New opportunities for quantitative analysis as applied to reflected electron energy loss spectroscopy of Fe/Si structures

The feasibility of determining the elemental composition, chemical state, and element distribution across the depth in a subsurface region using the computer simulation of the electron inelastic scattering cross section is demonstrated with iron layers on silicon substrates. Analysis is carried out based on the dielectric theory and on the experimental determination of the product of the electron inelastic mean free path by the inelastic scattering cross section from reflected electron energy loss spectra.     

Study of He bubbles in Al by electron energy loss spectroscopy

High energy electron energy loss spectroscopy (ELS) has been used to study He bubbles in Al, which were obtained by irradiation of He ions or α-particles of energy ranging from 500 eV to 8 keV and fluences 1x10²⁰m^?2 and 5x10²⁰m^?2. ELS reveals surface plasmon losses of the Al cavities as well as pressure shifts of the He-resonance lines as large as 1 eV. This is viewed as evidence for the existence of a so-called super-dense He in the bubbles. ELS is therefore a promising tool for obtaining information on the He pressure within the bubbles.     

K-shell excitation of HCN by electron energy loss spectroscopy

Small angle inelastic scattering of fast electrons has been used to study carbon and nitrogen K-shell excitation and ionization of HCN. The K→π* transitions in HCN have been investigated with high resolution.     

Reflection electron energy loss spectroscopy of aluminum

Reflection electron energy loss spectra (REELS) of Al(111) single crystal and of the aluminum polycrystalline (poly Al) film were measured at 200 eV and 1000 eV electron energies for a variety of experimental geometries and were mutually compared. No anisotropy was found for the poly Al, as expected. Polar intensity plots evaluated from the elastic (no loss) and inelastic first surface plasmon- and first bulk plasmon-loss intensities of the Al(111) surface show clearly discernable peaks for both considered electron energies. Their positions on the angular axis are the same for the elastic as well as for the inelastic, surface and bulk plasmon-loss peaks. The polar plots of intensities of the elastically and inelastically reflected electrons were compared to calculated intensities of photoelectrons emitted from the Al 2s core level to the same kinetic energy. Peak positions in the theoretically determined polar plots of electron intensities agree with those obtained experimentally in REELS.     

Image simulation for electron energy loss spectroscopy

Aberration correction of the probe forming optics of the scanning transmission electron microscope has allowed the probe-forming aperture to be increased in size, resulting in probes of the order of 1 A in diameter. The next generation of correctors promise even smaller probes. Improved spectrometer optics also offers the possibility of larger electron energy loss spectrometry detectors. The localization of images based on core-loss electron energy loss spectroscopy is examined as function of both probe-forming aperture and detector size. The effective ionization is nonlocal in nature, and two common local approximations are compared to full nonlocal calculations. The affect of the channelling of the electron probe within the sample is also discussed.     

Background subtraction in electron spectroscopy by use of reflection electron energy loss spectra

The use of reflected electron energy loss spectra (REELS) in deconvoluting the inelastic background signal from XPS and AES spectra from homogeneous samples is studied. It is demonstrated that under certain assumptions, the cross section for inelastic electron scattering can be extracted from a REELS spectrum. This cross section is applied to deconvolute an experimental XPS spectrum of aluminium. The method, its limitations and its relation to other methods are discussed.     

Study of thickness-dependent magnetic and transport properties of Fe/Al nanostructures

The paper presents magnetic and transport properties of compositionally modulated Fe/Al multilayer structures (MLS), with an overall atomic concentration ratio of Fe:Al = 3:1, 2:1 and 1:1. All MLS show soft ferromagnetic behaviour at room temperature (RT) with an in-plane easy axis of magnetization. In each case, coercivity increases continuously and magnetization decreases with an increase in temperature due to enhancement in the anisotropy as a result of non-uniform and disordered formation of thin intermixed (dead) FeAl layer at the interfaces. The Curie temperature obtained for the MLS is much less than that of bcc Fe but is well above RT. The observed magnetic behaviour is mainly attributed to the formation of different FeAl phases and increase in anti-ferromagnetic interlayer coupling with addition of Al. The formation of these phases is also supported by resistivity results. The results of this research enabled us to understand that by controlling of layers thickness and temperature in multilayer systems, the nanogranular thin films with good resistive and soft magnetic properties can be obtained.     

High-detective-quantum-efficiency fast-electron detector for electron energy loss spectroscopy

In order to increase the sensitivity of the parallel electron detector used in electron energy loss spectroscopy (EELS), we have developed a direct electron exposed detector, based on a photodiode array (PDA). This work investigates the performance of this detector at 100 keV incident electrons in terms of the detective quantum efficiency (DQE), the modulation transfer function (MTF) and radiation damage.     

Study of the optical properties of pyridine intercalated niobium disulphide by electron energy loss spectroscopy

High energy electron energy loss spectroscopy (EELS) has been used to study the influence of the intercalation with pyridine on the optical properties of the host material 2H-NbS₂. The optical joint density of states function OJDOS(ω) of the intercalated material is found to be well represented by a linear superposition of the OJDOS of pure NbS₂ and that of pyridine. A charge transfer from pyridine to NbS₂ of (0.27±0.05) electrons per NbS₂ formula unit is deduced from the energy shift of the d-band plasmon loss relative to the pure host material. Assuming this shift to be caused by a change in the charge density contributing to the d-band plasmon is confirmed by an unchanged effective mass of the electrons in the d_z2 band upon intercalation as has been deduced from the dispersion of the plasmon in question.     

Determination of valence-electron concentration in AgTe3 by electron energy loss spectroscopy

Electron energy loss studies have shown that the valence-electrons in AgTe₃ give rise to an electron plasma oscillation with a frequency that can be related to the density of these electrons. The density of valence-electrons in AgTe₃ is similar to that in Te but larger than that in Ag. The EELS spectra of AgTe₃ and Te are similar with the exception of an interband transition of 5 eV which occurs in Te but not in AgTe₃. These similarities support the view that AgTe₃ is a silver-stabilized cubic Te.     

Auger electron spectroscopy and electron energy loss spectroscopy studies on carbonization of Si(100) and (111) surfaces with ethylene

The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C₂H₄) at a pressure of 1.3×10⁻⁴ Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C₂H₄ exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C_KLL spectra from the sample subjected to various C₂H₄ exposure levels, and from β-SiC crystal.     

Room temperature hydrogenation of the Si(001)2 × 1 surface studied by angle-resolved electron energy loss spectroscopy

We observed the hydrogen adsorption on the Si(001)2 × 1 surface achieved at room temperature by angle-resolved electron energy loss spectroscopy (AR-ELS) and elastic low-energy electron diffraction. From measurements of the intensities of elastically diffracted beams, we found a characteristic hydrogen covered surface (called Si(001)2 × 1H(RT) surface in this paper), where all the diffracted beam intensities were enhanced drastically and a sharp 2 × 1 LEED pattern was observed. The angular dependence of the elastically diffracted beams on the 2 × 1H(RT) surface was different from that on the monohydride 2 × 1:H surface. On the 2 × 1H(RT) surface the S₃, transition from the back bond surface state disappeared in contrary to the 2 × 1:H surface and two hydrogen induced transitions were observed at 7.0 and 8.0 eV in AR-ELS spectra. We revealed that the 2 × 1H(RT) surface consisted of the monohydride and the dihydride phases with comparable weights. Additionally, we found the new transition S'₁, ascribed to the newly produced dangling bond surface state due to the rupture of the dimerization bond with hydrogen adsorption.     

Silicon surface passivation by aluminium oxide studied with electron energy loss spectroscopy

The origin behind crystalline silicon surface passivation by Al₂O₃ films is studied in detail by means of spatially‐resolved electron energy loss spectroscopy. The bonding configurations of Al and O are studied in as‐deposited and annealed Al₂O₃ films grown on c‐Si substrates by plasma‐assisted and thermal atomic layer deposition. The results confirm the presence of an interfacial SiO₂‐like film and demonstrate changes in the ratio between tetrahedrally and octahedrally coordinated Al in the films after annealing. These observations reveal the underlying origin of c‐Si surface passivation by Al₂O₃. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of atomic structures of diamond-like amorphous carbon by electron energy loss spectroscopy

Research into amorphous carbon films has been developed to such an extent that the film property can be fine tuned to mimic that of the crystalline counterparts, be it diamond, graphite, or even fullerene-like. This flexibility makes such films ideal for a wide range of applications from anti-abrasive window coating to lubricating layers on the surface of magnetic hard-disk. Not only are their mechanical properties interesting, electrically the diamond-like amorphous carbon films are also easier to dope than crystalline diamond, making them potentially a better alternative to amorphous silicon for photovoltaic devices. We will show that electron energy loss spectroscopy, in particular the carbon 1s core absorption spectroscopy, has been instrumental in revealing the nature of the bonding between carbon atoms. Such information allows microstructure models to be developed for proper understanding of the observed properties and providing scientific basis for future improvement.     

Frenkel exciton model of electron energy loss spectroscopy in -PTCDA

Recent experimental findings concerning electron energy loss spectroscopy in -Perylene-tetracarboxylic-dianhydride are analysed in terms of a Frenkel exciton model. Taking into account the energy dispersion of excitations with finite momentum transfer, the k-dependence of the dielectric tensor and the corresponding electron energy loss functions can be calculated. The exciton dispersion with a minimum at k≠0 yields a red shift of the lineshape of loss functions at large k, as observed experimentally.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

Azimuthal dependence of impact scattering in electron energy loss spectroscopy

The azimuthal dependence of electron energy loss spectroscopy (EELS) dipole and impact scattering intensity has been measured. Spectra for a saturation coverage of H adsorbed on W(110) exhibit loss peaks due to impact scattering from adsorbate vibrational modes. The intensity of the 160 meV loss peak has been measured as a function of the azimuthal angle between the scattering plane and a mirror plane of the $\text{s}$urface. The angular pattern has strong maxima oriented perpendicular to the <111 > rows of atoms on the surface, and has the C_2v symmetry of the W(110) surface. This azimuthal dependence is strikingly different from the nearly isotropic angular dependence of dipole scattering from Cl adsorbed on W(110). Selection rules for impact scattering account for the general features of the angular pattern based on asymmetric stretch modes associated with bridge site H atoms.We have shown that the azimuthal dependence of the 36 meV Cl/W(110) dipole scattering loss peak is isotropic and that the 160 meV H/W(110) impact scattering loss peak exhibits a striking azimuthal pattern with C_2v symmetry. The symmetry and deep minima suggest that selection rules play a central role in determining the azimuthal pattern. Application of these rules to two orthogonal directions (as in ref. 6) may be misleading, as is clear from Fig. 2, because essential features of the pattern will not be observed. Our analysis of the full pattern has suggested two bridge sites may be occupied at saturation coverage, but has still not resolved certain questions about the H/W(110) system.

1. Impact scattering selection rules for potential adsorbate sites. The listed directions are the intersections of the planes with the (110) surface for the mirror planes and the scattering planes, and the displacement directions for the adsorbate vibrational modes. Modes are assumed to be strictly parallel to the surface. The long bridge site is between two W atoms along the <001 > direction, the short bridge site is between two W atoms along the <$\text{111}\text{-}$ > direction, and the distorted bridge site is displaced from the long bridge site along the <$\text{110}\text{-}$ > direction (ref. 6) The asterisks (*) denote that the scattering amplitude is zero for all directions in the scattering plane, otherwise it is zero only in the specular direction. The <$\text{110}\text{-}$ > mode of the distorted bridge is not covered by the selection rules of ref. 2.

     

19.

Exciton coherence and electron energy loss spectroscopy of conjugated molecules     

Chernyak V  Volkov SN  Mukamel S 《Physical review letters》2001,86(6):995-998

Signatures of the exciton coherence size, which controls the nonlinear optical response and luminescence of conjugated systems, in the electronic dynamic structure factor S(q,omega) are calculated. We find that for small molecules the momentum dependence of the lowest exciton resonance is purely geometric, reflecting the molecular size rather than a universal exciton size, as suggested recently. For long chains the q dependence is determined by the interplay of the exciton size and the bond-alternation length scales.     

20.

Probing the electronic structure of ZnO nanowires by valence electron energy loss spectroscopy     

Wang J  Li Q  Egerton RF 《Micron (Oxford, England : 1993)》2007,38(4):346-353

Valence electron energy loss spectroscopy in a transmission electron microscope is employed to investigate the electronic structure of ZnO nanowires with diameter ranging from 20 to 100 nm. Its excellent spatial resolution enables this technique to explore the electronic states of a single nanowire. We found that all of the basic electronic structure characteristics of the ZnO nanowires, including the 3.3 eV band gap, the single electron interband transitions at approximately = 9.5, approximately = 13.5,and approximately = 21.8 eV, and the bulk plasmon oscillation at approximately 18.8 eV, resemble those of the bulk ZnO. Momentum transfer resolved energy loss spectra suggest that the 13.5 eV excitation is actually consisted of two weak excitations at approximately = 12.8 and approximately = 14.8 eV, which originate from transitions of two groups of the Zn 3d electrons to the empty density of states in the conduction band, with a dipole-forbidden nature. The energy loss spectra taken from single nanowires of different diameters show several size-dependent features, including an increase in the oscillator strength of the surface plasmon resonance at approximately = 11.5 eV, a broadening of the bulk plasmon peak, and splitting of the O 2s transition at approximately = 21.8 eV into two peaks, which coincides with a redshift of the bulk plasmon peak, when the nanowire diameter decreases. All these observations can be well explained by the increased surface/volume ratio in nanowires of small diameter.     

SITE	LONG BRIDGE	SHORT BRIDGE	DISTORTED BRIDGE
MIRROR PLANES	[001], [1$\text{1}\text{-}$0]	NONE	[1$\text{1}\text{-}$0]
2-FOLD ABOUT Z	YES	YES	NO
PARALLEL MODES	[001]	[1$\text{1}\text{-}$0]	[1$\text{1}\text{-}$1]	[$\text{1}\text{-}$12]	[001]	[1$\text{1}\text{-}$0]
DIRECTIONS OF ZERO SCATTERING	[001] * [1$\text{1}\text{-}$0]	[001] * [1$\text{1}\text{-}$0]	[1$\text{1}\text{-}$1] [$\text{1}\text{-}$12]	[1$\text{1}\text{-}$1] [$\text{1}\text{-}$12]	[001] * [1$\text{1}\text{-}$0]	NA

Exciton coherence and electron energy loss spectroscopy of conjugated molecules

Signatures of the exciton coherence size, which controls the nonlinear optical response and luminescence of conjugated systems, in the electronic dynamic structure factor S(q,omega) are calculated. We find that for small molecules the momentum dependence of the lowest exciton resonance is purely geometric, reflecting the molecular size rather than a universal exciton size, as suggested recently. For long chains the q dependence is determined by the interplay of the exciton size and the bond-alternation length scales.     

Probing the electronic structure of ZnO nanowires by valence electron energy loss spectroscopy

Valence electron energy loss spectroscopy in a transmission electron microscope is employed to investigate the electronic structure of ZnO nanowires with diameter ranging from 20 to 100 nm. Its excellent spatial resolution enables this technique to explore the electronic states of a single nanowire. We found that all of the basic electronic structure characteristics of the ZnO nanowires, including the 3.3 eV band gap, the single electron interband transitions at approximately = 9.5, approximately = 13.5,and approximately = 21.8 eV, and the bulk plasmon oscillation at approximately 18.8 eV, resemble those of the bulk ZnO. Momentum transfer resolved energy loss spectra suggest that the 13.5 eV excitation is actually consisted of two weak excitations at approximately = 12.8 and approximately = 14.8 eV, which originate from transitions of two groups of the Zn 3d electrons to the empty density of states in the conduction band, with a dipole-forbidden nature. The energy loss spectra taken from single nanowires of different diameters show several size-dependent features, including an increase in the oscillator strength of the surface plasmon resonance at approximately = 11.5 eV, a broadening of the bulk plasmon peak, and splitting of the O 2s transition at approximately = 21.8 eV into two peaks, which coincides with a redshift of the bulk plasmon peak, when the nanowire diameter decreases. All these observations can be well explained by the increased surface/volume ratio in nanowires of small diameter.