Current Image Diffraction Research Articles

Surface potential effects in low‐energy current image diffraction patterns observed on the Al(001) surface

Fine structure observed in high‐resolution low‐energy electron diffraction (LEED) measurements near the energy threshold for emergence of new beams has been attributed to surface barrier effects. Recently, the surface barrier has been suggested as the source of the fine structure observed in current image diffraction (CID) patterns obtained by rastering the primary beam across an Al(001) crystal surface at a constant energy. This suggestion was based on kinematic arguments which correlated the emergence angle for a new electron beam with the observed structure in the CID pattern. In this work, the angular dependence of the elastic component of the total crystal reflectivity is calculated at constant energy. The calculations are based on full dynamical theories such as used in LEED but incorporating different surface barrier models to account for the saturating image potential. The results are compared with the experimental CID results.

Surface resonance effects in high- and low-energy electron diffraction patterns in molybdenite

The high-energy electron diffraction surface analysis method of convergent beam reflection high-energy electron diffraction is compared with its low-energy counterpart current image diffraction. For the particular case of molybdenite it is demonstrated how a multislice diffraction computation scheme is able to faithfully reproduce experimental results for both techniques. The nature of surface resonance effects in the patterns is explored by means of this dynamical calculation. While many of the features can be explained by simple geometrical and Bragg’s law arguments, dynamical calculations are shown to be necessary for the detailed understanding of contrast distributions and their application to surface characterization.

Current image diffraction patterns of molybdenum disulphide

Current image diffraction patterns of molybdenum disulphide

Current image diffraction patterns of molybdenum disulphide

Current image diffraction (CID) patterns are reported for very low energy electron incidence on the basal (001) MoS 2 surface. Comparisons are made between such patterns and the results of low energy electron diffraction calculations performed using the multislice scattering matrix method. From the good agreement between experiment and calculation it is concluded that the contrast changes in the CID patterns are determined by elastic diffraction effects and that the CID technique can provide information on surfaces of such layered transition metal dichalcogenide compounds.

Interference effects in two-dimensional electron channeling

Interference effects have been identified with two-dimensional channeling features in current image diffraction (CID) patterns from Al(001) and Al(111) surfaces. These channeling lines are associated with the emergence conditions of {10} type LEED beams for both surfaces. The interference is seen as pairs of narrow, closely-spaced, parallel lines which move spatially with energy changes as calculated from theory. Because the surface potential barrier is a factor at these low energies in the scattering process, dynamic calculations can be expected to provide information about this part of the surface structure.

A comparison for the Al (001) and (111) surfaces between scattering matrix calculations and current image diffraction patterns

A comparison for the Al (001) and (111) surfaces between scattering matrix calculations and current image diffraction patterns

A comparison for the Al (001) and (111) surfaces between scattering matrix calculations and current image diffraction patterns

Results of low energy electron diffraction calculations using the scattering matrix method are presented for the incident energy range 22 to 29 eV and 7.5 to 11 eV for, respectively, the (001) and (111) Al faces. Comparisons are made with reported measured current image diffraction patterns. In particular it is shown that even at the low range of energies considered in this paper, the structure of the measured patterns is dominated by the elastically backscattered electrons. The role of geometrical analysis in these type of results is discussed.

Calculation of current image diffraction patterns

The formation of current image diffraction patterns is explained by calculating the effective field intensity inside a crystal. The current image diffraction patterns are related to surface state resonance effects in LEED and are well reproduced by calculating the effective transmitted intensity inside a crystal. An example given in this paper is for Al(001) with very low energy incident beams between 22 and 28 eV. The effects of surface potential barrier and an absorption part of the potential on CID patterns have been examined for studying the sensitivity of the patterns to surface states of the crystal.

Calculation of current image diffraction patterns

Calculation of current image diffraction patterns

Low energy electron channeling observed by current image diffraction (CID)

Electron channeling patterns observed at LEED energies contain surface structural information. Calculations indicate that surface layer relaxations contribute to the width of the channeling lines and large relaxations can result in line splitting. Possible evidence of this has been observed on aluminum. CID images containing lines due to elastic scattering from different sets of planes can be used to determine the inner potential and planar separation simultaneously at a specified primary electron beam energy. Generally, these channeling patterns observed experimentally are in good agreement with computations based on the simple Bragg theory.

Low-energy electron current image diffraction (CID) of the basal plane of titanium

The basal plane of titanium has been studied by low-energy electron current image diffraction. Using a simple model, the average inner potential and surface interlayer distance were deduced from Bragg scattering features in the diffraction patterns which were obtained from a series of photographs of crystal surface images obtained at various primary beam energies. Computed geometric changes of Bragg scattering as a function of angle of incidence are presented for the energy range 0–200 eV. It is suggested that an energy monochromator incorporated in the electron gun would improve the pattern resolution and bring out energy sensitive lines which are not observed under the present conditions. Image enhancement methods would also increase the precision of the measurements.

Low-energy electron current image diffraction (CID) of the basal plane of titanium

Abstract The basal plane of titanium has been studied by low-energy electron current image diffraction. Using a simple model, the average inner potential and surface interlayer distance were deduced from Bragg scattering features in the diffraction patterns which were obtained from a series of photographs of crystal surface images obtained at various primary beam energies. Computed geometric changes of Bragg scattering as a function of angle of incidence are presented for the energy range 0–200 eV. It is suggested that an energy monochromator incorporated in the electron gun would improve the pattern resolution and bring out energy sensitive lines which are not observed under the present conditions. Image enhancement methods would also increase the precision of the measurements.

Surface state resonance in LEED and its relation to CID pattern

The effective transmitted intensity of LEED is calculated to show a strong damping of the incident beam inside a crystal under surface state resonance conditions, and is compared with recently reported current image diffraction (CID) patterns. Good agreement between the transmitted intensities and CID patterns have been obtained, which shows that CID patterns originate mainly from surface state resonances. This result implies that CID patterns should be sensitive to surface states of a crystal.

Current image diffraction (CID) of single crystal metal surfaces

Images of single crystal metal surfaces made with a low energy (<400 eV) scanning electron beam exhibit diffraction patterns in the absorbed current. Dynamical theoretical calculations of the total reflectivity of the surface as a function of incident polar and azimuthal angles at a specified energy indicate that these patterns are primarily due to the elastically backscattered electrons. In this paper, several patterns on aluminum (100), (110), and (111) surfaces and the copper(100) surface are presented, some of which have features quite similar to those of electron channeling patterns that have been observed at much higher energies (several keV). That is, patterns containing dark and light lines corresponding to high and low reflectivities from Bragg planes are observed. The energy dependence of these lines correspond to simple Bragg theory. Thus, the inner potential and layer displacement can immediately be extracted from the images.

Current image diffraction patterns of metal single-crystal surfaces

Diffraction patterns in the specimen current images at low electron beam energies of the (100), (110), and (111) surfaces of aluminum and the (100) surface of copper are presented along with theoretical computations for the aluminum (100) surface. With the use of computer imaging techniques, a two-dimensional diffraction pattern for the (100) face of aluminum is generated based on a dynamical calculation of the reflectivity, as a function of the polar and azimuthal angles of the incident electron beam, due to the elastically backscattered electrons. This pattern reproduces the overall experimental features leading to the conclusion that the diffraction patterns are due to the elastically backscattered electrons.

Oxygen adsorption on the aluminum (111) surface by low-energy current image diffraction: A new approach

Scanning an aluminum (111) surface with low-energy electrons (20–200 eV), diffraction patterns in the speciment current image were observed to change upon exposure to oxygen. In particular at a primary beam energy of 20.8 eV relative to the vacuum, the pattern of the clean surface includes both hexagonal and trigonal features. Upon exposure to oxygen the hexagonal part of the image fades continuously so that by 290 L of oxygen only the feature of trigonal symmetry remains. Since oxygen is known from low-energy electron diffraction (LEED) and surface extended X-ray adsorption fire structure (SEXAFS) to form an ordered ad-layer on the (111) surface of aluminum, this suggests that current image diffraction (CID) can also be used to investigate adsorption.

Oxygen adsorption on the aluminum (111) surface by low-energy current image diffraction: A new approach

Scanning an aluminum (111) surface with low-energy electrons (20–200 eV), diffraction patterns in the speciment current image were observed to change upon exposure to oxygen. In particular at a primary beam energy of 20.8 eV relative to the vacuum, the pattern of the clean surface includes both hexagonal and trigonal features. Upon exposure to oxygen the hexagonal part of the image fades continuously so that by 290 L of oxygen only the feature of trigonal symmetry remains. Since oxygen is known from low-energy electron diffraction (LEED) and surface extended X-ray adsorption fine structure (SEXAFS) to form an ordered ad-layer on the (111) surface of aluminum, this suggests that current image diffraction (CID) can also be used to investigate adsorption.