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The initial stages of room temperature growth of Pb overlayers on commercial Si(100) p (2 × 1) surface have been investigated using low-energy electron diffraction and low-energy electron microscopy (LEEM) techniques. A well-ordered reconstructed Si(100) p (10 × 2) surface phase has been observed for 0.5 monolayers of Pb deposition and is found to vanish for higher Pb coverages. We do not observe any island formation in our LEEM studies during the early stages of growth, unlike earlier studies on low-miscut substrates. Our dark-field LEEM experiments suggest the observed high step density with low terrace widths is responsible for this behaviour. [ABSTRACT FROM AUTHOR]

Low‐energy electron microscopy (LEEM) taken as intensity–voltage (I–V) curves provides hyperspectral images of surfaces, which can be used to identify the surface type, but are difficult to analyse. Here, we demonstrate the use of an algorithm for factorizing the data into spectra and concentrations of characteristic components (FSC3) for identifying distinct physical surface phases. Importantly, FSC3 is an unsupervised and fast algorithm. As example data we use experiments on the growth of praseodymium oxide or ruthenium oxide on ruthenium single crystal substrates, both featuring a complex distribution of coexisting surface components, varying in both chemical composition and crystallographic structure. With the factorization result a sparse sampling method is demonstrated, reducing the measurement time by 1–2 orders of magnitude, relevant for dynamic surface studies. The FSC3 concentrations are providing the features for a support vector machine‐based supervised classification of the surface types. Here, specific surface regions which have been identified structurally, via their diffraction pattern, as well as chemically by complementary spectro‐microscopic techniques, are used as training sets. A reliable classification is demonstrated on both example LEEM I–V data sets. Lay description: Low‐energy electron microscopy (LEEM) is a powerful experimental method to image surfaces, thin films and nanoparticles. An incident beam of low energy electrons (<50eV) is reflected from the surface and used to create an image of the investigated sample. The structure of the first few atomic layers of the investigated sample is encoded in the energy dependent electron reflectivity of the surface, so called intensity versus electron energy, or in short LEEM I‐V, spectra, which however are difficult and time‐consuming to interpret. In this paper we present a factorization method to describe the LEEM I‐V hyperspectral data as a combination of characteristic components which are defined by their concentrations and spectra. Using the concentration maps, we demonstrate a supervised classification method which provides a fast and reliable classification of surface reconstructions, as shown on two examples, ruthenium oxide (RuO2), and praseodymium oxide (PrOx). For PrOx, the factorization and classification reveals that the surface consists of a flat substrate with bands of coalesced oxide islands which nucleated at the atomic step edges of the Ru(0001) substrate. The PrOx regions comprise a complex substructure of five distinguishable phases. For RuO2, the method reveals the different types of islands that exist in the rich RuO2/Ru system, where different RuO2 orientations characteristic of the Ru oxidation can be separated by their I‐V spectra. Furthermore, using the extracted component spectra and the classification of the concentrations, demonstrate a sparse sampling method to reduce the number of acquired spectral points required for classification. A reduction of the acquisition time by a factor of 30 per classification is achieved for the example data. [ABSTRACT FROM AUTHOR]

This paper aims to elucidate the effect of an air‐formed native oxide covering mild steel surface on the contrast in the scanning electron microscopy (SEM) images obtained with the landing energy from 5 keV down to 0 eV. Part of the mild steel surface was in‐situ cleaned by Ar+ ion sputtering process in order to remove native oxide from the surface. It enabled us to observe the oxide‐free and the naturally oxidized area on the mild steel surface simultaneously in the SEM micrographs. Presence of the native oxide starts to play a role in the SEM images acquired at landing energy below roughly 3 keV. Contrast between differently oriented grains situated inside the area covered by the native oxide starts to be negligible with landing energy decreasing below 3 keV, up to some ultra‐low values where the contrast increases again. Total reflectivity contrast between the clean and the oxidized area increases exponentially with landing energy decreasing below 3 keV. The reflectivity‐versus‐energy curves of the cleaned and the naturally oxidized mild steel surface are markedly different. The reflectivity of the electrons is correlated with the density of states (DOS), as is demonstrated at very low landing energies. Sensitivity of the very low‐energy electrons to the electronic structure was verified by comparison of the experimental data with the simulations of reflectivities, band structure, and DOS. The theoretical predictions are based on the density‐functional theory calculations and they have been performed in energy range corresponding to specular reflectivities of the Fe‐BCC (001) orientation. We have also observed that close to the mirror condition, that is, near‐zero landing energies, the primary electrons become sensitive to the surface potential differences caused by the work function differences of clean and native oxide‐covered steel surfaces. [ABSTRACT FROM AUTHOR]

Intercalation and de-intercalation processes below graphene (g) grown by chemical vapor deposition (CVD) on Cu foils (g/Cu) were investigated in a combined low-energy electron microscopy and X-ray photoemission electron microscopy study. Exposure of g/Cu to air induces oxygen and water intercalation which can be removed by annealing in vacuum leading to clean and well-ordered graphene on Cu. However, prolonged air exposure leads to intercalation of large amounts of oxygen, most likely inducing the formation of copper oxides. If sufficient intercalated oxygen remains at the interface when exceeding 320 °C, graphene is oxidized and burned off. Cu foils can be loaded with silicon on purpose during foil pre-treatment or accidently during long growth time when applying high temperatures at elevated H 2 pressure inducing the reactive removal of Si species from the quartz reactor wall. Due to the dissolution of Si in the Cu bulk, the Si surface concentration remains below detection limit and graphene of equal crystalline quality is grown as if the Cu foil was silicon-free. However, oxygen intercalation underneath graphene on Si-containing Cu foils can induce Si segregation towards the surface and formation of intercalated silica without attacking the covering graphene. Even at high temperatures, segregating Si acts as oxygen scavenger so that graphene resists oxidation. The observed effect explains the usefulness of certain synthesis protocols and paves the way towards large-scale fabrication of electronically decoupled graphene. The effect can be used to immobilize adsorbing oxygen at the interface and image the initial steps of intercalation below graphene in-situ. [Display omitted] • Exposure of graphene (g) on Cu to air leads to intercalation of oxygen and water. • Annealing of graphene leads to de-intercalation or oxidative graphene removal. • Cu foils are loaded with Si by released material from the quartz reactor wall. • Segregating silicon pins intercalated oxygen as SiO x to the g/Cu interface. • Si-loaded Cu foils act as oxygen scavenger protecting and decoupling graphene. [ABSTRACT FROM AUTHOR]

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.

The crystallinity of graphene flakes and their orientation with respect to the Cu(111) substrate are investigated by means of low‐energy electron microscopy (LEEM). The interplay between graphene and the metal substrate during chemical vapor deposition (CVD) introduces a restructuring of the metal surface into surface facets, which undergo a step bunching process during the growth of additional layers. Moreover, the surface facets introduce strain between the successively nucleated layers that follow the topography in a carpet‐like fashion. The strain leads to dislocations in between domains of relaxed Bernal stacking. After the transfer onto an epitaxial buffer layer, the imprinted rippled structure of even monolayer graphene as well as the stacking dislocations are preserved. A similar behavior might also be expected for other CVD grown 2D materials such as hexagonal boron nitride or transition metal dichalcogenides, where stacking relations after transfer on a target substrate or heterostructure could become important in future experiments. [ABSTRACT FROM AUTHOR]

Crystal structure and composition are inter-dependent and decoupling their effects on surface reactivity is challenging. Using low-energy electron microscopy to spatially and temporally resolve the oxide film growth during the oxidation of NiAl(100), we differentiate such coupled effects by monitoring oxide growth while simultaneously fine-tuning the surface structure and composition during oxidation. We demonstrate that the oxidation of chemically ordered surfaces results in compact oxide island growth whereas non-compact oxide growth during the surface phase transition. By incorporating the surface phase transition induced chemical disordering into kinetic Monte Carlo simulations, we show that the non-compact oxide growth is induced by the composition effect on the surface diffusion of oxygen, which can be described by the concept of "ant in the labyrinth". Image, graphical abstract [ABSTRACT FROM AUTHOR]

The capability of controlling the motion of nanoobjects on a surface would open perspectives in nanofabrication. Here, we show the proof of concept that the displacement of a 2D nanostructure on a surface can be controlled by means of two perpendicular electric fields. With a specifically designed sample holder, we displace a 2D negative island on Si(001) along a close loop, in a low-energy electron microscope. Our technique could be applied to other systems to assemble nanomaterials. [ABSTRACT FROM AUTHOR]

The interface between a metal electrode and an organic semiconductor (OS) layer has a defining role in the properties of the resulting device. To obtain the desired performance, interlayers are introduced to modify the adhesion and growth of OS and enhance the efficiency of charge transport through the interface. However, the employed interlayers face common challenges, including a lack of electric dipoles to tune the mutual position of energy levels, being too thick for efficient electronic transport, or being prone to intermixing with subsequently deposited OS layers. Here, we show that monolayers of 1,3,5-tris(4-carboxyphenyl)benzene (BTB) with fully deprotonated carboxyl groups on silver substrates form a compact layer resistant to intermixing while capable of mediating energy-level alignment and showing a large insensitivity to substrate termination. Employing a combination of surface-sensitive techniques, i.e., low-energy electron microscopy and diffraction, X-ray photoelectron spectroscopy, and scanning tunneling microscopy, we have comprehensively characterized the compact layer and proven its robustness against mixing with the subsequently deposited organic semiconductor layer. Density functional theory calculations show that the robustness arises from a strong interaction of carboxylate groups with the Ag surface, and thus, the BTB in the first layer is energetically favored. Synchrotron radiation photoelectron spectroscopy shows that this layer displays considerable electrical dipoles that can be utilized for work function engineering and electronic alignment of molecular frontier orbitals with respect to the substrate Fermi level. Our work thus provides a widely applicable molecular interlayer and general insights necessary for engineering of charge injection layers for efficient organic electronics.

• Low-energy electron microscopy images are distorted at work function discontinuities. • Ray-tracing simulations reveal the size of those work-function-induced artifacts. • They cause standard methods to greatly overestimate work function differences. • Combining simulations with experimental data yields a more robust measure. For many applications, it is important to measure the local work function of a surface with high lateral resolution. Low-energy electron microscopy is regularly employed to this end since it is, in principle, very well suited as it combines high-resolution imaging with high sensitivity to local electrostatic potentials. For surfaces with areas of different work function, however, lateral electrostatic fields inevitably associated with work function discontinuities deflect the low-energy electrons and thereby cause artifacts near these discontinuities. We use ray-tracing simulations to show that these artifacts extend over hundreds of nanometers and cause an overestimation of the true work function difference near the discontinuity by a factor of 1.6 if the standard image analysis methods are used. We demonstrate on a mixed-terminated strontium titanate surface that comparing LEEM data with detailed ray-tracing simulations leads to much a more robust estimate of the work function difference. [ABSTRACT FROM AUTHOR]

• Dark-field imaging of layer stackings in molecular multilayers. • Identification of lateral molecular displacements. • Visualization of domain boundaries on mesoscale. • Conceptual diffraction model based on electron path differences. The design of metal-organic interfaces with atomic precision enables the fabrication of highly efficient devices with tailored functionality. The possibility of fast and reliable analysis of molecular stacking order at the interface is of crucial importance, as the interfacial stacking order of molecules directly influences the quality and functionality of fabricated organic-based devices. Dark-field (DF) imaging using Low-Energy Electron Microscopy (LEEM) allows the visualization of areas with a specific structure or symmetry. However, distinguishing layers with different stacking orders featuring the same diffraction patterns becomes more complicated. Here we show that the top layer shift in organic molecular bilayers induces measurable differences in spot intensities of respective diffraction patterns that can be visualized in DF images. Scanning Tunneling Microscopy (STM) imaging of molecular bilayers allowed us to measure the shift directly and compare it with the diffraction data. We also provide a conceptual diffraction model based on the electron path differences, which qualitatively explains the observed phenomenon. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref [1] . We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination. [ABSTRACT FROM AUTHOR]

Based on its electronic, structural, chemical, and mechanical properties, many potential applications have been proposed for graphene. In order to realize these visions, graphene has to be synthesized, grown, or exfoliated with properties that are determined by the targeted application. Growth of so-called epitaxial graphene on silicon carbide by sublimation of silicon in an argon atmosphere is one particular method that could potentially lead to electronic applications. In this contribution we summarize our recent work on different aspects of epitaxial graphene growth and interface manipulation by intercalation, which was performed by a combination of low-energy electron microscopy, low-energy electron diffraction, atomic force microscopy and photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref [1] . We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination. [ABSTRACT FROM AUTHOR]

Quantized magnetotransport is observed in 5.6 × 5.6 mm 2 epitaxial graphene devices, grown using highly constrained sublimation on the Si-face of SiC(0001) at high temperature (1900 °C). The precise quantized Hall resistance of R x y = h 2 e 2 is maintained up to record level of critical current I xx = 0.72 mA at T = 3.1 K and 9 T in a device where Raman microscopy reveals low and homogeneous strain. Adsorption-induced molecular doping in a second device reduced the carrier concentration close to the Dirac point ( n ≈ 10 10 cm −2 ), where mobility of 18760 cm 2 /V is measured over an area of 10 mm 2 . Atomic force, confocal optical, and Raman microscopies are used to characterize the large-scale devices, and reveal improved SiC terrace topography and the structure of the graphene layer. Our results show that the structural uniformity of epitaxial graphene produced by face-to-graphite processing contributes to millimeter-scale transport homogeneity, and will prove useful for scientific and commercial applications. [ABSTRACT FROM AUTHOR]

The thickness and interfacial geometry of hexagonal boron nitride (hBN) films grown by chemical vapor deposition on polycrystalline nickel foils is studied using low-energy electron microscopy (LEEM). The reflectivity of the electrons, measured over an energy range of 0–20 eV, reveals distinct minima and maxima. The measured data is compared with simulations based on a first-principles description of the electronic structure of the material. From this comparison, the number of hBN layers and the separation between the lowest hBN layer and the nickel surface is deduced. The coupling of interlayer states of the hBN to both image-potential and Shockley-type surface states of the nickel is discussed, and the dependence of the reflectivity spectra on the surface orientation of nickel grains is examined. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A monolayer of graphene was prepared by thermal decomposition of ethylene gas on Pt(111). The graphene can be readily removed by dosing O 2 at pressures in 10 − 8 mbar range and surface temperatures (T s ) 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 T s range of 927–1014 K, yielding an apparent activation energy of 479 kJ/mol. [ABSTRACT FROM AUTHOR]

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 H2 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 H2 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Selective enhancement of a reaction step by low-energy electrons. • Distinct self-assembled phases depending on electron energy. • A unique non-thermal self-assembled phase. • A stable intermediate state for on-surface deprotonation. Self-assembly and on-surface synthesis are vital strategies used for fabricating surface-confined 1D or 2D supramolecular nanoarchitectures with atomic precision. In many systems, the resulting structure is determined by the kinetics of the processes involved, i.e., reaction rate, on-surface diffusion, nucleation, and growth, all of which are typically governed by temperature. However, other external factors have been only scarcely harnessed to control the on-surface chemical reaction kinetics and self-assembly. Here, we show that a low-energy electron beam can be used to steer chemical reaction kinetics and induce the growth of molecular phases unattainable by thermal annealing. The electron beam provides a well-controlled means of promoting the elementary reaction step, i.e., deprotonation of carboxyl groups. The reaction rate increases with the increasing electron beam energy beyond the threshold energy of 6 eV. Our results offer the novel prospect of controlling self-assembly, enhancing the rate of reaction steps selectively, and thus altering the kinetic rate hierarchy. [ABSTRACT FROM AUTHOR]

In-plane heteroatom substitution of graphene is a promising strategy to modify its properties. The ability to dope graphene with electron-donor nitrogen heteroatoms is highly important for modulating electrical properties of graphene. Here we demonstrate a transfer-free method to directly grow large area quasi free-standing N-doped graphene bilayers on an insulating substrate (SiN). Electron-bombardment heating under nitrogen flux results in simultaneous growth of N-doped graphene and a SiN layer on the SiC surface. The decoupling of N-doped graphene from the substrate and the presence of SiN are identified by X-ray photoemission spectroscopy and low-energy electron diffraction. The substitution of nitrogen atoms in the graphene planes was confirmed using high resolution X-ray photoemission spectroscopy which reveals several atomic configurations for the nitrogen atoms: Graphitic-like, pyridine-like, and pyrroliclike. Furthermore, we demonstrated for the first time that N-doped graphene could be used to efficiently probe oxygen molecules via nitrogen atom defects. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: Three different types of multislice algorithms, namely the conventional multislice (CMS) algorithm, the propagator-corrected multislice (PCMS) algorithm and the fully-corrected multislice (FCMS) algorithm, have been evaluated in comparison with respect to the accelerating voltages in transmission electron microscopy. Detailed numerical calculations have been performed to test their validities. The results show that the three algorithms are equivalent for accelerating voltage above 100kV. However, below 100kV, the CMS algorithm will introduce significant errors, not only for higher-order Laue zone (HOLZ) reflections but also for zero-order Laue zone (ZOLZ) reflections. The differences between the PCMS and FCMS algorithms are negligible and mainly appear in HOLZ reflections. Nonetheless, when the accelerating voltage is further lowered to 20kV or below, the PCMS algorithm will also yield results deviating from the FCMS results. The present study demonstrates that the propagation of the electron wave from one slice to the next slice is actually cross-correlated with the crystal potential in a complex manner, such that when the accelerating voltage is lowered to 10kV, the accuracy of the algorithms is dependent of the scattering power of the specimen. [Copyright &y& Elsevier]

Abstract: A computational method is developed whereby the reflectivity of low-energy electrons from a surface can be obtained from a first-principles solution of the electronic structure of the system. The method is applied to multilayer graphene. Two bands of reflectivity minima are found, one at 0–8eV and the other at 14–22eV above the vacuum level. For a free-standing slab with n layers of graphene, each band contains zeroes in the reflectivity. Two additional image-potential type states form at the ends of the graphene slab, with energies just below the vacuum level, hence producing a total of 2n states. A tight-binding model is developed, with basis functions localized in the spaces between the graphene planes (and at the ends of the slab). The spectrum of states produced by the tight-binding model is found to be in good agreement with the zeros of reflectivity (i.e. transmission resonances) of the first-principles results. [Copyright &y& Elsevier]

Spin-polarized low-energy electron microscopy was used to image a magnetite crystal with (001) surface orientation. Sets of spin-dependent images of magnetic domain patterns observed in this surface were used to map the direction of the magnetization vector with high spatial and angular resolution. We find that domains are magnetized along the surface <110> directions, and domain wall structures include 90° and 180° walls. A type of unusually curved domain walls are interpreted as Néel-capped surface terminations of 180° Bloch walls. [ABSTRACT FROM AUTHOR]

Abstract: The formation and nitridation of indium (In) droplets on Si (111)7×7, with regard to In droplet epitaxy growth of InN nanostructures, were studied using a spectroscopic photoemission and low energy electron microscopy, for the In coverages from 0.07 to 2.3 monolayer (ML). The results reveal that the In adatoms formed well-ordered clusters while keeping the Si (111)7×7 surface periodicity at 0.07ML and a single phase at 0.3ML around 440–470°C. At 0.82ML, owing to the presence of structurally defect areas beside the 7×7 domains, 3-D In droplets evolved concomitantly with the formation of 4×1-In cluster chains, accompanied by a transition in surface electric property from semiconducting to metallic. Further increasing the In to 2.3ML led to a moderate increase in number density and an appreciable lateral growth of the droplets, as well as the multi-domain In phases. Upon nitridation with NH3 at ~480°C, besides the nitridation of the In droplets, the N radicals also dissociated the Into form Siaused a partial disintegration of the ordered In phase and removal of the In adatoms between the In droplets. [Copyright &y& Elsevier]

Abstract: The unique properties of graphene can vary greatly depending on the number of graphene layers; therefore, spatial control of graphene thickness is desired to fully exploit these properties in promising new devices. Using low energy electron microscopy (LEEM), we investigate how scratches on the surface of 6H–SiC(0001) affect the epitaxial growth of graphene. Oscillations in the LEEM-image intensity as a function of electron energy (I–V LEEM analysis) show that the number of graphene layers clearly differs between regions of scratched and smooth substrate. The extent of the thicker graphene layers formed above scratches is found to be significantly larger than the width of the scratch itself. This finding can be implemented as an additional technique for spatially modulating graphene thickness. [Copyright &y& Elsevier]

Abstract: The lateral resolution of a surface sensitive low-energy electron microscope (LEEM) has been improved below 4nm for the first time. This breakthrough has only been possible by simultaneously correcting the unavoidable spherical and chromatic aberrations of the lens system. We present an experimental criterion to quantify the aberration correction and to optimize the electron optical system. The obtained lateral resolution of 2.6nm in LEEM enables the first surface sensitive, electron microscopic observation of the herringbone reconstruction on the Au(111) surface. [ABSTRACT FROM AUTHOR]

Abstract: We describe the design of a tandem instrument combining a low-energy electron microscope (LEEM) and a negative ion accelerator. This instrument provides video rate imaging of surface microtopography and the dynamics of its evolution during irradiation by energetic ions, at temperatures up to 1700K. The negative ion beam is incident on the sample at normal incidence with impact energies selectable in the range 0–5keV, and with current densities up to 30μA/cm2 (∼2×1014 ions/cm2 s or ∼0.2ML/s). The LEEM operates at a base pressure in the 10−9 Pa range. We describe the design and operating principles of the instrument and present examples of Pt(111) and Si(001) self-ion irradiation experiments. [Copyright &y& Elsevier]

Abstract: We report the present status of a newly installed low-energy electron microscopy and photoelectron microscopy end station at beamline U5UA at the National Synchrotron Light Source. In first test experiments on sub-monolayer Au coverages deposited on Ru(0001) we demonstrate core-level and valence band photoelectron imaging with a lateral resolution of about 65nm at a field of view of 10μm. In contrast to other installations for photoelectron microscopy, the new NSLS end station uses illumination under normal incidence. The implications of this geometry for different applications are discussed. [Copyright &y& Elsevier]

Abstract: Thin film growth of 6,13-pentacenequinone (C24H12O2, PnQ) on Si(111)-7×7 at room temperature (RT) was studied by low-energy electron microscopy (LEEM) and ab initio density functional theory (DFT) calculations. Our experiments yielded direct microscopic observation of enantiomorphic evolution mechanism in the initial stage of the chiral-like growth of PnQ islands, under kinetic growth conditions. We observed that the faster growth direction aligns with the direction of easier molecule incorporation, or lowest kink formation energy, rather than along the lowest energy step. Real time observation of the growth and subsequent relaxation of island shape revealed that kinetically stiff direction differs from the thermodynamic one. This feature together with anisotropic mass incorporation determines the enantiomorphic evolution and rotational arrangement of crystallites during the growth of elongated organic molecules, like PnQ. [Copyright &y& Elsevier]

Abstract: The density of two-dimensional Ag adatom gases on W(110) is determined by monitoring local electron reflectivity using low-energy electron microscopy (LEEM). This method of adatom concentration measurement can detect changes in adatom density at least as small as 10−3 ML for a μm size region of the surface. Using this technique at high temperatures we measure the sublimation rates of Ag adatoms on W(110). At lower temperatures, where Ag adatoms condense into monolayer islands, we determine the temperature dependence of the density of adatoms coexisting with this condensed phase and compare it with previous estimates. [Copyright &y& Elsevier]

Abstract: We present a method for the determination of orientation-dependent mobilities Γ eff(φ) based upon analyses of the detachment-limited coarsening/decay kinetics of equilibrium-shaped two-dimensional islands. An exact analytical expression relating the orientation-dependence of Γ eff(φ) to that of the anisotropic step energies β(φ) is derived. This provides relative values of Γ eff(φ) to within an orientation-independent scale factor that is proportional to the decay rate of the island area. Using in situ high temperature (T =1550–1700 K) low-energy electron microscopy measurements of two-dimensional TiN island coarsening/decay kinetics on TiN(111) terraces for which β(φ) values are known [Phys. Rev. B 67 (2003) 35409], we demonstrate the applicability of our analytic formulation for the determination of absolute Γ eff(φ) values. [Copyright &y& Elsevier]

Abstract: We have used scanning tunneling microscopy and low-energy electron microscopy to measure the thermal decay of two-dimensional Cu, Pb-overlayer, and Pb–Cu alloy islands on Pb–Cu(111) surface alloys. Decay rates covering 6–7 orders of magnitude are accessible by applying the two techniques to the same system. We find that Cu adatom diffusion across the surface alloy is rate-limiting for the decay of both Pb and Pb–Cu islands on the surface alloy and that this rate decreases monotonically with increasing Pb concentration in the alloy. The decrease is attributed to repulsive interactions between Cu adatoms and embedded Pb atoms in the surface alloy. The measured temperature dependences of island decay rates are consistent with first-principles calculations of the Cu binding and diffusion energies related to this “site-blocking” effect. [Copyright &y& Elsevier]

Abstract: Studies of surface dynamics, including the kinetics of two-dimensional (2D) island coarsening/decay, shape fluctuations, and shape evolution, enable determination of the rate-limiting mechanisms, corresponding surface mass transport parameters, and step energies. Most models describing these phenomena assume isotropic (circular) or near-isotropic island shapes and, hence, isotropic step energies. However, even simple elemental metal surfaces are anisotropic and more complex compound surfaces such as the low-index planes of TiN, GaAs, GaN, ZnO, Al2O3, ZrO2, are highly anisotropic. Here, we describe recent progress toward developing generalized theoretical and experimental approaches, applicable for analyses of 2D island coarsening/decay kinetics, coalescence kinetics, and determination of orientation-dependent step energies and step stiffnesses, on both isotropic and highly anisotropic surfaces. [Copyright &y& Elsevier]

We used low-energy electron microscopy to study structural and morphological changes of Si(1 1 1) on which “1 × 1” and 7 × 7 coexist. Because “1 × 1” and 7 × 7 have different thermodynamic and kinetic properties, various interesting phenomena unique to the two-phase surface take place. The difference in the surface mass diffusion constant, coupled with the preferential nucleation of 7 × 7 at the upper step edges, effectively causes a diffusion barrier at the upper side of the step, resulting in the selective slowing down of vacancy island decay and step wandering during homoepitaxial growth. The difference in the surface mass diffusion constant also influences the step motion caused by the difference in the atom density. The 7 × 7 domains coarsen to reduce the energetic cost of the boundaries consisting of narrow “1 × 1” regions. We demonstrate that von Neumann’s law governs the coarsening. [Copyright &y& Elsevier]

To demonstrate the potential of low energy in line projection holography, we study the reconstruction of the experimental holograms of three electrically different objects: a conducting, a semiconducting and an insulating object. The reconstructions of these holograms provide meaningful results for a large range of magnification of the object. The comparison between the reconstructed images and the scanning electron microscopy (SEM) micrographs of the same objects shows that the shapes and the dimensions of the reconstructed objects are identical to those obtained by conventional SEM. So, the simple assumptions needed to the reconstruction are justified. The reconstructions show a 2 nm resolution and appear superior than the best obtained SEM micrographs, when available. We also show some limitations of the reconstruction process. We point out that both numerical artifacts and experimental conditions are responsible for these limitations. [Copyright &y& Elsevier]

[Display omitted] • LEEM enables to follow phase transformations in self-assembled molecular layers in real time. • In the full layer, the phase transformations proceed differently and new phases appear. • In the full layer, deprotonation up to 50% does not lead to phase transformation. • The molecular overlayer (α phase) shows no long-range periodicity. Self-assembly is a bottom-up approach to prepare nanostructures with tailored properties. However, to utilize them as functional 2D layers, it is mandatory to understand their formation and its kinetics. Here, we describe the phase transformations in the full monolayer coverage of 4,4′-biphenyl-dicarboxylic acid (BDA) on the Ag(0 0 1) substrate. The phase transformations occur as a response to a thermally induced deprotonation of carboxyl groups. Contrary to the sub-monolayer coverage, the mass transport via on-surface diffusion in the full monolayer is hindered. We highlight two possible scenarios for the deprotonation of BDA in the full layer. For deprotonation of up to half of the carboxyl groups, the altered molecules can be incorporated within the existing α phase structure, forming a new binding motif. For structural changes that are required to respond to the further deprotonation, free substrate areas are necessary. The free surface is provided either by voids in the original layer or by removing excessive molecules, resulting in incomplete layers. This knowledge is essential in the fabrication of molecular layers that are part of hybrid organic–inorganic devices based on layered materials. [ABSTRACT FROM AUTHOR]

• Detector artefact correction enables true reflectivity measurements in LEEM. • Sub-pixel accurate image registration of LEEM images achieved. • Principal Component Analysis applicable to dimension reduction of spectra. • Rich color visualization of 90% of spatial spectrum information in two images. • Data clustering used to automatically distinguish stacking domains in samples. For many complex materials systems, low-energy electron microscopy (LEEM) offers detailed insights into morphology and crystallography by naturally combining real-space and reciprocal-space information. Its unique strength, however, is that all measurements can easily be performed energy-dependently. Consequently, one should treat LEEM measurements as multi-dimensional, spectroscopic datasets rather than as images to fully harvest this potential. Here we describe a measurement and data analysis approach to obtain such quantitative spectroscopic LEEM datasets with high lateral resolution. The employed detector correction and adjustment techniques enable measurement of true reflectivity values over four orders of magnitudes of intensity. Moreover, we show a drift correction algorithm, tailored for LEEM datasets with inverting contrast, that yields sub-pixel accuracy without special computational demands. Finally, we apply dimension reduction techniques to summarize the key spectroscopic features of datasets with hundreds of images into two single images that can easily be presented and interpreted intuitively. We use cluster analysis to automatically identify different materials within the field of view and to calculate average spectra per material. We demonstrate these methods by analyzing bright-field and dark-field datasets of few-layer graphene grown on silicon carbide and provide a high-performance Python implementation. [ABSTRACT FROM AUTHOR]

Silver is one of the most promising cathode materials for low temperature (300–500 °C) solid oxide fuel cells. The most important disadvantage of silver is its migration in the electric field. For better understanding of this phenomenon, an in situ observation of the migration mechanism was undertaken with the use of high-temperature microscopes. Scandia stabilised ceria doped zirconia CeScSZ electrolyte prepared from commercial powder was examined before and after silver migration experiments using scanning electron microscope. X-ray diffraction, broadband electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. The silver electrodes for solid oxide fuel cells were prepared using magnetron sputtering. The described cells under polarisation were examined using a high-temperature low energy electron microscope. Reference cells and post-mortem cells were observed using a scanning electron microscope equipped with high temperature stage. Under polarisation, silver moved inside the electrolyte and along the surface towards the region between electrodes. The structures thus formed were similar to those previously described in the literature; however, direct observation of the deposit growth was unsuccessful. In situ scanning electron microscopy observations of the silver electrode at 650 °C revealed neither melting of the smallest silver particles nor movement of silver structures. Silver migration through the electrolyte caused a reduction in grain interior conductivity of the electrolyte, whereas its grain boundary conductivity remained unaffected. [ABSTRACT FROM AUTHOR]

Understanding the nucleation and growth kinetics of thin films is a prerequisite for their large-scale utilization in devices. For self-assembled molecular phases near thermodynamic equilibrium the nucleation-growth kinetic models are still not developed. Here, we employ real-time low-energy electron microscopy (LEEM) to visualize a phase transformation induced by the carboxylation of 4,4'-biphenyl dicarboxylic acid on Ag(001) under ultra-high-vacuum conditions. The initial (α) and transformed (β) molecular phases are characterized in detail by X-ray photoemission spectroscopy, single-domain low-energy electron diffraction, room-temperature scanning tunneling microscopy, noncontact atomic force microscopy, and density functional theory calculations. The phase transformation is shown to exhibit a rich variety of phenomena, including Ostwald ripening of the α domains, burst nucleation of the β domains outside the α phase, remote dissolution of the α domains by nearby β domains, and a structural change from disorder to order. We show that all phenomena are well described by a general growth-conversion-growth (GCG) model. Here, the two-dimensional gas of admolecules has a dual role: it mediates mass transport between the molecular islands and hosts a slow deprotonation reaction. Further, we conclude that burst nucleation is consistent with a combination of rather weak intermolecular bonding and the onset of an additional weak many-body attractive interaction when a molecule is surrounded by its nearest neighbors. In addition, we conclude that Ostwald ripening and remote dissolution are essentially the same phenomenon, where a more stable structure grows at the expense of a kinetically formed, less stable entity via transport through the 2D gas. The proposed GCG model is validated through kinetic Monte Carlo (kMC) simulations.

Extreme ultraviolet (EUV) lithography (13.5 nm) is the newest technology that allows high-throughput fabrication of electronic circuitry in the sub-20 nm scale. It is commonly assumed that low-energy electrons (LEEs) generated in the resist materials by EUV photons are mostly responsible for the solubility switch that leads to nanopattern formation. Yet, reliable quantitative information on this electron-induced process is scarce. In this work, we combine LEE microscopy (LEEM), electron energy loss spectroscopy (EELS), and atomic force microscopy (AFM) to study changes induced by electrons in the 0-40 eV range in thin films of a state-of-the-art molecular organometallic EUV resist known as tin-oxo cage. LEEM-EELS uniquely allows to correct for surface charging and thus to accurately determine the electron landing energy. AFM postexposure analyses revealed that irradiation of the resist with LEEs leads to the densification of the resist layer because of carbon loss. Remarkably, electrons with energies as low as 1.2 eV can induce chemical reactions in the Sn-based resist. Electrons with higher energies are expected to cause electronic excitation or ionization, opening up more pathways to enhanced conversion. However, we do not observe a substantial increase of chemical conversion (densification) with the electron energy increase in the 2-40 eV range. Based on the dose-dependent thickness profiles, a simplified reaction model is proposed where the resist undergoes sequential chemical reactions, first yielding a sparsely cross-linked network and then a more densely cross-linked network. This model allows us to estimate a maximum reaction volume on the initial material of 0.15 nm 3 per incident electron in the energy range studied, which means that about 10 LEEs per molecule on average are needed to turn the material insoluble and thus render a pattern. Our observations are consistent with the observed EUV sensitivity of tin-oxo cages.

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. [ABSTRACT FROM AUTHOR]

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. Advanced low‐energy electron microscopy techniques are used to unravel important properties of Van der Waals heterostacks: using local diffraction spectroscopy, the stacking order, twist angle, and local orientation of the topmost layer are determined. [ABSTRACT FROM AUTHOR]

Abstract: The formation of anti-phase disorder is a major obstacle in the heteroepitaxy of III–V semiconductors on silicon. For an investigation of the anti-phase domain (APD) structure of GaP/Si(100) samples on mesoscopic length scales, we applied dark-field imaging in a low-energy electron microscope (LEEM) to thin GaP films grown on Si(100) substrates by metal organic vapor phase epitaxy (MOVPE). A contamination-free transfer of the samples from the MOVPE ambient to the ultra-high vacuum chamber of the microscope ensured that the atomically well-ordered, P-rich (2×2)/c(4×2) reconstruction of the surface was preserved. Mutually perpendicular oriented domains of the characteristic GaP(100) reconstruction identify the APDs in the GaP film at the surface and enabled us to achieve high contrast LEEM images. Striped patterns of APDs reflect the regular terrasse structure of the two-domain Si(100)(2×1) substrate far away from defects. APDs in the proximity of the defects have larger lateral extensions and are arranged in target pattern-like structures around the defects. In contrast to transmission electron microscopy, which was also applied in a specific dark-field mode for comparison, the characterization of anti-phase disorder by LEEM is non-destructive, does not require elaborate sample preparation, and addresses extended length scales. [Copyright &y& Elsevier]

Abstract: We show using low-energy electron microscopy that Au deposition at around 400°C leads to the arrangement of three-dimensional islands at single-layer steps on Si(111). Because the islands nucleate within a narrow coverage window, they have a small size distribution. After the coarsening of the islands during the interruption of the Au deposition, further deposition of Au results in the motion of islands into upper terraces with trenches left behind. This indicates that the islands are Au–Si alloy droplets. Additionally, the islands moved on terraces almost at constant velocities, but when they approach the upper-side steps, they jumped up to the steps. The atomic steps provide stable positions for Au–Si alloy islands, which means that the Au–Si alloy islands are suitable for arrangement using atomic steps. [Copyright &y& Elsevier]

Using low-energy electron microscopy, we find that the mechanisms of graphene growth on Ir(111) depend sensitively on island orientation with respect to Ir. In the temperature range of 750-900 °C, we observe that growing rotated islands are more faceted than islands aligned with the substrate. Further, the growth velocity of rotated islands depends not only on the C adatom supersaturation but also on the geometry of the island edge. We deduce that the growth of rotated islands is kink-nucleation-limited, whereas aligned islands are kink-advancement-limited. These different growth mechanisms are attributed to differences in the graphene edge binding strength to the substrate.