Dependence of the atomic structure and surface relief of platinum foils on the annealing and rolling conditions

The effect of cold rolling, polishing, and thermal annealing conditions on the atomic structure and surface geometry of platinum foils has been studied. The surface morphology has been analyzed using low-energy electron diffraction, atomic force microscopy, and scanning tunneling microscopy. The chemical composition of the surface has been evaluated by Auger electron spectroscopy. It has been demonstrated that a variation in the conditions used for the preparation of the samples makes it possible to produce surfaces with different degrees of perfection from atomically smooth to rippled, fractal, and diffraction-disordered surfaces.     

Transformation of graphite islets on the surface of recrystallized platinum foil under the action of mechanical loading

The effect of uniaxial tension on the behavior of graphite segregates (islets) on the surface of recrystallized platinum foil is studied in situ by low-energy electron diffraction and Auger electron spectroscopy. Mechanical stresses are shown to have a significant influence on the size and orientation of the islets. The alignment of the islets along steps on the platinum surface is discovered.     

Structural transformations on a platinum surface under mechanical tension

The effect of uniaxial tension on the structure of a recrystallized platinum surface is investigated using low-energy electron diffraction (LEED). The initial LEED patterns indicate that the (111) facet emerges on the surface of a platinum foil after a series of heating cycles under vacuum and in oxygen. After loading at ～80 MPa, the clean platinum surface is characterized by systems of regular and irregular atomic terraces. The regular terraces have a (9(111) × 100) structure. As the load increases to 90–100 MPa, the ordered arrangement of terraces transforms into a disordered arrangement. After the samples are held under these loads for ～2 h, the surface structure undergoes a transformation into the diffraction-disordered state. Under tensile deformation, the island structure of graphite molecules on the recrystallized platinum surface containing ～10 at. % C also undergoes a transformation.     

Atomic structures by direct transform of diffraction patterns

We propose all the diffraction patterns can be directly transformed to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either the holography or Patterson transform scheme is used. For diffraction patterns which are generated from a localized emitter source or dominated by an inelastic-scattering feature like core-level photoelectron or low-energy Kikuchi electron, holography transform is needed. On the other hand, for diffraction patterns which were dominated by elastic-scattering, like grazing-incidence X-ray diffraction, electron correlated thermal diffuse scattering or low-energy electron diffraction curves, Patterson transform is needed. To prove our point, high-fidelity and artifact-free three-dimensional atomic structures obtained by transform of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The future of these direct methods by transforming diffraction patterns will be discussed.     

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.     

Evidence for subsurface atomic displacements of the GaAs(110) surface from LEED/CMTA analysis

The low-energy electron diffraction constant momentum transfer average (LEED/CMTA) technique is applied to the GaAs(110) surface. In addition to verifying our earlier model of the top layer atomic displacements, atomic displacements in the first subsurface layer, opposite in direction to those in the top layer, are suggested. Best estimates for atomic positions in the surface and first subsurface layers are given.     

Determining the structure of Ru(0 0 0 1) from low-energy electron diffraction of a single terrace

While a perfect hcp (0 0 0 1) surface has threefold symmetry, the diffraction patterns commonly obtained are sixfold symmetric. This apparent change in symmetry occurs because on a stepped surface, the atomic layers on adjacent terraces are rotated by 180°. Here we use a low-energy electron microscope to acquire the threefold diffraction pattern from a single hcp Ru terrace and measure the intensity vs. energy curves for several diffracted beams. By means of multiple scattering calculations fitted to the experimental data with a Pendry R-factor of 0.077, we find that the surface is contracted by 3.5(±0.9)% at 456 K.     

A method to obtain kinematic intensities from low-energy electron diffraction data

In order to use low-energy electron diffraction as a tool for surface crystallography, it would be useful to extract the single scattering, or kinematic part, from the intensity data. Details of a method to do this by taking data with different diffraction geometry but at fixed momentum transfer are presented. The method is illustrated by data from Ni and Ag. The temperature dependence of the averaged intensity is presented. The multiple scattering contributions to the averaged intensity can be accounted for by an effective atomic scattering factor. Conditions for the applicability of the method are discussed. From the resulting kinematic intensities, the surface atomic arrangements can be determined by simple modifications of conventional crystallographic analysis.     

Solution of a multiple-scattering inverse problem: electron diffraction from surfaces

We present a solution to the multiple-scattering inverse problem for low-energy electron diffraction that enables the determination of the three-dimensional atomic structure of an entire surface unit cell directly from measured data. The solution requires a knowledge of the structure of the underlying bulk crystal and is implemented by a maximum entropy algorithm.     

Incident-energy dependence of crystalline structures of ion beam deposited Au thin films

The energy dependence of crystalline structures in Au thin-film deposition processes was investigated with the use of a low-energy mass-selected ion beam system. Au films deposited on Si(100) untreated wafer surfaces by the beam system at different ion energies in the range of 20–200?eV were analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD) and in-situ reflection high-energy electron diffraction (RHEED). The XRD results show that the kinetic energy provided by ion bombardment can facilitate crystal growth with specific orientations such as (100) or (110), the surfaces of which have relatively high surface energies. Our observations also suggest that each crystalline orientation appears only in a specific energy range of ion bombardment. These results indicate that Au crystalline orientations may be controlled by the ion irradiation energy during deposition processes.     

Transport characteristics of a single-layer graphene field-effect transistor grown on 4H-silicon carbide

We have investigated transport characteristics of epitaxial graphene grown on semi-insulating silicon-face 4H-silicon carbide (SiC) substrate by thermal decomposition method in relatively high N₂ pressure atmosphere. We have succeeded in forming 1–2 layers of graphene on SiC in controlled manner. The surface morphology of formed graphene was analyzed by atomic force microscopy (AFM), low-energy electron diffraction (LEED) and low-energy electron microscope (LEEM). We have confirmed single-layer graphene growth in average by this method. Top-gated, single-layer graphene field-effect transistors (FETs) were fabricated on epitaxial graphene grown on 4H-SiC. Increased on/off ratio of nearly 100 at low temperature and extremely small minimum conductance (0.018–0.3 in 4 e²/h) in gated Hall-bar samples suggest possible band-gap opening of single-layer epitaxial graphene grown on Si-face SiC.     

Low-temperature method of cleaning <Emphasis Type="Italic">p</Emphasis>-GaN(0001) surfaces for photoemitters with effective negative electron affinity

The changes in the chemical composition, atomic structure, and electronic properties of the p-GaN(0001) surface upon chemical treatment in an HCl-isopropanol solution and vacuum annealing are investigated by x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, and low-energy electron diffraction. It is demonstrated that a considerable part of the surface gallium oxide is removed upon chemical treatment of the GaN surface. Subsequent annealing of the surface under vacuum at temperatures of 400–450°C leads to a decrease in the residual carbon and oxygen contamination to 3–5% of the monolayer. The preparation of a clean p-GaN(0001) surface with a (1×1) structure identical to that of the bulk unit cells is confirmed by the low-energy electron diffraction data. The cesium adsorption on the clean p-GaN surface results in a decrease in the work function by ～2.5 eV and the appearance of an effective negative electron affinity on the surface. The quantum efficiency of the GaN photocathode at a wavelength of 250 nm is equal to 26%.     

ZnS(110)表面原子几何与电子特性的计算

用总能量最小方法，确定了ZnS(110)表面的原子几何结构，得到与弹性低能电子衍射实验相符的结果．利用格林函数的散射理论方法，计算了题ZnS(110)表面的电子结构，与第一性原理的计算结果进行了比较．讨论了晶格弛豫对表面电子特性的影响． 关键词：     

Influence of the thermal power of a Fe atomic flux on the formation of Cu/Fe nanofilms on a Si(001) substrate

The growth of a Fe sublayer 1.5–14.0 monolayers (MLs) thick and a Cu film (about 5 MLs) on this sublayer is studied at a reduced temperature (1240°C) and an elevated temperature (1400°C) of a Fe source and at a reduced temperature (900°C) of a Cu source. The films are examined by Auger electron spectroscopy, low-energy electron diffraction, and atomic force microscopy. As metal sources, thin Fe and Cu strips on a Ta foil are used. It is shown that a nonequilibrium 2D phase forms in the Fe-on-Si(001) film up to a thickness of 4–5 MLs. This phase appears as closely packed atomically smooth nanoislands. When the thickness of the film exceeds 4–5 MLs, the nonequilibrium Fe phase changes to the bulk (3D) phase of Fe and its silicide Fe _x Si. At Fe source temperatures of 1240 and 1400°C, the nonequilibrium phase consists of Fe with Si segregated on the Fe surface, and a Fe-Si mixture. Copper on the nonequilibrium Fe and Fe-Si phases grows, respectively, as a smooth layer Cu with Si segregated on the top and in the form of Cu-Fe and Cu-Si mixtures. Cu islands growing on the bulk Fe and Fe _x Si phases have smaller and larger sizes, respectively.     

Theoretical descriptions of surface state photoelectron spectroscopy

Various approximations which are commonly used in describing photoemission from solids are reviewed. To analyze experimental spectra quantitatively recourse must be had to theoretical calculations of surface states. Over a valence band width of 10 eV these calculations can be accurate to less than 0.5 eV for an assumed atomic surface configuration. For many purposes photoemission spectra may yield more information on allowed atomic configurations than LEED (low-energy electron diffraction) for current levels of sophistication.     

Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

The locally-resolved reaction kinetics of CO oxidation on individual (100)-type grains of a polycrystalline Pt foil was monitored in situ using photoemission electron microscopy (PEEM). Reaction-induced surface morphology changes were studied by optical differential interference contrast microscopy and atomic force microscopy (AFM). Regions of high catalytic activity, low activity and bistability in a (p,T)-parameter space were determined, allowing to establish a local kinetic phase diagram for CO oxidation on (100) facets of Pt foil. PEEM observations of the reaction front propagation on Pt(100) domains reveal a high degree of propagation anisotropy both for oxygen and CO fronts on the apparently isotropic Pt(100) surface. The anisotropy vanishes for oxygen fronts at temperatures above 465?K, but is maintained for CO fronts at all temperatures studied, i.e. in the range of 417 to 513?K. A change in the front propagation mechanism is proposed to explain the observed effects.     

Low-energy electron diffraction structure determination of an ultrathin CoO film on Ag(0 0 1)

The atomic structure of a four layer thick film of CoO on a Ag(0 0 1) substrate has been determined by comparing experimental low-energy electron diffraction (LEED) I(V) curves with multiple scattering calculations. The CoO film has been prepared using reactive evaporation of Co in an oxygen atmosphere leading to almost layer-by-layer growth. Contrary to the surface of CoO crystals an outward relaxation of the two outermost CoO layers as well as rumpling in the top layer has been found. The supposed driving force of this relaxation is the in-plane compressive stress, which results from the pseudomorphic growth of the CoO film on the Ag(0 0 1) substrate and the lattice mismatch of the two materials.     

Inversion of low-energy electron diffraction data with no pre-knowledge factor beyond optical holography

In this review, I describe how low-energy (typically below 500eV) electron diffraction spectra can be inverted to produce three-dimensional coordinates of atoms neighbouring a reference atom with no prior knowledge of what type or types of atom are present. The reference atom may be one of many equivalent near-surface atoms from which a core-level photoelectron is excited or, in the case of diffuse low-energy electron diffraction, one of many equivalent adsorbate atoms (lacking in long-range order) on the surface of a crystalline substrate. Other variants apply to low-energy electron diffraction, Kikuchi electron diffraction and time-reversed versions in which the wavenumber (energy) and direction of the incident beam are varied. I show that, for such low-energy electron diffraction spectra, the relative phases between the reference wave and scattered waves have a known geometric form if the spectra are taken from within a small angular cone in the near-back-scattering directions. By using the back-scattering small cone at each direction of interest, a simple algorithm is developed to invert the spectra and extract object atomic positions with no input of calculated dynamical factors. The article also reviews key ideas and works which led to the current understanding of this field.     

Strain-Induced Surface Roughening in Polycrystalline VT1-0 Titanium Specimens under Uniaxial Tension

Surface roughening in uniaxially tensile specimens of commercially pure titanium VT1-0 has been investigated using electron backscatter diffraction, optical and atomic force microscopy, and numerical simulation. It is shown that intragranular slip leads to the rotation of surface grains, due to which the grain surface is inclined and a terrace is formed at the interface with neighboring grains. The effect of the crystallographic grain orientation on the grain shape change and the degree of grain rotation occurring under constrained plastic deformation is demonstrated.     

Structure determination of Pb/Ge(111)-β-(√3 × √3)R30° by dynamical low-energy electron diffraction analysis and first-principles calculation

We have determined the atomic structure of the Pb/Ge(111)-β-(√3 × √3)R30° surface, which was shown to exhibit a large Rashba spin splitting in a metallic surface state by dynamical low-energy electron diffraction analysis. The Pb coverage for the optimized atomic structure is 4/3 with one Pb atom located at every third H(3) site of the bulk-truncated Ge(111) surface and the other three near the T(1) sites but slightly displaced towards the T(4) sites. The determined atomic structure agrees well with the energetically optimized one obtained from the first-principles calculation. The calculation also revealed that the potential for the Pb atoms on the H(3) sites is very soft along the surface normal, suggesting that their vertical position is distributed within a range of about 0.2-0.3 ?. The previously proposed phase transition associated with the surface melting is discussed.