High-resolution Electron Microscopy Micrographs Research Articles

Phosphosulfate Nasicon NaFe2(PO4)(SO4)2 As Electroactive Material for Sodium Batteries

NASICON compounds have been well investigated for their luminescence and gas sensor-properties, and as potential hosts for radioactive waste family [1-3]. Owing to their high ionic conductivity, these compounds were extensively studied as solid electrolytes since the mid-70s [4]. When incorporated with transition metals such nickel, manganese and iron cations, the materials regained tremendous attention as possible electroactive materials for sodium-, lithium-, and magnesium-ion batteries [5-7]. In this paper, we report on the structural, electrochemical and conductivity properties of the NASICON phase NaFe2(PO4)(SO4)2. The crystal structure was solved by the Rietveld method using powder x-ray diffraction (PXRD) data. SAED and HRTEM experiments confirmed the proposed structural model. The electrochemical performances were examined by galvanometric cycling, cyclic voltammetry, and impedance spectroscopy. The stability of the NASICON structure during cycling was also followed by ex-situ PXRD experiments. NaFe2(PO4)(SO4)2 was synthesized via a solid state synthesis route from stoichiometric mixtures of NaNO3, Fe(NO₃)₃.9H₂O, (NH4)2SO4, and NH4H2PO4. The starting raw materials, dissolved in an aqueous medium, were stirred at 80oC until evaporation of water. The resulting orange powder was calcined at 550oC for 12h under air, then a pure green powder of NaFe2(PO4)(SO4)2 was obtained. TG-DTA-MS experiments resulted in a 30% weight loss which corresponds to the departure of two SO2 molecules above 600oC, and hence confirm the chemical composition of NaFe2(PO4)(SO4)2. The crystal structure of the latter was solved using the structure of the NASICON NaTi2(PO4)3 as a starting structural model. The Rietveld analysis of the x-ray powder diffraction data led to the reliability factors, Rp = 0.5%, wRp = 0.8%, RB = 9.17%, which corroborates the NASICON crystalline structure of NaFe2(PO4)(SO4)2. When compared to Na4Zr2(SiO4)3, the formula can be rewritten as Na1□3Fe2(PO4)(SO4)2 with the 6b and 18e atomic positions occupied by sodium and vacancies, respectively (□ stands for vacancy). Sodium atoms and vacancies are located in two type of interstitial positions within 3D-interconnected channels formed by the covalent skeleton [Fe2P3O12]- built of FeO6 octahedra and XO4 tetrahedra (X= S, P). To further confirm the x-ray structural model, high resolution electron microscopy (HREM) study was performed along the [010] and [2-21] zone axes. In the case of the [010] zone axis, the corresponding HREM micrograph shows an apparently well-ordered NASICON material with d-spacings of 6.2 and 4.3 Å, corresponding to d10-2 and d104, respectively. The cycling performances of NaFe2(PO4)(SO4)2 were studied in cells embedding (NaPF6:EC:FEC) electrolyte and Na-metal. During the first discharge at a rate of C/20, the title compound delivers a capacity of 89 mAh/g indicating that 70% of the theoretical specific capacity (127mAh.g-1) was achieved (Fig. 1). The material was cycled at the C/5 rate and a 96% capacity retention was obtained after 30 cycles. After the first cycle, the powder XRD pattern was very similar to the one of the initial phase indicating a stable Nasicon structure during cycling. Figure 1. Charge/discharge curves of NaFe2(PO4)(SO4)2 at the rate of C/5, C/10, and C/20 vs. Na+/Na (a), capacity retention (b), and cyclic voltammetry curves at the scan rates of 0.1, 0.25, and 0.5mV/s (c).

Nanostructures of defects in CdZnTe single crystals

Nanostructures of Cd 1− x Zn x Te (CZT) grown by the vertical Bridgman method have been studied. Selected area diffraction patterns from various local areas indicate the homogenous zinc-blende structure of CZT crystals with the lattice constant of 6.442±0.003 Å. High-resolution electron microscopy micrographs have demonstrated two distinct nanostructures for dislocations in CZT. One has an extra line of atoms and the other shows a half atom-spacing shear displacement. The former has one order of magnitude lower density than the latter, indicating that it has much higher formation energy. Te precipitates also show two types of nanostructures including ellipse shaped and linear ones that are dependent on the Te-rich percentage in the precipitates. High Te-rich precipitates have an ellipse-shaped structure while the low Te-rich ones have a linear shape. Twins in CZT are usually grouped in 3–6 microtwins parallel to each other with twinning planes lying exclusively in CZT {1 1 1} planes.

Microstructural characterization and optical properties of ZnSe thin films

Zinc Selenide (ZnSe) thin films (d=0.09–1.25μm) on glass substrates were prepared by physical vapour deposition under vacuum, using the quasi-closed volume technique. X-ray diffraction (XRD) analysis indicated that the samples are polycrystalline, have a cubic (zinc blende) structure and that the crystallites are preferentially oriented with the (111) planes parallel to the substrates. The microstructure of ZnSe films examined by transmission electron microscopy (TEM), including electron diffraction (ED) and high-resolution electron microscopy (HRTEM), confirms the preferential orientation of crystallites. The HRTEM micrographs show that preferred orientation of crystallites, along (111) planes, increases if, after deposition, the samples are subjected to a heat treatment. The spectral dependences of the transmission and absorption coefficients were studied in the range 300–1400nm. The values of the bandgap energy, Eg, calculated from the absorption spectra, ranged between 2.50eV and 2.80eV.

Nickel–tungsten bimetallic sulfide nanostructures of fullerene type

Bimetallic NiW sulfide nanostructures of the inorganic fullerene-like (IF-like) type were prepared by a chemical method employing ammonium thiotungstate and nickel nitrate as metal-sulfide precursors followed by sulfidation in H2S/H2 at 400 °C. The nanostructures were grown with a Ni excess, at an atomic ratio R = 0.85 (R = Ni/Ni + W). The x-ray diffraction patterns showed poorly crystalline WS2, WO2, NiS, and Ni9S8 phases. High-resolution electron microscopy micrographs revealed the formation of two fullerene-like nanostructures, nickel sulfide nanoparticles and long nanotubes filled with tungsten suboxide, both coated by several WS2 layers. The surface area of 18 m2/g measured by nitrogen adsorption (BET surface-area) revealed that these materials contained micropororosity.

TEM Investigation on Stress Contrast and Interfaces of Contacting Particles

An investigation on nanoparticle contacting using convergent beam electron diffraction (CBED) and high-resolution electron microscopy (HREM) is reported. Cobalt particles (5–200nm) from a solution-treated Cu–2(w/o)Co alloy were extracted and allowed to contact without pressure. The contacting stress field due to adhesion was clearly observed, and the stress field had a dipole character. Free particles were observed to contact along low-index zone axes and with specific orientation relationships. Fourier transformation of the HREM micrographs revealed a highly distorted area along the contacting boundary containing dislocations. Electron-diffraction contrast from the stress fields between contacting particles was simulated, and agreed well with the experiments.

XPS analysis and microstructural characterization of a Ce/Tb mixed oxide supported on a lanthana-modified transition alumina

In the framework of a research project aimed at developing alternative materials for application in three-way catalysis, this work reports on the preparation and characterization of a ceria-terbia mixed oxide supported on a lanthana-modified transition alumina. This complex multicomponent oxide system has been characterized by combining x-ray diffraction (XRD), high-resolution electron microscopy (HREM), temperature-programmed desorption mass spectrometry (TPD-MS), Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS). Following a dry impregnation procedure, the lanthana-containing phase was deposited first, the ceria-terbia oxide being dispersed on the modified alumina in a subsequent step. The structural and chemical characterization studies carried out on the La 2 O 3 /Al 2 O 3 binary system provide no evidence of a crystalline lanthana-containing phase. Neither lanthanum oxide nor the phases resulting from its aging in air could be identified. Likewise, no lanthana-alumina mixed oxide phases could be deduced from analysis of the HREM micrographs, selected-area electron diffraction (SAED) or XRD patterns. As deduced from the H 2 O and CO 2 TPD studies, the chemisorptive properties of the La 2 O 3 /Al 2 O 3 are significantly different from those of the alumina and the bulk lanthana. We conclude, accordingly, that the supported lanthana consists of a highly dispersed oxide in strong interaction with the alumina support. By contrast, after deposition and further calcination of the cerium-terbium nitrate precursors, the presence of fluorite microcrystals of average size 5 nm could be identified both by XRD and HREM. The XPS studies carried out on the alumina support, La 2 O 3 /Al 2 O 3 and CeTbO x /La 2 O 3 /Al 2 O 3 , as well as a series of reference systems consisting of aged-in-air lanthana, CeLaO x and CeTbO x mixed oxides, have revealed the presence in the ternary system (CeTbO x /La 2 O 3 /Al 2 O 3 ) of at least two different types of chemical environments for the lanthanum ions. With the help of the La 3d 5/2 spectra recorded for the different reference systems, we conclude that these two environments correspond to highly dispersed lanthana strongly interacting with OH- and/or CO 3 2- groups, and to La 3+ species incorporated in the Ce/Tb mixed oxide phase.

HREM study of Co/Cu/Co/MnFe spin valves

High resolution electron microscopy (HREM) has been used to investigate the structure of Co(6 nm)/Cu(2.8 nm)/Co(4 nm)/MnFe(12 nm) spin valves so as to correlate the structure with the magnetic properties. The presence of a Ti seed layer enhances the structural quality of the film giving a strong <111> texture and large grain size. It is shown that the <111> texture favours a high exchange bias field. A numerical analysis of the HREM micrographs has been carried out in order to investigate more quantitatively the structure of the interfaces. It has been shown that locally they are diffuse over 2 monolayers in the growth direction.

Structural characterization of SinC δ layers embedded in a silicon matrix

Besides a great variety of carbon-containing atomic complexes and precipitates in crystalline silicon, new ordered silicon–carbon structures have recently been predicted theoretically by ab initio calculations [Rücker, Methfessel, Bugiel, and Osten, Phys. Rev. Lett. 72, 3578 (1994)] to be possible in highly C-doped epitaxial layers. These layers are strain stabilized. Due to the high mismatch in lattice parameters, pseudomorphic growth is restricted to layer thicknesses of some monolayers only. Four-hundred kV high resolution electron microscopy (HREM) has been applied to study the microscopic structure of the SinC layers grown by molecular beam epitaxy on (001)-oriented silicon substrate. Local ordering of the carbon with a quasiperiodical variation of its distribution over the layer has been observed on HREM micrographs at certain imaging conditions.

Structure and chemistry of interfaces

It is most desirable to understand the structure and chemistry of the internal interfaces for all classes of materials since the materials' properties often depend on the properties of the interfaces which, in turn, are controlled by their structure and chemistry. In contrast to surface science, there exist only a few techniques for studying the structure and chemistry of internal interfaces. One of the most powerful techniques seems to be transmission electron microscopy (TEM) by which short segments of interfaces can be analyzed. In high-resolution electron microscopy (HREM) a direct image is formed of the projection of the interfaces. A simple analysis of HREM micrographs is not possible owing to the complex image forming processes within HREM. In addition to experimental investigations, calculations of the structures must be performed using material specific interatomic potentials. From the calculated structure, HREM images must be simulated for the specific imaging conditions. The experimental micrographs must be compared to simulated images. An agreement between experimental micrographs and the simulated images results in the best possible atomistic configuration. A quantitative measure for this agreement is the difference image, D, between the experimental micrograph and the simulated image. Best agreement is reached if only the noise is visible in the difference image D. Analytical electron microscopy with high-spatial resolution (typical probe size <0.05 nm) allows the identification of impurities segregated at the interface. However the limit of detectability depends sensitively on the combination between the different elements. Recently developed techniques on spatially resolved electron energy loss spectra give information on bonding and coordination. As an example, the different TEM techniques have been applied to the investigation of grain boundaries in α-Al2O3. It should be emphasized, however, that the TEM techniques could also be applied to internal interfaces in different boundaries.

Statistical Analysis of the Intergranular Film Thickness in Silicon Nitride Ceramics

Silicon nitride materials typically reveal thin amorphous intergranular films along grain boundaries, with only the exception of special boundaries. It is known that such grainboundary films strongly affect the high‐temperature properties of the bulk material. High‐resolution electron microscopy (HREM) was used to study these amorphous films in different Si3N4 ceramics. The observed film thicknesses at grain boundaries in these materials varied between 5and 15 Å. It was shown that the grain‐boundary film thickness strongly depends on film chemistry. Careful inspections of film‐thickness measurements across grain boundaries in a given material suggest that the film widths vary on the order of 1 Å. Therefore, a quantitative evaluation should allow for the determination of the standard deviation of the film thickness. The amorphous film widths along grain boundaries in four materials were measured over the entire length (up to 1 μm) of the grain boundary between two triple points. Forty to fifty data points were evaluated for each boundary, giving a Gaussian‐like distribution of the film thickness around a median value, which corresponded well with the film width measured from single HREM micrographs. The accuracy achieved by the statistical method was better than ± 1 Å.

HREM of epitaxially grown Fe/TiO2 and Cu/TiO2 interfaces

High-resolution electron microscopy (HREM) has been used to provide information on atomic structures of metal/oxide interfaces, which are of both scientific and technological interest. In this report, we present results of a study on Fe/TiO2 and Cu/TiO2 interfaces by HREM. The Fe/TiO2 and Cu/TiO2 interfaces were formed by vapor deposition of Fe and Cu on TiO2 (110) surface, respectively, in a UHV chamber with a base pressure of ∽1x10−10 torr. Cross sectional HREM specimens were prepared using standard techniques involving mechanical polishing, dimpling and ion-milling. The samples were examined in an Topcon-002B high-resolution electron microscope. HREM simulations were performed using the EMS program.Figs, 1a and 1b show a HREM micrograph and a select area diffraction pattern of Fe/TiO2 interface, respectively, taken along the TiO2 [001] direction. From Fig.la and Fig.1b, the following orientation relationship is obtained: [001]Fe//[001]TiO2 and (100)Fe//(110)TiO2. With this orientation, there is about 12.6% lattice misfitt along the TiO2 [10] direction.

High resolution electron microscopic study of a Ga-containing carbonate apatite

Gallium-containing carbonate apatite was prepared by a precipitation method and studied by high resolution electron microscopy (HREM). HREM micrographs were observed for individual crystals along the 〈2 1 1 1〉, 〈5 1 4 0〉 , 〈30 3 1〉, 〈30 3 4〉, 〈2 1 1 0〉, 〈2 1 1 2〉, 〈22 4 1〉, 〈30 3 0〉 and 〈0001〉 axes. The observed images of these crystals were closely related to the computer-simulated image of the unit cell of hydroxyapatite. About 40% of the crystals showed structural defects, dislocations and grain boundaries. Dislocations have mixed edge and screw character. The structural defects observed apparently demonstrate the disturbing effect Ga has, under the conditions of this work, on the crystallization process of these crystals, probably due to the size and charge difference between Ga and Ca. Consequently, the noticed clinical observations concerning bone resorption decrease may be linked to pure solubility effects since the presence of dislocations induced higher reactivity of the crystals.

Studies on microstructure and microproperties of PLZT ceramics

The microstructure of PLZT transparent ferroelectric ceramics were studied by high-resolution electron microscopy (HREM) and electron diffraction. The following experimental results were obtained: 1. (1) The structure of micro-domains in the cubic α phase is exactly the same as that of the ferroelectric β phase after poling. The ‘fringe’-shaped micro-regions in the HREM micrograph of 7·9/70/30 PLZT is the result of lattice overlapping of the orthorhombic micro-domains and the cubic α phase matrix. 2. (2) A 2 × d 111 superlattice along the 〈111〉 directiion was observed in 7·9/70/30 PLZT ceramics. It is probably the result of gliding or rotation on the {111} plane due to self-regulation of the structure in the presence of point defects. 3. (3) The lattice image micrographs taken near the grain boundary show that the grain boundaries in PLZT ceramics are very fine. Most of their widths are less than 6Å; some are only 2–3 Å. However, in some places the grain boundaries are broadened, and may reach several tens of ångströms. The structure of broadened grain boundaries appears amorphous. In addition, using specially constructed instruments, the electrical and electrooptical properties of single grains in PLZT ceramics were measured and compared with the properties of the ceramic materials.

An HREM study of the defects in ZnS

Abstract High-resolution electron microscopy (HREM) has been used to investigate the defect structures in ZnS crystals of zincblende type with 〈110〉 beam incidence. It is concluded that there is a high density of 180° rotation twins. Under different imaging conditions the bright dots in the HREM micrographs may either correspond to the atomic column pairs or the structural channels surrounded by atoms, and these make the twin boundary appear in different forms by changing experimental parameters. End-on extended dislocations which are formed by the dissociation of 60° mixed dislocation and 0° screw dislocation with Burgers vectors of 1/2〈110〉 type were analysed and characterized Using anisotropic elasticity theory of dislocations and the width of dissociation measured from HREM images, the stacking fault energy on the {111} plane in ZnS has been determined to be 2·7±0·1 mJm−2. Frank dislocations located at the twin boundary and a Hirth dislocation lock having a Burgers vector of the 1/6〈002〉 type are also...

High energy heavy ion irradiation effects in yttrium iron garnet

The effects of heavy ion irradiation 1.8 GeV Ar and 2.9 GeV Kr in yttrium iron garnet have been investigated by means of electron diffraction, high resolution electron microscopy (HREM) and magnetization measurements. In the high energy zone (1.2 < E < 1.8 GeV for Ar ions and 2.1 < E < 2.9 GeV for Kr ions) a contrast of black and white stripes has been observed on HREM micrographs and assumed to be due to displacement cascade damage type. In the low energy zone (0 < E < 0.6 GeV for Ar ions and 0 < E < 0.9 GeV for Kr ions), a drastic change of the saturation magnetization has been observed and associated to a transition from a crystalline to amorphous state. The effect has been interpreted in term of atomic displacements rather than in term of tracks.

Tilt grain boundaries in NiO

High-resolution Electron Microscopy (HREM) show that crystallinity is maintained right up to the interface in a high-purity NiO 〈001〉 tilt grain boundary (GB). The strong Fresnel contrast effects that are found in high-angle NiO GBs by both conventional TEM and HREM are largely due to a locally reduced atomic density at the GB and are not caused by the presence of an amorphous phase. The HREM micrographs indicate several atomic-scale GB features.