I09-Surface and Interface Structural Analysis
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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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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
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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
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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
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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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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15899, 18191]
Abstract: The structure of coadsorption phases formed on Ag(111) by TCNQ (7,7,8,8-tetracyanoquinodimethane) with Cs are compared with previously reported coadsorption phases formed with K, following investigation by scanning tunnelling microscopy (STM), low energy electron diffraction, soft X-ray photoelectrons spectroscopy and normal incidence X-ray standing waves (NIXSW). For each alkali we identify two ordered phases, one with an alkali: TCNQ stoichiometry of 1:1 and the other 2:1. STM images show the molecular organisation is the same for Cs and K, although only the K2TCNQ phase is commensurate with the substrate. A previously-published detailed structure determination of the K2TCNQ phase, complemented by density function theory calculations that identify bonding strengths, showed that the binding within the layer is much stronger than that of the layer to the substrate. Insensitivity to commensuration is thus to be expected. The situation for KTCNQ and CsTCNQ is less clear; these ordered incommensurate overlayers clearly have strong intralayer bonding, but the relative strength of the average overlayer-substrate bonding is unknown. NIXSW data show that the alkalis in these phases occupy adsorption sites far more distant from the substrate than the TCNQ molecules when compared to the near coplanar alkali-TCNQ geometry of K2TCNQ and Cs2TCNQ. Ultraviolet photoelectron spectra show increasing bonding shifts of TCNQ orbital states with alkali coverage.
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Nov 2020
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[16243]
Open Access
Abstract: Interesting electronic properties arise in vertically stacked graphene sheets, some of which can be controlled by mutual orientation of the adjacent layers. In this study, we investigate the MBE grown multilayer graphene on Ir(111) by means of STM, LEED and XPS and we examine the influence of the substrate on the geometric and electronic properties of bilayer graphene by employing XSW and ARPES measurements. We find that the MBE method does not limit the growth to two graphene layers and that the wrinkles, which arise through extended carbon deposition, play a crucial role in the multilayer growth. We also find that the bilayer and trilayer graphene sheets have graphitic-like properties in terms of the separation between the two layers and their stacking. The presence of the iridium substrate imposes a periodic potential induced by the moiré pattern that was found to lead to the formation of replica bands and minigaps in bilayer graphene. From tight-binding fits to our ARPES data we find that band renormalization takes place in multilayer graphene due to a weaker coupling of the upper-most graphene layer to the iridium substrate.
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Sep 2020
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I09-Surface and Interface Structural Analysis
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Jack E. N.
Swallow
,
Christian
Vorwerk
,
Piero
Mazzolini
,
Patrick
Vogt
,
Oliver
Bierwagen
,
Alexander
Karg
,
Martin
Eickhoff
,
Jörg
Schörmann
,
Markus R.
Wagner
,
Joseph William
Roberts
,
Paul R.
Chalker
,
Matthew J.
Smiles
,
Philip
Murgatroyd
,
Sara
Mohamed
,
Zachary W.
Lebens-higgins
,
Louis F. J.
Piper
,
Leanne A. H.
Jones
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Joel B.
Varley
,
Juergen
Furthmüller
,
Claudia
Draxl
,
Tim D.
Veal
,
Anna
Regoutz
Diamond Proposal Number(s):
[21430, 24670]
Abstract: The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy effcient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.
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Sep 2020
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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
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Christopher H.
Don
,
Huw
Shiel
,
Theodore D. C.
Hobson
,
Christopher N.
Savory
,
Jack E. N.
Swallow
,
Matthew J.
Smiles
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Ken
Durose
,
Jonathan D.
Major
,
Vinod R.
Dhanak
,
David O.
Scanlon
,
Tim D.
Veal
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The presence of a lone pair of 5s electrons at the valence band maximum (VBM) of Sb2Se3 and the resulting band alignments are investigated using soft and hard X-ray photoemission spectroscopy in parallel with density functional theory (DFT) calculations. Vacuum-cleaved and exfoliated bulk crystals of Sb2Se3 are analysed using laboratory and synchrotron X-ray sources to acquire high resolution valence band spectra with both soft and hard X-rays. Utilising the photon-energy dependence of different orbital cross-sections and corresponding DFT calculations, the various orbital contributions to the valence band could be identified, including the 5s orbital's presence at the VBM. The ionization potential is also determined and places the VBM at 5.13 eV below the vacuum level, similar to other materials with 5s2 lone pairs, but far above those of related materials without lone pairs of electrons.
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Aug 2020
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