High-resolution Low-energy Electron Research Articles

Dimer Coupling Energies of the Si(001) Surface.

The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its order-disorder phase transition. Spot profiles in high-resolution low-energy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic two-dimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, ξ_{∥}^{+}/ξ_{⊥}^{+}=5.2 of the fluctuating c(4×2) domains above the critical temperature T_{c}=(190.6±10) K. We obtain effective couplings J_{∥}=(-24.9±1.3) meV along the dimer rows and J_{⊥}=(-0.8±0.1) meV across the dimer rows, i.e., antiferromagneticlike coupling of the dimers with c(4×2) symmetry.

Evolving surface morphology during epitaxy of NiO on Ag(001)

Reactive molecular beam epitaxy of NiO on Ag(001) has been characterized by high-resolution low energy electron diffraction performed during deposition of Ni on Ag(001) in molecular oxygen atmosphere for deposition temperatures of 300–420 K. The initial stage of film growth at high deposition temperature is determined by competing processes of Ni oxidation and Ni interdiffusion into Ag(001) with Ni-Ag alloying. Ni oxidation is favored at low deposition temperatures. Thus, NiO bilayers are immediately formed. At high deposition temperatures, however, alloying with Ag is enhanced and the formation of the initial NiO bilayer is delayed. After formation of a closed NiO bilayer, alloying is prevented and all deposited Ni is oxidized and NiO growth proceeds in the layer-by-layer growth mode. Later stages of epitaxy are governed by relaxation processes induced by the formation of mosaics due to interface dislocations. The tilt angle of the mosaic decreases with increasing film thickness due to elastic deformation of the NiO lattice. Dislocation related mosaics are paired and form grooves. Postdeposition characterization shows periodic arrangement of grooves related to the formation of a regular dislocation network.

Strain measurement of ultrathin epitaxial films using electron diffraction techniques

Many diffraction and imaging techniques have been developed and used to measure the strains in epitaxial films with different degrees of accuracy. In this tutorial, we will focus on measurements of strains in epitaxial films using electron diffraction techniques. These include reflection high-energy electron diffraction (RHEED), azimuthal reflection high-energy electron diffraction, low-energy electron diffraction (LEED), and high-resolution low-energy electron diffraction techniques. These diffraction techniques are surface sensitive and are suitable for in situ measurements of strains in ultrathin epitaxial films quantitatively. In particular, RHEED and LEED are simple to operate and readily available in many laboratories. We discuss the instrument limit of strain measurement in each technique. A nominal definition of the resolving power of an instrument to resolve a strain state is given as the percentage of intrinsic instrumental broadening in the reciprocal unit with respect to the first reciprocal Brillouin zone (2π/lattice constant) of a sample. It is shown that the resolving power of these diffraction techniques ranges from 2% to 0.2%. Experimental strategies without modifying the instrument and to achieve a strain detection capability beyond the definition of the nominal resolving power of these techniques are presented.

Surface buckling of black phosphorus: Determination, origin, and influence on electronic structure

The surface structure of black phosphorus materials is determined using surface-sensitive dynamical microspot low energy electron diffraction ($\ensuremath{\mu}\mathrm{LEED}$) analysis using a high spatial resolution low energy electron microscopy (LEEM) system. Samples of (i) crystalline cleaved black phosphorus (BP) at 300 K and (ii) exfoliated few-layer phosphorene (FLP) of about 10 nm thickness which were annealed at 573 K in vacuum were studied. In both samples, a significant surface buckling of 0.22 \AA{} and 0.30 \AA{}, respectively, is measured, which is one order of magnitude larger than previously reported. As direct evidence for large buckling, we observe a set of (for the flat surface forbidden) diffraction spots. Using first-principles calculations, we find that the presence of surface vacancies is responsible for the surface buckling in both BP and FLP, and is related to the intrinsic hole doping of phosphoresce materials previously reported.

Long Vertical Distance Bonding of the Hexagonal Boron Nitride Monolayer on the Cu(111) Surface

The hexagonal boron nitride (hBN) monolayer on the Cu(111) surface has recently been considered an example of an extremely weak hBN/metal interaction, as indicated, e.g., from the presence of an only electronic Moire-like superstructure that was observed in scanning tunneling microscopy images. From these results, a large bonding distance of the hBN sheet to the topmost Cu layer can be envisaged but has not been proven so far. We report a structural analysis of the hBN/Cu(111) interface based on high resolution low energy electron diffraction and normal incidence X-ray standing wave experiments. We find that both the boron and nitrogen atoms are located at very large vertical distances of dB = 3.25 ± 0.02 A and dN = 3.22 ± 0.03 A with respect to the nominal position of the topmost Cu(111) layer. Significant vertical buckling and lateral distortions of the hBN layer can be excluded. These results demonstrate that the hBN monolayer on the Cu(111) surface is indeed well described by a rigid and geometrically...

Converged and consistent high-resolution low-energy electron–hydrogen scattering. I. Data below [formula omitted] threshold for applications in stellar physics

In this article we present converged datasets containing scattering data for collisions of electrons on the atomic hydrogen for total energies below the n=4 excitation threshold. The data have been obtained from an ab initio solution of the two-electron Schrödinger equation in the B-spline basis with the exterior complex scaling boundary condition and are well converged both radially and in terms of partial waves, often to a greater accuracy than currently available data. The data consist of partial T-matrices and can be combined to various secondary quantities, most notably the differential and integral cross sections. We compare the cross sections with previously published theoretical and experimental results and with available data from on-line databases. It is demonstrated that the new data are superior to the generally available results. The consistency of the cross section datasets is checked using the theorem of detailed balance. The energy sampling is fine enough to contain all major resonances in the considered energy range.

Tailoring metal–organic hybrid interfaces: heteromolecular structures with varying stoichiometry on Ag(111)

The physical properties of interfaces between organic semiconductors and metal surfaces crucially influence the performance of organic electronic devices. In order to enable the tailoring of such metal–organic hybrid interfaces we study the adsorption of heteromolecular thin films containing the prototypical molecules copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) on the Ag(111) surface. Here, we demonstrate how the lateral order can be tuned by changing the relative coverage of both adsorbates on the surface. The layer growth has been studied in real time with low energy electron microscopy, and—for different stoichiometries—the geometric properties of three heteromolecular submonolayer phases have been investigated using high resolution low energy electron diffraction and low temperature scanning tunneling microscopy. Furthermore, we have used a theoretical approach based on van der Waals and electrostatic potentials in order to reveal the influence of the intermolecular and the molecule–substrate interactions on the lateral order of heteromolecular films.

In-situ high-resolution low energy electron diffraction study of strain relaxation in heteroepitaxy of Bi(111) on Si(001): Interplay of strain state, misfit dislocation array and lattice parameter

The relief of lattice mismatch-induced strain in Bi(111) on Si(001) heteroepitaxial system was investigated in real time as the Bi film relaxes, by means of high resolution low-energy electron diffraction (LEED). The inherent lattice mismatch of 2.5% at room temperature is accommodated through the formation of an ordered misfit dislocation array confined to the interface. The strain fields of the dislocations cause a periodic height undulation of the surface in the sub-Ångström regime, which is observed through spot splitting in LEED. From a simultaneous measurement of the position of the first-order LEED spots, which corresponds to the lattice parameter of the film, and of the separation of satellite spots, which corresponds to the ordering of the dislocation array, the evolution of the strain state during annealing of a 6nm Bi film was determined. The strain is solely relaxed by full edge-type dislocations arranged in the ordered array at the interface. From the remaining strain of ε=0.6% the critical thickness for generation of misfit dislocations under equilibrium conditions can be derived.

Temperature-dependent formation and evolution of the interfacial dislocation network ofAg/Pt(111)

We have investigated the formation of a dislocation network that develops at the interface of a Ag film on $\mathrm{Pt}(111)$ and its evolution with film thickness at 600 K and above. This system is commonly considered to be representative of a surface-confined alloy, and we have studied the structure and topography of the films' surfaces up to thicknesses of 35 ML using high-resolution low-energy electron diffraction. The network is formed during the growth of the second layer and is fully developed already for films only a few layers thick. The spacing of the network varies between 5.8 and 6.6 nm when grown at 600 and 800 K, respectively. This dependence on the growth temperature is attributed to a temperature-dependent material mixing in the first (few) layers of the film. This mixing is irreversibly set by the deposition temperature and even persists during prolonged annealing at a temperature close to the desorption of Ag from $\mathrm{Pt}(111)$.

Strain state, film and surface morphology of epitaxial topological insulator Bi2Se3 films on Si(111)

Epitaxial Bi2Se3 films were grown by molecular beam epitaxy on Si(111)-Bi(3×3)R30° at temperatures between 200 and 250°C. The surface and bulk morphology was characterized by high resolution low energy electron diffraction, X-ray diffraction, and atomic force microscopy for various film thicknesses between 6 and 90nm. The films are atomically smooth without small angle mosaics or small angle rotational domains. The precise determination of lattice parameter reveals that films grown at higher temperature exhibit a smaller value for the vertical lattice parameter. The presence of random stacking faults in the film is reflected by a parabolic increase of the width of the diffraction peaks in X-ray diffraction.

The uptake of ClNO3 on film coatings of natural sea salt

Using a coated-insert flow tube reactor coupled to a high-resolution low-energy electron impact mass spectrometer, the uptake of chlorine nitrate on a salt film coating of natural sea salt (NSS) was studied under variation in the reactant concentration ([ClNO3] = (1–15) · 1012 cm−3), humidity ([H2O] = (0.01–2.2) · 1015 cm−3), and reactor temperatures of 295 and 423 K. The time-dependent character of the uptake coefficient γ(t) = γ ini exp(−t/τ) + γ ss [1 − exp(−t/τ)] with parameters of the initial (γ ini ) and steady-state (γ ss ) uptake and characteristic time dependent on [ClNO3] was established. The only detected gas phase uptake product was Cl2. The absence of the dependence of the steady-state uptake coefficent γ ss on humidity in the aforementioned [H2O] range was established. Based on the model description of experimental dependences in terms of the modified Langmuir adsorption model, the adsorption heat of chlorine nitrate on NSS and the constant of unimolecular decay rate of the ClNO3…Z surface complex with the formation of the Cl2 product were estimated. It follows from the comparison of these parameters for the initial and steady-state uptake that their adsorption heats are similar within the experimental error and the difference between the coefficients γ ini and γ ss is caused by the difference in activation energies of unimolecular decay. The coefficients γ ini , γ ss , and τ were estimated under tropospheric chlorine nitrate concentrations that are unavailable in laboratory conditions.

Hysteresis proves that the In/Si(111)(8×2)to(4×1)phase transition is first-order

Indium on silicon (111) exhibits a Peierls-like phase transition from a $(4\ifmmode\times\else\texttimes\fi{}1)$ reconstructed high-temperature phase to a $(8\ifmmode\times\else\texttimes\fi{}2)$ reconstructed ground state. A controversial debate is going on what kind of phase transition it is: first- or second-order. We employed high-resolution low-energy electron diffraction under slow thermal heating and cooling cycles to follow the phase transition. A robust hysteresis of diffraction spot intensities with a width of 8.6 K has been observed, which is independent of the heating and cooling rate. This hysteresis directly proves the existence of an energy barrier at the phase transition temperature and thus the first-order behavior of this phase transition.

High resolution low-energy electron attachment to molecular clusters of sulfur dioxide

Using the laser photoelectron attachment method, we have investigated the formation of cluster anions in low-energy free electron attachment (E=1–200meV) as well as in Rydberg electron transfer (at principal quantum numbers 20–260) to molecular clusters of sulfurdioxide (SO2) in a collimated supersonic beam at high electron energy resolution (energy width ≤2meV). At these low energies (almost) only homogeneous cluster anions (SO2)q− (q≥1) are found with energy dependences which show – superimposed on a monotonically decreasing continuum cross section – characteristic peaks which are interpreted as vibrational Feshbach resonances (VFR), associated with excitation of the three fundamental vibrational modes in the SO2 molecule, namely bending (010), symmetric stretch (100), and asymmetric stretch (001). The VFRs are clear for q=2–12, but only weak for q=1. The energy positions of the VFRs are progressively red-shifted with increasing cluster anion size q by about 1.7meV per added molecular unit, and the peaks broaden towards larger q. The cross section enhancement in the VFRs – compared to the size of the continuum – is substantial, indicating that the VFRs act as important doorway states for electron capture. Model R-matrix calculations are presented which recover the main features of the experimental attachment spectra for cluster anion formation and suggest that VFRs occur for neutral (SO2)N clusters with sizes N≥4. With growing N, the electron binding energy increases and the capture cross section decreases. In contrast, the Vogt–Wannier model for electron capture into a polarization well predicts a growth of the cross section with the cluster size.

Surface morphology of ultrathin hex-Pr2O3 films on Si(1 1 1)

In this work, the morphology of the surface of hexagonal Pr2O3(0 0 0 1) films grown on Si(1 1 1) is studied by high-resolution low-energy electron diffraction combined with spot profile analysis. For this purpose, praseodymia films prepared by molecular beam epitaxy were capped with protecting amorphous germanium films. After removal of the capping layers due to heating in diluted oxygen atmosphere the surface properties of the oxide film were investigated in situ with Auger electron spectroscopy and spot profile analysis low energy electron diffraction. The removal of the capping layer has no impact on the hexagonal Pr2O3(0 0 0 1) film structure which is shown by x-ray diffraction. Surface sensitive electron diffraction confirms that the surface of the oxide film has hexagonal structure. Diffraction spot profile analysis shows that the film surface has grain structure without any mosaic spread due to the negligible lateral lattice mismatch between hexagonal Pr2O3(0 0 0 1) and Si(1 1 1). In addition, single atomic steps with complete bulk unit cell height are present at the surface. The density of the atomic steps is small pointing again to the high quality of the surface of hexagonal Pr2O3 films compared to cubic Pr2O3 films grown on Si(1 1 1).

Stabilization of the ceria ι-phase (Ce7O12) surface on Si(111)

In this work a 250 nm CeO2(111) film grown on a hex-Pr2O3(0001)/Si(111) system is annealed at 660 °C for 30 min to form the ι bulk phase of Ce7O12 as controlled by x-ray photoelectron spectroscopy. The (111) surface of the stabilized ι phase is characterized via high-resolution low-energy electron diffraction. The ι-phase surface exhibits a (7×7)R19.1° superstructure with two mirror domains. This structure is attributed to a periodic ordering of oxygen vacancies compared to the fluorite structure of CeO2.

Structural transitions of epitaxial ceria films on Si(111)

The structural changes of a (111) oriented CeO2 film grown on a Si(111) substrate covered with a hex-Pr2O3(0001) interface layer due to post deposition annealing are investigated. X-ray photoelectron spectroscopy measurements revealing the near surface stoichiometry show that the film reduces continuously upon extended heat treatment. The film is not homogeneously reduced since several coexisting crystalline ceria phases are stabilized due to subsequent annealing at different temperatures as revealed by high resolution low energy electron diffraction and X-ray diffraction. The electron diffraction measurements show that after annealing at 660 °C the ι-phase (Ce7O12) is formed at the surface which exhibits a (√7 × √7)R19.1° structure. Furthermore, a (√27 × √27)R30° surface structure with a stoichiometry close to Ce2O3 is stabilized after annealing at 860 °C which cannot be attributed to any bulk phase of ceria stable at room temperature. In addition, it is shown that the fully reduced ceria (Ce2O3) film exhibits a bixbyite structure. Polycrystalline silicate (CeSi(x)O(y)) and crystalline silicide (CeSi1.67) are formed at 850 °C and detected at the surface after annealing above 900 °C.

Tailor-made ultrathin manganese oxide nanostripes: ‘magic widths’ on Pd(1 1 N) terraces

The growth of ultrathin two-dimensional manganese oxide nanostripes on vicinal Pd(1 1 N) surfaces leads to particular stable configurations for certain combinations of oxide stripe and substrate terrace widths. Scanning tunneling microscopy and high-resolution low-energy electron diffraction measurements reveal highly ordered nanostructured surfaces with excellent local and long-range order. Density functional theory calculations provide the physical origin of the stabilization mechanism of ‘magic width’ stripes in terms of a finite-size effect, caused by the significant relaxations observed at the stripe boundaries.

Optical characterization of the evolution of ion-induced anisotropic nanopatterns on Ag(001)

Reflection anisotropy spectroscopy is used as an in situ probe for the emergence and evolution of surface patterns on Ag(001) during oblique incidence ion bombardment. The information is extracted from plasmon resonances induced by the nanoscale patterns, utilizing the fact that smooth Ag(001) is optically isotropic. The Rayleigh-Rice perturbation approach delivers the temporal development of the average periodicity and amplitude of the surface patterns. For ion bombardment at a polar angle of incidence of ${70}^{\ensuremath{\circ}}$ along a $\ensuremath{\langle}110\ensuremath{\rangle}$ azimuth, strongly anisotropic surface features develop, giving rise to a single plasmon resonance, which is described well with a one-dimensional power spectral density function. For a smaller polar angle of incidence of $61.{5}^{\ensuremath{\circ}}$ multiple plasmon resonances are observed, which demand a two-dimensional power spectral density function for a perfect description. These result compare well with high-resolution low-energy electron diffraction data, taken after ion bombardment at both angles of incidence. The optical data, obtained at $61.{5}^{\ensuremath{\circ}}$, show coarsening and seem to suggest scaling of the periodicity and roughness, with critical exponents $0.27$ and $0.40,$ respectively.

Growth temperature dependent graphene alignment on Ir(111)

The morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moiré spots reflect graphene in a single orientation epitaxial incommensurate phase.

Submonolayer growth of copper-phthalocyanine on Ag(111)

The growth of high-quality thin films is a key issue in the ability to design electronic devices based on organic materials and to tune their properties. In this context, the interfaces between metals and organic films play a decisive role. Here, we report on the interface formation between copper-phthalocyanine (CuPc) and an Ag(111) surface using various complementary methods. High-resolution low-energy electron diffraction revealed a rich phase diagram for this system with disordered (two-dimensional (2D)-gas-like) and ordered structures (commensurate and point-on-line). In particular, a continuous change in lattice parameters with increasing coverage was found for long-range ordered structures, indicating a substrate-mediated repulsive intermolecular interaction similar to the case of tin-phthalocyanine/Ag(111). Chemisorptive bonding to the substrate was found by x-ray standing waves and ultraviolet photoelectron spectroscopy, and this weakened with increasing coverage at low temperature. This remarkable effect is correlated to a shift in the highest occupied molecular orbital (HOMO) and a HOMO-1 split off band to higher binding energies. Based on our experimental results, we present a comprehensive study of the adsorption behavior of CuPc/Ag(111), including the mechanisms for phase formation and molecular interaction.