I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[9717, 12044, 11394, 14432, 15455]
Abstract: More than a decade after the discovery of the two-dimensional electron system (2DES) at the interface between the band insulators
LaAlO
3
(LAO) and
SrTiO
3
(STO) its microscopic origin is still under debate. Several explanations have been proposed, the main contenders being electron doping by oxygen vacancies and electronic reconstruction, i.e., the redistribution of electrons to the interface to minimize the electrostatic energy in the polar LAO film. However, no experiment thus far could provide unambiguous information on the microscopic origin of the interfacial charge carriers. Here we utilize a novel experimental approach combining photoelectron spectroscopy (PES) with highly brilliant synchrotron radiation and apply it to a set of samples with varying key parameters that are thought to be crucial for the emergence of interfacial conductivity. Based on microscopic insight into the electronic structure, we obtain results tipping the scales in favor of polar discontinuity as a generic, robust driving force for the 2DES formation. Likewise, other functionalities such as magnetism or superconductivity might be switched in all-oxide devices by polarity-driven charge transfer.
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May 2017
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[9202]
Abstract: The structure of a chloride terminated copper monolayer electrodeposited onto Au(111) from a CuSO4/KCl electrolyte was investigated ex situ by three complementary experimental techniques (scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), X-ray standing wave (XSW) excitation) and density functional theory (DFT) calculations. STM at atomic resolution reveals a stable, highly ordered layer which exhibits a Moiré structure and is described by a (5×5) unit cell. The XSW/PES data yield a well-defined position of the Cu layer and the value of 2.16 Å above the topmost Au layer suggests that the atoms are adsorbed in threefold hollow sites. The chloride exhibits some distribution around a distance of 3.77 Å in agreement with the observed Moiré pattern due to a higher order commensurate lattice. This structure, a high order commensurate Cl overlayer on top of a commensurate (1×1) Cu layer with Cu at threefold hollow sites, is corroborated by the DFT calculations.
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Aug 2017
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I09-Surface and Interface Structural Analysis
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Zachary W.
Lebens-Higgins
,
Nicholas
Faenza
,
Pinaki
Mukherjee
,
Shawn
Sallis
,
Fadwa
Badway
,
Nathalie
Pereira
,
Christoph
Schlueter
,
Tien-Lin
Lee
,
Frederic
Cosandey
,
Glenn
Amatucci
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[12764, 16005]
Abstract: For layered oxide cathodes, aluminum doping has widely been shown to improve performance, particularly at high degrees of delithiation. While this has led to increased interest in Al-doped systems, including LiNi0.8Co0.15Al0.05O2 (NCA), the aluminum surface environment has not been thoroughly investigated. Using hard x-ray photoelectron spectroscopy measurements of the Al 1s core region for NCA electrodes, we examined the evolution of the surface aluminum environment under electrochemical and thermal stress. By correlating the aluminum environment to transition metal reduction and electrolyte decomposition, we provide further insight into the cathode-electrolyte interface layer. A remarkable finding is that Al-O coatings in LiPF6 electrolyte mimic the evolution observed for the aluminum surface environment in doped layered oxides.
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Dec 2017
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I09-Surface and Interface Structural Analysis
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S. A.
Chambers
,
M. H.
Engelhard
,
L.
Wang
,
T. C.
Droubay
,
M. E.
Bowden
,
M. J.
Wahila
,
N. F.
Quackenbush
,
L. F. J.
Piper
,
Tien-Lin
Lee
,
C. J.
Nelin
,
P. S.
Bagus
Diamond Proposal Number(s):
[16630]
Abstract: We have measured high-resolution core-level and valence-band x-ray photoemission spectra for single-crystal Ti2O3 cleaved anoxically. The Ti(III) spectra for this lattice are considerably more complex than those measured for Ti(IV)-based oxides due to the presence of a single unpaired electron in the conduction band. This open-shell electron configuration leads to ligand-field split and frequently unresolved multiplets. The Ti 2p and 3p spectra have been calculated using relativistic Dirac-Hartree-Fock (DHF) theory with the sudden approximation for the intensities. Agreement between theory and experiment is excellent for the 3p spectrum, and very good for the 2p spectrum, the primary deficiency being a pair of features not captured by theory for the latter. The spectral line shapes are driven by final-state effects associated with angular momentum coupling of the unpaired valence electron with the core hole, one- and two-electron ligand-to-metal charge-transfer (shake) processes accompanying core photoionization, and core-hole screening by conduction-band electrons. The first two of these are accurately predicted by DHF theory with a small embedded cluster containing a single Ti cation and six oxygen ligands. The third effect is not predicted using this cluster in which screening of the core hole from electrons associated with more distant atoms is not possible.
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Nov 2017
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I09-Surface and Interface Structural Analysis
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Paul C.
Rogge
,
Ravini U.
Chandrasena
,
Antonio
Cammarata
,
Robert J.
Green
,
Padraic
Shafer
,
Benjamin M.
Lefler
,
Amanda
Huon
,
Arian
Arab
,
Elke
Arenholz
,
Ho Nyung
Lee
,
Tien-Lin
Lee
,
Slavomir
Nemsak
,
James M.
Rondinelli
,
Alexander
Gray
,
Steven J.
May
Diamond Proposal Number(s):
[17824]
Abstract: We investigated the metal-insulator transition for epitaxial thin films of the perovskite CaFeO3, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard x-ray photoelectron spectroscopy, soft x-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe sites undergo no observable spectroscopic change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for CaFeO3 and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by ∼5–10% in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.
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Jan 2018
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I09-Surface and Interface Structural Analysis
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Jia-Ye
Zhang
,
Weiwei
Li
,
Robert L. Z.
Hoye
,
Judith
Macmanus-Driscoll
,
Melanie
Budde
,
Oliver
Bierwagen
,
Le
Wang
,
Yingge
Du
,
Matthew
Wahila
,
Louis F. J.
Piper
,
Tien-Lin
Lee
,
Holly
Edwards
,
Vinod R.
Dhanak
,
Hongliang
Zhang
Diamond Proposal Number(s):
[16005]
Abstract: NiO is a p-type wide bandgap semiconductor of use in various electronic devices ranging from solar cells to transparent transistors. Understanding and improving its optical and transport properties have been of considerable interest. In this work, we have investigated the effect of Li doping on the electronic, optical and transport properties of NiO epitaxial thin films grown by pulsed laser deposition. We show that Li doping significantly increases the p-type conductivity of NiO, but all the films have relatively low room-temperature mobilities (< 0.05 cm2 V−1s−1). The conduction mechanism is better described by small-polaron hoping model in the temperature range of 200 K < T <330 K, and variable range hopping at T <200 K. A combination of x-ray photoemission and O K-edge x-ray absorption spectroscopic investigations reveal that the Fermi level gradually shifts toward the valence band maximum (VBM) and a new hole state develops with Li doping. Both the VBM and hole states are composed of primarily Zhang-Rice bound states, which accounts for the small polaron character (low mobility) of hole conduction. Our work provides guidelines for the search for p-type oxide materials and device optimization.
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Jan 2018
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I09-Surface and Interface Structural Analysis
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Zachary W.
Lebens-Higgins
,
Shawn
Sallis
,
Nicholas V.
Faenza
,
Fadwa
Badway
,
Nathalie
Pereira
,
David M.
Halat
,
Matthew
Wahila
,
Christoph
Schlueter
,
Tien-Lin
Lee
,
Wanli
Yang
,
Clare P.
Grey
,
Glenn G.
Amatucci
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[12764, 16005]
Abstract: For layered oxide cathodes, impedance growth and capacity fade related to reactions at the cathode-electrolyte interface (CEI) are particularly prevalent at high voltage and high temperatures. At a minimum, the CEI layer consists of Li2CO3, LiF, reduced (relative to the bulk) metal-ion species, and salt decomposition species but conflicting reports exist regarding their progression during (dis)charging. Utilizing transport measurements in combination with x-ray and nuclear magnetic resonance spectroscopy techniques, we study the evolution of these CEI species as a function of electrochemical and thermal stress for LiNi0.8Co0.15Al0.05O2 (NCA) particle electrodes using a LiPF6 ethylene carbonate: dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li2CO3 and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF6 breakdown at room temperature above 4.25 V, thermal aggravation is found to strongly promote salt breakdown and contributes to surface degradation even at lower voltages (4.1 V). An interesting finding of our work was the partial reformation of LiF upon discharge which warrants further consideration for understanding CEI stability during cycling.
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Jan 2018
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15716]
Open Access
Abstract: We use 9 nm and 15 nm thin membranes for determining the effective attenuation length of photoelectrons in silicon. One side of silicon membranes was covered with a thin film of aluminium and exposed to X-rays with energies from 3 to 8 keV. We recorded Al 1s and 2s photoelectrons that were (a) emitted from the Al film directly and (b) transmitted through the membranes. With the help of the ratio of both yields, we obtained values for the effective attenuation length (EAL) of electrons with kinetic energies up to 7.9 keV in silicon. The experimentally determined EAL values are smaller than obtained from different predictive equations. Using a power law fit View the MathML sourceEAL(k,p)=kEkinp to the experimental and predicted EAL values we find that mainly different is the pre-factor of the power law, k, while the exponent, i.e. the dependence on kinetic energy Ekin is represented well. Our study underlines the feasibility of using membranes for investigating surfaces under (near) ambient pressure conditions by photoelectron spectroscopy and points out the advantages of employing hard X-rays.
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Mar 2018
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I09-Surface and Interface Structural Analysis
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Matthias
Meier
,
Zdeněk
Jakub
,
Jan
Balajka
,
Jan
Hulva
,
Roland
Bliem
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Cesare
Franchini
,
Michael
Schmid
,
Ulrike
Diebold
,
Francesco
Allegretti
,
David
Duncan
,
Gareth S.
Parkinson
Diamond Proposal Number(s):
[13817]
Open Access
Abstract: Accurately modelling the structure of a catalyst is a fundamental prerequisite for correctly predicting reaction pathways, but a lack of clear experimental benchmarks makes it difficult to determine the optimal theoretical approach. Here, we utilize the normal incidence X-ray standing wave (NIXSW) technique to precisely determine the three dimensional geometry of Ag1 and Cu1 adatoms on Fe3O4(001). Both adatoms occupy bulk-continuation cation sites, but with a markedly different height above the surface (0.43 ± 0.03 Å (Cu1) and 0.96 ± 0.03 Å (Ag1)). HSE-based calculations accurately predict the experimental geometry, but the more common PBE + U and PBEsol + U approaches perform poorly.
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Jan 2018
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I09-Surface and Interface Structural Analysis
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Martin
Schwarz
,
Manuela
Garnica
,
David A.
Duncan
,
Alejandro
Pérez Paz
,
Jacob
Ducke
,
Peter S.
Deimel
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Angel
Rubio
,
Johannes V.
Barth
,
Francesco
Allegretti
,
Willi
Auwärter
Diamond Proposal Number(s):
[14624]
Abstract: The tetrapyrrole macrocycle of porphine is the common core of all porphyrin molecules, an interesting class of π-conjugated molecules with relevance in natural and artificial systems. The functionality of porphines on a solid surface can be tailored by the central metal atom and its interaction with the substrate. In this study, we present a local adsorption geometry determination for cobalt porphine on Cu(111) by means of complementary scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy and X-ray standing wave measurements, and density functional theory calculations. Specifically, the Co center was determined to be at an adsorption height of 2.25 ± 0.04 Å occupying a bridge site. The macrocycle adopts a moderate asymmetric saddle-shape conformation, with the two pyrrole groups that are aligned perpendicular to the densely packed direction of the Cu(111) surface tilted away from the surface plane.
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Feb 2018
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