A new application of electron energy loss spectroscopy technique for a non-destructive study of the Si-SiO2 interface

Electronic structures of Si(substrate)-SiO₂ (film) interface were examined non-destructively through an application of electron energy loss spectroscopy (ELS) by utilizing variable sampling depth (10 to several tens Å) of probing electrons with different energies (200–2000 eV). The ELS measurements on Si specimens with top oxide layers explored a new feature at the energy of ～ 8 eV in the ELS spectra, which did not exist in the spectra of pure (clean) Si and thick SiO₂. The sharp interfacial structure was proposed and the possible existence of undulation in the Si-SiO₂ interface was discussed.     

Characterization of ultrathin nickel layers on Si(111) using RHEED and RBS

Ultrathin nickel layers (0–40 Å) on Si(111) were characterized using reflection high energy electron diffraction (RHEED) in conjunction with high-resolution Rutherford backscattering spectrometry (RBS). RBS shows that Ni, when deposited on Si(111) at 300 K, reacts with the Si to form clusters with an average composition close to Ni₂Si, until the clusters coalesce into a continuous layer. RHEED measurements show that the microstructure of this layer also matches the Ni₂Si composition: the film consists mainly of very fine-grained Ni₂Si crystallites (∼ 15 Å grain size) which are randomly oriented. Additional Ni deposition results in the accumulation of an unreacted fine-grained Ni layer on top of the silicide film.     

Influence of a Cr seed layer on the magnetic anisotropy of epitaxial Fe/Ag films on MgO(001)

The magnetic anisotropy of epitaxial 300 Å thick Fe films on Ag and Ag/Cr buffer layers on MgO(001) has been studied by ferromagnetic resonance and magneto-optic Kerr effect measurements. The samples were grown by molecular beam epitaxy at ambient temperature. A reduction of the effective magnetization for the samples with a Ag buffer layer is attributed to strain and dislocation formation as seen from X-ray diffraction measurements at low and high angles. In the samples with a Cr seed layer, a higher magnetic anisotropy is found which correlates with a reduced roughness.     

The kinetics of Cr layer coated on TiNi films for hydrogen absorption

The effect of hydrogen absorption on electrical resistance with temperature for TiNi and TiNi-Cr thin films was investigated. The TiNi thin films of thickness 800 Å were deposited at different angles (? = 0°, 30°, 45°, 60° and 75°) under 10^?5 Torr pressure by thermal evaporation on the glass substrate at room temperature. A layer of Cr of thickness 100 Å was coated on the TiNi thin films. The changing rate of hydrogen absorption increases after Cr layer coating because Cr enhances the catalytic properties of hydrogen absorption in thin films. The rate of hydrogen absorption increases with temperature at lower range but at higher range of temperature it was found to decrease and also it was found that the hydrogen absorption increases with angle of deposition.     

Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr₂O₃ before and after surface reduction by 2 keV Ar⁺ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr₂O₃ exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar⁺ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr²⁺ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr₂O₃ surface exhibit features characteristic of both Cr⁰ and Cr₂O₃, indicating that Ar⁺ sputtering reduces Cr₂O₃. The fact that neither the surface- nor the bulk-plasmon features of Cr⁰ can be observed in the ELS spectra obtained from sputtered Cr₂O₃ while the loss features due to Cr⁰ interband transitions are clearly present indicates that Cr⁰ atoms form small clusters lacking a bulk metallic nature during Ar⁺ bombardment of Cr₂O₃.     

The structure of the p(2×2) tellurium overlayer on the (001) surface of copper investigated by leed

Low-energy electron diffraction (LEED) intensities have been calculated for the p(2 × 2) tellurium overlayer on the (001) surface of copper and compared with measurements. The tellurium atoms are found to sit in the hollow position (coordination number 4) 1.70 ± 0.15 Å above the atomic plane of the uppermost copper layer. This distance corresponds to a copper-tellurium bond length of 2.48 ± 0.10 Å which indicates a value less than 2.65–2.67 Å typical of copper-tellurium bulk compounds. Five different potentials were used for the adsorbate atoms and a potential from overlap of atomic charge densities with the Kohn-Sham-Gaspar expression for the exchange clearly gave the best agreement with experiment.     

Structure determination of the (100) surface of rhodium by low energy electron diffraction

Elastic low-energy electron diffraction intensity data have been measured as a function of energy for two directions of incidence for the (100) surface of rhodium. The dynamical perturbation programs of Van Hove and Tong have been used for analysing these new experimental data, and it is concluded that the normal face-centered cubic registry is maintained to the surface layer. A preliminary comparison between measured and calculated I(E) curves indicates the topmost interlayer spacing to be 1.96 ± 0.10 Å, and therefore possibly slightly expanded from the bulk interlayer spacing of 1.90 Å.     

New relaxation phenomena in the outer atomic layers of Fe{211}

A low-energy electron diffraction analysis of a {211} surface of body-centered cubic iron reveals relaxations in the directions perpendicular and parallel to the surface plane. Both relaxations alternate in successive layers. The perpendicular relaxation goes from contraction of 10.5% to expansion of 5% to contraction of 1%. The parallel relaxation goes from a shift of the first layer of 0.24Å (10% of the nearest neighbor distance) toward more symmetrical registration with the second layer, to an opposite shift of 0.035 Å of the second layer with respect to the third.     

Exhibition of surface steps by high resolution electron microscopy of monoclinic rare earth sesquioxides

On high resolution structure images, the existence of a superperiod has been observed, linked with the apparition of extraspots in the diffraction diagram. The specimen (B-Sm₂O₃) can be described as composed of layers of thickness 6 Å, nearly perpendicular to the incident electron beam. These layers are of 3 different kinds. We have shown that the apparition of the superperiod can be linked with steps of height 6 or 12 Å (i.e. 1 or 2 layers): computed images, using this hypothesis exhibit the observed superperiod, even for thicknesses around 230 Å. This allows atomic steps to be imaged with a high resolution structure image.     

Application of maximum entropy method for the study of electron density distribution in SrS, BaS and PuS using powder X-ray data

A study of the electronic structure of the three sulphides, SrS, BaS and PuS has been carried out in this work, using the powder X-ray intensity data from JCPDS powder diffraction data base. The statistical approach, MEM (maximum entropy method) is used for the analysis of the data for the electron density distribution in these materials and an attempt has been made to understand the bonding between the metal atom and the sulphur atom. The mid-bond electron density is found to be maximum for PuS among these three sulphides, being 0.584 e/Å3 at 2.397 Å. SrS is found to have the lowest electron density at the mid-bond (0.003 e/ų) at 2.118 Å from the origin leaving it more ionic than the other two sulphides studied in this work. The two-dimensional electron density maps on (100) and (110) planes and the one-dimensional profiles along the bonding direction [111] are used for these analyses. The overall and individual Debye-Waller factors of atoms in these systems have also been studied and analyzed. The refinements of the observed X-ray data were carried out using standard softwares and also a routine written by the author     

Magnetotransmission and magnetoreflection in multilayer FeCr nanostructures

The magnetotransmission and magnetoreflection spectra of a seven-layer Cr(28 Å)/Fe(36 Å)/Cr(13 Å)/Fe(18 Å)/Cr(13 Å)/Fe(36 Å)/Cr(28 Å) film, their temperature and field dependences measured at various directions of a magnetic field with respect to the film plane, and the correlations between these effects are studied. The experimental results are compared with the data on the magnetization and transverse Kerr effect. The magnetotransmittance and magnetoreflectance are calculated for this film in terms of the magnetorefractive effect theory. Apart from the film layer thickness, the effective relaxation time, the plasma frequency, and the spin asymmetry parameter are shown to strongly affect both the magnitudes and spectral dependences of the magnetotransmittance and magnetoreflectance. Although the calculation results qualitatively describe the signs and the spectral and field dependences of the magnetotransmittance and magnetoreflectance, they do not agree quantitatively with the experimental results, in particular, near the visible region. The causes of the quantitative discrepancies are discussed.     

Synthesis of BaSO4 nanoparticles by precipitation method using sodium hexa metaphosphate as a stabilizer

We describe the synthesis and structure of Barium sulfate nanoparticles by precipitation method in the presence of water soluble inorganic stabilizing agent, sodium hexametaphosphate, (NaPO₃)₆. The structural parameters were refined by the Rietveld refinement method using powder X-ray diffraction data. Barium sulfate nanoparticles were crystallized in the orthorhombic structure with space group Pbnm (No. 62) having the lattice parameters a=7.215(1) (Å), b=8.949(1) (Å) and c=5.501(1) (Å) respectively. Transmission electron microscopy study reveals that the nanoparticles are size range, 30–50 nm. Fourier transform infrared spectra showed distinct absorption due to the SO₄^2? moiety at 1115 and 1084 cm^?1 indicating formation of barium sulfate nanoparticles free from the phosphate group from the stabilizer used in the synthesis.     

Order-disorder effects in GaAs(110)-oxygen interaction: A LEED-ups analysis

Cleaved GaAs(110) surfaces were exposed to oxygen (10⁶ ? 5 × 10¹⁰ L) and analysed by LEED and UPS. Very important changes in the electron diffraction occur at 10⁸ L and the analysis of the LEED pattern shows a bulk-like diffraction superposed on a high background, implying that a thin layer (1–3 Å) disordered by the oxidation covers the GaAs lattice relaxted to its bulk structure.     

Oxygen chemisorption and initial oxidation of Cr(110)

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ～5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase (Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value (Δφ_min = ?0.24 eV at6L), and five additional ELS peaks were observed at ～2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr³⁺ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr³⁺ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again (Δφ_max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M_2,3(Cr)L_2,3(O)L_2,3(O) cross transition and the M_2,3VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr₂O₃. Finally, above 15 L, the work function decreased slowly (Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr₂O₃ film, and (4) slow thickening of Cr₂O₃ above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.     

Dispersion of surface plasmons in dielectric-metal coatings on concave diffraction gratings

Anomalies in the intensity ofp-polarized light from concave diffraction gratings (Wood's anomalies) have been used to obtain surface plasmon dispersion curves for dielectric-metal layers on the grating surface. These include a 350 Å MgF₂ layer and Al₂O₃ layers varying from a few angstroms to over 800 Å on an Al substrate. The wavelength range of the incident and diffracted light is from the visible to the vacuum ultraviolet (7,500?500 Å). Anomalous polarization peaks for Al₂O₃ layers in the vacuum ultraviolet (at ～ 1,600 Å) are shown to shift significantly to longer wavelengths for only a few angstroms (up to 50 Å) of oxide thickness, while for thicknesses greater than ～ 50 Å any further shift is small. In the visible region (> 4,000 Å), on the other hand, the wavelength shifts are small for dielectric thicknesses up to ～ 50 Å, but are large for thicknesses of several hundred angstroms. These results are in substantial agreement with the theoretical dispersion curves for these cases. Also considered are some of the effects of diffusion pump oil.     

Structural Analysis of Aluminum Oxyhydroxide Aerogel by Small Angle X-Ray Scattering

The work presents studies on the microstructure and mesostructure of nanostructured aluminum oxyhydroxide formed as a high porous monolithic material through the surface oxidation of aluminum liquidmetal solution in mercury in a temperature- and humidity-controlled air atmosphere. The methods of X-ray diffraction analysis, thermal analysis, the low temperature adsorption of nitrogen vapors, transmission electron microscopy, small-angle and very small-angle neutron scattering, and small-angle X-ray scattering are used for comprehensive investigation of the samples synthesized at 25°С as well as that annealed at temperatures up to 1150°C. It is found that the structure of the monolithic samples can be described within the framework of a three-level model involving primary heterogeneities (typical length scale of rc ≈ 9–19 Å), forming fibrils (cross-sectional radius R ≈ 36–43 Å and length L ≈ 3200–3300 Å) or lamellae (thickness T ≈ 110 Å and width W ≈ 3050 Å) which, in turn, are integrated into large-scale aggregates (typical size R _c ≈ 1.25–1.4 μm) with an insignificant surface roughness. It is shown that a high specific surface (~200 m²/g) typical for the initial sample is maintained upon its thermal annealing up to 900°С, and it decreases to 100 m²/g after heat treatment at 1150°С due to fibrillary agglomeration.     

Search for ferromagnetism in ultrathin epitaxial films: Cr/Au(100), Cr/Cu(100), and Fe/Cu(100)

The surface magneto-optic Kerr effect (SMOKE) technique was used to search for ferromagnetism in monolayer-range films of Cr and Fe grown on Au(100) and Cu(100). The growth modes were characterized using low energy electron diffraction (LEED) and Auger electron spectroscopy. The fcc structure of Cr could not be stabilized on Cu(100). Ferromagnetism was not observed for the Cr/Au(100) films at temperature above 100 K. Ferromagnetism also was not observed for fcc Fe/Cu(100) grown at room temperature; but for growth at >150°C, a ferromagnetic, metastable state was observed for the top layer of the Fe film, in the absence of bulk ferromagnetism. The ferromagnetic Fe/Au(100) system was used to establish the sensitivity of the approach.     

Multi-walled boron nitride nanotubes composed of diverse cross-section and helix type shells

High-resolution transmission electron microscopy (HRTEM) and electron diffraction analysis were performed on pure multi-walled novel type boron nitride nanotubes (BNNTs) by using a field-emission high-resolution 300 kV transmission electron microscope JEM-3100FEF (Omega filter). In contrast to commonly observed monochiral multi-walled BNNTs, the present tubular shells within every individual BNNT (up to ～50 shells) revealed the whole range of helixes, i.e., from zig-zag to arm-chair. This being paired with the wide variations in cross-sectional tubular shapes, i.e., from a faceted polygon to a cylinder, results in complex overall tubular structures. The BN atomic lattice fringe separations, down to 1.25 Å, on most symmetrical arm-chair (d_10-10=1.25 Å) and zig-zag (d_11-20=2.15 Å) tubular wall fragments within the same multi-walled BN nanotubes were ultimately achieved during HRTEM.     

A leed crystallographic analysis for the Cu(100)-(2 × 2)-S surface structure

An intensity analysis with low-energy electron diffraction is reported for the (2 × 2) surface structure obtained by the adsorption and presumed dissociation of H₂S on the (100) surface of copper. Intensity-versus-energy curves were measured with a video LEED analyser for five diffracted beams at normal incidence and for eleven beams at an off-normal direction with a polar angle of incidence equal to 14°. Comparisons were made with intensity curves calculated with the renormalised forward scattering method for three types of structural models in which the metal atoms remain in their regular bulk positions. The best correspondence between experimental and calculated intensities occurs with sulphur atoms adsorbed in the “expected” 4-coordinate adsorption sites. The reliability index proposed by Pendry is minimised with S atoms 1.32 Å above the topmost metal layer; this corresponds to nearest-neighbour S-Cu bond distances equal to 2.24 Å. This value appears broadly consistent with a measurement by photoelectron diffraction, as well as from model predictions.