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A UHV-compatible MOKE magnetometer for in situ studies operating in tandem with the PEEM at the Nanospectroscopy beamline of the Elettra synchrotron., We report on a custom-built UHV-compatible Magneto-Optical Kerr Effect (MOKE) magnetometer for applications in surface and materials sciences, operating in tandem with the PhotoEmission Electron Microscope (PEEM) endstation at the Nanospectroscopy beamline of the Elettra synchrotron. The magnetometer features a liquid-nitrogen-cooled electromagnet that is fully compatible with UHV operation and produces magnetic fields up to about 140 mT at the sample. Longitudinal and polar MOKE measurement geometries are realized. The magneto-optical detection is based on polarization analysis using a photoelastic modulator. The sample manipulation system is fully compatible with that of the PEEM, making it possible to exchange samples with the beamline endstation, where complementary X-ray imaging and spectroscopy techniques are available. The magnetometer performance is illustrated by experiments on cobalt ultra-thin films, demonstrating close to monolayer sensitivity. The advantages of combining in situ growth, X-ray Magnetic Circular Dichroism imaging (XMCD-PEEM) and MOKE magnetometry into a versatile multitechnique facility are highlighted.

After the discovery of graphene, many other 2D materials have been predicted theoretically and successfully prepared. In this context, single-sheet black phosphorus - phosphorene - is emerging as a viable contender in the field of (2D) semiconductors. Phosphorene offers high carrier mobility and an anisotropic structure that gives rise to a modulation of physical and chemical properties. This opens the way to many novel and fascinating applications related to field-effect transistors and optoelectronic devices. In previous studies, a single layer of blue phosphorene intermixed with Au atoms was grown using purified black phosphorus as a precursor. Starting from the observation that phosphorus vapor mainly consists of P clusters, in this work we aimed at obtaining blue phosphorus using much less expensive purified red phosphorus as an evaporant. By means of microscopy, spectroscopy and diffraction experiments, we show that black or red phosphorus deposition on Au(111) substrates yields the same blue phosphorus film.

Advanced electronic materials 2100936 (2022). doi:10.1002/aelm.202100936, Published by Wiley-VCH Verlag GmbH & Co. KG, Weinheim

Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene. The linear energy dispersion and nontrivial Berry phase play a pivotal role in the electronic, optical, mechanical, and chemical properties of 2D Dirac materials. The known 2D Dirac materials are gapless only within certain approximations, for example, in the absence of spin-orbit coupling (SOC). Here, we report a route to establishing robust Dirac cones in 2D materials with nonsymmorphic crystal lattice. The nonsymmorphic symmetry enforces Dirac-like band dispersions around certain high-symmetry momenta in the presence of SOC. Through μ-ARPES measurements, we observe Dirac-like band dispersions in α-bismuthene. The nonsymmorphic lattice symmetry is confirmed by μ-low-energy electron diffraction and scanning tunneling microscopy. Our first-principles simulations and theoretical topological analysis demonstrate the correspondence between nonsymmorphic symmetry and Dirac states. This mechanism can be straightforwardly generalized to other nonsymmorphic materials. The results enlighten the search of symmetry-enforced Dirac fermions in the vast uncharted world of nonsymmorphic 2D materials.

The crystalline quality of the graphene lattice is a crucial parameter that not only rules the electronic and transport properties of the carbon film, but also its interaction with the substrate. Elucidating the effect of different growth pathways on the resulting graphene-substrate structural configurations and the microscopic mechanisms for their formation is, therefore, a goal of utmost importance. By using electron spectro-microscopy with high chemical and structural sensitivity, we image the structural transformation that graphene on cobalt undergoes at temperatures above 500°C, from a rotationally-incoherent, defective layer to a high quality epitaxial one. We find that the transformation takes place via the growth and propagation of mesoscopic carbidic islands. We identify the underlying mechanism for the formation of epitaxial graphene to involve the dissolution and recondensation of carbon within these regions. The activation energy of the process is estimated to be 1.84 ± 0.11 eV, indicating that the carbon detachment is the rate-limiting step. With the aid of theoretical calculations, we show that the martensitic phase transition occurring in cobalt above 420°C does not affect the graphene transformation. These findings help to establish the optimal parameters to grow high-quality graphene epilayers on Co, opening viable routes towards usage in artificially fabricated magnetic heterostructures.

We report on a simple and cost-effective device for high-speed gating in photoemission electron microscopy (PEEM) with pulsed photon sources. This device is based on miniaturized electrode plates, which deflect the photoelectron beam inside the imaging column of the microscope so that it is either accepted or blocked in its path towards the detector. The gating device is optimized for installation on the Elmitec SPELEEM III microscope. Due to the compact design, it can be driven by voltage pulses of low amplitude (few volts), delivered by commercially available signal generators. Most notably, our device allows for stroboscopic data collection with on-time of less than 10 ns and at a rate in the range from 1 MHz to 250 MHz, making it suitable for usage in both hybrid and standard multi-bunch operation of the synchrotron ring. We demonstrate applications of pump-probe imaging at high lateral resolution, namely magnetic imaging and PEEM imaging of surface acoustic waves.

By using X-ray magnetic circular dichroism-photoemission electron microscopy, we study the magnetic behavior of ultra-thin Co on Re(0001) covered by a single layer of graphitic carbon. The carbon overlayer is lithographically fabricated via electron-stimulated dissociative adsorption of CO and subsequent annealing to 650 K. Graphitization is found to induce perpendicular magnetic anisotropy in cobalt at a thickness in the range 4-5 atomic layers, where out-of-plane domains separated by chiral Neel type walls are observed. The graphitic overlayer efficiently protects Co from oxidation upon exposure to ambient conditions, leaving its magnetic structure completely unaffected. The magnetic response of the Co film to an external field is imaged in situ across the hysteresis loop, allowing a quantitative analysis of the field-dependent domain area and structure. Our observations pinpoint the influence of the step morphology on the local magnetic behavior. Moreover, below the saturation field, the domains with chiral walls evolve into nanometric-sized magnetic structures resembling skyrmionic bubbles for applied fields

We outline our achievements in developing electron transparent, leak-tight membranes required for environmental photoelectron spectroscopy (PES). We discuss the mechanical constraints limiting the achievable membrane size and review the development of growth protocols for the chemical vapor deposition (CVD) of single-crystalline graphene on highly (111) textured Cu foils serving as membrane material. During CVD growth, Cu tends to develop a mesoscopic staircase morphology consisting of alternating inclined surface planes, irrespective of whether the covering graphene film or the substrate are single-crystalline. This morphology remains imprinted even when converting the film into freestanding graphene, which affects its mechanical properties. Determining the number of carbon layers in freestanding graphene, we show that membranes reported to suspend over distances larger than 20 µm most likely consist of few-layer graphene. The Raman band signature often used to confirm monolayer graphene rather relates to graphene with turbostratic stacking. The vertical corrugation of freestanding graphene was shown to be almost absent for tri- and four-layer-thick graphene but substantial for bilayer and especially for monolayer graphene. The corrugation is reduced when mechanically straining the freestanding graphene through thermal expansion of the supporting frame, especially flattening membrane areas with imprinted staircase morphology. The electron signal attenuation through supported and freestanding graphene was determined as a function of the electron kinetic energy, verifying that large-area graphene-based electron windows have sufficient electron transparency required for environmental PES. Meanwhile, we managed to cover 100 µm-sized single holes by few-layer graphene up to a coverage fraction of over 99.9998%, as deduced when applying 10 mbar air on one side of the sealing membrane without detecting any measurable pressure increase on its ultrahigh vacuum side. The reported achievements will pave the way toward the development of laboratory-based environmental PES.

The oxidation and reduction of a bimetallic Ni/Rh model catalyst during the water forming ${\mathrm{O}}_{2}+{\mathrm{H}}_{2}$ reaction is studied with low-energy electron microscopy, microspot-low-energy electron diffraction, and x-ray photoemission electron microscopy. Oxidation of a submonolayer Ni film results in the formation of three-dimensional (3D) NiO nanoparticles. Reduction of 3D-NiO in ${\mathrm{H}}_{2}$ produces a dispersed two-dimensional film of metallic Ni. Chemical waves during the ${\mathrm{O}}_{2}+{\mathrm{H}}_{2}$ reaction involve a cyclic transformation between 3D-NiO and 2D-NiO.

We studied the symmetry of magnetic properties and the resulting magnetic textures in ultra-thin epitaxial Au0.67Pt0.33/Co/W(110), a model system exhibiting perpendicular magnetic anisotropy and interface Dzyaloshinskii–Moriya interaction (DMI). As a peculiar feature, the C2v crystal symmetry induced by the Co/W interface results in an additional uniaxial in-plane magnetic anisotropy in the cobalt layer. Photo-emission electron microscopy with magnetic sensitivity reveals the formation of self-organised magnetic stripe domains oriented parallel to the hard in-plane magnetisation axis. We attribute this behavior to the lower domain wall energy when oriented along this axis, where both the DMI and the in-plane magnetic anisotropy favor a Néel domain wall configuration. The anisotropic domain wall energy also leads to the formation of elliptical skyrmion bubbles under a weak out-of-plane magnetic field.

In this work we demonstrate the general characteristics of hcp and fcc stacking in low-energy electron reflectivity for transition metal surfaces, by following the restacking during homoepitaxial growth in real-time. For this purpose, the stacking of a model system, single-crystalline Ag islands during layer-by-layer growth at high temperature on O/W(110), is chosen. Multiple scattering calculations are used to model the relation between electron reflectivity and the crystal geometry. The changes in the electron reflectivity are shown to derive from the changes in the stacking sequence of the topmost surface layers. The results allow to distinguish between the hcp and fcc crystalline arrangements at a surface based on typical differences in the reflectivity curves, making the Ag results relevant for a variety of materials with hexagonal surface geometry. In particular, the multiplet structure within the first Bragg peak in the very low electron energy regime is identified with the fcc structure and thus it can be utilized as a fingerprint to determine the stacking sequence.

X-ray photoemission electron microscopy, one of the most successful imaging tools at synchrotrons, is known to have limitations related to the application of external fields and to the short electron mean free path. In order to overcome such issues, we adapt an existing XPEEM instrument to simultaneously perform coherent x-ray scattering measurements in reflectivity mode, thus adding a complementary method to XPEEM. Photon-in photon-out x-ray scattering provides the sensitivity to buried interfaces as well as the possibility to work under external fields, which is challenging when using charged particles for imaging. XPEEM, in turn, greatly alleviates the difficulties associated with the reconstruction methods used in coherent diffraction imaging. The combination of the two methods is demonstrated for an artifical spin-ice lattice showing both chemical and magnetic contrast.

The recent discovery of magnetic van der Waals (vdW) materials triggered a wealth of investigations in materials science and now offers genuinely new prospects for both fundamental and applied research. Although the catalog of vdW ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective X-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domain walls in micron-sized flakes of the CrTe

Pnictogens have multiple allotropic forms resulting from their ns2 np3 valence electronic configuration, making them the only elemental materials to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light group VA elements are found in the layered orthorhombic A17 phase such as black phosphorus, and can transition to the layered rhombohedral A7 phase at high pressure. On the other hand, bulk heavier elements are only stable in the A7 phase. Herein, we demonstrate that these two phases not only co-exist during the vdW growth of antimony on weakly interacting surfaces, but also undertake a spontaneous transformation from the A17 phase to the thermodynamically stable A7 phase. This metastability of the A17 phase is revealed by real-time studies unraveling its thickness-driven transition to the A7 phase and the concomitant evolution of its electronic properties. At a critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle transition from AB to AA stacked alpha-antimonene followed by a gradual relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of this intermediate phase is found to be determined by surface self-passivation and the associated competition between A7- and A17-like bonding in the bulk. These results highlight the critical role of the atomic structure and interfacial interactions in shaping the stability and electronic characteristics of vdW layered materials, thus enabling a new degree of freedom to engineer their properties using scalable processes.

According to a recently proposed mechanism, the silver-catalyzed industrial synthesis of ethylene oxide (EO) involves adsorbed SO4. The O atoms that are added to the ethylene molecules to give EO originate from SO4, which may solve the long-standing question about the active oxygen species in this reaction. Here, we report a low-energy electron diffraction structure analysis of an ordered phase of SO4 on the Ag(111) surface, forming a (7 × √3)rect structure and containing the oxygen species that before had been spectroscopically identified on the active catalyst. Using I(V) data from a low-energy electron microscope and an input model from density functional theory, the complex structure could be solved. It contains SO4 moieties on a reconstructed Ag(111) surface in which all four O atoms bind to Ag atoms. In the proposed ethylene epoxide reaction model, the structure represents the parent phase from which the active SO4 phase is formed by a lifting of the reconstruction.

The irradiation with photons or electrons can dramatically influence the chemical stability of a molecule, either free or adsorbed on a surface, inducing its fragmentation or desorption. We revisit here the exostimulated dissociation of CO, a prototypical case, choosing hcp thin cobalt films as model support. Intense, microfocused soft X-rays or electron beams are used to locally stimulate CO dissociation. Fast-XPS gives direct access to the adsorbates' chemical state and coverage during irradiation, enabling the kinetics of the process to be monitored in real time. The energy-dependent cross sections for photon and electron stimulated molecular dissociation and desorption are estimated for a fixed initial CO coverage of 1 / 3 ML. In the soft X-ray regime, the desorption channel always prevails over dissociation and is significantly enhanced above the O K edge. The relative dissociation probability increases steadily with increasing photon energy, reaching 30% at 780 eV. Furthermore, we show that low energy electrons in the range 50 to 200 eV dissociate CO more efficiently than X-rays. The prolonged irradiation of the Co surface in CO ambient is found to produce a continuous increase of the carbon coverage, initially promoting the formation of carbides and subsequently accumulating sp 2 carbon on the surface. Far from being a detrimental effect, the CO stimulated dissociation can be exploited to lithographically graft carbon-rich microscopic patterns on Co, with resolution well into the nanometer scale. A brief thermal treatment following irradiation results in the formation of a graphitic carbon overlayer, which effectively protects Co from oxidation upon exposure to ambient conditions, preserving its out-of-plane magnetic anisotropy and domain configuration.

We report on the proof of principle of a scalable method for writing the magnetic state by electron-stimulated molecular dissociative adsorption on ultrathin Co on Re(0001). Intense microfocused low-energy electron beams are used to promote the formation of surface carbides and graphitic carbon through the fragmentation of carbon monoxide. Upon annealing at the CO desorption temperature, carbon persists in the irradiated areas, whereas the clean surface is recovered elsewhere, giving origin to chemical patterns with nanometer-sharp edges. The accumulation of carbon is found to induce an in-plane to out-of-plane spin reorientation transition in Co, manifested by the appearance of striped magnetic domains. Irradiation at doses in excess of 1000 L of CO followed by ultrahigh vacuum annealing at 380 °C determines the formation of a graphitic overlayer in the irradiated areas, under which Co exhibits out-of-plane magnetic anisotropy. Domains with opposite magnetization are separated here by chiral Neél walls. Our fabrication protocol adds lateral control to spin reorientation transitions, permitting to tune the magnetic anisotropy within arbitrary regions of mesoscopic size. We envisage applications in the nano-engineering of graphene-spaced stacks exhibiting the desired magnetic state and properties.

Lateral quantum confinement is of great interest in tuning the electronic properties of graphene-based nanostructures, making them suitable for technological applications. In principle, these properties might be controlled through the edge topology: for example, zigzag nanoribbons are predicted to have spin-polarized edge states. The practical realization of these structures is of utmost importance in fully harnessing the electronic properties of graphene. Here, the formation of regular, 1.4 nm wide ribbon-like graphene structures with zigzag edges are reported, showing 1D electronic states. It is found that these “pseudo-ribbons” embedded in single-layer graphene supported on Ni(100) can spontaneously form upon carbon segregation underneath 1D graphene moiré domains, extending hundreds of nanometers in length. On the basis of both microscopy/spectroscopy/diffraction experiments and theoretical simulations, it is shown that these structures, even though seamlessly incorporated in a matrix of strongly interacting graphene, exhibit electronic properties closely resembling those of zigzag nanoribbons.

We have studied the epitaxial growth of thin layers of vanadium and vanadium oxide on a Rh(110) surface with low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). Up to six monolayers of V/VOx were deposited with temperatures varying between 370 and 770 K. For V deposition, (1 × n) and (n × 1) overlayers with n = 2 and 4 are observed. For the deposition of 3 MLE of VOx, a (12 × 1) structure is obtained as a stable structure. XPS of V 2p3/2 reveals the simultaneous presence of an oxidic and a metallic or strongly reduced (V2+) V component.

A major obstacle for the implementation of redox-based memristive memory or logic technology is the large cycle-to-cycle and device-to-device variability. Here, we use spectromicroscopic photoemission threshold analysis and operando XAS analysis to experimentally investigate the microscopic origin of the variability. We find that some devices exhibit variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub)filaments and that the active, current-carrying filament may change from cycle to cycle. These findings account for the observed variability in device performance and represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices.

Tin diselenide (SnSe2) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO2-SnSe2 heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe2 surface increases by 0.23 and 0.40 eV upon exposure to O2 and air, respectively, with a charge transfer reaching 0.56 e-/SnO2 between the underlying SnSe2 and the SnO2 skin. Remarkably, both pristine SnSe2 and defective SnSe2 display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO2-SnSe2 interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe2-based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe2 should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules. © PID2019-109525RB-I00; Horizon 2020 Framework Programme, H2020: 730872; Ministerio de Economía y Competitividad, MINECO: CEX2018-000805-M, E12H1800010001; Ministero dell’Istruzione, dell’Università e della Ricerca, MIUR; Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2020-0060 This work has been partially supported by the Spanish Ministerio de Ciencia e Innovación under Project PID2019-109525RB-I00. D.F. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M). D.F. and A.A.T. acknowledge the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. A.P. and G.D. acknowledge the CERIC–ERIC Consortium for the access to the Nanospectroscopy facility and financial support. G.D. acknowledges funding of a Ph.D. fellowship from PON Ricerca e Innovazione 2014–2020 (Project E12H1800010001) by the Italian Ministry of University and Research (MIUR). D.W.B. acknowledges the support by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, Project No. FEUZ-2020-0060).

Gallium selenide (GaSe) is a van der Waals semiconductor widely used for optoelectronic devices, whose performances are dictated by bulk properties, including band-gap energy. However, recent experimental observations that the exfoliation of GaSe into atomically thin layers enhances performances in electrochemistry and photocatalysis have opened new avenues for its applications in the fields of energy and catalysis. Here, it is demonstrated by surface-science experiments and density functional theory (DFT) that the oxidation of GaSe into Ga2O3, driven by Se vacancies and edge sites created in the exfoliation process, plays a pivotal role in catalytic processes. Specifically, both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are energetically unfavorable in pristine GaSe, due to energy barriers of 1.9 and 5.7-7.4 eV, respectively. On the contrary, energy barriers are reduced concurrently with surface oxidation. Especially, the Heyrovsky step (Hads + H+ + e- -> H2) of HER becomes energetically favorable only in sub-stoichiometric Ga2O2.97 (-0.3 eV/H+). It is also discovered that the same mechanisms occur for the case of the parental compound indium selenide (InSe), thus ensuring the validity of the model for the broad class of III-VI layered semiconductors

It is well known that intercalated species can strongly affect the graphene-substrate interaction. As repeatedly shown by experiment and theory, the intercalation of atomic species may establish a free-standing character in chemisorbed graphene systems. Here, we focus on graphene grown on a strongly interacting support, cobalt, and demonstrate that the film electronic structure and doping can be tuned via the intercalation/removal of interfacial oxygen. Importantly, cathode lens microscopy reveals the main mechanism of oxygen intercalation, and in particular how microscopic openings in the mesh enable oxygen accumulation at the graphene-cobalt interface. Our experiments show that this process can be carefully controlled through temperature, without affecting the film morphology and crystalline quality. The presence of oxygen at the interface induces an upward shift of the graphene π band, moving its crossing above the Fermi level, accompanied by an increased Fermi velocity and reduced momentum width. Control on the graphene coupling to cobalt may enable one to alter the induced spin polarization in graphene’s electronic states.

In pattern-forming reaction systems, the conversion of macroscopic islands of chemisorbed oxygen into low work function (WF) areas has been attributed to formation of a subsurface oxygen species. We prepare micrometer-sized oxygen islands on Pt(100) and Pt(110) by reaction fronts in catalytic CO oxidation in the 10–6 mbar range. By applying electron microscopy with chemical and structural sensitivity, we characterize their chemical composition upon annealing in vacuum. On Pt(100) we reproduce the previously reported transformation from high to low WF, estimating a relative change Δϕ ≈-0.8 eV with respect to the CO covered surface. We demonstrate that the change in WF is due to a strong enrichment of electropositive contaminants, namely Ca and Ti, in the oxygen islands. On Pt(110) predosed with Cs, we find that the alkali metal accumulates in the oxygen islands, producing Δϕ ≈ −0.4 eV relative to CO–adlayer. Our experiments suggest that the interpretation of the low WF areas as due to “subsurface oxygen” s...

Interest in resolving the mechanisms behind ceria’s activity has been intense due to the numerous industrial applications including those in heterogeneous catalysis. In this work, we study the reduction and reoxidation of ultrathin CeO2(111) nanoislands on Rh(111) and Pt(111) substrates, so-called inverse model catalysts, with a combination of real and reciprocal space techniques based on X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy. Soft X-ray microfocused illumination was employed to reduce the ceria islands, which we are able to control by varying the oxygen partial pressure within the measurement chamber. Low energy electron diffraction measurements of the irradiated ceria films demonstrate the formation of an ordered array of oxygen vacancies leading to a (√7 × √7)R19.1° superstructure attributed to the ι-phase (Ce7O12)(111). Resonant photoelectron spectroscopy provides the required high sensitivity to detect small changes in Ce3+ concentration. The high spatial ...

We report a combined study of imaging the antiferromagnetic (AFM) spin structure and measuring the spin Hall magnetoresistance (SMR) in epitaxial thin films of the insulating non-collinear antiferromagnet SmFeO$_3$. X-ray magnetic linear dichroism photoemission electron microscopy measurements reveal that the AFM spins of the SmFeO$_3$(110) align in the plane of the film. Angularly dependent magnetoresistance measurements show that SmFeO$_3$/Ta bilayers exhibit a positive SMR, in contrast to the negative SMR expected in previously studied collinear AFMs. The SMR amplitude increases linearly with increasing external magnetic field at higher magnetic field, suggesting that field-induced canting of the AFM spins plays an important role. In contrast, around the coercive field, no detectable SMR signal is observed, indicating that SMR of AFM and canting magnetization components cancel out. Below 50~K, the SMR amplitude increases sizably by a factor of two as compared to room temperature, which likely correlates with the long-range ordering of the Sm ions. Our results show that the SMR is a sensitive technique for non-equilibrium spin system of non-collinear AFM systems., Comment: Accepted for publication in Journal of Physics: Condensed Matter. This is the version of the article before peer review or editing, as submitted by an author to Journal of Physics: Condensed Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it

We report the imaging of magnetic domains in ferromagnetic CoNiB nanotubes with very long aspect ratio, fabricated by electroless plating. While axial magnetization is expected for long tubes made of soft magnetic materials, we evidence series of azimuthal domains. We tentatively explain these by the interplay of anisotropic strain and/or grain size, with magneto-elasticity and/or anisotropic interfacial magnetic anisotropy. This material could be interesting for dense data storage, as well as curvature-induced magnetic phenomena such as the non-reciprocity of spin-wave propagation., Comment: Focus only on azimuthal domains, information on domain walls (part of v1) removed - title, abstract and discussion changed accordingly, tubular racetracks scheme removed; new references, figures. Submission to SciPost (reformatted, new abstract, DOI added to references). Enhanced discussion of anisotropy, disagreement of synchrotron data and magnetometry rectified, new MOKE data added

Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface.

We report on the effect of the lateral confinement and a perpendicular magnetic field on isolated room-temperature magnetic skyrmions in sputtered Pt/Co/MgO nanotracks and nanodots. We show that the skyrmions size can be easily tuned by playing on the lateral dimensions of the nanostructures and by using external magnetic field amplitudes of a few mT, which allow to reach sub-100 nm diameters. Our XMCD-PEEM observations also highlight the important role of the pinning on the skyrmions size and stability under an out-of-plane magnetic field. Micromagnetic simulations reveal that the effect of local pinning can be well accounted for by considering the thin film grain structure with local anisotropy variations and reproduce well the dependence of the skyrmion diameter on the magnetic field and the geometry., Comment: 17 pages, 4 figures

Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometer size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetisation at the nanoscale. To date, chiral skyrmion structures have been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films and under external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero applied magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral N\'eel internal structure which we explain as due to the large strength of the Dzyaloshinskii-Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions., Comment: Submitted version. Extended version to appear in Nature Nanotechnology

It is shown that on Pt(111) it is possible to prepare hexagonal boron nitride (h-BN) and graphene (G) in-plane heterojunctions from a single molecular precursor, by thermal decomposition of dimethylamine borane (DMAB). Photoemission, near-edge X-ray absorption spectroscopy, low energy electron microscopy, and temperature programmed desorption measurements indicate that the layer fully covers the Pt(111) surface. Evidence of in-plane layer continuity and weak interaction with Pt substrate has been established. The findings demonstrate that dehydrogenation and pyrolitic decomposition of DMAB is an efficient and easy method for obtaining a continuous almost freestanding layer mostly made of G, h-BN with only a low percentage (h-BN or boron carbonitride, BCN at the boundaries) in the same 2D sheet on a metal substrate, such as Pt(111), paving the way for the advancement of next-generation G-like-based electronics and novel spintronic devices.

We provide direct evidence that irradiation of a graphene membrane on Ir with low-energy Ar ions induces formation of solid noble-gas nanobubbles. Their size can be controlled by thermal treatment, reaching tens of nanometers laterally and height of 1.5 nm upon annealing at 1080 °C. Ab initio calculations show that Ar nanobubbles are subject to pressures reaching tens of GPa, their formation being driven by minimization of the energy cost of film distortion and loss of adhesion.

The distribution of ultrathin layers of vanadium oxide on Rh(110) (theta(V)

Metal nanostructures confined between sp2 hybridized 2D materials and solid supports are attracting attention for their potential application in new nanotechnologies. Model studies under well-defined conditions are valuable for understanding the fundamental aspects of the phenomena under 2D covers. In this work we investigate the intercalation of iron atoms through a single layer of mixed graphene and hexagonal boron nitride on Pt(111) using a combination of spectroscopic and microscopic techniques. Thermally activated diffusion of iron proceeds preferentially under graphene and only partially under hexagonal boron nitride areas. When oxygen is coadsorbed with iron, the intercalation rate is higher, and formation of B2O3 and oxygenated B-C species is observed. Our results suggest the possibility of confining ferromagnetic layers under heterostructures of graphene and hexagonal boron nitride with potential technological implications in the fields of spintronics, magnetic data storage or chemistry under 2D covers. © 2018 Elsevier Ltd

Nature Nanotechnology 11, 449–454 (2016); published online 25 January 2016; corrected after print 18 July 2017 In the version of this Article originally published, the Dzyaloshinskii–Moriya interaction parameter, D, should have been multiplied by √3. The vertical scale of Fig. 2 has been updated accordingly as has the following sentence concerning D values: “For 5 monolayers (ML) of Co, equivalent to a total Co thickness of 1 nm, the ab initio calculations predict = 4.

Nanometer-sized gold particles supported on ceria are an important catalyst for the low-temperature water–gas shift reaction. In this work, we prepared a model system of epitaxial, ultrathin (1–2 nm thick) CeO2–x(111) crystallites on a Rh(111) substrate. Low-energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) were employed to characterize the in situ growth and morphology of these films, employing Ce 4f resonant photoemission to probe the oxidation state of the ceria. The deposition of submonolayer amounts of gold at room temperature was studied with scanning tunneling microscopy (STM) and XPEEM. Spatially resolved, energy-selected XPEEM at the Au 4f core level after gold adsorption indicated small shifts to higher binding energy for the nanoparticles, with the magnitude of the shift inversely related to the particle size. Slight reduction of the ceria support was also observed upon increasing Au coverage. The initial oxidation state of the ceria film was shown to influen...

We report on the high temperature chemical vapor deposition of ethylene on Ir(1 0 0) and the resulting development of single and multi-layer graphene films. By employing X-ray photoemission electron spectromicroscopy, low energy electron microscopy and related microprobe methods, we investigate nucleation and growth of graphene as a function of the concentration of the chemisorbed carbon lattice gas. Further, we characterize the morphology and crystal structure of graphene as a function of temperature, revealing subtle changes in bonding occurring upon cooling from growth to room temperature. We also identify conditions to grow multi-layer flakes. Their thickness, unambiguously determined through the analysis of the intensity of the Ir 4f and C 1s emission, is correlated to the electron reflectivity at very low kinetic energy. The effective attenuation length of electrons in few-layer graphene is estimated to be 4.4 and 8.4 A at kinetic energies of 116 and 338 eV, respectively.

We investigated the structure and crystalline quality of monolayer graphene grown by hydrogen and methane chemical vapor deposition (CVD) on polycrystalline Cu foils. Our data show that the high temperature hydrogen pretreatment of the Cu foil has to be performed at a sufficiently high H 2 pressure in order to avoid graphene (g) formation already during the pretreatment, which limits the achievable domain size during subsequent growth in the CH 4 /H 2 mixture. Methane–hydrogen CVD sustains g growth but induces the faceting of the Cu substrate. Characterization by low energy electron microscopy evidenced a staircase Cu substrate morphology of alternating (4 1 0) and (1 0 0) planes interrupted by (n 1 1) type facets. The g flakes cover the staircase shaped support as a coherent layer. The polycrystalline film mostly contains rotational domains that are preferentially, but not strictly, aligned with respect to the stepped support surface. The substrate induced corrugated morphology occurs also underneath large single crystalline flakes and is transferred to suspended membranes, produced by etching the Cu underneath the graphene. Thus, membranes manufactured from g-Cu are non flat. This explains their reported softened elastic response and the formation of so called nanorippled graphene after transfer from the Cu support which deteriorates its electrical conductivity.

The loading of an Ag(1 1 1) sample with oxygen was monitored by in situ low-energy electron microscopy and X-ray photoemission electron microscopy during NO2 dosing at T≥480 K. At first, adsorbed oxygen populates the Ag(1 1 1) surface, which initiates the (4×4) reconstruction leading to the characteristic O 1s core level at 528.30 eV. The formation of this phase proceeds on a mesoscopic length scale by traveling fronts separating reconstructed from non-reconstructed surface areas. Continued NO2 dosing leads to the accumulation of a new oxygen species mainly at steps and step bunches. Characterized by an O 1s peak with two components at 530.20 eV and 530.75 eV, this species represents the active oxygen during the ethylene epoxidation reaction over Ag. The 530.20 eV component is attributed to surface oxygen, the 530.75 eV species to subsurface oxygen. This inhomogeneous accumulation of the active oxygen occurs at a very low rate. However, the preparation route can be changed, which strongly accelerates the population of the catalytically active oxygen species and leads to a homogeneous distribution of oxygen on the surface. This route involves the complete formation of the O(4×4) reconstruction by NO2 dosing, followed by a complete de-reconstruction of the surface by desorption of the oxygen adlayer. The faster population kinetics is related to the Ag adatom transport during such a reconstruction/de-reconstruction cycle.

This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulfide (WS2) on epitaxial graphene (EG) on SiC(0001). The WS2 is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS2/EG. The valence-band (VB) structure emerging from this alignment is investigated by means of photoelectron spectroscopy measurements, with both high space and energy resolution. We find that the spin–orbit splitting of monolayer WS2 on graphene is of 462 meV, larger than what is reported to date for other substrates. We determine the value of the work function for the WS2/EG to be 4.5 ± 0.1 eV. A large shift of the WS2 VB maximum is observed as well, due to the lowering of the WS2 work function caused by the donor-like interfacial states of EG. Density functional theory (DFT) calculations carried out on a coincidence supercell confirm the experimental band structure to an excellent degree. X-ray photoemission electron microscopy (XPEEM) measurements performed on single WS2 crystals confirm the van der Waals nature of the interface coupling between the two layers. In virtue of its band alignment and large spin–orbit splitting, this system gains strong appeal for optical spin-injection experiments and opto-spintronic applications in general.

All rights reserved. The production of high-quality graphene-oxide interfaces is normally achieved by graphene growth via chemical vapour deposition on a metallic surface, followed by transfer of the C layer onto the oxide, by atomic layer and physical vapour deposition of the oxide on graphene or by carbon deposition on top of oxide surfaces. These methods, however, come with a series of issues: they are complex, costly and can easily result in damage to the carbon network, with detrimental effects on the carrier mobility. Here we show that the growth of a graphene layer on a bimetallic Ni3Al alloy and its subsequent exposure to oxygen at 520 K result in the formation of a 1.5 nm thick alumina nanosheet underneath graphene. This new, simple and low-cost strategy based on the use of alloys opens a promising route to the direct synthesis of a wide range of interfaces formed by graphene and high-κ dielectrics., The research activities of L.O., E.M., F.O. and A.B. have received funding from the MIUR within the programme PRIN 2010-2011 for the project entitled ‘GRAF. Frontiers in Graphene Research: understanding and controlling Advanced Functionalities’ (No.20105ZZTSE_001). E.R.H. acknowledges support of the Spanish Research and Innovation Office through project FIS2012-31713. E.R.H. and D.A. acknowledge support from the Leverhulme Trust. M.B., S.U. and P.H. acknowledge support from the VILLUM foundation, The Danish Council for Independent Research/Technology and Production Sciences.

Geometrical frustration in magnetic materials often gives rise to exotic, low-temperature states of matter, such as the ones observed in spin ices. Here we report the imaging of the magnetic states of a thermally active artificial magnetic ice that reveal the fingerprints of a spin fragmentation process. This fragmentation corresponds to a splitting of the magnetic degree of freedom into two channels and is evidenced in both real and reciprocal space. Furthermore, the internal organization of both channels is interpreted within the framework of a hybrid spin–charge model that directly emerges from the parent spin model of the kagome dipolar spin ice. Our experimental and theoretical results provide insights into the physics of frustrated magnets and deepen our understanding of emergent fields through the use of tailor-made magnetism., By nanofabricating arrays of dipolar-coupled bistable single-domain nanomagnets, artificial model systems exhibiting collective ordering may be realized. Here, the authors present signatures of spin fragmentation in low-energy states of an artificial kagome ice.