Under Ultrahigh Vacuum Research Articles

Norbornadiene photoswitches anchored to well-defined oxide surfaces: From ultrahigh vacuum into the liquid and the electrochemical environment.

Employing molecular photoswitches, we can combine solar energy conversion, storage, and release in an extremely simple single molecule system. In order to release the stored energy as electricity, the photoswitch has to interact with a semiconducting electrode surface. In this work, we explore a solar-energy-storing model system, consisting of a molecular photoswitch anchored to an atomically defined oxide surface in a liquid electrolyte and under potential control. Previously, this model system has been proven to be operational under ultrahigh vacuum (UHV) conditions. We used the tailor-made norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) and characterized its photochemical and electrochemical properties in an organic electrolyte. Next, we assembled a monolayer of CNBD on a well-ordered Co3O4(111) surface by physical vapor deposition in UHV. This model interface was then transferred into the liquid electrolyte and investigated by photoelectrochemical infrared reflection absorption spectroscopy experiments. We demonstrate that the anchored monolayer of CNBD can be converted photochemically to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC) under potential control. However, the reconversion potential of anchored CQC overlaps with the oxidation and decomposition potential of CNBD, which limits the electrochemically triggered reconversion.

On-Surface Synthesis of Nonmetal Porphyrins.

We report the on-surface synthesis of a nonmetal porphyrin, namely, silicon tetraphenylporphyrin (Si-TPP), by the deposition of atomic silicon onto a free-base TPP layer on a Ag(100) surface under ultrahigh vacuum (UHV) conditions. Scanning tunneling microscopy provides insights into the self-assembly of the TPP molecules before and after Si insertion. Silicon coordinates with all four nitrogen atoms of the TPP macrocycle and interacts with a silver atom of the substrate as confirmed by scanning tunneling spectroscopy, X-ray photoelectron spectroscopy, and complementary density functional theory calculations. The Si-TPP complex presents a saddle-shaped conformation that is stable under STM manipulation. Our study shows how protocols established for the on-surface metalation of tetrapyrroles can be adopted to achieve nonmetal porphyrins. Complementary experiments yielding Si-TPP and Ge-TPP on Ag(111) highlight the applicability to different main group elements and supports. The success of our nonmetal porphyrin synthesis procedure is further corroborated by a temperature-programmed desorption experiment, revealing the desorption of Ge-TPP. This extension of interfacial complex formation beyond metal elements opens promising prospects for new tetrapyrrole architectures with distinct properties and functionalities.

Electrifying Oxide Model Catalysis: Complex Electrodes Based on Atomically-Defined Oxide Films

By studying complex model catalysts based on well-defined oxide surfaces, fundamental insights have been obtained into the surface chemistry of many heterogeneously catalyzed processes. In this perspective, we summarize a series of studies, in which we have transferred this model catalysis approach to the field of electrocatalysis. Our model electrocatalysts consisted of Pt nanoparticles (NPs) grown on atomically-defined oxide films. Specifically, we used well-ordered Co3O4(111) thin films on an Ir(100) support. The Pt NPs were prepared by physical vapor deposition (PVD) and the particle size was varied from a few nanometers to the sub-nanometer size range. We prepared all model catalysts under ultra-high vacuum (UHV) conditions using a dedicated preparation system. This setup enables us to transfer the model catalysts from UHV into the electrochemical environment to apply various in-situ techniques without exposure to air. We investigated the stability window of pristine Co3O4(111) and Pt/Co3O4(111) using online inductively coupled plasma mass spectrometry (ICPMS), electrochemical infrared reflection absorption spectroscopy (EC-IRRAS), scanning tunneling microscopy (STM), ex-situ emersion X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). Within the stability window (pH 10, 0.3–1.1 VRHE) the surface structure of the model electrocatalysts is preserved. We analyzed identical samples both in UHV and in the electrochemical environment. Specifically, we applied synchrotron radiation photoelectron spectroscopy (SR-PES) and ex-situ emersion XPS to analyze the electronic structure and we used infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), EC-IRRAS, and cyclic voltammetry (CV) to study CO adsorption and oxidation. The model electrocatalysts show pronounced particle size effects and metal support interactions are shown to play a key role in their catalytic reactivity. Of particular importance is an interfacial Pt oxide, which is stabilized by the oxide support and exists at electrode potentials as low as 0.5 VRHE. Moreover, spillover effects enable new reaction mechanisms, which involve oxygen from the oxide support. This review demonstrates the potential of the model electrocatalysis approach to provide fundamental insights into complex oxide-based electrocatalysis.

Selective Modification and Probing of the Electrocatalytic Activity of Step Sites.

Employing Pt(111) supported 2D Pt-core Au-shell model catalysts, we demonstrate that 2D core-shell surfaces prepared under ultrahigh vacuum (UHV) conditions constitute excellent model systems to determine the activity of step sites in electrocatalysis, especially because UHV-scanning tunneling microscopy (STM) enables control of the quality of narrow step modifications with high accuracy on such systems. As verified with STM, cyclic voltammetry (CV), and temperature-programmed desorption (TPD) measurements, this approach allows us (i) to increase the step density by homoepitaxial growth of monolayer high islands on the respective single crystal and (ii) to modify the step sites for adsorption of reactants by selective deposition of a guest metal. Herein, STM imaging in combination with electrochemical characterization provides a direct control to ascertain a selective modification of the entire steps. Comparing the electrocatalytic activity of 2D core-shell systems with and without the shell enables us to identify the activity of step sites for electrocatalytic reactions, as demonstrated for the bulk CO electro-oxidation.

Near ambient pressure adsorption of nickel carbonyl contaminated CO on Cu(111) surface

Formation of volatile nickel carbonyls with CO in catalytic reaction is one of the mechanisms of catalyst deactivation. CO is one of the most popular probe molecules to study the surface properties in model catalysis. Under ultra-high vacuum (UHV) conditions, the problem of nickel carbonyl impurity almost does not exist in the case that a high purity of CO is used directly. While in the near ambient pressure (NAP) range, nickel carbonyl is easily found on the surface by passing through the Ni containing tubes. Here, the NAP techniques such as NAP-X-ray photoelectron spectroscopy and NAP-scanning tunneling microscopy are used to study the adsorption of nickel carbonyl contaminated CO gas on Cu(111) surface in UHV and NAP conditions. By controlling the pressure of contaminated CO, the Ni-Cu bimetallic catalyst can form on Cu(111) surface. Furthermore, we investigate the process of CO adsorption and dissociation on the formed Ni-Cu bi-metal surface, and several high-pressure phases of CO structures are reported. This work contributes to understanding the interaction of nickel carbonyl with Cu(111) at room temperature, and reminds the consideration of CO molecules contaminated by nickel carbonyl especially in the NAP range study.

Recent Progress with In Situ Characterization of Interfacial Structures under a Solid-Gas Atmosphere by HP-STM and AP-XPS.

Surface science is an interdisciplinary field involving various subjects such as physics, chemistry, materials, biology and so on, and it plays an increasingly momentous role in both fundamental research and industrial applications. Despite the encouraging progress in characterizing surface/interface nanostructures with atomic and orbital precision under ultra-high-vacuum (UHV) conditions, investigating in situ reactions/processes occurring at the surface/interface under operando conditions becomes a crucial challenge in the field of surface catalysis and surface electrochemistry. Promoted by such pressing demands, high-pressure scanning tunneling microscopy (HP-STM) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS), for example, have been designed to conduct measurements under operando conditions on the basis of conventional scanning tunneling microscopy (STM) and photoemission spectroscopy, which are proving to become powerful techniques to study various heterogeneous catalytic reactions on the surface. This report reviews the development of HP-STM and AP-XPS facilities and the application of HP-STM and AP-XPS on fine investigations of heterogeneous catalytic reactions via evolutions of both surface morphology and electronic structures, including dehydrogenation, CO oxidation on metal-based substrates, and so on. In the end, a perspective is also given regarding the combination of in situ X-ray photoelectron spectroscopy (XPS) and STM towards the identification of the structure–performance relationship.

Research On Secondary Electron Emission Characteristics of Diamond-Like Carbon Thin Films

In modern particle accelerators, the build-up of electron cloud is a main limiting factor for the achievement of high-quality beam. Among the techniques to mitigate it, coating the internal walls of the beam pipes with a thin film which has a low secondary electron yield (SEY) is considered to be one of the most effective means. From several earlier studies, it was found that diamond-like carbon (DLC) thin films are potential coatings. This paper is mainly about the research on secondary electron emission characteristics of DLC thin films. The secondary electron emission (SEE) tests were done at temperature of 298 K and vacuum pressure of 2 × 10–9 Torr. Here, we obtained the characteristics of the SEE from DLC film coatings with different thickness under ultrahigh-vacuum (UHV) conditions. The maximum secondary electron yield (SEY), δmax, of the DLC thin films under different primary electron doses were also obtained, respectively.

Surface chemistry and electrochemistry of an ionic liquid and lithium on Li4Ti5O12(111)-A model study of the anode|electrolyte interface.

Aiming at a detailed molecular understanding of the initial stage of the solid|electrolyte interphase (SEI) formation in Li-ion batteries, we have investigated the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) (solvent/electrolyte) and Li (Li+ ion shuttle) on well-defined Li-poor Li4Ti5O12(111) and Li-rich Li4.3Ti5O12(111) surfaces/electrodes in a combined surface science and electrochemical model study. X-ray photoelectron spectroscopy (XPS) measurements reveal that postdeposition of Li0 under ultrahigh vacuum (UHV) conditions on a Li-poor Li4Ti5O12(111) surface precovered with a molecularly adsorbed [BMP][TFSI] adlayer leads to little IL decomposition at 80 and 300 K. We assume that most of the Li diffuses through the IL adlayer and rapidly inserts into the Li4Ti5O12(111) bulk. More pronounced IL decomposition was obtained upon IL deposition on a Li-rich Li4.3Ti5O12 phase at 80 K and subsequent heating to 300 K. Cyclic voltammograms (CVs) recorded on the Li4Ti5O12(111) electrodes in Li-TFSI/[BMP][TFSI] indicate an almost reversible Li (de-)insertion, with a slight decay of the amount of (de-)inserted Li with increasing cycle number. XPS measurements performed on the electrode after potential cycling show low intensity signals of IL decomposition products, in addition to dominant signals from residual IL electrolyte, which are related to reaction of the adsorbed IL with Li inserted into/extracted from Li4Ti5O12 during the CV. The results indicate a close similarity between IL decomposition products formed under UHV and under electrochemical conditions, underlining the validity of this experimental approach and the potential of such kind of model studies for obtaining detailed understanding of the SEI formation.

A first-principles DFT dispersion-corrected C60/Au(111) Raman study

We present an extensive density functional theory (DFT) study on the interaction of C60 molecules with Au(111), involving both periodic metal surfaces and clusters. The aim of this work is 2-fold: First, to quantify the adsorption energies per molecule (Eads) of the C60/Au(111) system from periodic calculations using a dispersion-corrected functional (DFT-D3). Secondly, to determine the effect of the metal substrate on the adsorbed C60 molecule via calculated non-resonant Raman vibrational frequencies. Using a commensurate (23×23) R30° phase – as proposed by experimental work as a stable C60/Au(111) monolayer – our DFT-D3 periodic calculations show a slight C60 molecule preference to adsorb on its hexagonal facet, with a calculated adsorption energy value of 2.40 eV. Our DFT-D3 calculations further reveal an energetic gain when the C60 molecule is adsorbed on an HCP surface (2.40 eV) site compared to an FCC one (2.31 eV). Using a variety of finite-size, slab-type Au clusters (e.g.Au24, Au32 and Au50), our DFT-D3 calculated Raman intensity profile shows a strong interaction between the C60 fullerene molecule and the gold substrate. This influences new vibrational modes over a wide range of frequencies, compared to those active modes for the isolated C60 gas-phase single molecule, and an overall shift of frequencies and symmetries. The DFT-D3 calculated Raman spectra of the C60 molecule supported on Au clusters are in good agreement when confronted to those recently obtained experimentally under ultra-high vacuum (UHV) conditions using tip-enhanced Raman spectroscopy (TERS).

Oxygen Adsorption on Graphene-Encapsulated Palladium Nanoparticles Imaged by Kelvin Probe Force Microscopy

Graphene-encapsulated metal nanoparticles (G@NPs) offer a possibility to observe confined reactions in the nanocontainer formed by the NP's facets and graphene. However, direct experimental detection of adsorbed atomic and molecular species under the graphene cover is still challenging, and the mechanisms of intercalation and adsorption are not well understood. Here, we show that Kelvin probe force microscopy can largely contribute to the understanding of adsorption and desorption at the single NP level, which we exemplify by comparing oxygen adsorption experiments obtained at as-prepared PdNPs and G@PdNPs, both supported on highly oriented pyrolytic graphite and studied under ultrahigh vacuum (UHV) conditions. We show that oxygen adsorption at room temperature occurs at a much higher partial oxygen pressure on G@PdNPs compared to as-prepared PdNPs. Similarly, the removal of oxygen via a reaction with the residual gas of the UHV is slower on the G@PdNPs compared to as-prepared PdNPs. The differences can be explained by a limited facility for reactant and product molecules to enter and desorb from the nanocontainer via the defects of the graphene. Experimental observations are supported by assisting density functional theory calculations.

Oxygen Reduction Reaction Activity for Surface-Magnetic-Anisotropy-Controlled Pt-Co/Pt(111) Bimetallic Surface

Introduction Developments in effective oxygen reduction reaction (ORR) catalysts for the cathode of Polymer Electrolyte Fuel Cell is important to achieve “hydrogen society”. Because oxygen molecules (O2), the reactant of ORR, is paramagnetic having a triplet spin state, ORR that proceeds on the cathode catalyst surfaces might modify by magnetism of the catalyst materials. Indeed, ORR activity was improved by the influence of an external magnetic field applied to the catalyst [1]. The alloys of 3d transition magnetic elements M (M=Co, Fe, Ni) are known to exhibit ferromagnetism. Etz et al. reported that the direction of magnetization easy axis of the epitaxial Co/Pt(111) surface depends on the Co deposition thickness [2]. However, to our best knowledge, relation between the magnetic anisotropy of the cathode electrode surface and ORR activity is yet to be clarified. Therefore, to investigate influence of spontaneous magnetization of the material’s surface on the ORR should cast some light on the ORR activity enhancement mechanisms for such the Pt-M alloy surfaces with the magnetic. In this study, we fabricate Pt-Co/Pt(111) ferromagnetic bimetallic surfaces under ultra-high vacuum (UHV) through 1 monolayer(ML)-thick epitaxial growth of Co on the clean Pt(111) substrate at various temperatures and discuss the influence of magnetic anisotropy of the Pt-Co/Pt(111) bimetallic surface on corresponding ORR activity. Experimental The sample fabrication process of Pt-Co/Pt(111) bimetallic surface is as follows. After surface cleaning of the Pt(111) single crystal substrate through Ar+ sputtering and annealing at 1173 K in ultra-high vacuum (UHV: 10-8 Pa), 1ML-thick-Co was deposited on the clean Pt(111) by the molecular-beam epitaxy (MBE) method at elevated substrate temperature (723 K to 973 K) to generate the Pt(111)-skin through the surface segregation of substrate Pt atoms [3]. The Pt-Co/Pt(111) bimetallic surfaces are described hereafter by the Co deposition temperatures, for example Pt-Co773K/Pt(111), etc. The resulting surface structures were verified by the scanning tunneling microscopy (STM) and low-energy ion scattering spectroscopy (LE-ISS) in UHV. Then, polar magneto optical Kerr effect (polar MOKE), and in-plane XRD measurements are conducted in air to evaluate the surface magnetic anisotropies and compressive strains of the generated Pt(111)-skin surfaces. The MBE-prepared samples were transferred to electrochemical evaluation systems set in a N2–purged glove box without being exposed to air. Then, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were conducted in N2–purged and O2–saturated 0.1 M HClO4, respectively, in the glove box. ORR activities of the samples were evaluated based on j k values at 0.9 V vs. RHE by using Koutecky–Levich equation. Results and Discussion An LE-ISS spectrum of the Pt-Co773K/Pt(111) is shown in Fig. 1(a). Only the signal due to of the Pt atoms at the topmost surface is appeared on the spectrum, indicating that the Pt(111)-skin is generated for the Pt-Co773K/Pt(111) surface and the deposited Co should be located below the second atomic layer of the Pt-Co773K/Pt(111) as a result of surface segregation of the substrate Pt atoms at deposition temperature (of 773 K). Fig. 1(b) shows an STM image of the corresponding Pt-Co773K/Pt(111) surface. As shown in Fig. 1(b), the sample surface is atomically flat. One might notice that Moiré-pattern-like surface corrugations on the image (highlighted by white line), probably stemming from the lattice mismatch between Pt(111)-skin and underlying Co (Pt-Co alloy) [4]. The result also supports the Pt(111)-skin. Fig. 1(c) shows the polar MOKE curves of the Pt-Co773K/Pt(111) (red) and Pt-Co823K/Pt(111) (blue). Both the samples show the hysteresis loops of Kerr rotation angle vs. magnetic field H, indicating that perpendicular magnetic anisotropy (PMA) of the sample surfaces. Relation between the ORR activities and in-plane-XRD-estimated compressive strains of the Pt-Co773K/Pt(111) (red) and Pt-Co873K/Pt(111) (blue) surfaces are shown in Fig. 1(d). For comparison, the DFT-calculated ORR activities of the strained Pt(111) surfaces (green) are plotted in the figure [5]. The estimated ORR activities for both the Pt-Co773K/Pt(111) and Pt-Co873K/Pt(111) are 20 to 30 % higher than the DFT-calculated ones. The results suggest that PMA might affect ORR activity enhancements for the prepared PMA surfaces of the Pt-Co/Pt(111). At the meeting, effects of magnetic anisotropies (perpendicular and in-plane) are discussed more in detail.

In Situ Ambient Pressure X-Ray Photoelectron Spectroscopy Studies of Potassium-Oxygen Redox Reactions

Rechargeable potassium−oxygen (K–O2) batteries have received much attention owing to their higher theoretical energy density (935 Wh/kg, K + O2 = KO2, E 0 = 2.48 V) compared to conventional lithium−ion batteries and their superior round-trip efficiency.1-3 In order to realize commercialization of K–O2 batteries, identifying fundamental barriers and instability that hinder the development of K–O2 batteries is required. In situ characterization methods provide valuable insight into the discharge/charge mechanisms of metal-oxygen batteries, especially the K-O2 discharge products are highly sensitive to the uncontrolled contaminations. In this work, we employed in-situ ambient pressure X-ray photoelectron spectroscopy (APXPS)4, 5 and examine in situ the chemistry of K-O2 reaction products under ultra high vacuum (UHV) and oxygen condition using APXPS. We will discuss the chemical changes of the K-O2 system during discharge and charge conditions. We show that combining in situ APXPS and electrochemical characterization techniques, the formation and disappearance of K-O2 reaction products can be directly visualized. Acknowledgement: This work was supported by a grant from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region, China, T23-601/17-R. Reference: X. Ren and Y. Wu, J. Am. Chem. Soc., 135, 2923 (2013).G. Cong, W. Wang, N.-C. Lai, Z. Liang and Y.-C. Lu, Nat. Mater., 18, 390 (2019).W. Wang, N. C. Lai, Z. Liang, Y. Wang and Y. C. Lu, Angew. Chem., Int. Ed., 57, 5042 (2018).Y.-C. Lu, E. J. Crumlin, G. M. Veith, J. R. Harding, E. Mutoro, L. Baggetto, N. J. Dudney, Z. Liu and Y. Shao-Horn, Sci. Rep., 2, 715 (2012).Y.-C. Lu, E. J. Crumlin, T. J. Carney, L. Baggetto, G. M. Veith, N. J. Dudney, Z. Liu and Y. Shao-Horn, J. Phys. Chem. C, 117, 25948 (2013).

Adsorption of Formic Acid on CH3NH3PbI3 Lead–Halide Organic–Inorganic Perovskites

The adsorption kinetics of formic acid (HCOOH) was studied under ultrahigh vacuum (UHV) on lead–halide organic–inorganic perovskites (CH3NH3PbI3) supported on indium-doped tin oxide (ITO). The thin film samples were made at ambient pressure using the common liquid-phase synthesis, but samples were degassed under UHV. We studied real-world samples which do have some carbon impurities. Adsorption and decomposition kinetics were characterized with thermal desorption spectroscopy. Furthermore, samples’ composition (including impurities) were inspected under UHV with Auger electron spectroscopy and X-ray photoelectron spectroscopy. Accordingly, formic acid adsorbs molecularly at 100 K but decomposed above 600 K. On ITO or thermally decomposed perovskite samples, formic acid adsorbs only molecularly. The gas-phase species detected are consistent with a decomposition mechanism via the formation of formate (HCOO) and formyl (HCO) intermediates. The perovskite itself also decomposed thermally, but it is stabilized...

Synthesis of Nanographenes, Starphenes, and Sterically Congested Polyarenes by Aryne Cyclotrimerization.

In recent years, synthetic transformations based on aryne chemistry have become particularly popular, mostly due to the spread of methods to generate these highly reactive intermediates in a controlled manner under mild reaction conditions. In fact, aryne cycloadditions such as the Diels-Alder reaction are nowadays widely used for the efficient preparation of polycyclic aromatic compounds. In 1998, our group discovered that arynes can undergo transition metal-catalyzed reactions, a finding that opened new perspectives in aryne chemistry. In particular, Pd-catalyzed [2 + 2 + 2] cycloaddition of arynes allowed the straightforward synthesis of triphenylene derivatives such as starphenes or cloverphenes. We found that this reaction is compatible with different substituents and sterically demanding arynes as starting materials. This transformation is especially useful to increase the molecular complexity in one single step, transforming molecules formed by n cycles in structures with 3n + 1 cycles. In fact, we took advantage of this protocol to prepare a large variety of sterically congested polycyclic aromatic hydrocarbons such as helicenes or twisted polyarenes. Soon after the discovery of the reaction, the co-cyclotrimerization of arynes with other reaction partners, such as electron deficient alkynes, significantly expanded the potential of this transformation. Also the use of catalysts based on alternative metals besides Pd (e.g., Ni, Cu, Au) or the use of other strained intermediates such as cycloalkynes or cycloallenes added value to this reaction. In addition, we realized that the Pd-catalyzed aryne cyclotrimerization reaction is particularly useful for the bottom-up preparation of well-defined nanographenes by chemical methods. Although the extreme insolubility of these planar nanographenes hampered their manipulation and characterization by conventional methods, recent advances in single molecule on-surface characterization by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) with functionalized tips under ultrahigh vacuum (UHV) conditions, permitted the impressive visualization of these nanographenes with submolecular resolution, together with the examination of the corresponding molecular orbital densities. Moreover, arynes have been shown to possess a rich on-surface chemistry. In particular, arynes have been generated and studied on-surface, showing that the reactivity is preserved even at cryogenic temperatures. On-surface aryne cyclotrimerization was also demonstrated to obtain large starphene derivatives. Therefore, it is expected that the combination of aryne cycloadditions and on-surface synthesis will provide notable findings in the near future, including the "à la carte" preparation of graphene materials or the synthesis of elusive molecules with unique properties.

Magnetism and structure in nanocomposite Fe nanoparticle/Al matrix films

We describe magnetometry measurements performed on nanocomposite films formed of Fe nanoparticles embedded in Al matrix. The samples were prepared using a flexible co-deposition technique under Ultra-High Vacuum (UHV) environments. Fe nanoparticles, made by using a gas aggregation source, were co-deposited with an atomic Al beam, made by an MBE source. The Volume Filling Fraction (VFF) of nanoparticles was varied controllably between 4% and 45%, while the mean diameter of Fe nanoparticles produced by the source was ∼ 2 nm. Fe K edge extended x-ray absorption fine structure (EXAFS) experiments show that there is a high degree of alloying between Fe nanoparticles and Al atoms. Magnetism in the embedded Fe nanoparticle samples was investigated using a SQUID magnetometer. Atomic moment of Fe in the Fe/Al nanocomposite films increases slightly when the proportion of Fe nanoparticles is increased, but is still significantly lower than the bulk Fe value. In a more detailed analysis, the magnetisation curves were fitted using the Random Anisotropy Model (RAM) which allowed the exchange field and random anisotropy field to be evaluated. The findings and issues raised by this approach are discussed.

Argon Embedded by Ion Bombardment: Relevance of Hidden Dopants in Rutile TiO2

To obtain a mechanistic understanding of occurring processes on oxide surfaces at the atomic level, systematic studies under ultra-high vacuum (UHV) conditions on single crystalline surfaces are co...

On-Surface Activation of Trimethylsilyl-Terminated Alkynes on Coinage Metal Surfaces.

The controlled attachment of protecting groups combined with the ability to selectively abstract them is central to organic synthesis. The trimethylsilyl (TMS) functional group is a popular protecting group in solution. However, insights on its activation behavior under ultra-high vacuum (UHV) and surface-confined conditions are scarce. Here we investigate a series of TMS-protected alkyne precursors via scanning tunneling microscopy (STM) regarding their compatibility with organic molecular beam epitaxy (OMBE) and their potential deprotection on various coinage metal surfaces. After in-situ evaporation on the substrates held in UHV at room temperature, we find that all molecules arrived and adsorbed as intact units forming ordered supramolecular aggregates stabilized by non-covalent interactions. Thus, TMS-functionalized alkyne precursors with weights up to 1100 atomic mass units are stable against OMBE evaporation in UHV. Furthermore, the TMS activation through thermal annealing is investigated with STM and X-ray photoelectron spectroscopy (XPS). We observe that deprotection starts to occur between 400 K and 500 K on the copper and gold surfaces, respectively. In contrast, on silver surfaces, the TMS-alkyne bond remains stable up to temperatures where molecular desorption sets in (≈600 K). Hence, TMS functional groups can be utilized as leaving groups on copper and gold surfaces while they serve as protecting groups on silver surfaces.

Electronic properties of p-GaN co-doped with Mn by thermal process: Surface studies

In this paper we report the method of the GaN:Mg electron structure modification by Mn co-doping. Manganese layers were deposited on p-GaN(0001) then subjected for thermal treatment processes in situ under ultrahigh vacuum (UHV) condition resulting in a dissolution of Mn in the subsurface layers. Surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low energy diffraction (LEED) were employed in this experiment. Mn atoms introduced into the p-GaN show strong doping effect and no band bending at the surface. The valence band maximum is strongly shifted and located almost at the Fermi level (EF). The Ga-3d and N-1 s peaks are shifted by 0.4 eV towards the EF relative to the p-GaN substrate. After Mn incorporation into the p-GaN the LEED patterns revealed (1 × 1)–(0001) structure with evidence of the surface faceting.

Low friction of metallic multilayers by formation of a shear-induced alloy

During sliding of metallic surfaces, the near surfaces undergo significant changes in terms of topography, composition and microstructure. Since friction and wear behavior of the materials are strongly influenced by sub-surface deformations, it is fundamental to investigate these effects. Therefore, the present study aims towards a better understanding of the behavior of friction depending on well-defined initial microstructures. By performing sliding experiments on Au-Ni multilayer samples under ultrahigh vacuum (UHV) conditions, we observe that the individual layer thickness of multilayer systems has a strong influence on friction behavior due to the transition in the dominant deformation mechanism near the surface. The experiments reported here provide a new route for lowering the friction force of metallic material systems in dry contact by providing more stable microstructures and alloy formation. Through ultrafine grains present in the alloy formed by mechanical mixing the number of grain boundaries strongly increases and hence, grain boundary-mediated deformation results in the low friction coefficient.

Growth of Ultrathin Single-Crystalline IrO2(110) Films on a TiO2(110) Single Crystal.

The growth of a flat, covering, and single-crystalline IrO2(110) film with controlled film thickness on a single-crystalline TiO2(110) substrate is reported. The preparation starts with a deposition of metallic Ir at room temperature followed by a post-oxidation step performed in an oxygen atmosphere of 10-4 mbar at 700 K. On this surface, additional Ir can be deposited at 700 K in an oxygen atmosphere of 10-6 mbar to produce a IrO2(110) layer with variable thicknesses. To improve the crystallinity of the resulting IrO2(110) layer, the final film was post-oxidized in 10-4 mbar of O2 at 700 K for 5 min. The surface-sensitive techniques of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity, and electronic structure of the prepared ultrathin IrO2(110) films and how these films decompose upon annealing under ultrahigh vacuum (UHV) conditions. STM provides evidence that the IrO2(110) films start already to reduce at 465 K under UHV conditions. Upon annealing to 605 K under UHV the reduction of IrO2 intensifies (XPS), but the oxide film can readily be restored by re-oxidation in 10-4 mbar of O2 at 700 K. Thermal decomposition at 725 K leads, however, to severe reduction of the IrO2(110) layer (XPS, STM) that cannot be restored by a subsequent re-oxidation step. The utility of the IrO2(110)-TiO2(110) system as model electrodes is exemplified with the electrochemical oxygen evolution reaction in an acidic environment.