I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[18398]
Abstract: Using the normal incidence x-ray standing-wave technique as well as low-energy electron microscopy we have investigated the structure of quasifreestanding monolayer graphene (QFMLG) obtained by intercalation of antimony under the
(
6
√
3
×
6
√
3
)
R
30
∘
reconstructed graphitized
6
H
-SiC(0001) surface, also known as zeroth-layer graphene. We found that Sb intercalation decouples the QFMLG well from the substrate. The distance from the QFMLG to the Sb layer almost equals the expected van der Waals bonding distance of C and Sb. The Sb intercalation layer itself is monoatomic, flat, and located much closer to the substrate, at almost the distance of a covalent Sb-Si bond length. All data is consistent with Sb located on top of the uppermost Si atoms of the SiC bulk.
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Oct 2022
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I09-Surface and Interface Structural Analysis
|
You-Ron
Lin
,
Markus
Franke
,
Shayan
Parhizkar
,
Miriam
Raths
,
Victor
Wen-Zhe Yu
,
Tien-Lin
Lee
,
Serguei
Soubatch
,
Volker
Blum
,
F. Stefan
Tautz
,
Christian
Kumpf
,
Francois C.
Bocquet
Diamond Proposal Number(s):
[17737]
Abstract: In the field of van der Waals heterostructures, the twist angle between stacked two-dimensional layers has been identified to be of utmost importance for the properties of the heterostructures. In this context, we previously reported the growth of a single layer of unconventionally oriented epitaxial graphene that forms in a surfactant atmosphere [F. C. Bocquet et al., Phys. Rev. Lett. 125, 106102 (2020)]. The resulting G-
R
0
∘
layer is aligned with the SiC lattice, and hence represents an important milestone towards high-quality twisted bilayer graphene, a frequently investigated model system in this field. Here, we focus on the surface structures obtained in the same surfactant atmosphere, but at lower preparation temperatures at which a boron nitride template layer forms on SiC(0001). In a comprehensive study based on complementary experimental and theoretical techniques, we find—in contrast to the literature—that this template layer is a hexagonal
B
x
N
y
layer, but not high-quality hBN. It is aligned with the SiC lattice and gradually replaced by low-quality graphene in the
0
∘
orientation of the
B
x
N
y
template layer upon annealing.
|
Jun 2022
|
|
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
|
Diamond Proposal Number(s):
[17737, 20810, 20855]
Abstract: We report the use of a surfactant molecule during the epitaxy of graphene on SiC(0001) that leads to the growth in an unconventional orientation, namely
R
0
°
rotation with respect to the SiC lattice. It yields a very high-quality single-layer graphene with a uniform orientation with respect to the substrate, on the wafer scale. We find an increased quality and homogeneity compared to the approach based on the use of a preoriented template to induce the unconventional orientation. Using spot profile analysis low-energy electron diffraction, angle-resolved photoelectron spectroscopy, and the normal incidence x-ray standing wave technique, we assess the crystalline quality and coverage of the graphene layer. Combined with the presence of a covalently bound graphene layer in the conventional orientation underneath, our surfactant-mediated growth offers an ideal platform to prepare epitaxial twisted bilayer graphene via intercalation.
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Sep 2020
|
|
I09-Surface and Interface Structural Analysis
|
Christine
Bruelke
,
Timo
Heepenstrick
,
Ina
Krieger
,
Beatrice
Wolff
,
Xiaosheng
Yang
,
Ali
Shamsaddinlou
,
Simon
Weiss
,
Francois
Bocquet
,
Stefan
Tautz
,
Serguei
Soubatch
,
Moritz
Sokolowski
Diamond Proposal Number(s):
[14878]
Abstract: The adsorption geometry, the electronic properties, and the adsorption energy of the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a monolayer of hexagonal boron nitride (hBN) grown on the Cu(111) surface were determined experimentally. The perylene core is at a large height of 3.37
Å
and only a minute downward displacement of the functional anhydride groups (0.07 Å) occurs, yielding adsorption heights that agree with the sum of the involved van der Waals radii. Thus, already a single hBN layer leads to a decoupled (physisorbed) molecule, contrary to the situation on the bare Cu(111) surface.
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Mar 2019
|
|
I09-Surface and Interface Structural Analysis
|
Open Access
Abstract: We introduce a software, Torricelli, for the analysis of normal incidence x-ray standing wave data. In particular, given the experimental x-ray reflectivity and photoelectron yield of a data set (photon energy scan), Torricelli provides the corresponding structural parameters. The algorithm and equations on which Torricelli is based are explained here in detail. In particular, the model of the experimental reflectivity takes into account the theoretical reflectivity of the double crystal monochromator as well as the sample crystal, and a Gaussian broadening to account for mosaicity and photon energy spread. If statistical errors are provided together with the photoelectron yield data, these are propagated to produce the statistical errors of the structural parameters. For a more accurate analysis, angle-dependent correction parameters specific to the photoemission process, also beyond the dipole approximation, can be taken into account, especially in the case of non-perfect normal incidence. The obtained structural parameters can be compared, averaged, and displayed in an Argand diagram, along with statistical error bars.
|
Dec 2018
|
|
I09-Surface and Interface Structural Analysis
|
S.
Weiß
,
D.
Gerbert
,
A.
Stein
,
A. K.
Schenk
,
X.
Yang
,
C.
Brülke
,
R.
Kremring
,
S.
Feldmann
,
F. C.
Bocquet
,
M.
Gille
,
S.
Hecht
,
M.
Sokolowski
,
P.
Tegeder
,
S.
Soubatch
,
F. S.
Tautz
Diamond Proposal Number(s):
[12627, 16056]
Abstract: Comparing the adsorption heights of various graphene nanoribbons on Cu(111) and Au(111) surfaces to those of graphene and π-conjugated planar organic molecules, we observe that two-dimensional graphene adsorbs much further away from the surface than both one-dimensional graphene nanoribbons and π-conjugated planar molecules—which represent zero-dimensional graphene flakes. We show that this is a direct consequence of the adsorbates' dimensionality. Our results provide invaluable insights into the interplay of Pauli repulsion, pushback effect, and chemical interaction for graphenelike adsorbates of any dimensionality on metal surfaces.
|
Aug 2018
|
|
I09-Surface and Interface Structural Analysis
|
Xiaosheng
Yang
,
Ina
Krieger
,
Daniel
Lüftner
,
Simon
Weiss
,
Timo
Heepenstrick
,
Michael
Hollerer
,
Philipp
Hurdax
,
Georg
Koller
,
Moritz
Sokolowski
,
Peter
Puschnig
,
Michael G.
Ramsey
,
F. Stefan
Tautz
,
Serguei
Soubatch
Diamond Proposal Number(s):
[14878]
Abstract: Properties of organic molecules adsorbed on metal surfaces are usually affected by the interaction with the substrate. There is a widespread belief that a thin dielectric layer between the molecular layer and the metal is sufficient for decoupling the former both physically and electronically. Using the example of perylenetetracarboxylic dianhydride on a bilayer of MgO on Ag(100), we show that this strategy is not always successful, as we observe a substantial charge transfer from the metal into the molecules. To avoid this, we suggest an alternative approach. Specifically, we deposit oxygen atoms on Cu(100) surface resulting in immobilization of the surface electrons in Cu-O bonds. This achieves a true electronic and physical decoupling. This mechanism of electronic surface hardening, proposed and demonstrated here by a combination of photoemission tomography, x-ray standing wave technique, and density functional calculations, is of general applicability as a strategy to decouple molecules from metals.
|
Jul 2018
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[10370, 11915, 13837]
Abstract: Molecular monolayer films containing two different types of molecules (so called heteromolecular films) are promising candidates for the controlled functionalization of metal-organic hybrid interfaces. This is particularly true for blends formed by charge donor and acceptor molecules. Here we study heteromolecular monolayer systems containing 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) as charge acceptor, and either copper-II- or tin-II-phthalocyanine (CuPc or SnPc) as charge donor, adsorbed on Ag(111). We find that both systems exhibit structural phases with identical lateral ordering (iso-structural phases), which is an important prerequisite for studying the role of the central metal atom without competing effects caused by different lateral structures. Using normal incidence x-ray standing waves and photoemission tomography we find distinct differences in the (vertical) geometric and electronic structure for the heteromolecular systems under study: While the vertical structure of CuPc is essentially unaffected by mixing with PTCDA, the SnPc clearly reacts to the formation of a blend by reducing its adsorption height by approx. 0.2 Å. Also, the vertical structure of the PTCDA anhydride groups changes strongly: While the anhydride oxygen atoms are located below the perylene core for most mixed phases, for one of the PTCDA+CuPc phases it is lying above the perylene core. Regarding the electronic structure we find that while mixing with PTCDA causes a complete depletion of the CuPc former lowest unoccupied molecular orbital (FLUMO), the SnPc FLUMO is pinned to the Fermi level instead, and thus it remains partially filled. We demonstrate that all these differences are driven by the rearrangement of the substrate electron density in the vicinity of the PTCDA molecules, which are caused by the interaction with the metal phthalocyanine molecules.
|
Mar 2018
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[11752]
Abstract: The hexagonal boron nitride (hBN) monolayer on the Cu(111) surface has recently been considered an example of an extremely weak hBN/metal interaction, as indicated, e.g., from the presence of an only electronic Moiré-like superstructure that was observed in scanning tunneling microscopy images. From these results a large bonding distance of the hBN sheet to the top most Cu layer can be envisaged, but has not been proven so far. We report a structural analysis of the hBN/Cu(111) interface based on high resolution low energy electron diffraction and normal incidence x-ray standing wave experiments. We find that both the boron and the nitrogen atoms are located at very large vertical distances of dB = 3.25 ± 0.02 Å and dN = 3.22 ± 0.03 Å with respect to the nominal position of the topmost Cu(111) layer. Significant vertical buckling and lateral distortions of the hBN layer can be excluded. These results demonstrate that the hBN monolayer on the Cu(111) surface is indeed well described by a rigid and geometrically well separated sheet.
|
Sep 2017
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