Recently 2D networks of sp2 hybridized B-atoms, called Borophene, have been grown on (111) faces of Cu, Ag and Au and on Ag(110). Borophene, presumably exhibiting highly interesting physical, chemical and electrical properties, does not exist as a bulk layered material, hence it cannot be studied via exfoliation. We investigated the growth of different Borophene phases via bulk segregation of...
The observation of h-BN single-domain growth on Pt(110) calls for an investigation of the mechanism eliminating rotational domains. We investigated the transformation of a chemisorbed layer of Borazine into h-BN islands upon heating via UV photoemission and STM. Evidence is presented for a non-classical, two-step nucleation process with a metastable 1D precursor phase. Non-classical nucleation...
We present a study of the electronic structure of a hexagonal boron nitride (h-BN) monolayer on Cu(111). Epitaxial growth is confirmed by X-Ray photoelectron spectroscopy (XPS) and diffraction (XPD), and ARPES reproduces the bands aligned to the vacuum level as seen on other metal substrates. A very small upwards shift of the Shockley surface state corroborates the small substrate interaction....
We have been studying various Dirac materials including topological insulators based on atom-surface scattering: A sensitive method to determine the electron-phonon coupling[[1][1],[2]][2], while lineshape broadening upon inelastic scattering allows to follow the nanoscale-nanosecond motion of water at Dirac materials[[3][3],[4]][4].
Furthermore, we will discuss the growth of hexagonal...
Layered silicate minerals, so-called phyllosilicates, were recently proposed as naturally occuring sources of 2D van der Waals (vdW) materials. Iron-rich members of the phyllosilicate family, were previously investigated for their magnetic properties and exhibit long-range magnetic ordering. However, probing the magnetism of 2D vdW magnets, especially in the monolayer limit, is challenging. To...
Transition metal dichalcogenides (TMDs) have revolutionised the field of electron layers, their spin orbit coupling (SOC) allowing an efficient control of the response in ,,spintronic'' applications. The band deformation induced by graphene's intrinsic SOC, though comparatively small, significantly influences the excitations. The resulting particle-hole bands cause the rapid Landau damping of...
We report on a low temperature (T=5K) measurement of striking singlets or multiplets of dissipation peaks above graphene nanodrums surface. The stress present in the structure leads to formation of few nanometre size graphene wrinkles and the observed dissipation peaks are attributed to tip-induced charge states transitions in quantum-dot-like entities. The dissipation peaks strongly depend on...
Nonlinear nanoscale optics, a success story driven by ultra-short laser pulses of immense power, commonly invokes surface plasmons, highly sensitive to details of the tightly confined electric field. Intrinsically 2D materials like graphene or so-called TMDs additionally offer gate-tunability of their nonlinear response.
We develop the spectral representation of $n$th order response functions...
Graphene nanoribbons (GNRs) synthesized with bottom-up technique allow electronic bandgap tuning, making GNRs an interesting candidate for room temperature switching applications as field-effect transistors (FET). We investigated various densities of aligned GNRs (by scanning tunneling microscopy) followed by transferring them to a target substrate. To investigate GNRs degree of alignment,...
Graphene nanoribbons (GNRs) with zigzag edge segments are able to host unpaired spins, which may exhibit topological end states via the interaction with superconductivity. Due to the need for a clean method to introduce superconductivity to GNRs, we propose to grow atomically precise GNRs via Ullmann coupling on the superconducting Ag/Nb(110) substrate. Through the investigation with scanning...
Cu2O, a natural p-type semiconductor with a direct bandgap of 2.17 eV, has a conventional conduction band position slightly above the water reduction potential and offers a low-cost photocathode for unassisted water splitting devices . Overlayers of n-type Ga2O3 can be employed to reduce the interfacial recombination effects due to the adequate conduction band alignment with Cu2O, leading to...
Polarons strongly influence the catalytic activity of transition metal oxides. The study of polaron formation and dynamics is fundamental to understanding the actual mechanisms and yields of catalytic reactions in these materials. A new method for the investigation of electron and hole polarons is demonstrated. Charge carriers are injected with the AFM tip into natural and Ti-doped...
Pt-based catalysts are among the most efficient catalysts for the hydrogen evolution, photocatalytic and CO-oxidation reactions. However, the high cost of Pt and its susceptibility to CO poisoning are major drawbacks. Downsizing catalyst to single atoms is an effective way to reach maximum efficiency. Nevertheless, stabilization of single atoms without compromising catalytic activity is a key...
Despite its high cost, rhodium is a widely applied catalyst. So-called single-atom catalysis offers an opportunity to reduce the amount of Rh required for traditional heterogeneous catalysis, and a path to heterogenize homogeneous reactions.
Using STM, nc-AFM and XPS we compare the stability of Rh adatoms on two different model supports: $\alpha$-Fe$_{2}$O$_{3}(1\overline{1}02)$ and...
Identification of the local environment of a single-atom catalyst on metal oxide surfaces is crucial for understanding the reactivity and the catalytic properties. On TiO$_{2}$(110), the stability and reactivity of adsorbed adatoms is further complicated by the presence of oxygen vacancies and associated polaron charge, as both can affect the electronic structure and local geometry. In this...
Indium oxide is widely used in semiconductor industry but it also displays promising performance in electro- and photo catalytic reactions. In all applications, surrounding water molecules may influence chemical processes at the atomic scale, and understanding the interaction of water with In$_2$O$_3$ is important. We focus on In$_2$O$_3$(111), which has an intrinsically large unit cell...
Polar surfaces offer intriguing physical and chemical properties applicable in electronics or catalysis. Cleaving the KTaO$_3$ perovskite along its polar (001) plane provides a well-defined, bulk-terminated surface with KO and TaO$_2$ terminations. As-cleaved surfaces exhibit a high concentration of in-gap states; these electrons predominantly reside at the TaO$_2$-terminated parts of the...
SrTiO3(001), a prototypical perovskite oxide surface, is a promising candidate for all-oxide electronics. Atomic configuration of a bare surface is commonly assumed to be bulk-terminated (1×1), which is far from warranted – noncontact atomic force microscopy (nc-AFM) reveals that only through ferroelectricity-assisted cleaving in ultrahigh vacuum [1], a (1×1) SrTiO3(001) closest to the...
We study the effect of antiferromagnetic buffer layers (insulating LaMnO$_3$ and metallic La$_{0.45}$Sr$_{0.55}$MnO$_3$) on the magnetism of epitaxial ultrathin La$_{0.8}$Sr$_{0.2}$MnO$_3$ (LSMO) films, grown by molecular beam epitaxy and studied by x-ray magnetic circular dichroism as a function of temperature and thickness. We find a non-monotonic variation of the moment in the LSMO films...
In this work, we report the extension of our setup by combining a sinusoidal modulation of the magnetic field with the synchronous detection of the reflectance difference spectroscopic MOKE (RD-MOKE) signal which allows recording hysteresis loops continuously as a function of coverage, time or temperature. We illustrate the capabilities of our setup for Ni thin films grown on a Cu(110)-(2x1)O...
Low-Energy Electron Diffraction (LEED) is a structure-sensitive technique commonly used to determine periodicity and order of a surface phase. Quantitative analysis of the modulation of beam intensities as a function of voltage (LEED $I(V)$) also gives access to the surface atom positions. This requires complex calculations and optimization of structural parameters. The Erlangen program...
Paper is increasingly used as a packaging material in particular also in food packaging. In the latter case it is necessary to get more insight into the adsorption and desorption behavior of aroma molecules on and in the paper network. Food packaging should keep the aroma molecules inside the food and protect the food from the environment. Paper is a porous material and this can be used as an...
In this work we present a new method of acquiring force volume data in a single scan using mixing products of different drive tones. The acquisition of such data is generally limited by the stand-by time of the system since it can easily take more than 10 hours. We address this limitation by combining our low temperature (LT) tuning fork atomic force microscopy (AFM) with two more advanced...
Photoemission Tomography has been widely used to study organic molecules adsorbed on metal substrates. Traditionally, the final state was taken as a plane wave in numerical simulations. This formalism has been very successful but discrepancies are also clearly observable, e.g. in the circular dichroism and the photon-energy dependence of the photocurrent.
In this work the...
Angular-Resolved Photoelectron Spectroscopy (ARPES) can benefit greatly from theoretical simulations as the observed momentum-space signature of the electronic structure is often quite involved. With the recent developments in ultra-fast laser physics, ARPES is now being used to also investigate time-resolved phenomena, e.g. for excited-states (pump-probe experiments).
In this talk we show...
In metals that form a bulk hydride upon hydrogen exposure, the surface properties depend on the state of the bulk. This impedes the use of surface science methods in general to post mortem analysis. We have developed a method to reversibly hydrogenate thin metal films in-situ under conditions suitable for electron spectroscopy measurements. As a proof of concept, we measure the temperature...
Carbon contamination is a notorious issue in surface science, especially in near-atmospheric conditions. Using APXPS we analysed the build-up of carbon on a rutile $TiO_2(110)$ single crystal when exposed to vapour and liquid water. Factors such as beam illumination, gas composition, and interaction with liquid water are shown to affect surface contamination. X-ray irradiation locally...
The IRAS system GRISU ( GRazing incident Infrared absorption Spectroscopy Unit) was developed for investigations in the research field of single atom catalysis. It combines the commercially available FTIR spectrometer Bruker Vertex 80v with an UHV chamber. The optics placed in HV and UHV was optimised for high throughput. In comparison to a system using two parabolic mirrors (f=250 mm)...
We investigate the optical dynamics of PbS nanocrystal layers on a gold thin film by microscopy-based ultrafast pump-probe spectroscopy. We probe with femtosecond resolution the transient absorption of nanocrystal films with specific thicknesses ranging from a few to 100 nm, as independently verified by atomic force microscopy. In stark contrast to individual nanocrystal and gold films, the...
Actinide-based metal-organic complexes and coordination architectures encompass intriguing properties and functionalities, but are still largely unexplored on surfaces. We report the first in situ synthesis of actinide tetrapyrrole complexes under ultra-high vacuum conditions, both on Ag(111) and h-BN/Cu(111) supports. Exposing a tetraphenylporphyrin (TPP) multilayer to an elemental beam of...
The structure and orientation of 5,14-dihydro-5,7,12,14-tetraazapentacene (DHTAP) layers deposited on Cu(110) was studied using reflectance difference spectroscopy (RDS), Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED). The evolution of the RDS signal allows to identify the sequential formation of up to three monolayers as well as a phase transition upon...
Molecular motors that convert external energy into controlled motion have seen great developments in the last decades. While many studies exist in solution, little is known how these functional molecules behave on surfaces. However, such solid support is advantageous as it offers a fixed point of reference and confinement in two dimensions, making the study of the directionality of their...
In this study the growth and energy level alignment of the rod-like, long chain acene heptacene on a Cu(110) surface is evaluated. Our results show that the orientation of the 7A molecules can be controlled by the preparation conditions. Our combined experimental and computational results show that for heptacene oriented along the Cu rows, the lowest unoccupied molecular orbital (LUMO) and the...
Self-metalation of 2H-tetraphenyl porphyrins (2H-TPP) on MgO thin films only occurs following charge transfer from the underlying surface. However, it is not clear whether the charging of the molecules is directly responsible for the metalation, or rather the charge-induced repositioning of the macrocycles.
Comparing the behavior of 2H-TPP with that of porphyne (2H-P) by angular resolved...
2D metal-organic networks show great promise for applications in catalysis, gas sensing or electronics. Ascertaining fundamental intrinsic properties of such systems requires their synthesis on weakly-interacting substrates. Here, we show Fe-TCNQ networks self-assembled on graphene/Ir(111), studied experimentally by Low Energy Electron Microscopy (LEEM), Scanning Tunneling Microscopy (STM) and...
Indium oxide (In$_2$O$_3$) is a ubiquitous anode material in OLEDs and photovoltaics due to its transmissivity to visible light and metal-like conductivity (when doped with Sn). When In$_2$O$_3$ is paired with organic materials, a thin organic buffer layer is often introduced to improve the charge injection from In$_2$O$_3$ to the organic active layers. We probe the adsorption behaviour and...
With the increasing average global temperature more and more households need a way to cool down. This study explores biomimetic passive cooling utilizing structured surfaces. The focus is put on structures that lower a body's average temperature without using electricity or replenishable resources. Biomimetics helps to find non-polluting ways to achieve such cooling structures.
At first,...
Nanoparticles comprising three different materials in a core@shell@shell configuration are synthesized in cold helium droplets by sequential doping. Rhodamine B molecules form complexes in helium droplets that give rise to a strong fluorescence upon laser excitation, enabling an in situ investigation of the synthesized structures. In the presence of a Au core, fluorescence from the...
High performance synthetic fibers are increasingly applied in fields where they might be exposed to elevated temperatures. Above a critical temperature, Tc, however, they lose their mechanical properties resulting in unexpected failure under load. To enable a passive sensor monitoring the temperature history, we introduce a method to functionalize polymer fibers with ultrathin thermochromic...
Lithium fluoride (LiF) is an interesting material for spintronic applications[1] and a potential candidate for decoupling single-molecule magnets from metallic substrates.[2] We have investigated the growth and morphology of LiF deposited onto the Ag(100) surface in the monolayer regime under different conditions. Scanning tunneling microscopy, low energy electron diffraction and polarized...
Recently, CdO has been proposed as a very promising material for IR plasmonics due to its high plasma frequency and low damping. However, low propagation lengths and broad minima are common limitations with CdO surface plasmons.
To overcome these limitations, we propose the use of thin high quality crystalline CdZnO layers grown on sapphire substrate. Hybrid surface modes arise from the...
A current-driven source of long-range
surface plasmons (LRSPs) on a duplex metal nanolayer
is reported. Electrical excitation of LRSPs was experimentally
observed in a planar structure, where an
organic light-emitting film was sandwiched between two
metal nanolayers that served as electrodes. To achieve
the LRSP propagation in these metal nanolayers at the
interface with air, the...
Electronic structure of heterointerfaces plays a pivotal role in their device functionality. Recently, ultrathin films of superconducting NbN have been integrated by MBE with the semiconducting GaN. We use soft X-ray angle-resolved photoelectron spectroscopy (ARPES) to directly measure the momentum-resolved electronic structure and band offsets at this Schottky heterointerface as well as the...
Successful implementation of cutting-edge flexible devices in various branches of modern life requires the synthesis of efficient materials with advanced properties. We address this challenge by mixing Al, Cu, and Ga via vacuum co-deposition applying the combinatorial libraries approach. Systematic screening of maximum current density and voltage, film thickness, and composition reveals the...
