Search

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.

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

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.

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.

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).

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 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

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.

The influence of Ga pre-adsorption on Si(111), Si(113) and Si(112) surfaces on Ge growth has been investigated by low-energy electron diffraction and microscopy as well as X-ray photoemission spectroscopy. On Si(111), step edges and substrate domain boundaries are decorated with Ga at high deposition temperatures, enabling selective growth and alignment of three-dimensional Ge islands on a chemically modulated surface. On Si(113), a morphological modulation is achieved by Ga saturation, as the Si substrate decomposes into an ordered array of (112) and (115) facets. This results in the growth of Ge islands aligned at the facets. These islands exhibit an anisotropy, as they are elongated along [1{bar 1}0]. Ga pre-adsorption on Si(112) smoothens the initially faceted bare surface, and subsequent Ge growth leads to the formation of nanoscale Ge wires. The results are discussed in terms of surface chemistry, as well as diffusion and strain relaxation anisotropy.

International audience; Epitaxial graphene is of particular interest because of its tunable electronic structure. One important approach to tune the electronic properties of graphene relays on intercalating atomic species between graphene and the topmost silicon carbide layer. Here, we investigated the morphology and electronic structure of gold-intercalated epitaxial graphene using a multitechnique approach combining spectroscopic photoemission low-energy electron microscopy (SPELEEM) for chemical and structural characterization at mesoscopic length scale and with transmission electron microscopy (STEM) at the atomic level. Deposition of gold on ex situ prepared graphene on SiC(0 0 0 1) results in the partial intercalation of Au adatoms under graphene, with the formation of a buffer layer of variable thickness. Gold has also shown to aggregate in nanometer-sized clusters lying on top of the same graphene film. X-ray photo-emission electron microscopy measurements indicate that Au induces only small changes in the doping of the graphene layer, which does not develop a quasi free-standing behavior.

Reorganization of Au + Pd submonolayers on a Rh(1 1 0) surface occurring during the water formation reaction has been observed and characterized by low energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM). The results demonstrate segregation of Au + Pd and oxygen into separate surface phases, the morphology and size of the O and Au + Pd patterns being governed by the reaction parameters and adsorbate coverage. At moderate Au + Pd coverages and temperatures in the range 760–860 K, lamellar periodic Au + Pd/O micro-structures are generated. The results are interpreted in terms of kinetic and thermodynamic considerations.

In catalytic methanol oxidation on ultrathin vanadium oxide layers on Rh(111) (Θ_{V}≈0.2 monolayer equivalent) we observe a 2D ripening of the VO_{x} islands that is controlled by the catalytic reaction. Neighboring VO_{x} islands move under reaction conditions towards each other and coalesce. The motion and the coalescence of the islands are explained by a polymerization-depolymerization equilibrium that is sensitive to gradients in the adsorbate coverages.

The instrumentation for synchrotron radiation X-ray photoemission electron microscopy (XPEEM) has recently undergone significant improvements, finding application in diverse fields such as magnetism, chemistry, surface science and nanostructure characterization. The spectroscopic photoemission and low energy electron microscope (SPELEEM) operational at the 'Nanospectroscopy beamline' at the Elettra synchrotron facility combines structural and spectroscopic analysis methods in a single instrument, exploiting the inherent chemical sensitivity of X rays. The SPELEEM reaches an energy resolution of 0.2 eV and a lateral resolution of few tens of nanometers in XPEEM. Selected results are used to illustrate the spectro-microscopic capabilities of the SPELEEM, and the usefulness of available complementary methods such as low energy electron microscopy (LEEM) and micro-spot low energy electron diffraction (LEED).

X-ray photoemission electron microscopy (XPEEM) is a full-field imaging technique giving access to the chemical state and magnetic order of laterally inhomogeneous surfaces, interfaces and thin films. In its simplest variant, synchrotron-based PEEM uses secondary emission to map local differences in the oxidation state, valence, and bond orientation around the emitter. The combination with X-ray circular and linear dichroism techniques represents the most frequent application, and has found extensive use in imaging ferromagnetic and antiferromagnetic domains. XPEEM instruments with energy filter can implement laterally resolved X-ray photoelectron spectroscopy (XPS) and angle resolved photoelectron spectroscopy (ARPES), reaching high chemical and electronic structure sensitivity. Here, we describe the basic aspects and methods of synchrotron-based spectromicroscopy with the PEEM, and its combination with low energy electron microscopy (LEEM). The present state of the art of the technique will be illustrated by applications in diverse fields, spanning from surface and materials sciences to biology and magnetism.

The impact of silver pre-adsorption on germanium growth on Si(113) was investigated using in-situ low-energy electron microscopy (LEEM) as well as low-energy electron diffraction (LEED). The adsorption of silver leads to the formation of a regular pattern of nanofacets along the [10] direction. The periodicity of this pattern in [33] direction was determined to (44 ± 4) nm. From LEED series at different energies the facets were identified to be of (111) and (115) orientation. While the (111) facets show a (√3 × √3)-R30° reconstruction, the (115) facets exhibit a (2 × n) superstructure. The subsequent growth of Ge results in the formation of nanoislands that are aligned along the facets. These Ge islands have an anisotropic shape with typical sizes of about 100 nm in [33] direction and 400 nm in [10] direction. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fe3S4 (greigite), a ferrimagnetic compound widely encountered in nature and of increasing technical interest, is prepared by vapor–solid interaction. This is done in a laterally resolving ultrahigh vacuum multi-method instrument, which not only allows in situ analysis of the reaction product without influence of the environment but also follows its evolution during the reaction. The physical method employed here avoids formation of unwanted reaction products that are produced by the usual wet chemical synthesis methods. The resulting greigite crystals exhibit complex intergrowth, faceting and nanoparticle substructure and are somewhat distorted from a pure cubic symmetry. A new Curie temperature above 450 °C is established.

High resolution X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy (LEEM) have been used to investigate the growth of ultrathin CeOx(111) on Re(0001), a model catalyst system. Rotational domains of CeOx(111) are identified with microprobe low energy electron diffraction (LEED) and dark-field LEEM. In the regions not covered by the ceria islands, a surface rhenium-oxide layer has been observed using energy-filtered XPEEM imaging and spectroscopy. The oxidation state of the ceria is key to its catalytic activity. For this reason we have employed resonant photoelectron spectroscopy of the Ce 4f contributions to the valence band to monitor the relative Ce3+ and Ce4+ concentrations. The overall stoichiometry of the moderately reduced film was CeO1.63. Resonant energy-filtered XPEEM imaging of the Ce oxidation state allowed us to confirm the uniformity of this stoichiometry across the ceria islands that constituted the film.

The NO + H2 reaction on a potassium promoted Rh(110) surface is shown to sustain the formation of spatio-temporal periodic patterns leading to mass transport phenomena. The excitation of pulses and the mass transport mechanism are studied in the 10-7 and 10-6 mbar pressure range, with the potassium coverage varying between K = 0.05 and K = 0.12 ML. Using spectroscopic photoemission and spectroscopic low energy electron microscopy (SPELEEM) as well as related microprobe diffraction techniques, we show that the excitation mechanism comprises a cyclic structural transformation: K + O-coadsorbate → (2 × 1)-N → c(2 × 4)-2O,N → K + O coadsorbate. Laterally resolved spectroscopy demonstrates that potassium is accumulated in front of the nitrogen pulses, suggesting that adsorbed nitrogen acts as a diffusion barrier for potassium. © 2013 The Owner Societies.

We show that bimetallic surface alloying provides a viable route for governing the interaction between graphene and metal through the selective choice of the elemental composition of the surface alloy. This concept is illustrated by an experimental and theoretical characterization of the properties of graphene on a model PtRu surface alloy on Ru(0001), with a concentration of Pt atoms in the first layer between 0 and 50%. The progressive increase of the Pt content determines the gradual detachment of graphene from the substrate, which results from the modification of the carbon orbital hybridization promoted by Pt. Alloying is also found to affect the morphology of graphene, which is strongly corrugated on bare Ru, but becomes flat at a Pt coverage of 50%. The method here proposed can be readily extended to several supports, thus opening the way to the conformal growth of graphene on metals and to a full tunability of the graphene-substrate interaction.

Synchrotron based photoemission electron microscopy with energy filter combines real space imaging with microprobe diffraction ($\mu$-ARPES), giving access to the local electronic structure of laterally inhomogeneous materials. We present here an overview of the capabilities of this technique, illustrating selected applications of angle resolved photoemission electron microscopy and related microprobe methods. In addition, we report the demonstration of a darkfield XPEEM (df-XPEEM) imaging method for real space mapping of the electronic structure away from $\Gamma$ at a lateral resolution of few tens of nm. The application of df-XPEEM to the (1$\times$12)-O/W(110) model oxide structure shows the high sensitivity of this technique to the local electronic structure, allowing to image domains with inequivalent adsorption site symmetry. Perspectives of angle resolved PEEM are discussed., Comment: 10 pages, 6 figures, the article belongs to a special issue "Photoelectron microscopy, Time resolved pump-probe PES" (Edited By Maya Kiskinova and Andreas Scholl)

The effect of oxygen adsorption on the local structure and electronic properties of monolayer graphene grown on SiC(0001) has been studied by means of Low Energy Electron Microscopy (LEEM), microprobe Low Energy Electron Diffraction (\muLEED) and microprobe Angle Resolved Photoemission (\muARPES). We show that the buffer layer of epitaxial graphene on SiC(0001) is partially decoupled after oxidation. The monitoring of the oxidation process demonstrates that the oxygen saturates the Si dangling bonds, breaks some Si-C bonds at the interface and intercalates the graphene layer. Accurate control over the oxidation parameters enables us to tune the charge density modulation in the layer., 12 pages, 4 figures

PACS number(s): 75.70.−i, 75.47.Lx, 68.37.Nq, 78.20.Ls.-- et al., The oldest known magnetic material, magnetite, is of current interest for use in spintronics as a thin film. An open question is how thin can magnetite films be and still retain the robust ferrimagnetism required for many applications. We have grown 1-nm-thick magnetite crystals and characterized them in situ by electron and photoelectron microscopies including selected-area x-ray circular dichroism. Well-defined magnetic patterns are observed in individual nanocrystals up to at least 520 K, establishing the retention of ferrimagnetism in magnetite two unit cells thick., This researchwas supported by the Spanish Ministry of Science and Innovation through Projects No. MAT2009-14578-C03-01, MAT2009-14578-C03-02 and MAT2010-21156-C03-02, by the Office of Basic Energy Sciences, Division of Materials and Engineering Sciences, US Department of Energy under Contract No. DE-AC04-94AL85000, and by the European Union through 226716-ELISA. M. M. and B. S. thank the Spanish Ministry of Science and Innovation for supporting them through FPI fellowships.

We have studied the composition profiles of Stranski–Krastanov InAs/GaAs quantum dots grown on patterned nanohole arrays. Two dimensional surface chemical maps obtained by X-ray photoemission electron microscopy reveal a non-uniform composition profile similar to that of standard dots grown on planar surfaces, with an enhanced In concentration at the center of the islands, with respect to the wetting layer. A statistical analysis, however, revealed an improvement of about 50% of dot composition uniformity associated with the already reported enhancement of size uniformity. No significant size or composition variation was found by changing the period of the hole pattern from 250 to 600 nm.

Upon exposure of ultrathin vanadium oxide layers on Rh(111) (θV ∼ 0.2 MLE) to the H2 + O2 reaction, we observe the reversible formation of stationary macroscopic stripe patterns caused by lateral redistribution of the vanadium oxide. These patterns are investigated in the 10–6 mbar range at around 750 K, employing LEEM (low energy electron microscopy), microprobe-LEED and microprobe-XPS. The stripe patterns evolve via several stages and structures: essentially, vanadium oxide nanoislands with a (√7 × √7)R19.1° structure are reduced from V+5 to V3+ and condense into a macroscopic (2 × 2) oxide phase upon increasing p(H2). The orientation of the stripe patterns is determined by step bunches, which introduce a diffusional anisotropy. The stationary patterns are discussed within the concept of reactive phase separation.

The chemical and structural evolution of supported PtRh alloy nano- and microparticles during oxidation has been studied by means of scanning photoelectron microscopy (SPEM) and low energy electron...

It is shown that the pulses which develop in the NO+H 2 reaction on an alkali promoted Rh(110) surface reaction can transport alkali metal. This leads to the accumulation of a substantial alkali-metal concentration in the collision area of pulse trains. Realistic simulations revealed that the effect is due to the strong energetic interactions of the alkali metal with coadsorbates, i.e., the attractive interaction with coadsorbed oxygen and the effectively repulsive interaction with coadsorbed nitrogen.

The stress-induced lattice changes in a p(1x2) ordered oxygen layer on W(110) are measured by low-energy electron diffraction. We have observed that small oxygen islands show a mismatch with the underlying lattice. Our results indicate that along [1-10] the average mismatch scales inversely with the island size as 1/L for all oxygen coverages up to 0.5 ML, while along [001] it is significant only for the smallest oxygen islands and scales as a higher power of the inverse island size. The behaviour along [1-10] is described by a one-dimensional finite-size Frenkel-Kontorova model. Using this model, together with calculated force constants, we make a quantitative estimate for the change of surface-stress upon oxygen adsorption. The result is consistent with our ab-initio calculations, which give a relative compressive stress of -4.72 N/m along [1-10] and a minute relative tensile stress of 0.15 N/m along [001]. The scaling along [001] is qualitatively explained as an effect induced by the lattice relaxation in the [1-10] direction., 22 pages, 5 figures

By using X‐Ray Photoemission Electron Microscopy we have obtained two‐dimensional maps of the in‐plane surface composition of InAs/GaAs self‐assembled quantum dots. The chemical analysis reveals a higher In composition on the dot surface, with respect to the surrounding wetting layer, with a concentration increasing from the sides to the center of the dots. Comparison with concentrations found in the core of similar dots suggests strong In segregation on their surface, similarly to the surrounding wetting layer.

The impact of Ga preadsorption on the spatial correlation of nanoscale three-dimensional (3D) Ge-islands has been investigated by low-energy electron microscopy and low- energy electron diffraction. Submonolayer Ga adsorption leads to the formation of a 2D chemical nanopattern, since the Ga-terminated (2 x 2) domains exclusively decorate the step edges of the Si(113) substrate. Subsequent Ge growth on such a partially Ga-covered surface results in Ge 3D islands with an increased density as compared to Ge growth on clean Si(113). However, no pronounced alignment of the Ge islands is observed. Completely different results are obtained for Ga saturation coverage, which results in the formation of (112) and (115) facets regularly arranged with a periodicity of about 40 nm. Upon Ge deposition, Ge islands are formed at a high density of about 1.3 x 10(10) cm(-2). These islands are well ordered as they align at the substrate facets. Moreover, the facet array induces a reversal of the Ge islands' shape anisotropy as compared to growth on planar Si(113) substrates.

The interaction of molecular hydrogen with ceria is of important relevance for heterogeneous catalysis related to green chemistry and renewable energy. Here, the complex structural transformations of a well-defined cerium oxide model catalyst are followed in situ and in real time when exposed to a reactive H2 environment. By using electron spectromicroscopy and diffraction with chemical and structural sensitivities, it is demonstrated that the transition from CeO2 to crystalline Ce2O3 occurs through a mixture of transient, coexisting phases on the nanoscale. The findings establish a clear relationship between structure and functionality for hydrogen dissociation over ceria(111), bearing profound implications on the nature of the reduction (Ce4+ Ce3+) and mechanism for H2 scission.

We investigated the initial Ge nucleation and Ge island growth on a Si(113) surface using low energy electron microscopy and low energy electron diffraction. The sample temperature was varied systematically between 380 degrees C and 590 degrees C. In this range, a strong temperature dependence of the island shape is observed. With increasing temperature the Ge islands are elongated in the [332] direction. Simultaneously, the average island size increases while their density decreases. From the Arrhenius-like behaviour of the island density, a Ge adatom diffusion barrier height of about 0.53 eV is deduced. (c) 2005 Elsevier B.V. All rights reserved.

We investigate thin films of diindenoperylene deposited on Au(100) single crystals by using real time in situlow energy electron microscopy (LEEM) in synergy with X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. We find evidence for Stranski–Krastanov growth, with island nucleation prevalently at step bunches, and a lower probability of island nucleation at terraces. We also directly observe the effect of the Ehrlich–Schwoebel barrier at the substrate and in the film morphology, in particular inside the islands that show a non-homogeneous thickness due to a rough front growth, because of the additional energy required for the diffusing molecules to surmount a downward step.

The deposition of ceria on a preoxidized W(110) crystal at 870 K has been studied in situ by photoelectron spectroscopy and low-energy electron diffraction. Formation of an epitaxial layer of crystalline cerium tungstate Ce(6)WO(12)(100), with the metals in the Ce(3+) and W(6+) chemical states, has been observed. The interface between the tungsten substrate and the tungstate film consists of WO suboxide. At thicknesses above 0.89 nm, cerium dioxide grows on the surface of Ce(6)WO(12), favoured by the limited diffusion of tungsten from the substrate.

The correlation between the thickness-dependent oxidation rate of ultrathin Al films on W(110) and the quantum-well states (QWS) resulting from electron confinement in the Al film has been explored by combined x-ray photoemission electron microscopy (XPEEM), low energy electron microscopy (LEEM), and first-principles calculations. Hybridization with substrate electronic states is observed to alter the film electronic structure, strongly modifying the electron density decay length in vacuum. The decay length, rather than the density of states at the Fermi energy, is found to dominate the observed reactivity trends.

Low energy electron irradiation of a Ag(111) surface during NO2 adsorption at 300 K induces formation of Ag oxide. Using a spatially confined electron beam, small Ag2O spots could be grown with a sharp, ∼100 nm wide, boundary to the nonirradiated metallic surface. Since the structure size will mainly depend on the sharpness of the irradiating electron beam, this process has the potential of a single step nanostructuring process. Temperature treatment offers an easy way to manipulate the boundary between oxide and metallic silver by steering a chemical front.

One-dimensional Au rows are produced on the TiO2(110) surface by controlled photon-stimulated desorption followed by Au deposition at 750 K. In the resulting (1 × 2) structure, O vacancies are replaced by Au atoms, as demonstrated by combined laterally resolved x-ray spectroscopy and micro-LEED (low-energy electron diffraction) measurements. The experimental results are confirmed by first-principles calculations, which indicate strong Au–Ti bonding and a very stable configuration of the Au rows. The calculations determine the detailed atomic structure of the Au rows, with an inter-row spacing of 13 A and an intra-row Au distance of 2.95 A.

With laterally resolved photoemission spectroscopy, we obtained In and Ga surface concentration maps of InAs/GaAs quantum dots. Our data demonstrate that the dot composition is neither pure InAs nor homogeneous InxGa1-xAs, but presents an In concentration increasing from the borders to the center of the dots. Besides, our observations suggest strong In segregation (x ~ 0.9) on the surface of the dots and of the surrounding wetting layer. Such segregation, well known for two-dimensional InAs/GaAs growth, had not been directly observed so far on the dots, and should be taken into account to model size and composition of GaAs-overgrown structures.

The transition from single-layer to bilayer growth of molybdenum disulfide on the Au(111) surface is investigated by in situ low-energy electron and photoemission microscopy. By mapping the film morphology with nanometer resolution, we show that a MoS2 bilayer forms at the boundaries of single-layer single-domain MoS2 islands and next to merging islands whereas bilayer nucleation at the island centers is found to be suppressed, which may be related to the usage of dimethyl disulfide as sulfur precursor in the growth process. This approach, which may open up the possibility of growing continuous films over large areas while delaying bilayer formation, is likely transferable to other transition metal dichalcogenide model systems.