I09-Surface and Interface Structural Analysis
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Abstract: Superconducting microwave circuits are a promising physical implementation of quantum computing. A significant challenge with the development of superconducting qubits is the materials source of loss leading to qubit decoherence. Much of this loss is dielectric loss or quasiparticle dissipation at superconductor surfaces and interfaces. In order to understand the loss mechanisms microscopically, it is necessary to have an accurate picture of the atomic structure and microstructure. In this work, we explore the structure of three interfaces. First, we present non- destructive X-ray characterization of the NbH surface precipitation in Nb thin films. Unwanted hydride precipitation in niobium-based superconducting circuits is a side effect of hydrofluoric acid etching of the Nb surface oxide. The precipitate microstructure is challenging to probe because of the high mobility of hydrogen in niobium. Using X-rays diffraction, we show evidence supporting phase-field simulations that the nucleation of NbH occurs at free surfaces. Using darkfield X-ray nanoprobe microscopy, we identify a complex microstructure suggesting a martensitic nucleation that transitions to a dendritic growth. Next, we present X-ray standing wave excited X-ray photoelectron spectroscopy of the annealed, Nb(110) ordered oxide surface layer. We discover the existence of an oxygen interstitial rich subsurface layer and identify the origin of two distinct oxygen chemical states at the surface: one coming from this subsurface layer, and the other from the surface NbO termination. Last, we present the heteroepitaxy of single crystal Al2O3 with a TiN. We identify TiN as an ideal substrate for the epitaxial growth of Al2O3 in a capacitor geometry based on the high degree of crystallinity and sharp interfaces with minimal diffusion and
3
we measure the two-level-state loss of the Al2O3 junction dielectric layer. We present this interface as an alternative to the commonly used amorphous alumina in Josephson Junctions.
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Dec 2025
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[36085]
Open Access
Abstract: For establishing a fundamental understanding of the emerging properties of two-dimensional (2D) materials, a reliable determination of the crystallographic structure is essential, as we demonstrate in this work for the specific case of the quantum spin Hall insulator bismuthene. Diffraction-based methods are widely used for structure determination, however, they suffer from a fundamental shortcoming, the phase retrieval problem, that is the inability to directly measure the phase of scattered waves. The (normal incidence) X-ray standing wave (NIXSW) technique circumvents this problem by introducing a Bragg-generated X-ray standing wave field throughout the sample, relative to which any atomic species can be localized. In essence, a single NIXSW measurement captures the complex scattering factor (amplitude and phase) corresponding to one single Bragg reflection. Collecting data for multiple reflections enables a three-dimensional reconstruction of the scattering density as the Fourier sum of all measured scattering factors. Here, we utilize this technique to reveal the mechanism of a reversible switching process that has been reported for a 2D Bi layer recently (Tilgner et al., Nat. Commun. 16, 6171, 2025). In this prominent example, the Bi layer is confined between a 4H-SiC substrate and an epitaxial graphene layer, and can be reversibly switched between an electronically inactive precursor state and the bismuthene state. In our NIXSW imaging experiment, we clearly identify the change of the adsorption site of the Bi atoms, caused by H-saturation of one out of three Si dangling bonds per unit cell, as the key feature leading to the formation of the characteristic band structure of the 2D bismuthene honeycomb.
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Oct 2025
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I09-Surface and Interface Structural Analysis
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Niclas
Tilgner
,
Susanne
Wolff
,
Serguei
Soubatch
,
Tien-Lin
Lee
,
Andres David
Peña Unigarro
,
Sibylle
Gemming
,
F. Stefan
Tautz
,
Thomas
Seyller
,
Christian
Kumpf
,
Fabian
Göhler
,
Philip
Schädlich
Diamond Proposal Number(s):
[36085]
Open Access
Abstract: Quantum spin Hall insulators have been extensively studied both theoretically and experimentally because they exhibit robust helical edge states driven by spin-orbit coupling and offer the potential for applications in spintronics through dissipationless spin transport. Here we show that a single layer of elemental Bi, formed by intercalation of an epitaxial graphene buffer layer on SiC(0001), is a promising candidate for a quantum spin Hall insulator. This layer can be reversibly switched between an electronically inactive precursor state and a bismuthene state, the latter exhibiting the predicted band structure of a true two-dimensional bismuthene layer. Switching is accomplished by hydrogenation (dehydrogenation) of the sample. A partial passivation (activation) of Si dangling bonds causes a lateral shift of Bi atoms involving a change of the adsorption site. In the bismuthene state, the Bi honeycomb layer is a prospective quantum spin Hall insulator, inherently protected by the graphene sheet above and the H-passivated substrate below.
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Jul 2025
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I09-Surface and Interface Structural Analysis
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Connor
Fields
,
Aleksandra
Foerster
,
Sadegh
Ghaderzadeh
,
Ilya
Popov
,
Bang
Huynh
,
Filipe
Junqueira
,
Tyler
James
,
Sofia
Alonso Perez
,
David A.
Duncan
,
Tien-Lin
Lee
,
Yitao
Wang
,
Sally
Bloodworth
,
Gabriela
Hoffman
,
Mark
Walkey
,
Richard J.
Whitby
,
Malcolm H.
Levitt
,
Brian
Kiraly
,
James N.
O'Shea
,
Elena
Besley
,
Philip
Moriarty
Diamond Proposal Number(s):
[31574]
Open Access
Abstract: Charge transfer is fundamentally dependent on the overlap of the orbitals comprising the transport pathway. This has key implications for molecular, nanoscale, and quantum technologies, for which delocalization (and decoherence) rates are essential figures of merit. Here, we apply the core hole clock technique—an energy-domain variant of ultrafast spectroscopy—to probe the delocalization of a photoexcited electron inside a closed molecular cage, namely the Ar 2p54s1 state of Ar@C60. Despite marginal frontier orbital mixing in the ground configuration, almost 80% of the excited state density is found outside the buckyball due to the formation of a markedly diffuse hybrid orbital. Far from isolating the intracage excitation, the surrounding fullerene is instead a remarkably efficient conduit for electron transfer: we measure characteristic delocalization times of 6.6 ± 0.3 fs and ≲ 500 attoseconds, respectively, for a 3D Ar@C60 film and a 2D monolayer on Ag(111).
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May 2025
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I09-Surface and Interface Structural Analysis
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Open Access
Abstract: The synthesis of large, freestanding, single-atom-thick two-dimensional (2D) metallic materials remains challenging due to the isotropic nature of metallic bonding. Here, we present a bottom-up approach for fabricating macroscopically large, nearly freestanding 2D gold (Au) monolayers, consisting of nanostructured patches. By forming Au monolayers on an Ir(111) substrate and embedding boron (B) atoms at the Au/Ir interface, we achieve suspended monoatomic Au sheets with hexagonal structures and triangular nanoscale patterns. Alternative patterns of periodic nanodots are observed in Au bilayers on the B/Ir(111) substrate. Using scanning tunneling microscopy, X-ray spectroscopies, and theoretical calculations, we reveal the role of buried B species in forming the nanostructured Au layers. Changes in the Au monolayer’s band structure upon substrate decoupling indicate a transition from 3D to 2D metal bonding. The resulting Au films exhibit remarkable thermal stability, making them practical for studying the catalytic activity of 2D gold.
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Dec 2024
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[33305, 33757]
Open Access
Abstract: On-surface synthesis of functional molecular structures provides a route to the fabrication of materials tailored to exhibit bespoke catalytic, (opto)electronic, and magnetic properties. The fabrication of graphene nanoribbons via on-surface synthesis, where reactive precursor molecules are combined to form extended polymeric structures, provides quasi-1D graphitic wires that can be doped by tuning the properties/composition of the precursor molecules. Here, we combine the atomic precision of solution-phase synthetic chemistry with on-surface protocols to enable reaction steps that cannot yet be achieved in solution. Our focus of this work is the inclusion of porphyrin species within graphene nanoribbons to create porphyrin-fused graphene nanoribbons. A combination of scanning tunneling microscopy and photoelectron spectroscopy techniques is used to characterize a porphyrin-fused graphene nanoribbon formed on-surface from a linear polymer consisting of regularly spaced Ni-porphyrin units linked by sections of aryl rings which fuse together during the reaction to form graphitic regions between neighboring Ni-porphyrin units.
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Nov 2024
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I09-Surface and Interface Structural Analysis
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Pablo
Vezzoni Vicente
,
Tobias
Weiss
,
Dennis
Meier
,
Wenchao
Zhao
,
Birce Sena
Tömekçe
,
Marc
G. Cuxart
,
Benedikt P.
Klein
,
David A.
Duncan
,
Tien-Lin
Lee
,
Anthoula C.
Papageorgiou
,
Matthias
Muntwiler
,
Ari Paavo
Seitsonen
,
Willi
Auwärter
,
Peter
Feulner
,
Johannes V.
Barth
,
Francesco
Allegretti
Diamond Proposal Number(s):
[25907]
Abstract: In light of the recent research interest in low-dimensional bismuth structures as spin-active materials and topological insulators, we present a comprehensive characterization of the Bi/Au(111) interface. The nuanced evolution of Bi phases upon deposition in ultrahigh vacuum (UHV) on a Au(111) surface is investigated from semidisordered clusters to few-layer Bi(110) thin films. Particular attention is devoted to the high-coverage, submonolayer phases, commonly grouped under the (𝑃×√3) nomenclature. We bring forth a new model, refining the current understanding of the Bi/Au(111) interface and demonstrating the existence of submonolayer moiré superstructures, whose geometry and superperiodicity depend on their coverage. This tuneable periodicity paves the way for their use as tailored buffer and templating layers for epitaxial growth of thin films on Au(111). Finally, we clarify the growth mode of multilayer Bi(110) as bilayer-by-bilayer, allowing precise thickness control of anisotropically strained thin films. This holistic understanding of the structural properties of the material was enabled by the synergy of several experimental techniques, namely low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy and spectroscopy (STM, STS), and x-ray standing waves (XSW), further corroborated by density functional theory (DFT) simulations.
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Oct 2024
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[27468]
Open Access
Abstract: Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, we demonstrate that the X-ray standing wave technique allows the surface polarization profile of a ferroelectric thin film, as opposed to the average film polarity, to be probed directly. The X-ray standing wave technique provides the average Ti and Ba atomic positions, along the out-of-plane direction, near the surface of three differently strained
thin films. This technique gives direct access to the local ferroelectric polarization at and below the surface. By employing X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing species adsorbed on the surface is obtained. The different amplitude and orientation of the local ferroelectric polarizations are associated with surface charges attributed to different type, amount and spatial distribution of the oxygen-containing adsorbates.
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Oct 2024
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I09-Surface and Interface Structural Analysis
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Paul
Ryan
,
Panukorn
Sombut
,
Ali
Rafsanjani-Abbasi
,
Chunlei
Wang
,
Fulden
Eratam
,
Francesco
Goto
,
Cesare
Franchini
,
Ulrike
Diebold
,
Matthias
Meier
,
David A.
Duncan
,
Gareth S.
Parkinson
Diamond Proposal Number(s):
[31726]
Open Access
Abstract: Water–solid interfaces pervade the natural environment and modern technology. On some surfaces, water–water interactions induce the formation of partially dissociated interfacial layers; understanding why is important to model processes in catalysis or mineralogy. The complexity of the partially dissociated structures often makes it difficult to probe them quantitatively. Here, we utilize normal incidence X-ray standing waves (NIXSW) to study the structure of partially dissociated water dimers (H2O–OH) at the α-Fe2O3(012) surface (also called the (11̅02) or “R-cut” surface): a system simple enough to be tractable yet complex enough to capture the essential physics. We find the H2O and terminal OH groups to be the same height above the surface within experimental error (1.45 ± 0.04 and 1.47 ± 0.02 Å, respectively), in line with DFT-based calculations that predict comparable Fe–O bond lengths for both water and OH species. This result is understood in the context of cooperative binding, where the formation of the H-bond between adsorbed H2O and OH induces the H2O to bind more strongly and the OH to bind more weakly compared to when these species are isolated on the surface. The surface OH formed by the liberated proton is found to be in plane with a bulk truncated (012) surface (−0.01 ± 0.02 Å). DFT calculations based on various functionals correctly model the cooperative effect but overestimate the water–surface interaction.
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Sep 2024
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[26188, 33755]
Open Access
Abstract: The intercalation of graphene with suitable atomic species is one of the most frequently applied methods to decouple the graphene layer from the substrate in order to establish the classical electronic properties of graphene. In this context, we studied the bismuth (Bi) intercalation of the (6√3×6√3)R30° reconstructed so-called "zeroth layer graphene'' on SiC(0001). 
As reported earlier by Sohn et al. [J. Korean Phys. Soc. 78, 157 (2021)], two phases are formed depending on the amount of intercalated Bi, which in turn is controlled by the annealing temperature: The α phase, showing a 1×1 periodicity with respect to the substrate, and, at higher temperatures, the √3×√3 reconstructed β phase. We characterise both phases and the transformation from the α to the β phase by photoelectron spectroscopy, normal incidence x-ray standing waves, electron diffraction and electron microscopy. We clearly see an almost complete intercalation of the graphene layers in both phases, with strong (covalent) interaction between the topmost Si atoms of the substrate and the Bi intercalant, but only weak (van der Waals) interaction between Bi and the graphene layer. The n-doping of the graphene found for the α phase decreases continuously during the phase transformation, in agreement with a reduced density of the Bi intercalating layer. Missing core level shifts of the surface species as well as the normal incidence x-ray standing waves results indicate that all surface Si atoms remain saturated during the transition and no dangling bonds are formed. Low energy electron microscopy and diffraction reveal the coexistance of both phases after annealing to intermediate temperatures and allow a quantitative analysis of island sizes and numbers.
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Sep 2024
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