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Abstract: A noncollinear relativistic PBEsol+U study of the low-index actinide dioxides (AnO2, An = U, Np, Pu) surfaces has been conducted. The surface properties of the AnO2 have been investigated and the importance of the reorientation of magnetic vectors relative to the plane of the surface is highlighted. In collinear nonrelativistic surface models, the orientation of the magnetic moments is often ignored; however, the use of noncollinear relativistic methods is key to the design of reliable computational models. The ionic relaxation of each surface is shown to be confined to the first three monolayers and we have explored the configurations of the terminal oxygen ions on the reconstructed (001) surface. The reconstructed (001) surfaces are ordered as (001)αβ < (001)α < (001)β in terms of energetics. Electrostatic potential isosurface and scanning tunneling microscopy images have also been calculated. By considering the energetics of the low-index AnO2 surfaces, an octahedral Wulff crystal morphology has been calculated.
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Dec 2018
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I09-Surface and Interface Structural Analysis
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Daniel W.
Davies
,
Aron
Walsh
,
James J.
Mudd
,
Chris F.
Mcconville
,
Anna
Regoutz
,
J. Matthias
Kahk
,
David J.
Payne
,
Vin R.
Dhanak
,
David
Hesp
,
Katariina
Pussi
,
Tien-Lin
Lee
,
Russell G.
Egdell
,
Kelvin H. L.
Zhang
Diamond Proposal Number(s):
[8441]
Abstract: Indium oxide is widely used as transparent electrode in optoelectronic devices and as a photocatalyst with activity for reduction of CO2. However, very little is known about the structural and electronic properties of its surfaces, particularly those prepared under reducing conditions. In this report, directional ‘lone-pair’ surface states associated with filled 5s2 orbitals have been identified on vacuum-annealed In2O3(111) through a combination of hard and soft X-ray photoemission spectroscopy and density functional theory calculations. The lone pairs reside on indium ad-atoms in a formal +1 oxidation state, each of which traps two electrons into a localised hybrid orbital protruding away from the surface and lying just above the valence band maximum in photoemission spectra. The third electron associated with the ad-atoms is delocalised into the conduction band, thus producing the surface electron accumulation layer identified previously on vacuum-annealed In2O3(111) (1×1) surfaces. The surface structure is further supported by low energy electron diffraction, but there is no chemical shift in indium core level x-ray photoelectron spectra between surface In(I) ad-atoms and bulk In(III). The 5s2 lone pairs confer Lewis basicity on the surface In sites and may have a pronounced impact on the catalytic or photo-catalytic activity of reduced In2O3.
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Dec 2018
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[11145]
Abstract: Four new symmetrical donor–acceptor–donor (D–A–D)-type molecules are reported with diphenylamine (DPA) or 10,11-dihydro-5H-dibenz[b,f]azepine (Az) as electron donors and 9,9-dimethylthioxanthene-S,S-dioxide (TXO2) as the electron acceptor. The donors are attached at different positions on the acceptor core: either para or meta to the sulfone unit. This series provides new insights into the effects of chromophore rigidity/flexibility on the efficiency of thermally activated delayed fluorescence (TADF). The molecules have been characterized by X-ray crystallography, by in-depth photophysical studies, and by theoretical calculations. The clear differences observed in the photophysical properties when using DPA or Az as a donor are shown to originate from different geometries of the donor unit which, in turn, influence the geometry of the nitrogen lone pair and the donating strength of the corresponding fragment. Thus, a para-substituted Az derivative demonstrated blue TADF in polar media, while the compounds with more flexible DPA units did not show delayed fluorescence. To obtain deep-blue emitters, weaker donating units are needed. A more flexible donor unit leads to increased local excited state (donor) LE emission and reduced TADF. However, a certain amount of flexibility has to be present to ensure deep-blue TADF.
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Dec 2018
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I09-Surface and Interface Structural Analysis
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P. J.
Blowey
,
R. J.
Maurer
,
L. A.
Rochford
,
D. A.
Duncan
,
Jie Hun
Kang
,
D. A.
Warr
,
A. J.
Ramadan
,
T.-L.
Lee
,
P. K.
Thakur
,
G.
Costantini
,
K.
Reuter
,
D. P.
Woodruff
Diamond Proposal Number(s):
[9459, 14524, 15899]
Open Access
Abstract: The local structure of the non-planar phthalocyanine, vanadyl phthalocyanine (VOPc), adsorbed on Cu(111) at a coverage of approximately one half of a saturated molecular layer, has been investigated by a combination of normal-incidence X-ray standing waves (NIXSW), scanned-energy mode photoelectron diffraction (PhD) and density-functional theory (DFT), complemented by scanning tunnelling microscopy (STM). Qualitative assessment of the NIXSW data clearly shows that both ‘up’ and ‘down’ orientations of the molecule (with V=O pointing out of, and into, the surface) must coexist on the surface. O 1s PhD proves to be inconclusive regarding the molecular orientation. DFT calculations, using two different dispersion correction schemes, show good quantitative agreement with the NIXSW structural results for equal co-occupation of the two different molecular orientations and clearly favour the Many Body Dispersion (MBD) method to deal with long-range dispersion forces. The calculated relative adsorption energies of the differently-oriented molecules at the lowest coverage show a strong preference for the ‘up’ orientation, but at higher local coverages, this energetic difference decreases and mixed orientation phases are almost energetically equivalent to pure ‘up’ oriented phases. DFT-based Tersoff-Hamann simulations of STM topographs for the two orientations cast some light on the extent to which such images provide a reliable guide to molecular orientation.
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Oct 2018
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I09-Surface and Interface Structural Analysis
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Caio C.
Silva
,
Marcella
Iannuzzi
,
David A.
Duncan
,
Paul
Ryan
,
Katherine T.
Clarke
,
Johannes T.
Kuchle
,
Jiaqi
Cai
,
Wouter
Jolie
,
Christoph
Schlueter
,
Tien-Lin
Lee
,
Carsten
Busse
Diamond Proposal Number(s):
[12558, 16796]
Abstract: The structure of graphene on Ru(0001) has been extensively studied over the last decade, yet with no general agreement. Here we analyse graphene's valleys and hills using a combination of x-ray standing waves (XSW) and density functional theory (DFT). The chemical specicity of XSW allows an independent analysis of valleys and hills which, together with DFT model, results in the precise determination of the distance between the at, strongly bonded valleys of graphene and the substrate, as well the corrugation presented in the weakly bounded hills. From the theoretical viewpoint, the good agreement with experiment validates the choices regarding the unit cell size and the non-local correlation functional.
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Jul 2018
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I07-Surface & interface diffraction
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Giuliano
Duva
,
Linus
Pithan
,
Clemens
Zeiser
,
Berthold
Reisz
,
Johannes
Dieterle
,
Bernd
Hofferberth
,
Paul
Beyer
,
Laura
Bogula
,
Andreas
Opitz
,
Stefan
Kowarik
,
Alexander
Hinderhofer
,
Alexander
Gerlach
,
Frank
Schreiber
Abstract: In this work two novel donor:acceptor (D:A) complexes, namely diindenoperylene(DIP):1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane(F6TCNNQ) and alpha-sexithiophene(6T):F6TCNNQ, are studied. The D:A complexes segregate in form of π-π stacked D:A co-crystals and can be observed by X-ray scattering. The different conformational degrees of freedom of the donor molecules, respectively, seem to affect the thin film crystalline texture and composition of the D:A mixtures significantly. In equimolar mixtures, for DIP:F6TCNNQ the crystallites are mostly uniaxially oriented and homogeneous, whereas for 6T:F6TCNNQ a mostly 3D (isotropic) orientation of the crystallites and coexistence of domains of pristine compounds and D:A complex, respectively, are observed. Using optical absorption spectroscopy we observe for each of the two mixed systems a set of new, strong transitions located in the near-IR range below the gap of the pristine compounds: such transitions are related to charge-transfer (CT) interactions between donor and acceptor. The optical anisotropy of domains of the D:A complexes with associated new electronic states is studied by ellipsometry. We infer that the CT-related transition dipole moment is perpendicular to the respective π-conjugated planes in the D:A complex.
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Jul 2018
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[16332]
Abstract: Aurivillius-phase ferroelectrics with high Curie temperature can be turned into multiferroic materials by incorporating magnetic ions. The four-layer Aurivillius-type system Bi5FeTi3O15 is well known to show a strong magnetoelectric effect; however, much controversy exists on its magnetic state and possible multiferroicity at room temperature. In this paper, we report on a detailed investigation on the interconnections between crystal chemistry and magnetic properties of Bi5FeTi3O15 ceramics chemically modified by the A-site gadolinium substitution. The structural studies showed that all Bi5-xGdxFeTi3O15 (0 ≦ x ≦ 1) samples adopt the polar orthorhombic space group symmetry A21am at room temperature. The unit cell volume and the orthorhombic distortion decrease alongside the reduction of octahedral tilts with increasing amount of Gd added. The decrease in tilting distortion of [Ti/Fe]O6 octahedra was further evidenced by the suppression of the Raman A1[111] tilt mode at 233 cm-1. By using the superconducting quantum interference device and vibrating sample magnetometry, it was demonstrated that all the ceramic series is paramagnetic from 5 K up to 700 K. It was thus concluded that the A-site substitution of Bi5FeTi3O15 with magnetic Gd ions brings about a slight structural relaxation of the parental orthorhombic lattice, but is not effective way to induce multiferroic properties in the Aurivillius compound. We suggest that the room-temperature (ferri/ferro/antiferro-) magnetism in Bi5FeTi3O15 previously reported in the literature might be due to the presence of magnetic impurities or local short-range magnetic ordering formed during material processing under different conditions.
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Jun 2018
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I09-Surface and Interface Structural Analysis
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Qi
Wang
,
Antoni
Franco-Cañellas
,
Penghui
Ji
,
Christoph
Buerker
,
Rong-Bin
Wang
,
Katharina
Broch
,
Pardeep Kumar
Thakur
,
Tien-Lin
Lee
,
Haiming
Zhang
,
Alexander
Gerlach
,
Lifeng
Chi
,
Steffen
Duhm
,
Frank
Schreiber
Diamond Proposal Number(s):
[10443]
Abstract: Organic heterostructures are a central part of a manifold of (opto)electronic devices and serve a variety of functions. Particularly, molecular monolayers on metal electrodes are of paramount importance for device performance as they allow tuning energy levels in a versatile way. However, this can be hampered by molecular exchange, i.e., by interlayer diffusion of molecules toward the metal surface. We show that the organic–metal interaction strength is the decisive factor for the arrangement in bilayers, which is the most fundamental version of organic–organic heterostructures. The subtle differences in molecular structure of 6,13-pentacenequinone (P2O) and 5,7,12,14-pentacenetetrone (P4O) lead to antithetic adsorption behavior on Ag(111): physisorption of P2O but chemisorption of P4O. This allows providing general indicators for organic–metal coupling based on shifts in photoelectron spectroscopy data and to show that the coupling strength of copper-phthalocyanine (CuPc) with Ag(111) is in between that of P2O and P4O. We find that, indeed, CuPc forms a bilayer when deposited on a monolayer P4O/Ag(111) but molecular exchange takes place with P2O, as shown by a combination of scanning tunneling microscopy and X-ray standing wave experiments.
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Apr 2018
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Abstract: Understanding an interaction at an interface between a topological insulator and a metal is of critical importance when designing electronic and spintronic devices or when such systems are used in catalysis. In this paper, we report on a chemical instability of the interface between Bi2Se3 and Ag studied by X-Ray Powder Diffraction and Electron Microscopy. We present strong experimental evidence of a redox solid-state reaction occurring at the interface with kinetics that is significant already at room temperature. The reaction yields Ag2Se, AgBiSe2 and Bi. The unexpected room-temperature chemical instability of the interface should be considered for all future theoretical and applicative studies involving the interface between Bi2Se3 and Ag.
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Apr 2018
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Abstract: The enantioselective hydrogenation of methyl acetoacetate (MAA) over modified Ni-based catalysts is a key reaction in the understanding of enantioselective heterogeneous catalysis as it represents the only example of this class of reactions catalyzed by base metals. Yet, there is very little molecular-level information available about the adsorption complex formed by the reactants on Ni surfaces. Here, we report a combined experimental and theoretical study of the adsorption of MAA on the Ni{100} surface. X-ray photoelectron spectroscopy shows that MAA forms stable multilayers at low temperatures, which desorb between 200 and 220 K. At higher temperatures a single chemisorbed layer is formed, which decomposes between 300 and 350 K. Density functional theory modeling predicts an enolate species with bidentate coordination as the most stable chemisorbed species. Comparison of photoelectron spectroscopy and X-ray absorption data with simulations using this adsorption model show good qualitative and quantitative agreement. The molecular plane is tilted with respect to the surface plane by about 50°. This breaking of symmetry provides a mechanism for the enantioselective hydrogenation.
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Mar 2018
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