Electron Energy Loss Spectroscopy Results Research Articles

Adsorption of CO on Cs-precovered Ru(001): formation of a double-layer structure [formula omitted] and complete screening of the CO dipole

Adsorption of CO on Cs-precovered Ru(001) with one-monolayer coverage ( θ Cs = 0.33) has been investigated by means of electron energy loss spectroscopy (EELS), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). We found unusual adsorption of CO on the Cs(1 ML) Ru(001) surface. Gaseous CO adsorbs on the completely Cs-precovered surface at 85 K to a saturation coverage of 0.23. Although the low-resolution EELS and UPS results indicate that CO molecularly adsorbs on this surface, the CO stretching mode is not observed in the specular mode of high-resolution EELS but clearly appears at quite a low frequency of 1245 cm −1 in the off-specular mode. From a combination of the EELS, TPD and XPS experiments, we propose an adsorption model in which the CO molecules displace one-fourth of the Cs adatoms from the preadsorbed Cs monolayer, resulting in formation of a double-layer structure, Cs Cs + CO Ru(001) ; the displaced Cs atoms adsorb on the “rotated-p(2 × 2)” lattice which is composed of Cs and CO with a ratio of 1 : 1. In this adsorption state, it is considered that the CO species has an inclined or a side-on-bonded geometry and that the CO dipole is effectively screened by the second-layer Cs. This complete screening of the CO dipole is explained in terms of combined effects of the embedding of CO into the double-layer structure, a large polarizability of Cs and laterally delocalized electrons in the double-layer structure.

An EELS and EXELFS study of amorphous hydrogenated silicon carbide

A series of thin films of amorphous hydrogenated silicon carbide (a-Si∶C∶H) produced by RF plasma decomposition of propane and silane has been studied by electron energy-loss spectroscopy (EELS) and extended energy-loss fine structure (EXELFS) studies. The composition of the films has been determined by EELS and the nearest neighbour spacings have been determined by EXELFS. These results, along with the energy of the plasmon loss peaks, have been compared with the deposition conditions for each film. The results show that for a large gas ratio (C3H8/(C3H8+SiH4)) the films have a high proportion of carbon and are similar to a-C∶H in structure, whereas those films prepared with Y = 0.4 or 0.5 have nearest neighbour spacings consistent with those for tetrahedrally bonded carbon. The films prepared with lowest Y have nearest neighbour spacings similar to those for amorphous silicon carbide. The results for a-Si∶C∶H have been compared with the results of EELS and EXELFS of CVD diamond films, amorphous carbon and amorphous silicon.

Interaction of oxygen with a Ge-covered Si(100) surface

The interaction of oxygen with a Ge-covered Si(100)×2 × 1 surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), thermal desorption and work function (WF) measurements. It has been found that the initial oxygen sticking coefficient of the Ge-covered (1 < θ Ge < 3) Si(100) is almost independent of the Ge coverage, θ Ge, and is close to that of a bare Si surface. Strong suppression of the oxygen uptake rate has been observed at 300 K for moderate oxygen coverages. This effect is weaker for the oxygen adsorption kinetics at 500 K. AES and EELS results show that short time annealing above 370 K can induce oxygen transfer from Ge to Si atoms. This is accompanied by diffusion of Si and formation of Si oxides over the Ge layer, as judged from the WF and AES data. The annealing experiments show that the shape of the GeO TPD curves is determined by complicated reduction-oxidation reactions in which diffusion of oxygen is involved.

FORMATION AND CHEMICAL REACTION OF THE Mn/GaAs(100) INTERFACE

The interface formation of room temperature deporsited Mn on GaAs(100)(4×1) surface was studied by low energy electron diffraction (LEED), X-ray photoelectron spec-troscopy (XPS), electron energy loss spectroscopy (EELS) and ultra-violet photoelectron spectroscopy (UPS). It was found that above the Mn coverage of 0. 25 nm, the LEED pattern disappeared completely, which indicated that the Mn overlayer did not form single crystalline structure. The layer-by-layer growth mode at the initial stage was proved by the results of both LEED and EELS. By careful study of the intensities and line shapes of the Ga2p3/2 and As2p3/2 core level photoemission spectra, we observed that the exchange reaction between the deposited Mn and the substrate occurred at very early stage. The Ga atoms released by the exchange reaction distributed within a region about 3 nm above the original interface, while the As atoms which combined with Mn to form As-Mn compound segregated on the top of Mn overlayer. The layer-by-layer growth mode persisted until the latest stage of the deposition. Some of the above results were intuitively confirmed by UPS measurements.

Identification of segregation from borophosphosilicate glass films during annealing

The structure and crystallinity of a segregation on a borophosphosilicate glass (BPSG) film have been clarified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), transmission electron diffraction (TED), and electron energy loss spectroscopy (EELS). The segregation arises on the BPSG film surface after annealing in nitrogen ambient. Its size is below 0.5 μm. The segregation is identified as crystalline BPO4 from the results of TED and EELS. In this letter, TED pattern of crystalline BPO4 is indicated in detail. The growth mechanism of crystalline BPO4 is proposed on the basis of a series of the results.

Models for the Interpretation of the Different Interfaces Sb/InP(I00) Using Quantitative Results of AES and EELS

The model of quantitative interpretation of Auger electron spectroscopy (AES) results is described and some complementary electron energy loss spectroscopy (EELS) results are also reported. It is shown that the InP(100) surfaces perturbed by the Ar+ cleaning treatment can be ordered by Sb deposition. The variations of ditferent Auger lines are interpreted, and the transformations 3D → 2D of the initially formed In clusters can be well-followed during the frrst stages of the deposition. PACS numbers: 68.35.Βs, 68.35.Fx, 72.80.Ey, 82.80.Pv

Ordered Hg0.8Cd0.2Te (111)A and (111)B surfaces: Preparation by annealing in Hg vapor and characterization by low-energy electron diffraction, Auger electron spectroscopy, and electron energy-loss spectroscopy

Reported here are studies in which an oriented single-crystal, Hg0.8Cd0.2Te (111), was etched with methanolic bromine solution, ion bombarded with Ar+, and annealed in Hg vapor. Both parallel faces of the crystal, the metal-terminated (111)A surface and the Te-terminated (111)B surface, were characterized after each of the above stages of preparation by Auger electron spectroscopy (AES), electron energy-loss spectroscopy (EELS) and low-energy electron diffraction (LEED). After polishing in air the (111)A and (111)B surfaces were found to be contaminated with C and S-containing substances, and deficient in Cd; the LEED patterns were diffuse. EELS spectra showed a broad C–H stretching band near 2980 cm−1 and another broad band centered at 1300 cm−1 due to C–H bending and C–C stretching of surface hydrocarbons. Etching with bromine solution in air removed S-containing surface contaminants, without significantly changing the LEED and EELS results. Argon ion-bombardment removed the C contamination and increased the Cd content to expected levels, while decreasing the Hg content substantially. LEED showed a (111)A[1×1] pattern with strong diffuse scattering, while the (111)B[1×1] pattern was comparatively weak, indicating that detectable long-range order existed somewhere within the outermost few atomic layers. EELS of both faces displayed a peak at about 140 cm−1 attributable to the excitation of surface phonons. When the ion-bombarded crystal was annealed in Hg vapor, the Hg surface concentration increased. The ratios of metal to Te Auger intensity (IHg/ITe and ICd/ITe ) were larger for the (111)A than for the (111)B surface. The A-face yielded a sharp (2×2) LEED pattern, while the B-face displayed a reproducible, complex, three-fold symmetric pattern indicative of a faceted surface. The annealed surfaces exhibited virtually the same EELS spectra as bombarded surfaces. The annealed surfaces are stable in vacuum, as judged by AES, LEED, and EELS.

Application of electron energy loss spectroscopy to microchemical analysis of Ti-modified austenitic stainless steel irradiated in HFIR

Microchemical analysis on Ti-modified austenitic stainless steel specimens, irradiated to 56 dpa at 773 K in the High Flux Isotope Reactor (HFIR) was conducted by utilizing electron energy loss spectroscopy (EELS). EELS results of precipitates extending through the foil surface were compared with the typical results by energy dispersive X-ray spectroscopy (EDS) on the precipitates, which were extracted on carbon replica films. Reasonably good agreement was obtained between the results by EELS and those by EDS. Comparing the EELS spectrum from a precipitate and the adjacent matrix, spatial resolution was obtained to be about 20 nm or better. EELS results on dislocation loops with sizes of about 20 nm in diameter and oriented normal to the foil surface revealed that the enrichment of Ti and Cr had occurred by irradiation. The pressure of helium gas atoms in a cavity of about 10 nm in diameter was estimated from the EELS spectra. The energy shift of the plasmon loss peak indicated that the pressure was of the order of 10 8 Pa.

Small metallic particles studied by optical and electron-optical spectroscopy

Electron energy loss spectroscopy (EELS) in a transmission electron microscope (TEM) and photoacoustic spectroscopy (PAS) has been used to study the electronic structure of small Ag, In and Au particles, deposited on thin carbon foils, quartz supports or as self-supporting layers, respectively.In the low energy-loss region the EELS spectra of Ag particles with diameters of ca. 50 nm are different from the spectra of the homogeneous Ag layers, mainly owing to the influence of the carbon foil. By decreasing the packing density of the Ag particles, a shift of the plasma losses to lower energies is observed in the energy region 10 eV. The optical behaviour of homogeneous metallic and small particle layers on thin quartz supports was studied by PAS in the ultraviolet/visible (UV/VIS) spectral region. The homogeneous layers, e.g. of Ag, show, in the UV range, the well known change of reflectance due to an interband transition of bare silver. For Ag particle layers surface plasma resonances are excited. With increasing packing density of the particles the absorption peak observed is red-shifted and becomes broader, whereas the results of EELS in the low energy-loss region revealed a shift to higher energies.To obtain additional information investigations of In and Au particles were performed.

Oxygen K-edge fine structure of Tl2Ba2Can−1CuxO2n+4 studied by electron energy loss spectroscopy

The fine structures of the oxygen K-edge in Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 ( n =1−3) with double Tl-O layers were investigated by electron energy loss spectroscopy (EELS). A peak at 528 eV becomes smaller with the increase of the number of Cu-O layers, n , in Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 , indicating that the average hole content in a Cu-O bonding decreases with the increase of n . Reducing the hole content in Tl 2 Ba 2 Cu 1 O 6 by replacing a part of Ba(+2) with La(+3), the peak at 528 eV becomes smaller and T c of the compounds increases up to 55 K. It was shown that holes of Tl 2 Ba 2 Cu 1 O 6 are overdoped. The present EELS results were compared with the previous results of Tl 1 Ba 2 Ca n −1 Cu n O 2 n +3 with single Tl-O layers.

CHaracterization of CO binding sites on Rh{111} and Rh{331} surfaces by XPS and LEED: Comparison to EELS results

Changes in the nature of the binding site of chemisorbed CO on the Rh{111} and Rh{331} single crystal surfaces during adsorption and desorption have been monitored by X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). Two bonding states of molecular CO have been identified from the O 1s photoemission line. These states are assigned as atop and bridge-bonded species and are observed to be coverage and temperature dependent. On both surfaces atop sites are populated first and at higher CO coverages bridge sites are filled. On Rh{111} the bridge sites are filled at a CO coverage of θ CO ∼ 0.50 and their presence is correlated with a change in the LEED pattern. The presence of the step atoms on the Rh{331} surface markedly influenced the sequential filling of binding sites in comparison to that observed on the Rh{111} surface. A comparison of our data to previous Electron Energy Loss Spectroscopy (EELS) work on Rh{111} is in remarkable quantitative agreement with EELS peak heights.

Origin of the Coverage-Dependent Vibrational Shift for O on Ni(100)

The $p(2\ifmmode\times\else\texttimes\fi{}2)$ and $c(2\ifmmode\times\else\texttimes\fi{}2)$ coverages of O on Ni(100) are studied with a cluster model. The $p(2\ifmmode\times\else\texttimes\fi{}2)$ coverage is modeled by the addition of a single O at a fourfold site. The $c(2\ifmmode\times\else\texttimes\fi{}2)$ is modeled by the explicit inclusion of four additional O atoms. Consistent with electron energy-loss spectroscopy results, the O-Ni vibration is 85 ${\mathrm{cm}}^{\ensuremath{-}1}$ lower for the $c(2\ifmmode\times\else\texttimes\fi{}2)$ cluster than for the $p(2\ifmmode\times\else\texttimes\fi{}2)$. The height of O above the surface is 0.11 \AA{} smaller for the $c(2\ifmmode\times\else\texttimes\fi{}2)$ coverage. The nature of the O-to-Ni bonding is similar for both coverages.