High-resolution Transmission Electron Microscopy Micrographs Research Articles

Microstructural investigation of CaTiO 3 formed mechanochemically by dry grinding of a CaO–TiO 2 mixture

Microstructure of the CaO–TiO 2 mixture ground under dry atmospheric condition using a planetary ball mill was investigated by transmission electron microscopy–energy dispersive X-ray spectroscopy (TEM-EDX) and high resolution transmission electron microscopy (HR-TEM) with selected area electron diffraction (SAED) analysis. According to the TEM observation, the mixtures ground for about 2 h or more consist of many fine particles of about 20 nm. The CaTiO 3 microcrystals are formed in the mixture ground for about 2 h or more. One of the lattice planes, {121} is clearly observed in the HR-TEM micrograph of the 5-h ground mixture, while the lattice image is partly detected in the mixture ground for about 2 h or less. The grains grow in size in the prolonged grinding, and their sizes reach about 20 nm at 5 h grinding by the TEM observation. The size of microcrystals becomes larger with increasing grinding time.

Analysis of composition fluctuations in Al xGa 1 −xN

Composition fluctuations in Al x Ga 1 −x N-layers ( x=0.25 and 0.35) are investigated on an atomic scale by high-resolution transmission electron microscopy (HRTEM). The samples were grown by plasma induced molecular beam epitaxy on Al 2O 3 (0001). A strain state analysis of the cross-sectional HRTEM micrographs is performed with the digital analysis of lattice images (DALI) program package. Composition profiles on an atomic scale are derived by the measurement of the distances between intensity maxima positions in the HRTEM image. The analyses revealed different areas in the Al x Ga 1 −x N-layers with either homogeneous or ‘striped’ contrast. In the striped areas the analyses indicate a strong decomposition that leads to the formation of self-organized superlattice structures.

Cutting Single Wall Carbon Nanotubes

ABSTRACTIn this contribution, we report on a mechanical method to cut and open single walled carbon nanotubes. This technique is based on using an abrasive material (diamond powder) without any chemical treatments or oxidation in air at high temperature. We present highresolution transmission electron microscopy micrographs, which show firstly that the tubes are unambiguously opened and secondly, that the nanotubes have not suffered the treatment. x-ray diffraction pattern confirms a well-defined bundle organisation. A breaking mechanism of the nanotube bundles is proposed. Nitrogen adsorption measurements at 77K reveal the presence of new microporosities ranging in the average nanotube diameter, which confirm the opening of some tubes.

Colossal magnetoresistance of thin films of La 0.66Ba 0.33MnO 3 as a function of film thickness

La 0.66Ba 0.33MnO 3 shows a strong change of its resistivity if a magnetic field is applied. The strength of this colossal magneto resistance (CMR) effect depends of the thickness of epitaxial thin films prepared on MgO (100) and SrTiO 3 (100). We have prepared films in a thickness range of 5 to 250 nm by pulsed laser deposition. The epitaxial La 0.66Ba 0.33MnO 3 thin films were characterized by Rutherford backscattering spectrometry and channeling, X-ray diffraction measurements as well as by high resolution transmission electron microscopy (HRTEM). The high crystalline quality was proved by ion channeling (minimum yield of χ min≈2%) and by HRTEM micrographs. By X-ray diffraction measurements, no additional phases were detected beside the cubic phase with a lattice constant a=3.915 Å. The CMR effect of 10 nm thin films is enlarged by 45% as compared to 150 nm thick films on MgO (100) substrates. The saturation magnetization decreases with decreasing film thickness.

Growth of Ain on Si(111) by Plasma-Assisted Molecular Beam Epitaxy: Application to Surface Acoustic Wave Devices

AbstractEpitaxial wurtzite aluminum nitride (AIN) films have been grown on (111) oriented silicon substrates by plasma-assisted solid source molecular beam epitaxy (MBE). The growth has been performed in the two-dimensional (2D) growth mode. Occuring surface reconstructions have been monitored by reflection of high-energy electron diffraction (RHEED). The films have been characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). 2D grown films exhibit atomically smooth surfaces, are singlecrystalline and void of oriented domains. High-resolution (HR) TEM micrographs of the heterointerface show the presence of an interfacial atomic arrangement where (5:4) unit cells coincide along the AIN[2110]/Si[110] direction.Surface acoustic wave (SAW) properties of MBE-grown AIN(0001)/Si(ll1) have been studied for the first time. The AIN film exhibits very good piezoelectricity. The SAW time response indicates low scattering of the waves during their propagation. An electromechanical coupling coefficient (EMC) of 0.07% has been measured for the present interdigital transducer (IDT) geometry.

FCC Superlattice Packing of Faceted Silver Nanocrystals

The ordering of size selected nanocrystaline particles into large, well ordered supercrystals has been under recent study. Here we describe the ordering of silver nanocrystals, of core diameter ca. 4.5nm, passivated with alkythiolate self-assembled monolayers. The faceted nanocrystals are found to size selectively condense into ordered supercrystals with edge lengths on the order of 0.1 to 0.5 microns. Further, evidence has been found in High Resolution TEM micrographs of packed monolayers of these same nanocrystals implying an orientational ordering of the silver nanocrystals’ atomic lattices within and co-aligning with the supercrystal’s “superlattice”. A three dimensional model using a truncated octahedron for the nanocrystal core morphology is constructed from this evidence into an FCC superlattice which fits the remaining experimental observations.Micrographs of the same region of a nanocrystal Ag supercrystal (NCASX) are shown in figure 1 under differing defocus conditions.

Effects of the Elastic Stress Relaxation on the HRTEM Image Contrast of Strained Heterostructures

In this work, the effects of the elastic relaxation of compositional stresses caused by the finite size of transmission electron microscopy (TEM) specimens on the image contrast of high-resolution transmission electron microscopy (HRTEM) micrographs of strained heterostructures made by cubic materials are investigated. The reduced spatial dimensions, owing to the thinning process of strained heterostructures, cause modification of the atomic positions in the thinned specimens with respect to the bulk ones. This deformation is a function not only of the specimen thickness but also of the thinning crystallographic direction. The results show that the strains of an elastically relaxed structure can vary by 15% as a function of the thinning direction ([100] or [011]). The bending of the atomic columns caused by the elastic relaxation phenomena in HRTEM specimens of strained semiconductor materials can cause a strong background-intensity variation in the HRTEM images. This effect is a function of the structure of the investigated materials, indicating that information on the background intensity variation, owing to the non-uniform lattice distortion of an elastically relaxed heterostructure made by cubic materials, is contained in the {200} beams. Thus, the influence of the elastic relaxation cannot be neglected whenever HRTEM is used to deduce the local chemical composition or the local unit cell in strained cubic materials.

Differential Contrast Imaging of High Resolution TEM Micrographs of Rotary Shadowed Heterodouplex Protein Complexes

The resolution attainable from rotary shadowed molecules is limited by sample preparation method, the distribution of the metal through the shadowing procedure and the grain structure of the metal film. Utilizing very thin metal coats (less than a few 0.1 nm measured perpendicular to the substrate) and metals that form small grain sizes improves the resolution. Differential hysteresis (DH) processing of the micrograph data offers the opportunity to utilize the full range of contrast information present in the images and to improve the structural analysis and interpretation of high resolution rotary shadowed macromolecules. A protein complex of interest that shows varying structures in the presence or absence of ATP was prepared by glycerol spray onto freshly cleaved mica. The mica was placed on the stage of a Cressington CFE-50 and dried in vacuo for 5 min. Samples were rotary shadowed with 0.2 nm W/Ta (measured on the support) at 5° incident angle, followed by stabilization with 2.5 to 3.0 nm C. Replicas were examined in a Hitachi H-7000 with a LaB6gun at 100 kV at direct magnifications of 100,000x. Micrographs were taken on Kodak type 4489 EM film at one f-stop higher than for standard silver halide printing to provide a full gray scale data file during digitization.

Nanocrystalline high coercivity PrCo//Cr thin films: Potential high density magnetic recording media

PrCo thin films with a Cr underlayer (PrCo//Cr) have been prepared by dc magnetron sputtering. The as-deposited PrCo films with a Cr underlayer of 80 nm have coercivity values of about 300 to 400 Oe but after annealing at 400°C for 20 min a large enhancement in the coercivity is observed. The coercivity of the PrCo films after annealing at 400°C varies from 2 kOe to as high as 8 kOe depending on the film thickness and deposition conditions. The as-deposited PrCo//Cr films have a mostly amorphous PrCo layer but after annealing at 400°C the PrCo layer is nearly 100% crystalline with a grain size of about 10 nm as revealed by the high resolution TEM micrographs. The coercivity of the films depends strongly on the Ar pressure during sputtering of the PrCo layer, while the Ar pressure for the Cr underlayer does not significantly change the coercivity of the films. The Cr underlayer thickness was varied from 10 to 160 nm and only a slight change in coercivity was observed. The magnetic switching volume was estimated by the dependence of apparent coercivity on the sweep rate of the magnetic field and also through magnetic viscosity and remanence measurements. The measured switching volume is of the order of 1 to 1.5 × 10 −18 cm 3.

Effect of Calcium Doping on the Atomistic Structure of the (1104) Twin Boundary in Alumina

This high‐resolution transmission electron microscopy (HRTEM) study revealed changes due to calcium segregation in the atomistic structure of the (1104) twin boundary in α‐Al2O3. Calcium grain‐boundary doped and undoped twins were prepared using ultrahigh vacuum (UHV) diffusion bonding of two highly pure single crystals. Two distinct boundary configurations—each having mirror and glide mirror symmetry—were found in the calcium‐doped bicrystal. Thus far, only the mirror‐related configuration could be identified in the undoped boundary. To estimate the atomistic structure of the grain boundary, simulated images of four supercells with different calcium contents were compared to experimental HRTEM micrographs. The amount of calcium found at the (1104) twin boundary using this method was consistent with results that were obtained by energy‐dispersive X‐ray spectroscopy.

Structural relationship between pyrite and marcasite

We studied pyrite spherules built of radiating crystals to establish the relationships among microstructure, composition, and macroscopic appearance. Results of high-resolution transmission electron microscopy (HRTEM) imaging and selected-area electron diffraction (SAED) show that many pyrite crystals contain planar faults perpendicular to one of the [001] axes. A comparison between HRTEM micrographs and images simulated for defect model structures indicates that the faults can be interpreted as single (101) layers of marcasite that disrupt the regular sequence of (002) layers in pyrite. Such a marcasite layer can be described as a boundary between two pyrite crystals that are related by a 21 screw axis parallel to [100]. Energy-dispersive X-ray spectrometry (EDS) indicates that such disordered pyrite crystals contain about 3 at% As. However, the distribution of As is uniform over heavily faulted and fault-free regions, indicating the As content is not related to the occurrence of marcasite lamellae in pyrite.

Interface simulation of strained and non-abrupt III–V quantum wells. Part 2: energy level calculation versus experimental data

This work describes a program able to compute the allowed energy levels and the respective wavefunctions of strained In 1 − x Ga x As y P 1 − y In 1 − z Ga z As w P 1 − w for electrons, light and heavy holes in single- and multi-quantum wells and superlattices. Ground and excited states can be detected. The problem of non-abrupt interfaces has been taken into account. The computation on intentionally strained QW structures can be performed. The simulation of coupled wells may also be done, allowing theoretical prediction on the 4 K photoluminescence emission of superlattices. The adopted mathematical approach has been treated in details. The results of the presented simulations performed on heterostructures grown by low-pressure metallorganic vapor phase epitaxy and by chemical beam epitaxy heterostructures are compared with 4 K Fourier transform photoluminescence and with room temperature IR absorption data. The data used as input by this program are previously computed by the program BANDSTRAIN (described in a previous paper). In this work also the simulation of high resolution X-ray diffraction patterns is briefly presented and compared with the experimental curves; it is shown how the combined simulations of PL and X-ray data is a powerful tool in the interfaces characterization. Finally, qualitative information is extracted from high-resolution transmission electron microscopy micrographs.

Growth and characterization of two-dimensional GaP on Si by metalorganic chemical vapor deposition

Extremely flat GaP layers grown on a Si substrate thinner than the critical thickness were characterized by high resolution transmission electron microscopy (HRTEM) and spectroscopic ellipsometry (SE). The HRTEM micrograph shows that the interface roughness of 20 nm thick GaP on Si is 10–15 atomic layers. The SE measurement shows that the compressive stress of the GaP layer caused by the lattice mismatch decreases gradually with increasing the thickness, which means that the lattice relaxation of GaP occurs gradually with increasing the thickness.

Influence of phosphorus on the structure and the hydrodesulphurization and hydrodenitrogenation activity of W/Al 2O 3 catalysts

A series of W/Al 2O 3 catalysts containing various amounts of phosphorus (0–6 wt.-% P 2O 5) were characterized, in the oxide state by X-ray diffraction and temperature-programmed reduction (TPR), and in the sulphide form by high-resolution transmission electron microscopy (HRTEM); their activities for simultaneous hydrodesulphurization (HDS) of gas oil and hydrodenitrogenation (HDN) were also evaluated. The TPR results indicated that phosphorus addition increased the amount of those tungsten species which are more easily reduced, such as octahedral polytungstates. Consistently, the HRTEM micrographs showed that when phosphorus content was increased the lateral dimension of the WS 2 structures did not significantly vary, while the number of stacks was clearly increased. Both HDS and HDN were notably enhanced by increasing the phosphorus content; the promotion being relatively larger for HDS than for HDN. Possible explanations for the phosphorus promotion on W/Al 2O 3 catalysts are discussed.

Fine structure and magnetic properties of Mn- and Co-doped nanocrystalline γ-Fe2O3

High resolution TEM (HRTEM), Mössbauer spectroscopy and SQUID magnetometry were utilized to study the fine structure of equiaxed, Mn- and Co-doped γ-Fe2O3 nanocrystals with a particle size of 30–50 nm. The allocation of cation distribution, based on a redox mechanism and known cation site preferences is (Mn2+0.11Fe3+0.89) [Mn4+0.07Mn3+0.16) Co2+0.34Fe3+1.23⧠0.20]O4, in which ⧠ is the vacancy, (⋅⋅) is the A site, and [⋅⋅] is the B site. This leads to an Fe occupancy ratio of B sites to A sites of 1.38, as verified by Mössbauer spectroscopy. The calculated saturation magnetic moment of 2.99μB/MF is very close to the measured 2.84μB/MF. HRTEM micrographs of the air-cooled nanocrystals fits well with the simulated lattice images using the proposed cation distribution (without directional ordering). While the HRTEM micrograph of the core of an annealed nanocrystal are not in good agreement. This is thought to arise from directional ordering inside the annealed particles.

Origin of agglomerates observed in high resolution transmission electron microscope (HRTEM) micrographs of a catalyst containing platinum (II) complexes anchored on phosphinated SiO 2: Computer simulation of HRTEM images

Computer simulation has been applied to the interpretation of “spots on stems” images observed in HRTEM micrographs of a catalyst containing PtCl 2 complexes anchored on phosphinated silica. Molecular models of isolated molecules P 2 or P 2 ·PtCl 2 ( P 2 = diphosphine ligand covalently attached to the amorphous SiO 2 layer), aggregates (“islands”) of anchored PT(II) complexes, [ P 2 ·PtCl 2] n ( n=2–5), and metel clusters Pt n ( n=1–20) attached to the SiO 2 surface via a graphite “stem” were constructed. Multislice programs were used to simulate HRTEM images of these models. The comparison of real images obtained using a JEM-100CX microscope with those simulated for various models shows that experimental HRTEM micrographs exhibit Pt n metal clusters on graphite “stems”. These objects appear to be the products of decomposition of initial [ P 2 ·PtCl 2] n “islands” under the electron beam. The results emphasize the necessity of computer simulation for correct interpretation of HRTEM images, especially when the objects are sensitive to the heating effects of the electron beam.

High‐Resolution Transmission Electron Microscopy Studies of a Near Σ11 Grain Boundary in α‐Alumina

The atomistic structure of a near Σ11 (N211) grain boundary in ultrapure α‐AI2O3 bicrystals was determined by quantitative high‐resolution transmission electron microscopy (HRTEM). High‐resolution imaging revealed an atomically sharp interface with a characteristic periodic pattern at (0111) ‖ (0111) facets. The pattern was analyzed by comparing the HRTEM micrographs with simulated images of different structures that had been relaxed by static‐lattice calculations. The best agreement with experiment was reached for the grain boundary structure with the lowest energy (1.7 J·m−2). Although this structure reproduces the relative translation state of the adjacent crystals to within 0.02 nm, the calculated structure was not in complete agreement with the periodic pattern at the grain boundary. The origin of this difference is probably due to a slight misalignment of the two crystals adjacent to the boundary.

Nanostructures of Sm-Co on Cr thin films

The nanostructures of Sm-Co on Cr thin films prepared by dc magnetron sputtering were investigated by high resolution transmission electron microscopy (HRTEM) and diffraction techniques. HRTEM micrographs show that the crystallites in the Sm-Co films, as revealed by the lattice fringes, are distributed discontinuously in the matrix. The matrix is amorphous as indicated by the microdiffraction study. The size of the crystallites is in the range of 2/spl sim/5 nanometers. The volume fraction of the crystallites in the film decreases from 91% to 54% as the argon pressure is increased from 5 mTorr to 30 mTorr. Micrographs recorded in a bright field transmission electron microscope (TEM) with a defocus of a few micrometers reveal grain-like structures of about 25 nm in some but not all films. This grain-structure is found to be inherited from the Cr underlayer. >

Hrem imaging and simulation with image processing for the study of phase transformations in nanometric zirconia powders

Abstract Submicronic or nanometric particles now used as powder materials for the sintering of oxide-based ceramics, like zirconia, require techniques having the appropriate resolution for characterization. Here are presented high resolution transmission electron microscopy (HREM) studies of nanometric (non-doped) ZrO2 powders processed by low-temperature routes for industrial use. It is shown that careful HREM imaging of such powders allows one to study the morphological and structural changes accommodating the phase transformations which can occur in these powders calcined at increasing temperatures. Three-dimensional reconstruction of the shape of the particles has been achieved by comparison of experimental HREM micrographs with simulated images. Internal strains have been revealed in the same way. Finally, image processing which allows one to separate the contributions of the different beams to the HREM image, can reveal structural defects hidden in the initial image.

Characterization of ZnS Layers Grown by MOCVD for Thin Film Electroluminescence (TFEL) Devices

ABSTRACTWe have characterized MOCVD grown ZnS layers for thin film electroluminescence (TFEL) devices. Films with thicknesses ranged from several Å to 2 μm were studied by x-ray diffractometry (XRD), cross-sectional transmission electron microscopy (XTEM), high-resolution transmission electron microscopy (HRTEM), and ultraviolet-visible photospectrometry (UVS). From HRTEM micrographs, it was observed that ZnS films consist of a mixture of hexagonal and cubic phases. Correlation of UV absorption spectra with XRD analysis resulted in a method for quantitative determination of the cubic packing fraction of polymorphic ZnS films. The initial ZnS layer (< 1000 Å) deposited on BaTa2O6 had more hexagonal than cubic phase because of denser crystal defects. However, the fraction of cubic phase increased with the film thickness. In addition to film microstructures, the mean grain size, growth rate, film uniformity, and surface roughness of MOCVD grown ZnS thin films as functions of film thicknesses and substrate temperatures were also calibrated by XTEM results.