I09-Surface and Interface Structural Analysis
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Abhinav
Prakash
,
Nicholas F.
Quackenbush
,
Hwanhui
Yun
,
Jacob T.
Held
,
Tianqi
Wang
,
Tristan
Truttmann
,
James M.
Ablett
,
Conan
Weiland
,
Tien-lin
Lee
,
Joseph C.
Woicik
,
K. Andre
Mkhoyan
,
Bharat
Jalan
Diamond Proposal Number(s):
[15845]
Abstract: Separating electrons from their source atoms in La-doped BaSnO3, the first perovskite oxide semiconductor to be discovered with high room-temperature electron mobility, remains a subject of great interest for achieving high-mobility electron gas in two dimensions. So far, the vast majority of work in perovskite oxides has focused on heterostructures involving SrTiO3 as an active layer. Here we report the demonstration of modulation doping in BaSnO3 as high room temperature mobility host without the use of SrTiO3. Significantly, we show the use of angle- resolved hard X-ray photoelectron spectroscopy (HAXPES) as a non-destructive approach to not only determine the location of electrons at the buried interface but also to quantify the width of electron distribution in BaSnO3. The transport results are in good agreement with the results of self-consistent solution to one-dimensional Poisson and Schrödinger equations. Finally, we discuss viable routes to engineer two-dimensional electron gas density through band-offset engineering.
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Nov 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[12975]
Abstract: A combination of scanning tunneling microscopy, low-energy electron diffraction,and low-energy electron microscopy (LEEM) has been used to identify the structural phases formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) on Ag(111). These comprise a two-dimensional gas phase, a low-density commensurate (LDC) phase, and a higher-density incommensurate (HDI) phase. LEEM also shows the presence of an additional “precursor-HDI” phase with a surface unit mesh area only ≈3% less than the HDI phase. Normal incidence x-ray standing-wave measurements of the HDI phase yield almost identical structural parameters to the LDC phase for which a full structure determination has been previously reported. The results show TCNQ does not adopt the inverted bowl distortion favored in earlier density functional theory calculations of TCNQ on coinage metal surfaces, but the N atoms are twisted out of the molecular plane, an effect found for the LDC phase to be due to incorporation of Ag adatoms. The possible role of Ag adatoms in the HDI phase, and in the transition from the precursor-HDI phase, is discussed.
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Nov 2019
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I09-Surface and Interface Structural Analysis
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Zachary W.
Lebens-higgins
,
David M.
Halat
,
Nicholas V.
Faenza
,
Matthew J.
Wahila
,
Manfred
Mascheck
,
Tomas
Wiell
,
Susanna K.
Eriksson
,
Paul
Palmgren
,
Jose
Rodriguez
,
Fadwa
Badway
,
Nathalie
Pereira
,
Glenn G.
Amatucci
,
Tien-lin
Lee
,
Clare P.
Grey
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[22250, 22148]
Open Access
Abstract: Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2−yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes.
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Dec 2019
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I09-Surface and Interface Structural Analysis
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G.
Vinai
,
C.
Bigi
,
A.
Rajan
,
M. D.
Watson
,
T.-l.
Lee
,
F.
Mazzola
,
S.
Modesti
,
S.
Barua
,
M.
Ciomaga Hatnean
,
G.
Balakrishnan
,
P. D. C.
King
,
P.
Torelli
,
G.
Rossi
,
G.
Panaccione
Diamond Proposal Number(s):
[21429]
Abstract: Among transition-metal dichalcogenides, mono and few-layers thick
VSe
2
has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of
Fe
/
VSe
2
heterostructure by combining element sensitive x-ray absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk
VSe
2
do not show intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the
Fe
/
VSe
2
interface and its relation with interfacial magnetic coupling.
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Jan 2020
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I09-Surface and Interface Structural Analysis
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Anna
Regoutz
,
Alex M.
Ganose
,
Lars
Blumenthal
,
Christoph
Schlueter
,
Tien-lin
Lee
,
Gregor
Kieslich
,
Anthony K.
Cheetham
,
Gwilherm
Kerherve
,
Ying-sheng
Huang
,
Ruei-san
Chen
,
Giovanni
Vinai
,
Tommaso
Pincelli
,
Giancarlo
Panaccione
,
Kelvin H. L.
Zhang
,
Russell G.
Egdell
,
Johannes
Lischner
,
David O.
Scanlon
,
David J.
Payne
Diamond Proposal Number(s):
[12673]
Abstract: Theory and experiment are combined to gain an understanding of the electronic properties of OsO2, a poorly studied metallic oxide that crystallizes in the rutile structure. Hard and soft valence-band x-ray photoemission spectra of OsO2 single crystals are in broad agreement with the results of density-functional-theory calculations, aside from a feature shifted to high binding energy of the conduction band. The energy shift corresponds to the conduction electron plasmon energy measured by reflection electron energy loss spectroscopy. The plasmon satellite is reproduced by many-body perturbation theory.
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Feb 2019
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I09-Surface and Interface Structural Analysis
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Wei-cheng
Lee
,
Matthew
Wahila
,
Shantanu
Mukherjee
,
Christopher N.
Singh
,
Tyler
Eustance
,
Anna
Regoutz
,
Hanjong
Paik
,
Jos E.
Boschker
,
Fanny
Rodolakis
,
Tien-lin
Lee
,
Darrell G.
Schlom
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[20647, 21430]
Open Access
Abstract: We investigate the electronic structure of epitaxial VO
2
2
films in the rutile phase using density functional theory combined with the slave-spin method (DFT + SS). In DFT + SS, multi-orbital Hubbard interactions are added to a DFT-fit tight-binding model, and slave spins are used to treat electron correlations. We find that while stretching the system along the rutile
c
c
-axis results in a band structure favoring anisotropic orbital fillings, electron correlations favor equal filling of the
t
2g
t2g
orbitals. These two distinct effects cooperatively induce an orbital-dependent redistribution of the electron occupations and spectral weights, driving strained VO
2
2
toward an orbital selective Mott transition (OSMT). The simulated single-particle spectral functions are directly compared to V L-edge resonant X-ray photoemission spectroscopy of epitaxial 10 nm VO
2
2
/TiO
2
2
(001) and (100) strain orientations. Excellent agreement is observed between the simulations and experimental data regarding the strain-induced evolution of the lower Hubbard band. Simulations of rutile NbO
2
2
under similar strain conditions are performed, and we predict that an OSMT will not occur in rutile NbO
2
2
. Our prediction is supported by the high-temperature hard x-ray photoelectron spectroscopy measurement on relaxed NbO
2
2
(110) thin films with no trace of the lower Hubbard band. Our results indicate that electron correlations in VO
2
2
are important and can be modulated even in the rutile phase before the Peierls instability sets in.
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Feb 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[9012, 12558]
Abstract: We study the effect of alkali metal intercalation (Cs and Li) on the geometry of graphene on Ir(111) using the x-ray standing waves technique. For both alkali metals, the increase in the mean height of the carbon layer does not depend on the lateral structure or the density of the intercalated layer. For Li, full delamination of graphene from the metal substrate is found already for a small amount of intercalant. Even though Lithium lifts graphene to a smaller height, it is much more efficient in ironing out the corrugation of pristine graphene on Ir(111).
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May 2019
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I09-Surface and Interface Structural Analysis
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Anders L.
Jørgensen
,
David A.
Duncan
,
Claus F. P.
Kastorp
,
Line
Kyhl
,
Zeyuan
Tang
,
Albert
Bruix
,
Mie
Andersen
,
Bjork
Hammer
,
Tien-lin
Lee
,
Liv
Hornekaer
,
Richard
Balog
Diamond Proposal Number(s):
[16243]
Abstract: Functionalization of graphene on Ir(111) is a promising route to modify graphene by chemical means in a controlled fashion at the nanoscale. Yet, the nature of such functionalized sp3 nanodots remains unknown. Density functional theory (DFT) calculations alone cannot differentiate between two plausible structures, namely true graphane and substrate stabilized graphane-like nanodots. These two structures, however, interact dramatically differently with the underlying substrate. Discriminating which type of nanodots forms on the surface is thus of paramount importance for the applications of such prepared nanostructures. By comparing X-ray standing wave measurements against theoretical model structures obtained by DFT calculations we are able to exclude the formation of true graphane nanodots and clearly show the formation graphane-like nanodots.
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Jun 2019
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I09-Surface and Interface Structural Analysis
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Peter
Knecht
,
Bodong
Zhang
,
Joachim
Reichert
,
David A.
Duncan
,
Martin
Schwarz
,
Felix
Haag
,
Paul
Ryan
,
Tien-Lin
Lee
,
Peter S.
Deimel
,
Peter
Feulner
,
Francesco
Allegretti
,
Willi
Auwärter
,
Guillaume
Médard
,
Ari Paavo
Seitsonen
,
Johannes V.
Barth
,
Anthoula C.
Papageorgiou
Diamond Proposal Number(s):
[24320]
Abstract: The controlled arrangement of N-heterocyclic carbenes (NHCs) on solid surfaces is a current challenge of surface functionalization. We introduce a strategy of using Ru porphyrins in order to control both the orientation and lateral arrangement of NHCs on a planar surface. The coupling of the NHC to the Ru porphyrin is a facile process which takes place on the interface: we apply NHCs as functional, robust pillars on well-defined, preassembled Ru porphyrin monolayers on silver and characterize these interfaces with atomic precision via a battery of experimental techniques and theoretical considerations. The NHCs assemble at room temperature modularly and reversibly on the Ru porphyrin arrays. We demonstrate a selective and complete functionalization of the Ru centers. With its binding, the NHC modifies the interaction of the Ru porphyrin with the Ag surface, displacing the Ru atom by 1 Å away from the surface. This arrangement of NHCs allows us to address individual ligands by controlled manipulation with the tip of a scanning tunneling microscope, creating patterned structures on the nanometer scale.
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Mar 2021
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I09-Surface and Interface Structural Analysis
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B. F.
Spencer
,
S.
Maniyarasu
,
B.
Reed
,
D. J. H.
Cant
,
R.
Ahumada-lazo
,
A. G.
Thomas
,
C. A.
Muryn
,
M.
Maschek
,
S. K.
Eriksson
,
T.
Wiell
,
T.-l.
Lee
,
S.
Tougaard
,
A. G.
Shard
,
W. R.
Flavell
Diamond Proposal Number(s):
[20059]
Abstract: Hard X-ray Photoelectron Spectroscopy (HAXPES) provides minimally destructive depth profiling into the bulk, extending the photoelectron sampling depth. Detection of deeply buried layers beyond the elastic limit is enabled through inelastic background analysis. To test the robustness of this technique, we present results on a thin (18 nm) layer of buried metal-organic complex buried below up to 200 nm of organic material. Overlayers with thicknesses 25-140 nm were measured using photon energies ranging 6-10 keV at the I09 end station at Diamond Light Source, and a new fixed energy Ga Kα (9.25 keV) laboratory-based HAXPES spectrometer was also used to measure samples with overlayers up to 200 nm thick. The sampling depth was varied: at Diamond Light Source by changing the photon energy, and in the lab system by performing angle-resolved measurements. For all the different overlayers and sampling depths, inelastic background modelling consistently provided thicknesses which agreed, within reasonable error, with the ellipsometric thickness. Relative sensitivity factors were calculated, and these factors consistently provided reasonable agreement with the expected nominal stoichiometry, suggesting the calculation method can be extended to any element. These results demonstrate the potential for the characterisation of deeply buried layers using synchrotron and laboratory-based HAXPES.
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Dec 2020
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