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.     

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.     

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.     

Quantitative analysis of reflection electron energy loss spectra of aluminum

Reflection electron energy loss spectra of aluminium were studied for primary energies in the 500–2000 eV and loss energies in the 0–80 eV range. The absolute intensity observed could be well explained by using an electron-gas model for the inelastic electron scattering cross section and by assuming that the distribution of the path lengths travelled in the solid is exponentially decreasing. The attenuation length in this distribution is found to be on the order of the transport mean free path for elastic electron scattering.     

Photoelectron and electron energy loss spectra of epitaxial aluminum nitride

The valence bands of epitaxial layers of A1N were studied by ultraviolet photoelectron spectroscopy and electron energy loss spectroscopy. A self-consistent band structure was calculated and the resultant density of states compared with the UPS spectra. Qualitative agreement was good, with discrepancies arising primarily from the neglect of dipole matrix elements.     

An electron energy loss spectroscopy study of hydrogen physisorption on aluminum and alumina surfaces at room temperature

This paper deals with electron energy loss spectra (EELS) of hydrogen adsorbed at room temperature on Al and Al₂O₃ surfaces. It is shown that while hydrogen physisorption on Al₂O₃ is detected under H₂ pressures in the 10^?4 Torr range, no hydrogen physisorption is observed on clean Al under the same experimental conditions. These observations are consistent with calculations of the potential well depth of interaction between the H₂ molecule and the aluminum and alumina surfaces.     

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.

     

8.

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.     

9.

Frenkel exciton model of electron energy loss spectroscopy in -PTCDA     

I. Vragovic  M. Schreiber  R. Scholz   《Journal of luminescence》2004,110(4):284-289

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.     

10.

Joint experimental and computational study of aluminum electron energy loss spectra     

Lucia Calliari  Massimiliano Filippi 《Surface science》2007,601(10):2270-2276

Experimental reflection electron energy loss (REEL) spectra are measured from aluminum for primary energies ranging from 130 eV to 2 keV. A Monte Carlo simulation is shortly described and used to calculate the same spectra. The focus is on reproducing the variable weight of surface and bulk losses as the surface sensitivity of spectra changes by changing the primary electron energy. The intensity of surface losses in the simulations is modulated by the thickness of the region where surface excitations occur. Simulations based either on a constant or an energy-dependent thickness for this layer are considered. In both cases, simulated spectra reproduce the experimental trend as a function of energy, though the correct surface-to-bulk intensity ratio for each energy is either underestimated or overestimated.     

11.

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

《Applied Surface Science》1987,29(1):101-112

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.     

12.

Ultrahigh-resolution electron energy loss spectroscopy via digital signal processing techniques     

Wang Y  Weinberg WH 《Physical review letters》1992,69(23):3326-3329

     

13.

Low frequency surface resonance modes in electron energy loss spectroscopy     

Talat S. Rahman  D.L. Mills  J.E. Black 《Journal of Electron Spectroscopy and Related Phenomena》1983,29(1):199-212

Electron energy loss spectroscopy has proved a powerful probe of vibrational modes of a wide variety of adsorbed species. Here the primary focus has been on modes with frequency well above the maximum phonon frequency of the substrate. Examples are internal vibration modes of adsorbed molecules, possibly shifted significantly in frequency from their gas phase analogues, and high frequency vibrations of an adsorbed molecule or atom against the substrate. Recent experiments explore features in the energy loss spectrum with frequency below the maximum phonon frequency of the substrate, for ordered overlayers of atoms adsorbed on low index metal surfaces. We shall summarize our theoretical studies of such spectra for several adsorbate/substrate combinations, with emphasis oh the physical origin of the features which appear in the calculations. We obtain a good account of the existing data, within the framework of a rather simple lattice dynamical model, and the calculations show that the features which appear are quite sensitive to the symmetry of the adsorption sits, and other details of the surface geometry. We shall illustrate this with several specific examples.     

14.

Comment on "Ultrahigh resolution electron energy loss spectroscopy"     

Frederick BG  Richardson NV 《Physical review letters》1994,73(5):772

     

15.

Atomic-resolution electron energy loss spectroscopy imaging in aberration corrected scanning transmission electron microscopy     

Allen LJ  Findlay SD  Lupini AR  Oxley MP  Pennycook SJ 《Physical review letters》2003,91(10):105503

The "delocalization" of inelastic scattering is an important issue for the ultimate spatial resolution of innershell spectroscopy in the electron microscope. It is demonstrated in a nonlocal model for electron energy loss spectroscopy (EELS) that delocalization of scanning transmission electron microscopy (STEM) images for single, isolated atoms is primarily determined by the width of the probe, even for light atoms. We present experimental data and theoretical simulations for Ti L-shell EELS in a [100] SrTiO3 crystal showing that, in this case, delocalization is not significantly increased by dynamical propagation. Issues relating to the use of aberration correctors in the STEM geometry are discussed.     

16.

Modelling relativistic effects in momentum-resolved electron energy loss spectroscopy of graphene     

K. Lyon  D. J. Mowbray 《辐射效应与固体损伤》2018,173(1-2):8-21

We present an analytical model for the electron energy loss through a two-dimensional (2D) layer of graphene, fully taking into account relativistic effects. Using two different models for graphene's 2D conductivity, one a two-fluid hydrodynamic model with an added correction to account for the inter-band electron transitions near the Dirac point in undoped graphene, the other derived from ab initio plane-wave time-dependent density functional theory in the frequency domain (PW-TDDFT-ω) calculations applied on a graphene superlattice, we derive various different expressions for the probability density of energy and momentum transfer from the incident electron to graphene. To further compare with electron energy loss spectroscopy (EELS) experiments that use setups like scanning Transmission Electron Microscopy, we integrated our energy loss functions over a range of wavenumbers, and compared how the choice of range directly affects the shape, position, and relative heights of graphene's π → π* and σ → σ* transition peaks. Comparisons were made with experimental EELS data under different model inputs, revealing again the strong effect that the choice of wavenumber range has on the energy loss.     

17.

Study of He bubbles in Al by electron energy loss spectroscopy     

R. Manzke  M. Campagna 《Solid State Communications》1981,39(2):313-317

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.     

18.

Theory of dielectric screening and electron energy loss spectroscopy at surfaces     

《Comptes Rendus Physique》2009,10(6):560-574

We present an overview of theoretical techniques for describing electron energy loss processes in a reflection geometry. We start from a fundamental representation of the dielectric susceptibility tensor of the semi-infinite crystal, and illustrate how the screening becomes modified by the presence of the surface. A new formalism is also presented which improves upon existing techniques for modeling energy loss, is fully q-dependent, and accounts for nonlocality. The impact of nonlocality, local field effects and other many-body effects is discussed. The theory is supported by some explicit calculations on the GaAs(001)–$c(4\times 4)$ surface. To cite this article: C. Hogan et al., C. R. Physique 10 (2009).     

19.

Comparison of cross sections in high resolution electron energy loss spectroscopy and infrared reflection spectroscopy     

H Ibach 《Surface science》1977,66(1):56-66

Electron energy losses and absorption of infrared radiation are both caused by the dipole moment of the surface vibration. A comparison of absolute intensities between both techniques should therefore be possible. In this paper the appropriate formulas are derived. For the adsorption system CO on Pt(111) which has been investigated by both techniques a perfect agreement is found. For a number of adsorbate systems the effective ionic charge is calculated from previously published electron energy loss data.     

20.

Electron energy loss spectroscopy of A {111} oriented aluminum single crystal     

B.H. Nall  A.N. Jette  C.B. Bargeron 《Surface science》1981,110(1):L606-L610

Recently, the electron energy loss spectroscopy (ELS) and secondary electron spectroscopy (SES) of the interaction of oxygen with polycrystalline aluminum were reported for primary energies in the range 30 ? E_p ? 250 eV. Two new transitions were resolved in the ELS spectra (one at 4 eV for clean aluminum and the other at 12 eV for oxide-covered aluminum) for low primary energies (E_p ~ 30 eV). In this paper we report on experiments utilizing a {111} oriented single crystal of aluminum that confirm the existence of these loss peaks for low primary energies and show that the 4 eV peak position for pure aluminum depends on the primary beam energy. This suggests that this low energy loss peak is due to direct nonvertical inter- and intraband transitions which differs from the previous assignment.     

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.     

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.     

Joint experimental and computational study of aluminum electron energy loss spectra

Experimental reflection electron energy loss (REEL) spectra are measured from aluminum for primary energies ranging from 130 eV to 2 keV. A Monte Carlo simulation is shortly described and used to calculate the same spectra. The focus is on reproducing the variable weight of surface and bulk losses as the surface sensitivity of spectra changes by changing the primary electron energy. The intensity of surface losses in the simulations is modulated by the thickness of the region where surface excitations occur. Simulations based either on a constant or an energy-dependent thickness for this layer are considered. In both cases, simulated spectra reproduce the experimental trend as a function of energy, though the correct surface-to-bulk intensity ratio for each energy is either underestimated or overestimated.     

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.     

Low frequency surface resonance modes in electron energy loss spectroscopy

Electron energy loss spectroscopy has proved a powerful probe of vibrational modes of a wide variety of adsorbed species. Here the primary focus has been on modes with frequency well above the maximum phonon frequency of the substrate. Examples are internal vibration modes of adsorbed molecules, possibly shifted significantly in frequency from their gas phase analogues, and high frequency vibrations of an adsorbed molecule or atom against the substrate. Recent experiments explore features in the energy loss spectrum with frequency below the maximum phonon frequency of the substrate, for ordered overlayers of atoms adsorbed on low index metal surfaces. We shall summarize our theoretical studies of such spectra for several adsorbate/substrate combinations, with emphasis oh the physical origin of the features which appear in the calculations. We obtain a good account of the existing data, within the framework of a rather simple lattice dynamical model, and the calculations show that the features which appear are quite sensitive to the symmetry of the adsorption sits, and other details of the surface geometry. We shall illustrate this with several specific examples.     

Atomic-resolution electron energy loss spectroscopy imaging in aberration corrected scanning transmission electron microscopy

The "delocalization" of inelastic scattering is an important issue for the ultimate spatial resolution of innershell spectroscopy in the electron microscope. It is demonstrated in a nonlocal model for electron energy loss spectroscopy (EELS) that delocalization of scanning transmission electron microscopy (STEM) images for single, isolated atoms is primarily determined by the width of the probe, even for light atoms. We present experimental data and theoretical simulations for Ti L-shell EELS in a [100] SrTiO3 crystal showing that, in this case, delocalization is not significantly increased by dynamical propagation. Issues relating to the use of aberration correctors in the STEM geometry are discussed.     

Modelling relativistic effects in momentum-resolved electron energy loss spectroscopy of graphene

We present an analytical model for the electron energy loss through a two-dimensional (2D) layer of graphene, fully taking into account relativistic effects. Using two different models for graphene's 2D conductivity, one a two-fluid hydrodynamic model with an added correction to account for the inter-band electron transitions near the Dirac point in undoped graphene, the other derived from ab initio plane-wave time-dependent density functional theory in the frequency domain (PW-TDDFT-ω) calculations applied on a graphene superlattice, we derive various different expressions for the probability density of energy and momentum transfer from the incident electron to graphene. To further compare with electron energy loss spectroscopy (EELS) experiments that use setups like scanning Transmission Electron Microscopy, we integrated our energy loss functions over a range of wavenumbers, and compared how the choice of range directly affects the shape, position, and relative heights of graphene's π → π* and σ → σ* transition peaks. Comparisons were made with experimental EELS data under different model inputs, revealing again the strong effect that the choice of wavenumber range has on the energy loss.     

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.     

Theory of dielectric screening and electron energy loss spectroscopy at surfaces

We present an overview of theoretical techniques for describing electron energy loss processes in a reflection geometry. We start from a fundamental representation of the dielectric susceptibility tensor of the semi-infinite crystal, and illustrate how the screening becomes modified by the presence of the surface. A new formalism is also presented which improves upon existing techniques for modeling energy loss, is fully q-dependent, and accounts for nonlocality. The impact of nonlocality, local field effects and other many-body effects is discussed. The theory is supported by some explicit calculations on the GaAs(001)–$c(4\times 4)$ surface. To cite this article: C. Hogan et al., C. R. Physique 10 (2009).     

Comparison of cross sections in high resolution electron energy loss spectroscopy and infrared reflection spectroscopy

Electron energy losses and absorption of infrared radiation are both caused by the dipole moment of the surface vibration. A comparison of absolute intensities between both techniques should therefore be possible. In this paper the appropriate formulas are derived. For the adsorption system CO on Pt(111) which has been investigated by both techniques a perfect agreement is found. For a number of adsorbate systems the effective ionic charge is calculated from previously published electron energy loss data.     

Electron energy loss spectroscopy of A {111} oriented aluminum single crystal

Recently, the electron energy loss spectroscopy (ELS) and secondary electron spectroscopy (SES) of the interaction of oxygen with polycrystalline aluminum were reported for primary energies in the range 30 ? E_p ? 250 eV. Two new transitions were resolved in the ELS spectra (one at 4 eV for clean aluminum and the other at 12 eV for oxide-covered aluminum) for low primary energies (E_p ~ 30 eV). In this paper we report on experiments utilizing a {111} oriented single crystal of aluminum that confirm the existence of these loss peaks for low primary energies and show that the 4 eV peak position for pure aluminum depends on the primary beam energy. This suggests that this low energy loss peak is due to direct nonvertical inter- and intraband transitions which differs from the previous assignment.