Resolution Electron Energy Loss Spectra Research Articles

Electronic Structure and Band Alignment of LaMnO3/SrTiO3 Polar/Nonpolar Heterojunctions

AbstractThe behavior of polar LaMnO3 (LMO) thin films deposited epitaxially on nonpolar SrTiO3(001) (STO) is dictated by both the LMO/STO band alignment and the chemistry of the Mn cation. Using in situ X‐ray photoelectron spectroscopy, the valence band offset (VBO) of LMO/STO heterojunctions is directly measured as a function of thickness, and found that the VBO is 2.5 eV for thicker (≥3 u.c.) films. No evidence of a built‐in electric field in LMO films of any thickness is found. Measurements of the Mn valence by Mn L‐edge X‐ray absorption spectroscopy and by spatially resolved electron energy loss spectra in scanning transmission electron microscopy images reveal that Mn2+ is present at the LMO surface, but not at the LMO/STO interface. These results are corroborated by density functional theory simulations that confirm a VBO of ≈2.5 eV for both ideal and intermixed interfaces. A model is proposed for the behavior of polar/nonpolar LMO/STO heterojunctions in which the polar catastrophe is alleviated by the formation of oxygen vacancies at the LMO surface.

Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS

The weak signal strength in electron magnetic circular dichroism (EMCD) measurements remains one of the main challenges in the quantification of EMCD related EELS spectra. As a consequence, small variations in peak intensity caused by changes of background intervals, choice of method for extraction of signal intensity and equally differences in sample quality can cause strong changes in the EMCD signal. When aiming for high resolution quantitative EMCD, an additional difficulty consists in the fact that the two angular resolved EELS spectra needed to obtain the EMCD signal are taken at two different instances and it cannot be guaranteed that the acquisition conditions for these two spectra are identical. Here, we present an experimental setup where we use a double hole aperture in the transmission electron microscope to obtain the EMCD signal in a single acquisition. This geometry allows for the parallel acquisition of the two electron energy loss spectra (EELS) under exactly the same conditions. We also compare the double aperture acquisition mode with the qE acquisition mode which has been previously used for parallel acquisition of EMCD. We show that the double aperture mode not only offers better signal to noise ratio as compared to qE mode but also allows for much higher acquisition times to significantly improve the signal quality which is crucial for quantitative analysis of the magnetic moments.

Electron-impact electronic-state excitation of para-benzoquinone.

Angle resolved electron energy loss spectra (EELS) for para-benzoquinone (C6H4O2) have been recorded for incident electron energies of 20, 30, and 40 eV. Measured differential cross sections (DCSs) for electronic band features, composed of a combination of energetically unresolved electronic states, are subsequently derived from those EELS. Where possible, the obtained DCSs are compared with those calculated using the Schwinger multichannel method with pseudopotentials. These calculations were performed using a minimum orbital basis single configuration interaction framework at the static exchange plus polarisation level. Here, quite reasonable agreement between the experimental cross sections and the theoretical cross sections for the summation of unresolved states was observed.

Tensor decompositions for the analysis of atomic resolution electron energy loss spectra

A selection of tensor decomposition techniques is presented for the detection of weak signals in electron energy loss spectroscopy (EELS) data. The focus of the analysis lies on the correct representation of the simulated spatial structure. An analysis scheme for EEL spectra combining two-dimensional and n-way decomposition methods is proposed. In particular, the performance of robust principal component analysis (ROBPCA), Tucker Decompositions using orthogonality constraints (Multilinear Singular Value Decomposition (MLSVD)) and Tucker decomposition without imposed constraints, canonical polyadic decomposition (CPD) and block term decompositions (BTD) on synthetic as well as experimental data is examined.

Imaging Energy Transfer in Pt-Decorated Au Nanoprisms via Electron Energy-Loss Spectroscopy.

Driven by the desire to understand energy transfer between plasmonic and catalytic metals for applications such as plasmon-mediated catalysis, we examine the spatially resolved electron energy-loss spectra (EELS) of both pure Au nanoprisms and Pt-decorated Au nanoprisms. The EEL spectra and the resulting surface-plasmon mode maps reveal detailed near-field information on the coupling and energy transfer in these systems, thereby elucidating the underlying mechanism of plasmon-driven chemical catalysis in mixed-metal nanostructures. Through a combination of experiment and theory we demonstrate that although the location of the Pt decoration greatly influences the plasmons of the nanoprism, simple spatial proximity is not enough to induce significant energy transfer from the Au to the Pt. What matters more is the spectral overlap between the intrinsic plasmon resonances of the Au nanoprism and Pt decoration, which can be tuned by changing the composition or morphology of either component.

Spatially and momentum resolved energy electron loss spectra from an ultra-thin PrNiO3 layer

We present an experimental approach which allows for the acquisition of spectra from ultra-thin films at high spatial, momentum, and energy resolutions. Spatially and momentum (q) resolved electron energy loss spectra have been obtained from a 12 nm ultra-thin PrNiO3 layer using a nano-beam electron diffraction based approach which enabled the acquisition of momentum resolved spectra from individual, differently oriented nano-domains and at different positions of the PrNiO3 thin layer. The spatial and wavelength dependence of the spectral excitations are obtained and characterized after the analysis of the experimental spectra using calculated dielectric and energy loss functions. The presented approach makes a contribution towards obtaining momentum-resolved spectra from nanostructures, thin film, heterostructures, surfaces, and interfaces.

Electron irradiation of nuclear graphite studied by transmission electron microscopy and electron energy loss spectroscopy

Structural and chemical bonding changes in nuclear graphite have been investigated during in-situ electron irradiation in a transmission electron microscope (TEM); electron beam irradiation has been employed as a surrogate for neutron irradiation of nuclear grade graphite in nuclear reactors. This paper aims to set out a methodology for analysing the microstructure of electron-irradiated graphite which can then be extended to the analysis of neutron-irradiated graphites. The damage produced by exposure to 200keV electrons was examined up to a total dose of approximately 0.5dpa (equivalent to an electron fluence of 5.6×1021electronscm−2). During electron exposure, high resolution TEM images and electron energy loss spectra (EELS) were acquired periodically in order to record changes in structural (dis)order and chemical bonding, by quantitatively analysing the variation in phase contrast images and EEL spectra.

Surface structure and surface kinetics of InN grown by plasma-assisted atomic layer epitaxy: A HREELS study

The surface bonding configuration and kinetics of hydrogen desorption from InN grown by plasma-assisted atomic layer epitaxy have been investigated. High resolution electron energy loss spectra exhibited loss peaks assigned to a Fuchs–Kliewer surface phonon, N-N and N-H surface species. The surface N-N vibrations are attributed to surface defects. The observation of N-H but no In-H surface species suggested N-terminated InN. Isothermal desorption data were best fit by the first-order desorption kinetics with an activation energy of (0.88 ± 0.06) eV and pre-exponential factor of (1.5 ± 0.5) × 105 s−1.

Phase separation and electronic structure of ZnS0.3O0.7 alloy thin film with and without (Ag, Li) co-doping

Abstract ZnS 0.3 O 0.7 alloy thin film with and without Ag and Li co-doping are grown by pulsed laser deposition on c -plane sapphire substrate. The films are phase separated in S-rich and S-poor regions. Two and four different phases are observed to form in (Ag, Li) 0.05 :Zn 0.95 S 0.3 O 0.7 and ZnS 0.3 O 0.7 films respectively. Different phases and their relative volume fractions have been identified by electron diffraction pattern. The band gap corresponding to each phase is identified by low loss region of high resolution electron energy loss spectra. Band bowing parameter upon S doping is found to be 4.12 eV which closely match with Wien2k based density functional theory calculation utilizing mBJLDA exchange correlation potential. Oxygen positions have been replaced by sulphur in the lattice as confirmed by S L 3,2 electron energy loss near edge absorption spectra. High resolution electron energy loss spectroscopy has been used to collect core level spectra of various dopants in order to identify their locations in the lattice. Experimental Ag M 5,4 extended energy loss fine structure and Li K electron energy loss near edge structure in (Ag, Li):ZnS 0.3 O 0.7 alloy have been compared with calculated spectra using FEFF code, suggesting that Ag and Li have taken up both the substitution and interstitial positions in the lattice. All the samples are resistive with resistance in the range of a few mega-ohms.

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.

Dissociative adsorption of hydrogen on the ZrB2(0001) surface

We have studied the dissociation of H2 on the ZrB2(0001) surface using density functional theory and reflection absorption infrared spectroscopy (RAIRS). Our results show that H2 readily dissociates on the Zr-terminated (0001) surface up to a H coverage of 1/2ML. Furthermore, we show that H is very mobile on the surface and that it desorbs between 545 and 625K. The calculated vibrational frequencies for the adsorbed H are in excellent agreement with our RAIRS measurements and with previously reported high resolution electron energy loss spectra.

The micro-structural studies of Ni-BaTiO3 nanocomposite films by TEM and EELS

Epitaxial BaTiO3 films with embedded metallic Ni nanocrystal (Ni-BaTiO3) were successfully fabricated on SrTiO3 (001) single-crystalline substrate through the laser molecular beam epitaxial (L-MBE) technique. High resolution transmission electron microscopy (HRTEM) and electron energy loss spectrum (EELS) with Kramers-Kronig analysis methods were employed to characterize the microstructures, elementary distribution and the electron structure of these films. HRTEM results suggested that the structure of BaTiO3 was tetragonal with lattice parameters of a=0.399 nm and c=0.403 nm. Energy dispersive X-Ray spectroscopy (EDX) confirmed metallic Ni nanocrystal embedded successfully in BaTiO3 epitaxial films. The Ni-BaTiO3 composite films were compound of the epitaxial BaTiO3 (110) layers alternating with Ni NCs array (111) layers. Furthermore, the existence of the misfit dislocations induced by the embedding of Ni nanoparticles was also clearly demonstrated by the HRTEM images. The Ni L2,3 edges of EELS revealed that Ni NCs in their metallic state were embedded uniformly in the BaTiO3 matrix. A chemical shift of about 7 eV regarding L3 edges in the Ni EELS was also observed. The optical band gap of BaTiO3 in these films was about 3.84 eV, higher than 3.55 eV for pure BaTiO3 films at room temperature.

Quantitative statistical analysis, optimization and noise reduction of atomic resolved electron energy loss spectrum images

In this work we investigate methods of statistical processing and background fitting of atomic resolution electron energy loss spectrum image (SI) data. Application of principal component analysis to SI data has been analyzed in terms of the spectral signal-to-noise ratio (SNR) and was found to improve both the spectral SNR and its standard deviation over the SI, though only the latter was found to improve significantly and consistently across all data sets analyzed. The influence of the number of principal components used in the reconstructed data set on the SNR and resultant elemental maps has been analyzed and the experimental results are compared to theoretical calculations.

Observation of NH2 species on tilted InN (011−1) facets

The structural properties and surface bonding configuration of InN layers grown by high-pressure chemical vapor deposition have been characterized using Raman spectroscopy, x-ray diffraction (XRD), and high resolution electron energy loss spectroscopy. The appearance of the A1(TO) mode at 447 cm−1 in unpolarized z(·)z− Raman spectrum indicates distortions in the crystal lattice due to the growth of tilted plane crystallites. A Bragg reflex in the x-ray diffraction spectrum at 2Θ ≈ 33° has been assigned to tilted InN facets in the polycrystalline InN layer. The high resolution electron energy loss spectrum for this InN layer features vibration modes assigned to NH2 species indicating a surface orientation consistent with the crystalline properties observed in Raman spectroscopy and XRD. The appearance of tilted planes is suggested to be due to the effects of high V–III ratio and lattice mismatch on the growth mechanism.

Shallow Donor States Induced by In-Diffused Cu in ZnO: A Combined HREELS and Hybrid DFT Study

A combined experimental and first principles study of Cu defects in bulk ZnO is presented. Cu particles are epitaxially deposited on the polar O-ZnO(0001) surface at room temperature. Upon heating, a broadening of the quasielastic peak in high resolution electron energy loss spectra is observed, corresponding to an electronic doping effect of Cu atoms in bulk ZnO with an ionization energy of 88 meV. Cu impurities in ZnO, although commonly acting as acceptors, are presently observed to induce shallow donor states. We assign these to interstitial Cu species on the basis of a hybrid density functional study.

Multiple scattering calculations of relativistic electron energy loss spectra

A generalization of the real-space Green's-function approach is presented for ab initio calculations of relativistic electron energy loss spectra (EELS) which are particularly important in anisotropic materials. The approach incorporates relativistic effects in terms of the transition tensor within the dipole-selection rule. In particular, the method accounts for relativistic corrections to the magic angle in orientation resolved EELS experiments. The approach is validated by a study of the graphite $\text{C}\text{ }K$ edge, for which we present an accurate magic angle measurement consistent with the predicted value.

From nanoparticle to nanocable: Impact of size and geometrical constraints on the optical modes of Si/SiO2 core/shell nanostructures

In the extinction spectra of Si/SiO2 core/shell nanostructures, peak features in the near UV region (3–5 eV) appear when the nanostructure geometrical configuration changes from spherical nanoparticles to cylindrical nanocables, with the peak features become more intense in the nanocables of smaller core diameter. Similar feature at ∼4.2 eV is also observed in the spatially resolved electron energy loss spectra (SREELS) of individual nanocable, but not in the nanoparticle. The EELS simulations unravel the origin of such excitation as the monopolar interface plasmon in cylindrical nano-objects, being responsible for the observed near UV extinction modes in nanocables.

StripeSTEM, a technique for the isochronous acquisition of high angle annular dark-field images and monolayer resolved electron energy loss spectra

A technique capable of producing monolayer resolved electron energy loss (EEL) spectroscopy data along one direction in crystal structures is introduced. Unambiguous assignment of EEL spectra to atomic planes is possible via the execution of high angle annular dark-field (HAADF) imaging and EEL spectrum acquisition in parallel. The recording of instrumental instabilities in the HAADF image during the measurement enables a proper quantification by virtue of post-acquisition correction. Compared to the conventional line profile technique a dose reduction by several orders of magnitude can be achieved. The technique is applied to bulk SrTiO3 and ZnO:In2O3 in order to explore its capabilities and limits. Monolayer resolution was achieved for the Ti–L23 and In–M45 core-losses. Multislice calculations were carried out for the purpose of assessing the residual delocalisation of the inelastic signal. Fundamental limits to the resolution are imposed by dynamical dispersion of the electron wave in the crystal combined with the extension of the inelastic potential. In the present case, owing to the requirement of a high beam current, the geometrical probe size cannot be neglected when compared to the width of an inelastic scattering potential.

Triple-twin domains in Mg doped GaN wurtzite nanowires: structural and electronicproperties of this zinc-blende-like stacking

We report on the effect of Mg doping on the properties of GaN nanowires grownby plasma assisted molecular beam epitaxy. The most significant feature is thepresence of triple-twin domains, the density of which increases with increasing Mgconcentration. The resulting high concentration of misplaced atoms gives rise to localchanges in the crystal structure equivalent to the insertion of three non-relaxedzinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ)nanowire growth axis. High resolution electron energy loss spectra were obtainedexactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolvedmeasurements, which allow us to identify modifications in the local density of states,revealed changes in the band to band electronic transition energy from 3.4 eV forwurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreementwith specific ab initio atomistic simulations and demonstrate that the redshiftobserved in previous photoluminescence analyses is directly related to the presenceof these zinc-blende domains, opening up new possibilities for band-structureengineering.

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.