Publications

@article{Ranalli2023,

title = {Temperature-dependent anharmonic phonons in quantum paraelectric KTaO_{3} by first principles and machine-learned force fields},

author = {Luigi Ranalli and Carla Verdi and Lorenzo Monacelli and Georg Kresse and Matteo Calandra and Cesare Franchini},

year  = {2023},

date = {2023-02-01},

urldate = {2023-02-01},

journal = {Advanced Quantum Technology},

keywords = {P03, P07},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Corrias2023,

title = {Automated Real-Space Lattice Extraction for Atomic Force Microscopy Images},

author = {Marco Corrias and Lorenzo Papa and Igor Sokolovíc and Viktor Birschitzky and Alexander Gorfer and Martin Setvin and Michael Schmid and Ulrike Diebold and Michele Reticcioli and Cesare Franchini},

doi = {10.1088/2632-2153/acb5e0},

year  = {2023},

date = {2023-01-24},

urldate = {2023-01-24},

journal = {Machine Learning: Science and Technology},

abstract = {Analyzing atomically resolved images is a time-consuming process requiring solid experience and substantial human intervention. In addition, the acquired images contain a large amount of information such as crystal structure, presence and distribution of defects, and formation of domains, which need to be resolved to understand a material's surface structure. Therefore, machine learning techniques have been applied in scanning probe and electron microscopies during the last years, aiming for automatized and efficient image analysis. This work introduces a free and open source tool (AiSurf: Automated Identification of Surface Images) developed to inspect atomically resolved images via Scale-Invariant Feature Transform (SIFT) and Clustering Algorithms (CA). AiSurf extracts primitive lattice vectors, unit cells, and structural distortions from the original image, with no pre-assumption on the lattice and minimal user intervention. The method is applied to various atomically resolved non-contact atomic force microscopy (AFM) images of selected surfaces with different levels of complexity: anatase TiO_{2}(101), oxygen deficient rutile TiO_{2}(110) with and without CO adsorbates, SrTiO_{3}(001) with Sr vacancies and graphene with C vacancies. The code delivers excellent results and is tested against atom misclassification and artifacts, thereby facilitating the interpretation of scanning probe microscopy images.},

keywords = {P02, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Zeininger2022,

title = {Reaction Modes on a Single Catalytic Particle: Nanoscale Imaging and Micro-Kinetic Modeling},

author = {Johannes Zeininger and Maximilian Raab and Yuri Suchorski and Sebastian Buhr and Michael Stöger-Pollach and Johannes Bernardi and Günther Rupprechter},

doi = {10.1021/acscatal.2c02901},

year  = {2022},

date = {2022-10-07},

journal = {ACS Catalysis},

volume = {12},

number = {20},

pages = {12774--12785},

publisher = {American Chemical Society (ACS)},

abstract = {The kinetic behavior of individual Rh(\textit{hkl}) nanofacets coupled in a common reaction system was studied using the apex of a curved rhodium microcrystal (radius of 0.65 μm) as a model of a single catalytic particle and field electron microscopy for in situ imaging of catalytic hydrogen oxidation. Depending on the extent of interfacet coupling via hydrogen diffusion, different oscillating reaction modes were observed including highly unusual multifrequential oscillations: differently oriented nanofacets oscillated with differing frequencies despite their immediate neighborhood. The transitions between different modes were induced by variations in the particle temperature, causing local surface reconstructions, which create locally protruding atomic rows. These atomic rows modified the coupling strength between individual nanofacets and caused the transitions between different oscillating modes. Effects such as entrainment, frequency locking, and reconstruction-induced collapse of spatial coupling were observed. To reveal the origin of the different experimentally observed effects, microkinetic simulations were performed for a network of 105 coupled oscillators, modeling the individual nanofacets communicating via hydrogen surface diffusion. The calculated behavior of the oscillators, the local frequencies, and the varying degree of spatial synchronization describe the experimental observations well.},

keywords = {P08},

pubstate = {published},

tppubtype = {article}

}

@article{Zeininger2022a,

title = {Pattern Formation in Catalytic H_{2} Oxidation on Rh: Zooming in by Correlative Microscopy},

author = {Johannes Zeininger and Philipp Winkler and Maximilian Raab and Yuri Suchorski and Mauricio J. Prieto and Liviu C. Tănase and Lucas Souza Caldas and Aarti Tiwari and Thomas Schmidt and Michael Stöger-Pollach and Andreas Steiger-Thirsfeld and Beatriz Roldan Cuenya and Günther Rupprechter},

doi = {10.1021/acscatal.2c03692},

year  = {2022},

date = {2022-09-19},

urldate = {2022-09-19},

journal = {ACS Catalysis},

volume = {12},

number = {19},

pages = {11974--11983},

publisher = {American Chemical Society (ACS)},

abstract = {Spatio-temporal nonuniformities in H_{2} oxidation on individual Rh(\textit{h k l}) domains of a polycrystalline Rh foil were studied in the 10^{–6} mbar pressure range by photoemission electron microscopy (PEEM), X-ray photoemission electron microscopy (XPEEM), and low-energy electron microscopy (LEEM). The latter two were used for in situ correlative microscopy to zoom in with significantly higher lateral resolution, allowing detection of an unusual island-mediated oxygen front propagation during kinetic transitions. The origin of the island-mediated front propagation was rationalized by model calculations based on a hybrid approach of microkinetic modeling and Monte Carlo simulations.},

keywords = {P08},

pubstate = {published},

tppubtype = {article}

}

@article{Wanzenboeck2022,

title = {Neural-network-backed evolutionary search for SrTiO_{3}(110) surface reconstructions},

author = {Ralf Wanzenböck and Marco Arrigoni and Sebastian Bichelmaier and Florian Buchner and Jesús Carrete and Georg K. H. Madsen},

doi = {10.1039/d2dd00072e},

year  = {2022},

date = {2022-08-26},

journal = {Digital Discovery},

volume = {1},

number = {5},

pages = {703--710},

publisher = {Royal Society of Chemistry (RSC)},

abstract = {The determination of atomic structures in surface reconstructions has typically relied on structural models derived from intuition and domain knowledge. Evolutionary algorithms have emerged as powerful tools for such structure searches. However, when density functional theory is used to evaluate the energy the computational cost of a thorough exploration of the potential energy landscape is prohibitive. Here, we drive the exploration of the rich phase diagram of TiO_{x} overlayer structures on SrTiO_{3}(110) by combining the covariance matrix adaptation evolution strategy (CMA-ES) and a neural-network force field (NNFF) as a surrogate energy model. By training solely on SrTiO_{3}(110) 4×1 overlayer structures and performing CMA-ES runs on 3×1, 4×1 and 5×1 overlayers, we verify the transferability of the NNFF. The speedup due to the surrogate model allows taking advantage of the stochastic nature of the CMA-ES to perform exhaustive sets of explorations and identify both known and new low-energy reconstructions.},

keywords = {P09},

pubstate = {published},

tppubtype = {article}

}

@article{Wang2022,

title = {Surface chemistry on a polarizable surface: Coupling of CO with KTaO _{3}(001)},

author = {Zhichang Wang and Michele Reticcioli and Zdenek Jakub and Igor Sokolović and Matthias Meier and Lynn A Boatner and Michael Schmid and Gareth S. Parkinson and Ulrike Diebold and Cesare Franchini and Martin Setvin},

url = {https://www.science.org/doi/10.1126/sciadv.abq1433},

doi = {10.1126/sciadv.abq1433},

year  = {2022},

date = {2022-08-19},

urldate = {2022-08-19},

journal = {Science Advances},

volume = {8},

issue = {33},

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

abstract = {Polarizable materials attract attention in catalysis because they have a free parameter for tuning chemical reactivity. Their surfaces entangle the dielectric polarization with surface polarity, excess charge, and orbital hybridization. How this affects individual adsorbed molecules is shown for the incipient ferroelectric perovskite KTaO_{3}. This intrinsically polar material cleaves along (001) into KO- and TaO_{2}-terminated surface domains. At TaO_{2} terraces, the polarity-compensating excess electrons form a two-dimensional electron gas and can also localize by coupling to ferroelectric distortions. TaO_{2} terraces host two distinct types of CO molecules, adsorbed at equivalent lattice sites but charged differently as seen in atomic force microscopy/scanning tunneling microscopy. Temperature-programmed desorption shows substantially stronger binding of the charged CO; in density functional theory calculations, the excess charge favors a bipolaronic configuration coupled to the CO. These results pinpoint how adsorption states couple to ferroelectric polarization.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Reticcioli2022,

title = {Competing electronic states emerging on polar surfaces},

author = {Michele Reticcioli and Zhichang Wang and Michael Schmid and Dominik Wrana and Lynn A. Boatner and Ulrike Diebold and Martin Setvin and Cesare Franchini},

url = {https://www.nature.com/articles/s41467-022-31953-6},

doi = {10.1038/s41467-022-31953-6},

year  = {2022},

date = {2022-07-25},

urldate = {2022-07-25},

journal = {Nature Communications},

volume = {13},

number = {4311},

publisher = {Springer Science and Business Media LLC},

abstract = {Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO_{2} termination of KTaO_{3}(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.},

keywords = {P02, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Sombut2022,

title = {Role of Polarons in Single-Atom Catalysts: Case Study of Me_{1}[Au_{1},Pt_{1} and Rh_{1}] on TiO_{2}(110)},

author = {Panukorn Sombut and Lena Puntscher and Marlene Atzmüller and Zdenek Jakub and Michele Reticcioli and Matthias Meier and Gareth S. Parkinson and Cesare Franchini},

doi = {10.1007/s11244-022-01651-0},

year  = {2022},

date = {2022-07-25},

journal = {Topics in Catalysis},

volume = {65},

pages = {1620--1630},

abstract = {The local environment of metal-oxide supported single-atom catalysts plays a decisive role in the surface reactivity and related catalytic properties. The study of such systems is complicated by the presence of point defects on the surface, which are often associated with the localization of excess charge in the form of polarons. This can affect the stability, the electronic configuration, and the local geometry of the adsorbed adatoms. In this work, through the use of density functional theory and surface-sensitive experiments, we study the adsorption of Rh_{1}, Pt_{1}, and Au_{1} metals on the reduced TiO_{2}(110) surface, a prototypical polaronic material. A systematic analysis of the adsorption configurations and oxidation states of the adsorbed metals reveals different types of couplings between adsorbates and polarons. As confirmed by scanning tunneling microscopy measurements, the favored Pt_{1} and Au_{1} adsorption at oxygen vacancy sites is associated with a strong electronic charge transfer from polaronic states to adatom orbitals, which results in a reduction of the adsorbed metal. In contrast, the Rh_{1} adatoms interact weakly with the excess charge, which leaves the polarons largely unaffected. Our results show that an accurate understanding of the properties of single-atom catalysts on oxide surfaces requires a careful account of the interplay between adatoms, vacancy sites, and polarons.},

keywords = {P04, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Yigit2022,

title = {Active sites and deactivation of room temperature CO oxidation on Co_{3}O_{4} catalysts: combined experimental and computational investigations},

author = {Nevzat Yigit and Alexander Genest and Schamil Terloev and Jury Möller and Günther Rupprechter},

doi = {10.1088/1361-648x/ac718b},

year  = {2022},

date = {2022-06-29},

urldate = {2022-06-29},

journal = {Journal of Physics: Condensed Matter},

volume = {34},

number = {35},

pages = {354001},

publisher = {IOP Publishing},

abstract = {Co_{3}O_{4} is a well-known low temperature CO oxidation catalyst, but it often suffers from deactivation. We have thus examined room temperature (RT) CO oxidation on Co_{3}O_{4} catalysts by operando DSC, TGA and MS measurements, as well as by pulsed chemisorption to differentiate the contributions of CO adsorption and reaction to CO_{2}. Catalysts pretreated in oxygen at 400 °C are most active, with the initial interaction of CO and Co_{3}O_{4} being strongly exothermic and with maximum amounts of CO adsorption and reaction. The initially high RT activity then levels-off, suggesting that the oxidative pretreatment creates an oxygen-rich reactive Co_{3}O_{4} surface that upon reaction onset loses its most active oxygen. This specific active oxygen is not reestablished by gas phase O_{2} during the RT reaction. When the reaction temperature is increased to 150 °C, full conversion can be maintained for 100 h, and even after cooling back to RT. Apparently, deactivating species are avoided this way, whereas exposing the active surface even briefly to pure CO leads to immediate deactivation. Computational modeling using DFT helped to identify the CO adsorption sites, determine oxygen vacancy formation energies and the origin of deactivation. A new species of CO bonded to oxygen vacancies at RT was identified, which may block a vacancy site from further reaction unless CO is removed at higher temperature. The interaction between oxygen vacancies was found to be small, so that in the active state several lattice oxygen species are available for reaction in parallel.},

keywords = {P08},

pubstate = {published},

tppubtype = {article}

}

@article{Birschitzky2022,

title = {Machine learning for exploring small polaron configurational space},

author = {Viktor C Birschitzky and Florian Ellinger and Ulrike Diebold and Michele Reticcioli and Cesare Franchini},

url = {https://www.nature.com/articles/s41524-022-00805-8},

doi = {10.1038/s41524-022-00805-8},

year  = {2022},

date = {2022-06-06},

urldate = {2022-06-06},

journal = {npj Computational Materials},

volume = {8},

number = {125},

publisher = {Springer Science and Business Media LLC},

abstract = {Polaron defects are ubiquitous in materials and play an important role in many processes involving carrier mobility, charge transfer and surface reactivity. Determining small polarons’ spatial distributions is essential to understand materials properties and functionalities. However, the required exploration of the configurational space is computationally demanding when using first principles methods. Here, we propose a machine-learning (ML) accelerated search that determines the ground state polaronic configuration. The ML model is trained on databases of polaron configurations generated by density functional theory (DFT) via molecular dynamics or random sampling. To establish a mapping between configurations and their stability, we designed descriptors modelling the interactions among polarons and charged point defects. We used the DFT+ML protocol to explore the polaron configurational space for two surface-systems, reduced rutile TiO_{2}(110) and Nb-doped SrTiO_{3}(001). The ML-aided search proposes additional polaronic configurations and can be utilized to determine optimal polaron distributions at any charge concentration.},

keywords = {P02, P07},

pubstate = {published},

tppubtype = {article}

}