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In this PhD thesis, the recombination of different atomic lattices in stacked 2D materials such as twisted bilayer graphene is studied. Using the different possibilities of Low-Energy Electron Microscopy (LEEM), the domain forming between the two atomic layers with small differences is studied. Superlattices in three such 2D material systems are studied. In twisted bilayer graphene, the small difference is caused by a twist of approximately one degree between the layers. In graphene on SiC, the difference is caused by the lattice mismatch between a buffer layer bound to the substrate and the next graphene layer. For both, we show that domains of different shapes and sizes occur and relate them to strain and lattice mismatch. The third system studied is tantalum disulfide. In this layered material, two different superlattices occur: a superlattice between atomic layers with different atomic arrangements in the layers, so-called polytypes, and the superlattices between the atomic lattice and the Charge Density Waves (CDW). CDWs cause a large temperature dependent resistivity change. The influence of a mixture of different polytypes on the precise CDW states is studied using LEEM spectroscopy and local Low-Energy Electron Diffraction.

Understanding of thin film growth mechanism is crucial for tailoring film growth behaviors, which in turn determine physicochemical properties of the resulting films. Here, vapor-growth of tungsten carbide overlayers on W(110) surface is investigated by real time low energy electron microscopy. The surface growth is strongly confined by surface steps, which is in contrast with overlayer growth crossing steps in a so-called carpet-like growth mode for example in graphene growth on metal surfaces. Density functional theory calculations indicate that the step-confined growth is caused by the strong interaction of the forming carbide overlayer with the substrate blocking cross-step growth of the film. Furthermore, the tungsten carbide growth within each terrace is facilitated by the supply of carbon atoms from near-surface regions at high temperatures. These findings suggest the critical role of near-surface atom diffusion and step confinement effects in the thin film growth, which may be active in many film growth systems.

5 pags., 5 figs., Li metal has been deposited on the surface of a Ru(0001) single crystal containing patches of monolayer-thick epitaxial graphene islands. The use of low-energy electron microscopy and diffraction allowed us to {\em in situ} monitor the process by measuring the local work function as well as to study the system in real and reciprocal space, comparing the changes taking place on the graphene with those on the bare Ru(0001) surface. It is found that Li deposition decreases the work function of the graphene islands but to a much smaller degree than of the Ru(0001) surface, as corresponds to its intercalation below the graphene overlayer. Finally, the diffusion process of Li out of the graphene islands has been monitored by photoelectron microscopy using a visible-light laser., This work is supported by grant RTI2018-095303-B-C51, funded by MCIN/AEI/10.13039/501100011033 and by ‘‘ERDF A way of making Europe’’, by grant PID2020-117024GB-C43 (Ministerio de Ciencia e Innovación) and by grant S2018-NMT-4321, funded by the Regional Government of Madrid, Spain and by ‘‘ERDF A way of making Europe’’. L.M.G. acknowledges a sabbatical grant from DGAPA-UNAM

Using low energy electron microscopy, Au on Ge(111) is determined to follow a Stranski–Krastanov growth mode consisting of a single layer up to one monolayer (ML), followed by three-dimensional Au–Ge alloy droplets. Near 600 °C, we report the first observation of a reversible first-order phase transition that occurs from the (3 × 3)R30° phase to a (1 × 1) phase, which has a coverage of 0.367 ML. The transition gradually occurs through a coexistence region with a temperature range of about 2 °C and weakly depends on coverage, varying from 640 °C at 1 ML down to 580 °C at 0.8 ML. The phase transition is accompanied by phase fluctuations of small domains or the fluctuations of phase boundaries of large domains. At coverage >1 ML and above 250 °C, the 3D droplets move with stick-slip hopping behavior that has previously been explained by dissolution of Ge at step edges into the alloy droplet, which then comes to concentration and thermal equilibrium via the island motion.

This work illustrates the impact of atmospheric gases on the surface of epitaxial graphene. The different rate of adsorption on different parts of graphene samples provides a concrete evidence that the surface morphology of graphene plays a significant role in this process. The uneven adsorption occurs only on the surface of the monolayer graphene and not on bilayer graphene. The second monolayer is distinguished and verified by the phase contrast mode of atomic force microscopy and the low energy electron microscopy, respectively. Raman spectroscopy is used to study the strain on the surface of graphene; results indicate that monolayer and bilayer graphene exhibit different types of strain. The bilayer is under more compressive strain in comparison with monolayer graphene that hinders the process of adsorption. However, the wrinkles and edges of steps of the bilayer are under tensile strain, hence, facilitate adsorption. Samples were subjected to X-ray photoelectron spectroscopy which confirms that the adsorbates on the epitaxial graphene are carbon clusters with nitrogen and oxygen contamination. For reversing the adsorption process the samples are annealed and a method for the removal of these adsorbates is proposed.

Using spin-polarized low energy electron microscopy (SPLEEM), we observed surface step bunch induced perpendicular magnetic anisotropy in Fe/Ni bilayers grown on Cu(001) single crystal as well as in Ni/Co/Pd trilayers grown on W(110) crystal. On Cu(100) the formation of step bunches can be stimulated or suppressed by high- or low-temperature annealing cycles, respectively. SPLEEM images resolving the three dimensional magnetization vector in the Fe/Ni films grown on step bunched Cu(100) reveal an additional perpendicular magnetic anisotropy in regions near step bunches. In contrast, no extra perpendicular magnetic anisotropy is observed on low-temperature annealed Cu(100) featuring single-atom height step arrays. Additional investigation of Ni/Co/Pd trilayers on W(110) reveals the influence of step bunch orientation on magnetic anisotropy. Our observations may lead to opportunities for tailoring or patterning anisotropy in magnetic thin-films by controlling film morphology.

The magnetic properties of ultrathin ferromagnetic films are studied by means of Spin Polarized Low Energy Electron Microscopy. Measurements of the onset of ferromagnetic order, distribution and shape of magnetic domains, magnetization direction and their change are now well established standards for this technique. Here, the asymmetry parameter has been determined as a function of film coverage and energy of the incident electron beam. It reveals oscillatory behavior which is usually described as due to the quantum size effect (QSE). We explore the origin of the characteristic features observed in the asymmetry curves and distinguish between the QSE oscillations and other phenomena influencing the shape of the asymmetry curves. As an example we discuss the asymmetry of ultrathin iron films grown on the W(1 1 0) surface.

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.

Two-dimensional materials, such as graphene, are usually prepared by chemical vapor deposition (CVD) on selected substrates, and their transfer is completed with a supporting layer, mostly polymethyl methacrylate (PMMA). Indeed, the PMMA has to be removed precisely to obtain the predicted superior properties of graphene after the transfer process. We demonstrate a new and effective technique to achieve a polymer-free CVD graphene — by utilizing low-energy electron irradiation in a scanning low-energy electron microscope (SLEEM). The influence of electron-landing energy on cleaning efficiency and graphene quality was observed by SLEEM, Raman spectroscopy (the presence of disorder D peak) and XPS (the deconvolution of the C 1s peak). After removing the absorbed molecules and polymer residues from the graphene surface with slow electrons, the individual graphene layers can also be distinguished outside ultra-high vacuum conditions in both the reflected and transmitted modes of a scanning low-energy (transmission) electron microscope.

We present a study of temperature dependent growth of nano-sized ceria islands on a Cu (100) substrate. Low-energy electron microscopy, micro-electron diffraction, X-ray absorption spectroscopy, and photoemission electron microscopy are used to determine the morphology, shape, chemical state, and crystal structure of the grown islands. Utilizing real-time observation capabilities, we reveal a three-way interaction between the ceria, substrate, and local oxygen chemical potential. The interaction manifests in the reorientation of terrace boundaries on the Cu (100) substrate, characteristic of the transition between oxidized and metallic surface. The reorientation is initiated at nucleation sites of ceria islands, whose growth direction is influenced by the proximity of the terrace boundaries. The grown ceria islands were identified as fully stoichiometric CeO2 (100) surfaces with a (2 × 2) reconstruction.

Surface processes such as metal oxidation and metal oxide growth invariably influence the physical and chemical properties of materials and determine their interaction with their surroundings and hence their functionality in many technical applications. On a fundamental level, these processes are found to be governed by a complex interplay of thermodynamic variables and kinetic constraints, resulting in a rich variety of material-specific phenomena. In this review article, we discuss recent results and insights on transition metal oxidation and rare-earth oxide growth acquired by low-energy electron microscopy and related techniques. We demonstrate that the use of in situ surface sensitive methods is a prerequisite to gaining a deeper understanding of the underlying concepts and the mechanisms responsible for the emerging oxide structure and morphology. Furthermore, examples will be provided on how structural and chemical modifications of the oxide films and nanostructures can be followed in real-time and analyzed in terms of local reactivity and cooperative effects relevant for heterogeneous model catalysis.

Fractional order (1/2, 0) spots appear in the electron diffraction from NiO/Ag(0 0 1) films due to exchange scattering of low energy electrons by the antiferromagnetically ordered surface Ni moments. Utilizing these beams, imaging of the nanosized surface magnetic domains were carried out employing the high spatial resolution ( ∼ 10 nm) of the Low Energy Electron Microscopy (LEEM) in the dark-field (DF) mode. While selected through a contrast aperture, the four magnetic reflections produced by the p ( 2 × 2 ) antiferromagnetic sub-lattice lead to the visualization of the different magnetic twin domains. The intensity variations of different twin domains were measured as a function of electron beam energies via domain resolved LEEM I-V plots. The surface N e el temperatures ( T N ) of the films were measured using the temperature dependence of these half-order spot intensities. Detailed morphological studies of the size and shape of these nanodomains and their evolution as a function of the film thickness have been carried out with the help of pair-correlation function and fractal analysis. The size, shape and distribution of these magnetic domains are modified significantly by the strain relaxation mechanism beyond the critical film thickness. A method to estimate the relative domain sizes from a quantitative measure of the half-order spot intensities is manifested well below T N .

Interaction of electrons with chiral matter gives rise to interesting phenomena such as the chirality-induced spin selectivity. The interdependence of reflectivity of spin-polarized low energy electrons and the absolute handedness of chiral molecules is investigated. First, the growth of homochiral films of helical aromatic hydrocarbons, so-called helicenes, on a Cu(100) surface is studied by means of low energy electron microscopy and scanning tunneling microscopy in ultrahigh vacuum. As soon as the coverage exceeds one monolayer, double-layer nucleation and growth is favored such that depletion in the first layer occurs. Spatially resolved work function measurements show that second-layer patches have a lower work function than first-layer areas. Reflectivity spectra of spin-polarized electrons do not show any asymmetry between homochiral films of the enantiomers. Laterally resolved work function measurements do not confirm work function differences such as those reported earlier for photoelectron studies of chiral peptide films on ferromagnetic substrates.

Thin, stable and inert dielectric spacers are essential for manufacturing electronic devices based on 2D materials. However, direct synthesis on top of 2D materials is difficult due to their inert nature. In this work, we studied how an electron beam induces fragmentation of borazine and enables spatially confined synthesis of amorphous-BNx on graphene at room temperature. Using a combination of X-ray Photoelectron Spectroscopy, Low Energy Electron Microscopy, and Scanning Tunneling Microscopy we studied the morphology of the heterostructure, its chemical composition, and finally its temperature evolution. We find that electron-beam induced deposition starts by the binding of heavily fragmentized borazine, including atomic boron, followed by the growth of a multilayer with a 1:0.7 B:N ratio. The final structure exhibits a thermal stability up to 1400 K and ~ 50 nm spatial control provided by the electron beam. Our studies provide surface science insight into the use of electron beams for synthesis and lateral control of stable and inert layers in 2D heterostructures.

The growth of Ir on Ge(111) as a function of temperature between 23 °C and 820 °C is characterized with low energy electron microscopy (LEEM), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and x-ray photoemission spectroscopy (XPS). Deposition onto a substrate at 350 °C revealed a novel growth mode consisting of multilayer Ir islands with (√3 × √3)R30° (abbreviated as √3) structure interconnected by “bridges” of single-layer Ir several atoms wide. For deposition onto substrates above 500 °C, the √3 Ir phase grows with dendritic morphology, and substrate step bunches act as barriers to √3 Ir growth. LEEM images showed Stranski–Krastanov growth for 650–820 °C: after the √3 phase covers the surface, corresponding to 2 monolayers (ML) Ir coverage, multilayer hexagonal-shaped Ir islands form, surrounded by regions of IrGe alloy. Hexagonal-shaped Ir islands also formed upon heating 1.2 ML of √3 Ir beyond 830 °C, which resulted in the elimination of √3 structure from the surface. The transformation from √3 to (1 × 1) structure upon heating to 830 °C was an irreversible surface phase transition. Annealing > 2.0 ML of Ir in the √3 phase above the 830 °C disorder temperature, followed by cooling, produced a (3 × 1) structure. Subsequent heating and cooling through 830 °C give evidence for a reversible (3 × 1) to (1 × 1) phase transition.

A review is presented that covers the experimental and theoretical literature relating to the preparation, electronic structure and chemical and physical properties of the surfaces of the wurtzite form of GaN. The discussion includes the adsorption of various chemical elements and of inorganic, organometallic and organic species. The focus is on work that contributes to a microscopic, atomistic understanding of GaN surfaces and interfaces, and the review concludes with an assessment of possible future directions.

This paper describes a newly designed set-up which is intended for automated preparation both for STM tips and for ultra-sharp field emission cathodes made of polycrystalline and single crystal tungsten in laboratory conditions. The newly designed set-up incorporates electrochemical etching of a wire in the surface layer of an electrolyte and also the so called drop-off method which consists of two steps and requires more precise wire and current setting. Additionally, the method was extended for polycrystalline wires of tungsten using a special cut-off algorithm, which deals with variable etching speed for various crystallographic orientation of the current grain. Produced tips are examined using scanning low energy electron microscopy as for the surface and by the occurrence of electron field-emission as for the geometry based on the Fowler-Nordheim analysis. STM performance of the produced tip is discussed and examined as well.

Nucleation, growth and coalescence of 3D Au-Ge droplets on Ge(1 1 1) surface have been studied in real time by low energy electron microscopy. The surface diffusion energy barrier of Au on Ge(1 1 1) is estimated as 1.8 ± 0.2 eV from the dependence of the number of droplets per surface unit with temperature and Au deposition rate. At long time scale it is also shown that the droplets have a diffusive motion that results in dynamic coalescence events. From the decrease of the droplet number, the size-dependence of the diffusion coefficient of the droplets is shown to decay as R - 1.9 ± 0.5 where R is the droplet radius. This result indicates that the spontaneous motion of the droplets is controlled by an interfacial mass-transport phenomenon.

Transport experiments in twisted bilayer graphene have revealed multiple superconducting domes separated by correlated insulating states1–5. These properties are generally associated with strongly correlated states in a flat mini-band of the hexagonal moire superlattice as was predicted by band structure calculations6–8. Evidence for the existence of a flat band comes from local tunnelling spectroscopy9–13 and electronic compressibility measurements14, which report two or more sharp peaks in the density of states that may be associated with closely spaced Van Hove singularities. However, direct momentum-resolved measurements have proved to be challenging15. Here, we combine different imaging techniques and angle-resolved photoemission with simultaneous real- and momentum-space resolution (nano-ARPES) to directly map the band dispersion in twisted bilayer graphene devices near charge neutrality. Our experiments reveal large areas with a homogeneous twist angle that support a flat band with a spectral weight that is highly localized in momentum space. The flat band is separated from the dispersive Dirac bands, which show multiple moire hybridization gaps. These data establish the salient features of the twisted bilayer graphene band structure. Spectroscopic measurements using nano-ARPES on twisted bilayer graphene directly highlight the presence of the flat bands.

PACS 75.50.Gg · 76.80.+y · 75.70.i · 71.30.+h .-- This article is part of the Topical Collection on Proceedings of the 16th Latin American Conference on the Applications of the M¨ossbauer Effect (LACAME 2018), 18–23 November 2018, Santiago de Chile, Chile, Magnetite, a semiconducting ferrimagnetic iron spinel with a metal-insulator phase transition, the Verwey transition, has long been the subject of Mössbauer spectroscopy studies, which continue today. We review the current status of the understanding of the Mössbauer spectra of magnetite. Furthermore, magnetite is a very attractive material in current topics such as spintronics. In this particular subject, to determine the behavior of magnetic domains is paramount, and the changes ocurring on the near surface region upon undergoing the Verwey transition are relevant. In order to advance in this area, we have incorporated some new techniques, namely microscopy observations made with low-energy electrons. These observations can be performed upon changing the temperature, and can provide magnetic contrast through the use of spin-polarized electrons. By this means, we have observed the ferroelastic transformation associated with the Verwey transition, discovered an order-disorder transition of the (001) surface of magnetite and observed the changes in the magnetic domains on the same surface by changing the temperature. Low-energy electrons also are the key to the Mössbauer experiments of magnetite films and surfaces, with the promise of providing surface-sensitive spatially resolved Mössbauer spectra., This research was partially supported by the Spanish Ministry of Economy and Competitiveness No. MAT2015-64110-C2-1-P, the Comunidad Autónoma de Madrid NANOMAGCOST-CM project with Ref. P2018/NMT-4321, the European Commission through Project H2020 No. 720853 (Amphibian) and the Chilean FONDECYT Project No. 1161117.

In contrast to the in-plane transport electron mean-free path in graphene, the transverse mean-free path has received little attention and is often assumed to follow the 'universal' mean-free path (MFP) curve broadly adopted in surface and interface science. Here we directly measure transverse electron scattering through graphene from 0 to 25 eV above the vacuum level both in reflection using Low Energy Electron Microscopy and in transmission using electron-Volt Transmission Electron Microscopy. From this data, we obtain quantitative MFPs for both elastic and inelastic scattering. Even at the lowest energies, the total MFP is just a few graphene layers and the elastic MFP oscillates with graphene layer number, both refuting the 'universal' curve. A full theoretical calculation taking the graphene band structure into consideration agrees well with experiment, while the key experimental results are reproduced even by a simple optical toy model.

International audience; Nanostructures on surfaces can be displaced by applying an electric eld or electric currents through a material. This induced mass transport is referred to as electromigration. In this article we show that the anisotropy of diusion may control the direction of motion of electromigrating nanostructures. For this purpose we study in situ and in real time, by Low Energy Electron Microscopy, the motion of 2D one-atom thick islands or one-atom deep holes on a highly anisotropic surface (reconstructed Si(100)). Based on experiments and Kinetic Monte Carlo simulations, we propose a simple analytical model that explains most of the observations. In particular, the direction of motion of the nanostructures depends on the diusion anisotropy and does not necessarily coincide with the electric eld direction. This work opens a way for the manipulation of 2D nanostructures by means of an electric eld on anisotropic surfaces.

A novel method to fabricate uniform epitaxial graphene on C-face SiC substrates was investigated. Graphene was grown on the C-face 6H-SiC substrates with a sputtered SiC film by annealing temperatures ranging from 1400 to 1900 °C under an Ar ambient. The fractional area of the graphene having the layer number of two was about 95% in a 75×75 μm square by a Raman mapping and a low energy electron microscopy. Graphene on the C-face SiC fabricated by this method is quite uniform compared to that made by a conventional method without the sputtered SiC films and is thus suitable for high frequency analog devices.

The structural and electronic properties of Pt induced nanowires on Ge(110) surfaces have been studied by scanning tunneling microscopy and low energy electron microscopy. The deposition of a sub-monolayer amount of Pt and subsequent annealing at 1100 (±30) K results into nanowires which are aligned along the densely packed [1–10] direction of the Ge(110) surface. With increasing Pt coverage the nanowires form densely packed arrays with separations of 1.1 ± 0.1 nm, 2.0 ± 0.1 nm and 3.4 ± 0.1 nm. Ge pentagons reside in the troughs for nanowire separations of 3.4 nm, however for smaller nanowire separations no pentagons are found. Spatially resolved scanning tunneling spectroscopy measurements reveal a filled electronic state at −0.35 eV. This electronic state is present in the troughs as well as on the nanowires. The −0.35 eV state has the strongest intensity on the pentagons. For Pt depositions exceeding two monolayers, pentagon free nanowire patches are found, that coexist with Pt/Ge clusters. Upon annealing at 1040 K these Pt/Ge clusters become liquid-like, indicating that we are dealing with eutectic Pt0.22Ge0.78 clusters. Low energy electron microscopy videos reveal the formation and spinodal decomposition of these eutectic Pt/Ge clusters.

Graphene and related two-dimensional (2D) materials are attracting huge attention due to their wide-range potential applications. Because large-scale, high-quality 2D crystals are prerequisites for many of the applications, crystal growth of 2D materials has been intensively studied. We have also been conducting research to understand the growth mechanism of 2D materials and have been developing growth techniques of high-quality materials based on the understandings, in which detailed structural characterizations using low-energy electron microscopy (LEEM) have played essential roles. In this paper, we explain the principles of obtaining various structural features using LEEM, and then we review the status of our current understanding on the growth of graphene and hexagonal boron nitride.

The surface diffusion energy of Au on the 3 × 3 Au–Si(111) reconstructed surface is determined from low energy electron microscopy experiments. We have used two methods, one based on the nucleation of Au particles, the other one on the island growth by Ostwald ripening. The two methods give Ed = 1.10 ± 0.34 eV and Ed = 1.56 ± 0.31 eV respectively and with a weighted mean we obtain Ed = 1.3 ± 0.2 eV. We suggest that this high activation energy could be due to a mechanism of diffusion by exchange.

A monolayer of graphene was prepared by thermal decomposition of ethylene gas on Pt(111). The graphene can be readily removed by dosing O2 at pressures in 10− 8 mbar range and surface temperatures (Ts) near 1000 K. Residual gas analysis during the oxygen treatment of graphene layer detected CO to be the only formed product. The oxidation process has been continuously imaged by Low-energy Electron Microscope (LEEM) operated in mirror-electron mode. LEEM observations revealed that the oxidation of graphene on Pt(111) occurs simultaneously at the outer island perimeter and in the interior of the graphene island. Symmetric hexagonal pits were observed to form continuously within graphene sheets, the pits proceeded isotropically. The etch rate was determined to be equal for both modes and independent of the surface environment with the exception of areas above Pt step edges. The pit growth rate at constant oxygen pressure was found to increase exponentially with respect to temperature over the investigated Ts range of 927–1014 K, yielding an apparent activation energy of 479 kJ/mol.

Spin Polarized Low Energy Electron Microscopy (SPLEEM) is a powerful tool to reveal the magnetic structure of ferromagnetic surfaces on the atomic depth scale level[1-3]. With aberration corrected LEEM and a high brightness spin polarized electron gun, high spatial resolution will provide more details for ultra-thin ferromagnetic film studies. This study reports the first realization of aberration corrected SPLEEM (AC-SPLEEM). The performance of the setup was tested on ferromagnetic Fe nanoscale islands on a W(110) single crystal, with spatial resolution of 3.3 nm in spin asymmetry images.

Hexagonal boron nitride, often entitled the ‘white graphene’ because of its large band gap, is one of the most important two-dimensional (2D) materials and frequently investigated in context with stacked arrays of single 2D layers, so called van der Waals heterostructures. Here, we concentrate on the growth of hBN on the coinage metal surface Cu(1 1 1). Using low energy electron microscopy and diffraction, we investigate the self-terminated growth of the first layer in situ and in real time. Most prominently, we find dendritic structures with three strongly preferred growth branches that are mostly well aligned with the Cu(1 1 1) substrate and exhibit a three-fold symmetric shape. The observation of dendritic structures is very surprising since hBN was found to grow in compact, triangular-shaped islands on many other metal substrates, in particular, on transition metal surfaces where it shows a much stronger interaction to the surface. We explain the unexpected dendritic growth by an asymmetry of the bonding energy for the two possible ways a borazine molecule can attach to an existing hBN island, namely either with one of its boron or one of its nitrogen atoms. We suggest that this asymmetry originates from different dehydrogenation states of the adsorbed borazine molecules and the hBN islands. We call this mechanism ‘Dehydrogenation Limited Aggregation’ since it is generic in the sense that it is merely based on different dehydrogenation energies for the involved building blocks forming the 2D layer.

Important applications of spin polarized low energy electron microscopy (SPLEEM) employ this technique's vector imaging capability to resolve domain wall (DW) spin textures. Studying several thin film systems including Co/W(110), Co/Cu(001) and (Co/Ni)n/W(110), we show that an additional contrast can appear at magnetic DWs. By imaging the magnetization as a function of electron landing energy, electron energies are selected at which the magnetic domain contrast vanishes. Surprisingly, under such conditions of zero contrast between magnetic domains, we observe the appearance of magnetic contrast outlining the DWs. This DW contrast does not depend on the DW spin texture. Instead, our measurements show that this DW contrast results from a combination of the energy-dependence of the spin reflectivity asymmetry of the magnetic film, the finite energy width of the spin polarized electron source, and the dispersion of the magnetic prism array that separates the illumination and imaging columns of the instrument. Awareness of this DW contrast mechanism is useful to aid correct interpretation of SPLEEM images.

The two-dimensional melting/solidification transition of the high density [2012] phase of Bi on Cu(111) has been studied by means of low energy electron microscopy (LEEM). This well defined phase has an ideal concentration of one Bi atom per two Cu surface atoms (θ Bi = 0.500). The Bi density is determined accurately in situ and the highest melting temperature of 538 K occurs at exactly θ Bi = 0.500. A significantly reduced melting temperature is observed for lower Bi densities (θ Bi < 0.500) and, surprisingly, also for θ Bi > 0.500. At |Δ θ Bi| = 0.015 the melting temperature is reduced by about 20 K. This lowering of the melting temperature is attributed to a critical vacancy density at melting and we propose that this quantity triggers the 2D solid-liquid phase transition. For this particular system, the critical vacancy fraction for melting amounts to 5%-6%. Above θ Bi = 0.500 and near melting a homogeneous, unilaterally compressed phase, '[2012]' is observed, with a density that increases continuously with coverage. It is commensurate along and incommensurate along The ability to distinguish between Bi accommodated within the '[2012]' phase and Bi residing on top as a lattice gas by applying LEEM is of crucial importance for the analysis.

The properties of Van der Waals (VdW) heterostructures are determined by the twist angle and the interface between adjacent layers as well as their polytype and stacking. Here, the use of spectroscopic low energy electron microscopy (LEEM) and micro low energy electron diffraction (µLEED) methods to measure these properties locally is described. The authors present results on a MoS2/hBN heterostructure, but the methods are applicable to other materials. Diffraction spot analysis is used to assess the benefits of using hBN as a substrate. In addition, by making use of the broken rotational symmetry of the lattice, the cleaving history of the MoS2 flake is determined, that is, which layer stems from where in the bulk.

International audience; We have used low energy electron microscopy (LEEM) to study the evolution of the surface structure of BaTiO3(001) during the ferroelectric-paraelectric phase transition (120 °C, P4mm-Pm3m). Transient surface structures appear under heating at temperatures slightly above TC. Intersections between polar domain walls and the sample surface persist in the surface layer at 126.0 °C while the bulk has already transformed into the cubic phase. The wall signals are criss-crossed by a second set of stripe patterns with roughly perpendicular orientation at 126.3 °C. These surface patterns coarsen under further heating to 126.9 °C. The LEEM image is dominated in the ferroelectric state by intersections of ferroelastic/ferroelectric 90° walls and the surface. The intersection lines are charged.

We review the results of our achievements on thin film processes in organic materials obtained with the use of low energy electron microscopy (LEEM) and photoelectron emission microscopy (PEEM). We address the growth and the locally resolved electronic structure of an organic semiconductor, diindenoperylene, deposited on Au(1 0 0) and Au(1 1 1) single crystals, taking into account also the lattice geometry of the substrates. In addition, we present the results that we have obtained on a metal-free organic magnet, a derivative of the nitronyl nitroxide, grown on Si(1 1 1) surfaces. By taking great advantage of the LEEM and PEEM capabilities, our work has shown that it is possible to deeply understand which parameters influence the XPS spectroscopic lines in phenomena such as photoemission in organic materials. We have also elucidated with the simultaneous investigation of morphology, photoemission and absorption, the existing interplay between morphology, structure and electronic properties. The fact that the different modes of a LEEM–PEEM can be used to comprehensively investigate thin films in situ gives further relevance to the use of these microscopies, when describing fundamental chemical and physical phenomena at the interface and surface of materials.

A well-ordered surface oxide grown on Fe(110) has been studied using scanning tunneling microscopy (STM), low energy electron diffraction, low energy electron microscopy, and core level photoelectr ...

article i nfo By Low Energy Electron Microscopy, we show that the spontaneous motion of gold droplets on silicon (111) is chemically driven: the droplets tend to dissolve silicon monoatomic steps to reach the temperature-dependent Au-Si equilibrium stoichiometry. According to the droplet size, the motion details are different. In the first stages of Au deposition small droplets nucleate at steps and move continuously on single terraces. The droplets tempo- rarily pin at each step they meet during their motion. During pinning, the growing droplets become supersatu- rated in Au. They depin from the steps when a notch nucleate on the upper step. Then the droplets climb up and locally dissolve the Si steps, leaving behind them deep tracks formed by notched steps. Measurements of the dissolution rate and the displacement lengths enable us to describe quantitatively the motion mechanism, also in terms of anisotropy of Si dissolution kinetics. Scaling laws for the droplet position as a function of time are proposed: x ∝ t n

MnAs epitaxial thin films on GaAs(001) single crystalline substrates crystallize at room temperature (RT) in a mixture of two crystalline phases with distinct magnetic properties, organized as stripes along the MnAs [0001] direction. This particular morphology is driven by anisotropic epitaxial strain. We elucidate here the physical mechanisms at the origin of size reduction effect on the MnAs crystalline phase transition. We investigated the structural and magnetic changes in MnAs patterned microstructures (confined geometry) when the lateral dimension is reduced to values close to the periodicity and width of the stripes observed in continuous films. The effects of the microstructure’s lateral size, shape and orientation (with respect to the MnAs $$\mathrm{[11}\bar{2}\mathrm{0]}$$ [11 2 ¯ 0] direction) were characterized by local probe synchrotron X-ray diffraction (μ-XRD) using a focused X-ray beam, X-ray Magnetic Circular Dichroïsm - Photo Emission Electron Microscopy (XMCD-PEEM) and Low Energy Electron Microscopy (LEEM). Changes in the transition temperature and the crystalline phase distribution inside the microstructures are evidenced and quantitatively measured. The effect of finite size and strain relaxation on the magnetic domain structure is also discussed. Counter-intuitively, we demonstrate here that below a critical microstructure size, bulk MnAs structural and magnetic properties are restored. To support our observations we developed, tested and validated a model based on the size-dependence of the elastic energy and strain relaxation to explain this phase re-distribution in laterally confined geometry.

The positions of the low energy electron diffraction (LEED) spots from ferroelectric single crystal films depend on its polarization state, due to electric fields generated outside of the sample. One may derive the surface potential energy, yielding the depth where the mobile charge carriers compensating the depolarization field are located ($\ensuremath{\delta}$). On ferroelectric $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$(001) samples, surface potential energies are between 6.7 and 10.6 eV, and $\ensuremath{\delta}$ values are unusually low, in the range of $1.8\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{0.16em}{0ex}}\AA{}$. When $\ensuremath{\delta}$ is introduced in the values of the band bending inside the ferroelectric, a considerably lower value of the dielectric constant and/or of the polarization near the surface than their bulk values is obtained, evidencing either that the intrinsic `dielectric constant' of the material has this lower value or the existence of a `dead layer' at the free surface of clean ferroelectric films. The inwards polarization of these films is explained in the framework of the present considerations by the formation of an electron sheet on the surface. Possible explanations are suggested for discrepancies between the values found for surface potential energies from LEED experiments and those derived from the transition between mirror electron microscopy and low energy electron microscopy.

The lack of large-area synthesis processes on substrates compatible with industry requirements has been one of the major hurdles facing the integration of 2D materials in mainstream technologies. This is particularly the case for the recently discovered monoelemental group V 2D materials which can only be produced by exfoliation or growth on exotic substrates. Herein, to overcome this limitation, we demonstrate a scalable method to synthesize antimonene on germanium substrates using solid-source molecular beam epitaxy. This emerging 2D material has been attracting a great deal of attention due to its high environmental stability and its outstanding optical and electronic properties. In situ low energy electron microscopy allowed the real time investigation and optimization of the 2D growth. Theoretical calculations combined with atomic-scale microscopic and spectroscopic measurements demonstrated that the grown antimonene sheets are of high crystalline quality, interact weakly with germanium, exhibit semimetallic characteristics, and remain stable under ambient conditions. This achievement paves the way for the integration of antimonene in innovative nanoscale and quantum technologies compatible with the current semiconductor manufacturing.

Controllable synthesis of large domain, high-quality monolayer MoS2 is the basic premise both for exploring some fundamental physical issues, and for engineering its applications in nanoelectronics, optoelectronics, etc. Herein, by introducing H-2 as carrier gas, the successful synthesis of large domain monolayer MoS2 triangular flakes on Au foils, with the edge length approaching to 80 mm is reported. The growth process is proposed to be mediated by two competitive effects with H-2 acting as both a reduction promoter for efficient sulfurization of MoO3 and an etching reagent of resulting MoS2 flakes. By using low-energy electron microscopy/diffraction, the crystal orientations and domain boundaries of MoS2 flakes directly on Au foils for the first time are further identified. These on-site and transfer-free characterizations should shed light on the initial growth and the aggregation of MoS2 on arbitrary substrates, further guiding the growth toward large domain flakes or monolayer films.

Growth processes of Au on Ni (111) surface at various substrate temperatures between 150 °C and 400 °C were investigated with low energy electron microscopy (LEEM). Au–Ni surface alloy forms in the beginning of growth and the following de-alloying results in the formation of the Au overlayer above 0.3 ML. The growth modes after that depend on the substrate temperature. Above about 300 °C, Au grows with layer-by-layer fashion up to 2 ML and the multilayer growth follows. On the contrary, the multilayer growth takes place from the initial stage below about 300 °C. Areas with the different epitaxial orientations of Au(111) film, Au 111 11 2 ¯ / / Ni 111 11 2 ¯ and Au 111 1 ¯ 1 ¯ 2 / / Ni 111 11 2 ¯ , were separately imaged using dark-field LEEM. But the clear correlation between the area distribution with different epitaxial orientation and surface morphology was not recognized on the terrace.

We synthesized twisted bilayer graphene on single crystalline Ni(111) thin films to analyze the statistical twist angle distribution on a large scale. The twisted bilayer graphene was formed by combining two growth methods, namely the catalytic surface reaction of hydrocarbons and carbon segregation from Ni. Low energy electron diffraction (LEED) investigations directly revealed dominant twist angles of 13 degrees, 22 degrees, 38 degrees, and 47 degrees. We show that the angle distribution is closely related to the sizes of Moire superlattices which form at commensurate rotation angles. In addition to the commensurate angles, quasi-periodic Moire structures were also formed in the vicinity of the dominant angles, confirmed by microscopic observations with low energy electron microscopy and scanning tunneling microscopy (STM). The quasi-periodic Moire patterns are presumably caused by insufficient mobility of carbon atoms during the segregation growth while cooling. Micro-LEED studies reveal that the size of single twisted domains is below 400 nm. Atomic-scale characterization by STM indicates that the twisted layer grown by segregation is located underneath the layer grown by surface reaction, i.e. between the Ni surface and the top single-crystal graphene layer. (C) 2014 Elsevier B.V. All rights reserved.

The ionic component of the strong bond in hexagonal boron nitride (hBN) has been grossly disregarded in literature. Precisely this quantity is demonstrated to govern the shape of monolayer hBN islands grown at high temperatures. HBN zigzag edges are charged and energetically less favorable than the neutral armchair edges, in contrast to those of the purely covalent graphene. Nucleation of hBN islands occurs exclusively on either the inner or the outer corners of substrate steps. Taking into account the charge at edges of hBN islands offers a powerful framework to understand the nucleation of the islands and their orientation with respect the founding steps, as well as various equilibrium shapes, including prominently a right-angled trapezoid. BN dimers are identified as basic building blocks for hBN. A surprisingly strong interaction between hBN and the pre-existing steps on the moderately reactive Ir(1 1 1) substrate is uncovered. Localized charges are probably relevant for all 2D-materials lacking inversion symmetry.

Development of surface morphology in Si molecular beam epitaxy on mesa-structured Si(111) at temperatures around the (7 × 7)-“1 × 1” surface phase transition was studied by atomic force microscopy. Significant changes in surface morphology were found for a small increase in temperature near the surface phase transition, accompanied by a strong increase in step-free area dimension. This behavior is reported for the first time for epitaxial silicon growth, but supports earlier in situ studies of the thermal decay of 2-dimensional islands and voids on Si(111) close to the surface phase transition using low energy electron microscopy by Hibino et al . (Phys. Rev. B 63, 245402, 2001). The observed changes in surface morphology clearly demonstrate the interplay of crystal growth and the surface phase transformation. In particular, the simultaneous appearance of two surface phases under certain conditions and their specific influence on the growth behavior are discussed with regard to this matter.

Both Ni and Pb intercalation reactions at graphene/Ru(0001) interface were studied by low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM). It is suggested that the Ni intercalation is dominated by an exchange intercalation mechanism, in which Ni adatoms produce transient atomic-scale defects in the graphene lattice and penetrate through the carbon monolayer. In contrast, the Pb intercalation process needs to be facilitated by the diffusion of Pb atoms through extended defect sites of graphene, such as open edges and domain boundaries. The two contrast intercalation mechanisms originate from the different interaction strength of the intercalated elements with carbon. Different responses of the graphene electronic structure to the Ni and Pb intercalation reactions were observed by PEEM.

Nano research 6(8), 562 - 570 (2013). doi:10.1007/s12274-013-0331-9, The atomic and electronic structure of graphene synthesized on commercially available cubic-SiC(001)/Si(001) wafers have been studied by low energy electron microscopy (LEEM), scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and angle resolved photoelectron spectroscopy (ARPES). LEEM and STM data prove the wafer-scale continuity and uniform thickness of the graphene overlayer on SiC(001). LEEM, STM and ARPES studies reveal that the graphene overlayer on SiC(001) consists of only a few monolayers with physical properties of quasi-freestanding graphene. Atomically resolved STM and micro-LEED data show that the top graphene layer consists of nanometersized domains with four different lattice orientations connected through the 〈110〉-directed boundaries. ARPES studies reveal the typical electron spectrum of graphene with the Dirac points close to the Fermi level. Thus, the use of technologically relevant SiC(001)/Si(001) wafers for graphene fabrication represents a realistic way of bridging the gap between the outstanding properties of graphene and their applications., Published by Tsinghua Press, [S.l.]

Progress in materials science is inseparably connected with development of new analytical methods which make possible to observe the materials microstructure with high sensitivity. The aim of the present study is shown that scanning low energy electron microscopy (SLEEM) has a significant impact in advance of a fundamental understanding of the evolution of microstructure upon semi-solid processing. This paper deals with the application of the ultra high vacuum scanning low energy electron microscopy (UHV SLEEM) to the study of microstructure of X210Cr12 steel after the formation in semi-solid state and the study of the annealing of deformed metastable austenite. Examples from these specimens show that the contrast between differently oriented grains in polycrystalline materials is very sensitive to the parameters such as energy of the primary beam, working distance and detection of high angle backscattered electrons.

Low-energy electron microscopy (LEEM) has been used to study the structure, initial growth orientation, growth progression, and the number of layers of atomically thin hexagonal boron nitride (h-BN) films. The h-BN films are grown on heteroepitaxial Co using chemical vapor deposition (CVD) at low pressure. Our findings from LEEM studies include the growth of monolayer film having two, oppositely oriented, triangular BN domains commensurate with the Co lattice. The growth of h-BN appears to be self-limiting at a monolayer, with thicker domains only appearing in patches, presumably initiated between domain boundaries. Reflectivity measurements of the thicker h-BN films show oscillations resulting from the resonant electron transmission through quantized electronic states of the h-BN films, with the number of minima scaling up with the number of h-BN layers. First principles density functional theory (DFT) calculations show that the positions of oscillations are related to the electronic band structure of h-BN. Open image in new window