Morphology of monolayer MgO films on Ag(100): switching from corrugated islands to extended flat terraces

Charge and adsorption height dependence of the self-metalation of porphyrins on ultrathin MgO(001) films

We have experimentally determined the adsorption structure, charge state, and metalation state of porphin, the fundamental building block of porphyrins, on ultrathin Ag(001)-supported MgO(001) films by scanning tunneling microscopy and photoemission spectroscopy, supported by calculations based on density functional theory. By tuning the substrate work function to values below and above the critical work function for charging, we succeeded in the preparation of 2H-P monolayers which contain negatively charged and uncharged molecules. It is shown that the porphin molecules self-metalate at room temperature, forming the corresponding Mg-porphin, irrespective of their charge state. This is in contrast to self-metalation of tetraphenyl porphyrin (TPP), which occurs on planar MgO(001) only if the molecules are negatively charged. The different reactivity is explained by the reduced molecule-substrate distance of the planar porphin molecule compared to the bulkier TPP. The results of this study shed light on the mechanism of porphyrin self-metalation on oxides and highlight the role of the adsorption geometry on the chemical reactivity.

Negative electron affinity opens quantum well in MgO layers on Ag(100)

Epitaxial MgO films on Ag(100) were studied by photoelectron spectroscopy. From the low-energy part of the spectra we obtain a negative electron affinity of about -0.9 eV. Even though electrons in the lowest conduction band are not confined by a potential barrier at the surface, quantum-well resonances are observed. The dispersion of the conduction band is determined in good agreement with theoretical calculations. Aspects of observing image-potential states predicted by theory on MgO films are discussed.

Growth-mode and interface structure of epitaxial ultrathin MgO/Ag(001) films

MgO ultrathin films are of great technological importance as electron tunneling barrier in electronics and spintronics, and as template for metallic clusters in catalysis and for molecular networks for 2D electronics. The wide band-gap of MgO allows for a very effective decoupling from the substrate. The films morphology and the detailed structure of the interface are crucial for applications, controlling the electronic transfer. Using surface x-ray diffraction, we studied the growth-mode and the structure of MgO/Ag(001) ultrathin films elaborated by reactive molecular beam epitaxy as function of the substrate temperature. We observed that deposition of about 1 monolayer results in an MgO(001) film in coherent epitaxy, with the oxygen atoms on top of silver as predicted by DFT calculations, and an interlayer distance at the interface of about 270 pm. Under well-defined conditions, a sharp MgO bilayer is formed covering a fraction of the substrate surface.

Morphology dependent interaction between Co(II)-tetraphenylporphyrin and the MgO(100) surface

Porphyrins are key elements in organic–inorganic hybrid systems for a wide range of applications. Understanding their interaction with the substrate gives a handle on structural and electronic device properties. Here we investigate a single transition-metal porphyrin, namely Co(ii)-tetraphenylporphyrin (CoTPP), on the MgO(100) surface and the effect of multilayer film formation within hybrid density-functional theory and many-body perturbation theory. We focus on the relevant adsorption sites, simulate their photoemission spectra as a key fingerprint and compare with experiments on MgO(100) films on Ag(100). While we find only weak interaction between the cobalt centre and terrace sites on the MgO(100) surface, a strong interaction manifests itself with the low-coordinated sites. This leads to distinct features in both the valence and core-level regions of the electronic structure, as observed in the ultraviolet and X-ray photoemission spectra, corroborated by simulated spectra and calculated cobalt core-level shifts. Our work thus demonstrates the relevance of morphology-related low-coordinated sites and their properties in the adsorption of CoTPP on the MgO(100) surface.

The adsorption of Co-tetraphenylporphyrin at relevant low-coordinated sites on MgO(100) shows distinct features from terrace-site and multilayer films in the near-valence and corelevel regions of the electronic structure.

Controlling the electronic and physical coupling on dielectric thin films

Ultrathin dielectric/insulating films on metals are often used as decoupling layers to allow for the study of the electronic properties of adsorbed molecules without electronic interference from the underlying metal substrate. However, the presence of such decoupling layers may effectively change the electron donating properties of the substrate, for example, by lowering its work function and thus enhancing the charging of the molecular adsorbate layer through electron tunneling. Here, an experimental study of the charging of para-sexiphenyl (6P) on ultrathin MgO(100) films supported on Ag(100) is reported. By deliberately changing the work function of the MgO(100)/Ag(100) system, it is shown that the charge transfer (electronic coupling) into the 6P molecules can be controlled, and 6P monolayers with uncharged molecules (Schottky-Mott regime) and charged and uncharged molecules (Fermi level pinning regime) can be obtained. Furthermore, it was found that charge transfer and temperature strongly influence the orientation, conformation, and wetting behavior (physical coupling) of the 6P layers on the MgO(100) thin films.

Chapter model systems in heterogeneous catalysis at the atomic level: a personal view

The review presents an overview of studies in the surface science of oxide and related surfaces with an emphasis of the studies performed in the authors’ group. Novel instruments and technique developments, as well as their applications are reported, in an attempt to cover studies on model systems of increasing complexity, including some of the key ingredients of an industrially applied heterogeneous catalyst and its fabrication. The review is intended to demonstrate the power of model studies in understanding heterogeneous catalysis at the atomic level. The studies include those on supported nano-particles, both, prepared in vacuum and from solution, interaction of surfaces and the underlying bulk with molecules from the gas phase, strong metal support interaction, as well as the first attempt to include studies on reactions in confined spaces.

2D oxides on metal materials: concepts, status, and perspectives

Two-dimensional oxide-on-metal materials: concepts, methods, and link to technological applications, with 5 subtopics: structural motifs, robustness, catalysis, ternaries, and nanopatterning.

Upgrade of a low-temperature scanning tunneling microscope for electron-spin resonance

Electron spin resonance with a scanning tunneling microscope (ESR-STM) combines the high energy resolution of spin resonance spectroscopy with the atomic scale control and spatial resolution of STM. Here we describe the upgrade of a helium-3 STM with a 2D vector-field magnet (B_z = 8.0 T, B_x = 0.8 T) to an ESR-STM. The system is capable of delivering radio frequency (RF) power to the tunnel junction at frequencies up to 30 GHz. We demonstrate magnetic field-sweep ESR for the model system TiH/MgO/Ag(100) and find a magnetic moment of (1.004 ± 0.001) μ_B. Our upgrade enables to toggle between a DC mode, where the STM is operated with the regular control electronics, and an ultrafast-pulsed mode that uses an arbitrary waveform generator for pump-probe spectroscopy or reading of spin-states. Both modes allow for simultaneous radiofrequency excitation, which we add via a resistive pick-off tee to the bias voltage path. The RF cabling from room temperature to the 350 mK stage has an average attenuation of 18 dB between 5 and 25 GHz. The cable segment between the 350 mK stage and the STM tip presently attenuates an additional 34_-3 ⁺⁵ dB from 10 to 26 GHz and 38_-2 ⁺³ dB between 20 and 30 GHz. We discuss our transmission losses and indicate ways to reduce this attenuation. We finally demonstrate how to synchronize the arrival times of RF and DC pulses coming from different paths to the STM junction, a prerequisite for future pulsed ESR experiments.

Spin Excitations in a 4f-3d Heterodimer on MgO

We report on the magnetic properties of HoCo dimers as a model system for the smallest intermetallic compound of a lanthanide and a transition metal atom. The dimers are adsorbed on ultrathin MgO(100) films grown on Ag(100). New for 4f elements, we detect inelastic excitations with scanning tunneling spectroscopy and prove their magnetic origin by applying an external magnetic field. In combination with density functional theory and spin Hamiltonian analysis, we determine the magnetic level distribution, as well as sign and magnitude of the exchange interaction between the two atoms. In contrast to typical 4f-3d bulk compounds, we find ferromagnetic coupling in the dimer.

1st Symposium on Polydopamine and NanoTech Poland 2018: Conference Report

The NanoTech Poland is an annual international conference with a strong scientific agenda focused on nanotechnology in energy, environment, and biomedicine. The Nanotech Poland 2018 was held at the NanoBioMedical Centre and Department of Physics at Adam Mickiewicz University in Poznań from June 6th to June 9th. The aim of NanoTech Poland 2018 was to bring together the scientific community's principal investigators, scientists, researchers, analysts, clinicians, policy makers, industry experts, and well-established and budding entrepreneurs to discuss the present and future perspectives in nanotechnology and nanoscience research and development. This year, the 1st Symposium on Polydopamine was held on June 6th. This forum was dedicated to the application of polydopamine and related catechol materials in a variety of research fields, both at the nano- and macroscale. The symposium gathered leading scientists from this important research field from top universities and institutions that have been involved in the research revolved around polydopamine. With over 200 national and international participants, NanoTech Poland 2018 and the 1st Symposium on Polydopamine provided a forum to present and discuss the latest scientific news from the field of nanotechnology with a strong interdisciplinary aspect and bioinspired materials.

On the Structure of Ultrathin FeO Films on Ag(111)

Ultrathin transition metal oxide films exhibit unique physical and chemical properties not observed for the corresponding bulk oxides. These properties, originating mainly from the limited thickness and the interaction with the support, make those films similar to other supported 2D materials with bulk counterparts, such as transition metal dichalcogenides. Ultrathin iron oxide (FeO) films, for example, were shown to exhibit unique electronic, catalytic and magnetic properties that depend on the type of the used support. Ag(111) has always been considered a promising substrate for FeO growth, as it has the same surface symmetry, only ~5% lattice mismatch, is considered to be weakly-interacting and relatively resistant to oxidation. The reports on the growth and structure of ultrathin FeO films on Ag(111) are scarce and often contradictory to each other. We attempted to shed more light on this system by growing the films using different preparation procedures and studying their structure using scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). We observed the formation of a previously unreported Moiré superstructure with 45 Å periodicity, as well as other reconstructed and reconstruction-free surface species. The experimental results obtained by us and other groups indicate that the structure of FeO films on this particular support critically depends on the films' preparation conditions. We also performed density functional theory (DFT) calculations on the structure and properties of a conceptual reconstruction-free FeO film on Ag(111). The results indicate that such a film, if successfully grown, should exhibit tunable thickness-dependent properties, being substrate-influenced in the monolayer regime and free-standing-FeO-like when in the bilayer form.

Thermal and Magnetic-Field Stability of Holmium Single-Atom Magnets

We use spin-polarized scanning tunneling microscopy to demonstrate that Ho atoms on magnesium oxide exhibit a coercive field of more than 8 T and magnetic bistability for many minutes, both at 35 K. The first spontaneous magnetization reversal events are recorded at 45 K, for which the metastable state relaxes in an external field of 8 T. The transverse magnetic anisotropy energy is estimated from magnetic field and bias voltage dependent switching rates at 4.3 K. Our measurements constrain the possible ground state of Ho single-atom magnets to either J_{z}=7 or 8, both compatible with magnetic bistability at fields larger than 10 mT.

Interaction of free-base tetraphenylporphyrin with magnesium oxide: Influence of MgO morphology on metalation

Using x-ray photoemission spectroscopy, we investigated the self-metalation of free-base tetraphenylporphyrin (2HTPP) on thin MgO(100) films on Ag(100). The deposition of one monolayer 2HTPP on MgO results in the formation of magnesium(ii) tetraphenylporphyrin (MgTPP) at room temperature. We demonstrate that the efficiency of the reaction drastically depends on the morphology of the oxide layers. The latter is changed by varying the substrate temperature during the oxide growth. We observe the complete metalation of the 2HTPP monolayer when the MgO films are grown at 393 K. The increase of the growth temperature to 573 K leads to the reduction of the percentage of metalated molecules to ∼50%. We ascribe these results to the fact that MgTPP formation takes place through the hydroxilation of steps and defects on the MgO surface, which leads to an increase of the OH component in the O 1s line.

Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems

Model systems are very important to identify the working principles of real catalysts, and to develop concepts that can be used in the design of new catalytic materials.

Charge Control in Model Catalysis: The Decisive Role of the Oxide-Nanoparticle Interface

In chemistry and physics the electronic charge on a species or material is one important determinant of its properties. In the present Minireview, the essential requirements for a model catalyst system suitable to study charge control are discussed. The ideal model catalyst for this purpose consists of a material system, which comprises a single crystal metal support, covered by an epitaxially grown ultrathin oxide film, and flat, two-dimensional nanoparticles residing on this film. Several examples from the literature are selected and presented, which illustrate various aspects of electron transport from the support to the nanoparticle and vice versa. Key experiments demonstrate charge control within such model catalysts and give direct evidence for a chemical reaction at the perimeter of Au nanoparticles. The concepts derived from these studies are then taken a step further to see how they may be applied for bulk powder oxide supported nanoparticles as they are frequently found in catalytically active materials.

Reading and writing single-atom magnets

The single-atom bit represents the ultimate limit of the classical approach to high-density magnetic storage media. So far, the smallest individually addressable bistable magnetic bits have consisted of 3-12 atoms. Long magnetic relaxation times have been demonstrated for single lanthanide atoms in molecular magnets, for lanthanides diluted in bulk crystals, and recently for ensembles of holmium (Ho) atoms supported on magnesium oxide (MgO). These experiments suggest a path towards data storage at the atomic limit, but the way in which individual magnetic centres are accessed remains unclear. Here we demonstrate the reading and writing of the magnetism of individual Ho atoms on MgO, and show that they independently retain their magnetic information over many hours. We read the Ho states using tunnel magnetoresistance and write the states with current pulses using a scanning tunnelling microscope. The magnetic origin of the long-lived states is confirmed by single-atom electron spin resonance on a nearby iron sensor atom, which also shows that Ho has a large out-of-plane moment of 10.1 ± 0.1 Bohr magnetons on this surface. To demonstrate independent reading and writing, we built an atomic-scale structure with two Ho bits, to which we write the four possible states and which we read out both magnetoresistively and remotely by electron spin resonance. The high magnetic stability combined with electrical reading and writing shows that single-atom magnetic memory is indeed possible.

Charge Transfer and Orbital Level Alignment at Inorganic/Organic Interfaces: The Role of Dielectric Interlayers

It is becoming accepted that ultrathin dielectric layers on metals are not merely passive decoupling layers, but can actively influence orbital energy level alignment and charge transfer at interfaces. As such, they can be important in applications ranging from catalysis to organic electronics. However, the details at the molecular level are still under debate. In this study, we present a comprehensive analysis of the phenomenon of charge transfer promoted by a dielectric interlayer with a comparative study of pentacene adsorbed on Ag(001) with and without an ultrathin MgO interlayer. Using scanning tunneling microscopy and photoemission tomography supported by density functional theory, we are able to identify the orbitals involved and quantify the degree of charge transfer in both cases. Fractional charge transfer occurs for pentacene adsorbed on Ag(001), while the presence of the ultrathin MgO interlayer promotes integer charge transfer with the lowest unoccupied molecular orbital transforming into a singly occupied and singly unoccupied state separated by a large gap around the Fermi energy. Our experimental approach allows a direct access to the individual factors governing the energy level alignment and charge-transfer processes for molecular adsorbates on inorganic substrates.

Structural motifs of water on metal oxide surfaces

This review describes the state-of-the-art of the molecular-level understanding of water adsorption, dissociation and clustering on model surfaces of metal oxides.

Magnetic remanence in single atoms

A permanent magnet retains a substantial fraction of its saturation magnetization in the absence of an external magnetic field. Realizing magnetic remanence in a single atom allows for storing and processing information in the smallest unit of matter. We show that individual holmium (Ho) atoms adsorbed on ultrathin MgO(100) layers on Ag(100) exhibit magnetic remanence up to a temperature of 30 kelvin and a relaxation time of 1500 seconds at 10 kelvin. This extraordinary stability is achieved by the realization of a symmetry-protected magnetic ground state and by decoupling the Ho spin from the underlying metal by a tunnel barrier.

Generation of highly reactive oxygen species on metal-supported MgO(100) thin films

A series of highly reactive oxygen species are formed with the assistance of water on an insulating surface.

Supports and modified nano-particles for designing model catalysts

In order to design catalytic materials, we need to understand the essential causes for material properties resulting from its composite nature. In this paper we discuss two, at first sight, diverse aspects: (a) the effect of the oxide–metal interface on metal nanoparticle properties and (b) the consequences of metal particle modification after activation on the selectivity of hydrogenation reactions. However, these two aspects are intimately linked. The metal nanoparticle’s electronic structure changes at the interface as a catalyst is brought to different reaction temperatures due to morphological modifications in the metal and, as we will discuss, these changes in the chemistry lead to changes in the reaction path. As the morphology of the particle varies, facets of different orientations and sizes are exposed, which may lead to a change in the surface chemistry as well. We use two specific reactions to address these issues in some detail. To the best of our knowledge, the present paper reports the first observations of this kind for well-defined model systems. The changes in the electronic structure of Au nanoparticles due to their size and interaction with a supporting oxide are revealed as a function of temperature using CO₂ activation as a probe. The presence of spectator species (oxopropyl), formed during an activation step of acrolein hydrogenation, strongly controls the selectivity of the reaction towards hydrogenation of the unsaturated CO bond vs. the CC bond on Pd(111) when compared with oxide-supported Pd nanoparticles.

Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen

We report the study of Ni nanoclusters deposited on MgO/Ag(100) ultrathin films (one monolayer) at T = 200 K. We show by STM analysis and DFT calculations that in the limit of low Ni coverage the formation of nanoclusters of four to six atoms occurs and that these aggregates are flat rather than 3D, as expected for Ni tetramers, pentamers, or hexamers. Both the shape of the clusters and the interatomic distance between neighboring Ni atoms are indicative that the nanoparticles do not consist of pure metal atoms but that a NiyOx structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface, with Ni clusters acting as oxygen pumps. Besides being of relevance in view of the use of metal nanoclusters in catalysis and other applications, this finding gives a further proof of the peculiar behavior of ultrathin oxide films.

Properties of two-dimensional insulators: a DFT study of Co adsorption on NaCl and MgO ultrathin films

The interaction of TM atoms like Co with two-dimensional NaCl/Au(111) and MgO/Ag(001) ultrathin films is completely different.