I09-Surface and Interface Structural Analysis
|
Galo J.
Paez
,
Christopher N.
Singh
,
Matthew J.
Wahila
,
Keith E.
Tirpak
,
Nicholas F.
Quackenbush
,
Shawn
Sallis
,
Hanjong
Paik
,
Yufeng
Liang
,
Darrell G.
Schlom
,
Tien-Lin
Lee
,
Christoph
Schlueter
,
Wei-Cheng
Lee
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[13812, 25355]
Abstract: Recent reports have identified new metaphases of
VO
2
with strain and/or doping, suggesting the structural phase transition and the metal-to-insulator transition might be decoupled. Using epitaxially strained
VO
2
/
Ti
O
2
(001) thin films, which display a bulklike abrupt metal-to-insulator transition and rutile to monoclinic transition structural phase transition, we employ x-ray standing waves combined with hard x-ray photoelectron spectroscopy to simultaneously measure the structural and electronic transitions. This x-ray standing waves study elegantly demonstrates the structural and electronic transitions occur concurrently within experimental limits (±1K).
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May 2020
|
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I09-Surface and Interface Structural Analysis
|
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
|
Z. h.
Lim
,
N. F.
Quackenbush
,
A. n.
Penn
,
M.
Chrysler
,
M.
Bowden
,
Z.
Zhu
,
J. M.
Ablett
,
T.-L.
Lee
,
J. m.
Lebeau
,
J. C.
Woicik
,
P. v.
Sushko
,
S. a.
Chambers
,
J. h.
Ngai
Diamond Proposal Number(s):
[17449]
Abstract: We report charge transfer and built-in electric fields across the epitaxial
SrNb
x
Ti
1
−
x
O
3
−
δ
/
Si
(
001
)
interface. Electrical transport measurements indicate the formation of a hole gas in the Si and the presence of built-in fields. Hard x-ray photoelectron measurements reveal pronounced asymmetries in core-level spectra that arise from these built-in fields. Theoretical analysis of core-level spectra enables built-in fields and the resulting band bending to be spatially mapped across the heterojunction. The demonstration of tunable charge transfer, built-in fields, and the spatial mapping of the latter, lays the groundwork for the development of electrically coupled, functional heterojunctions.
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Jul 2019
|
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[17449]
Abstract: The electronic properties of epitaxial heterojunctions consisting of the prototypical perovskite oxide semiconductor,
n
−
SrTiO
3
, and the high-mobility Group IV semiconductor
p
-Ge have been investigated. Hard x-ray photoelectron spectroscopy with a new method of analysis has been used to determine band alignment while at the same time quantifying a large built-in potential found to be present within the Ge. Accordingly, the built-in potential within the Ge has been mapped in a layer-resolved fashion. Electron transfer from donors in the
n
−
SrTi
O
3
to the
p
-Ge creates a space-charge region in the Ge resulting in downward band bending, which spans most of the Ge gap. This strong downward band bending facilitates visible light, photogenerated electron transfer from Ge to STO, favorable to drive the hydrogen evolution reaction associated with water splitting. Ti 2p and
Sr
3
d
core-level line shapes reveal that the STO bands are flat despite the space-charge layer therein. Inclusion of the effect of Ge band bending on band alignment is significant, amounting to a
∼
0.4
eV
reduction in valence band offset compared to the value resulting from using spectra averaged over all layers. Density functional theory allows candidate interface structural models deduced from scanning transmission electron microscopy images to be simulated and structurally optimized. These structures are used to generate multislice simulations that reproduce the experimental images quite well. The calculated band offsets for these structures are in good agreement with experiment.
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Sep 2018
|
|
I09-Surface and Interface Structural Analysis
|
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
|
|
I09-Surface and Interface Structural Analysis
|
Nicholas F.
Quackenbush
,
Hanjong
Paik
,
Megan E.
Holtz
,
Matthew J.
Wahila
,
Jarrett A.
Moyer
,
Stefan
Barthel
,
Tim O.
Wehling
,
Dario A.
Arena
,
Joseph C.
Woicik
,
David A.
Muller
,
Darrell G.
Schlom
,
Louis
Piper
Diamond Proposal Number(s):
[12546, 14730]
Abstract: The characteristics of the cooperative Mott-Peierls metal-insulator transition (MIT) of VO2 can be altered by employing epitaxial strain. While the most commonly used substrate for this purpose is isostructural rutile TiO2, thin films often suffer from interdiffusion of Ti ions near the interface. Exploiting this phenomena, we investigate the nature of interfacial V4+/Ti4+ cation intermixing and its effects on the MIT using scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS), soft x-ray absorption spectroscopy (XAS), and hard x-ray photoelectron spectroscopy (HAXPES), along with supporting density functional theory (DFT) calculations. We find that the reduced orbital occupancy in highly Ti incorporated VO2 is responsible for suppressing the MIT. Interdiffused films are found to be metallic at all measured temperatures, despite a resolute dimerization inferred from x-ray absorption data at lower temperatures. Our results demonstrate that the Mott physics can be suppressed in doped VO2, while a lattice dimerization remains thermodynamically favorable.
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Aug 2017
|
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I09-Surface and Interface Structural Analysis
|
Nicholas
Quackenbush
,
H.
Paik
,
Matthew
Wahila
,
Shawn
Sallis
,
M. E.
Holtz
,
X.
Huang
,
A.
Ganose
,
B. J.
Morgan
,
David O.
Scanlon
,
Y.
Gu
,
F.
Xue
,
L.-Q.
Chen
,
G. E.
Sterbinsky
,
Christoph
Schlueter
,
Tien-Lin
Lee
,
J. C.
Woicik
,
J.-H.
Guo
,
J. D.
Brock
,
D. A.
Muller
,
D. A.
Arena
,
D. G.
Schlom
,
Louis
Piper
Diamond Proposal Number(s):
[12546]
Abstract: Tensile strain along the cR axis in epitaxial VO2 films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surface-sensitive x-ray spectroscopy to distinguish from the TiO2 substrate and identify the phases of VO2 as a function of temperature in epitaxial VO2/TiO2 thin films with well-defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the M2 phase is stabilized by nearly an order of magnitude more strain than expected for the measured temperature window. Our results reveal that the elongation of the cR axis is insufficient for describing the transition pathway of VO2 epitaxial films and that a strain induced increase of electron correlation effects must be considered.
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Aug 2016
|
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I09-Surface and Interface Structural Analysis
|
Linda W.
Wangoh
,
Shawn
Sallis
,
Kamila M.
Wiaderek
,
Yuh-Chieh
Lin
,
Bohua
Wen
,
Nicholas
Quackenbush
,
Natasha A.
Chernova
,
Jinghua
Guo
,
Lu
Ma
,
Tianpin
Wu
,
Tien-Lin
Lee
,
Christoph
Schlueter
,
Shyue Ping
Ong
,
Karena W.
Chapman
,
M. Stanley
Whittingham
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[11246, 12546]
Abstract: Full, reversible intercalation of two Li+ has not yet been achieved in promising VOPO4 electrodes. A
pronounced Li+ gradient has been reported in the low voltage window (i.e., second lithium reaction)
that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium
reaction). Here, we employ a combination of hard and soft x–ray photoelectron and absorption
spectroscopy techniques to depth profile solid state synthesized LiVOPO4 cycled within the low
voltage window only. Analysis of the vanadium environment revealed no evidence of a Li+ gradient,
which combined with almost full theoretical capacity confirms that disrupted kinetics in the high
voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that
the uniform Li+ intercalation is a prerequisite for the formation of intermediate phases Li1.50VOPO4
and Li1.75VOPO4. The evolution from LiVOPO4 to Li2VOPO4 via the intermediate phases is
confirmed by direct comparison between O K–edge absorption spectroscopy and density functional
theory.
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Aug 2016
|
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I09-Surface and Interface Structural Analysis
|
Shawn
Sallis
,
N.
Pereira
,
P.
Mukherjee
,
Nicholas
Quackenbush
,
N.
Faenza
,
Christoph
Schlueter
,
Tien-Lin
Lee
,
W. L.
Yang
,
F.
Cosandey
,
G. G.
Amatucci
,
Louis
Piper
Diamond Proposal Number(s):
[12764]
Abstract: The pronounced capacity fade in Ni-rich layered oxide lithium ion battery cathodes observed when cycling above 4.1 V (versus Li/Li+) is associated with a rise in impedance, which is thought to be due to either bulk structural fatigue or surface reactions with the electrolyte (or combination of both). Here, we examine the surface reactions at electrochemically stressed Li1– x Ni 0.8Co0.15Al0.05O2 binder-free powder electrodes with a combination of electrochemical impedance spectroscopy, spatially resolving electron microscopy, and spatially averaging X-ray spectroscopy techniques. We circumvent issues associated with cycling by holding our electrodes at high states of charge (4.1 V, 4.5 V, and 4.75 V) for extended periods and correlate charge-transfer impedance rises observed at high voltages with surface modifications retained in the discharged state (2.7 V). The surface modifications involve significant cation migration (and disorder) along with Ni and Co reduction, and can occur even in the absence of significant Li2CO3 and LiF. These data provide evidence that surface oxygen loss at the highest levels of Li+ extraction is driving the rise in impedance.
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Jun 2016
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[12546]
Abstract: We present evidence of strain-induced modulation of electron correlation effects and increased orbital
anisotropy in the rutile phase of epitaxial VO2/TiO2 films from hard x-ray photoelectron spectroscopy and
soft V L-edge x-ray absorption spectroscopy, respectively. By using the U(1) slave spin formalism, we further argue that the observed anisotropic correlation effects can be understood by a model of orbital selective Mott transition at a filling that is noninteger but close to the half filling. Because the overlaps of wave functions between d orbitals are modified by the strain, orbital-dependent renormalizations of the bandwidths and the onsite energy
occur. These renormalizations generally result in different occupation numbers in different orbitals. We find that if the system has a noninteger filling number near the half filling such as for VO2, certain orbitals could reach an occupation number closer to half filling under the strain, resulting in a strong reduction in the quasiparticle weight Zα of that orbital. Our work demonstrates that such an orbital selective Mott transition, defined as the case with Zα = 0 in some but not all orbitals, could be accessed by epitaxial-strain engineering of correlated electron systems
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Jun 2016
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