High-resolution Electron Energy Loss Spectra Research Articles

High resolution electron energy loss spectroscopy of hydrogenated polycrystalline diamond: Assignment of peaks through modifications induced by isotopic exchange

In this work we unambiguously determine the origin of the different peaks which appear in the High Resolution Electron Energy Loss Spectrum (HREELS) of hydrogenated polycrystalline diamond films for an incident electron energy of 5 eV and loss energies extending to 700 meV. High quality diamond films deposited by hot filament chemical vapor deposition from various isotopic gas mixtures: 12CH 4 + H 2, 12CD 4 + D 2, 12CH 4 + D 2, 12CD 4 + D 2, 13CH 4 + H 2 were characterized. The different vibrational modes, fundamentals and overtones, were directly identified through the modifications of the HREEL spectra induced by the isotopic exchange of H by D and 12C by 13C Three types of peaks were identified: (1) pure C–C related peaks (a diamond optical phonon at ∼ 155 meV and its overtones at 300, 450 and 600 meV), (2) pure C–H related peaks (C–H bend at ∼ 150 meV and C–H stretch of sp 3 carbon at 360 meV), (3) coupling of C–H and C–C peaks (510 meV peak due to coupling of the C–H stretch at 360 meV with either the C–C stretch or the C–H bend at ∼ 155 meV). The overtones at 300, 450 and 600 meV (associated with electron scattering at diamond optical phonons) indicate a well defined hydrogenated diamond surface since they are absent in the HREEL spectrum of low energy ion beam damaged diamond surface.

The Atomic, Electronic and Defect Structure of the Dynamically Formed Cu2O/Cu Interfaces

AbstractTo gain fundamental insights into metal oxidation, the dynamically formed Cu/Cu2O interface was investigated by cross-sectional TEM (Transmission Electron Microscopy) methods. Copper (001) films were oxidized in oxygen within a UHV chamber to create Cu2O islands that formed epitaxially with respect to the Cu film. The cross-sectional Cu2O/Cu TEM sample was prepared by dual beam (DB) focused ion beam (FIB) instrument and the interface was probed by high-resolution TEM (HREM) and electron energy loss spectrum (EELS). It is found that Cu2O {110} layer distance significantly decreases from the interface area to the bulk Cu2O region, which is about 3∼4 unit cell thickness in Cu2O side; while the {100Cu2O layer distance increases with increasing distance from the interface region. The chemical Cu/Cu2O interface thickness has been measured with EELS analysis, which is about 2nm where the oxidation state of Cu gradually changes from Cu0 to Cu+1. This transition region indicates the area where Cu/Cu2O interface exists and suggests the existence of metastable Cu oxides. The Cu2O island growth mechanism of predominantly anion interfacial diffusion at the initial stage oxidation has been proposed.

Fate of Reactive Intermediates Formed in Acetaldehyde under Exposure to Low-Energy Electrons

Chemical reactions have been induced in condensed acetaldehyde by exposure to low-energy electrons as demonstrated by use of high-resolution electron energy loss (HREEL) spectroscopy and thermal desorption spectroscopy (TDS). The HREEL spectra show that the acetaldehyde film is modified significantly when the incident electron energy is increased above 10 eV. Release of CO upon exposure to electrons at an incident energy of 14 eV is observed by electronic HREEL spectra. The changes in the vibrational spectra upon exposure of the molecular film to electrons at an incident energy of 15.5 eV are analyzed in detail using reference spectra for a number of potential product molecules. TDS data reveal clearly the decomposition of acetaldehyde and formation of CO and CH4 as major products under electron exposure. The combined results also give evidence for the formation of propionaldehyde. Other less prominent products are acetone and possibly an alcoholic species. A reaction mechanism is suggested that rationalizes the formation of larger products. It involves fragmentation of the molecule releasing CO, abstraction of an H atom from an adjacent acetaldehyde by the H radical fragment, and, finally, recombination of the resulting radical with the remaining CH3 fragment.

Surface structure, composition, and polarity of indium nitride grown by high-pressure chemical vapor deposition

The structure and surface bonding configuration of InN layers grown by high-pressure chemical vapor deposition have been studied. Atomic hydrogen cleaning produced a contamination free surface. Low-energy electron diffraction yielded a 1×1 hexagonal pattern demonstrating a well-ordered c-plane surface. High-resolution electron energy loss spectra exhibited a Fuchs–Kliewer surface phonon and modes assigned to a surface N–H species. Assignments were confirmed by observation of isotopic shifts following atomic deuterium cleaning. No In–H species were observed, and since an N–H termination of the surface was observed, N-polarity indium nitride is indicated.

Competition of different scattering channels in electron impact vibrational excitation of thin solid films of sulfur hexafluoride

Vibrational high-resolution electron energy loss spectra of solid films of SF 6 have been recorded at incident energies ( E 0) ranging from 1.5 to 16.5 eV. All possible vibrations of SF 6 are observed. The spectra reveal characteristic variations of the vibrational band intensities as a function of E 0. In addition, an analysis of the multiple scattering spectral region using intensity ratios predicted by a simple multiple scattering model is applied to locate overtone and combination bands. In agreement with earlier gas phase results, the characteristic energy dependence of the intensity of the totally symmetric vibration ν 1 gives evidence for three different resonant scattering processes. The energy dependence of the strong dipole scattering band ν 3 closely follows the behavior of the elastic intensity that is characterized by minima at approximately the positions of the resonance maxima. This is explained by competing dipole and resonant scattering channels.

Electron energy loss spectra of C 60 and C 70 fullerenes

The electron energy loss spectra of C 60 and C 70 fullerenes are investigated using spectroscopy measurements and theoretical calculations. An additional peak has been identified for C 70 above the carbon K-edge onset from our high-resolution electron energy loss spectra and analysed by density functional theory. Good agreement between theory and experiment is obtained, and the additional peak is found to be due to the electronic states associated with the 10 equatorial atoms in C 70.

Electrochemically Driven Assembly of Mixed Dithiol Bilayers via Sulfur Dimers

We report on the electroformation of mixed bilayers of rigid (1,4-benzenedimethanethiol) and flexible (1,6-hexanedithiol) dithiols on Au(111) from alkaline aqueous solution. These bilayers are formed by first electrodepositing a monolayer of vertically aligned dithiols. The unreacted dithiols are then oxidatively dimerized with the other dithiols, and a bilayer is formed. ACIS (ac impedance spectroscopy) measurements indicate that the monolayer of the rigid dithiol is a better substrate for dimerization than the flexible dithiol. ACIS also reveals that the electroformation of a bilayer is more complete, and the bilayer has fewer defects than one obtained through chemical incubation. Infrared reflection-absorption spectroscopy provides proof for mixed-bilayer formation ofthe alkanedithiol with the benzenedimethanethiol having its methylene groups deuterated. The mechanism for bilayer formation via the oxidative dimerization of thiols is supported by high-resolution electron energy loss spectra of bilayers, which show a S-S stretch at 515 cm - 1 . Our results show that this electrochemical approach is a viable methodology for the formation of complex organic interfaces.

Cross sections for low energy (1–12 eV) inelastic electron scattering from condensed thymine

We describe a new method to obtain the absolute integral cross sections per scatterer for inelastic electron scattering from sub- and monolayer film of molecules condensed on an inert surface. These cross sections are determined by means of a high-resolution electron-energy loss (HREEL) spectrometer for electrons of incident energies similar to those of secondary electrons generated by high energy radiation, i.e. 1–12 eV. Using the linear relationship between the reflected and transmitted current, a HREEL spectrum can be normalized in such a way that the energy integral performed over an energy loss feature can be expressed in terms of an absolute reflectivity value. Given the thickness of the film and assuming thin film and single collision conditions, those reflectivity values can then be converted into absolute integral cross sections for inelastic electron scattering. As an example, we report the energy dependence of the cross sections for the vibrational excitations of thymine. The maximum of 1.6×10 −16 cm 2 at about 4 eV in the cross section for an energy loss of 95 meV is attributed to the formation of a transient anion state that is decaying into vibrational excitation channels.

Adsorption sites and microstructures ofCO2on Fe(111) derived from specular and off-specular HREELS

The dependence on surface temperature and adsorbate pressure of static and time-dependent HREEL (high-resolution electron-energy-loss) spectra provides direct experimental evidence for molecular adsorption of ${\mathrm{CO}}_{2}$ on Fe(111) at 100 K in one linear state and two bent states. The data demonstrate that these adsorption states can be distinguished unambiguously by their binding energies and their thermal stabilities. Comparison of HREELS data taken under specular geometry with those taken at off-specular angles allows a complete determination of the individual symmetries and microstructures of ${\mathrm{CO}}_{2}$ in these states.

Temperature effects on the coadsorption of K–CO on Ni(100)

High-resolution electron energy-loss spectra of the coadsorption of potassium and CO on the Ni(100) surface show the role of temperature in driving chemical species along the surface towards the formation of ordered islands made of alkali atoms and CO species. At low potassium precoverages, the formation of K–CO islands occurs by heating the sample to 420 K. On the contrary, for a potassium coverage near 0.38 ML, K–CO patches of ordered phases grow spontaneously even at 220 K. The interpretation of the present data relies essentially on two literature works on the K–CO/Ni(100) system, one of them presenting calorimetric measurements and the other reoprting the theoretical temperature behaviour of the chemical species on the nickel surface.

Ab Initio Study of Adsorption and Decomposition of NH3 on Si(100)-(2×1)

We investigate the mechanism of NH3 adsorption and initial decomposition on the (2×1) reconstructed Si(100) surface using B3LYP density functional theory. The Si(100)-(2×1) surface is described using cluster approximation. Ammonia is found to adsorb on the “down” atom of the buckled silicon dimer with no activation barrier. We also find that only half of the surface silicon atoms are active sites for ammonia adsorption. Ammonia adsorption on the Si(100)-(2×1) is exothermic with an adsorption energy of 29 kcal/mol. Dissociation of the adsorbed ammonia to form NH2(a) and H(a) proceeds with a low activation energy of 5 kcal/mol below the NH3(g) and bare Si(100)-(2×1) energy. Our calculated recombination desorption energy of 51 kcal/mol is found to be in good agreement with the temperature-programmed desorption experimental result of 47 kcal/mol. Additionally, our calculated vibrational spectra of NH2(a) and H(a) agree within 2% of the experimental high-resolution electron energy loss spectra.

Composition and structure of HCl-isopropanol treated and vacuum annealed GaAs(100) surfaces

The GaAs(100) surfaces chemically treated in HCl-isopropanol solution and annealed in vacuum were studied by means of Auger electron spectroscopy, x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectra, and Low-energy electron diffraction (LEED). Chemical treatment and sample transfer into ultrahigh vacuum were performed under nitrogen atmosphere. The HCl-isopropanol treatment removes gallium and arsenic oxides from the surface, with about 2 monolayers of excess arsenic being left on it. The residual carbon contaminations were around 0.2–0.4 ML and consisted of the hydrocarbon molecules. These hydrocarbon contaminations were removed from the surface together with the excess arsenic by vacuum annealing at 300–420 °C. With increased annealing temperature, a sequence of six reconstructions were identified by LEED: (1×1), (2×4)/c(2×8), (2×6), (3×6), (4×1) and c(8×2) in the temperature intervals of 250–400, 420–480, 480–500, 500–520, 520–560 and 560–600 °C, respectively. All surface reconstructions were irreversible. The structural properties of chemically prepared GaAs(100) surfaces were found to be similar to those obtained by molecular-beam epitaxy-growth and by decapping of As-capped epitaxial layers.

Chemistry of Cyclopropane on Pt(111): Thermal, Electron, and Photon Activation

The thermal, electron, and photon-induced chemistry of cyclopropane, c-C3H6, adsorbed on Pt(111) at 100 K has been studied. The thermal chemistry is simple. Adsorption is saturable, no multilayer accumulates, and thermal desorption exhibits only c-C3H6 with one peak at 144 K. High-resolution electron energy loss spectra results indicate that c-C3H6 adsorbs with the plane of the ring tilted away from the surface normal. The electron-induced chemistry is more complex. Irradiation with 50 eV electrons activates dissociation of adsorbed c-C3H6 at 100 K, and there are thermally activated reactions during subsequent temperature-programmed desorption. The total cross-section for destruction of c-C3H6 is 8.2 (±0.2) × 10-17 cm2. For low electron fluences, <1016 cm-2, the chemistry involved is describable in terms of C−C bond breaking to open the ring and form metallacyclic species, some of which promptly lose H to form allylic (C3H5) moieties. In subsequent thermally activated reaction-limited processes, propylene...

Low-Energy Electron Damage to Condensed-Phase Deoxyribose Analogues Investigated by Electron Stimulated Desorption of H- and Electron Energy Loss Spectroscopy

We report the 5−20 eV electron-stimulated desorption (ESD) yields of H- produced by dissociative electron attachment (DEA) to the DNA backbone sugar-like analogues tetrahydrofuran (I), 3-hydroxytetrahydrofuran (II), and α-tetrahydrofurfuryl alcohol (III) physisorbed on a polycrystalline Pt substrate. For the pure disordered solid films, we observe one peak in the H- yield function at an incident electron energy, Ei, of ∼10 eV, which is attributed to selective dissociation of endocyclic α-CH bonds via the formation of a core-excited shape resonance; no desorbing polyatomic fragments were detected. A second peak is also observed in the H- ESD yield function of II; it appears as a weak shoulder superimposed on the low-energy side of the 10 eV structure displaying a sharp vertical onset near 6.7 eV and a peak maximum around 7.3 eV. The sharp onset and narrow energy width are characteristic of a core-excited Feshbach resonance; it is attributed to H- produced via DEA to the OH substituent whose corresponding parent state may be similar to that observed in CH3OH, where the 7.3 eV resonance originating from the hydroxyl group was assigned to a 2A‘ ‘ Feshbach resonance. High-resolution electron energy loss (HREEL) spectra recorded with 11 and 14 eV incident electrons are also reported for solid I. They show that, with increasing electron dose, degradation of the solid leads to formation of a C−O π-bonded product believed to arise from fragmentation of an α-cleaved transient intermediate following direct electronic excitation of the parent molecule. The HREEL spectra suggest a rather complex fragmentation pathway following low-energy electron bombardment. In vacuo kinetic energy (Ek) distribution measurements of desorbed H- in the 10 eV resonance suggest that for I, II, and III a dissociation mechanism similar to that proposed for the 14 eV HREELS spectra occurs, whereby α-cleavage of the C−O bond occurs within the lifetime of the dissociative negative ion state.

Hreels studies on the electronic structure of oligothiophene films

Abstract High resolution electron energy loss spectra (HREELS) were recorded for quaterthiophene (4T) films and compared to those of quinquethiophene (5T) and sexithiophene (6T). Excitonic gap was evaluated to be 2.64±0.02 eV. A second threshold appearing in HREELS but not in optical spectra, is assigned to excitation of charge transfer levels with thresholds of 3.6Q±0.02, 3.24±0.02 and 2.97±0.02 eV for 4T, 5T and 6T respectively. Losses between 1.0 and 2.3 eV in 4T and 5T are assigned to S 0 →T n transitions. Resonance mechanisms are responsible for induced excitations in the oligothiophene film surfaces.

Computation of adsorbate vibrational frequencies: methyl, methylidyne and methoxy adsorbed on the Al(111) surface

A general strategy for computing adsorbate vibrational frequencies using cluster calculations is illustrated for methyl, methylidyne and methoxy adsorbed on an Al(111) surface. The method uses energy second derivatives rather than the numerical fitting procedures often used in cluster calculations and avoids having to make guesses at the form of the normal modes for the vibrating polyatomic adsorbate. Using energy derivatives also facilitates performing a convergence check on the adsorbate frequency values. The methyl and methylidyne computed frequencies are in good agreement with vibrational frequencies for the CH3I dissociation product observed by high resolution electron energy loss spectra (HREELS). The computed methoxy adsorbate frequencies are also consistent with HREELS experiments, but the optimized methoxy structure, while agreeing with chemical intuition, differs from the methoxy adsorption at the hcp threefold site recently determined by normal incidence standing X-ray wavefield absorption (NISXW) experiments.

Electronic and vibrational structure of thin films of bithiophene: Undoped and alkali-doped states

The complete electronic (occupied and unoccupied states) and vibrational structure of thin films of bithiophene in undoped form and after alkali doping have been investigated experimentally and theoretically. The ultraviolet (UV) photoemission and inverse photoemission spectra demonstrate clearly the filling of the LUMO of bithiophene in the doping process due to charge transfer from the alkali atoms, and density functional theory (DFT) calculations confirm the formation of dianion (bipolaronlike) states in the HOMO–LUMO band gap at high doping levels, respectively. High-resolution electron energy loss spectra (HREELS) have been used to probe the vibrational structure of the bithiophene films, and the formation of a quinoidic structure on Cs doping is well reflected in HREELS by the advent of an intense vibrational structure at 206 meV, which is due to the stiffened C–C interring bond as also predicted by theory.

Surface plasmons at MOCVD-grown GaN

We report on the first observation of surface plasmons at a GaN surface by high-resolution electron energy loss (HREEL) spectroscopy. Dipole scattering theory using self-consistent electron density profiles is employed to calculate the surface energy loss function and hence the expected energy loss spectra. The characteristic features of both the surface plasmons and the Fuchs-Kliewer (FK) surface optical phonons are well reproduced in the framework of this model. Fitting measured HREEL spectra allows us to give estimates of important semiconductor parameters such as electron density, electron mobility and band bending. Moreover, it is found that the plasmon damping strongly depends on primary beam energy. Similarly, the FK phonon damping and frequency are also affected by the variation of primary beam energy.

Characterization of thin films of chloroaluminum phthalocyanine on MoS 2: HREELS, LEET and PIES study

High-resolution electron energy loss spectra (HREELS), low-energy electron transmission spectra (LEET) and Penning ionization electron spectra (PIES) were measured for thin films of chloroaluminum phthalocyanine (ClAlPc) deposited on a cleaved MoS 2 surface in order to investigate the molecular orientation and aggregation in the monolayer range. It was found that the as-grown film of monolayer equivalence consists of ClAlPc islands and the substrate surface is not fully covered by the molecules. Upon heating the film, the molecules spread over the substrate surface to form a uniform monolayer with a flat-lie orientation, where the Cl atoms protrude outside the film.

Alkali metal/GaAs(110) interfaces: correlation effects and sub-gap electron energy loss spectra

The sub-gap high-resolution electron energy loss spectra (HREELS) of the interfaces between Rb, Cs, K,Na and GaAs(110) are presented. The spectra of the Rb/GaAs(110) interface show that this interface is insulating up to half-monolayer coverage when two strong loss peaks appear at 0.6 and 1.3 eV, in analogy to that already found for Cs and K and confirmed here. By comparing the energies of these two losses in the different interfaces, interesting regularities are found as well as a possible connection with the corresponding bulk metal surface plasmon. This comparison also suggests that the formerly proposed link between the two peaks and the presence of correlation effects is not as direct. The two loss features could be associated with coupled collective excitations of the small clusters originated from the decay, at above half a monolayer, of the multi-row atomic chains formed by Cs, K and presumably also by Rb atoms on the GaAs(110) surface. In the case of the Na/GaAs(110) interface, on the contrary, the formation of bi-polarons seems to prevent the build-up of multi-rows chains and their decay into small bi-dimensional clusters. Accordingly, no sub-gap features are found in the corresponding spectra.