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John Pendry: His Contributions to the Development of LEED Surface Crystallography

Description: In this paper we discuss the pivotal role played by Sir John Pendry in the development of Low Energy Electron Diffraction (LEED) during the past three decades; the earliest understanding on the physics of LEED to the development of sophisticated methods for the structural solution of complex surfaces.
Date: October 15, 2007
Creator: Somorjai, Gabor A. & Rous, P. J.
Partner: UNT Libraries Government Documents Department

Surface structural analysis of LiF(100) thin films grown on Pt(111)

Description: The surface structure of a multilayer LiF(100) thin film grown on Pt(111) from the vapor has been determined by the automated tensor low energy electron diffraction (LEED) method. The final structure, which refined to a Pendry R-factor (RP) of 0.24, had a surface corrugation (D1) of 0.24+-0.04 Angstrom due to the Li+ being displaced towards the bulk, leaving the initially coplanar F - unshifted. A similar intralayer corrugation due to the movement of the Li+ was also observed in the layer immediately under the surface layer, although to a lesser degree: D2=0.07+-0.04 Angstrom. This asymmetric relaxation resulted in the reduction of the first interlayer spacing, d(F2-Li1), to 1.77+-0.0 6 Angstrom from the ideal value of 2.01 Angstrom. The second interlayer spacing, d(Li3-F2), was within error bars of the bulk value, 2.01 Angstrom.
Date: August 29, 2002
Creator: Roberts, J.G.; Van Hove, M.A. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Atomic resolution 3D electron diffraction microscopy

Description: Electron lens aberration is the major barrier limiting the resolution of electron microscopy. Here we describe a novel form of electron microscopy to overcome electron lens aberration. By combining coherent electron diffraction with the oversampling phasing method, we show that the 3D structure of a 2 x 2 x 2 unit cell nano-crystal (framework of LTA [Al12Si12O48]8) can be ab initio determined at the resolution of 1 Angstrom from a series of simulated noisy diffraction pattern projections with rotation angles ranging from -70 degrees to +70 degrees in 5 degrees increments along a single rotation axis. This form of microscopy (which we call 3D electron diffraction microscopy) does not require any reference waves, and can image the 3D structure of nanocrystals, as well as non-crystalline biological and materials science samples, with the resolution limited only by the quality of sample diffraction.
Date: March 1, 2002
Creator: Miao, Jianwei; Ohsuna, Tetsu; Terasaki, Osamu & O'Keefe, Michael A.
Partner: UNT Libraries Government Documents Department

Low-energy electron diffraction study of the thermal expansion of Ag(111)

Description: The temperature dependence of the first three interlayer distances of the Ag(111) surface was studied by low-energy electron diffraction (LEED) over the temperature range 128K to 723 K. The first three interlayer spacings and the effective Debye temperatures were extracted from the LEED analysis. At the lowest temperature, the first two interlayer spacings are slightly (0.5 percent) contracted. All three interlayer spacings increase with temperature, finally reaching expansions relative to the bulk of about 0.8 percent at the highest temperature studied. The effective surface Debye temperature is lowest for the outermost layer, increasing toward the bulk value for successive layers.
Date: May 19, 2000
Creator: Soares, E.A.; Leatherman, G.S.; Diehl, R.D. & Van Hove, M.A.
Partner: UNT Libraries Government Documents Department

The atomic structure of the cleaved Si(111)-(2x1) surface refined by dynamical LEED

Description: New or modified models have been proposed for the much-studied Si(111)-(2x1) surface structure, including: a reverse-tilted p-bonded chain model (by Zitzlsperger et al); a three-bond scission model (by Haneman et al); and a p-bonded chain model with enhanced vibrations (present work). These models are compared here to the generally accepted modified p-bonded chain model (by Himpsel et al, 1984), by analyzing low-energy electron diffraction (LEED) I-V curves measured earlier. Using the efficient automated tensor LEED technique, the models can be refined to a much greater degree than with earlier methods of LEED analysis. This study distinctly favors the earlier modified p-bonded chain model, but with strongly enhanced vibrations. To compare models that have different numbers of adjustable free parameters a Hamilton ratio test is used: it can distinguish between improvement due to a better model and improvement due only to more parameters.
Date: March 1, 2004
Creator: Xu, Geng; Deng, Bingcheng; Yu, Zhaoxian; Tong, S.Y.; Van Hove, M.A.; Jona, F. et al.
Partner: UNT Libraries Government Documents Department

Revealing Charge Density Wave Formation in the LaTe2 System byAngle Resolved Photoemission Spectroscopy

Description: We present the first direct study of charge density wave(CDW) formation in quasi-2D single layer LaTe2 using high-resolutionangle resolved photoemission spectroscopy (ARPES) and low energy electrondiffraction (LEED). CDW formation is driven by Fermi surface (FS)nesting, however characterized by a surprisingly smaller gap (~;50 meV)than seen in the double layer RTe3 compounds, extending over the entireFS. This establishes LaTe2 as the first reported semiconducting 2D CDWsystem where the CDW phase is FS nesting driven. In addition, the layerdependence of this phase in the tellurides and the possible transitionfrom a stripe to a checkerboard phase is discussed.
Date: November 15, 2006
Creator: Garcia, D. R.; Gweon, G.-H.; Zhou, S. Y.; Graf, J.; Jozwiak, C. M.; Jung, M. H. et al.
Partner: UNT Libraries Government Documents Department

Using Pattern Search Methods for Surface Structure Determinationof Nanomaterials

Description: Atomic scale surface structure plays an important roleindescribing many properties of materials, especially in the case ofnanomaterials. One of the most effective techniques for surface structuredetermination is low-energy electron diffraction (LEED), which can beused in conjunction with optimization to fit simulated LEED intensitiesto experimental data. This optimization problem has a number ofcharacteristics that make it challenging: it has many local minima, theoptimization variables can be either continuous or categorical, theobjective function can be discontinuous, there are no exact analyticderivatives (and no derivatives at all for categorical variables), andfunction evaluations are expensive. In this study, we show how to apply aparticular class of optimization methods known as pattern search methodsto address these challenges. These methods donot explicitly usederivatives, and are particularly appropriate when categorical variablesare present, an important feature that has not been addressed in previousLEED studies. We have found that pattern search methods can produceexcellent results, compared to previously used methods, both in terms ofperformance and locating optimal results.
Date: June 9, 2006
Creator: Zhao, Zhengji; Meza, Juan & Van Hove, Michel
Partner: UNT Libraries Government Documents Department

RHEED studies of Ag/Si(111) growth at low temperatures

Description: This thesis showed that it is possible to achieve well ordered growth at low temperatures when chaing fluxes during the course of the deposition. It was also demonstrated that nucleation theory fails to predict or explain at least part of the results, in particular when deposition takes place at an initially low rate, with presumably a relatively low nucleation density, followed by a change to a high flux rate. This points to an inherent lack of nucleation theory; alternative explanations are presented based on flux-independent growth as reported by Roos (Surf. Sci. 302 (1994) 37).
Date: January 2, 1996
Creator: Koehler, U.
Partner: UNT Libraries Government Documents Department

The Growth of Thin Epitaxial Copper Films on Ruthenium (0001)and Oxygen-Precovered Ruthenium (0001) as studied by x-rayphotoelectron diffraction. University of California, Davis, Department of Physics, Ph.D. Thesis

Description: In the first part of this dissertation, the variation of mean emitter depths with direction for core photoelectron emission from single crystals, including the effects of both isotropic inelastic scattering and single and multiple elastic scattering was theoretically studied. The mean emitter depth was found to vary by as much as &plusmn;30% with direction. In the second part of this dissertation, x-ray photoelectron diffraction (XPD) was used to study the structure and growth mechanisms of Cu films grown on a clean and an oxygen-precovered Ru(OOO1) surface. Experimental Cu 2p3/2 (E<sub>kin</sub> = 556 eV) and Ru 3d (E<sub>kin</sub> = 1205 eV) intensities were measured for Cu coverages from submonolayer up to several monolayer (ML) on the clean Ru(OOO1) surface. In addition, the O 1s (E<sub>kin</sub> = 958 eV) intensity was measured for Cu grown on oxygen precovered Ru(OOO1). These XPD intensities have been analyzed using single scattering cluster (SSC) and multiple scattering cluster (MSC) models. The first Cu layer has been found to grow pseudomorphically on the Ru(OOO1) surface in agreement with prior studies of the Cu/Ru(OOO 1) system. Thus, the initial growth is layer-by-layer. For higher coverages, XPD shows that the short-range structure of the Cu films is fcc Cu(l 11), but with significant interlayer relaxation (compared to bulk Cu(l 11)) that persists up to {ge}8 ML. When oxygen is preadsorbed on the Ru(OOO1) surface before Cu film growth (possibly to act as a surfactant promoting smoother growth), XPD shows that the first ~3 ML of Cu grow as 3-D islands. In addition, XPD shows that, during Cu growth, all of the oxygen "floats" on the CU surface, in contrast to prior studies which found that 30% of the oxygen remains at the Cu/Ru intetiace. XPD also indicates that the oxygen is highly disordered on the Cu overlayer surface. In ...
Date: June 1, 1997
Creator: Ruebush, Scott Daniel
Partner: UNT Libraries Government Documents Department

Investigation of medium to high strain deformation microstructures using an automated electron back scatter pattern (EBSP) system

Description: The speed of automated EBSP orientation gathering has now increased to a point where it is possible to examine large areas of deformed material at a resolution close to the length scale of the dislocation substructures involved (typically < 1 {micro}m for strains of greater than {epsilon} = 1). Investigation of such deformed samples invariably gives rise to diffraction images of low quality and to many double patterns. To assist in a critical selection of the data, it is usual to store a measure of the indexing confidence for each point. Here the authors describe how deformed samples, which provide the appropriate range of pattern qualities, can be used to calibrate acceptance criteria for unsupervised EBSP measurements on deformation microstructures. An application in the use of such criteria is also described. Two crystals chosen with orientations that are known to develop a well defined texture spread during deformation were channel die deformed to a strain of {epsilon} = 1.5 (78% reduction). The texture spread was determined for each sample from a large number of transmission electron microscope (TEM) orientation measurements taken using a fast semi-automatic technique. EBSP scans were then made on the samples. Orientations failing within the expected (TEM determined) texture spread are assumed to be correct. While those falling outside the expected spread are assumed to be incorrect. Unsupervised acceptance criteria based on an indexing confidence parameter (in their case the fraction of located Kikuchi bands matching the indexing solution) were then tested by comparing the set of orientations selected using a given approval criterion with the set of the orientations falling within the expected spread. Criteria were appraised by determining both the fraction of all orientations that were misclassified (incorrect orientations selected plus correct orientations not selected) or by the fraction of incorrect orientations in the selected ...
Date: March 1, 1998
Creator: Godfrey, A.; Hughes, D.A.; Krieger Lassen, N.C. & Jensen, D.J.
Partner: UNT Libraries Government Documents Department

Growth and magnetic properties of ultrathin Fe on Pd(110).

Description: We have investigated the growth and magnetic properties of 0-3 ML (monolayer) Fe on stepped Pd(110) with reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED) and the surface magneto-optic Kerr effect (SMOKE) in order to relate the morphology, structure and magnetic properties in a low-dimensional system. The Fe films, grown at 340 K, are smooth and pseudomorphic up to 1.5 ML, where three-dimensional growth and lateral lattice relaxation ensues. The in-plane row spacing along the [110] decreases by {approximately}5-6 % at 3 ML. RHEED oscillations with l-ML period are observed in the (1,0), (2,0) and the center of the (0,0) streak intensity. The tail of the (0,0) streak at low exit angle, however, has a 0.5-ML period oscillation, which suggests step decoration growth. Submonolayer Fe films remain ferromagnetic above {approximately}0.3 ML. The magnetic easy axis is initially perpendicular to the surface and is in-plane for Fe thickness &gt;1.5 ML. Between 0.9-1.2 ML, there appear to be mixed magnetic phases as indicated by an increase in coercivity.
Date: November 1, 2000
Creator: Cuenya, B. R.; Pearson, J.; Yu, C.; Li, D. & Bader, S. D.
Partner: UNT Libraries Government Documents Department

Electron backscatter diffraction: A powerful tool for phase identification in the SEM

Description: EBSD in the SEM has been developed into a tool that can provide identification of unknown crystalline phases with a spatial resolution that is better than one micrometer. This technique has been applied to a wide range of materials. Use of the HOLZ rings in the EBSD patterns has enabled the reduced unit cell to be determined from unidexed EBSD patterns. This paper introduces EBSD for phase identification and illustrates the technique with examples from metal joining and particle analysis. Reduced unit cell determination from EBSD patterns is then discussed.
Date: December 2, 1999
Creator: Michael, J.R. & Goehner, R.P.
Partner: UNT Libraries Government Documents Department

Application of Electron Backscattered Diffraction (EBSD) and Atomic Force Microscopy (AFM) to Determine Texture, Microtexture, and Grain Boundary Energies in Ceramics

Description: Crystallographic orientations in alumina (Al<sub>2</sub>0<sub>3</sub>) and magnesium aluminate spinel (MgAl<sub>2</sub>0<sub>4</sub>) were obtained using electron backscattered diffraction (EBSD) patterns. The texture and mesotexture (grain boundary misorientations) were random and no special boundaries were observed. The relative grain boundary energies were determined by thermal groove geometries using atomic force microscopy (AFM) to identify relationships between the grain boundary energies and misorientations.
Date: May 19, 1999
Creator: Glass, S.J.; Rohrer, G.S.; Saylor, D.M. & Vedula, V.R.
Partner: UNT Libraries Government Documents Department

Use of Reciprocal Lattice Layer Spacing in Electron Backscatter Diffraction Pattern Analysis

Description: In the scanning electron microscope (SEM), using electron backscattered diffraction (EBSD), it is possible to measure the spacing of the layers in the reciprocal lattice. These values are of great use in confirming the identification of phases. The technique derives the layer spacing from the HOLZ rings which appear in patterns from many materials. The method adapts results from convergent-beam electron diffraction (CBED) in the transmission electron microscope (TEM). For many materials the measured layer spacing compares well with the calculated layer spacing. A noted exception is for higher atomic number materials. In these cases an extrapolation procedure is described that requires layer spacing measurements at a range of accelerating voltages. This procedure is shown to improves the accuracy of the technique significantly. The application of layer spacing measurements in EBSD is shown to be of use for the analysis of two polytypes of SiC.
Date: May 10, 1999
Creator: Eades, J.A. & Michael, J.R.
Partner: UNT Libraries Government Documents Department

Phase Identification of Individual Crystalline Particles by Electron Backscatter Diffraction (EBSD)

Description: Recently, an EBSD system was developed that uses a 1024 x 1024 CCD camera coupled to a thin phosphor. This camera has been shown to produce excellent EBSD patterns. In this system, crystallographic information is determined from the EBSD pattern and coupled with the elemental information from energy or wavelength dispersive x-ray spectrometry. Identification of the crystalline phase of a sample is then made through a link to a commercial diffraction database. To date, this system has been applied almost exclusively to conventional, bulk samples that have been polished to a flat surface. In this investigation, the authors report on the application of the EBSD system to the phase identification analysis (PIA) of individual micrometer and submicrometer particles rather than flat surfaces.
Date: June 12, 2000
Partner: UNT Libraries Government Documents Department

Crystal structure dependence of antiferromagnetic coupling in Fe/Si multilayers

Description: Recent reports of temperature dependent antiferromagnetic coupling in Fe/Si multilayers have motivated the generalization of models describing magnetic coupling in metal/metal multilayers to metal/insulator and metal/semiconductor layered systems. Interesting dependence of the magnetic properties on layer thickness and temperature are predicted. We report measurements that show the antiferromagnetic (AF) coupling observed in Fe/Si multilayers is strongly dependent on the crystalline coherence of the silicide interlayer. Electron diffraction images show the silicide interlayer has a CsCl structure. It is not clear at this time whether the interlayer is a poor metallic conductor or a semiconductor so the relevance of generalized coupling theories is unclear.
Date: April 1, 1995
Creator: Michel, R.P.; Chaiken, A. & Wall, M.A.
Partner: UNT Libraries Government Documents Department

Microstructures of GaN and In{sub x}Ga{sub 1-x}N films grown by MOCVD on free-standing GaN templates

Description: We summarize structural properties of thick HVPE GaN templates from the point of view of their application as substrates for growth of nitride layers. This is followed by the results of optical and structural studies, mostly transmission electron microscopy, of nitride layers grown by MOCVD on top of the HVPE substrates. The results indicate high structural quality of these layers with a low density of threading dislocations (in the range of 10{sup 6} cm{sup -2}). Convergent beam electron diffraction studies showed that the MOCVD GaN films have Ga-polarity, the same polarity as the HVPE GaN substrates. Structural studies of an InGaN layer grown on top of the MOCVD GaN film showed the presence of two layers, which differed in lattice parameter and composition. The upper layer, on the top of the structure had a c-lattice parameter about 2% larger than that of GaN and contained 10.3 {+-} 0.8% of In. Values measured for the thinner, intermediate layer adjacent to the GaN layer were about 2 .5 times lower.
Date: April 30, 2002
Creator: Jasinski, J.; Liliental-Weber, Z.; Huang, D.; Reshchikov, M.A.; Yun, F.; Morkoc, H. et al.
Partner: UNT Libraries Government Documents Department

Three-fold diffraction symmetry in epitaxial graphene and the SiC substrate

Description: The crystallographic symmetries and spatial distribution of stacking domains in graphene films on 6H-SiC(0001) have been studied by low energy electron diffraction (LEED) and dark field imaging in a low energy electron microscope (LEEM). We find that the graphene diffraction spots from 2 and 3 atomic layers of graphene have 3-fold symmetry consistent with AB (Bernal or rhombohedral) stacking of the layers. On the contrary, graphene diffraction spots from the buffer layer and monolayer graphene have apparent 6-fold symmetry, although the 3-fold nature of the satellite spots indicates a more complex periodicity in the graphene sheets.
Date: December 10, 2009
Creator: Siegel, D A; Zhou, S Y; El Gabaly, F; Schmid, A K; McCarty, K F & Lanzara, A
Partner: UNT Libraries Government Documents Department

Effect of NiO spin orientation on the magnetic anisotropy of the Fe film in epitaxially grown Fe/NiO/Ag(001) and Fe/NiO/MgO(001)

Description: Single crystalline Fe/NiO bilayers were epitaxially grown on Ag(001) and on MgO(001), and investigated by Low Energy Electron Diffraction (LEED), Magneto-Optic Kerr Effect (MOKE), and X-ray Magnetic Linear Dichroism (XMLD). We find that while the Fe film has an in-plane magnetization in both Fe/NiO/Ag(001) and Fe/NiO/MgO(001) systems, the NiO spin orientation changes from in-plane direction in Fe/NiO/Ag(001) to out-of-plane direction in Fe/NiO/MgO(001). These two different NiO spin orientations generate remarkable different effects that the NiO induced magnetic anisotropy in the Fe film is much greater in Fe/NiO/Ag(001) than in Fe/NiO/MgO(001). XMLD measurement shows that the much greater magnetic anisotropy in Fe/NiO/Ag(001) is due to a 90{sup o}-coupling between the in-plane NiO spins and the in-plane Fe spins.
Date: February 10, 2010
Creator: Kim, W.; Jin, E.; Wu, J.; Park, J.; Arenholz, E.; Scholl, A. et al.
Partner: UNT Libraries Government Documents Department