Transmission electron microscopy studies of nanophase TiO2

The chemical and microstructural properties of nanosized TiO₂, obtained by sol—gel procedure, were investigated. The samples were characterized by analytical electron microscopy combining electron diffraction, high resolution electron microscopy and atomic force microscopy. These methods were supported by X-ray diffraction and thermal analysis. The chemical and structural properties, as well as the size of grains, depended on the temperature of TiO₂ preparation. In the temperature range between 293 and 820 K the TiO₂ grains consisted of anatase, as the dominant phase, and brookite. High resolution electron microscopy showed that the sample heated at 573 K had grain sizes from 2 to 10 nm, the average being (5 ± 1) nm, and the pore sizes from 4.5 to 12 nm. The sample heated at820 K revealed larger grain sizes, from 7 to 12 nm, the average being (9 ± 1) nm, and the pore sizes from 6 to 10 nm. These values were confirmed by X-ray diffraction broadening.     

Structural, dielectric, impedance and optical properties of CaBi2B2O7 glasses and glass-nanocrystal composites

Optically clear glasses were fabricated by quenching the melt of CaCO₃–Bi₂O₃–B₂O₃ (in equimolecular ratio). The amorphous and glassy characteristics of the as-quenched samples were confirmed via the X-ray powder diffraction (XRD) and differential scanning calorimetric (DSC) studies. These glasses were found to have high thermal stability parameter (S). The optical transmission studies carried out in the 200–2500 nm wavelength range confirmed both the as-quenched and heat-treated samples to be transparent between 400 nm and 2500 nm. The glass-plates that were heat-treated just above the glass transition temperature (723 K) for 6 h retained ≈60% transparency despite having nano-crystallites (≈50–100 nm) of CaBi₂B₂O₇ (CBBO) as confirmed by both the XRD and transmission electron microscopy (TEM) studies. The dielectric properties and impedance characteristics of the as-quenched and heat-treated (723 K/6 h) samples were studied as a function of frequency at different temperatures. Cole–Cole equation was employed to rationalize the impedance data.     

Microstructures of YBa2Cu3O7/La0.7Ca0.3MnO3 and La0.7Ca0.3MnO3/YBa2Cu3O7 bi-layers grown on (001)LaAlO3

Epitaxial YBa₂Cu₃O₇/La_0.7Ca_0.3MnO₃ (YBCO/LCMO) bi-layers and La_0.7Ca_0.3MnO₃/YBa₂Cu₃O₇ (LCMO/YBCO) bi-layers were grown on (001)LaAlO₃ by pulsed laser deposition, and their microstructures were compared by transmission electron microscopy investigation. In the YBCO(100 nm)/LCMO(150 nm) bi-layers, the LCMO layer consists of columnar grains of ~ 17 nm in diameter and contains mixed orientation domains of [100]_c, [010]_c and [001]_c. The YBCO layer is totally c-axis oriented and the YBCO lattices are tilted − 2.5° to + 2.5° as they grew on the rough surfaces of LCMO columnar grains. For the LCMO(140 nm)/YBCO(140 nm) bi-layers, the LCMO/YBCO interface is sharp and flat. The initial 12-nm thickness of the YBCO layer is composed of c-axis oriented domains, and the upper part of YBCO layer is [100] oriented. The LCMO layer was predominantly [001]_c oriented while [100]_c-oriented domains were occasionally observed.     

Transmission electron microscopy studies of nanostructured TiO2 films on various substrates

A chemical vapour deposition (CVD) synthetic route to the production of nanocrystalline titanium dioxide has been carefully investigated on various substrates. CVD was performed at a relatively low temperature of 320 °C on KCl crystal, Al foil, KBr pellet and freshly sliced MICA substrates. The influence of substrate material on film formation was studied in order to find a titanium dioxide film with good intercalation properties for an electrode in a dye-sensitized solar cell. Intercalation properties depend on average grain sizes and porosity in nanophased materials. These films were thoroughly characterized with respect to their surface morphology, crystal structure and the phase composition. Transmission electron microscopy (TEM) accompanied by selected area electron diffraction (SAED) was employed for structural characterization of TiO₂ films. The studies showed that films deposited on KCl crystal, KBr pellet and MICA are solely composed of an anatase phase whereas in the film deposited onto Al foil, the brookite phase of TiO₂ is also present. The structural parameters of anatase were determined using the Rietveld refinement of electron diffraction data. By comparison of anatase lattice parameters with their corresponding bulk values, the significant deviation in values of lattice parameters a and c in anatase phase was observed and attributed to the thin-film features. The average grain size and the grain size distribution obtained by TEM were compared for TiO₂ films deposited on different substrates.     

Quantitative characterization of clay dispersion in polypropylene-clay nanocomposites by combined transmission electron microscopy and optical microscopy

This paper presents a novel method to describe the microstructure of polymer/clay nanocomposites quantitatively. Based on the image analyses of transmission electron microscopy (TEM) and optical microscopy micrographs, two parameters, degree of dispersion (χ) and mean interparticle distance per unit volume of clay (λ_V) are proposed to describe the level of clay dispersion. The degree of dispersion gives the percentage of exfoliation, and λ_V is a measure of spatial separation between particles relative to clay loading. A polypropylene/clay system was chosen as an example to show the effects of processing conditions and biaxial stretching on clay dispersion using the proposed quantifiers. It provides insights into the ‘real’ clay dispersion using a combination of both microscopical and macroscopical aspects.     

Densification of In2O3:Sn multilayered films elaborated by the dip-coating sol-gel route

Indium Tin Oxide (ITO) thin films have been deposited by the Sol-Gel Dip-Coating technique, the starting solutions being prepared from chlorides. These multilayered films were crystallized by means of a classical heat treatment at temperatures ranging from 500 to 600 °C. Five stacked layers are necessary to obtain a global electrical resistivity value of 2.9×10⁻³ Ω cm, for 500 °C annealed film. The paper focuses on the study of the structure of such multilayered deposits, and on the densification process, using transmission electron microscopy, Rutherford Back-scattering Spectrometry and electrical resistivity measurements. This analysis reveals structural inhomogeneities and different crystallite growth processes as a function of annealing temperature and number of deposited layers.     

Grain size and strain in thin sputter-deposited Ni0.8Fe0.2 and Cu films

The average grain size and strain in the direction parallel to the surface of thin Ni_0.8Fe_0.2 and Cu films, sandwiched between Ta layers, have been determined as a function of layer thickness by grazing incidence X-ray diffraction. The in-plane grain size and grain size distribution were also assessed by plan-view transmission electron microscopy. Standard θ-2θ X-ray powder diffraction was used to determine the uniform strain in the direction perpendicular to the surface. Both for Ni_0.8Fe_0.2 and Cu, an elongation of the lattice parameter perpendicular to the surface and a compression of the lattice parameter in the plane of the film is observed, which decreases with increasing film thickness. Additionally, for Ni_0.8Fe_0.2 a non-uniform elongation of the perpendicular interactomic distance at the Ta interfaces is deduced by fitting a kinematical model to the θ-2θ diffraction spectrum. This study illustrates the strength and the complementary character of standard powder X-ray diffraction, grazing incidence X-ray diffraction and transmission electron microscopy for the structural analysis of thin metal films.     

High resolution electron microscopy of Nb3Sn and its defects

Powdered samples of polycrystalline Nb₃Sn superconductor filaments with cubic A 15-structure have been investigated by high resolution electron microscopy (HREM). Structure images representing the projected electric potential of the structure in the (001)- plane are presented. The constrast pattern shows the position of the individual chains of Nb-atoms arranged parallel to the electron beam. This pattern is compared with the contrast calculated on the basis of the dynamical theory of electron diffraction by the multislice method. The observed contrast agrees with the calculated pattern. A structure image made in the [012]-projection containing one dislocation reveals a large region of strain in the crystal lattice surrounding that defect.     

Transmission electron microscopy application for the analysis of carbon black and metal-carbon nanostructures

Abstract

The article presents the analytical possibilities of the combined use of transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) methods to study of carbon black and metal-carbon nanostructures. Platinum nanoparticles were shown to be formed on the carbon polyhedral structures and nanocapsules as a result of laser exposure to the carbon black suspensions in H₂PtCl₆ aqueous solution. Phase composition of nickel-carbon and cobalt-carbon core-shell structures was identified.     

Transmission electron microscopy investigation into the recrystallization of carbon resulting from laser processing of carbon-implanted copper

The present paper is a contribution to the discussion about the laser-pulse-assisted synthesis of carbon from copper implanted with carbon atoms. In this study, in contrast to others, the phase composition of the products of such synthesis was examined, observing the electron diffraction from the carbon film formed on a copper substrate without removing it from this substrate. The phase identification performed in this experiment has shown that the carbon phase generated by the laser-assisted synthesis from carbon-implanted copper is chaoite.     

Transmission electron microscopy/electron energy loss spectroscopy measurements and ab initio calculation of local magnetic moments at nickel grain boundaries

We have determined local magnetic moments at nickel grain boundaries using a transmission electron microscopy/electron energy loss spectroscopy method assuming that the magnetic moment of Ni atoms is a linear function of the L₃/L₂ (white-line ratio) in the energy loss spectrum. The average magnetic moment measured in the grain interior was 0.55 μ_B, which agrees well with the calculated magnetic moment of pure nickel (0.62 μ_B). The local magnetic moments at the grain boundaries increased up to approximately 1.0 μ_B as the mis-orientation angle increased, and showed a maximum around 50°. The respective enhancement of local magnetic moments at the Σ5 (0.63 μ_B) and random (0.90 μ_B) grain boundaries in pure nickel was approximately 14 and 64% of the grain interior. In contrast, the average local magnetic moment at the (111) Σ3 grain boundary was found to be 0.55 μ_B and almost the same as that of the grain interior. These results are in good agreement with available ab initio calculations.     

Nano-structure formation of Fe-Pt perpendicular magnetic recording media co-deposited with MgO, Al2O3 and SiO2 additives

Perpendicular magnetic recording media samples were prepared by sputter deposition on sapphire with a layer sequence of MgO seed-layer/Cr under-layer/FeSi soft magnetic under-layer/MgO intermediate layer/FePt-oxide recording layer. The effects of MgO, Al₂O₃ and SiO₂ additives on the morphology and orientation of the FePt layer were investigated by transmission electron microscopy. The samples exhibited (001) orientation of the L1₀ FePt phase with the mutual orientations of sapphire substrate//MgO(100)[001]//Cr(100)[11¯0]//FeSi(100)[11¯0]//MgO(100)[001]//FePt(001)[100]. The morphology of the FePt films varied due to the co-deposited oxides: The FePt layers were continuous and segmented by stacking faults aligned at 54° to the surface. Films with SiO₂ addition, beside the oriented columnar FePt grains, exhibited a fraction of misoriented crystallites due to random repeated nucleation. Al₂O₃ addition resulted in a layered structure, i.e. an initial continuous epitaxial FePt layer covered by a secondary layer of FePt-Al₂O₃ composite. Both components (FePt and MgO) of the MgO-added samples were grown epitaxially on the MgO intermediate layer, so that a nano-composite of intercalated (001) FePt and (001) MgO was formed.The revealed microstructures and formation mechanisms may facilitate the improvement of the structural and magnetic properties of the FePt-oxide composite perpendicular magnetic recording media.     

Growth structure investigation of MgF2 and NdF3 films grown by molecular beam deposition on CaF2(111) substrates

The growth structure of MgF₂ and NdF₃ films grown on polished CaF₂(111) substrates deposited by molecular beam deposition has been investigated using transmission electron microscopy (TEM) of microfractographical and surface replications as well as cross-sectional TEM, atomic force microscopy, packing density, and absorption measurements. It has been shown that by taking advantage of ultrahigh vacuum environments and a special stratification property of MgF₂ and NdF₃ films, the preparation of nanocrystalline films of high packing density and low optical absorption is possible at a substrate temperature of 425 K.     

Microstructural changes induced by the pyrolysis step on epitaxial La2Zr2O7 thin films grown by metalorganic decomposition

La₂Zr₂O₇ (LZO) thin films are used as buffer layers in second generation high Tc superconductor tapes. The microstructure of LZO films grown by metalorganic decomposition is characterized by the presence of nanovoids throughout the whole thickness of the films. We introduced an out-gassing plateau under vacuum during the pyrolysis process to decrease the size of voids. The temperature of this plateau was determined by Fourier transformed infrared spectroscopy, electron back scattering diffraction and X-Ray diffraction characterizations. The dwelling time was also varied. Transmission Electron Microscopy (TEM) studies revealed that a high heating ramp in combination with a less than an hour pyrolysis plateau decreased pore size. The deposition rate during dip-coating was also decreased to enhance out-gassing at the plateau. Successive LZO layers were deposited and energy filtered TEM images at C K-edge were performed to identify the role of carbon in the nucleation mechanisms.     

Raman study of stress effect on Ge nanocrystals embedded in Al2O3

Ge nanocrystals (NCs) embedded in Al₂O₃ were grown by RF-sputtering. X-ray diffraction, high resolution transmission electron microscopy, and Raman spectroscopy techniques were used to characterize the stresses on the NCs. While small NCs (< 10 nm) have been observed to be spherical and fully relaxed in the matrix, the larger ones (> 17 nm) demonstrated a compressive stress effect. This could be linked to the crystal structure of the adjacent Al₂O₃ matrix.     

Ellipsometry and transmission electron microscopy study of MoSi2 coatings after oxidation at high temperature in air

MoSi₂ coatings were deposited on Si (100) substrates by means of magnetron sputtering. The structural and optical properties of the coatings were then investigated through transmission electron microscopy (TEM) and UV-visible-near IR ellipsometry after heat treatments in air at two temperatures (1100 K and 1600 K). After annealing at 1100 K, no silica layer could be observed by TEM at the surface of the MoSi₂ coating, whereas annealing at 1600 K gave rise to a protective silica layer. As observed by TEM, this silica layer contained nanometric molybdenum silicide crystals. Increasing the annealing time at 1600 K from 20 min to 2 h resulted in both a higher concentration and growth of these molybdenum silicide crystals in the silica layer. An optical model, taking into account the observed microstructure and allowing a good fit of the ellipsometry data, was developed. It is also shown that the dielectric constants of the non-oxidized part of the MoSi₂ layer became modified when the annealing temperature (1100 K versus 1600 K) and the annealing time at 1600 K (20 min versus 2 h) were increased.     

Transmission electron microscopy characterization of Al2Cu precipitate growth kinetics by in situ heat treatment

Growth kinetics of the Al₂Cu (θ) precipitate in an Al-1.5Cu alloy thin film were characterized in situ using transmission electron microscopy (TEM). The sizes of θ precipitates during isothermal growth in TEM were measured during aging at 250, 275 and 300 °C. The results of precipitate growth were analyzed based on the model of the “collector plate mechanism” for bulk alloys. The lengthening of the θ precipitate in thin film varies as t^0.29 for short annealing times (up to ∼10-20 min) at temperatures between 250 and 300 °C. This is in good agreement with the analysis in bulk alloys. For longer annealing time, however, the t^0.29 time law does not fit as the distances of Cu volume diffusion exceed one half of the film thickness.     

Characterisation of dislocations, nanopipes and inversion domains in GaN by transmission electron microscopy

Transmission electron microscopy is used to analyse a range of defects observed in hexagonal GaN films grown on sapphire and GaN substrates by metalorganic chemical vapour deposition. Large angle convergent beam electron diffraction is used to analyse the Burgers vectors of dislocations and to show that hollow tubes, or nanopipes, are associated with screw dislocations having Burgers vectors±c. Weak-beam electron microscopy shows that dislocations are dissociated into partials in the (0001) basal plane, but that threading segments are generally undissociated. The presence of high densities of inversion domains in GaN/sapphire films is confirmed using convergent beam electron diffraction and the atomic structure of the {

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} inversion domain boundary is determined by an analysis of displacement fringes seen in inclined domains.     

La0.7Ca0.3MnO3薄膜微观结构的HRTEM表征

利用透射电子显微镜研究了衬底温度600～800℃条件下采用脉冲激光法在(001)LaAlO3(LAO)衬底上制备的La0.7Ca0.3MnO3(LCMO)薄膜的微观结构。结果表明薄膜由大量柱状晶组成且与LAO衬底形成良好外延关系。LCMO薄膜与LAO衬底的取向关系可以描述为:(ⅰ)(100)f∥(001)s、[011]f∥[100]s;(ⅱ)(011)f∥(001)s、[100]f∥[100]s。LCMO显示层状畴结构,即在衬底上初始生长的薄膜为(ⅱ)型畴,在此之上生长的薄膜为(ⅰ)类和(ⅱ)类型畴的混合体。薄膜中可观察到一些反向畴与孪晶等缺陷。薄膜与衬底界面少见错配位错,薄膜以Stranski-Krastanov模式生长。     

Magnetic properties of nano-crystalline Li0.35Cd0.3Fe2.35O4 ferrite prepared by modified citrate precursor method

Single phase nano-crystalline lithium cadmium ferrite Li_0.35Cd_0.3Fe_2.35O₄ is synthesized by a modified citrate gel precursor technique in different pH media. The modified citrate precursor technique reduces the formation of the impurity phase α-Fe₂O₃ in the inverse spinel phase of lithium ferrite. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the average crystallite size. As-prepared sample shows the paramagnetic behaviour of M–H curve measured by vibrating sample magnetometer (VSM). The coercivity (H_c) and magnetization (M) both increase with decrease in temperature from 300 K to 80 K. Temperature dependent magnetic properties below the Curie point are defined by the Bloch's law and Neel relaxation relations. The effect of annealing on magnetic properties at different temperature is studied.