Characterising the surface and interior chemistry of core-shell nanoparticles using scanning transmission electron microscopy

A method for extracting core and shell spectra from core-shell particles with varying core to shell volume fractions is described. The method extracts the information from a single EELS spectrum image of the particle. The distribution of O and N was correctly reproduced for a nanoparticle with a TiN core and Ti-oxide shell. In addition, the O distribution from a nanoparticle with a Cu core and a Cu-oxide shell was obtained, and the extracted Cu L_2,3-core and shell spectra showed the required change in EELS near edge fine structure. The extracted spectra can be used for multiple linear least squares fitting to the raw data in the spectrum image. The effect of certain approximations on numerical accuracy, such as treating the nanoparticle as a perfect sphere, as well as the intrinsic detection limits of the technique have also been explored. The technique is most suitable for qualitative, rather than quantitative, work.     

A new analytical method for characterising the bonding environment at rough interfaces in high-k gate stacks using electron energy loss spectroscopy

Determining the bonding environment at a rough interface, using for example the near-edge fine structure in electron energy loss spectroscopy (EELS), is problematic since the measurement contains information from the interface and surrounding matrix phase. Here we present a novel analytical method for determining the interfacial EELS difference spectrum (with respect to the matrix phase) from a rough interface of unknown geometry, which, unlike multiple linear least squares (MLLS) fitting, does not require the use of reference spectra from suitable standards. The method is based on analysing a series of EELS spectra with variable interface to matrix volume fraction and, as an example, is applied to a TiN/poly-Si interface containing oxygen in a HfO₂-based, high-k dielectric gate stack. A silicon oxynitride layer was detected at the interface consistent with previous results based on MLLS fitting.     

Specimen thickness dependence of hydrogen evolution during cryo‐transmission electron microscopy of hydrated soft materials

The evolution of hydrogen from many hydrated cryo‐preserved soft materials under electron irradiation in the transmission electron microscope can be observed at doses of the order of 1000 e nm^?2 and above. Such hydrogen causes artefacts in conventional transmission electron microscope or scanning transmission electron microscopy (STEM) imaging as well as in analyses by electron energy‐loss spectroscopy. Here we show that the evolution of hydrogen depends on specimen thickness. Using wedge‐shaped specimens of frozen‐hydrated Nafion, a perfluorinated ionomer, saturated with the organic solvent DMMP together with both thin and thick sections of frozen‐hydrated porcine skin, we show that there is a thickness below which hydrogen evolution is not detected either by bubble observation in transmission electron microscope image mode or by spectroscopic analysis in STEM electron energy‐loss spectroscopy mode. We suggest that this effect is due to the diffusion of hydrogen, whose diffusivity remains significant even at liquid nitrogen temperature over the length scales and time scales relevant to transmission electron microscopy analysis of thin specimens. In short, we speculate that sufficient hydrogen can diffuse to the specimen surface in thin sections so that concentrations are too low for bubbling or for spectroscopic detection. Significantly, this finding indicates that higher electron doses can be used during the imaging of radiation‐sensitive hydrated soft materials and, consequently, higher spatial resolution can be achieved, if sufficiently thin specimens are used in order to avoid the evolution of hydrogen‐based artefacts.     

The importance of microscopy in studying the wear behaviour of ceramic surfaces

This paper demonstrates how careful microscopy of worn ceramic surfaces can be used to provide information on the mechanisms of material removal. This information is necessary as a critical complement to wear-rate data obtainable from simple wear tests alone (e.g. lapping with diamond grits). Scanning electron microscopy has been extensively used to investigate the changing appearance of worn surfaces as plasticity and fracture processes compete as materials removal and redistribution mechanisms. Examples of the use of secondary electron imaging at different surface tilts, back-scattered electron imaging and stereo imaging are shown. Further, transmission electron microscopy of samples specially prepared to contain the worn surface layer can reveal the presence of phase changes accompanying wear. Furthermore, observations have been made of instances whereby brittle fracture has unexpectedly occurred as a result of repeated plastic deformation of surfaces at low contact severities. Some conclusions are drawn regarding the influence of specimen microstructure, abrasive grit size and environment of the wear of glass-bonded (debased) alumina and titania materials.     

Application of recording and processing of energy-filtered image sequences for the elemental mapping of biological specimens: Imaging-Spectrum

Computerized energy-filtered transmission electron microscope (EFTEM) permits the recording and the processing of energy-filtered images, allowing a part of an electron energy-loss spectrum for each picture element to be obtained. This method, called ‘Imaging-Spectrum’, uses a Zeiss CEM902 coupled to several image analysis systems. The actual configuration records sequences of 48 images, 256 × 256 pixels, in steps of the energy loss, ΔE. Processing these sequences results in part of a core-loss EELS-spectrum for each pixel. This approach produces elemental maps with a short processing time. We have implemented three kinds of background calculation for the image subtraction. The influence of the irradiation dose and of the energy selecting slit width on the quality of the spectra is investigated. The method is applied to the analysis of some biological specimens (pericellular coat behaviour during adhesion between macrophages and red blood cells and location of calcite microcrystals in dental pulp cells). The Imaging-Spectrum method appears to be suitable for the analysis of large areas.     

Quantitative study of spurious X-ray production in thin film microanalysis

When X-ray microanalysis is performed in a TEM on a thin area of a specimen, some a priori indistinguishable spurious photons produced in other zones of this specimen are always recorded. Several mechanisms contribute to this production. For instance, some Bremsstrahlung and characteristic photons are generated by secondary and Auger electrons; a conservative upper bound to this particular contribution is calculated for several materials, and the present approach is compared to the Monte Carlo simulation. It is then shown that, in special test-specimens, the total extraneous contribution of the thick parts of the specimen to the spectra recorded in a thin zone can be measured; different instruments may now be compared in this respect. In the HB5 STEM, this total contribution remains low; its main cause is beam scattering in the specimen, not before it. Finally, an experimental procedure for estimating this bulk contribution in any specimen of interest is proposed. Calculations and experiments are illustrated for the case of gallium arsenide.     

Hydrothermal synthesis of a lamellar zinc dimolybdate hydroxide with application as the anode for lithium-ion batteries

Abstract

A lamellar zinc dimolybdate hydroxide Zn₃Mo₂O₈(OH)₂ was prepared for the first time by a facile hydrothermal synthesis. The electrochemical properties of the lamellar Zn₃Mo₂O₈(OH)₂ as an anode material were investigated by cyclic voltammetry (CV), reversible capacity, cycling stability and rate capability for lithium ion batteries (LIB). The lamellar zinc dimolybdate hydroxide exhibits a reversible capacity of 404.6 mAh g^?1 at a current density of 100?mA g^?1 after 200 cycles. The reversible capacity of the lamellar Zn₃Mo₂O₈(OH)₂ remained at 60 mAh g^?1 even at a current density of 3000?mA g^?1. When the current density was returned to 100?mA g^?1, a discharge capacity of 380 mAh g^?1 was maintained after 200 discharge/charge cycles. The excellent electrochemical performance may be due to its unique lamellar structure, which buffers the volume change during the Li⁺ intercalation/de-intercalation and provides the electrode with convenient lithium ions and electron transport pathways. These results suggest the promising potential application of the lamellar zinc dimolybdate hydroxide in lithium-ion batteries.     

Microstructure analysis of Nd-Fe-B sintered magnets improved by Tb-metal vapour sorption

Behaviours of constituent elements in the Nd-Fe-B sintered magnets improved by Tb-metal vapour sorption have been investigated by using an analytical transmission microscopy. It was found that a triple junction of the grain boundaries consists of fine Nd-O crystalline and amorphous phase. The energy dispersive X-ray spectroscopy (EDS) analysis showed that the amorphous phase mainly consists of Co, Nd and Tb. The Tb-treatment causes the formation of the amorphous Co-Nd(Tb) wetting-layer phase which wraps each Nd₂Fe₁₄B grain. The results suggest that the wrapped structure prevents the nucleation of magnetic reversed domains and then improves significantly the coercivity of the magnet.     

The effect of vacancies on the annular dark field image contrast of grain boundaries: a SrTiO(3) case study

The analysis of grain boundary structure in high resolution electron microscopy is often hindered by contrast variation within the grain boundary region which is not explained by simple models of the grain boundary structure. Recent work suggests that structural disorder along the beam direction and the presence of vacancies contribute significantly to this effect. One might expect a significant reduction in contrast in a Z-contrast image of a grain boundary would imply that vacancies present must result from the absence of heavier elements. Using a [001](210) Σ5 grain boundary in SrTiO₃ as a test case and first principles structure relaxation to calculate stable defect structures, we show that the reduction in the intensity from fully occupied Sr columns due to the structural distortion resulting from a nearby O vacancy can be as great as that due to introducing a Sr vacancy in the column itself. The effect on energy dispersive X-ray spectroscopy signals is also considered, but found to be smaller than that on Z-contrast images.     

Influence of weave structures on the tribological properties of hybrid Kevlar/PTFE fabric composites

The existing research of the woven fabric self-lubricating liner mainly focus on the tribological performance improvements and the service life raised by changing different fiber type combinations, adding additive modification, and performing fiber surface modification. As fabric composites, the weave structures play an important role in the mechanical and tribological performances of the liners. However, hardly any literature is available on the friction and wear behavior of such composites with different weave structures. In this paper, three weave structures (plain, twill 1/3 and satin 8/5) of hybrid Kevlar/PTFE fabric composites are selected and pin-on-flat linear reciprocating wear studies are done on a CETR tester under different pressures and different frequencies. The relationship between the tensile strength and the wear performance are studied. The morphologies of the worn surfaces under the typical test conditions are analyzed by means of scanning electron microscopy (SEM). The analysis results show that at 10 MPa, satin 8/5 performs the best in friction-reduction and antiwear performance, and plain is the worst. At 30 MPa, however, the antiwear performance is reversed and satin 8/5 does not even complete the 2 h wear test at 16 Hz. There is no clear evidence proving that the tensile strength has an influence on the wear performance. So the different tribological performance of the three weave structures of fabric composites may be attributed to the different PTFE proportions in the fabric surface and the different wear mechanisms. The fabric composites are divided into three regions: the lubrication region, the reinforced region and the bonding region. The major mechanisms are fatigue wear and the shear effects of the friction force in the lubrication region. In the reinforced region fiber-matrix de-bonding and fiber breakage are involved. The proposed research proposes a regional wear model and further indicates the wear process and the wear mechanism of fabric composites.     

Polyphosphate at the Streptomyces lividans cytoplasmic membrane is enhanced in the presence of the potassium channel KcsA

The distribution of polyphosphate (polyP) within the cytoplasmic membrane of Streptomyces lividans hyphae or protoplasts has been determined at high spatial resolution by elemental mapping using energy‐filtered electron microscopy (EFTEM). The results revealed that polyP was best traceable after its interaction with lead ions followed by their precipitation as lead sulphide. Concomitant studies of the S.lividans wildtype (WT) strain and its co‐embedded mutant ΔK (lacking a functional kcsA gene) were conducted by labelling as the surface matrix of either one was labelled by cationic colloidal thorium dioxide. Within the WT strain, additional polyP was found to accumulate distinctly at the inner face of the cytoplasmic membrane. After removal of the cell wall (within protoplasts), the polyP‐derived lead‐sulphide (PbS) precipitate formed clusters of fibrillar material extending up to 50 nm into the cytoplasm. This feature was absent in the ΔK mutant strain. Together the results revealed that the presence of the KcsA channel and the structured polyP coincide.     

Tribological properties of carbon nanotube-doped carbon/carbon composites

Carbon nanotube (CNT)-doped carbon/carbon (C/C) composites were fabricated by the chemical vapor infiltration (CVI) method to investigate the effect of CNTs on tribological properties of C/C composites. CNTs, which had been synthesized by catalytic pyrolysis of hydrocarbons, were added to carbon fiber formed preforms before CVI process. Ring-on-block-type wear tests were performed to evaluate the frictional properties of CNT-doped C/C composites. Results show that CNTs can not only increase wear resistance of C/C composites but also maintain stable friction coefficients under different loads. Polarized light microscopy, X-ray diffraction, scanning electron microscopy and Raman spectroscopy analyses demonstrate that favorable effects of CNTs on tribological properties of C/C composites have been achieved indirectly by altering microstructure of pyrocarbons and directly by serving as high-strength lubricative frictional media at the same time. Electron dispersive spectroscopy (EDS) analyses verify the existence of adhesive wear mechanism in both pure C/C composites and CNT-doped C/C composites albeit the two-body abrasive mechanism dominates in pure C/C composites.     

Carbon-fibre reinforced magnesium alloys: nanostructure and chemistry of interlayers and their effect on mechanical properties

The micromechanical fracture behaviour of C/Mg–Al composites of varying interface reactivity was investigated by scanning electron microscope bending tests. Structure and chemistry of fibre/matrix interlayers were studied down to the atomic scale by imaging and spectroscopical transmission electron microscope techniques (high-resolution electron microscopy, energy dispersive X-ray spectroscopy, parallel-recording electron energy loss spectroscopy and energy-filtered transmission electron microscopy). The chemical reactions at the fibre/matrix interfaces of the C/Mg–Al composites were found to form plate-shaped carbidic precipitates, mainly Al₂MgC₂, which strongly influence the composite's mechanical properties by changing the fibre/matrix bonding strength.     

The role of sulphur in ZDDP-induced reaction films formed in the presence of ZDDP: Contribution of electron spectroscopic imaging technique

The anti-wear action of zinc dithiophosphate (ZDDP), extensively used in engine oils, is here studied by analysis of wear particles generated during a friction test between two ferrous substrates. It is known in this case that the interfacial film is complex, and contains elements from surfaces, environment and degradation products of the additive. This paper deals with the presence and the form of sulphur species in the constitution of this film. Analytical Transmission Electron Microscopy (TEM, STEM, EDS and EELS filtered images) of particles shows that sulphur is in the form of small crystallites of iron sulphide Fe₇S₈, embedded in an amorphous phosphate matrix. Wear tests performed at different normal loads show the variation of the P/S ratio in the particles. The results are discussed.     

Friction studies on the process in circular sawing of granites

An investigation is reported of the friction characteristics of the process in circular sawing of granites with diamond segmented sawblade. The sawing forces were measured and specific sawing energies were calculated in sawing five types of typical granite at specific removal rates ranging from 25 to 200 mm²/s. Coupled with SEM observations of the sawn granite surfaces, a quantitative analysis for sawing energy suggests that most of the sawing energy should be expended by friction of sliding between diamonds and granites. The effect of sliding friction was discussed by introducing a new quantity which is called mean chip longitudinal area. This revised version was published online in August 2006 with corrections to the Cover Date.     

Defect formation in self-assembling quantum dots of InGaAs on GaAs: a case study of direct measurements of local strain from HREM

The growth and defect structures in free-standing self-assembled In_0.6Ga_0.4As quantum dots (QDs) grown on (001) GaAs by solid source molecular beam epitaxy has been investigated. The QDs are elongated along [1¯ 1 0]. At a nominal thickness of eight monolayers defect complexes, associated with intrinsic stacking faults, have been generally observed on both sides of a QD in (1¯ 1 0) cross-section. The total defect vector of such defect complexes is a /3 <111>. Local strain components on {111} slip planes in the QDs without defects have been measured directly from digitized high-resolution electron microscopy images. The distortion on the two sets of {111} planes of a (1¯ 1 0) cross-section is different owing to elastic relaxation. The results of strain measurements suggest that a 60° dislocation nucleates first on the set of {111} planes of higher contractive shear strain, i.e. (111) planes on the right side of the QDs, and (1¯ 1¯ 1) planes on the left side. A 30° partial dislocation forms subsequently on the other set of {111} planes, i.e. (1¯ 1¯ 1) planes on the right side of the QDs and (111) planes on the left side, when the 60° dislocation glides down towards the In_0.6Ga_0.4As/GaAs interface, as a result of the additional strain field of the 60° dislocation. The efficiency of the defect complexes in strain relaxation of the QDs has been shown by strain measurements in QDs with the presence of defects.     

Generalization of the polar representation in time domain fluorescence lifetime imaging microscopy for biological applications: practical implementation

The polar representation or phasor, which provides a fast and visual indication on the number of exponentials present in the intensity decay of the fluorescence lifetime images is increasingly used in time domain fluorescence lifetime imaging microscopy experiments. The calculations of the polar coordinates in time domain fluorescence lifetime imaging microscopy experiments involve several experimental parameters (e.g. instrumental response function, background, angular frequency, number of temporal channels) whose role has not been exhaustively investigated. Here, we study theoretically, computationally and experimentally the influence of each parameter on the polar calculations and suggest parameter optimization for minimizing errors. We identify several sources of mistakes that may occur in the calculations of the polar coordinates and propose adapted corrections to compensate for them. For instance, we demonstrate that the numerical integration method employed for integrals calculations may induce errors when the number of temporal channels is low. We report theoretical generalized expressions to compensate for these deviations and conserve the semicircle integrity, facilitating the comparison between fluorescence lifetime imaging microscopy images acquired with distinct channels number. These theoretical generalized expressions were finally corroborated with both Monte Carlo simulations and experiments.     

Quantitative and qualitative characterization of aquatic iron oxyhydroxide particles by EF-TEM

Electron energy-loss spectroscopy (EELS) and elemental imaging under the energy-filtered transmission electron microscope are powerful tools for the characterization of iron-rich particles present in natural waters. Features present in EEL spectra (Fe-M_2,3 Fe-L_2,3 and O-K ionization edges) of goethite (α-FeOOH) have been studied with an energy filter operated at 80 keV to determine optimal quantification and elemental imaging of Fe-rich natural aquatic particles in the 30–200 nm range of thickness. For quantitative aims, the Fe-M_2,3 ionization edge cannot be used easily, but the Fe-L_2,3 edge provides more accurate results owing to a better background extrapolation. The partial cross-section of the Fe(III) M shell has been determined for iron oxide. The use of two-windows (jump-ratio) and three-windows (background stripping) imaging methods is discussed in relation to the specimen thickness.