High-resolution Electron Energy Loss Spectra Research Articles

Tailoring microstructure, mechanical and tribological properties of amorphous CNx films by incorporating Fe

Amorphous carbon nitride (α-CNx) films are perfect protective materials for advanced tribological application due to low coefficients of friction and chemical stability. However, their lower nano-hardness and poor degree of graphitization during sliding, result in the limited lubrication effect and inevitable wear, which greatly limits its wider potential for protective films. Herein, α-CNx/Fe films were constructed at different deposition temperatures by magnetron sputtering. The results indicated that the α-CNx/Fe film deposited at 600 °C has the highest hardness of ~27.3 GPa (2.13 times that of the pure α-CNx film), which is mainly attributed to the high sp3/sp2 carbon ratio. Besides, the FeN bond of the film can induce the formation of self-lubricating tribofilm on the worn surface, which leads to low friction coefficient (μ = 0.18) of the α-CNx/Fe film during sliding, only 46 % of pure α-CNx films. High-resolution transmission electron microscopy, Raman spectra, and Electron energy loss spectra further confirmed that most of the tribofilm on the worn surface is composed of graphitized onion-like carbon structure scrolls, which is the critical factor for the decrease of friction coefficient. These results demonstrated that the obtained CNx/Fe films have excellent lubrication characteristics, providing guidance for CNx-based films in advanced precision machining and tribological applications.

Investigation of Deoxidation Process of MoO3 Using Environmental TEM.

In situ environmental transmission electron microscope (ETEM) could provide intuitive and solid proof for the local structure and chemical evolution of materials under practical working conditions. In particular, coupled with atmosphere and thermal field, the behavior of nano catalysts could be directly observed during the catalytic reaction. Through the change of lattice structure, it can directly correlate the relationship between the structure, size and properties of materials in the nanoscale, and further directly and accurately, which is of great guiding value for the study of catalysis mechanism and the optimization of catalysts. As an outstanding catalytic material in the application of methane reforming, molybdenum oxide (MoO3)-based materials and its deoxidation process were studied by in situ ETEM method. The corresponding microstructures and components evolution were analyzed by diffraction, high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectrum (EELS) techniques. MoO3 had a good directional deoxidation process accompanied with the process of nanoparticles crushing and regrowth in hydrogen (H2) and thermal field. However, in the absence of H2, the samples would exhibit different structural evolution.

Surficial Structure Retention Mechanism for LiNi0.8Co0.15Al0.05O2 in a Full Gradient Cathode.

A LiNi0.8Co0.15Al0.05O2 (NCA) solid battery was assembled by coupling an NCA cathode with a polyvinylidene fluoride (PVDF)/Li6.75La3Z1.75Ta0.25O12 (LLZTO) composite polymer electrolyte and a Li anode. A NiO-like phase transition is regarded as the reason for interface impedance, which leads to drastic capacity fading during cycling. A gradient cathode with excessive addition of Li6.75La3Zr1.75Ta0.25O12 was fabricated at the cathode/electrolyte interface to overcome the increased impedance. Using high-resolution transmission electron microscopy and electron energy loss spectra, the formation of a unique localized cation migration (LCM) region was confirmed on the NCA surface. The formation of an LCM region hindered further distortion and enhanced the capacity retention of the spinel lattice.

Synthesis of diamond nanostructures from carbon nanotube and formation of diamond-CNT hybrid structures

We report direct conversion of multiwall carbon nanotubes (CNTs), synthesized by chemical vapor deposition, into diamond by nanosecond pulsed laser melting process at ambient temperature and pressure in air without any catalysts. The Raman spectroscopy of the CNTs after the laser irradiation showed the characteristic diamond peak at around 1324-1325 cm−1. The downshift of this peak from its theoretical position (at 1332 cm−1) is explained by phonon confinement in nanostructured diamond. The SEM and TEM images show the formation of diamond mostly at the tip and bends of the CNTs. The grain size distribution and the shape of the converted nanodiamonds suggest that the transformation takes place by melting of the CNTs in a super undercooled state by nanosecond laser pulses, and subsequent rapid quenching to convert it into phase-pure diamond. The EBSD analysis illustrates the phase-pure single crystal diamond formation at the tips and bends of the CNTs. The high-resolution electron energy-loss spectrum in the STEM contains characteristic σ* peak at 292 eV for sp3 bonding of diamond. This study on the laser-induced direct conversion of CNTs to diamond marks a major breakthrough in the formation of diamond nanostructures and diamond-CNT hybrid for a variety of potential applications.

Uptake and persistence of bacterial magnetite magnetosomes in a mammalian cell line: Implications for medical and biotechnological applications.

Magnetotactic bacteria biomineralize intracellular magnetic nanocrystals surrounded by a lipid bilayer called magnetosomes. Due to their unique characteristics, magnetite magnetosomes are promising tools in Biomedicine. However, the uptake, persistence, and accumulation of magnetosomes within mammalian cells have not been well studied. Here, the endocytic pathway of magnetite magnetosomes and their effects on human cervix epithelial (HeLa) cells were studied by electron microscopy and high spatial resolution nano-analysis techniques. Transmission electron microscopy of HeLa cells after incubation with purified magnetosomes showed the presence of magnetic nanoparticles inside or outside endosomes within the cell, which suggests different modes of internalization, and that these structures persisted beyond 120 h after internalization. High-resolution transmission electron microscopy and electron energy loss spectra of internalized magnetosome crystals showed no structural or chemical changes in these structures. Although crystal morphology was preserved, iron oxide crystalline particles of approximately 5 nm near internalized magnetosomes suggests that minor degradation of the original mineral structures might occur. Cytotoxicity and microscopy analysis showed that magnetosomes did not result in any apparent effect on HeLa cells viability or morphology. Based on our results, magnetosomes have significant biocompatibility with mammalian cells and thus have great potential in medical, biotechnological applications.

Band gap measurement of Bi2MoxW1-xO6 by low loss electron energy loss spectroscopy

This work shows the comparison of high-resolution electron energy loss spectra (HR-EELS) in the low loss region (0−15eV) to investigate the electronic structure from koechilinite Bi2MoO6 to rusellite Bi2WO6 varying the stoichiometric relation Bi2MoxW1−xO6. The effect of the Mo to W ratio on the bandgap energy was evaluated on individual particles. Two approximations were considered in order to determine the band gap energy value, the first one was a linear fit and the second one was a mathematical fit. Both analyses are in agreement with those ones collected and analyzed by UV–Vis characterization. Our results suggest a direct electronic transition that increases from about 2.53eV to about 3eV as the W content increase from 0% to 100% wt. X-ray diffraction was used to corroborate the crystal structure and crystal size; transmission electron microscopy was used to monitor the morphology evolution and UV–Vis spectroscopy in diffuse reflectance mode to determine the Eg. These techniques complement the characterization of these materials.

Reflections on the value of electron microscopy in the study of heterogeneous catalysts.

Electron microscopy (EM) is arguably the single most powerful method of characterizing heterogeneous catalysts. Irrespective of whether they are bulk and multiphasic, or monophasic and monocrystalline, or nanocluster and even single-atom and on a support, their structures in atomic detail can be visualized in two or three dimensions, thanks to high-resolution instruments, with sub-Ångstrom spatial resolutions. Their topography, tomography, phase-purity, composition, as well as the bonding, and valence-states of their constituent atoms and ions and, in favourable circumstances, the short-range and long-range atomic order and dynamics of the catalytically active sites, can all be retrieved by the panoply of variants of modern EM. The latter embrace electron crystallography, rotation and precession electron diffraction, X-ray emission and high-resolution electron energy-loss spectra (EELS). Aberration-corrected (AC) transmission (TEM) and scanning transmission electron microscopy (STEM) have led to a revolution in structure determination. Environmental EM is already playing an increasing role in catalyst characterization, and new advances, involving special cells for the study of solid catalysts in contact with liquid reactants, have recently been deployed.

Synthesis and Characterization of Atomically Flat Methyl-Terminated Ge(111) Surfaces.

Atomically flat, terraced H-Ge(111) was prepared by annealing in H2(g) at 850 °C. The formation of monohydride Ge-H bonds oriented normal to the surface was indicated by angle-dependent Fourier-transform infrared (FTIR) spectroscopy. Subsequent reaction in CCl3Br(l) formed Br-terminated Ge(111), as shown by the disappearance of the Ge-H absorption in the FTIR spectra concomitant with the appearance of Br photoelectron peaks in X-ray photoelectron (XP) spectra. The Br-Ge(111) surface was methylated by reaction with (CH3)2Mg. These surfaces exhibited a peak at 568 cm(-1) in the high-resolution electron energy loss spectrum, consistent with the formation of a Ge-C bond. The absorption peaks in the FTIR spectra assigned to methyl "umbrella" and rocking modes were dependent on the angle of the incident light, indicating that the methyl groups were bonded directly atop surface Ge atoms. Atomic-force micrographs of CH3-Ge(111) surfaces indicated that the surface remained atomically flat after methylation. Electrochemical scanning-tunneling microscopy showed well-ordered methyl groups that covered nearly all of the surface. Low-energy electron diffraction images showed sharp, bright diffraction spots with a 3-fold symmetry, indicating a high degree of order with no evidence of surface reconstruction. A C 1s peak at 284.1 eV was observed in the XP spectra, consistent with the formation of a C-Ge bond. Annealing in ultrahigh vacuum revealed a thermal stability limit of ∼400 °C of the surficial CH3-Ge(111) groups. CH3-Ge(111) surfaces showed significantly greater resistance to oxidation in air than H-Ge(111) surfaces.

Topological effect of surface plasmon excitation in gapped isotropic topological insulator nanowires

We present a theoretical investigation of the surface plasmon (SP) at the interface between a topologically nontrivial cylindrical core and a topologically trivial surrounding material, from the axion electrodynamics and modified constitutive relations. We find that the topological effect always leads to a red-shift of SP energy, while the energy red-shift decreases monotonically as core diameter decreases. A qualitative picture based on classical perturbation theory is given to explain these phenomena, from which we also infer that to enhance the shift, the difference between the inverse of dielectric constants of two materials must be increased. We also find that the surrounding magnetic environment suppresses the topological effect. All these features can be well described by a simple ansatz surface wave, which is in good agreement with full electromagnetic eigenmodes. In addition, bulk plasmon energy at ωp = 17.5 ± 0.2 eV for a semiconducting Bi2Se3 nanoparticle is observed from high-resolution electron energy loss spectrum measurements.

Thin single-wall BN-nanotubes formed inside carbon nanotubes.

We report a high yield synthesis of single-wall boron nitride nanotubes (SWBNNTs) inside single-wall carbon nanotubes (SWCNTs), a nano-templated reaction, using ammonia borane complexes (ABC) as a precursor. Transmission electron microscope (TEM), high angle annular dark field (HAADF)-scanning TEM (STEM), electron energy loss spectra (EELS) and high resolution EELS mapping using aberration-corrected TEM system clearly show the formation of thin SWBNNTs inside SWCNTs. We have found that the yield of the SWBNNT formation is high and that the most of ABC molecules decompose and fuse to form the thin BNNTs at a temperature of 1,673 K having a narrow diameter distribution of 0.7 ± 0.1 nm. Optical absorption measurements suggest that the band gap of the thin SWBNNTs is about 6.0 eV, which provide the ideal insulator nanotubes with very small diameters.

Simulation of the oxidation pathway on Si(100) using high‐resolution EELS

AbstractWe compute high‐resolution electron energy loss spectra (HREELS) of possible structural motifs that form during the dynamic oxidation process on Si(100), including the important metastable precursor silanone and an adjacent‐dimer bridge (ADB) structure that may seed oxide formation. Spectroscopic fingerprints of single site, silanone, and “seed” structures are identified and related to changes in the surface bandstructure of the clean surface. Incorporation of oxygen into the silicon lattice through adsorption and dissociation of water is also examined. Results are compared to available HREELS spectra and surface optical data, which are closely related. Our simulations confirm that HREELS offers complementary evidence to surface optical spectroscopy, and show that its high sensitivity allows it to distinguish between energetically and structurally similar oxidation models.

Diazonium-Derived Aryl Films on Gold Nanoparticles: Evidence for a Carbon–Gold Covalent Bond

Tailoring the surface chemistry of metallic nanoparticles is generally a key step for their use in a wide range of applications. There are few examples of organic films covalently bound to metal nanoparticles. We demonstrate here that aryl films are formed on gold nanoparticles from the spontaneous reduction of diazonium salts. The structure and the bonding of the film is probed with surface-enhanced Raman scattering (SERS). Extinction spectroscopy and SERS show that a nitrobenzene film forms on gold nanoparticles from the corresponding diazonium salt. Comparison of the SERS spectrum with spectra computed from density functional theory models reveals a band characteristic of a Au-C stretch. The observation of this stretch is direct evidence of a covalent bond. A similar band is observed in high-resolution electron energy loss spectra of nitrobenzene layers on planar gold. The bonding of these types of films through a covalent interaction on gold is consistent with their enhanced stability observed in other studies. These findings provide motivation for the use of diazonium-derived films on gold and other metals in applications where high stability and/or strong adsorbate-substrate coupling are required.

Common fingerprint of hydroxylated non-polar steps on MgO smoke and MgO films

The infrared spectra of MgO smoke exposed to water are compared to high-resolution electron energy loss spectra (HREELS) of hydroxylated ultrathin MgO(1 0 0)/Ag(1 0 0) films. Very similar bands are observed at 3458–3480 cm−1 and 3710–3714 cm−1. On the basis of first principle calculations, these bands are interpreted as the stretching modes of the two distinct OH groups that are formed at monatomic steps parallel to 〈1 0 0〉 crystallographic directions. The lower frequency band is due to H adsorbed on O at the step edge, while the higher frequency one originates from OH groups that are twofold coordinated with Mg. Consistently, scanning tunnelling microscopy images of MgO films, prepared in similar conditions as during the HREELS experiments, show that the MgO/Ag(1 0 0) island edges are mainly non-polar, i.e. oriented along the 〈1 0 0〉 direction. In spite of that, a minor contribution to the OH-stretch intensity from hydroxylated polar 〈1 1 0〉 steps cannot be excluded; for such geometry density functional theory predicts indeed a single OH species with a vibrational frequency very close to the high-frequency band of OH adsorbed at 〈1 0 0〉 steps.

Electron-Induced Reactions of MeCpPtMe3 Investigated by HREELS

Multilayer condensed films of trimethyl(methylcyclopentadienyl)-platinum(IV) (MeCpPtMe 3 ) have been exposed to low-energy electrons at incident electron energies of 15, 30, and 500 eV. The reactions occurring under exposure have been investigated using high-resolution electron energy loss (HREEL) spectroscopy. The observed changes in the HREEL spectra upon exposure are similar for the three different energies. Contrary to previous results obtained for electron irradiation at 500 eV using reflection absorption infrared spectroscopy (RAIRS) and pointing toward a complete loss of hydrogen a comparable electron exposure in the present experiment yields a product that still contains a significant amount of C—H bonds. In accord with results from thermal desorption spectrometry also obtained after exposure to electrons at lower electron energy, a material is obtained that does not evaporate at room temperature. A crude estimate suggests that the overall reaction rate is consistent with previous results.

High-resolution electron energy loss spectra of reconstructed Si(100) surfaces: First-principles study

We present ab initio calculations of electron energy loss spectroscopy in the reflection geometry (REELS) for the Si(100) surface for which several experimental data are available. The standard surface models [$p(2\ifmmode\times\else\texttimes\fi{}1)$, $c(4\ifmmode\times\else\texttimes\fi{}2)$, and $p(2\ifmmode\times\else\texttimes\fi{}2)$] are structurally very similar in nature, and precise calculations are necessary to differentiate between them. Starting from optimized geometries we compute REELS spectra within the framework of the three-layer model. We adopt several methodologies to ensure a realistic model of the experiment, including a precise partitioning of the surface and bulk dielectric functions and a numerical integration over the detector aperture. We obtain good agreement with the various available experimental energy loss and reflectance anisotropy spectra. The calculations allow us to definitively rule out the presence of the $p(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction. We interpret the ${S}_{0}$ peak observed by Farrell et al. [Phys. Rev. B 30, 721 (1984)] in high resolution REELS. Furthermore, we explain the observed dependence of the spectra on temperature by inferring the presence of dimer flipping at room temperature.

Nano size effects in the high resolution electron energy loss spectra and excitation function of hydrogenated diamond films

The present work studies the high resolution electron energy loss spectra (HR-EELS) and excitation function of hydrogenated diamond films composed of diamond grains of varying size of ∼ 300, ∼ 20 nm and ∼ 5 nm. It is reported that HR-EELS modes associated with C–C lattice phonons are comparably more intense than modes associated with the C–H surface vibrational modes in the case of nano-crystalline compared to micro-crystalline diamond films. Excitation function measurements of the different modes show a pronounced maximum at ∼ 8 eV for the nano-diamond films, whereas for micro-crystalline films similar modes showed maximum intensities at 13 eV with a smaller contribution at 8 eV. It is reported that the elastic electron reflectivity is substantially larger for the nano-diamond films, showing broad maxima at ∼ 8 and ∼ 13 eV in both cases. It is suggested that these effects are associated with the larger probability for inelastic scattering of electrons by lattice phonons mediated by scattering with internal grain boundaries and defects in the case of nano-crystalline diamond films.

High-Resolution Vibrational Electron-Energy-Loss Spectroscopy of Gaseous and Solid Glycine

The vibrational spectra of thin solid films of glycine deposited at temperatures between 12 and 130 K, as well as gaseous glycine, have been measured by high-resolution electron-energy-loss (HREEL) spectroscopy. The spectra are dominated by hydrogen-bonded glycine dimers when the films are deposited at low temperatures. The HREEL spectra of films deposited at higher temperature (130 K) have a different habitus that can be explained on the basis of a proton transfer reaction leading to the formation of zwitterionic glycine. Gas-phase HREEL spectra show bands characteristic of the neutral monomer of glycine. The results are discussed in relation to a previous study aiming at glycine synthesis in mixed ices under the effect of low-energy electrons.

Electronic State Spectroscopy of c-C5F8 Explored by Photoabsorption, Electron Impact, Photoelectron Spectroscopies and Ab Initio Calculations

The electronic transitions and resonance-enhanced vibrational excitations of octafluorocyclopentene (c-C5F8) have been investigated using high-resolution photoabsorption spectroscopy in the energy range 6-11 eV. In addition, the high-resolution electron energy loss spectrum (HREELS) was recorded under the electric dipolar excitation conditions (100 eV incident energy, approximately 0 degrees scattering angle) over the 5-14 eV energy loss range. A He(I) photoelectron spectrum (PES) has also been recorded between 11 and 20 eV, allowing us to derive a more precise value of (11.288 +/- 0.002) eV for the ground neutral state adiabatic ionization energy. All spectra presented in this paper represent the first and highest resolution data yet reported for octafluorocyclopentene. Ab initio calculations have been performed for helping in the assignment of the spectral bands for both neutral excited states and ionic states.

Desorption of hydrogen from InN [formula omitted] observed by HREELS

The kinetics of isothermal desorption of hydrogen from InN ( 000 1 ¯ ) have been investigated using surface vibrational spectroscopy. Reductions in intensity of the N–H stretching and bending vibrations in high resolution electron energy loss spectra (HREELS) upon annealing indicated loss of surface hydrogen and was attributed to recombinative desorption. Hydrogen completely desorbs from the InN surface upon annealing for 900 s at 425 °C or upon annealing for 30 s at 500 °C. Surface hydrogen coverage was determined using the intensity of the N–H stretching vibrational loss peak. Fitting the coverage versus temperature for anneals of either 30 or 900 s indicated that the desorption was best described by second-order desorption kinetics with an activation energy and pre-exponential factor of 1.3 ± 0.2 eV and 10 –7.3±1.0 cm 2/s, respectively. In addition to thermal desorption, an increase in the carrier concentration in the film was also observed upon annealing to 450 °C or higher as shown in HREELS by a shift of the conduction band plasmon excitation to higher energy.

Evaluation of Infrared Absorption Spectra and High Resolution Electron Energy Loss Spectra by First-principles Calculation

We review first-principles theoretical simulations of surface vibrational spectra such as infrared absorption spectroscopy and high-resolution electron energy loss spectra. We compare theoretical and experimental vibrational spectra for two cases, i.e., n-alkane on Cu(100) and methylthiolate on Au(111) and see the predictive power of the first-principles method for vibrational spectra.