Showing 1–50 of 89 results for author: Diebold, U

An improved reliability factor for quantitative low-energy electron diffraction

Authors: Alexander M. Imre, Lutz Hammer, Ulrike Diebold, Michele Riva, Michael Schmid

Abstract: Quantitative low-energy electron diffraction [LEED $I(V)$ or LEED $I(E)$, the evaluation of diffraction intensities $I$ as a function of the electron energy] is a versatile technique for the study of surface structures. The technique is based on optimizing the agreement between experimental and calculated intensities. Today, the most commonly used measure of agreement is Pendry's $R$ factor… ▽ More Quantitative low-energy electron diffraction [LEED $I(V)$ or LEED $I(E)$, the evaluation of diffraction intensities $I$ as a function of the electron energy] is a versatile technique for the study of surface structures. The technique is based on optimizing the agreement between experimental and calculated intensities. Today, the most commonly used measure of agreement is Pendry's $R$ factor $R_\mathrm{P}$. While $R_\mathrm{P}$ has many advantages, it also has severe shortcomings, as it is a noisy target function for optimization and very sensitive to small offsets of the intensity. Furthermore, $R_\mathrm{P} = 0$, which is meant to imply perfect agreement between two $I(E)$ curves can also be achieved by qualitatively very different curves. We present a modified $R$ factor $R_\mathrm{S}$, which can be used as a direct replacement for $R_\mathrm{P}$, but avoids these shortcomings. We also demonstrate that $R_\mathrm{S}$ is as good as $R_\mathrm{P}$ or better in steering the optimization to the correct result in the case of imperfections of the experimental data, while another common $R$ factor, $R_\mathrm{ZJ}$ (suggested by Zanazzi and Jona) is worse in this respect. △ Less

Submitted 7 November, 2025; originally announced November 2025.

When Surface Dynamics Fakes Symmetry -- Oxygen on Rh(100) Revisited

Authors: Lutz Hammer, Tilman Kißlinger, Margareta Wagner, Reinhard B. Neder, Michael Schmid, Ulrike Diebold, M. Alexander Schneider

Abstract: Heating a long-range ordered adsorbate phase beyond its stability temperature does not necessarily result in a disordered phase, it can also break up into heavily fluctuating ordered domains. Temporal and/or spatial averaging over these fluctuations may give the impression of both a wrong periodicity and a false local symmetry. This can happen even below liquid-nitrogen temperature, so that the tr… ▽ More Heating a long-range ordered adsorbate phase beyond its stability temperature does not necessarily result in a disordered phase, it can also break up into heavily fluctuating ordered domains. Temporal and/or spatial averaging over these fluctuations may give the impression of both a wrong periodicity and a false local symmetry. This can happen even below liquid-nitrogen temperature, so that the true nature of the phase might remain undetected. We demonstrate this scenario at the catalytically active Rh(100) surface covered by 1/2 monolayer of oxygen, using quantitative low energy electron diffraction (LEED), variable-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). Using the example of CO adsorption, we show that local symmetry can have a decisive influence on the binding energy and thus the chemical reactivity. △ Less

Submitted 6 October, 2025; v1 submitted 11 August, 2025; originally announced August 2025.

Molecular Arrangements in the First Monolayer of Cu-Phthalocyanine on In$_2$O$_3$(111)

Authors: Matthias Blatnik, Fabio Calcinelli, Andreas Jeindl, Moritz Eder, Michael Schmid, Jan Čechal, Ulrike Diebold, Peter Jacobson Oliver T. Hofmann, Margareta Wagner

Abstract: Well-ordered organic molecular layers on oxide surfaces are key for organic electronics. Using a combination of scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) we probe the structures of copper phthalocyanine (CuPc) on In$_2$O$_3$, a model for a prototypical transparent conductive oxide (TCO). These scanning-probe images allow the direct determination of the ad… ▽ More Well-ordered organic molecular layers on oxide surfaces are key for organic electronics. Using a combination of scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) we probe the structures of copper phthalocyanine (CuPc) on In$_2$O$_3$, a model for a prototypical transparent conductive oxide (TCO). These scanning-probe images allow the direct determination of the adsorption site and distortions of the molecules, which are corroborated by DFT calculations. Isolated CuPc molecules adsorb in a flat, slightly tilted geometry in three symmetry-equivalent configurations on the stoichiometric In$_2$O$_3$(111) surface. Increasing the coverage leads to densely-packed 1D chains oriented along $\langle1\bar{1}0\rangle$ directions, which dissolve into a highly ordered (2$\times$2) superstructure upon increasing the CuPc density to 3/4 per surface unit cell. At a coverage of one CuPc per surface unit cell, a densely packed (1$\times$1) superstructure fully covers the surface. The molecules still assume the same site and orientation as before, but they partially overlap to accommodate the high packing density, leading to a bending of the molecules. These results are compared to the behavior of CoPc on In$_2$O$_3$(111). In summary, we demonstrate that a uniform first layer of metal-phthalocyanine molecules can be realized on the In$_2$O$_3$(111) surface when using the proper metal atom in the molecule. △ Less

Submitted 11 July, 2025; originally announced July 2025.

Journal ref: J. Mater. Chem. C 13, 17650 (2025)

Multi-Technique Characterization of Rhodium Gem-Dicarbonyls on TiO$_2$(110)

Authors: Moritz Eder, Faith J. Lewis, Johanna I. Hütner, Panukorn Sombut, Maosheng Hao, David Rath, Jan Balajka, Margareta Wagner, Matthias Meier, Cesare Franchini, Ulrike Diebold, Michael Schmid, Florian Libisch, Jiří Pavelec, Gareth S. Parkinson

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 applied catalysts, the ill-defined crystallographic environment of species observed on powder catalysts renders data interpretation challenging. In this work, we apply a multi-technique surfa… ▽ More 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 applied catalysts, the ill-defined crystallographic environment of species observed on powder catalysts 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. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Total Variation-Based Image Decomposition and Denoising for Microscopy Images

Authors: Marco Corrias, Giada Franceschi, Michele Riva, Alberto Tampieri, Karin Föttinger, Ulrike Diebold, Thomas Pock, Cesare Franchini

Abstract: Experimentally acquired microscopy images are unavoidably affected by the presence of noise and other unwanted signals, which degrade their quality and might hide relevant features. With the recent increase in image acquisition rate, modern denoising and restoration solutions become necessary. This study focuses on image decomposition and denoising of microscopy images through a workflow based on… ▽ More Experimentally acquired microscopy images are unavoidably affected by the presence of noise and other unwanted signals, which degrade their quality and might hide relevant features. With the recent increase in image acquisition rate, modern denoising and restoration solutions become necessary. This study focuses on image decomposition and denoising of microscopy images through a workflow based on total variation (TV), addressing images obtained from various microscopy techniques, including atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning electron microscopy (SEM). Our approach consists in restoring an image by extracting its unwanted signal components and subtracting them from the raw one, or by denoising it. We evaluate the performance of TV-$L^1$, Huber-ROF, and TGV-$L^1$ in achieving this goal in distinct study cases. Huber-ROF proved to be the most flexible one, while TGV-$L^1$ is the most suitable for denoising. Our results suggest a wider applicability of this method in microscopy, restricted not only to STM, AFM, and SEM images. The Python code used for this study is publicly available as part of AiSurf. It is designed to be integrated into experimental workflows for image acquisition or can be used to denoise previously acquired images. △ Less

Submitted 13 May, 2025; originally announced May 2025.

How to cleave cubic perovskite oxides

Authors: Igor Sokolović, Michael Schmid, Ulrike Diebold, Martin Setvín

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 modelling 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… ▽ More 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 modelling 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 SrTiO$_3$. The large-area morphology and typical distribution of surface terminations on cleaved SrTiO$_3$(001) is 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. △ Less

Submitted 16 August, 2024; originally announced August 2024.

ViPErLEED package I: Calculation of $I(V)$ curves and structural optimization

Authors: Florian Kraushofer, Alexander M. Imre, Giada Franceschi, Tilman Kißlinger, Erik Rheinfrank, Michael Schmid, Ulrike Diebold, Lutz Hammer, Michele Riva

Abstract: Low-energy electron diffraction (LEED) is a widely used technique in surface-science. 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 I(V), is underutilized. To acquire these data, minor adjustments would be required in most… ▽ More Low-energy electron diffraction (LEED) is a widely used technique in surface-science. 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 I(V), is underutilized. To acquire these data, minor adjustments would be required in most experimental setups, but existing analysis software is cumbersome to use. ViPErLEED (Vienna package for Erlangen LEED) lowers these barriers, introducing a combined solution for 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 automated LEED-I(V) calculations and structural optimization. Minimal user input is required, and the functionality is significantly enhanced compared to existing solutions. Computation is performed by embedding the Erlangen tensor-LEED package (TensErLEED). ViPErLEED manages parallelization, monitors convergence, and processes input and output. This makes LEED I(V) more accessible to new users while minimizing the potential for errors and the manual labor. Added functionality includes structure-dependent defaults, automatic detection of bulk and surface symmetries and their relationship, automated symmetry-preserving search procedures, adjustments to the TensErLEED code to handle larger systems, 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 >=3.7) and provided as an open-source package (GNU GPLv3 or later). A structure determination of the $α$-Fe2O3(1-102)-(1x1) surface is presented as an example for the application of the software. △ Less

Submitted 26 June, 2024; originally announced June 2024.

Journal ref: Phys. Rev. Research 7, 013005 (2025)

Digging its own Site: Linear Coordination Stabilizes a Pt1/Fe2O3 Single-Atom Catalyst

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

Abstract: Determining the local coordination of the active site is a pre-requisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems, so much emphasis is placed on density functional theory-based computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (1-102) facet of Fe2… ▽ More Determining the local coordination of the active site is a pre-requisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems, so much emphasis is placed on density functional theory-based computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (1-102) facet of Fe2O3, a common support material in SAC. Using a combination of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudo-linear 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, and that extensive structural searches are likely necessary to determine realistic active site geometry in single-atom catalysis. △ Less

Submitted 26 June, 2024; originally announced June 2024.

Journal ref: ACS Nano 18, 26920 (2024)

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

Authors: Michael Schmid, Florian Kraushofer, Alexander M. Imre, Tilman Kißlinger, Lutz Hammer, Ulrike Diebold, Michele Riva

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… ▽ More 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 I(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. △ Less

Submitted 26 June, 2024; originally announced June 2024.

Journal ref: Phys. Rev. Research 7, 013006 (2025)

Quantitative measurement of cooperative binding in partially dissociated water dimers at the hematite R-cut surface

Authors: Paul T. P. Ryan, Panukorn Sombut, Ali Rafsanjani Abbasi, Chunlei Wang, Fulden Eratam, Francesco Goto, Ulrike Diebold, Matthias Meier, David A. Duncan, Gareth S. Parkinson

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 make it difficult to probe them in a quantitative manner. Here, we utilize… ▽ More 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 make it difficult to probe them in a quantitative manner. Here, we utilize normal incidence x-ray standing waves (NIXSW) to study the structure of partially dissociated water dimers (H2O-OH) at the Fe2O3(012) surface (also called (1-102) or R-cut surface); a system simple enough to be tractable, yet complex enough to capture the essential physics. We find the H2O and terminal OH groups to be the same height above the surface within experimental error (1.45 +/- 0.04 Angstrom and 1.47 +/- 0.02 Angstrom, 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 H2O and OH induces the H2O to bind more strongly, and 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 Angstrom). DFT calculations based on various functionals correctly model the cooperative effect, but overestimate the water-surface interaction. △ Less

Submitted 26 June, 2024; originally announced June 2024.

A Multi-Technique Study of C2H4 Adsorption on a Model Single-Atom Rh1 Catalyst

Authors: Chunlei Wang, Panukorn Sombut, Lena Puntscher, Manuel Ulreich, Jiri Pavelec, David Rath, Jan Balajka, Matthias Meier, Michael Schmid, Ulrike Diebold, Cesare Franchini, Gareth S. Parkinson

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 C2H4 with a model Rh/Fe3O4(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 calc… ▽ More 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 C2H4 with a model Rh/Fe3O4(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. C2H4 adsorption is strongest at the 2-fold coordinated Rh1 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. Adsorption at the 5-fold coordinated Rh sites is predicated to -1.49 eV, but the superposition of this signal with that from small Rh clusters and additional heterogeneity leads to a broad C2H4 desorption shoulder in TPD above room temperature. △ Less

Submitted 5 June, 2024; originally announced June 2024.

Journal ref: J. Phys. Chem. C 127, 18378 (2023)

Stoichiometric reconstruction of the Al$_{2}$O$_{3}$(0001) surface

Authors: Johanna I. Hütner, Andrea Conti, David Kugler, Florian Mittendorfer, Georg Kresse, Michael Schmid, Ulrike Diebold, Jan Balajka

Abstract: Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. The basal (0001) plane of $α$-Al$_{2}$O$_{3}$ was imaged with noncontact atomic force microscopy with an atomically-defined tip apex. The surface forms a complex $({\sqrt31} {\times} {\sqrt31})R{\pm}9°$ reconstruction. The lateral positions of the in… ▽ More Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. The basal (0001) plane of $α$-Al$_{2}$O$_{3}$ was imaged with noncontact atomic force microscopy with an atomically-defined tip apex. The surface forms a complex $({\sqrt31} {\times} {\sqrt31})R{\pm}9°$ reconstruction. The lateral positions of the individual O and Al surface atoms come directly from experiment; how these connect to the underlying crystal bulk was determined based on computational modeling. 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$_{3}$. △ Less

Submitted 16 September, 2024; v1 submitted 29 May, 2024; originally announced May 2024.

Journal ref: Science 385, 6714, 1241-1244 (2024)

Duality and degeneracy lifting in two-dimensional electron liquids on SrTiO$_3$(001)

Authors: Igor Sokolović, Eduardo B. Guedes, Thomas P. van Waas, Samuel Poncé, Craig Polley, Michael Schmid, Ulrike Diebold, Milan Radović, Martin Setvín, J. Hugo Dil

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 for… ▽ More 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, that share 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. △ Less

Submitted 31 May, 2024; v1 submitted 29 May, 2024; originally announced May 2024.

NH$_3$ adsorption and competition with H$_2$O on a hydroxylated aluminosilicate surface

Authors: Giada Franceschi, Andrea Conti, Luca Lezuo, Rainer Abart, Florian Mittendorfer, Michael Schmid, Ulrike Diebold

Abstract: The interaction between ammonia (NH$_3$) and (alumino)silicates is of fundamental and applied importance, yet the specifics of NH$_3$ adsorption on silicate surfaces remain largely unexplored, mainly because of experimental challenges related to their electrically insulating nature. An example of this knowledge gap is evident in the context of ice nucleation on silicate dust, wherein the role of N… ▽ More The interaction between ammonia (NH$_3$) and (alumino)silicates is of fundamental and applied importance, yet the specifics of NH$_3$ adsorption on silicate surfaces remain largely unexplored, mainly because of experimental challenges related to their electrically insulating nature. An example of this knowledge gap is evident in the context of ice nucleation on silicate dust, wherein the role of NH$_3$ for ice nucleation remains debated. This study explores the fundamentals of the interaction between NH$_3$ and microcline feldspar (KAlSi$_3$O$_8$), a common aluminosilicate with outstanding ice nucleation abilities. Atomically resolved non-contact atomic force microscopy, x-ray photoelectron spectroscopy, and density functional theory-based calculations elucidate the adsorption geometry of NH$_3$ on the lowest-energy surface of microcline, the (001) facet, and its interplay with surface hydroxyls and molecular water. NH$_3$ and H$_2$O are found to adsorb molecularly in the same adsorption sites, creating H-bonds with the proximate surface silanol (Si-OH) and aluminol (Al-OH) groups. Despite the closely matched adsorption energies of the two molecules, NH$_3$ readily yields to replacement by H$_2$O, challenging the notion that ice nucleation on microcline proceeds via the creation of an ordered H$_2$O layer atop pre-adsorbed NH$_3$ molecules. △ Less

Submitted 26 April, 2024; originally announced April 2024.

Comments: 14 pages, 3 figures

Journal ref: J. Chem. Phys. 160, 164312 (2024)

Infrared Reflection Absorption Spectroscopy Setup with Incidence Angle Selection for Surfaces of Non-Metals

Authors: David Rath, Vojtěch Mikerásek, Chunlei Wang, Moritz Eder, Michael Schmid, Ulrike Diebold, Gareth S. Parkinson, Jiří Pavelec

Abstract: Infrared Reflection Absorption Spectroscopy (IRAS) on dielectric single crystals is challenging because the optimal incidence angles for light-adsorbate interaction coincide with regions of low IR reflectivity. Here, we introduce an optimized IRAS setup that maximizes the signal-to-noise ratio for non-metals. This is achieved by maximizing light throughput, and by selecting optimal incidence angle… ▽ More Infrared Reflection Absorption Spectroscopy (IRAS) on dielectric single crystals is challenging because the optimal incidence angles for light-adsorbate interaction coincide with regions of low IR reflectivity. Here, we introduce an optimized IRAS setup that maximizes the signal-to-noise ratio for non-metals. This is achieved by maximizing light throughput, and by selecting optimal incidence angles that directly impact the peak heights in the spectra. The setup uses a commercial FTIR spectrometer and is usable in ultra-high vacuum (UHV). Specifically, the design features sample illumination and collection mirrors with a high numerical aperture inside the UHV system, and an adjustable aperture to select the incidence angle range on the sample. This is important for p-polarized measurements on dielectrics, because the peaks in the spectra reverse direction at the Brewster angle (band inversion). The system components are connected precisely via a single flange, ensuring long-term stability. We studied the signal-to-noise (SNR) variation in p-polarized IRAS spectra for one monolayer of CO on TiO2(110) as a function of incidence angle range, where a maximum signal-to-noise ratio of 70 was achieved at 4 cm-1 resolution in five minutes measurement time. The capabilities for s-polarization are demonstrated by measuring one monolayer D2O adsorbed on a TiO2(110) surface, where a SNR of 65 was achieved at a delta_R/R0 peak height of 1.4x10-4 in twenty minutes. △ Less

Submitted 28 March, 2024; originally announced March 2024.

Comments: supplementary information included

Journal ref: Rev. Sci. Instrum. 95, 065106 (2024)

Machine Learning Based Prediction of Polaron-Vacancy Patterns on the TiO$_2$(110) Surface

Authors: Viktor C. Birschitzky, Igor Sokolovic, Michael Prezzi, Krisztian Palotas, Martin Setvin, Ulrike Diebold, Michele Reticcioli, Cesare Franchini

Abstract: The multifaceted physics of oxides is shaped by their composition and the presence of defects, which are often accompanied by the formation of polarons. The simultaneous presence of polarons and defects, and their complex interactions, pose challenges for first-principles simulations and experimental techniques. In this study, we leverage machine learning and a first-principles database to analyze… ▽ More The multifaceted physics of oxides is shaped by their composition and the presence of defects, which are often accompanied by the formation of polarons. The simultaneous presence of polarons and defects, and their complex interactions, pose challenges for first-principles simulations and experimental techniques. In this study, we leverage machine learning and a first-principles database to analyze the distribution of surface oxygen vacancies (V$_{\rm O}$) and induced small polarons on rutile TiO$_2$(110), effectively disentangling the interactions between polarons and defects. By combining neural-network supervised learning and simulated annealing, we elucidate the inhomogeneous V$_{\rm O}$ distribution observed in scanning probe microscopy (SPM). Our innovative approach allows us to understand and predict defective surface patterns at previously inaccessible length scales, identifying the specific role of individual types of defects. Specifically, surface-polaron-stabilizing V$_{\rm O}$-configurations are identified, which could have consequences for surface reactivity. △ Less

Submitted 22 January, 2024; originally announced January 2024.

Evolution of the surface atomic structure of multielement oxide films: curse or blessing?

Authors: Giada Franceschi, Renè Heller, Michael Schmid, Ulrike Diebold, Michele Riva

Abstract: Atomically resolved scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) are used to gain atomic-scale insights into the heteroepitaxy of lanthanum-strontium manganite (LSMO, La$_{1-x}$Sr$_x$MnO$_{3-δ}$, $x$ $\approx$ 0.2) on SrTiO$_3$(110). LSMO is a perovskite oxide characterized by several composition-dependent surface reconstructions. The flexibility of the surface al… ▽ More Atomically resolved scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) are used to gain atomic-scale insights into the heteroepitaxy of lanthanum-strontium manganite (LSMO, La$_{1-x}$Sr$_x$MnO$_{3-δ}$, $x$ $\approx$ 0.2) on SrTiO$_3$(110). LSMO is a perovskite oxide characterized by several composition-dependent surface reconstructions. The flexibility of the surface allows it to incorporate nonstoichiometries during growth, which result in composition-dependent surface atomic structures. This happens up to a critical point, where phase separation occurs, clusters rich in the excess cations form at the surface, and films show a rough morphology. To limit the nonstoichiometry introduced by non-optimal growth conditions, it proves useful to monitor the changes in surface atomic structures as a function of the PLD parameters and tune the latter accordingly. △ Less

Submitted 27 November, 2023; originally announced November 2023.

Comments: 8 pages, 5 figures

How water binds to microcline feldspar (001)

Authors: Giada Franceschi, Andrea Conti, Luca Lezuo, Rainer Abart, Florian Mittendorfer, Michael Schmid, Ulrike Diebold

Abstract: Microcline feldspar (KAlSi$_3$O$_8$) is a common mineral with important roles for Earth's ecological balance. It participates in the carbon, potassium, and water cycles, contributing to CO$_2$ sequestration, soil formation, and atmospheric ice nucleation. To understand the fundamentals of these processes, it is essential to establish microcline's surface atomic structure and its interaction with t… ▽ More Microcline feldspar (KAlSi$_3$O$_8$) is a common mineral with important roles for Earth's ecological balance. It participates in the carbon, potassium, and water cycles, contributing to CO$_2$ sequestration, soil formation, and atmospheric ice nucleation. To understand the fundamentals of these processes, it is essential to establish microcline's surface atomic structure and its interaction with the omnipresent water molecules. This work presents atomic-scale results on microcline's lowest-energy surface and its interaction with water, combining ultrahigh vacuum investigations by non-contact atomic force microscopy and X-ray photoelectron spectroscopy with density functional theory calculations. An ordered array of hydroxyls bonded to silicon or aluminum readily forms on the cleaved surface at room temperature. The distinct proton affinities of these hydroxyls influence the arrangement and orientation of the first water molecules binding to the surface, holding potential implications for the subsequent condensation of water. △ Less

Submitted 27 November, 2023; originally announced November 2023.

Comments: 14 pages, 5 figures

Interaction of surface cations of cleaved mica with water in vapor and liquid forms

Authors: Giada Franceschi, Sebastian Brandstetter, Jan Balajka, Igor Sokolović, Jiri Paveleć, Martin Setvín, Michael Schmid, Ulrike Diebold

Abstract: Natural minerals contain ions that become hydrated when they come into contact with water in vapor and liquid forms. Muscovite mica -- a common phyllosilicate with perfect cleavage planes -- is an ideal system to investigate the details of ion hydration. The cleaved mica surface is decorated by an array of K$^+$ ions that can be easily exchanged with other ions or protons when immersed in an aqueo… ▽ More Natural minerals contain ions that become hydrated when they come into contact with water in vapor and liquid forms. Muscovite mica -- a common phyllosilicate with perfect cleavage planes -- is an ideal system to investigate the details of ion hydration. The cleaved mica surface is decorated by an array of K$^+$ ions that can be easily exchanged with other ions or protons when immersed in an aqueous solution. Despite the vast interest in the atomic-scale hydration processes of these K$^+$ ions, experimental data under controlled conditions have remained elusive. Here, atomically resolved non-contact atomic force microscopy (nc-AFM) is combined with X-ray photoelectron spectroscopy (XPS) to investigate the cation hydration upon dosing water vapor at 100 K in ultra-high vacuum (UHV). The cleaved surface is further exposed to ultra-clean liquid water at room temperature, which promotes ion mobility and partial ion-to-proton substitution. The results offer the first direct experimental views of the interaction of water with muscovite mica in UHV. The findings are in line with previous theoretical predictions. △ Less

Submitted 28 August, 2023; originally announced August 2023.

Comments: 8 pages, 4 figures

Journal ref: Faraday Discussions (2023)

Resolving the intrinsic short-range ordering of K$^+$ ions on cleaved muscovite mica

Authors: Giada Franceschi, Pavel Kocán, Andrea Conti, Sebastian Brandstetter, Jan Balajka, Igor Sokolović, Markus Valtiner, Florian Mittendorfer, Michael Schmid, Martin Setvín, Ulrike Diebold

Abstract: Muscovite mica, KAl$_2$(Si$_3$Al)O$_{10}$(OH)$_2$, is a common layered phyllosilicate with perfect cleavage planes. The atomically flat surfaces obtained through cleaving lend themselves to scanning probe techniques with atomic resolution and are ideal to model minerals and clays. Despite the importance of the cleaved mica surfaces, several questions remain unresolved. It is established that K… ▽ More Muscovite mica, KAl$_2$(Si$_3$Al)O$_{10}$(OH)$_2$, is a common layered phyllosilicate with perfect cleavage planes. The atomically flat surfaces obtained through cleaving lend themselves to scanning probe techniques with atomic resolution and are ideal to model minerals and clays. Despite the importance of the cleaved mica surfaces, several questions remain unresolved. It is established that K$^+$ ions decorate the cleaved surface, but their intrinsic ordering -- unaffected by the interaction with the environment -- is not known. This work presents clear images of the K$^+$ distribution of cleaved mica obtained with low-temperature non-contact atomic force microscopy (AFM) under ultra-high vacuum (UHV) conditions. The data unveil the presence of short-range ordering, contrasting previous assumptions of random or fully ordered distributions. Density functional theory (DFT) calculations and Monte Carlo simulations show that the substitutional subsurface Al$^{3+}$ ions have an important role for the surface K$^+$ ion arrangement. △ Less

Submitted 27 August, 2023; originally announced August 2023.

Comments: 23 pages, 4 figures

Journal ref: Nature Communications, 14, 208 (2023)

Reconstruction changes drive surface diffusion and determine the flatness of oxide surfaces

Authors: Giada Franceschi, Michael Schmid, Ulrike Diebold, Michele Riva

Abstract: Surface diffusion on metal oxides is key in many areas of materials technology, yet it has been scarcely explored at the atomic scale. This work provides phenomenological insights from scanning tunneling microscopy on the link between surface diffusion, surface atomic structure, and oxygen chemical potential based on three model oxide surfaces: Fe$_2$O$_3(1\overline{1}02)$, La$_{1-x}$Sr$_x$MnO… ▽ More Surface diffusion on metal oxides is key in many areas of materials technology, yet it has been scarcely explored at the atomic scale. This work provides phenomenological insights from scanning tunneling microscopy on the link between surface diffusion, surface atomic structure, and oxygen chemical potential based on three model oxide surfaces: Fe$_2$O$_3(1\overline{1}02)$, La$_{1-x}$Sr$_x$MnO$_3$(110), and In$_2$O$_3$(111). In all instances, changing the oxygen chemical potential used for annealing stabilizes reconstructions of different compositions while promoting the flattening of the surface morphology -- a sign of enhanced surface diffusion. It is argued that thermodynamics, rather than kinetics, rules surface diffusion under these conditions: The composition change of the surface reconstructions formed at differently oxidizing conditions drives mass transport across the surface. △ Less

Submitted 27 August, 2023; originally announced August 2023.

Comments: 20 pages, 3 figures

Journal ref: Journal of Vacuum Science & Technology A 40, 023206 (2022)

The effect of different In$_2$O$_3$(111) surface terminations on CO$_2$ adsorption

Authors: Sabrina M. Gericke, Minttu M. Kauppinen, Margareta Wagner, Michele Riva, Giada Franceschi, Alvaro Posada-Borbón, Lisa Rämisch, Sebastian Pfaff, Erik Rheinfrank, Alexander M. Imre, Alexei B. Preobrajenski, Stephan Appelfeller, Sara Blomberg, Lindsay R. Merte, Johan Zetterberg, Ulrike Diebold, Henrik Grönbeck, Edvin Lundgren

Abstract: In$_2$O$_3$-based catalysts have shown high activity and selectivity for CO$_2$ hydrogenation to methanol, however the origin of the high performance of In$_2$O$_3$ is still unclear. To elucidate the initial steps of CO$_2$ hydrogenation over In$_2$O$_3$, we have combined X-ray Photoelectron Spectroscopy (XPS) and Density Functional Theory (DFT) calculations to study the adsorption of CO$_2$ on th… ▽ More In$_2$O$_3$-based catalysts have shown high activity and selectivity for CO$_2$ hydrogenation to methanol, however the origin of the high performance of In$_2$O$_3$ is still unclear. To elucidate the initial steps of CO$_2$ hydrogenation over In$_2$O$_3$, we have combined X-ray Photoelectron Spectroscopy (XPS) and Density Functional Theory (DFT) calculations to study the adsorption of CO$_2$ on the In$_2$O$_3$(111) crystalline surface with different terminations, namely the stoichiometric, the reduced, and the hydroxylated surface, respectively. The combined approach confirms that the reduction of the surface results in the formation of In ad-atoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level-shifts (using methanol and formic acid as benchmark molecules) suggests that CO$_2$ adsorbs as a carbonate on all surface terminations. We find that CO$_2$ adsorption is hindered by hydroxyl groups on the hydroxylated surface. △ Less

Submitted 24 August, 2023; originally announced August 2023.

Comments: 49 pages, 18 figures

Oxygen-Terminated (1x1) Reconstruction of Reduced Magnetite Fe$_3$O$_4$(111)

Authors: Florian Kraushofer, Matthias Meier, Zdeněk Jakub, Johanna Hütner, Jan Balajka, Jan Hulva, Michael Schmid, Cesare Franchini, Ulrike Diebold, Gareth S. Parkinson

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 Fe$_{\rm oct2}$ termination in reducing conditions. All three struct… ▽ More 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 Fe$_{\rm oct2}$ termination in reducing conditions. All three structures change the coordination of iron in the kagome Fe$_{\rm oct1}$ layer to tetrahedral. With atomically-resolved microscopy techniques, we show that the termination that coexists with the Fe$_{\rm tet1}$ termination consists of tetrahedral iron capped by three-fold coordinated oxygen atoms. This structure explains the inert nature of the reduced patches. △ Less

Submitted 23 August, 2023; originally announced August 2023.

Journal ref: Journal of Physical Chemistry C Letters, 14 (2023) 3258 - 3265

A study of Pt, Rh, Ni and Ir dispersion on anatase TiO2(101) and the role of water

Authors: Lena Puntscher, Kevin Daninger, Michael Schmid, Ulrike Diebold, Gareth S. Parkinson

Abstract: Understanding how metal atoms are stabilized on metal oxide supports is important for predicting the stability of single-atom catalysts. In this study, we use scanning tunnelling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) to investigate four catalytically active metals - Platinum, Rhodium, Nickel and Iridium - on the anatase TiO2(101) surface. The metals were vapor deposited at ro… ▽ More Understanding how metal atoms are stabilized on metal oxide supports is important for predicting the stability of single-atom catalysts. In this study, we use scanning tunnelling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) to investigate four catalytically active metals - Platinum, Rhodium, Nickel and Iridium - on the anatase TiO2(101) surface. The metals were vapor deposited at room temperature in ultrahigh vacuum (UHV) conditions, and also with a background water pressure of 2x10-8 mbar. Pt and Ni exist as a mixture of adatoms and nanoparticles in UHV at low coverage, with the adatoms immobilized at defect sites. Water has no discernible effect on the Pt dispersion, but significantly increases the amount of Ni single atoms. Ir is highly dispersed, but sinters to nanoparticles in the water vapor background leading to the formation of large clusters at step edges. Rh forms clusters on the terrace of anatase TiO2(101) irrespective of the environment. We conclude that introducing defect sites into metal oxide supports could be a strategy to aid the dispersion of single atoms on metal-oxide surfaces, and that the presence of water should be taken into account in the modelling of single-atom catalysts. △ Less

Submitted 23 August, 2023; originally announced August 2023.

Journal ref: Electrochimica Acta, 449 (2023) 142190

The prototypical organic-oxide interface: intra-molecular resolution of sexiphenyl on In$_2$O$_3$(111)

Authors: Margareta Wagner, Jakob Hofinger, Martin Setvín, Lynn A. Boatner, Michael Schmid, Ulrike Diebold

Abstract: The performance of an organic-semiconductor device is critically determined by the geometric alignment, orientation, and ordering of the organic molecules. While an organic multilayer eventually adopts the crystal structure of the organic material, the alignment and configuration at the interface with the substrate/electrode material is essential for charge injection into the organic layer. This w… ▽ More The performance of an organic-semiconductor device is critically determined by the geometric alignment, orientation, and ordering of the organic molecules. While an organic multilayer eventually adopts the crystal structure of the organic material, the alignment and configuration at the interface with the substrate/electrode material is essential for charge injection into the organic layer. This work focuses on the prototypical organic semiconductor para-sexiphenyl (6P) adsorbed on In$_2$O$_3$(111), the thermodynamically most stable surface of the material that the most common transparent conducting oxide, indium tin oxide (ITO) is based on. The onset of nucleation and formation of the first monolayer are followed with atomically-resolved scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Annealing to 200$^\circ$C provides sufficient thermal energy for the molecules to orient themselves along the high-symmetry directions of the surface, leading to a single adsorption site. The AFM data suggests a twisted adsorption geometry. With increasing coverage, the 6P molecules first form a loose network with poor long-range order. Eventually the molecules re-orient and form an ordered monolayer. This first monolayer has a densely packed, well-ordered (2$\times$1) structure with one 6P per In$_2$O$_3$(111) substrate unit cell, i.e., a molecular density of 5.64$\times$10$^{13}$ cm$^{-2}$. △ Less

Submitted 22 August, 2023; originally announced August 2023.

Journal ref: ACS Applied Materials & Interfaces 10, 2018, 14175-14182

Direct assessment of the proton affinity of individual surface hydroxyls with non-contact atomic force microscopy

Authors: Margareta Wagner, Bernd Meyer, Martin Setvin, Michael Schmid, Ulrike Diebold

Abstract: The state of protonation/deprotonation of surfaces has far-ranging implications in all areas of chemistry: from acid-base catalysis$^1$ and the electro- and photocatalytic splitting of water$^2$, to the behavior of minerals$^3$ and biochemistry$^4$. The acidity of a molecule or a surface site is described by its proton affinity (PA) and pK$_\mathrm{a}$ value (the negative logarithm of the equilibr… ▽ More The state of protonation/deprotonation of surfaces has far-ranging implications in all areas of chemistry: from acid-base catalysis$^1$ and the electro- and photocatalytic splitting of water$^2$, to the behavior of minerals$^3$ and biochemistry$^4$. The acidity of a molecule or a surface site is described by its proton affinity (PA) and pK$_\mathrm{a}$ value (the negative logarithm of the equilibrium constant of the proton transfer reaction in solution). For solids, in contrast to molecules, the acidity of individual sites is difficult to assess. For mineral surfaces such as oxides they are estimated by semi-empirical concepts such as bond-order valence sums$^5$, and also increasingly modeled with first-principles molecular dynamics simulations$^{6,7}$. Currently such predictions cannot be tested - the experimental measures used for comparison are typically average quantities integrated over the whole surface or, in some cases, individual crystal facets$^8$, such as the point of zero charge (pzc)$^9$. Here we assess individual hydroxyls on In$_2$O$_3$(111), a model oxide with four different types of surface oxygen atoms, and probe the strength of their hydrogen bond with the tip of a non-contact atomic force microscope (AFM). The force curves are in quantitative agreement with density-functional theory (DFT) calculations. By relating the results to known proton affinities and pK$_\mathrm{a}$ values of gas-phase molecules, we provide a direct measure of proton affinity distributions at the atomic scale. △ Less

Submitted 22 August, 2023; originally announced August 2023.

Journal ref: Nature 592, 2021, 722-725

Adsorption configurations of Co-phthalocyanine on In2O3(111)

Authors: Margareta Wagner, Fabio Calcinelli, Andreas Jeindl, Michael Schmid, Oliver T. Hofmann, Ulrike Diebold

Abstract: Indium oxide offers optical transparency paired with electric conductivity, a combination required in many optoelectronic applications. The most-stable In2O3(111) surface has a large unit cell (1.43 nm lattice constant). It contains a mixture of both bulk-like and undercoordinated O and In atoms and provides an ideal playground to explore the interaction of surfaces with organic molecules of simil… ▽ More Indium oxide offers optical transparency paired with electric conductivity, a combination required in many optoelectronic applications. The most-stable In2O3(111) surface has a large unit cell (1.43 nm lattice constant). It contains a mixture of both bulk-like and undercoordinated O and In atoms and provides an ideal playground to explore the interaction of surfaces with organic molecules of similar size as the unit cell. Non-contact atomic force microscopy (nc-AFM), scanning tunneling microscopy (STM), and density functional theory (DFT) were used to study the adsorption of Co-phthalocyanine (CoPc) on In2O3(111). Isolated CoPc molecules adsorb at two adsorption sites in a 7:3 ratio. The Co atom sits either on top of a surface oxygen ('F configuration') or indium atom ('S configuration'). This subtle change in adsorption site induces bending of the molecules, which is reflected in their electronic structure. According to DFT the lowest unoccupied molecular orbital of the undistorted gas-phase CoPc remains mostly unaffected in the F configuration but is filled by one electron in S configuration. At coverages up to one CoPc molecule per substrate unit cell, a mixture of domains with molecules in F and S configuration are found. Molecules at F sites first condense into a F-(2x2) structure and finally rearrange into a F-(1x1) symmetry with partially overlapping molecules, while S-sited molecules only assume a S-(1x1) superstructure. △ Less

Submitted 22 August, 2023; originally announced August 2023.

Journal ref: Surface Science 722, 2022, 122065

Water Structures Reveal Local Hydrophobicity on the In2O3(111) Surface

Authors: Hao Chen, Matthias A. Blatnik, Christian L. Ritterhoff, Igor Sokolović, Francesca Mirabella, Giada Franceschi, Michele Riva, Michael Schmid, Jan Čechal, Bernd Meyer, Ulrike Diebold, Margareta Wagner

Abstract: Clean oxide surfaces are generally hydrophilic. Water molecules anchor at undercoordinated surface metal atoms that act as Lewis-acid sites, and they are stabilized by H bonds to undercoordinated surface oxygens. The large unit cell of In2O3(111) provides surface atoms in various configurations, which leads to chemical heterogeneity and a local deviation from this general rule. Experiments (TPD, X… ▽ More Clean oxide surfaces are generally hydrophilic. Water molecules anchor at undercoordinated surface metal atoms that act as Lewis-acid sites, and they are stabilized by H bonds to undercoordinated surface oxygens. The large unit cell of In2O3(111) provides surface atoms in various configurations, which leads to chemical heterogeneity and a local deviation from this general rule. Experiments (TPD, XPS, ncAFM) agree quantitatively with DFT calculations and show a series of distinct phases. The first three water molecules dissociate at one specific area of the unit cell and desorb above room temperature. The next three adsorb as molecules in the adjacent region. Three more water molecules rearrange this structure and an additional nine pile up above the OH groups. Despite offering undercoordinated In and O sites, the rest of the unit cell is unfavorable for adsorption and remains water-free. The first water layer thus shows ordering into nanoscopic 3D water clusters separated by hydrophobic pockets. △ Less

Submitted 22 August, 2023; originally announced August 2023.

Journal ref: ACS Nano 16, 2022, 21163-21173

A Multi-Technique Study of C2H4 Adsorption on Fe3O4(001)

Authors: Lena Puntscher, Panukorn Sombut, Chunlei Wang, Manuel Ulreich, Jiri Pavelec, Ali Rafsanjani-Abbasi, Matthias Meier, Adam Lagin, Martin Setvin, Ulrike Diebold, Cesare Franchini, Michael Schmid, Gareth S. Parkinson

Abstract: The adsorption/desorption of ethene (C2H4), also commonly known as ethylene, on Fe3O4(001) was studied under ultrahigh vacuum conditions using temperature programmed desorption (TPD), scanning tunneling microscopy, x-ray photoelectron spectroscopy, and density functional theory (DFT) based computations. To interpret the TPD data, we have employed a new analysis method based on equilibrium thermody… ▽ More The adsorption/desorption of ethene (C2H4), also commonly known as ethylene, on Fe3O4(001) was studied under ultrahigh vacuum conditions using temperature programmed desorption (TPD), scanning tunneling microscopy, x-ray photoelectron spectroscopy, and density functional theory (DFT) based computations. To interpret the TPD data, we have employed a new analysis method based on equilibrium thermodynamics. C2H4 adsorbs intact at all coverages and interacts most strongly with surface defects such as antiphase domain boundaries and Fe adatoms. On the regular surface, C2H4 binds atop surface Fe sites up to a coverage of 2 molecules per (rt2xrt2)R45° unit cell, with every second Fe occupied. A desorption energy of 0.36 eV is determined by analysis of the TPD spectra at this coverage, which is approximately 0.1-0.2 eV lower than the value calculated by DFT + U with van der Waals corrections. Additional molecules are accommodated in between the Fe rows. These are stabilized by attractive interactions with the molecules adsorbed at Fe sites. The total capacity of the surface for C2H4 adsorption is found to be close to 4 molecules per (rt2xrt2)R45° unit cell. △ Less

Submitted 22 August, 2023; originally announced August 2023.

Real-space investigation of polarons in hematite Fe2O3

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

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,… ▽ More 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 Fe2O3 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. △ Less

Submitted 31 March, 2023; originally announced March 2023.

Hematite $α-Fe_{2}O_{3}(0001)$ in top and side view: resolving long-standing controversies about its surface structure

Authors: Jesús Redondo, Jan Michalička, Giada Franceschi, Břetislav Šmid, Nishant Kumar, Ondřej Man, Matthias Blatnik, Dominik Wrana, Florian Kraushofer, Benjamin Mallada, Martin Švec, Gareth S. Parkinson, Martin Setvin, Michele Riva, Ulrike Diebold, Jan Čechal

Abstract: Hematite $α-Fe_{2}O_{3}(0001)$ is the most-investigated iron oxide model system in photo and electrocatalytic research. The rich chemistry of Fe and O allows for many bulk and surface transformations, but their control is challenging. This has led to controversies regarding the structure of the topmost layers. This comprehensive study combines surface methods (nc-AFM, STM, LEED, and XPS) complemen… ▽ More Hematite $α-Fe_{2}O_{3}(0001)$ is the most-investigated iron oxide model system in photo and electrocatalytic research. The rich chemistry of Fe and O allows for many bulk and surface transformations, but their control is challenging. This has led to controversies regarding the structure of the topmost layers. This comprehensive study combines surface methods (nc-AFM, STM, LEED, and XPS) complemented by structural and chemical analysis of the near-surface bulk (HRTEM and EELS). The results show that a compact 2D layer constitutes the topmost surface of $α-Fe_{2}O_{3}(0001)$; it is locally corrugated due to the mismatch with the bulk. Assessing the influence of naturally-occurring impurities shows that these can force the formation of surface phases that are not stable on pure samples. Impurities can also cause the formation of ill-defined inclusions in the subsurface and modify the oxidation phase diagram of hematite. The results provide a significant step forward in determining the hematite surface structure that is crucial for accurately modeling catalytic reactions. Combining surface and cross-sectional imaging provided the full view that is essential for understanding the evolution of the near-surface region of oxide surfaces under oxidative conditions. △ Less

Submitted 10 March, 2023; originally announced March 2023.

CO oxidation by Pt2/Fe3O4: metastable dimer and support configurations facilitate lattice oxygen extraction

Authors: Matthias Meier, Jan Hulva, Zdenek Jakub, Florian Kraushofer, Mislav Bobić, Roland Bliem, Martin Setvin, Michael Schmid, Ulrike Diebold, Cesare Franchini, Gareth S. Parkinson

Abstract: Heterogeneous catalysts based on sub-nanometer 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 utilize a combination of atom… ▽ More Heterogeneous catalysts based on sub-nanometer 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 utilize 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/Fe3O4(001) single-atom catalyst. We demonstrate that (PtCO)2 dimers, formed dynamically through the agglomeration of mobile Pt-carbonyl species, catalyse a reaction involving the oxide support to form CO2. Pt2 dimers produce one CO2 molecule before falling apart into two adatoms, releasing the second CO. Interestingly, Olattice extraction only becomes facile when both the Pt-dimer and the Fe3O4 support can access metastable configurations, suggesting that substantial, concerted rearrangements of both cluster and support must be considered for reactions occurring at elevated temperature. △ Less

Submitted 9 September, 2022; originally announced September 2022.

Journal ref: Science Advances 8 (13), eabn4580, 2022

Single Rh adatoms stabilized on α-Fe2O3(1-102) by co-adsorbed water

Authors: Florian Kraushofer, Lena Haager, Moritz Eder, Ali Rafsanjani-Abbasi, Zdeněk Jakub, Giada Franceschi, Michele Riva, Matthias Meier, Michael Schmid, Ulrike Diebold, Gareth S. Parkinson

Abstract: Oxide-supported single-atom catalysts are commonly modelled as a metal atom substituting surface cation sites in a low-index surface. Adatoms with dangling bonds will inevitably coordinate molecules from the gas phase, and adsorbates such as water can affect both stability and catalytic activity. Here, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (ncAFM), and x-ra… ▽ More Oxide-supported single-atom catalysts are commonly modelled as a metal atom substituting surface cation sites in a low-index surface. Adatoms with dangling bonds will inevitably coordinate molecules from the gas phase, and adsorbates such as water can affect both stability and catalytic activity. Here, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (ncAFM), and x-ray photoelectron spectroscopy (XPS) to show that high densities of single Rh adatoms are stabilized on alpha-Fe2O3(1-102) in the presence of 2x10^(-8) mbar of water at room temperature, in marked contrast to the rapid sintering observed under UHV conditions. Annealing to 50 °C in UHV desorbs all water from the substrate leaving only the OH groups coordinated to Rh, and high-resolution ncAFM images provide a direct view into the internal structure. We provide direct evidence of the importance of OH ligands in the stability of single atoms, and argue that their presence should be assumed when modelling SAC systems. △ Less

Submitted 9 September, 2022; originally announced September 2022.

Journal ref: ACS energy letters 7 (1), 375-380, 2021

Rapid oxygen exchange between hematite and water vapor

Authors: Zdenek Jakub, Matthias Meier, Florian Kraushofer, Jan Balajka, Jiri Pavelec, Michael Schmid, Cesare Franchini, Ulrike Diebold, Gareth S. Parkinson

Abstract: Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surpris… ▽ More Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline r-cut (1-102) surface of hematite (α-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 °C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology. △ Less

Submitted 9 September, 2022; originally announced September 2022.

Journal ref: Nature communications 12 (1), 1-8, 2021

Automated Real-Space Lattice Extraction for Atomic Force Microscopy Images

Authors: Marco Corrias, Lorenzo Papa, Igor Sokolović, Viktor Birschitzky, Alexander Gorfer, Martin Setvin, Michael Schmid, Ulrike Diebold, Michele Reticcioli, Cesare Franchini

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… ▽ More 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 TiO2(101), oxygen deficient rutile TiO2(110) with and without CO adsorbates, SrTiO3(001) with Sr vacancies and graphene with C vacancies. The code delivers excellent results and has proved to be robust against atom misclassification and noise, thereby facilitating the interpretation scanning probe microscopy images. △ Less

Submitted 28 August, 2022; originally announced August 2022.

Journal ref: 2023 Mach. Learn.: Sci. Technol. 4 015015

Competing electronic states emerging on polar surfaces

Authors: Michele Reticcioli, Zhichang Wang, Michael Schmid, Dominik Wrana, Lynn A. Boatner, Ulrike Diebold, Martin Setvin, Cesare Franchini

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… ▽ More 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. Controlling the degree of charge ordering and the transition from ferroelectric to paraelectric states could be of great benefit for the generation and transport of carriers in electronic applications. △ Less

Submitted 1 July, 2022; originally announced July 2022.

Machine Learning for Exploring Small Polaron Configurational Space

Authors: Viktor C. Birschitzky, Florian Ellinger, Ulrike Diebold, Michele Reticcioli, Cesare Franchini

Abstract: Polaron defects are ubiquitous in materials and play an important role in many processes involving carrier mobility, charge transfer and surface reactivity. Determining the spatial distribution of small polarons is essential to understand materials properties and functionalities. This requires an exploration of the configurational space, which is computationally demanding when using standard first… ▽ More Polaron defects are ubiquitous in materials and play an important role in many processes involving carrier mobility, charge transfer and surface reactivity. Determining the spatial distribution of small polarons is essential to understand materials properties and functionalities. This requires an exploration of the configurational space, which is computationally demanding when using standard first principles methods, and technically prohibitive for many-polaron systems. Here, we propose a machine-learning (ML) accelerated search that compares the energy stability of different polaron patterns and determines the ground state configuration. The kernel-regression based ML model is trained on databases generated by density functional theory (DFT) calculations on a minimal set of initial polaron patterns, obtained by using either molecular dynamics simulations or a random sampling approach. To establish an efficient mapping between training data and configuration stability we designed simple descriptors that model the interactions among polarons and charged point defects. The proposed DFT+ML protocol is used here to explore millions of polaron configurations for two different systems, oxygen defective rutile TiO$_2$(110) and Nb-doped SrTiO$_3$(001). Our data shows that the ML-aided search correctly individuates the ground-state polaron patterns, proposes polaronic configurations not visited in the training and can be used to efficiently determine the optimal distribution of polarons at any charge concentration. △ Less

Submitted 2 February, 2022; originally announced February 2022.

Surface Reduction State Determines Stabilization and Incorporation of Rh on α-Fe2O3(1-102)

Authors: Florian Kraushofer, Nikolaus Resch, Moritz Eder, Ali Rafsanjani-Abbasi, Sarah Tobisch, Zdenek Jakub, Giada Franceschi, Michele Riva, Matthias Meier, Michael Schmid, Ulrike Diebold, Gareth S. Parkinson

Abstract: Iron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as-synthesized catalyst prior to use. Here, we study Rh adsorption and incorporation on the (1-102) surface of hematite (α-Fe2O3), which switches from a stoichiometric (1x1) termination to a reduced (… ▽ More Iron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as-synthesized catalyst prior to use. Here, we study Rh adsorption and incorporation on the (1-102) surface of hematite (α-Fe2O3), which switches from a stoichiometric (1x1) termination to a reduced (2x1) reconstruction in reducing conditions. Rh atoms form clusters at room temperature on both surface terminations, but Rh atoms incorporate into the support lattice as isolated atoms upon annealing above 400 °C. Under mildly oxidizing conditions, the incorporation process is so strongly favoured that even large Rh clusters containing hundreds of atoms dissolve into the surface. Based on a combination of low energy ion scattering and scanning tunnelling microscopy data, as well as density functional theory, we conclude that the Rh atoms are stabilized in the immediate subsurface, rather than the surface layer. △ Less

Submitted 27 September, 2021; originally announced September 2021.

Journal ref: Advanced Materials Interfaces 8, 2001908 (2021)

Unravelling CO adsorption on model single-atom catalysts

Authors: Jan Hulva, Matthias Meier, Roland Bliem, Zdenek Jakub, Florian Kraushofer, Michael Schmid, Ulrike Diebold, Cesare Franchini, Gareth S. Parkinson

Abstract: Understanding how the local environment of a single-atom catalyst affects stability and reactivity remains a significant challenge. We present an in-depth study of Cu1, Ag1, Au1, Ni1, Pd1, Pt1, Rh1, and Ir1 species on Fe3O4(001); a model support where all metals occupy the same 2-fold coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsor… ▽ More Understanding how the local environment of a single-atom catalyst affects stability and reactivity remains a significant challenge. We present an in-depth study of Cu1, Ag1, Au1, Ni1, Pd1, Pt1, Rh1, and Ir1 species on Fe3O4(001); a model support where all metals occupy the same 2-fold coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal-CO bond. These effects could strengthen the bond (as for Ag1-CO) or weaken it (as for Ni1-CO), but CO-induced structural distortions reduce adsorption energies from those expected based on electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry. △ Less

Submitted 27 September, 2021; originally announced September 2021.

Journal ref: Science 371, 375-379 (2021)

Atomic-scale studies of Fe3O4(001) and TiO2(110) surfaces following immersion in CO2-acidified water

Authors: Francesca Mirabella, Jan Balajka1, Jiri Pavelec, Markus Göbel, Florian Kraushofer, Michael Schmid1, Gareth S. Parkinson1, Ulrike Diebold

Abstract: Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water, and studied its interaction with TiO2 and Fe3O4 surfaces. Here, we describe a simple approach to vary the pH through the partial p… ▽ More Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water, and studied its interaction with TiO2 and Fe3O4 surfaces. Here, we describe a simple approach to vary the pH through the partial pressure of CO2 (pCO2) in the surrounding vacuum chamber, and use this to study how these surfaces react to an acidic solution. The TiO2(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe3O4(001)-(rt2 x rt2)R45 surface begins to dissolve at a pH 4.0-3.9 (pCO2 = 0.8-1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe3O4(001) surface layer remains visible in scanning tunneling microscopy (STM) images. X-ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced, and contains a significant accumulation of bicarbonate (HCO3-) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO3)2), which precipitate onto the surface when the water evaporates. △ Less

Submitted 27 September, 2021; originally announced September 2021.

Journal ref: Chemphyschem 21, 1788 (2020)

Ni modified Fe3O4(001) surface as a simple model system for understanding the Oxygen Evolution Reaction

Authors: Francesca Mirabella, Matthias Muellner, Thomas Touzalin, Michele Riva, Zdenek Jakub, Florian Kraushofer, Michael Schmid, Marc T. M. Koper, Gareth S. Parkinson, Ulrike Diebold

Abstract: Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the… ▽ More Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4 (001) single crystal, using a combination of surface science techniques in ultra-high-vacuum such as low energy electron diffraction (LEED), x-rays photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an Fe:Ni composition ratio in the range 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. △ Less

Submitted 27 September, 2021; originally announced September 2021.

Journal ref: Electrochimica Acta 389, 138638 (2021)

An oxygen-rich, tetrahedral surface phase on high-temperature rutile VO$_2$(110)$_\text{T}$ single crystals

Authors: Margareta Wagner, Jakub Planer, Bettina S. J. Heller, Jens Langer, Andreas Limbeck, Lynn A. Boatner, Hans-Peter Steinrück, Josef Redinger, Florian Maier, Florian Mittendorfer, Michael Schmid, Ulrike Diebold

Abstract: Vanadium dioxide undergoes a metal-insulator transition (MIT) from an insulating (monoclinic) to a metallic (tetragonal) phase close to room temperature, which makes it a promising functional material for many applications, e.g. as chemical sensors. Not much is known about its surface and interface properties, although these are critical in many of its applications. This work presents an atomic-sc… ▽ More Vanadium dioxide undergoes a metal-insulator transition (MIT) from an insulating (monoclinic) to a metallic (tetragonal) phase close to room temperature, which makes it a promising functional material for many applications, e.g. as chemical sensors. Not much is known about its surface and interface properties, although these are critical in many of its applications. This work presents an atomic-scale investigation of the tetragonal rutile VO$_2$(110)$_\text{T}$ single-crystal surface and reports results obtained with scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS), supported by density-functional theory-based (DFT) calculations. The surface reconstructs into an oxygen-rich (2$\times$2) superstructure that coexists with small patches of the underlying, unreconstructed (110)-(1$\times$1) surface. The best structural model for the (2$\times$2) surface termination, conceptually derived from a vanadium pentoxide (001) monolayer, consists of rings of corner-shared tetrahedra. Over a wide range of oxygen chemical potentials this reconstruction is more stable than the unreconstructed (110) surface as well as models proposed in the literature. △ Less

Submitted 1 July, 2021; originally announced July 2021.

Does a pristine, unreconstructed SrTiO$_3$(001) surface exist?

Authors: Igor Sokolović, Giada Franceschi, Zhichang Wang, Jian Xu, Jiří Pavelec, Michele Riva, Michael Schmid, Ulrike Diebold, Martin Setvín

Abstract: The surfaces of perovskite oxides affect their functional properties, and while a bulk-truncated (1$\times$1) termination is generally assumed, its existence and stability is controversial. Here, such a surface is created by cleaving the prototypical SrTiO$_3$(001) in ultra-high vacuum, and its response to thermal annealing is observed. Atomically resolved nc-AFM shows that intrinsic point defects… ▽ More The surfaces of perovskite oxides affect their functional properties, and while a bulk-truncated (1$\times$1) termination is generally assumed, its existence and stability is controversial. Here, such a surface is created by cleaving the prototypical SrTiO$_3$(001) in ultra-high vacuum, and its response to thermal annealing is observed. Atomically resolved nc-AFM shows that intrinsic point defects on the as-cleaved surface migrate at temperatures above 200\,$^\circ$C. At 400--500\,$^\circ$C, a disordered surface layer forms, albeit still with a (1$\times$1) pattern in LEED. Purely TiO$_2$-terminated surfaces, prepared by wet-chemical treatment, are also disordered despite their (1$\times$1) periodicity in LEED. △ Less

Submitted 16 December, 2020; originally announced December 2020.

Journal ref: Phys. Rev. B 103, 241406 (2021)

2D Surface Phase Diagram of a Multicomponent Perovskite Oxide: La$_{0.8}$Sr$_{0.2}$MnO$_3$(110)

Authors: Giada Franceschi, Michael Schmid, Ulrike Diebold, Michele Riva

Abstract: The many surface reconstructions of (110)-oriented lanthanum--strontium manganite (La$_{0.8}$Sr$_{0.2}$MnO$_3$, LSMO) were followed as a function of the oxygen chemical potential ($μ_\text{O}$) and the surface cation composition. Decreasing $μ_\text{O}$ causes Mn to migrate across the surface, enforcing phase separation into A-site-rich areas and a variety of composition-related, structurally dive… ▽ More The many surface reconstructions of (110)-oriented lanthanum--strontium manganite (La$_{0.8}$Sr$_{0.2}$MnO$_3$, LSMO) were followed as a function of the oxygen chemical potential ($μ_\text{O}$) and the surface cation composition. Decreasing $μ_\text{O}$ causes Mn to migrate across the surface, enforcing phase separation into A-site-rich areas and a variety of composition-related, structurally diverse B-site-rich reconstructions. The composition of these phase-separated structures was quantified with scanning tunneling microscopy (STM), and these results were used to build a 2D phase diagram of the LSMO(110) equilibrium surface structures. △ Less

Submitted 11 October, 2020; originally announced October 2020.

Journal ref: Phys. Rev. Materials 5, 092401 (2021)

IrO2 Surface Complexions Identified Through Machine Learning and Surface Investigations

Authors: Jakob Timmermann, Florian Kraushofer, Nikolaus Resch, Peigang Li, Yu Wang, Zhiqiang Mao, Michele Riva, Yonghyuk Lee, Carsten Staacke, Michael Schmid, Christoph Scheurer, Gareth S. Parkinson, Ulrike Diebold, Karsten Reuter

Abstract: A Gaussian Approximation Potential (GAP) was trained using density-functional theory data to enable a global geometry optimization of low-index rutile IrO2 facets through simulated annealing. Ab initio thermodynamics identifies (101) and (111) (1x1)-terminations competitive with (110) in reducing environments. Experiments on single crystals find that (101) facets dominate, and exhibit the theoreti… ▽ More A Gaussian Approximation Potential (GAP) was trained using density-functional theory data to enable a global geometry optimization of low-index rutile IrO2 facets through simulated annealing. Ab initio thermodynamics identifies (101) and (111) (1x1)-terminations competitive with (110) in reducing environments. Experiments on single crystals find that (101) facets dominate, and exhibit the theoretically predicted (1x1) periodicity and X-ray photoelectron spectroscopy (XPS) core level shifts. The obtained structures are analogous to the complexions discussed in the context of ceramic battery materials. △ Less

Submitted 24 September, 2020; originally announced September 2020.

Comments: 13 pages 2 figures

Journal ref: Phys. Rev. Lett. 125, 206101 (2020)

Few-monolayer yttria-doped zirconia films: Segregation and phase stabilization

Authors: Peter Lackner, Amy J. Brandt, Ulrike Diebold, Michael Schmid

Abstract: For most applications, zirconia (ZrO2) is doped with yttria. Doping leads to the stabilization of the tetragonal or cubic phase, and increased oxygen ion conductivity. Most previous surface studies of yttria-doped zirconia were plagued by impurities, however. We have studied doping of pure, 5-monolayer ZrO2 films on Rh(111) by x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (… ▽ More For most applications, zirconia (ZrO2) is doped with yttria. Doping leads to the stabilization of the tetragonal or cubic phase, and increased oxygen ion conductivity. Most previous surface studies of yttria-doped zirconia were plagued by impurities, however. We have studied doping of pure, 5-monolayer ZrO2 films on Rh(111) by x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). STM and LEED show that the tetragonal phase is stabilized by unexpectedly low dopant concentrations, 0.5 mol% Y2O3, even when the films are essentially fully oxidized (as evidenced by XPS core level shifts). XPS also shows Y segregation to the surface with an estimated segregation enthalpy of $-23 \pm 4$\,kJ/mol. △ Less

Submitted 21 January, 2020; originally announced January 2020.

Comments: Accepted for Journal of Chemical Physics

Substoichiometric ultrathin zirconia films cause strong metal-support interaction

Authors: Peter Lackner, Joong-Il Jake Choi, Ulrike Diebold, Michael Schmid

Abstract: The strong metal-support interaction (SMSI) leads to substantial changes of the properties of an oxide-supported catalyst after annealing under reducing conditions. The common explanation is the formation of heavily reduced, ultrathin oxide films covering metal particles. This is typically encountered for reducible oxides such as TiO2 or Fe3O4. Zirconia (ZrO2), a typical catalyst support, is diffi… ▽ More The strong metal-support interaction (SMSI) leads to substantial changes of the properties of an oxide-supported catalyst after annealing under reducing conditions. The common explanation is the formation of heavily reduced, ultrathin oxide films covering metal particles. This is typically encountered for reducible oxides such as TiO2 or Fe3O4. Zirconia (ZrO2), a typical catalyst support, is difficult to reduce and therefore no obvious candidate for the SMSI effect. In this work, we use inverse model systems with Rh(111), Pt(111), and Ru(0001) as supports. Contrary to expectations, we show that SMSI is encountered for zirconia. Upon annealing in ultra-high vacuum, oxygen-deficient ultrathin zirconia films ($\approx$ZrO$_{1.5}$) form on all three substrates. However, Zr remains in its preferred charge state of 4+, as electrons are transferred to the underlying metal. At high temperatures, the stability of the ultrathin zirconia films depends on whether alloying of Zr and the substrate metal occurs. The SMSI effect is reversible; the ultrathin suboxide films can be removed by annealing in oxygen. △ Less

Submitted 17 October, 2019; originally announced October 2019.

Comments: Submitted to Journal of Materials Chemistry A (2019)

Journal ref: Journal of Materials Chemistry A 7, 24837-24846 (2019)

Self-limited Growth of an Oxyhydroxide Phase at the Fe3O4(001) Surface in Liquid and Ambient Pressure Water

Authors: Florian Kraushofer, Francesca Mirabella, Jian Xu, Jiří Pavelec, Jan Balajka, Matthias Müllner, Nikolaus Resch, Zdeněk Jakub, Jan Hulva, Matthias Meier, Michael Schmid, Ulrike Diebold, Gareth S. Parkinson

Abstract: Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g. dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the UHV-compatible dosing of liquids to explore the stability of the Fe3O4(001)-c(2x2) surface following exposure to liquid and… ▽ More Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g. dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the UHV-compatible dosing of liquids to explore the stability of the Fe3O4(001)-c(2x2) surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) data show that extensive hydroxylation causes the surface to revert to a bulk-like (1x1) termination. However, scanning tunnelling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers, and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM (EC-STM) studies lead us to believe a similar structure exists at the solid-electrolyte interface during immersion at pH 7. △ Less

Submitted 28 August, 2019; originally announced August 2019.

Small Polarons in Transition Metal Oxides

Authors: Michele Reticcioli, Ulrike Diebold, Georg Kresse, Cesare Franchini

Abstract: The formation of polarons is a pervasive phenomenon in transition metal oxide compounds, with a strong impact on the physical properties and functionalities of the hosting materials. In its original formulation the polaron problem considers a single charge carrier in a polar crystal interacting with its surrounding lattice. Depending on the spatial extension of the polaron quasiparticle, originati… ▽ More The formation of polarons is a pervasive phenomenon in transition metal oxide compounds, with a strong impact on the physical properties and functionalities of the hosting materials. In its original formulation the polaron problem considers a single charge carrier in a polar crystal interacting with its surrounding lattice. Depending on the spatial extension of the polaron quasiparticle, originating from the coupling between the excess charge and the phonon field, one speaks of small or large polarons. This chapter discusses the modeling of small polarons in real materials, with a particular focus on the archetypal polaron material TiO2. After an introductory part, surveying the fundamental theoretical and experimental aspects of the physics of polarons, the chapter examines how to model small polarons using first principles schemes in order to predict, understand and interpret a variety of polaron properties in bulk phases and surfaces. Following the spirit of this handbook, different types of computational procedures and prescriptions are presented with specific instructions on the setup required to model polaron effects. △ Less

Submitted 15 February, 2019; v1 submitted 11 February, 2019; originally announced February 2019.

Comments: 36 pages, 12 figures

Journal ref: In: Andreoni W., Yip S. (eds) Handbook of Materials Modeling. Springer, Cham. (2019)

Adsorption of CO on the Ca3Ru2O7(001) surface

Authors: Wernfried Mayr-Schmölzer, Daniel Halwidl, Florian Mittendorfer, Michael Schmid, Ulrike Diebold, Josef Redinger

Abstract: The adsorption of CO molecules at the Ca3Ru2O7(001) surface was studied using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). Ca3Ru2O7 can be easily cleaved along the (001) plane, yielding a smooth, CaO-terminated surface. The STM shows a characteristic pattern with alternating dark and bright stripes, resulting from the tilting of the RuO$_6$ octahedra.… ▽ More The adsorption of CO molecules at the Ca3Ru2O7(001) surface was studied using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). Ca3Ru2O7 can be easily cleaved along the (001) plane, yielding a smooth, CaO-terminated surface. The STM shows a characteristic pattern with alternating dark and bright stripes, resulting from the tilting of the RuO$_6$ octahedra. At 78,K, CO adsorbs at an apical surface O at the channel edge with a predicted binding energy of $E_{ads} = -0.85$eV. After annealing at room temperature, the CO forms a strong bond ($E_{ads}= -2.04$eV) with the apical O and the resulting carboxylate takes the place of the former surface O. This carboxylate can be decomposed by scanning the surface with a high sample bias voltage of +2.7V, restoring the original surface. △ Less

Submitted 2 October, 2018; originally announced October 2018.

Journal ref: Surf. Sci. 680, 18-23 (2019)