Surface and bulk plasmon coupling observed in reflection electron energy loss spectra

Reflection electron energy loss spectra have been measured for a semiconductor and some metals (Si, Cu, Ag and Au). A novel procedure is presented to rigorously decompose the spectra into contributions corresponding to surface and volume excitations. The resulting distributions of energy losses in an individual surface loss are in good agreement with theory. In particular, the begrenzungs effect occurring at the boundary of a solid state plasma, i.e. the reduction of the intensity of bulk modes due to the coupling with surface modes, can be clearly observed in the retrieved energy loss distribution.     

Angle-resolved elastic peak electron spectroscopy: Role of surface excitations

We analyze the possibility of determining the surface excitation parameter (SEP) from the dependence of the elastic backscattering signal intensity on the emission angle. It has been found that the shape of this dependence is reasonably well described by the theoretical model implemented in a typical Monte Carlo simulation strategy. As shown recently, the mean percentage deviation between the experimental angular dependence and the theoretical dependence is equal to 8.82% at 200 eV, 6.28% at 500 eV and 4.69% at 1000 eV. In the theoretical model used, the surface energy losses were ignored. Close inspection of the deviations between theory and experiments indicates systematic trends that can be ascribed to the surface energy losses. We found here that taking into the account the surface energy losses further improves the agreement between theory and experiment. The total mean percentage deviation, equal to 6.65%, decreases to 5.59% if the mathematical form of the Chen formula for SEP is used, or to 5.16% if the Oswald expression is used. The material dependent coefficients in the expression of SEP derived from the emission angle dependence of the elastic peak intensity differ from these coefficients resulting from other methods. We conclude that the determination of SEP from shape of the angular dependence requires the experimental data of high quality, and the reliable theoretical model describing elastic electron backscattering.     

Construction of an electron analyzer with a wide acceptance solid angle

We have developed a new electron energy analyzer with a large solid angle of 0.14π, which is comparable to that of cylindrical mirror analyzer. Typical energy resolution was ΔE/E₀ ∼ 0.016 for the aperture of 1 mm and central radius of 100 mm, and typical angular resolution was less than 0.5°.     

EELS studies of surface phonons on Ag(111)

High-resolution electron energy loss spectroscopy is used to study surface phonons in the direction of qG on Ag(111). The gap mode (S₂) at in the surface Brillouin zone is measured for the first time. We have also measured the Rayleigh mode and the resonance mode up to the zone boundary. The results are complementary to the earlier He atom scattering measurements and are in good agreement with the first-principles calculations performed recently.     

High-resolution study of quasi-elastic electron scattering from a two-layer system

In large-angle elastic scattering events of keV electrons a significant amount of momentum is transferred from the electron to a nucleus in the target. As a consequence kinetic energy is transferred from the energetic electron to the nucleus, and hence these processes can be referred to as ‘quasi-elastic’. How much energy is transferred depends on the mass of the nucleus. In this paper, we present measurements from a two-layer system (a germanium layer and a carbon layer), and at high energies the quasi-elastic peaks of Ge and C are clearly resolved. It is demonstrated that the sample geometry has a huge effect on the observed relative intensities. It is shown that the intensities are influenced by the elastic scattering cross-section of the atoms in the film, film composition and selective attenuation, due to varying amount of inelastic scattering for layers of the film. However truly quantitative agreement is not obtained.     

Surface excitation effects in elastic peak electron spectroscopy

Surface electron inelastic excitations, a consequence of electron-surface interaction, effect the measured intensities in surface-sensitive electron spectroscopic methods and distort the quantitative information. This phenomenon is more pronounced at low electron energy and glancing emission angles. In this work we investigate quantitatively the influence of the surface excitation effects on the measured electron elastic backscattering probability. As a model system we used Si, Cu and Al, i.e. materials with different surface excitation properties. Results obtained show that properly corrected measured elastic electron backscattering probabilities lead to inelastic mean free path values which compare well with the theory.     

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.     

Electronic properties of (3/2 × 3/2)-Na/Cu(1 1 1)

High-resolution electron energy loss spectroscopy was used to study the electronic properties of (3/2 × 3/2)-Na/Cu(1 1 1) at room temperature. Loss spectra showed two-well distinct losses at 114 and 180 meV assigned to not dispersive charge density waves. Mechanisms to explain their existence are proposed. Moreover, the expected 2D plasmon of the Na quantum well state was not observed. The strong influence of the underlying Cu substrate may be responsible for the absence of such mode.     

Electronic properties of self-assembled quantum dots of sodium on Cu(1 1 1) and their interaction with water

The electronic properties of thin films of Na on Cu(1 1 1) and their interaction with water have been investigated at room temperature by high resolution electron energy loss spectroscopy. The first Na layer is characterized by two features tentatively assigned to charge density waves. The second Na layer grows as small islands. The loss spectrum of this layer shows a feature at 3.0 eV identified as a Mie resonance. Increasing alkali coverage, Na islands form a continuous film, as indicated by the appearance of a Na surface plasmon and by the disappearance of the Mie resonance. Water vapour strongly interacts with Na layers as shown by the OH-Na vibration whose frequency shifts from 36 meV to 53 meV as a function of alkali coverage.     

Visible and near ultraviolet photocurrent generation in carbon nanotubes

Multiwall carbon nanotubes are found to generate photocurrent in the visible and near ultra violet spectral range using a photoelectrochemical technique. Peaks in the photocurrent are observed at excitation energies in the visible region. Their electron energy loss spectra exhibit the π plasmon feature, typical of graphite layers, and a peak at lower energy. Features at energies between 0 and 4 eV have been already observed for single wall carbon nanotubes and ascribed to interband electronic transitions due to the reduced dimensionality of these systems. The present measurements suggest that the usual identification of multiwall carbon nanotubes electronic density of states with that of graphite layers is not sufficient and more theoretical investigations are necessary to shed light on this point.     

Electronic modifications and surface reactivity of an ion bombarded graphite surface

The first steps of structural and electronic modifications of a graphite surface bombarded with argon, hydrogen and deuterium ions were investigated using high resolution electron energy loss spectroscopy (HREELS). The energy and the damping of the low energy plasmon mode of graphite (E//C mode) were studied with respect to the bombardment settings. We show that argon bombardment affects the energy of the plasmon mode, while no similar change is observed after hydrogen (deuterium) bombardments. This can be related to the variation of inter-planar distance between two graphene layers. Moreover, the damping of the plasmon mode can be correlated with the interstitial defect concentration. Concerning the reactivity of the bombarded surfaces, we demonstrate that deuterium bombardment produce a non-deuterated surface. This last is very reactive to a further atomic deuterium exposure, as it is shown by the formation of C-D bondings. The deuterated sites can be removed after thermal annealings between 473 and 783 K. The occurrence of a chemical erosion mechanism accompanying this deuteration is discussed.     

Surface dynamics of Mo(1 1 0)–H and Mo(1 1 0)–Li

We compare the surface dynamics of the adsorbate systems Mo(1 1 0)–H and Mo(1 1 0)–Li. In both cases electron energy loss spectroscopy measurements revealed strong substrate surface phonon anomalies. Whereas the phonon anomaly of the hydrogen-covered surface was unequivocally assigned to be of the Kohn type, the anomalous behavior of the surface phonons of the lithium-covered surface remained obscure. In this paper we develop an experimental criterion based on the dispersion of adsorbate phonons, which allows to decide whether the observed substrate surface phonon anomaly is of the Kohn type or not. Employing this criterion we now definitely rule out that the anomaly on Mo(1 1 0)–Li is due to the Kohn effect.     

Determination of Ce/Ce in electron-beam-damaged CeO2 by electron energy-loss spectroscopy

We followed the reduction of Ce⁴⁺ in CeO₂ by observing changes in the shape of the Ce M_4,5 edge by parallel electron energy-loss spectroscopy with a transmission electron microscope. The energy-loss near-edge structure of the beam-damaged CeO₂ exhibits Ce M_4,5 and O K-edge shapes that are consistent with reduction to a Ce³⁺ oxide. During the damage process the spectrum of CeO₂ changes as follows: (a) decreases in energies of the M₅ and M₄ maxima; (b) changes in shape of the near-edge structure; (c) inversion of the M₅ to M₄ branching ratio; and (d) increase in the M₅ to M₄ area ratio. We simulated M_4,5 edges of damaged CeO₂ with a linear combination of Ce⁴⁺ and Ce³⁺ spectra and observed no intermediate oxidation states.     

Anisotropy of the angle resolved electron attenuation at crystal surfaces

Photoemission multiple scattering theory is used to describe the electron transport in the surface region of a crystal. Intensities of photoemission from core levels of atoms situated in subsurface atomic layers are calculated as a function of the emitter distance from the surface. The electron angle resolved attenuation length (ARAL) is extracted from the exponential fitting of the intensity decays of photoemission into different directions. Substantial anisotropy of the electron ARAL is found for the Cu(1 1 1) surface in Mg Kα photoexcitation of Cu 2p_3/2 levels and correlated with the orientation of highly packed atomic rows. Enhanced photoemission contributions from specific subsurface layers, caused by electron forward focusing effects, are reported.     

Design of a novel correlative reflection electron microscope for in-situ real-time chemical analysis

A novel instrument that integrates reflection high energy electron diffraction (RHEED), electron energy loss spectroscopy (EELS), and imaging is designed and simulated. Since it can correlate the structural, elemental, and spatial information of the same surface region via the simultaneously acquired patterns of RHEED, EELS, and energy-filtered electron microscopy, it is named correlative reflection electron microscopy (c-REM). Our simulation demonstrates that the spatial resolution of this c-REM is lower than 50 nm, which meets the requirements for in-situ monitoring the structural and chemical evolution of surface in advanced material.     

Zone specificity in low energy electron stimulated desorption of Cl from reconstructed Si(1 1 1)-7 × 7:Cl surfaces

Diffraction in electron stimulated desorption has revealed a propensity for Cl⁺ desorption from rest atom vs. adatom areas and unfaulted vs. faulted zones of Cl-terminated Si(1 1 1)-(7 × 7) surfaces. We associate the 15 eV ± 1 eV threshold with ionization of Si-Cl σ-bonding surface states and formation of screened two-hole states with Si 3s character. Similar specificity is observed from A and B reconstructions. This can be due to reduced screening in unfaulted regions and increased hole localization in Si back-bonds within faulted regions.     

Interference between resonance and potential scattering of high energy electrons from crystal surfaces

A solution of the tight-binding model of resonance diffraction of high-energy electrons from a crystal surface is found which describes the behaviour of the reflectivity in the vicinity of the intersection of a resonace parabola and a horizontal Kikuchi line. A simple analytical formula is obtained which makes it possible to evaluate the wave function for the case where both resonance and potential contributions to the reflectivity are of the same order of magnitude. The intensity of the specular reflection is shown to be directly related to the effective displacement Δ of the beam in the direction parallel to the surface, and a general formula suitable for evaluation of Δ from calculated RHEED rocking curves is derived.     

Interaction of H2O with the RuO2(1 1 0) surface studied by HREELS and TDS

The adsorption of water on a RuO₂(1 1 0) surface was studied by using high-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). The first thermal desorption peak observed between 350 and 425 K is attributed to molecular water adsorbed on fivefold coordinated Ru_cus sites. Higher coverages of water give rise to TDS peaks between 190 and 160 K, which we attribute to water in the second layer bound to bridge oxygen, and multilayers, respectively. HREELS shows that H₂O chemisorbs on Ru_cus sites through oxygen inducing a slight red shift of the vibrational frequency of O_bridge atoms. Molecular adsorption is also confirmed by the presence of both the scissor and the libration modes showing the expected isotopic shift for D₂O. The water adsorbed on the Ru_cus sites also forms hydrogen bonds with the bridge oxygen indicated by the broad intensity at the lower frequency side of the O-H stretch mode. HREELS and TDS results suggest that on the perfect RuO₂(1 1 0) surface water dissociation is almost negligible.     

Lattice-controlled electron transport characteristics in quantized surface layers at low temperature

A theory of intravalley acoustic scattering of the carriers in a non-degenerate two-dimensional electron gas is developed here under the condition of low lattice temperature when the assumptions of the well-known traditional theory are not valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics in an n-channel Si inversion layer. It is found that the finite energy of the phonons makes the energy and lattice temperature dependence of the scattering rate, and consequently the lattice temperature dependence of the mobility, significantly different from what follows, in the light of traditional theory which assumes equipartition law for the phonon distribution and neglects the phonon energy in comparison to the carrier energy.