Area Low Energy Electron Diffraction Research Articles

Microscopic Study of the Spinodal Decomposition of Supported Eutectic Droplets During Cooling: PtGe/Ge{110}.

We embarked on an in situ low-energy electron microscopy, photo-electron emission microscopy, and selected area low-energy electron diffraction study during the cooling of huge eutectic droplets through the critical stages of the eutectic transition. On this journey through uncharted waters, we revealed an expected initial shrinking of the exposed area of the droplet, followed by an unanticipated expansion. We attribute this behavior to an initial fast amorphization of the interface between the droplet and surface, followed by the recrystallization of Ge expelled from the droplet at the interface. As a major surprise, we discovered the emergence of extensive “spaghetti”-like patterns, which are rationalized in terms of parallel Ge ripples oriented along, mainly, [−554] and [−55–4] directions. They emerge during spinodal decomposition when passing the eutectic temperature of the system. Their sides are defined by Ge{111} and Ge{11–1} vicinals covered with Pt-modified (√3 × √3) superstructures. The distance between adjacent ripples is about 18 nm.

Surface structure of bulk 2H-MoS2(0001) and exfoliated suspended monolayer MoS2: A selected area low energy electron diffraction study

We have used selected area low energy electron diffraction intensity-voltage (μLEED-IV) analysis to investigate the surface structure of crystalline 2H molybdenum disulfide (MoS2) and mechanically exfoliated and suspended monolayer MoS2. Our results show that the surface structure of bulk 2H-MoS2 is distinct from its bulk and that it has a slightly smaller surface relaxation at 320K than previously reported at 95K. We concluded that suspended monolayer MoS2 shows a large interlayer relaxation compared to the MoS2 sandwich layer terminating the bulk surface. The Debye temperature of MoS2 was concluded to be about 600K, which agrees with a previous theoretical study. Our work has shown that the dynamical μLEED-IV analysis performed with a low energy electron microscope (LEEM) is a powerful technique for determination of the local atomic structures of currently extensively studied two-dimensional (2-D) materials.

High thermal stability quasi-free-standing bilayer graphene formed on 4H–SiC(0 0 0 1) via platinum intercalation

Influences on electronic structure induced by platinum (Pt) deposited on monolayer graphene grown on SiC(0001) are investigated by photoelectron spectroscopy (PES), selected area low energy electron diffraction (μ-LEED) and angle resolved photoelectron spectroscopy (ARPES) techniques at the MAX Laboratory. Stable monolayer graphene electronic properties are observed after Pt deposition and after annealing at temperatures below 600°C. At ⩾600°C platinum silicide forms at the graphene/SiC interface. Annealing at 900°C results in an efficient decoupling of the carbon buffer layer from the SiC substrate and transformation into a second graphene layer. At this stage a quasi-free standing bi-layer graphene sample is obtained. The new superstructure spots then appearing in μ-LEED pattern suggest formation of an ordered platinum silicide at the interface. This silicide is found to be stable even after annealing at temperature up to 1200°C.

Is the Registry Between Adjacent Graphene Layers Grown on C-Face SiC Different Compared to That on Si-Face SiC

Graphene grown on C-face SiC substrates using two procedures, high and low growth temperature and different ambients, was investigated using Low Energy Electron Microscopy (LEEM), X-ray Photo Electron Electron Microscopy (XPEEM), selected area Low Energy Electron Diffraction (μ-LEED) and selected area Photo Electron Spectroscopy (μ-PES). Both types of samples showed formation of μm-sized grains of graphene. The sharp (1 × 1) μ-LEED pattern and six Dirac cones observed in constant energy photoelectron angular distribution patterns from a grain showed that adjacent layers are not rotated relative to each other, but that adjacent grains in general have different azimuthal orientations. Diffraction spots from the SiC substrate appeared in μ-LEED patterns collected at higher energies, showing that the rotation angle between graphene and SiC varied. C 1s spectra collected did not show any hint of a carbon interface layer. A hydrogen treatment applied was found to have a detrimental effect on the graphene quality for both types of samples, since the graphene domain/grain size was drastically reduced. From hydrogen treated samples, μ-LEED showed at first a clear (1 × 1) pattern, but within minutes, a pattern containing strong superstructure spots, indicating the presence of twisted graphene layers. The LEED electron beam was found to induce local desorption of hydrogen. Heating a hydrogenated C-face graphene sample did not restore the quality of the original as-grown sample.

In Situ Observation of a Deprotonation-Driven Phase Transformation: 4,4′-Biphenyldicarboxylic Acid on Au(111)

We have studied the growth of 4,4′-biphenyldicarboxylic acid (BDA) domains on Au(111) by means of low-energy electron microscopy (LEEM) and selective area low-energy electron diffraction (μLEED). Between 300 and 400 K, BDA forms three different 2-D crystalline phases, which are attributed to different degrees of deprotonation. At room temperature, BDA condenses in needle-shaped domains (α-phase) with the carboxylic acid–dimer as the main molecular bonding motif. Upon annealing from room temperature to above about 330 K, the needle-shaped domains change irreversibly into more compact ones. This remarkable feature is attributed to a partial deprotonation of the carboxylic end groups and thus breaking of the dimers, which allows the molecules to establish hydrogen bonds with adjacent molecules by a slight rotation (β-phase). A third phase is formed upon direct adsorption of BDA at substrate temperatures above 330 K. In this γ-phase the deprotonated BDA molecules form an open 2-D metal–organic coordination network, probably by incorporation of thermally excited Au adatoms.

Defects and inhomogeneities in Fe3O4(111) thin film growth on Pt(111)

Growth and surface termination of a Fe${}_{3}$O${}_{4}$(111) thin film on a Pt(111) surface were examined by a combination of low-energy electron microscopy, selected area low-energy electron diffraction (LEED), and x-ray-induced photoemission electron microscopy. The film exhibits the predominance of one out of two possible rotational domains, independent of film thickness. The morphology strongly depends on preparation conditions, e.g., at high oxidation temperature FeO/Pt(111) domains are formed that prevent the closure of the thin film. Dynamical LEED analysis and spot-profile analysis LEED (SPA-LEED) show that the surface exposes \textonequarter{} monolayer of Fe over a close-packed oxygen layer only when the sample is subsequently annealed in ultrahigh vacuum at 900 K. In contrast, the as-prepared films grown by oxidation at 1000 K and subsequent cooling down in oxygen, additionally exhibit small FeO${}_{x}$ agglomerates that rest upon the canonical surface termination. Their formation as a function of the various preparation conditions of the thin film is discussed.

Selected-area diffraction and spectroscopy in LEEM and PEEM

This paper addresses the effects of spherical and chromatic aberration of the objective lens, as well as chromatic dispersion of magnetic prism arrays, on the ability to perform selected area Low Energy Electron Diffraction, as well as (Angle Resolved) Photo Electron Spectroscopy experiments in today's advanced cathode lens microscopy instruments.

Growth and decay of hcp-like Cu hut-shaped structures on W(100)

We have studied both the morphology and structure of thin Cu deposits on W(100) during growth and desorption, using low-energy electron microscopy (LEEM) and selective area low-energy electron diffraction ($\ensuremath{\mu}$LEED). During growth at 674 K hut-shaped Cu crystallites with steep facets ($>\phantom{\rule{-0.16em}{0ex}}{54}^{\ensuremath{\circ}}$) coexist with a pseudomorphic Cu monolayer. The $\ensuremath{\mu}$LEED data suggest that these crystallites predominantly have a hcp structure with a high density of stacking faults and the (11$\overline{2}$0) plane parallel to W(100). The boundaries run along the [$\overline{1}$50] azimuth on W(100), which is explained by cancellation of shear stress exerted by Cu on the W(100) surface. Upon slow heating, Cu desorbs and the pseudomorphic wetting layer is transformed into coexisting surface alloy patches, with respectively, a Cu-rich $p$$(2\ifmmode\times\else\texttimes\fi{}2)$ and $p$$(2\ifmmode\times\else\texttimes\fi{}1)$ structure at 815 K. At about 950 K the islands are fully desorbed, leaving $p$$(2\ifmmode\times\else\texttimes\fi{}1)$ footprints behind. The $p$$(2\ifmmode\times\else\texttimes\fi{}2)$ patches disappear at about 1020 K, resulting in a homogeneous $p$$(2\ifmmode\times\else\texttimes\fi{}1)$ surface. Upon continued Cu desorption this surface transforms into small $c$$(2\ifmmode\times\else\texttimes\fi{}2)$ domains until all Cu has been desorbed at 1150 K.

Quantum Size Effect Driven Structure Modifications of Bi Films on Ni(111)

The quantum-size effect (QSE) driven growth of Bi film structures on Ni(111) was studied in situ using low energy electron microscopy and selective area low energy electron diffraction (μLEED). Domains with a (3×3), [(3)(1)(-1)(2)], and (7×7) film structure are found with a height of 3, 5, and 7 atomic layers, respectively. A comparison of I/V-μLEED curves with tensor LEED calculations shows perfectly accommodated Fermi wavelengths, indicative that not only the quantized height, but also the film structure is driven by QSE.

Formation of nanowires and their interaction with atomic steps during growth of Bi on Ni(111)

Using low-energy electron microscopy (LEEM) and selective area low-energy electron diffraction, we have characterized both the $(7\ifmmode\times\else\texttimes\fi{}7)$ wetting layer and the BiNi${}_{9}$ $[\begin{array}{cc}\hfill 2& 0\\ \hfill \ensuremath{-}2& 5\end{array}]$ nanowires that form during the growth of Bi on Ni(111). The 60 $\ifmmode\pm\else\textpm\fi{}$ 20 nm wide nanowires have lengths up to 10 $\ensuremath{\mu}$m and a height of 4--6 atomic layers. After the formation of the wetting layer and nanowires, quantum size effect driven growth ensues, accompanied by the gradual disappearance of the nanowires and resulting meandering of the substrate steps. The displacements of substrate steps, directly imaged with LEEM, can be traced back to dealloying.

Quantum size effects on surfaces without a projected bandgap: Pb/Ni(111)

We have studied the initial growth of Pb on Ni(111) using low-energy electron microscopy (LEEM) and selective area low-energy electron diffraction (μLEED). First, a one-layer-high wetting layer develops that consists of small (7 × 7) and (4 × 4) domains. For larger coverages, Pb mesas are formed that are embedded in the wetting layer. In spite of the absence of a projected bandgap on clean Ni(111), we observe distinct quantum size effect (QSE)-driven preferred heights. These are apparent from a variety of frequently occurring island height transitions during growth, both on wide terraces and across substrate steps. Also, the average island heights that evolve during deposition at 422 and 474 K show a clear signature of QSE-driven preferred heights. These distinctly include five, seven and nine layers and thus correspond nicely to the values obtained in the key examples of QSE: Pb films on Si(111) and Ge(111). We suggest that the Pb-induced surface modification of Ni(111) shifts the Fermi level into the gap of the interface projected Ni bulk bands, thereby effectively causing decoupling of the Pb states with the bulk Ni states.

A low energy electron microscopy study of the initial growth, structure, and thermal stability of 4,4′-biphenyldicarboxylic acid domains on Cu(001)

The growth of 4,4(')-biphenyldicarboxylic acid (BDA) on Cu(001) has been studied using low energy electron microscopy and selective area low energy electron diffraction. The emergence of large islands and hydrogen bonding to perpendicularly oriented, adjacent molecules is confirmed. The two benzene rings of adsorbed BDA are twisted along the molecular axis. Unconventional growth of the domains, followed by a second nucleation stage, is observed at room temperature. This unanticipated feature is attributed to the accumulation of stress in the islands. Ostwald ripening in the films and the decay of BDA domains at 448 K exhibits features that are consistent with diffusion limited behavior.

Anomalous Decay of Electronically Stabilized Lead Mesas on Ni(111)

With their low surface free energy, lead films tend to wet surfaces. However, quantum size effects (QSE) often lead to islands with distinct preferred heights. We study thin lead films on Ni(111) using low energy electron microscopy and selected area low energy electron diffraction. Indeed, the grown lead mesas show distinct evidence for QSE's. At about 526 K metastable mesas reshape into hemispheres within milliseconds, driven by a huge reduction in interfacial free energy. The underlying diffusion rate is many orders of magnitude faster than expected for lead on bulk lead.

Sb on In/Si(111) processes with dynamically observable LEEM, selected area LEED and chemically analyzed SR‐XPEEM

AbstractSb adsorption processes on In/Si(111) √3 × √3 and √31 × √31 surfaces were investigated in the present work. The dynamic growth behavior was observed with low energy electron microscopy (LEEM), and the local surface structure change was monitored with selected area low energy electron diffraction (LEED). X‐ray generated photo emission electron microscopy using synchrotron radiation (SR‐XPEEM) was employed in order to see the variation of the chemical state on the surface. On both √3 × √3 and √31 × √31 surfaces, Sb adsorption induces the surface structural change and modifies the chemical interaction of In atoms with Si. In atoms are replaced by impinged Sb atoms, and the discharged In atoms form islands. Copyright © 2006 John Wiley & Sons, Ltd.

Scanning Low-Energy Electron Diffraction Microscopy Combined with Scanning Tunnling Microscopy

A ultra-high vacuum scanning electron microscope (UHV-SEM) with a field emission gun (FEG) has been operated in an energy range of from 100 eV to 3 keV. A new technique of scanning low energy electron diffraction (LEED) microscopy has been added to the other techniques: scanning Auger microscopy (SAM), secondary electron microscopy, electron energy loss microscopy and the others available for the UHV-SEM. In addition to scanning LEED microscopy, a scanning tunneling microscope (STM) has been installed in the UHV-SEM-.The combination of STM with SEM covers a wide magnification range from 105 to 107 and is very effective for observation of surface structures with a high resolution of about 1 Å.A UHV-FEG-SEM is equipped in a chamber in which the vacuum is better than 2×10-10 Torr. A movable cylindrical mirror analyzer (CMA), a two dimensional detector of diffracted LEED beams, an ion gun and a deposition source are installed in this chamber. The concept of the scanning LEED microscope is comprised of two steps: (1) the formation of a selected area LEED pattern and (2) the generation of raster images with information contained in the diffraction pattern. In the present experiment, the LEED detector assembly shown in Fig.l has been used; it consists of two hemisherical grids, a two-stage channel-plate amplifier and a position-sensitive detector. The selection of one (or more) diffracted beam is performed electronically by a window using the two-dimensional analogue comparators. The intensity of a particular beam selected by the window modulates the brightness of the scanning image and a dark field image sensitive to the surface structure is formed. The experimental spatial resolutions of 150 Å and 500 Å have been attained at the primary electron energy 1 keV and 250 eV, respectively.

Selected area low energy electron diffraction and microscopy

The possibility of using the cathode lens for selected area low energy electron diffraction (LEED) and microscope in selected reflection is discussed. The column design for separated primary and diffracted beams is described.

942. Selected area low energy electron diffraction in the emission microscope: V Drahos et al, Proc 5th Czech Conf Electron Vacuum Phys, CzechAcad Sci, Brno 1972, I c−5

942. Selected area low energy electron diffraction in the emission microscope: V Drahos et al, Proc 5th Czech Conf Electron Vacuum Phys, CzechAcad Sci, Brno 1972, I c−5

Low-energy electron diffraction on the planes non parallel with the surface

Some problems of the interpretation of the geometry of low-energy electron diffraction patterns obtained on metal samples with a general crystallographic orientation near that of some densely occupied atomic plane are discussed. It appears that for the interpretation of these diffraction patterns the commonly used classical model of the diffraction on the surface plane cannot be used. The new geometrical model — model of the diffraction on densely occupied planes non parallel with the surface — in constructed. Using this model the calculation of the diffraciton patterns is made for a surface with an orientation close to the densely occupied (100) plane. The calculated diffraction patterns are compared with those obtained on monocrystalline niobium with known surface orientation near that of the (100) plane. Experimental results confirm the mentioned model to be rightful. This diffraction model can be used for determination of the polycrystalline grain orientation by means of the selected area low-energy electron diffraction. The experiments were made in the emission electron microscope adapted for low-energy electron diffraction.