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.
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Aug 2021
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I09-Surface and Interface Structural Analysis
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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
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I09-Surface and Interface Structural Analysis
|
Benedikt P.
Klein
,
Juliana M.
Morbec
,
Markus
Franke
,
Katharina K.
Greulich
,
Malte
Sachs
,
Shayan
Parhizkar
,
Francois C.
Bocquet
,
Martin
Schmid
,
Samuel J.
Hall
,
Reinhard J.
Maurer
,
Bernd
Meyer
,
Ralf
Tonner
,
Christian
Kumpf
,
Peter
Kratzer
,
J. Michael
Gottfried
Diamond Proposal Number(s):
[16259]
Abstract: Interfaces between polycyclic π-electron systems and metals play prominent roles in organic or graphene-based (opto)electronic devices, in which performance-related parameters depend critically on the properties of metal/semiconductor contacts. Here, we explore how the topology of the π-electron system influences the bonding and the electronic properties of the interface. We use azulene as a model for nonalternant pentagon-heptagon (5-7) ring pairs and compare it to its isomer naphthalene, which represents the alternant 6-6 ring pair. Their coverage-dependent interaction with Ag(111) and Cu(111) surfaces was studied with the normal-incidence X-ray standing wave (NIXSW) technique, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, UV and X-ray photoelectron spectroscopy (UPS, XPS), and density functional theory (DFT). Coverage-dependent adsorption heights and spectroscopic data reveal that azulene forms shorter interfacial bonds than naphthalene and engages in stronger electronic interactions with both surfaces. These differences are more pronounced on Cu. Increasing coverages lead to larger adsorption heights, indicating bond weakening by intermolecular repulsion. The extensive DFT calculations include dispersive interactions using: (1) the DFT-D3 scheme, (2) the vdWsurf correction based on DFT-TS, (3) a Many-Body Dispersion (MBD) correction scheme, and (4) the D3surf scheme. All methods predict the adsorption heights reasonably well with an average error below 0.1 Å. The stronger bond of azulene is attributed to its nonalternant topology, which results in a reduced HOMO-LUMO gap and brings the LUMO energetically close to the Fermi energy of the metal, causing stronger hybridization with electronic states of the metal surfaces.
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Nov 2019
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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
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I09-Surface and Interface Structural Analysis
|
Benedikt P.
Klein
,
Nadine J.
Van Der Heijden
,
Stefan R.
Kachel
,
Markus
Franke
,
Claudio K.
Krug
,
Katharina K.
Greulich
,
Lukas
Ruppenthal
,
Philipp
Müller
,
Phil
Rosenow
,
Shayan
Parhizkar
,
Francois C.
Bocquet
,
Martin
Schmid
,
Wolfgang
Hieringer
,
Reinhard J.
Maurer
,
Ralf
Tonner
,
Christian
Kumpf
,
Ingmar
Swart
,
J. Michael
Gottfried
Diamond Proposal Number(s):
[16259]
Open Access
Abstract: The interaction of carbon-based aromatic molecules and nanostructures with metals can strongly depend on the topology of their π-electron systems. This is shown with a model system using the isomers azulene, which has a nonalternant π system with a 5-7 ring structure, and naphthalene, which has an alternant π system with a 6-6 ring structure. We found that azulene can interact much more strongly with metal surfaces. On copper (111), its zero-coverage desorption energy is 1.86 eV, compared to 1.07 eV for naphthalene. The different bond strengths are reflected in the adsorption heights, which are 2.30 Å for azulene and 3.04 Å for naphthalene, as measured by the normal incidence x-ray standing wave technique. These differences in the surface chemical bond are related to the electronic structure of the molecular π systems. Azulene has a lowlying LUMO that is close to the Fermi energy of Cu and strongly hybridizes with electronic states of the surface, as is shown by photoemission, near-edge x-ray absorption fine-structure, and scanning tunneling microscopy data in combination with theoretical analysis. According to density functional theory calculations, electron donation from the surface into the molecular LUMO leads to negative charging and deformation of the adsorbed azulene. Noncontact atomic force microscopy confirms the deformation, while Kelvin probe force microscopy maps show that adsorbed azulene partially retains its in-plane dipole. In contrast, naphthalene experiences only minor adsorption-induced changes of its electronic and geometric structure. Our results indicate that the electronic properties of metal-organic interfaces, as they occur in organic (opto)electronic devices, can be tuned through modifications of the π topology of the molecular organic semiconductor, especially by introducing 5-7 ring pairs as functional structural elements.
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Feb 2019
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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.
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Dec 2018
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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.
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Aug 2018
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[8449, 15122]
Abstract: The normal incidence x-ray standing waves technique is one of the most well-established methods for investigating the geometric structure at interfaces and surfaces. It is able to measure vertical positions and distances of individual atomic species with highest precision (typically <0.02 Å). These data not only yield valuable structural information, but also represent an excellent benchmark for density functional theory and ab initio calculations. Non-dipolar effects are well known to strongly affect the result, in particular when light elements are involved. A correction mechanism for these effects is established, but in its commonly-used form it is based on one essential restriction, namely the assumption of perfect normal incidence of the x-rays with respect to the relevant lattice planes of the crystal. Here, we show that small deviations from normal incidence, as they are unavoidable in typical experimental setups, lead to significant systematic errors in the NIXSW results. The magnitude of this effect depends on the specific conditions in a non-linear way and may reach up to 5%, corresponding to several tenths of an Ångström in the adsorption height. We present a straightforward way of accounting for this effect, and demonstrate that recording the photoelectron yield in an angular-resolved mode is indispensable, since the correction parameters strongly depend on the electron take-off angle.
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Jul 2017
|
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I09-Surface and Interface Structural Analysis
|
J.
Sforzini
,
P.
Hapala
,
M.
Franke
,
G.
Van Straaten
,
A.
Stöhr
,
S.
Link
,
S.
Soubatch
,
P.
Jelínek
,
T.-l.
Lee
,
U.
Starke
,
M.
Švec
,
F.
Bocquet
,
S.
Tautz
Diamond Proposal Number(s):
[10271]
Open Access
Abstract: We investigate the structural and electronic properties of nitrogen-doped epitaxial monolayer graphene
and quasifreestanding monolayer graphene on 6H-SiCð0001Þ by the normal incidence x-ray standing wave
technique and by angle-resolved photoelectron spectroscopy supported by density functional theory
simulations. With the location of various nitrogen species uniquely identified, we observe that for the same
doping procedure, the graphene support, consisting of substrate and interface, strongly influences the
structural as well as the electronic properties of the resulting doped graphene layer. Compared to epitaxial
graphene, quasifreestanding graphene is found to contain fewer nitrogen dopants. However, this lack of
dopants is compensated by the proximity of nitrogen atoms at the interface that yield a similar number of
charge carriers in graphene.
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Mar 2016
|
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I09-Surface and Interface Structural Analysis
|
J.
Sforzini
,
L.
Nemec
,
T.
Denig
,
B.
Stadtmüller
,
T. L
Lee
,
C.
Kumpf
,
S.
Soubatch
,
U.
Starke
,
P.
Rinke
,
V.
Blum
,
Francois
Bocquet
,
Stefan
Tautz
Diamond Proposal Number(s):
[8449, 10271]
Open Access
Abstract: We measure the adsorption height of hydrogen-intercalated quasifreestanding monolayer graphene on the
(0001) face of 6H silicon carbide by the normal incidence x-ray standing wave technique. A density
functional calculation for the full ð6
ffiffiffi
3 p × 6
ffiffiffi
3 p Þ-R30° unit cell, based on a van derWaals corrected exchange
correlation functional, finds a purely physisorptive adsorption height in excellent agreement with experiments,
a very low buckling of the graphene layer, a very homogeneous electron density at the interface, and
the lowest known adsorption energy per atom for graphene on any substrate.Astructural comparison to other
graphenes suggests that hydrogen-intercalated graphene on 6H-SiCð0001Þ approaches ideal graphene.
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Mar 2015
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