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

@article{Wadhwa_2025a,

title = {Machine learning study of surface reconstructions of the Cu_{2}⁢O⁡(111) surface},

author = {Payal Wadhwa and Michael Schmid and Georg Kresse},

doi = {10.1103/sfjm-1gyr},

year  = {2025},

date = {2025-11-17},

journal = {Physical Review B},

volume = {112},

issue = {20},

pages = {205420},

abstract = {The atomic structure of the most stable reconstructed surface of cuprous oxide (Cu_{2}⁢O)⁢(111) surface has been a longstanding topic of debate. In this study, we develop on-the-fly machine-learned force fields (MLFFs) to systematically investigate the various reconstructions of the Cu_{2}⁢O⁡(111) surface under stoichiometric as well as O- and Cu-deficient or rich conditions, focusing on both (√3×√3⁢)R⁢30∘ and (2×2) supercells. By utilizing parallel tempering simulations supported by MLFFs, we confirm that the previously described nanopyramidal and Cu-deficient (1×1) structures are the lowest energy structures from moderately to strongly oxidizing conditions. In addition, we identify two promising nanopyramidal reconstructions at highly reducing conditions, a stoichiometric one and a Cu-rich one. Surface energy calculations performed using spin-polarized PBE, PBE+𝑈, r2⁢SCAN, and HSE06 functionals show that the previously known Cu-deficient configuration and nanopyramidal configurations are at the convex hull (and, thus, equilibrium structures) for all functionals, whereas the stability of the other structures depends on the functional and is therefore uncertain. Our findings demonstrate that on-the-fly trained MLFFs provide a simple, efficient, and rapid approach to explore the complex surface reconstructions commonly encountered in experimental studies, and also enhance our understanding of the stability of Cu_{2}⁢O⁡(111) surfaces.},

keywords = {P02, P03},

pubstate = {published},

tppubtype = {article}

}

@article{Huetner_2025a,

title = {Surface reconstructions govern ice nucleation on silver iodide},

author = {Johanna I. Hütner and Andrea Conti and David Kugler and Franziska Sabath and Kim Noelle Dreier and Hans-Georg Stammler and Florian Mittendorfer and Angelika Kühnle and Michael Schmid and Ulrike Diebold and Jan Balajka},

doi = {10.1126/sciadv.aea2378},

year  = {2025},

date = {2025-10-31},

journal = {Science Advances},

volume = {11},

number = {44},

pages = {eaea2378},

abstract = {Silver iodide (AgI) is among the most effective ice-nucleating agents, attributed to its close lattice match with hexagonal ice. However, the atomic-level mechanism behind its efficiency remains unclear. The basal surfaces of AgI are polar and inherently unstable, necessitating a compensation mechanism, such as surface reconstruction, which may disrupt the favorable lattice match with ice. We combine noncontact atomic force microscopy with advanced computational modeling to determine the atomic structure of basal AgI surfaces in ultrahigh vacuum. The Ag-terminated (0001) surface exhibits a (2 × 2) reconstruction with ordered Ag vacancies, preserving a hexagonal arrangement of surface atoms that facilitates epitaxial ice growth. In contrast, the I-terminated (0001) surface adopts a complex rectangular reconstruction, incompatible with continuous ice layer formation. These findings highlight the decisive role of surface atomic structure and indicate that the Ag-terminated basal plane is primarily responsible for efficient ice nucleation on AgI.},

keywords = {P02, P04},

pubstate = {published},

tppubtype = {article}

}

@article{Eder_2025a,

title = {Multi-technique characterization of rhodium gem-dicarbonyls on TiO_{2}(110)},

author = {Moritz Eder and Faith J. Lewis and Johanna I. Hütner and Panukorn Sombut and Maosheng Hao and David Rath and Paul Ryan and Jan Balajka and Margareta Wagner and Matthias Meier and Cesare Franchini and Gianfranco Pacchioni and Ulrike Diebold and Michael Schmid and Florian Libisch and Jiři Pavelec and Gareth S. Parkinson},

doi = {10.1039/D5SC04889C},

year  = {2025},

date = {2025-10-16},

journal = {Chemical Science},

abstract = {Gem-dicarbonyls of transition metals supported on metal (oxide) surfaces are common intermediates in heterogeneous catalysis. While infrared (IR) spectroscopy is a standard tool for detecting these species on powder catalysts, the ill-defined crystallographic environment renders data interpretation challenging. In this work, we apply a multi-technique surface science approach to investigate rhodium gem-dicarbonyls on a single-crystalline rutile TiO_{2}(110) surface. We combine spectroscopy, scanning probe microscopy, and density functional theory (DFT) to determine their location and coordination on the surface. IR spectroscopy shows the successful creation of gem-dicarbonyls on a titania single crystal by exposing deposited Rh atoms to CO gas, followed by annealing to 200–250 K. Low-temperature scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) data reveal that these complexes are mostly aligned along the [001] crystallographic direction, corroborating theoretical predictions. Notably, X-ray photoelectron spectroscopy (XPS) data reveal multiple rhodium species on the surface, even when the IR spectra show only the signature of rhodium gem-dicarbonyls. As such, our results highlight the complex behavior of carbonyls on metal oxide surfaces, and demonstrate the necessity of multi-technique approaches for the adequate characterization of single-atom catalysts.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}

@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{Sombut_2025a,

title = {The surface phase diagram of Fe_{3}O_{4}(001) revisited},

author = {Panukorn Sombut and Matthias Meier and Moritz Eder and Thomas Angerler and Oscar Gamba and Michael Schmid and Ulrike Diebold and Cesare Franchini and Gareth S. Parkinson},

doi = {10.1039/D5LF00022J},

year  = {2025},

date = {2025-03-14},

urldate = {2025-03-14},

journal = {RSC Applied Interfaces},

volume = {2},

issue = {3},

pages = {673-683},

abstract = {Understanding how the physical and electronic structures of metal-oxide surfaces evolve under varying conditions is crucial for optimizing their performance in applications such as catalysis. In this study, we compute the surface phase diagram of the Fe_{3}O_{4}(001) facet using density functional theory (DFT)-based calculations, with an emphasis on understanding the terminations observed in surface science experiments. Our results reveal two stable terminations in addition to the subsurface cation vacancy (SCV) structure, which dominates under oxidizing conditions. The commonly reported octahedral Fe pair, also known as the Fe-dimer termination, is stable within an oxygen chemical potential range of −3.1 eV < μ_{O} < −2.3 eV. We identify the lowest-energy structure of this surface as the one proposed by J. R. Rustad, E. Wasserman and A. R. Felmy, A Molecular Dynamics Investigation of Surface Reconstruction on Magnetite (001), \textit{Surf. Sci.}, 1999, \textbf{432}, 1–2, where a tetrahedrally coordinated Fe_{A} atom is replaced by two octahedrally coordinated Fe_{B} atoms in the surface layer. This transformation serves as a precursor to the emergence of an FeO-like termination under highly reducing conditions. A key insight from our study is the importance of thoroughly sampling different charge-order configurations to identify the global minima across varying stoichiometries.},

keywords = {P02, P04, P07},

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}

}