
,
Min
Tu
,
Helge
Reinsch
,
Marcel
Dickmann
,
Werner
Egger
,
Ivo F. J.
Vankelecom
,
Dirk E.
De Vos
,
Rob
Ameloot
Diamond Proposal Number(s):
[17279]
Abstract: A simple solvent- and catalyst-free method is presented for the synthesis of the large-pore metal-organic framework (MOF) MAF-6 (RHO-Zn(eIm)2) based on the reaction of ZnO with 2-ethylimidazole vapor at temperatures ≤ 100 °C. By translating this method to a chemical vapor deposition (CVD) protocol, crystalline films of a large-pore material could be deposited for the first time entirely from the vapor phase. A combination of PALS and Kr physisorption measurements confirmed the porosity of these MOF-CVD films and the size of the MAF-6 supercages (diam. ~2 nm), in close agreement with powder data and calculations. MAF-6 powders and films were further characterized by XRD, TGA, SEM, FTIR, PDF and EXAFS. The exceptional uptake capacity of MAF-6 in comparison to ZIF-8 is demonstrated by vapor-phase loading of a molecule larger than the ZIF-8 windows.
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Jan 2020
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Abstract: In recent years a considerable amount of effort has been put into a better understanding of the correlation of structure and electrochemical properties in Li- and Mn-rich NCM layered oxide Li ion battery cathode materials, such as the intensely investigated Li1.2Ni0.15Co0.1Mn0.55O2 composition. A gradual transformation from a trigonal R-3m layered structure towards a cubic Fd-3m spinel structure during electrochemical cycling results in an unwanted decay of the mean charge and discharge voltages, called ‘voltage fade’. This transformation proceeds via an interim phase, which is characterized by the local de-ordering of cations and the introduction of various lattice defects after the formation of the initially well-ordered material. In this study, these structural changes are studied in detail on a long-range atomic scale by synchrotron radiation powder diffraction as well as on a very-local atomic scale by 7Li nuclear magnetic resonance and X-ray absorption spectroscopy. A structural reordering was induced by a mild thermal treatment (150 °C – 300 °C) in lithiated (discharged to 2.0 V) as well as in delithiated (charged to 4.7 V) electrodes, which results either in a partial recovery of the initial well-ordered state or in an intensification of the structural degradation towards a spinel-type cation ordering, respectively. The structural reordering thus obtained was again studied on a long-range and local atomic scale and correlated with the electrochemical properties. In order to complement the experiment an electrochemically highly fatigued electrode (300 cycles, discharged to 2.0 V) showing a pronounced voltage fade was thermally treated, which resulted again in a partial recovery of the initial well-ordered structure and its accompanying electrochemical properties. Finally, the results are summarized in a model explaining the influence of the local cation ordering, lattice defects and the oxygen sublattice on the electrochemical properties, such as the oxygen redox activity and the voltage fade.
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Jan 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786]
Abstract: By tuning the tolerance factor, $t$, of the Ruddlesden--Popper oxide Ca$_2$MnO$_4$ through isovalent substitutions we show that the uniaxial coefficient of linear thermal expansion (CLTE) of these systems can be systematically changed through large negative to positive values. High-resolution X-ray diffraction measurements show that the magnitude of uniaxial negative thermal expansion (NTE) increases as $t$ decreases across the stability window of the NTE phase. Transitions to phases with positive thermal expansion (PTE) are found to occur at both the high-$t$ and low-$t$ limits of stability. First-principles calculations demonstrate that reducing $t$ enhances the contribution to thermal expansion from the lowest frequency phonons, which have the character of octahedral tilts and have negative mode Gr\"uneisen parameter components along the NTE axis. By tuning $t$ to the lower edge of the NTE phase stability window, we are hence able to maximise the amplitudes of these vibrations and thereby maximise NTE with a CLTE of -8.1~ppm/K at 125~K. We also illustrate, at the other end of the phase diagram, that an enhancement in compliance of these materials associated with the rotational instability provides another mechanism by which NTE could be yet further enhanced in this and related systems.
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Dec 2019
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