Electron energy loss spectroscopy and ultraviolet photoemission of TmSe

Intraionic Tm²⁺ and Tm³⁺ ? → d and p → d charge transfer transitions are identified in the energy loss spectra of 100eV electrons of TmSe by comparison with X-ray photoemission data. In contrast the ultraviolet photoemission spectra with photon energies up to 21.2 eV do not show any evidence for the intermediate valence character of TmSe due to matrix element effects.     

Electron energy loss processes in X-ray photoelectron spectroscopy

A technique is established in X-ray photoelectron spectroscopy (XPS), using spectra emitted from successively evaporated metallic films, to distinguish between electron energy loss mechanisms identified as, respectively, extrinsic and intrinsic to the photoelectron excitation process. It is demonstrated that tailing on the high kinetic energy side of many XPS peaks is due to intrinsic processes, while the background emission at energies generally some 30 eV below the peaks arises from extrinsic processes. Plasmon energy-loss peaks are believed to contain contributions from both intrinsic and extrinsic processes.     

Electron energy loss spectroscopy of TiC,ZrC and HfC

The dielectric properties of commercial TiC, ZrC and HfC powders were determined by analyzing the low loss region of the EELS spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) were obtained by Kramers–Kronig analysis. As maxima observed in the OJDS spectra are assigned to interband transitions across the energy gap, these spectra can be interpreted on the basis of existing energy-band calculations. Comparison between experimental results and theory shows good agreement.     

Electron energy loss spectroscopy of ultrathin pentacene field effect transistors

Electron energy loss spectra of ultrathin pentacene field effect transistors were measured by applying gate bias voltages. Tailing and shouldering of the primary peak on the energy loss side was observed for 1.5 nm thick films when a negative gate bias was applied. The energy loss spectra obtained from the deconvolution of the primary electron profile showed peaks, and the peak energies increased as a function of the gate bias voltage. This is consistent with the behavior of two-dimensional plasmons of field-induced carriers. The thickness dependence is explained by the thickness of the accumulation layer and the probing depth of the spectroscopy.     

Electron energy loss spectroscopy of sodium covered Ge(111) surfaces

The ELS spectra of sodium covered Ge(111) surfaces are studied. It was found that sodium coverage upto 1 ML leads to a extinction of the ELS features of the clean Ge surface. With increasing sodium coverage a metal induced peak at 3.4 eV and a peak at 31.6 eV due to a sodium 2p-core level transitions appear. ELS spectra of a heat-treated surface covered with sodium imply a formation of a surface Na-Ge “compound” as a result of Na-Ge interdiffusion.     

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.     

Electron energy loss spectroscopy analysis of the interaction of Cr and V with MWCNTs

The presented scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) results show the strong reaction of Cr and V with the graphitic walls of MWCNTs. For Vanadium, an interfacial VC layer could be observed at the interface between VN and MWCNTs, when the samples were heated in situ to 750 °C. Knowledge about this interfacial VC layer is important for the formation of VN-MWCNT hybrid materials, used in supercapacitor electrodes, often synthesized at high temperatures. Chromium reacts at 500 °C with the MWCNTs to form Cr₃C₂ and in some cases, dissolved the MWCNT completely. Together with the previously published results about the interaction of MWCNTs with Cu (no interaction) and Ni (a slight rehybridisation trend for the outermost MWCNT-wall observed with EELS) (Ilari et al., 2015) the influence of the valence d-orbital occupancy of 3d transition metals on the interaction strength with CNTs is shown experimentally. For a transition metal to form chemical bonds towards CNT-walls, unoccupied states in its valence d-orbitals are needed. While Ni (2 unoccupied states) interacts only slightly, Cr (5 unoccupied states) and V (7 unoccupied states) react much stronger and can dissolve the MWCNTs, at least partially.     

Electron energy loss spectroscopy studies of nitrogen adsorption on W(100)

Vibrational excitations of nitrogen on W(100) are investigated over the 100–300 K temperature range using elastic and inelastic electron scattering. New vibrational modes of nitrogen are identified that require different mode assignments from previous work. Experimental evidence for a molecular precursor to the atomic β₂ phase of adsorbed nitrogen is presented. Coverage dependent studies of vibrational modes suggests conversion between two different molecular surface phases and between atomic and molecular phases. A new ordered nitrogen phase characterized by a (4 × 1) LEED pattern is observed. The new phase appears to consist of orthogonal domains of p(4 × 1) symmetry that contain atomic nitrogen at the four fold sites (the β₂ atomic phase) with additional bridge-bonded nitrogen atoms in the unit cell.     

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.     

新型高分辨率电子能量损失谱仪与表面元激发研究

高分辨率电子能量损失谱仪利用单色平行电子束入射样品表面,与表面吸附基团的化学键振动、表面声子、电子及其集体激发模式等相互作用而被散射,通过分析散射电子的能量和动量,可以测量表面化学键、晶格动力学、电子态占据以及表面等离激元等的精确信息,是表面科学研究的有力工具.最近,能够对电子能量、动量做二维成像探测分析的半球形电子能量分析器被引入电子能量损失谱仪,实现了高能量、动量分辨率的高效率测量.在对FeSe/SrTiO_3界面超导增强物理机制的研究中,不同厚度的FeSe膜表面的电子能量损失谱表明衬底光学声子产生的偶极电场能够穿透到薄膜内部,诱导较强的电子-声子耦合作用,从而增强薄膜中电子的配对作用,进而使超导转变温度显著提高.三维拓扑绝缘体Bi_2Se_3表面大动量范围的电子能量损失谱还显示出一支奇异的电子集体激发模式,其色散特征不受晶格周期性的限制,而且其寿命和强度几乎不随动量的增加而衰减.这说明在拓扑绝缘体表面,不仅是狄拉克电子态本身,其集体激发也受到拓扑保护.充分发挥新型电子能量损失谱仪观测表面元激发分辨率高、动态范围大的优势,将有力地推动表面界面凝聚态物理问题研究的深入和发展.     

Electron energy loss spectroscopy of the decomposition of formic acid on Ru(001)

Electron energy loss spectroscopy has demonstrated the existence of both a monodentate and a symmetric bidentate bridging formate as stable intermediates in the decomposition of formic acid on the Ru(001) surface. The monodentate formate converts upon heating to the bidentate formate which decomposes via two pathways: CH bond cleavage to yield CO₂ and adsorbed hydrogen; and CO bond cleavage to yield adsorbed hydrogen, oxygen and CO. Thermal desorption spectra demonstrate the evolution of H₂,H₂O, CO and CO₂ as gaseous products of the decomposition reaction. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate, the existence of which is rendered questionable by the spectroscopic results reported here.     

Electron energy loss studies in solids; The transition metal dichalcogenides

The theory of characteristic electron energy losses is discussed in terms of the electronic band structure of a solid. The relationship between the observed plasmon energies, the average interband energy gap and the background dielectric constant of the solid is developed.

The transmission energy loss spectra of a number of the layer-type transition metal dichalcogenides, MX₂, where M=Zr, Hf, Nb, Ta, Mo and W and X=S and Se, have been measured in the range of 0–50 eV. In the experiments, a beam of 50 keV electrons is incident along the c-axis of the crystals and electrons inelastically scattered through an angle of 1 m radian are selected for energy analysis. This ensures that the momentum transfer and hence the electric vector for the excitations lies in the basal plane of the crystal (E⊥c).

Kramers-Kronig analysis has been applied to the energy loss data to deduce the complex dielectric function of each material. From this function, all other ‘optical’ constants, such as the reflectivity, and the oscillator integral function and joint density of states function have been calculated.

The results give substantial support to the existing band model for the family of materials and, in addition, provide the basis for a quantitative understanding of the band structure of individual compounds.     

Electron energy loss spectra of dye vapors

Electron energy loss studies of POPOP andp-terphenyl in the vapor phase at an incident electron energy of 25 eV have been performed. The spectra covering an energy loss between 0 and 9 eV revealed a considerable initial triplet population under electron impact excitation. The implications for electron-beam pumped vapor phase dye lasers are discussed.     

Electron energy loss spectroscopy measurement of the optical gaps on individual boron nitride single-walled and multiwalled nanotubes

Spatially resolved electron energy loss spectroscopy experiments have been performed in an electron microscope on several individual boron nitride (BN) single-, double-, and triple-walled nanotubes, whose diameters and number of shells have been carefully measured. In the low-loss region (from 2 to 50 eV) the spectra have been analyzed within the framework of the continuum dielectric theory, leading to the conclusion of a weak influence of out-of-plane contribution to the dielectric response of the tubes. The gap has been measured to be independent of the nanotubes geometry, and close to the in-plane gap value of hexagonal BN (5.8+/-0.2 eV).     

Electron energy loss spectroscopy measurement of surface vibration dispersion on clean and oxygen-covered Ni(100)

Inelastic electron scattering has been carried out at 300 K on Ni(100) and at 150 K on p(2 × 2) and c(2 × 2) oxygen overlayers adsorbed on Ni(100). Impact energies ranged from 4 to 300 eV in order to measure the dispersion curves of surface vibrations throughout the two-dimensional Brillouin zone in the [110] direction. The Rayleigh mode has been observed in all cases. On O-coved surfaces a surface resonance and two vibrations of different polarizations associated with oxygen motion have been detected. The polarizations of the detected modes and the origin of the resonances, arising from the folding of the Brillouin zone due to the adsorbate, have been analysed with the help of symmetry considerations and the EELS selection rules. The O-coverage dependence of the Rayleigh mode frequency suggests a continuous outwards expansion of the first Ni plane starting from a contracted clean surface. The O-dispersion data are consistent with an O layer distant from the first Ni plane by ～ 0.9 Å in both investigated overlayers.     

Photoemission and energy loss spectroscopy on semiconductor surfaces

The application of multiple experimental techniques to silicon surface studies is discussed. Two specific cases are considered: the chemisorption of oxygen on silicon at coverages up to one monolayer and the intrinsic surface states on cleaved and on annealed Si (111) surfaces. It is shown that by combining electron energy loss spectroscopy and ultraviolet photoemission spectroscopy, one can obtain an approximate energy level model for both occupied and unoccupied states     

Electron energy loss spectra and optical constants of Bismuth

Energy losses of fast electrons through thin films are used to compute the energy loss function, the complex dielectric constant and the optical absorption coefficient in the spectral region 2–150 eV. The existence and the origin of all the previously observed features are discussed.     

Electron energy loss and secondary emission mechanisms in BaO

Electron Loss Spectroscopy (ELS), X-ray Photoemission (XPS), Secondary Emission Energy Distribution, and Secondary Electron yield data have been obtained on both evaporated films and sprayed-on coatings of BaO. Using the ELS correlated with the XPS data, bulk and surface plasmon losses as well as excitonic and interband transition electron loss mechanisms have been identified. It was found that at low primary beam energies (<100 eV), structure in the secondary emission energy distribution could be correlated with a conduction band energy structure. This structure was consistent with the model used to explain the loss transitions. The structure in the energy distribution curves shows little, if any, correlation with plasmon decay mechanisms and other two-step electron emission processes. On the contrary, for the case of BaO (at least at low primary energies), the energy distribution data and structure in the secondary yield vs. primary beam energy data indicate that most secondaries are produced by direct excitation of secondaries by the primary electrons.