I09-Surface and Interface Structural Analysis
|
M.
Chrysler
,
J.
Gabel
,
T.-L.
Lee
,
A. N.
Penn
,
B. E.
Matthews
,
D. M.
Kepaptsoglou
,
Q. M.
Ramasse
,
J. R.
Paudel
,
R. K.
Sah
,
J. D.
Grassi
,
Z.
Zhu
,
A. X.
Gray
,
J. M.
Lebeau
,
S. R.
Spurgeon
,
S. A.
Chambers
,
P. V.
Sushko
,
J. H.
Ngai
Diamond Proposal Number(s):
[25582, 26487]
Abstract: We demonstrate that the interfacial dipole associated with bonding across the
SrTi
O
3
/
Si
heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into
SrTi
O
3
. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.
|
Oct 2021
|
|
I09-Surface and Interface Structural Analysis
|
F.
Offi
,
K.
Yamauchi
,
S.
Picozzi
,
V.
Lollobrigida
,
A.
Verna
,
C.
Schlueter
,
T.-L.
Lee
,
A.
Regoutz
,
D. J.
Payne
,
A.
Petrov
,
G.
Vinai
,
G. M.
Pierantozzi
,
T.
Pincelli
,
G.
Panaccione
,
F.
Borgatti
Diamond Proposal Number(s):
[11322]
Abstract: Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of
La
0.65
Sr
0.35
MnO
3
(LSMO), a landmark system for spintronics applications, a description based solely on Mn
3
d
and O
2
p
electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La
5
p
, Mn
4
s
, and O
2
s
orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.
|
Oct 2021
|
|
I09-Surface and Interface Structural Analysis
|
Maximilian
Bauernfeind
,
Jonas
Erhardt
,
Philipp
Eck
,
Pardeep K.
Thakur
,
Judith
Gabel
,
Tien-Lin
Lee
,
Jörg
Schäfer
,
Simon
Moser
,
Domenico
Di Sante
,
Ralph
Claessen
,
Giorgio
Sangiovanni
Diamond Proposal Number(s):
[26419, 25151]
Open Access
Abstract: Large-gap quantum spin Hall insulators are promising materials for room-temperature applications based on Dirac fermions. Key to engineer the topologically non-trivial band ordering and sizable band gaps is strong spin-orbit interaction. Following Kane and Mele’s original suggestion, one approach is to synthesize monolayers of heavy atoms with honeycomb coordination accommodated on templates with hexagonal symmetry. Yet, in the majority of cases, this recipe leads to triangular lattices, typically hosting metals or trivial insulators. Here, we conceive and realize “indenene”, a triangular monolayer of indium on SiC exhibiting non-trivial valley physics driven by local spin-orbit coupling, which prevails over inversion-symmetry breaking terms. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice and unveil how a hidden honeycomb connectivity emerges from interference patterns in Bloch px ± ipy-derived wave functions.
|
Sep 2021
|
|
I09-Surface and Interface Structural Analysis
|
Matthew J.
Smiles
,
Jonathan M.
Skelton
,
Huw
Shiel
,
Leanne A. H.
Jones
,
Jack E. N.
Swallow
,
Holly J.
Edwards
,
Thomas
Featherstone
,
Philip A. E.
Murgatroyd
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Vinod R.
Dhanak
,
Tim D.
Veal
Diamond Proposal Number(s):
[21431, 23160]
Open Access
Abstract: Germanium sulfide and germanium selenide bulk crystals were prepared using a melt growth technique. X-ray photoemission spectroscopy (XPS) was used to determine ionisation potentials of 5.74 and 5.48 eV for GeS and GeSe respectively. These values were used with the previously-measured band gaps to establish the natural band alignments with potential window layers for solar cells and to identify CdS and TiO2 as sensible choices. The ionisation potential of GeS is found to be smaller than in comparable materials. Using XPS and hard x-ray photoemission (HAXPES) measurements in conjunction with density-functional theory calculations, we demonstrate that stereochemically active Ge 4s lone pairs are present at the valence-band maxima. Our work thus provides direct evidence for active lone pairs in GeS and GeSe, with important implications for the applications of these and related materials, such as Ge-based perovskites.
|
Sep 2021
|
|
I09-Surface and Interface Structural Analysis
|
Samuel P.
Jarvis
,
Hongqian
Sang
,
Filipe
Junqueira
,
Oliver
Gordon
,
Joe E. A.
Hodgkinson
,
Alex
Saywell
,
Philipp
Rahe
,
Salvatore
Mamone
,
Simon
Taylor
,
Adam
Sweetman
,
Jeremy
Leaf
,
David A.
Duncan
,
Tien-Lin
Lee
,
Pardeep K.
Thakur
,
Gabriella
Hoffman
,
Richard J.
Whitby
,
Malcolm H.
Levitt
,
Georg
Held
,
Lev
Kantorovich
,
Philip
Moriarty
,
Robert G.
Jones
Diamond Proposal Number(s):
[12979, 15022]
Open Access
Abstract: Molecular surgery provides the opportunity to study relatively large molecules encapsulated within a fullerene cage. Here we determine the location of an H2O molecule isolated within an adsorbed buckminsterfullerene cage, and compare this to the intrafullerene position of HF. Using normal incidence X-ray standing wave (NIXSW) analysis, coupled with density functional theory and molecular dynamics simulations, we show that both H2O and HF are located at an off-centre position within the fullerene cage, caused by substantial intra-cage electrostatic fields generated by surface adsorption of the fullerene. The atomistic and electronic structure simulations also reveal significant internal rotational motion consistent with the NIXSW data. Despite this substantial intra-cage interaction, we find that neither HF or H2O contribute to the endofullerene frontier orbitals, confirming the chemical isolation of the encapsulated molecules. We also show that our experimental NIXSW measurements and theoretical data are best described by a mixed adsorption site model.
|
Sep 2021
|
|
I09-Surface and Interface Structural Analysis
|
X. C.
Huang
,
W.-W.
Li
,
S.
Zhang
,
F. E.
Oropeza
,
G.
Gorni
,
V. A.
De La Pena-O'Shea
,
T.-L.
Lee
,
M.
Wu
,
L.-S.
Wang
,
D.-C.
Qi
,
L.
Qiao
,
J.
Cheng
,
K. H. L.
Zhang
Diamond Proposal Number(s):
[24219]
Abstract: In this paper, we report insights into the local atomic and electronic structure of
NiCo
2
O
4
epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined
NiCo
2
O
4
epitaxial thin films with controlled properties on
Mg
Al
2
O
4
(
001
)
substrates using pulsed laser deposition. Films grown at low temperatures (
<
400
∘
C
) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the
Ni
3
+
valence state promoted at low growth temperature introduces delocalized
Ni
3
d
-derived states at the Fermi level (
E
F
), responsible for the metallic state in
NiCo
2
O
4
, while the
Co
3
d
-related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and
∼
+
2
. Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the
Ni
2
+
valence state in
NiCo
2
O
4
. The
Ni
3
d
-related state becomes localized away from
E
F
, opening a band gap for a semiconducting state. The band gap of the semiconducting
NiCo
2
O
4
is estimated to be
<
0.8
eV
, which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is
d
−
d
dipole forbidden, and therefore, the semiconducting
NiCo
2
O
4
still shows reasonable transparency in the infrared-visible light region. The present insights into the role of
Ni
3
+
in determining the electronic structure and defect chemistry of
NiCo
2
O
4
provide important guidance for use of
NiCo
2
O
4
in electrocatalysis and opto-electronics.
|
Sep 2021
|
|
I15-Extreme Conditions
|
Diamond Proposal Number(s):
[9902, 8858]
Abstract: The tensile fractured surfaces of ZrTi-based bulk metallic glass and composite samples were studied using synchrotron X-ray total scattering. The scanned areas contain different shear bands densities. The shear bands create localized atomic strains, which in turn cause more ordered atomic structures. Such structural changes were reflected in the scattering structure factor, i.e. the higher the density of the shear bands, the higher the scattering structure factor. Similar phenomenon was also found in the metallic glass composite.
|
Aug 2021
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[23317]
Abstract: The discovery of topological superconductivity in doped
Bi
2
Se
3
made this class of materials highly important for the field of condensed matter physics. However, the structural origin of the superconducting state remained elusive, despite being investigated intensively in recent years. We use scanning tunneling microscopy and the normal incidence x-ray standing wave (NIXSW) technique in order to determine the vertical position of the dopants—one of the key parameters for understanding topological superconductivity in this material— for the case of
Sr
x
Bi
2
Se
3
. In particular, we analyze the NIXSW data in consideration of the inelastic mean free path of the photoemitted electrons, which allows us to distinguish between symmetry-equivalent sites. We find that Sr atoms are not situated inside the van der Waals gap between the
Bi
2
Se
3
quintuple layers but rather in the quintuple layer close to the outer Se planes.
|
Aug 2021
|
|
I09-Surface and Interface Structural Analysis
|
M.
Stübinger
,
J.
Gabel
,
Philipp
Scheiderer
,
M.
Zapf
,
M.
Schmitt
,
P.
Schütz
,
B.
Leikert
,
J.
Küspert
,
M.
Kamp
,
P. K.
Thakur
,
T.-L.
Lee
,
P.
Potapov
,
A.
Lubk
,
B.
Büchner
,
M.
Sing
,
R.
Claessen
Diamond Proposal Number(s):
[17499, 23737]
Abstract: A heterostructure consisting of the Mott insulator LaVO3 and the band insulator SrTiO3 is considered
a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of
LaVO3 match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the
nonpolar/polar interface when integrated with SrTiO3, also offer remarkable advantages over conventional solar
cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition
are scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of
soft and hard x-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the
electrical properties, band bending, and band alignment of LaVO3/SrTiO3 heterostructures. We find a critical
LaVO3 thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3d states at the
interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as
the driving mechanism for the formation of the metallic interface in LaVO3/SrTiO3.
|
Jun 2021
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[11841]
Abstract: The self-assembly of a nickel-porphyrin derivative (Ni-DPPyP) containing two pyridyl coordinating sites and two pentyl chains at trans meso positions was studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) on Au(111). Deposition of Ni-DPPyP onto Au(111) gave rise to a close-packed network for coverages smaller or equal to one monolayer as revealed by STM and LEED. The molecular arrangement of this two-dimensional network is stabilized via hydrogen bonds formed between the pyridyl’s nitrogen and hydrogen atoms from the pyrrole groups of neighboring molecules. Subsequent deposition of cobalt atoms onto the close-packed network and post-deposition annealing at 423 K led to the formation of a Co-coordinated hexagonal porous network. As confirmed by XPS measurements, the porous network is stabilized by metal-ligand interactions between one cobalt atom and three pyridyl ligands, each pyridyl ligand coming from a different Ni-DPPyP molecule.
|
Jun 2021
|
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