Electron spectroscopy measurements on hydrogen implanted graphite and comparison to amorphous hydrogenated carbon films (a-C:H)

Highly oriented pyrolytic graphite (HOPG) as well as polycrystalline graphite (pc-graphite) were bombarded with 3.5 keV H⁺ ions by means of a Penning ion source. The implanted graphite was characterized by in situ electron spectroscopy techniques such as UPS, XPS and EELS. Our UPS valence band measurements of the hydrogen saturated graphite reveal it to be an insulating phase, and XPS measurements show a shift of the C1s core level to higher binding energy with respect to pristine graphite. This behavior is explained by a Fermi energy shift upon hydrogen bombardment of graphite.In addition, a close resemblance in the electronic structure of hydrogen bombarded graphite and amorphous hydrogenated carbon films (a-C:H) is shown which suggests the modification of pristine graphite to an amorphous network [1] of mostly tetrahedrally bonded carbon atoms by hydrogen implantation.     

Cluster growth of Cu on graphite: XPS,Auger and electron energy loss studies

Auger, XPS and EELS techniques have been used to investigate the core levels, the d-valence band and the electronic transitions of different UHV deposited Cu clusters on graphite. The decreasing of the Cu particle size produces core levels and valence band shifts towards higher binding energies. Lower extra-atomic screening of the conduction electrons near the excited atom and shift of the d-band towards the isolated atom levels are claimed to explain these effects. The EELS results suggest that, for smallest clusters, no structural change but only a lattice parameter contraction of the f.c.c. cage occur.     

Inversion of two-band superconductivity at the critical electron doping of (Mg, Al)B2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.     

Electron energy loss spectroscopy study of the initial stage of lanthanum oxidation

The electron energy loss spectra (EELS) of a pure metallic lanthanum surface and variations in these spectra at the initial stages of surface oxidation were studied. The measurements were performed at primary-electron beam energies E _p from 200 to 1000 eV. A very pronounced peak at a loss energy of about 7.5 eV arises due to transitions from the La4d electronic states of the valence band into the empty La4f electronic states of the conduction band at 5.0–5.5 eV above the Fermi level. Marked changes are observed in the EELS during the oxidation of lanthanum: the peak at an energy of 7.5 eV disappears, and the peak at 13.5 eV corresponding to bulk collective energy loss in lanthanum oxide becomes more pronounced. The results obtained are discussed in terms of the electronic structure of lanthanum and lanthanum oxide.     

Unraveling the symmetry of the hole states near the Fermi level in the MgB2 superconductor

We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the K edge of boron in MgB(2). We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron p states near the Fermi level. The changes in the near edge structure observed in EELS with direction of the momentum transfer imply that these states have p(x)p(y) symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty p(x)p(y) states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the p(x)p(y) feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.     

The near edge structure of cubic boron nitride

We compare the near edge structure (NES) of cubic boron nitride (cBN) measured using both electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) with that calculated using three commonly used theoretical approaches. The boron and nitrogen K-edges collected using EELS and XAS from cBN powder were found to be nearly identical. These experimental edges were compared to calculations obtained using an all-electron density functional theory code (WIEN2k), a pseudopotential density functional theory code (CASTEP) and a multiple scattering code (FEFF). All three codes were found to reproduce the major features in the NES for both ionisation edges when a core-hole was included in the calculations. A partial core hole (1/2 of a 1s electron) was found to be essential for correctly reproducing features near the edge threshold in the nitrogen K-edge and to correctly obtain the positions of all main peaks. CASTEP and WIEN2k were found to give almost identical results. These codes were also found to produce NES which most closely matched experiment based on χ² calculations used to qualitatively compare theory and experiment. This work demonstrated that a combined experimental and theoretical approach to the study of NES is a powerful way of investigating bonding and electronic structure in boron nitride and related materials.     

New point of view on anisotropy of electrical properties in graphite and some graphite intercalation compounds

The mechanism of the EPR line shift observed in some sp²-bonded carbon materials is discussed. An additional factor containing the carriers' concentration and their effective mass is included in g-factor, the aim being to link together the g-factor anisotropy observed in graphite with presence of the charge carriers. Good agreement with the experimental data obtained for graphite is shown. Effective masses for the carriers in graphite intercalation compounds (GIC) are calculated on the base of the g-factor values obtained from EPR measurements.     

Electronic structure of YMn2O5 studied by EELS and first-principles calculations

The electronic structure of multiferroic YMn₂O₅ material has been studied by use of the generalized gradient approximation (GGA). The results demonstrate that the oxygen 2p and manganese 3d orbitals are strongly hybridized. Considering the on-site Coulomb interaction U, we performed the GGA+U calculations for 0 < U ? 8 eV, and it is found that the increase of U could enlarge the band gap and, on the other hand, weaken the Mn-O hybridization. The experimental measurements of the electron energy-loss spectrometry (EELS) exhibit a rich variety of structural features in both O-K edge and Mn-L edges. A theoretical and experimental analysis on the O-K edge suggests that the on-site Coulomb interaction (U) in YMn₂ O₅ could be less than 4 eV. Certain electronic structural features of LaMn₂O₅ have been discussed in comparison with those of YMn₂O₅.     

Characterization of doped diamond-like carbon films deposited by hot wire plasma sputtering of graphite

Diamond-like carbon (DLC) films doped with nitrogen and oxygen were deposited on silicon(100) and polytetrafluoroethylene (PTFE) substrates by hot wire plasma sputtering of graphite. The morphology and chemical composition of deposited films has been characterized by scanning electron microscopy, XPS, Auger, FTIR spectroscopy and micro-Raman scattering. Plasmon loss structure accompanying the XPS C 1s peak and electron energy loss spectroscopy (EELS) in reflection mode was used to study the fraction of sp³ bonded C atoms and the density of valence electrons. Raman spectra show two basic C–C bands around 1575 cm^-1 (G line) and 1360 cm^-1 (D line) . Auger depth profiling spectroscopy was used to measure the spatial distributions of C, N and O atoms in the surface layer of DLC films. The fraction of sp³ bonded atoms of about 40% was detected in DLC films by XPS plasmon loss and EELS techniques. Nitrile and iso-nitrile groups observed in FTIR spectra demonstrated the existence of sp bonded carbon in doped DLC films. The typical for DLC films specific density 1.7–1.8 g/cm³ was obtained from EELS and XPS data. PACS 52.77.Dq; 81.65.-b; 82.80.Pv     

The diamond surface: atomic and electronic structure

Experimental studies of the diamond surface with primary emphasis on the (111) surface are presented. Aspects of the diamond surface which are addressed include (1) the electronic structure, (2) the atomic structure, and (3) the effect of termination of the lattice by foreign atoms. Limited studies of graphite are discussed for comparison with the diamond results. Experimental results from valence band and core level photoemission spectroscopy (PES), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) are used to study and characterize both the clean and hydrogenated surface. In addition, the interaction of hydrogen with the diamond surface is examined using results from vibrational high resolution low energy electron loss spectroscopy and photon stimulated ion desorption (PSID) yield at photon energies just above the carbon k-edge. Both EELS and PSID verify that the mechanically polished 1 × 1 surface is hydrogen terminated and also that the reconstructed 2×2/2×1 surface is hydrogen free. We apply this basic knowledge of the clean diamond surface and of diamond-hydrogen systematics to understanding of the fundamental growth characteristics of diamond films.     

Correlation of electronic and optical transitions: Mo(CO)6 adsorbed on clean and K-preadsorbed Si(111)7 × 7

Electron energy loss spectroscopy (EELS) is used to measure the electronic structure of adsorbed Mo(CO)₆ molecules on clean and potassium-preadsorbed Si(111)7 × 7 at 82 K. The electronic structure of physisorbed MoCCO)₆ is found to be not significantly (within 0.1 eV) perturbed from that of free molecules. The implication of electronic EELS measurements to surface photochemistry is discussed in this paper.     

A methodology to optimize the quantification of sp carbon fraction from K edge EELS spectra

A methodology is presented to select a consistent method, using electron energy loss spectroscopy (EELS), to extract the fraction of sp²-bonded carbon atoms in carbonaceous materials. According to this methodology, a reliable method has to conjointly fulfill two criteria. The first one consists in verifying, on a perfect graphite sample, that the experimental evolution of R = I_π*/(I_π* + I_σ*)-ratios is in good agreement with the one theoretically predicted as a function of experimental settings. The second criterion consists in measuring sp² fractions in amorphous carbon samples with a minimum of fluctuation. We test three commonly used R-extraction techniques, and we show that they exhibit some failures. We thus implement a more accurate R-extraction process that accounts for the predicted graphite R-evolution and exhibits a low intrinsic 4%-noise, as determined from the sp² fraction fluctuation. Moreover, we check the transferability of our method on a wide range of EELS spectra, acquired with different experimental resolutions on samples exhibiting various sp² contents.     

Yttrium segregation and intergranular defects in alumina

Intergranular segregation of yttrium is investigated in a rhombohedral twin grain boundary of alumina. The deviation from the twin orientation is compensated by a periodic arrangement of intergranular dislocations. TEM tools (CTEM and HREM, EDXS, EFTEM and EELS/ELNES) have been used to characterize changes in chemical and electronic environments along this twin. Within the experimental limits, no Y is detected in the perfect twin parts. On the other hand, Y segregation occurs very locally in the dislocation cores on about four atomic planes perpendicularly to the GB, which in fact corresponds to the step height associated with the interfacial dislocations. By comparison with an undoped bicrystal, both the dislocation distribution and the Y segregation localization confirm the influence of Y on the dislocation mobility. Furthermore, in the Y-rich defects, high spatial resolution analysis of the energy loss near edge structures (ELNES) of the Al-L₂₃ absorption edge brings some enlightenments about the Al³⁺ cation environment, provided the effects of local radiation damage are fully considered and under control.     

Nanoscale EELS analysis of dielectric function and bandgap properties in gaN and related materials

The low loss region of an EEL spectrum (<50 eV) contains information about excitations of outer shell electrons and thus the electronic structure of a specimen which determines its optical properties. In this work, dedicated electron energy loss spectroscopy (EELS) methods for the experimental acquisition and analysis of spectra are described, which give improved information about the electronic structure near the bandgap region at a spatial resolution in the range of nanometers. For this purpose, we made use of a cold field emission scanning transmission electron microscope (STEM) equipped with a dedicated EELS system. This device provides a subnanometer electron probe and offers an energy resolution of 0.35 eV. Application of suitable deconvolution routines for removal of the zero loss peak extracts information on the bandgap region while the Kramers-Kronig transformation deduces the dielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap materials for the case of III-nitride compounds, which are currently the most promising material for applications on optoelectronic devices working in the blue and ultraviolet spectral range. The obtained results are in excellent agreement with experimental measurements by synchrotron ellipsometry and theoretical studies. The potential of the superior spatial resolution of EELS in a STEM is demonstrated by the analysis of dielectric properties of individual layers of heterostructures and individual defects within wurtzite GaN.     

Experimental and theoretical determination of the low-loss electron energy loss spectroscopy of LiMn2O4

The dielectric properties of cubic spinel-type LiMn₂O₄, used as cathode material in lithium ion secondary batteries, are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) spectrum in a transmission electron microscope. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA) is presented. The origins of interband transitions are identified in the electronic band structure, by calculating the partial imaginary part of the dielectric function and the partial density of states of Li, Mn and O. Good agreement with experimental spectra is observed which allowed interpreting main features of the EELS spectrum.     

Elemental electron energy loss mapping of a precipitate in a multi-component aluminium alloy

The elemental distribution of a precipitate cross section, situated in a lean Al-Mg-Si-Cu-Ag-Ge alloy, has been investigated in detail by electron energy loss spectroscopy (EELS) and aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A correlative analysis of the EELS data is connected to the results and discussed in detail. The energy loss maps for all relevant elements were recorded simultaneously. The good spatial resolution allows elemental distribution to be evaluated, such as by correlation functions, in addition to being compared with the HAADF image.The fcc-Al lattice and the hexagonal Si-network within the precipitates were resolved by EELS. The combination of EELS and HAADF-STEM demonstrated that some atomic columns consist of mixed elements, a result that would be very uncertain based on one of the techniques alone. EELS elemental mapping combined with a correlative analysis have great potential for identification and quantification of small amounts of elements at the atomic scale.     

Adsorption and desorption kinetics of Cu and Au on (0001) graphite

The interaction of Au and Cu atoms with the (0001) plane of graphite was studied by mass spectrometric measurements of desorption flux both in the presence and absence of an incident atomic beam. For these systems the condensation coefficient increases from ～0.05 at θ = 0 to unity at θ = 1; furthermore the rate of thermal desorption has a kinetic order of one-half for both systems at low coverage. These observations are consistent with a kinetic model in which two-dimensional nucleation of mobile adsorbed atoms occurs upon adsorption, while the reverse process, loss of atoms from the edges of disc nuclei, is the rate controlling step for desorption. This model implies that bonding of metal atoms to the basal plane of graphite is weak and nonlocalized, with adsorption occurring only when two-dimensional nucleation permits metal-metal bonding.     

Thermal processing and native oxidation of silicon nanoparticles

In this study, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) were used to investigate in-air oxidation of silicon nanoparticles ca. 11 nm in diameter. Particle samples were prepared first by extracting them from an RF plasma synthesis reactor, and then heating them in an inert carrier gas stream. The resulting particles had varying surface hydrogen coverages and relative amounts of SiH _x (x = 1, 2, and 3), depending on the temperature to which they had been heated. The particles were allowed to oxidize in-air for several weeks. FTIR, XPS, and EELS analyses that were performed during this period clearly establish that adsorbed hydrogen retards oxidation, although in complex ways. In particular, particles that have been heated to intermediate hydrogen coverages oxidize more slowly in air than do freshly generated particles that have a much higher hydrogen content. In addition, the loss of surface hydride species at high processing temperatures results in fast initial oxidation and the formation of a self-limiting oxide layer. Analogous measurements made on deuterium-covered particles show broadly similar behavior; i.e., that oxidation is the slowest at some intermediate coverage of adsorbed deuterium.     

Metallization of the Ge(111) surface at high-temperature probed by energy-loss and Auger spectroscopies

We present new electron energy-loss spectroscopy (EELS) and Auger (AES) experiments aimed to study the structural transition of the Ge(111) surface taking place at high temperatures. Our advanced high-temperature set-up allowed us to collect accurate EELS spectra near the M_2,3 excitation edges and AES MMV and MVV spectra, corresponding to different probing depths ranging from 4 to 10 Å. The metallization of the surface has been clearly detected by the shift of the M_2,3 edge and of the MMV, MVV Auger energies. A detailed study of the transition has been performed using a fine temperature step under thermal equilibrium conditions. The AES and EELS experiments show that a sudden semiconductor-metal transition takes place at about 1000 K involving mainly the topmost layers. Deeper layers within 10 Å are also involved in the metallization process (in a range of 10 above 1010 K) and a smooth change in the topmost layers is also observed at higher temperatures up to 1070 K. These transitions are not fully reversible upon cooling (down to 870 K). Structural and electronic characteristics of the surface transition are discussed in light of available models.     

Vibrational spectra of acetylene and ethylene adsorbed on Fe(110)

Vibrational spectra of normal and deuterated acetylene and ethylene adsorbed on a Fe(110) surface have been measured by high resolution low energy electron loss spectroscopy (EELS). For both acetylene and ethylene molecular adsorption at 120 K is evident. For a proper assignment of the vibrational modes the angular profiles of most of the losses have also been measured. Application of the EELS dipole selection and the Teller-Redlich product rule favors a triangular adsorption site (μ₃ site) on Fe(110) in a trans-bent and a trans-twisted configuration for acetylene and ethylene, respectively. Furthermore, a strong distortion of the acetylene and ethylene molecules close to a sp³ hybridization state is suggested. Above 340 K acetylene starts to decompose and the formation of CH_x intermediates is indicated followed by a complete loss of hydrogen at temperatures above 540 K. Ethylene begins to decompose into acetylene and hydrogen below 300 K.