Surface structure and reactivity of
multi-component oxides at the atomic scale

@article{Sokolovic_2025a,

title = {Duality and degeneracy lifting in two-dimensional electron liquids on SrTiO_{3}(001)},

author = {Igor Sokolović and Eduardo B. Guedes and Thomas P. van Waas and Fei Guo and Samuel Poncé and Craig Polley and Michael Schmid and Ulrike Diebold and Milan Radović and Martin Setvín and J. Hugo Dil},

url = {https://doi.org/10.1038/s41467-025-59258-4},

year  = {2025},

date = {2025-05-17},

journal = {Nature Communications},

volume = {16},

issue = {1},

pages = {4594},

abstract = {Two-dimensional electron liquids (2DELs) have increasing technological relevance for ultrafast electronics and spintronics, yet significant gaps in their fundamental understanding are exemplified on the prototypical SrTiO_{3}. We correlate the exact SrTiO_{3}(001) surface structure with distinct 2DELs through combined microscopic angle-resolved photoemission spectroscopy and non-contact atomic force microscopy on truly bulk-terminated surfaces that alleviate structural uncertainties inherent to this long-studied system. The SrO termination is shown to develop a 2DEL following the creation of oxygen vacancies, unlike the intrinsically metallic TiO_{2} termination. Differences in degeneracy of the 2DELs, with nearly the same band filling and identical band bending, are assigned to polar distortions of the Ti atoms in combination with spin order, supported with the extraction of fundamental electron-phonon coupling strength. These results not only resolve the ambiguities regarding 2DELs on SrTiO_{3} thus far, but also pave the way to manipulating band filling and spin order in oxide 2DELs in general.},

keywords = {P02},

pubstate = {published},

tppubtype = {article}

}

@article{Sokolovic_2024a,

title = {How to cleave cubic perovskite oxides},

author = {Igor Sokolović and Michael Schmid and Ulrike Diebold and Martin Setvín},

doi = {10.1063/5.0233747},

year  = {2025},

date = {2025-03-20},

urldate = {2025-03-20},

journal = {Review of Scientific Instruments},

volume = {96},

issue = {3},

pages = {035113},

abstract = {Surfaces of cubic perovskite oxides attract significant attention for their physical tunability and high potential for technical applications. Bulk-terminated surfaces are desirable for theoretical modeling and experimental reproducibility, yet there is a lack of methods for preparing such well-defined surfaces. We discuss a method for strain-assisted cleaving of perovskite single crystals, using a setup easily transferable between different experimental systems. The details of the cleaving device and the procedure were optimized in a systematic study on the model cubic perovskite oxide SrTiO_{3}. The large-area morphology and typical distribution of surface terminations on cleaved SrTiO_{3}(001) are presented, with specific guidelines on how to distinguish well-cleaved surfaces from conchoidally fractured ones. The cleaving is applicable to other cubic perovskites, as demonstrated on KTaO_{3}(001) and BaTiO_{3}(001). This approach opens up a pathway for obtaining high-quality surfaces of this promising class of materials.},

keywords = {P02},

pubstate = {published},

tppubtype = {article}

}

@article{Schmid_2024a,

title = {ViPErLEED package II: Spot tracking, extraction and processing of I(V) curves},

author = {Michael Schmid and Florian Kraushofer and Alexander M. Imre and Tilman Kißlinger and Lutz Hammer and Ulrike Diebold and Michele Riva},

doi = {10.1103/PhysRevResearch.7.013006},

year  = {2025},

date = {2025-01-03},

urldate = {2025-01-03},

journal = {Physical Review Research},

volume = {7},

issue = {1},

pages = {013006},

abstract = {As part of the ViPErLEED project (Vienna package for Erlangen LEED, low-energy electron diffraction), computer programs have been developed for facile and user-friendly data extraction from movies of LEED images. The programs make use of some concepts from astronomical image processing and analysis. As a first step, flat-field and dark-frame corrections reduce the effects of inhomogeneities of the camera and screen. In a second step, for identifying all diffraction maxima ("spots"), it is sufficient to manually mark and label a single spot or very few spots. Then the program can automatically identify all other spots and determine the distortions of the image. This forms the basis for automatic spot tracking (following the "beams" as they move across the LEED screen) and intensity measurement. Even for complex structures with hundreds to a few thousand diffraction beams, this step takes less than a minute. The package also includes a program for further processing of these \textit{I}(\textit{V}) curves (averaging of equivalent beams, manual and/or automatic selection, smoothing) as well as several utilities. The software is implemented as a set of plugins for the public-domain image processing program ImageJ and provided as an open-source package.},

keywords = {P02},

pubstate = {published},

tppubtype = {article}

}

@article{Kraushofer_2024a,

title = {ViPErLEED package I: Calculation of curves and structural optimization},

author = {Florian Kraushofer and Alexander M. Imre and Giada Franceschi and Tilman Kißlinger and Erik Rheinfrank and Michael Schmid and Ulrike Diebold and Lutz Hammer and Michele Riva},

doi = {10.1103/PhysRevResearch.7.013005},

year  = {2025},

date = {2025-01-03},

urldate = {2025-01-03},

journal = {Physical Review Research},

volume = {7},

issue = {1},

pages = {013005},

abstract = {Low-energy electron diffraction (LEED) is a widely used technique in surface-science laboratories. Yet, it is rarely used to its full potential. The quantitative information about the surface structure, contained in the modulation of the intensities of the diffracted beams as a function of incident electron energy, LEED , is underutilized. To acquire these data, only minor adjustments would be required in most experimental setups, but existing analysis software is cumbersome to use and often computationally inefficient. The ViPErLEED (Vienna package for Erlangen LEED) project lowers these barriers, introducing a combined solution for user-friendly data acquisition, extraction, and computational analysis. These parts are discussed in three separate publications. Here, the focus is on the computational part of ViPErLEED, which performs highly automated LEED- calculations and structural optimizations. Minimal user input is required, and the functionality is significantly enhanced compared to existing solutions. Computation is performed by embedding the existing Erlangen tensor-LEED package (TensErLEED). ViPErLEED manages additional parallelization, monitors convergence, and processes all input and output. This makes LEED more accessible to new users while minimizing the potential for errors and the manual labor. Added functionalities include intelligent structure-dependent defaults for most calculation parameters, automatic detection of bulk and surface symmetries and their relationship, automated search procedures that preserve the symmetry and speed up convergence, adjustments to the TensErLEED code to handle larger systems than before, as well as parallelization and optimization. Modern file formats are used as input and output, and there is a direct interface to the atomic simulation environment (ASE) package. The software is implemented primarily in Python (version ) and provided as an open-source package (GNU GPLv3 or any later version). A structure determination of the surface is presented as an example for the application of the software.},

keywords = {P02},

pubstate = {published},

tppubtype = {article}

}

@article{Redondo2024,

title = {Real-space investigation of polarons in hematite Fe_{2}O_{3}},

author = {Jesus Redondo and Michele Reticcioli and Vit Gabriel and Dominik Wrana and Florian Ellinger and Michele Riva and Giada Franceschi and Erik Rheinfrank and Igor Sokolovic and Zdenek Jakub and Florian Kraushofer and Aji Alexander and Laerte L. Patera and Jascha Repp and Michael Schmid and Ulrike Diebold and Gareth S. Parkinson and Cesare Franchini and Pavel Kocan and Martin Setvin},

url = {https://arxiv.org/abs/2303.17945

https://www.science.org/doi/10.1126/sciadv.adp7833},

year  = {2024},

date = {2024-11-01},

urldate = {2024-09-27},

journal = {Science Advances},

volume = {10},

issue = {44},

pages = {eadp7833},

abstract = {In polarizable materials, electronic charge carriers interact with the surrounding ions, leading to quasiparticle behaviour. The resulting polarons play a central role in many materials properties including electrical transport, optical properties, surface reactivity and magnetoresistance, and polaron properties are typically investigated indirectly through such macroscopic characteristics. Here, noncontact atomic force microscopy (nc-AFM) is used to directly image polarons in Fe_{2}O_{3} at the single quasiparticle limit. A combination of Kelvin probe force microscopy (KPFM) and kinetic Monte Carlo (KMC) simulations shows that Ti doping dramatically enhances the mobility of electron polarons, and density functional theory (DFT) calculations indicate that a metallic transition state is responsible for the enhancement. In contrast, hole polarons are significantly less mobile and their hopping is hampered further by the introduction of trapping centres.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Ryan_2024a,

title = {Quantitative Measurement of Cooperative Binding in Partially Dissociated Water Dimers at the Hematite “R-Cut” Surface},

author = {Paul T. P. Ryan and Panukorn Sombut and Ali Rafsanjani-Abbasi and Chunlei Wang and Fulden Eratam and Francesco Goto and Ulrike Diebold and Matthias Meier and David A. Duncan and Gareth S. Parkinson},

url = {https://arxiv.org/abs/2406.18264

https://pubs.acs.org/doi/10.1021/acs.jpcc.4c04537},

year  = {2024},

date = {2024-09-30},

urldate = {2024-09-30},

journal = {The Journal of Physical Chemistry C},

volume = {128},

issue = {40},

pages = {16977–16985},

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 (H_{2}O–OH) at the α-Fe_{2}O_{3}(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 H_{2}O 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 H_{2}O and OH induces the H_{2}O 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.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Rafsanjani_2024a,

title = {Digging Its Own Site: Linear Coordination Stabilizes a Pt_{1}/Fe_{2}O_{3} Single-Atom Catalyst},

author = {Ali Rafsanjani-Abbasi and Florian Buchner and Faith J. Lewis and Lena Puntscher and Florian Kraushofer and Panukorn Sombut and Moritz Eder and Jiří Pavelec and Erik Rheinfrank and Giada Franceschi and Viktor Birschitzky and Michele Riva and Cesare Franchini and Michael Schmid and Ulrike Diebold and Matthias Meier and Georg K. H. Madsen and Gareth S. Parkinson},

url = {https://doi.org/10.1021/acsnano.4c08781},

year  = {2024},

date = {2024-09-18},

urldate = {2024-09-18},

journal = {ACS Nano},

volume = {18},

issue = {39},

pages = {26920–26927},

abstract = {Determining the local coordination of the active site is a prerequisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems and much emphasis is placed on density functional theory computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (11̅02) facet of α-Fe_{2}O_{3}; a common support material in SACs. Using a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudolinear coordination to surface oxygen atoms. Despite breaking three surface Fe–O bonds, this geometry is favored by 0.84 eV over the best configuration involving an unperturbed support. We suggest that the linear O–Pt–O configuration is common in reactive Pt-based SAC systems because it balances thermal stability with the ability to adsorb reactants from the gas phase. Moreover, we conclude that extensive structural searches are necessary to determine realistic active site geometries in single-atom catalysis.},

keywords = {P02, P04, P07, P09},

pubstate = {published},

tppubtype = {article}

}

@article{Wanzenboeck2024b,

title = {Exploring Inhomogeneous Surfaces: Ti-rich SrTiO_{3}(110) Reconstructions via Active Learning},

author = {Ralf Wanzenböck and Esther Heid and Michele Riva and Giada Franceschi and Alexander M. Imre and Jesús Carrete and Ulrike Diebold and Georg K. H. Madsen},

url = {https://doi.org/10.1039/D4DD00231H},

year  = {2024},

date = {2024-09-16},

urldate = {2024-09-16},

journal = {Digital Discovery},

volume = {3},

pages = {2137-2145},

abstract = {The investigation of inhomogeneous surfaces, where various local structures co-exist, is crucial for understanding interfaces of technological interest, yet it presents significant challenges. Here, we study the atomic configurations of the (2×m) Ti-rich surfaces at (110)-oriented SrTiO_{3} by bringing together scanning tunneling microscopy and transferable neural-network force fields combined with evolutionary exploration. We leverage an active learning methodology to iteratively extend the training data as needed for different configurations. Training on only small well-known reconstructions, we are able to extrapolate to the complicated and diverse overlayers encountered in different regions of the heterogeneous SrTiO_{3}(110)-(2×m) surface. Our machine-learning-backed approach generates several new candidate structures, in good agreement with experiment and verified using density functional theory. The approach could be extended to other complex metal oxides featuring large coexisting surface reconstructions.},

keywords = {P02, P09},

pubstate = {published},

tppubtype = {article}

}

@article{Huetner2024,

title = {Stoichiometric reconstruction of the Al_{2}O_{3}(0001) surface},

author = {Johanna I. Hütner and Andrea Conti and David Kugler and Florian Mittendorfer and Georg Kresse and Michael Schmid and Ulrike Diebold and Jan Balajka},

url = {https://www.science.org/doi/10.1126/science.adq4744

https://arxiv.org/abs/2405.19263},

issn = {1095-9203},

year  = {2024},

date = {2024-09-12},

urldate = {2024-09-12},

journal = {Science},

volume = {385},

pages = {1241--1244},

publisher = {American Association for the Advancement of Science (AAAS)},

abstract = {Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. We imaged the basal (0001) plane of α–aluminum oxide (α-Al_{2}O_{3}) using noncontact atomic force microscopy with an atomically defined tip apex. The surface formed a complex (√31 × √31)R±9° reconstruction. The lateral positions of the individual oxygen and aluminum surface atoms come directly from experiment; we determined with computational modeling how these connect to the underlying crystal bulk. Before the restructuring, the surface Al atoms assume an unfavorable, threefold planar coordination; the reconstruction allows a rehybridization with subsurface O that leads to a substantial energy gain. The reconstructed surface remains stoichiometric, Al_{2}O_{3}.},

keywords = {P02, P03},

pubstate = {published},

tppubtype = {article}

}

@article{Wang_2024b,

title = {A Multitechnique Study of C_{2}H_{4} Adsorption on a Model Single-Atom Rh_{1} Catalyst},

author = {Chunlei Wang and Panukorn Sombut and Lena Puntscher and Manuel Ulreich and Jiri Pavelec and David Rath and Jan Balajka and Matthias Meier and Michael Schmid and Ulrike Diebold and Cesare Franchini and Gareth S. Parkinson},

url = {https://doi.org/10.1021/acs.jpcc.4c03588},

year  = {2024},

date = {2024-09-05},

journal = {The Journal of Physical Chemistry C},

volume = {128},

issue = {37},

pages = {15404–15411},

abstract = {Single-atom catalysts are potentially ideal model systems to investigate structure–function relationships in catalysis if the active sites can be uniquely determined. In this work, we study the interaction of C_{2}H_{4} with a model Rh/Fe_{3}O_{4}(001) catalyst that features 2-, 5-, and 6-fold coordinated Rh adatoms, as well as Rh clusters. Using multiple surface-sensitive techniques in combination with calculations of density functional theory (DFT), we follow the thermal evolution of the system and disentangle the behavior of the different species. C_{2}H_{4} adsorption is strongest at the 2-fold coordinated Rh_{1} with a DFT-determined adsorption energy of −2.26 eV. However, desorption occurs at lower temperatures than expected because the Rh migrates into substitutional sites within the support, where the molecule is more weakly bound. The adsorption energy at the 5-fold coordinated Rh sites is predicated to be −1.49 eV, but the superposition of this signal with that from small Rh clusters and additional heterogeneity leads to a broad C_{2}H_{4} desorption shoulder in TPD above room temperature.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}