Surface Structure Determination Research Articles

Surface structure determination of Sn deposited on Pt(111) by low energy alkali ion scattering

Surface structures resulting from the deposition and annealing of Sn on Pt(111) and exhibiting p(2 × 2) and p(√3 × √3)−R30° LEED patterns have been studied by low energy alkali ion scattering. Azimuthal and polar incident angle dependencies were measured for 500 and 1000 eV Li + ions scattered from the clean and Sn covered surfaces. Shadowing and blocking effects adequately describe the single scattering from the clean Pt surface. The data support a structural model in which the Sn is incorporated into the Pt surface rather than existing as an overlayer. Analysis of the Sn angle dependence indicates that the Sn is not coplanar with the Pt but is protruding 0.022 ± 0.005 nm above the surface. The results are compared with related metal-on-metal systems.

Surface structure determination of Sn deposited on Pt(111) by low energy alkali ion scattering

Abstract Surface structures resulting from the deposition and annealing of Sn on Pt(111) and exhibiting p(2 × 2) and p(√3 × √3)−R30° LEED patterns have been studied by low energy alkali ion scattering. Azimuthal and polar incident angle dependencies were measured for 500 and 1000 eV Li + ions scattered from the clean and Sn covered surfaces. Shadowing and blocking effects adequately describe the single scattering from the clean Pt surface. The data support a structural model in which the Sn is incorporated into the Pt surface rather than existing as an overlayer. Analysis of the Sn angle dependence indicates that the Sn is not coplanar with the Pt but is protruding 0.022 ± 0.005 nm above the surface. The results are compared with related metal-on-metal systems.

Surface structure determination of the (2 × 1)OCu(110) reconstruction by low-energy ion scattering

Using impact collision ion scattering we show, that the reconstruction of the (2 × 1)OCu(110) surface is of the missing-row type where every other [001] row of copper atoms is missing. The oxygen atoms are located 0.08 ± 0.15 Å below the copper atoms of the remaining rows. The second layer copper atoms are shifted laterally 0.12 ± 0.07 Å away from the oxygen atoms in a pairing-row type reconstruction. The first layer is relaxed outward over 0.23 ± 0.04 Å compared to the ideal spacing, whereas the distance between the second and third layer remains unchanged. A critical comparison of our results with the data available in the literature shows that the position of copper and oxygen atoms can be determined with an accuracy of about 0.05 and 0.15 Å, respectively.

Surface structure determination of the (2 × 1)OCu(110) reconstruction by low-energy ion scattering

Surface structure determination of the (2 × 1)OCu(110) reconstruction by low-energy ion scattering

Structure Determination of Crystal Surfaces by Electron Diffraction

Structure Determination of Crystal Surfaces by Electron Diffraction

Grazing-incidence X-Ray Analysis of Surfaces and Thin Films

AbstractGrazing-incidence X-ray analysis techniques which are commonly used for the nondestructive characterization of surfaces and thin films are reviewed. The X-ray reflectivity technicue is used to study surface uniformity and oxidation, layer thickness and density, interface roughness and diffusion, etc. The grazing-incidence in-plane diffraction technique is used to determine in-plane crystallography of epitaxial films. The grazing-incidence asymmetric-Bragg diffraction is used for surface phase identification and structural depth profiling determination of polycrystalline films. Typical examples to illustrate the types of information that can be obtained by the techniques are presented.

LEIS for structure determination of surfaces and thin films

Ion scattering spectrometry (ISS) has developed as a useful method of analysing the structure of single-crystal surfaces. In this paper, the strengths and weaknesses of four techniques of low-energy ion scattering (LEIS) are discussed in reference to the analysis ofc(2 × 2)O/Ni(001), (2 × 1)O/Ni(110)and CaF 2/Si(111). The techniques are LENRS (low-energy negative recoil spectrometry), CAICISS (coaxial impact ion scattering spectrometry) and conventional LEIS using inert gas and alkali ions.

A real-space multiple scattering theory of low energy electron diffraction: a new approach for the structure determination of stepped surfaces

A new theory of Low Energy Electron Diffraction (LEED) is presented in which the relevant multiple scattering equations are solved in the angular momentum representation within the framework of real-space multiple scattering theory (RS-MST). This approach avoids the plane wave basis used in many conventional LEED techniques and its associated limitations when applied to the calculation of LEED intensities from open surfaces containing small bulk interplanar spacings. In particular, high Miller index, stepped surfaces which lie beyond the present capabilities of conventional LEED, can now be treated in a relatively efficient and convergent manner. The new theory is tested by evaluating I–V spectra from the (100), (311), (331) surfaces of Cu which are compared with the results of a layer doubling (LD) LEED calculation. Excellent agreement is obtained in the (100) and (311) cases, for which the LD approach is expected to be applicable. The (311) surface is about the highest index fcc surface which can reasonably be attempted with the existing approaches. The results obtained for the case of the (331) surface using the LD and the RS-MST approaches agree up to about E = 100 eV, beyond which the LD process fails to converge. We discuss and contrast the convergence properties of both methods.

An efficient method for LEED crystallography

Determination of periodic surface structures from analysis of LEED intensity data is usually based on the evaluation of continuous I/V-spectra for a large number of model structures for which all the structural parameters have to be refined simultaneously until the best agreement with the experimental data, as quantified by the minimum of R-factor, is achieved. It is demonstrated that analysis based on intensity data taken only at discrete energy intervals (of up to about 20 eV) leads to no loss in accuracy if compared with the evaluation of continuous I/V-spectra. The introduction of a novel R DE-factor permits in addition to replace the “grid search” technique by a “least-squares” optimisation scheme which enables automatic search of the R-factor minimum at considerably reduced computational efforts. The strength of this technique becomes particularly evident with more complex structures as is demonstrated for Ni(110)-(2 × 1)O and other systems.

Formation and chemical reactions of ruthenium tricarbonyls adsorbed on metal oxides. Use of chemical reactions as a method of surface structure determination

The adsorption of Ru(CO) 3 Cl 2 (THF) onto the surface of hydroxylated metal oxides yields one of two surface metal carbonyls depending on the metal oxide. Depending on the nature of the metal oxide, the surface species either retains or loses coordinated chloride

Directional variations in thermal emission from geologic surfaces

We investigated the directional emission properties of geologic surfaces using a ground‐based, hand‐held infrared radiometer and thermistor probe. Field sites involved surfaces ranging from smooth playa and sand surfaces to a very rough aa lava flow in the Lunar Crater Volcanic Field, NV, as part of the Geological Remote Sensing Field Experiment. Large directional variations in thermal emission were found; they result from the presence of surface roughness–at large scales producing spatial variations in kinetic temperature and at small scales producing emissivity variations. These variations are important in remotely determining surface structure and understanding surface energy balance and emission spectra.

Direct methods in surface crystallography

Conventional methods of determining surface structure from LEED intensity measurements face severe problems when confronted with complex or unconventional surface structures forcing a restricted search of parameter space, possibly leading to errors in the determination. Direct Methods sidestep this problem. In this paper we discuss several possible implementations of the Direct Methods and illustrate one of them with the example of p(2 × 2)O Rh(100) . The output from a Direct Methods calculation is the probability distribution of the surface atoms and the paper concludes with a discussion of the possibility for study of anharmonic thermal vibrations, and of translational motion which this opens up.

Time-of-flight scattering and recoiling spectrometer (TOF-SARS) for surface analysis

A UHV spectrometer system has been designed and constructed for time-of-flight scattering and recoiling spectrometry (TOF-SARS). The technique uses a pulsed primary ion beam and TOF methods for analysis of both scattered and recoiled neutrals (N) and ions (I) simultaneously with continuous scattering angle variation over a flight path of ≊1 m. The pulsed ion beam line uses an electron impact ionization source with acceleration up to 5 keV; pulse widths down to 20 ns with average current densities of 0.05–5.0 nA/mm2 have been obtained. Typical current densities used herein are ≊0.1 nA/mm2 and TOF spectra can be collected with a total ion dose of <10−3 ions/surface atom. A channel electron multiplier detector, which is sensitive to both ions and fast neutrals, is mounted on a long tube connected to a precision rotary motion feedthru, allowing continuous rotation over a scattering angular range 0°<θ<165°. The sample is mounted on a precision manipulator, allowing azimuthal δ and incident α angle rotation, as well as translation along three orthogonal axes. The system also accommodates standard surface analysis instrumentation for LEED, AES, XPS, and UPS. The capabilities of the system are demonstrated by the following examples: (A) TOF spectra versus scattering angle θ; (B) comparison to LEED and AES; (C) surface and adsorbate structure determinations; (D) monitoring surface roughness; (E) surface semichanneling measurements; (F) measurements of scattered ion fractions; and (G) ion induced Auger electron emission.

Directed search methods for surface structure determination by LEED

Conventional surface crystallography by low-energy electron diffraction employs a trial-and-error search controlled at each step by a human being. This trial-and-error approach becomes very cumbersome and unreliable when it is applied to complex surfaces with a large number of unknown structural parameters. We present an automatic optimisation procedure for LEED which combines the complementary techniques of Tensor LEED and a numerical search algorithm. The significant advantages of using Tensor LEED as the basis of a search strategy are illustrated by its application as part of an automatic directed search to the determination of two surface structures: clean Pd(111) and Pd(111)( 3 × 3 ) R30°- CO. This approach can reduce the computer time required for an entire structure determination by several orders of magnitude. It also allows all structural parameters to be found simultaneously; in particular complex, non-symmetric structures and adsorbate induced substrate distortions can be readily determined.

X-Ray scattering from semiconductor interfaces

X-ray diffraction has been used to determine the structure of surfaces over the past decade or so. The concepts involved in surface structure determination may be extended to the study of buried interfaces. A formalism for the determination of the structure of epitaxial interfaces is outlined, together with its application to the NiSi2/Si(111) interface. The use of a grazing incidence diffraction geometry to investigate strain relaxation in latticemismatched epilayers on the monolayer scale is discussed. The technique has been employed to monitor strain relaxation in thin Ge overlayers on Si(001) substrates with greater sensitivity than is attainable using other techniques. The results indicate that strain relaxation in this system relates primarily to islanding on the surface rather than the formation of misfit dislocations.

Photoelectron holography = holography + photoelectron diffraction

Photoelectron holography produces three dimensional images of surface sites with atomic scale (0.5Å) resolution. While experimentally identical to two angle photoelectron diffraction, photoelectron holography is a true holographic technique. The atomic resolution images can be obtained by Fourier transformation. I will review the principles of holography, focusing on the orginal work of the inventor of holography, D. Gabor, and the principles of photoelectron diffraction, discussing my own view of the important steps which brought us to our present understanding. Finally I will show how photoelectron holography partially fulfills the orginal goals of Gabor and provides the long sought direct-analysis of photoelectron diffraction for surface structure determination.

The sensitivity of diffuse leed experiments to atomic positions

The new technique of diffuse low energy electron diffraction (DLEED) is finding increasing use for surface structure determination. We explore its sensitivity to structural parameters, particularly to the lateral positioning of surface atoms. Our conclusions are that its sensitivity is uniform over a wide range of data. except for the general increase of sensivity with energy to be expected on the grounds of decreasing wavelength of the probe.

A novel procedure for fast surface structural analysis based on LEED intensity data

By evaluating LEED intensities from different diffraction beams taken only at discrete energy intervals (which may be as large as 15–20 eV) the same degree of reliability in surface structure determination can be reached as with the conventional techniques based on analysis of continuous I/V-spectra. The minimum of the corresponding R-factor can be found by a least-squares fit method, as will be exemplified with a system in which 8 structural parameters were subject to simultaneous refinement.

A novel procedure for fast surface structural analysis based on LEED intensity data

By evaluating LEED intensities from different diffraction beams taken only at discrete energy intervals (which may be as large as 15–20 eV) the same degree of reliability in surface structure determination can be reached as with the conventional techniques based on analysis of continuous I V -spectra. The minimum of the corresponding R-factor can be found by a least-squares fit method, as will be exemplified with a system in which 8 structural parameters were subject to simultaneous refinement.

The theory of tensor LEED

We present a method for high-speed surface structure determination by low energy electron diffraction called tensor LEED. Tensor LEED is a perturbative approach to the calculation of LEED I – V spectra in which an initial reference surface is distorted to generate a particular trial structure. If these atomic displacements are small then we can use a first order perturbation theory to calculate the resultant changes in the LEED I – V spectra. This leads to a highly efficient calculational technique which can reduce the computational effort expended upon a typical structure determination by several orders of magnitude.