I09-Surface and Interface Structural Analysis
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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
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I09-Surface and Interface Structural Analysis
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Slavomir
Nemsak
,
Mathias
Gehlmann
,
Cheng-tai
Kuo
,
Shih-chieh
Lin
,
Christoph
Schlueter
,
Ewa
Mlynczak
,
Tien-lin
Lee
,
Lukasz
Plucinski
,
Hubert
Ebert
,
Igor
Di Marco
,
Ján
Minár
,
Claus M.
Schneider
,
Charles S.
Fadley
Diamond Proposal Number(s):
[11516, 12032]
Open Access
Abstract: The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials.
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Aug 2018
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[9118]
Abstract: Variable-period x-ray standing wave (VPXSW) studies have been carried out using 3 keV x rays and
photoelectron detection. Two model surfaces have been used, a native SiO2 layer (20 Å thick) on bulk silicon, and
a purpose-built multilayer surface comprising a chloroform/water marker layer (12 Å thick) on an ionic liquid
spacer layer (211 Å thick) deposited on a SiO2/Si substrate at 90 K. By using photoelectron detection, both
chemical and elemental sensitivity were achieved. The surfaces were modeled using dynamic x-ray scattering
for x-ray intensity, and attenuation of photoelectrons transmitted through the layers, to produce simulations
which accurately reproduced the experimental VPXSW measurements. VPXSW measurements made using the
substrate, spacer layer, and marker layer photoelectron signatures produced consistent structural values. This
work demonstrates that VPXSW can be used to determine chemically specific layer thicknesses within thick
(300 Å) surface structures composed of the light elements B, C, N, O, F, and Cl with an accuracy of 10 to
15 Å, perpendicular to the surface.
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Oct 2018
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[16005]
Abstract: Zn
M
I
I
I
2
O
4
(
M
I
I
I
=
Co
, Rh, Ir) spinels have been recently identified as promising
p
-type semiconductors for transparent electronics. However, discrepancies exist in the literature regarding their fundamental optoelectronic properties. In this paper, the electronic structures of these spinels are directly investigated using soft/hard x-ray photoelectron and x-ray absorption spectroscopies in conjunction with density functional theory calculations. In contrast to previous results,
ZnCo
2
O
4
is found to have a small electronic band gap with forbidden optical transitions between the true band edges, allowing for both bipolar doping and high optical transparency. Furthermore, increased
d
−
d
splitting combined with a concomitant lowering of Zn
s
/
p
conduction states is found to result in a
ZnCo
2
O
4
(
ZCO
)
<
ZnRh
2
O
4
(
ZRO
)
≈
ZnIr
2
O
4
(
ZIO
)
band gap trend, finally resolving long-standing discrepancies in the literature.
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Aug 2019
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[19524]
Abstract: Delafossite CuFeO2 photocathodes have recently attracted attention for water splitting due to their suitable band gap (∼1.5 eV) and high stability in aqueous media. The preparation of CuFeO2 usually requires long and energy‐intense treatments in an inert atmosphere for the full conversion of spinel CuFe2O4 to delafossite CuFeO2. Herein, we report the preparation and characterization of highly uniform and stable CuFeO2 thin films obtained via a combination of inexpensive ultrasonic spray pyrolysis followed by a short hybrid microwave treatment (∼4 min). The resulting films show good stability in alkaline media and produce a photocurrent of ∼650 μA/cm2 under 1.5 AM simulated sunlight and with oxygen bubbling. The effect of the rapid transformation from the spinel to the delafossite phase induced by hybrid microwave annealing was investigated with synchrotron‐based X‐ray absorption spectroscopy (XAS) and X‐ray photoelectron spectroscopy (XPS).
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Sep 2019
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I09-Surface and Interface Structural Analysis
|
Zachary W.
Lebens-higgins
,
Nicholas V.
Faenza
,
Maxwell D.
Radin
,
Hao
Liu
,
Shawn
Sallis
,
Jatinkumar
Rana
,
Julija
Vinckeviciute
,
Philip J.
Reeves
,
Mateusz
Zuba
,
Fadwa
Badway
,
Nathalie
Pereira
,
Karena W.
Chapman
,
Tien-lin
Lee
,
Tianpin
Wu
,
Clare P.
Grey
,
Brent
Melot
,
Anton
Van Der Ven
,
Glenn G.
Amatucci
,
Wanli
Yang
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[19162]
Open Access
Abstract: Oxygen participation, arising from increased transition metal–oxygen covalency during delithiation, is considered essential for the description of charge compensation in conventional layered oxides. The advent of high-resolution mapping of the O K-edge resonant inelastic X-ray scattering (RIXS) provides an opportunity to revisit the onset and extent of oxygen participation. Combining RIXS with an array of structural and electronic probes for the family of Ni-rich LiNi0.8Co0.2−yAlyO2 cathodes, we identify common charge compensation regimes that are assigned to formal transition metal redox (<4.25 V) and oxygen participation through covalency (>4.25 V). From O K-edge RIXS maps, we find the emergence of a sharp RIXS feature in these systems when approaching full delithiation, which has previously been associated with lattice oxidized oxygen in alkali-rich systems. The lack of transition metal redox signatures and strong covalency at these high degrees of delithiation suggest this RIXS feature is similarly attributed to lattice oxygen charge compensation as in the alkali-rich systems. The RIXS feature's evolution with state of charge in conventional layered oxides is evidence that this feature reflects the depopulation of occupied O 2p states associated with oxygen participation.
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Jul 2019
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I09-Surface and Interface Structural Analysis
|
X. C.
Huang
,
J. Y.
Zhang
,
M.
Wu
,
S.
Zhang
,
H. Y.
Xiao
,
W. Q.
Han
,
T.-l.
Lee
,
A.
Tadich
,
D.-c.
Qi
,
L.
Qiao
,
L.
Chen
,
K. H. L.
Zhang
Diamond Proposal Number(s):
[21432]
Abstract: This work reports a fundamental study on the electronic structure, optical properties, and defect chemistry of a series of Co-based spinel oxide (
Co
3
O
4
,
ZnCo
2
O
4
, and
CoAl
2
O
4
) epitaxial thin films using x-ray photoemission and absorption spectroscopies, optical spectroscopy, transport measurements, and density functional theory. We demonstrate that
ZnCo
2
O
4
has a fundamental bandgap of 1.3 eV, much smaller than the generally accepted values, which range from 2.26 to 2.8 eV. The valence band edge mainly consists of occupied
Co
3
d
t
6
2
g
with some hybridization with O
2
p
/Zn
3
d
, and the conduction band edge of unoccupied
e
∗
g
state. However, optical transition between the two band edges is dipole forbidden. Strong absorption occurs at photon energies above 2.6 eV, explaining the reasonable transparency of
ZnCo
2
O
4
. A detailed defect chemistry study indicates that Zn vacancies formed at high oxygen pressure are the origin of a high
p
-type conductivity of
ZnCo
2
O
4
, and the hole conduction mechanism is described by small-polaron hoping model. The high
p
-type conductivity, reasonable transparency, and large work function make
ZnCo
2
O
4
a desirable
p
-type transparent semiconductor for various optoelectronic applications. Using the same method, the bandgap of
Co
3
O
4
is further proved to be ∼0.8 eV arising from the tetrahedrally coordinated
Co
2
+
cations. Our work advances the fundamental understanding of these materials and provides significant guidance for their use in catalysis, electronic, and solar applications.
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Sep 2019
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I09-Surface and Interface Structural Analysis
|
Abstract: We study
1
s
and
2
p
hard x-ray photoemission spectra (XPS) in a series of late transition metal oxides:
Fe
2
O
3
(
3
d
5
),
FeTiO
3
(
3
d
6
), CoO (
3
d
7
), and NiO (
3
d
8
). The experimental spectra are analyzed with two theoretical approaches:
MO
6
cluster model and local density approximation (LDA)
+
dynamical mean-field theory (DMFT). Owing to the absence of the core-valence multiplets and spin-orbit coupling,
1
s
XPS is found to be a sensitive probe of chemical bonding and nonlocal charge-transfer screening, providing complementary information to
2
p
XPS. The
1
s
XPS spectra are used to assess the accuracy of the ab initio
LDA
+
DMFT
approach, developed recently to study the material-specific charge-transfer effects in core-level XPS.
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Aug 2019
|
|
I09-Surface and Interface Structural Analysis
|
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]
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
|
Abstract: The electronic properties of
CaCuO
2
/
La
0.7
Sr
0.3
MnO
3
(LSMO) superlattices are determined by the electronic structure of the structural units and in particular their interfaces. The electronic structure of LSMO is governed by a metal-insulator transition, which is controlled by the thickness of the units and the sample temperature, resulting in a systematic downward band shift for metallic samples (i.e., thick LSMO units, low temperature). We present a systematic study of the changes in the valence-band structure and screening features in Mn
2
p
and Cu
2
p
core-level spectra. The results show that hybridization of Cu
3
d
orbitals with out-of-plane O
2
p
orbitals can be systematically tuned by controlling the band alignment at the interface via the metal-to-insulator transition of the LSMO units. This opens a new route to rational design of functional interfaces and control of orbital reconstructions.
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Sep 2019
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