Polaron pattern recognition
in correlated oxide surfaces

@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{Joseph_2025a,

title = {Coupling between small polarons and ferroelectricity in BaTiO_{3}},

author = {Darin Joseph and Cesare Franchini},

doi = {10.1103/5z43-rm34},

year  = {2025},

date = {2025-09-30},

urldate = {2025-09-30},

journal = {Physical Review Materials},

volume = {9},

pages = {094415},

abstract = {In this study, we investigate the formation of electron and hole small polarons in the prototypical ferroelectric material BaTiO_{3}, with a focus on their interaction with ferroelectric distortive fields. To accurately describe the ferroelectric phase in electronically correlated BaTiO_{3}, we employ the HSE06 hybrid density functional, which addresses the limitations of conventional density-functional theory (DFT) and Hubbard-corrected DFT+U models, providing a more precise depiction of both ferroelectric and polaronic behaviors. Our analysis spans three structural phases of BaTiO_{3}: cubic, tetragonal, and rhombohedral. We uncover a unique phase-dependent trend in electron-polaron stability, which progressively increases across the structural phases, peaking in the rhombohedral phase due to the constructive coupling between the polaron and ferroelectric phonon fields. In contrast, hole polarons exhibit a stability pattern largely unaffected by the phase transitions. Furthermore, we observe that polaron self-trapping significantly alters the local ferroelectric distortive pattern, which propagates to neighboring sites but has a minimal effect on the long-range macroscopic spontaneous polarization. Charge trapping is also associated with localized spin formation, opening new possibilities for enhanced functionalities in multiferroic materials.},

keywords = {P07},

pubstate = {published},

tppubtype = {article}

}

@article{Franchini_2025a,

title = {Spin Matters: A Multidisciplinary Roadmap to Understanding Spin Effects in Oxygen Evolution Reaction During Water Electrolysis},

author = {Emma van der Minne and Priscila Vensaus and Vadim Ratovskii and Seenivasan Hariharan and Jan Behrends and Cesare Franchini and Jonas Fransson and Sarnjeet S. Dhesi and Felix Gunkel and Florian Gossing and Georgios Katsoukis and Ulrike I. Kramm and Magalí Lingenfelde and Qianqian Lan and Yury V. Kolen’ko and Yang Li and Ramsundar Rani Mohan and Jeffrey McCord and Lingmei Ni and Eva Pavarini and Rossitza Pentcheva and David H. Waldeck and Michael Verhage and Anke Yu and Zhichuan J. Xu and Piero Torelli and Silvia Mauri and Narcis Avarvari and Anja Bieberle-Hütter and Christoph Baeumer},

doi = {10.1002/aenm.202503556},

year  = {2025},

date = {2025-09-01},

journal = {Advanced Energy Materials},

pages = {e03556},

abstract = {A central challenge in water electrolysis lies with the oxygen evolution reaction (OER) where the formation of molecular oxygen (O_{2}) is hindered by the constraint of angular momentum conservation. While the reactants OH^{-} or H_{2}O are diamagnetic (DM), the O_{2} product has a paramagnetic (PM) triplet ground state, requiring a change in spin configuration when being formed. This constraint has prompted interest in spin-selective catalysts as a means to facilitate OER. In this context, the roles of magnetism and chirality-induced spin selectivity (CISS) in promoting the OER reaction have recently been investigated through both theoretical and experimental studies. However, pinpointing the key principles and their relative contribution in mediating spin-enhancement remains a significant challenge. This roadmap offers a forward-looking perspective on current experimental trends and theoretical developments in spin-enhanced OER electrocatalysis and outlines strategic directions for integrating incisive experiments and operando approaches with computational modeling to disentangle key mechanisms. By providing a conceptual framework and identifying critical knowledge gaps, this perspective aims to guide researchers toward dedicated experimental and computational studies that will deepen the understanding of spin-induced OER enhancement and accelerate the development of next-generation catalysts.},

keywords = {P07},

pubstate = {published},

tppubtype = {article}

}

@article{Kender_2025a,

title = {Automatic Determination of Quasicrystalline Patterns from Microscopy Images},

author = {Tano Kim Kender and Marco Corrias and Cesare Franchini},

doi = {10.1002/aidi.202500043},

year  = {2025},

date = {2025-07-09},

journal = {Advanced Intelligent Discovery},

pages = {202500043},

abstract = {Quasicrystals are aperiodically ordered solids that exhibit long-range order without translational periodicity, bridging the gap between crystalline and amorphous materials. Due to their lack of translational periodicity, information on atomic arrangements in quasicrystals cannot be extracted by current crystalline lattice recognition softwares. This work introduces a method to automatically detect quasicrystalline atomic arrangements and tiling using image feature recognition coupled with machine learning, tailored toward quasiperiodic tilings with 8-, 10-, and 12-fold rotational symmetry. Atom positions are identified using clustering of feature descriptors. Subsequent nearest-neighbor analysis and border following on the interatomic connections deliver the tiling. Support vector machines further increase the quality of the results, reaching an accuracy consistent with those reported in the literature. A statistical analysis of the results is performed. The code is now part of the open-source package AiSurf.},

keywords = {P07},

pubstate = {published},

tppubtype = {article}

}

@article{Birschitzky_2024b,

title = {Machine Learning Small Polaron Dynamics},

author = {Viktor C. Birschitzky and Luca Leoni and Michele Reticcioli and Cesare Franchini},

url = {https://doi.org/10.1103/PhysRevLett.134.216301},

year  = {2025},

date = {2025-05-27},

urldate = {2024-09-24},

journal = {Physical Review Letters},

volume = {134},

issue = {21},

pages = {216301},

abstract = {Polarons are crucial for charge transport in semiconductors, significantly impacting material properties and device performance. The dynamics of small polarons can be investigated using first-principles molecular dynamics. However, the limited timescale of these simulations presents a challenge for adequately sampling infrequent polaron hopping events. Here, we introduce a message-passing neural network combined with first-principles molecular dynamics within the Born-Oppenheimer approximation that learns the polaronic potential energy surface by encoding the polaronic state, allowing for simulations of polaron hopping dynamics at the nanosecond scale. By leveraging the statistical significance of the long timescale, our framework can accurately estimate polaron (anisotropic) mobilities and activation barriers in prototypical polaronic oxides across different scenarios (hole polarons in rocksalt MgO and electron polarons in pristine and F-doped rutile TiO_{2}) within experimentally measured ranges.},

keywords = {P07},

pubstate = {published},

tppubtype = {article}

}

@article{Cao_2025a,

title = {Quantum Delocalization Enables Water Dissociation on Ru(0001)},

author = {Yu Cao and Jiantao Wang and Mingfeng Liu and Yan Liu and Hui Ma and Cesare Franchini and Yan Sun and Georg Kresse and Xing-Qiu Chen and Peitao Liu},

doi = {10.1103/PhysRevLett.134.178001},

year  = {2025},

date = {2025-04-30},

journal = {Physical Review Letters},

volume = {134},

issue = {17},

pages = {178001},

abstract = {We revisit the long-standing question of whether water molecules dissociate on the Ru(0001) surface through nanosecond-scale path-integral molecular dynamics simulations on a sizable supercell. This is made possible through the development of an efficient and reliable machine-learning potential with near first-principles accuracy, overcoming the limitations of previous ab initio studies. We show that the quantum delocalization associated with nuclear quantum effects enables rapid and frequent proton transfers between water molecules, thereby facilitating the water dissociation on Ru(0001). This work provides the direct theoretical evidence of water dissociation on Ru(0001), resolving the enduring issue in surface sciences and offering crucial atomistic insights into water-metal interfaces.},

keywords = {P03, P07},

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{Romano_2024a,

title = {Structure and Dynamics of the Magnetite(001)/Water Interface from Molecular Dynamics Simulations Based on a Neural Network Potential},

author = {Salvatore Romano and Pablo Montero de Hijes and Matthias Meier and Georg Kresse and Cesare Franchini and Christoph Dellago},

doi = {10.1021/acs.jctc.4c01507},

year  = {2025},

date = {2025-02-13},

urldate = {2024-08-21},

journal = {Journal of Chemical Theory and Computation},

volume = {21},

issue = {4},

pages = {1951–1960},

abstract = {The magnetite/water interface is commonly found in nature and plays a crucial role in various technological applications. However, our understanding of its structural and dynamical properties at the molecular scale remains still limited. In this study, we developed an efficient Behler-Parrinello neural network potential (NNP) for the magnetite/water system, paying particular attention to the accurate generation of reference data with density functional theory. Using this NNP, we performed extensive molecular dynamics simulations of the magnetite (001) surface across a wide range of water coverages, from single molecules to bulk water. Our simulations revealed several new ground states of low coverage water on the Subsurface Cation Vacancy (SCV) model and yielded a density profile of water at the surface that exhibits marked layering. By calculating mean square displacements, we obtained quantitative information on the diffusion of water molecules on the SCV for different coverages, revealing significant anisotropy. Additionally, our simulations provided qualitative insights into the dissociation mechanisms of water molecules at the surface.},

keywords = {P03, P07, P12},

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}

}