I09-Surface and Interface Structural Analysis
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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.
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Jun 2021
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I09-Surface and Interface Structural Analysis
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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.
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Jun 2021
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[15856]
Abstract: Depositing disordered Al on top of
Sr
Ti
O
3
is a cheap and easy way to create a two-dimensional electron system in the
Sr
Ti
O
3
surface layers. To facilitate future device applications, we passivate the heterostructure by a disordered
La
Al
O
3
capping layer to study the electronic properties by complementary x-ray photoemission spectroscopy and transport measurements on the very same samples. We also tune the electronic interface properties by adjusting the oxygen pressure during film growth.
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Jun 2021
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I09-Surface and Interface Structural Analysis
|
Huw
Shiel
,
Theodore D. C.
Hobson
,
Oliver S.
Hutter
,
Laurie J.
Phillips
,
Matthew J.
Smiles
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Jack E. N.
Swallow
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Jonathan D.
Major
,
Ken
Durose
,
Tim D.
Veal
Diamond Proposal Number(s):
[23160]
Open Access
Abstract: Antimony selenide (Sb2
2
Se3
3
) possesses great potential in the field of photovoltaics (PV) due to its suitable properties for use as a solar absorber and good prospects for scalability. Previous studies have reported the growth of a native antimony oxide (Sb2
2
O3
3
) layer at the surface of Sb2
2
Se3
3
thin films during deposition and exposure to air, which can affect the contact between Sb2
2
Se3
3
and subsequent layers. In this study, photoemission techniques were utilized on both Sb2
2
Se3
3
bulk crystals and thin films to investigate the band alignment between Sb2
2
Se3
3
and the Sb2
2
O3
3
layer. By subtracting the valence band spectrum of an in situ cleaved Sb2
2
Se3
3
bulk crystal from that of the atmospherically contaminated bulk crystal, a valence band offset (VBO) of −1.72
−
1.72
eV is measured between Sb2
2
Se3
3
and Sb2
2
O3
3
. This result is supported by a −1.90
−
1.90
eV VBO measured between Sb2
2
O3
3
and Sb2
2
Se3
3
thin films via the Kraut method. Both results indicate a straddling alignment that would oppose carrier extraction through the back contact of superstrate PV devices. This work yields greater insight into the band alignment of Sb2
2
O3
3
at the surface of Sb2
2
Se3
3
films, which is crucial for improving the performance of these PV devices.
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Jun 2021
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I09-Surface and Interface Structural Analysis
|
Peter
Knecht
,
Joachim
Reichert
,
Peter S.
Deimel
,
Peter
Feulner
,
Felix
Haag
,
Francesco
Allegretti
,
Manuela
Garnica
,
Martin
Schwarz
,
Willi
Auwärter
,
Paul T. P.
Ryan
,
Tien-Lin
Lee
,
David A.
Duncan
,
Ari Paavo
Seitsonen
,
Johannes V.
Barth
,
Anthoula Chrysa
Papageorgiou
Diamond Proposal Number(s):
[24320]
Open Access
Abstract: We assess the crucial role of tetrapyrrole flexibility in the CO ligation to distinct Ru‐porphyrins supported on an atomistically well‐defined Ag(111) substrate. Our systematic real space visualisation and manipulation experiments with scanning tunnelling microscopy directly probe the ligation, while bond‐resolving atomic force microscopy and X‐ray standing waves measurements characterise the geometry, X‐ray and ultraviolet photoelectron spectroscopy the electronic structure, and temperature programmed desorption the binding strength. Density functional theory calculations provide additional insight into the functional interface. We unambiguously demonstrate that the substituents regulate the interfacial conformational adaptability, either promoting or obstructing the uptake of axial CO adducts.
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May 2021
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I09-Surface and Interface Structural Analysis
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C.
Kalha
,
S.
Bichelmaier
,
N. K.
Fernando
,
J. V.
Berens
,
P. K.
Thakur
,
T.-L.
Lee
,
J. J.
Gutiérrez Moreno
,
S.
Mohr
,
L. E.
Ratcliff
,
M.
Reisinger
,
J.
Zechner
,
M.
Nelhiebel
,
A.
Regoutz
Diamond Proposal Number(s):
[19885]
Open Access
Abstract: The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallization scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high-temperature events or its behavior when exposed to air. Here, a combined soft and hard x-ray photoelectron spectroscopy (XPS) characterization approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behavior of a 300 nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales, and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the bulk titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft x-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO2, WO3, and a WO3 oxide coordinated with a titanium environment. This combinatorial characterization approach provides valuable insights into the thermal and oxidation stability of TiW alloys from two depth perspectives, aiding the development of future device technologies.
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May 2021
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I09-Surface and Interface Structural Analysis
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Peter
Knecht
,
Bodong
Zhang
,
Joachim
Reichert
,
David A.
Duncan
,
Martin
Schwarz
,
Felix
Haag
,
Paul
Ryan
,
Tien-Lin
Lee
,
Peter S.
Deimel
,
Peter
Feulner
,
Francesco
Allegretti
,
Willi
Auwärter
,
Guillaume
Médard
,
Ari Paavo
Seitsonen
,
Johannes V.
Barth
,
Anthoula C.
Papageorgiou
Diamond Proposal Number(s):
[24320]
Abstract: The controlled arrangement of N-heterocyclic carbenes (NHCs) on solid surfaces is a current challenge of surface functionalization. We introduce a strategy of using Ru porphyrins in order to control both the orientation and lateral arrangement of NHCs on a planar surface. The coupling of the NHC to the Ru porphyrin is a facile process which takes place on the interface: we apply NHCs as functional, robust pillars on well-defined, preassembled Ru porphyrin monolayers on silver and characterize these interfaces with atomic precision via a battery of experimental techniques and theoretical considerations. The NHCs assemble at room temperature modularly and reversibly on the Ru porphyrin arrays. We demonstrate a selective and complete functionalization of the Ru centers. With its binding, the NHC modifies the interaction of the Ru porphyrin with the Ag surface, displacing the Ru atom by 1 Å away from the surface. This arrangement of NHCs allows us to address individual ligands by controlled manipulation with the tip of a scanning tunneling microscope, creating patterned structures on the nanometer scale.
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Mar 2021
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I09-Surface and Interface Structural Analysis
|
Peter
Knecht
,
Paul T. P.
Ryan
,
David A.
Duncan
,
Li
Jiang
,
Joachim
Reichert
,
Peter S.
Deimel
,
Felix
Haag
,
Johannes T.
Kuchle
,
Francesco
Allegretti
,
Tien-Lin
Lee
,
Martin
Schwarz
,
Manuela
Garnica
,
Willi
Auwärter
,
Ari Paavo
Seitsonen
,
Johannes V.
Barth
,
Anthoula C.
Papageorgiou
Diamond Proposal Number(s):
[24320, 17634]
Abstract: The adsorption and monolayer self-assembly of functional metal–organic blocks on solid surfaces are critical for the physicochemical properties of these low-dimensional materials. Although modern microscopy tools are very sensitive to the lateral arrangement of such blocks, they are still unable to offer directly the complete structural analysis especially for nonplanar molecules containing different atoms. Here, we apply a combinatorial approach for the characterization of such interfaces, which enables unexpected insights. An archetypal metalloporphyrin on a catalytically active surface as a function of its molecular coverage and substituent arrangement is characterized by low-energy electron diffraction, scanning probe microscopy, X-ray photoelectron spectroscopy, normal-incidence X-ray standing waves, and density functional theory. We look into Ru tetraphenyl porphyrin (Ru-TPP) on Ag(111), which is also converted into its planarized derivates via surface-assisted cyclodehydrogenation reactions. Depending on the arrangement of the phenyl substituents, the Ru atoms have distinct electronic structures and the porphyrin macrocycles adapt differently to the surface: saddle shape (pristine Ru-TPP) or bowl shape (planarized Ru-TPP derivates). In all cases, the Ru atom resides close to the surface (2.59/2.45 Å), preferably located at hollow sites and in the interface between the plane of the porphyrin macrocycle and the Ag surface. For the more flexible pristine Ru-TPP, we identify an additional self-assembled structure, allowing the molecular density of the self-assembled monolayer to be tuned within ∼20%. This precise analysis is central to harnessing the potential of metalloporphyrin/metal interfaces in functional systems.
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Jan 2021
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I09-Surface and Interface Structural Analysis
|
B. F.
Spencer
,
S.
Maniyarasu
,
B.
Reed
,
D. J. H.
Cant
,
R.
Ahumada-Lazo
,
A. G.
Thomas
,
C. A.
Muryn
,
M.
Maschek
,
S. K.
Eriksson
,
T.
Wiell
,
T.-L.
Lee
,
S.
Tougaard
,
A. G.
Shard
,
W. R.
Flavell
Diamond Proposal Number(s):
[20059]
Abstract: Hard X-ray Photoelectron Spectroscopy (HAXPES) provides minimally destructive depth profiling into the bulk, extending the photoelectron sampling depth. Detection of deeply buried layers beyond the elastic limit is enabled through inelastic background analysis. To test the robustness of this technique, we present results on a thin (18 nm) layer of buried metal-organic complex buried below up to 200 nm of organic material. Overlayers with thicknesses 25-140 nm were measured using photon energies ranging 6-10 keV at the I09 end station at Diamond Light Source, and a new fixed energy Ga Kα (9.25 keV) laboratory-based HAXPES spectrometer was also used to measure samples with overlayers up to 200 nm thick. The sampling depth was varied: at Diamond Light Source by changing the photon energy, and in the lab system by performing angle-resolved measurements. For all the different overlayers and sampling depths, inelastic background modelling consistently provided thicknesses which agreed, within reasonable error, with the ellipsometric thickness. Relative sensitivity factors were calculated, and these factors consistently provided reasonable agreement with the expected nominal stoichiometry, suggesting the calculation method can be extended to any element. These results demonstrate the potential for the characterisation of deeply buried layers using synchrotron and laboratory-based HAXPES.
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Dec 2020
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I09-Surface and Interface Structural Analysis
|
Huw
Shiel
,
Oliver S.
Hutter
,
Laurie J.
Phillips
,
Jack E. N.
Swallow
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Vinod R.
Dhanak
,
Jonathan D.
Major
,
Tim D.
Veal
Diamond Proposal Number(s):
[23160]
Abstract: Sb2Se3 is a promising material for use in photovoltaics, but the optimum device structure has not yet been identified. This study provides band alignment measurements between Sb2Se3, identical to that used in high-efficiency photovoltaic devices, and its two most commonly used window layers, namely, CdS and TiO2. Band alignments are measured via two different approaches: Anderson’s rule was used to predict an interface band alignment from measured natural band alignments, and the Kraut method was used in conjunction with hard X-ray photoemission spectroscopy to directly measure the band offsets at the interface. This allows examination of the effect of interface formation on the band alignments. The conduction band minimum (CBM) of TiO2 is found by the Kraut method to lie 0.82 eV below that of Sb2Se3, whereas the CdS CBM is only 0.01 eV below that of Sb2Se3. Furthermore, a significant difference is observed between the natural alignment- and Kraut method-determined offsets for TiO2/Sb2Se3, whereas there is little difference for CdS/Sb2Se3. Finally, these results are related to device performance, taking into consideration how these results may guide the future development of Sb2Se3 solar cells and providing a methodology that can be used to assess band alignments in device-relevant systems.
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Dec 2020
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