Surface structure and reactivity of
multi-component oxides at the atomic scale
Subproject P02
Multi-component metal oxides exhibit a plethora of stoichiometry-dependent structural phases at the surface, even if the composition of the bulk is kept the same. The long-term objective of P02 is to unravel the relationship between surface electronic and geometric structure and reactivity, to ultimately tune these materials for energy-related reactions such as the ORR. The project applies the surface science approach. We will grow well-defined, epitaxial perovskite thin films of LSFO and LSMO in a UHV-based PLD/surface science apparatus under tight control of the surface stoichiometry in the first project period. We will determine the coordinates of surface atoms quantitatively using LEED-IV in close collaboration with theoretical groups.
Theoretical models will also help with interpreting atomically-resolved ncAFM/STM images. These images give direct insights into the behavior of polarons in these complex materials and show how adsorbates such as O2, H2O, CO, and CO2 interact with electronic and structural defects. XPS, TPD, and FTIR of these well-defined systems will deliver desorption energies, vibrational frequencies, and spectral fingerprints. These experimental data on well-defined systems will build a bridge when tested under ‘realistic’ environments at high pressure/temperature and in aqueous solutions. They will also serve to validate ML-based theory approaches.
Expertise
Our expertise is experimental surface science. We operate a total of seven ultrahigh-vacuum (UHV) chambers, which contain virtually all main experimental surface science techniques, as well as an (electro-)chemistry lab.
All chambers are equipped with facilities for sample preparation (sputtering/annealing/gas dosing), as well as various growth techniques (e-beam evaporators, Knudsen cells, UHV-compatible sputter deposition, pulsed laser deposition (PLD)).
Analysis techniques used in our research include:
- Scanning Tunneling Microscopy (STM) (in UHV 4K – 300 K, electrochemical STM)
- Atomic Force Microscopy (AFM): UHV-based (q+ sensor) and in the ambient (cantilever-based)
- Low-Energy Electron Diffraction (LEED)
- Reflection High Energy Diffraction (RHEED)
- X-ray Photoelectron Spectroscopy (XPS)
- Ultraviolet Photoelectron Spectroscopy (UPS)
- Auger Electron Spectroscopy (AES)
- Low-energy He+ ion scattering (LEIS)
- Thermal Programmed Desorption Spectroscopy (TPD)
Team
Former Members
Publications
2023
Kraushofer, Florian; Meier, Matthias; Jakub, Zdeněk; Hütner, Johanna; Balajka, Jan; Hulva, Jan; Schmid, Michael; Franchini, Cesare; Diebold, Ulrike; Parkinson, Gareth S.
Oxygen-Terminated (1 × 1) Reconstruction of Reduced Magnetite Fe3O4(111)
Journal ArticleOpen AccessIn: vol. 14, no. 13, pp. 3258–3265, 2023.
Abstract | Links | BibTeX | Tags: P02, P04, P07
@article{Kraushofer2023,
title = {Oxygen-Terminated (1 × 1) Reconstruction of Reduced Magnetite Fe_{3}O_{4}(111)},
author = {Florian Kraushofer and Matthias Meier and Zdeněk Jakub and Johanna Hütner and Jan Balajka and Jan Hulva and Michael Schmid and Cesare Franchini and Ulrike Diebold and Gareth S. Parkinson},
doi = {10.1021/acs.jpclett.3c00281},
year = {2023},
date = {2023-03-28},
urldate = {2023-03-28},
volume = {14},
number = {13},
pages = {3258--3265},
publisher = {American Chemical Society (ACS)},
abstract = {The (111) facet of magnetite (Fe_{3}O_{4}) has been studied extensively by experimental and theoretical methods, but controversy remains regarding the structure of its low-energy surface terminations. Using density functional theory (DFT) computations, we demonstrate three reconstructions that are more favorable than the accepted Feoct2 termination under reducing conditions. All three structures change the coordination of iron in the kagome Feoct1 layer to be tetrahedral. With atomically resolved microscopy techniques, we show that the termination that coexists with the Fetet1 termination consists of tetrahedral iron capped by 3-fold coordinated oxygen atoms. This structure explains the inert nature of the reduced patches.},
keywords = {P02, P04, P07},
pubstate = {published},
tppubtype = {article}
}
Verdi, Carla; Ranalli, Luigi; Franchini, Cesare; Kresse, Georg
Journal ArticleIn: Physical Review Materials, vol. 7, no. 3, pp. l030801, 2023.
Abstract | Links | BibTeX | Tags: P03, P07
@article{Verdi2023,
title = {Quantum paraelectricity and structural phase transitions in strontium titanate beyond density functional theory},
author = {Carla Verdi and Luigi Ranalli and Cesare Franchini and Georg Kresse},
doi = {10.1103/physrevmaterials.7.l030801},
year = {2023},
date = {2023-03-16},
journal = {Physical Review Materials},
volume = {7},
number = {3},
pages = {l030801},
publisher = {American Physical Society (APS)},
abstract = {We demonstrate an approach for calculating temperature-dependent quantum and anharmonic effects with beyond density-functional theory accuracy. By combining machine-learned potentials and the stochastic self-consistent harmonic approximation, we investigate the cubic to tetragonal transition in strontium titanate and show that the paraelectric phase is stabilized by anharmonic quantum fluctuations. We find that a quantitative understanding of the quantum paraelectric behavior requires a higher-level treatment of electronic correlation effects via the random phase approximation. This approach enables detailed studies of emergent properties in strongly anharmonic materials beyond density-functional theory.},
keywords = {P03, P07},
pubstate = {published},
tppubtype = {article}
}
Ranalli, Luigi; Verdi, Carla; Monacelli, Lorenzo; Kresse, Georg; Calandra, Matteo; Franchini, Cesare
Journal ArticleOpen AccessIn: Advanced Quantum Technology, vol. 6, iss. 4, 2023.
Abstract | Links | BibTeX | Tags: P03, P07
@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},
doi = {10.1002/qute.202200131},
year = {2023},
date = {2023-02-22},
urldate = {2023-02-22},
journal = {Advanced Quantum Technology},
volume = {6},
issue = {4},
abstract = {Understanding collective phenomena in quantum materials from first principles is a promising route toward engineering materials properties and designing new functionalities. This work examines the quantum paraelectric state, an elusive state of matter characterized by the smooth saturation of the ferroelectric instability at low temperature due to quantum fluctuations associated with anharmonic phonon effects. The temperature-dependent evolution of the soft ferroelectric phonon mode in the quantum paraelectric KTaO_{3} in the range 0–300 K is modeled by combining density functional theory (DFT) calculations with the stochastic self-consistent harmonic approximation assisted by an on-the-fly machine-learned force field. The calculated data show that including anharmonic terms is essential to stabilize the spurious imaginary ferroelectric phonon predicted by DFT in the harmonic approximation, in agreement with experiments. Augmenting the DFT workflow with machine-learned force fields allows for efficient stochastic sampling of the configuration space using large supercells in a wide temperature range, inaccessible to conventional ab initio protocols. This work proposes a robust computational workflow capable of accounting for collective behaviors involving different degrees of freedom and occurring at large time/length scales, paving the way for precise modeling and control of quantum effects in materials.},
keywords = {P03, P07},
pubstate = {published},
tppubtype = {article}
}
Corrias, Marco; Papa, Lorenzo; Sokolovíc, Igor; Birschitzky, Viktor; Gorfer, Alexander; Setvin, Martin; Schmid, Michael; Diebold, Ulrike; Reticcioli, Michele; Franchini, Cesare
Automated Real-Space Lattice Extraction for Atomic Force Microscopy Images
Journal ArticleOpen AccessIn: Machine Learning: Science and Technology, vol. 4, pp. 015015, 2023.
Abstract | Links | BibTeX | Tags: P02, P07
@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},
volume = {4},
pages = {015015},
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}
}
2022
Wang, Zhichang; Reticcioli, Michele; Jakub, Zdenek; Sokolović, Igor; Meier, Matthias; Boatner, Lynn A; Schmid, Michael; Parkinson, Gareth S.; Diebold, Ulrike; Franchini, Cesare; Setvin, Martin
Surface chemistry on a polarizable surface: Coupling of CO with KTaO 3(001)
Journal ArticleOpen AccessIn: Science Advances, vol. 8, iss. 33, 2022.
Abstract | Links | BibTeX | Tags: P02, P04, P07
@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}
}
Reticcioli, Michele; Wang, Zhichang; Schmid, Michael; Wrana, Dominik; Boatner, Lynn A.; Diebold, Ulrike; Setvin, Martin; Franchini, Cesare
Competing electronic states emerging on polar surfaces
Journal ArticleOpen AccessIn: Nature Communications, vol. 13, no. 4311, 2022.
Abstract | Links | BibTeX | Tags: P02, P07
@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}
}
Sombut, Panukorn; Puntscher, Lena; Atzmüller, Marlene; Jakub, Zdenek; Reticcioli, Michele; Meier, Matthias; Parkinson, Gareth S.; Franchini, Cesare
Role of Polarons in Single-Atom Catalysts: Case Study of Me1[Au1,Pt1 and Rh1] on TiO2(110)
Journal ArticleOpen AccessIn: Topics in Catalysis, vol. 65, pp. 1620–1630, 2022.
Abstract | Links | BibTeX | Tags: P04, P07
@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}
}
Birschitzky, Viktor C; Ellinger, Florian; Diebold, Ulrike; Reticcioli, Michele; Franchini, Cesare
Machine learning for exploring small polaron configurational space
Journal ArticleOpen AccessIn: npj Computational Materials, vol. 8, no. 125, 2022.
Abstract | Links | BibTeX | Tags: P02, P07
@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}
}
Meier, Matthias; Hulva, Jan; Jakub, Zdenek; Kraushofer, Florian; Bobić, Mislav; Bliem, Roland; Setvin, Martin; Schmid, Michael; Diebold, Ulrike; Franchini, Cesare; Parkinson, Gareth S.
Journal ArticleOpen AccessIn: ScienceAdvances, vol. 8, iss. 13, pp. eabn4580, 2022.
Abstract | Links | BibTeX | Tags: P02, P04, P07
@article{SCIADV2022,
title = {CO oxidation by Pt_{2}/Fe_{3}O_{4}: Metastable dimer and support configurations facilitate lattice oxygen extraction},
author = {Matthias Meier and Jan Hulva and Zdenek Jakub and Florian Kraushofer and Mislav Bobić and Roland Bliem and Martin Setvin and Michael Schmid and Ulrike Diebold and Cesare Franchini and Gareth S. Parkinson},
url = {https://www.science.org/doi/10.1126/sciadv.abn4580},
doi = {10.1126/sciadv.abn4580},
year = {2022},
date = {2022-04-01},
urldate = {2022-04-01},
journal = {ScienceAdvances},
volume = {8},
issue = {13},
pages = {eabn4580},
abstract = {Heterogeneous catalysts based on subnanometer metal clusters often exhibit strongly size-dependent properties, and the addition or removal of a single atom can make all the difference. Identifying the most active species and deciphering the reaction mechanism is extremely difficult, however, because it is often not clear how the catalyst evolves in operando. Here, we use a combination of atomically resolved scanning probe microscopies, spectroscopic techniques, and density functional theory (DFT)–based calculations to study CO oxidation by a model Pt/Fe_{3}O_{4}(001) “single-atom” catalyst. We demonstrate that (PtCO)_{2} dimers, formed dynamically through the agglomeration of mobile Pt-carbonyl species, catalyze a reaction involving the oxide support to form CO_{2}. Pt_{2} dimers produce one CO_{2} molecule before falling apart into two adatoms, releasing the second CO. Olattice extraction only becomes facile when both the Pt-dimer and the Fe_{3}O_{4} support can access metastable configurations, suggesting that substantial, concerted rearrangements of both cluster and support must be considered for reactions occurring at elevated temperature.},
keywords = {P02, P04, P07},
pubstate = {published},
tppubtype = {article}
}
Reticcioli, Michele; Diebold, Ulrike; Franchini, Cesare
Modeling polarons in density functional theory: lessons learned from TiO2
Journal ArticleOpen AccessIn: Journal of Physics: Condensed Matter, vol. 34, no. 20, pp. 204006, 2022.
Abstract | Links | BibTeX | Tags: P02, P07
@article{JPCM2022,
title = {Modeling polarons in density functional theory: lessons learned from TiO_{2}},
author = {Michele Reticcioli and Ulrike Diebold and Cesare Franchini},
url = {https://iopscience.iop.org/article/10.1088/1361-648X/ac58d7},
doi = {10.1088/1361-648X/ac58d7},
year = {2022},
date = {2022-03-14},
urldate = {2022-03-14},
journal = {Journal of Physics: Condensed Matter},
volume = {34},
number = {20},
pages = {204006},
abstract = {Density functional theory (DFT) is nowadays one of the most broadly used and successful techniques to study the properties of polarons and their effects in materials. Here, we systematically analyze the aspects of the theoretical calculations that are crucial to obtain reliable predictions in agreement with the experimental observations. We focus on rutile TiO_{2}, a prototypical polaronic compound, and compare the formation of polarons on the (110) surface and subsurface atomic layers. As expected, the parameter U used to correct the electronic correlation in the DFT+U formalism affects the resulting charge localization, local structural distortions and electronic properties of polarons. Moreover, the polaron localization can be driven to different sites by strain: Due to different local environments, surface and subsurface polarons show different responses to the applied strain, with impact on the relative energy stability. An accurate description of the properties of polarons is key to understand their impact on complex phenomena and applications: As an example, we show the effects of lattice strain on the interaction between polarons and CO adsorbates.},
keywords = {P02, P07},
pubstate = {published},
tppubtype = {article}
}