Analysis Of High-resolution Transmission Electron Microscopy Images Research Articles

Structural properties of ultrafine Ba-hexaferrite nanoparticles

Crystal structure of ultrafine Ba-hexaferrite (BaFe12O19) nanoparticles was studied using X-ray diffractometry (XRD), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDXS), X-ray absorption fine structure (XAFS), and Mössbauer spectroscopy (MS), to be compared to the structure of larger nanoparticles and the bulk. The nanoparticles were synthesized with hydrothermal treatment of an appropriate suspension of Ba and Fe hydroxides in the presence of a large excess of OH−. The ultrafine nanoparticles were formed in a discoid shape, ∼10nm wide and only ∼3nm thick, comparable to the size of the hexagonal unit cell in the c-direction. The HRTEM image analysis confirmed the hexaferrite structure, whereas EDXS showed the composition matching the BaFe12O19 formula. XAFS and MS analyses showed considerable disorder of the structure, most probably responsible for the low magnetization.

Application of the O-lattice theory for the reconstruction of the high-angle near 90° tilt Si(1 1 0)/(0 0 1) boundary created by wafer bonding

This work presents an experimental and theoretical identification of defects and morphologies of a high-angle near-90° tilt Si (1¯10)//(001) boundary created by direct wafer bonding. Two samples with different twist misorientations, between the (1¯10) layer and the (001) substrate, were studied using conventional transmission electron microscopy (TEM) and geometric phase analysis of high-resolution TEM images. The O-lattice theory was used for atom reconstruction of the interface along the [11¯0]sub//[001]lay direction. It is demonstrated that to preserve covalent bonding across the interface, it should consist of {11¯1}sub,lay//{1¯12}lay,sub facets intersected by maximum of six {11¯1}lay,sub planes with three 90° Shockley dislocations per facet. It is shown that a particular atom reconstruction is needed at transition points from one facet to another. The presence or absence of deviation from exact 90° tilt of the layer with respect to the substrate is shown to be related directly to the undulations of the interface. It is demonstrated that the latter has an influence on the Burgers vector of the dislocations adjusting in-plane twist misorientation. A general model for cubic face-centered materials for an arbitrary 〈110〉sub,lay tilt interface is proposed, which predicts the net Burgers vector and the spacing between dislocations necessary to realize transition from the lattice of the substrate (layer) to the layer (substrate).

Comparative study of nanopores in activated carbons by HRTEM and adsorption methods

Detailed analysis of nanopores (IUPAC micropores at pore half-width x<1nm) of carbonised porous phenolformaldehyde resin microbeads used as a precursor of activated carbon (AC) and CO2 activated carbon (at 50% burn-off) has been performed on the basis of high-resolution transmission electron microscopy (HRTEM) image analysis and nitrogen adsorption data analysed using several density functional theory (DFT) methods. The results of quenched solid DFT (QSDFT) and nonlocal (NLDFT) are in agreement with the pore size distributions of nanopores based on the HRTEM image analysis. Development of porosity with progressive activation degree in a series of ACs leads to enhancement of the deviation of the pore shape from the used pore models. The TG/DTA data and Raman spectra show nonlinear but weak changes in the AC characteristics with increasing burn-off degree.

Simulations of Hydroxyapatite Nanocrystals for HRTEM Images Calculations

Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is one of the most important biomaterials used in bone regeneration therapies due to their chemical properties are very similar to the inorganic phase found in bone tissues. The direct observation of the ultrastructure of HA is very important in the comprehension of their nucleation and interactions with the molecules involved in bone formation. High-resolution Transmission Electron Microscopy (HRTEM) is a currently technique used for this task. However, the interpretation of the images is not straightforward and needs the use of softwares dedicated to high-resolution images simulations. This work presents the applicability of MEGACELL software in the analysis of HRTEM images of HA nanoparticles. MEGACELL is the most newly software, developed to construct nanocrystals models for HRTEM multislice simulations. The output files generated by MEGACELL are raw data format (.xyz), containing all the atomic positions, as well as input files compatible with JEMS (Java Electron Microscopy Software) format files. High-resolution images were acquired using a JEM 3010 URP microscope, with a LaB6 thermionic electron gun operating at 300 kV, with a point-to-point resolution of 0.17 nm and a CCD Gatan 794SC multiscan digital camera, attached to the DigitalMicrographTM software for recording and image processing. Electron microscopy samples were prepared by dropping HA powder on copper TEM grids. HRTEM experimental images of HA particles, orientated along different zone axes, were interpreted applying the MEGACELL software to construct HA nanocrystal models and the multislice method to simulated them. MEGACELL improves the extraction of the ultrastructural features and facilitates a better interpretation of the phase-contrast images.

Development of an HRTEM image analysis method to quantify carbon nanostructure

An analytical method for analyzing high-resolution transmission electron microscopy (HRTEM) images is presented. The method is composed of two major parts: digital image processing and lattice fringe characterization. The digital image processing is composed of the following operations: negative transformation, region of interest (ROI) selection, contrast enhancement, Gaussian lowpass filter, top-hat transformation (used to correct uneven illumination across an image), thresholding to obtain a binary image, morphological modification, clearing fringes on the ROI border, skeletonization, and removing short fringes that lack physical meaning. These operations are defined by equations with effects illustrated by images. The lattice fringe characterization generates statistics on fringe length, tortuosity, and separation based on the skeletons of the graphene layers. Fringe length and tortuosity are obtained automatically from the features of the skeletons, while fringe separation is obtained by analyzing manually selected fringe pairs. The algorithms are implemented within MATLAB, and demonstrated upon an exemplary HRTEM image of carbonaceous material. The image analysis algorithm permits quantitative HRTEM, here specifically addressing nanostructure of carbonaceous materials.

Shape Transformation Mechanism of Silver Nanorods in Aqueous Solution

The spontaneous shape transformation of silver nanorods with an initial length of several hundred nanometers towards spherical particle shapes in aqueous solution is investigated by means of scanning electron microscopy, UV-vis absorption spectroscopy, anodic stripping voltammetry, and high-resolution transmission electron microscopy (HRTEM). The consolidation of the results reveals an increase in the particle number density with time. Moreover, HRTEM image analysis along the cross section of the rods evidences the presence of fivefold twinning defects which extend along the whole rod length. According to the analytical model of Monk et al. this kind of rod structure is only thermodynamically stable if the rod length is below a critical value at a given diameter. The rods investigated in the present work do not fulfill the stability criterion as they exceed the critical length. Thus, the rods decay into smaller "nanobuns" and defective as well as defect-free spheres. A mechanism based on findings from the literature, HRTEM image analysis of former rods, transition states, and the final particle structures is proposed. The defects along the surface are seen as starting points for the dissolution of material, which is reintegrated into the solid phase by homogeneous as well as heterogeneous nucleation and growth.

Diesel soot oxidation during the late combustion phase

The diesel soot was extracted from the chamber of a running diesel engine using a total cylinder sampling system. A structural description and size distribution of the soot spherules were performed by means of high-resolution transmission electron microscopy (HRTEM). HRTEM image analysis was carried out to achieve semi-quantitative information about the soot organization. Different nanostructures were found in dependence upon the engine test conditions. A better organization of the graphitic layers occurred for the in-cylinder soot as the expansion stroke developed. The oxygen concentrations at sampling angles and the cylinder pressure were measured. The oxidation rates of the in-cylinder soot during the late combustion phase were calculated according to the differences of spherules size distributions between two consecutive sampling angles. The soot reactivity in dependence on the nanostructure was shown after deducting the effect of oxygen concentration and temperature. The increase of resistance toward oxidation was associated to the increase of the structural order for the in-cylinder soot. The results are a useful database for testing the structure–property relationship between the nanostructure and the reactivity of the in-cylinder diesel soot.

Magnetic properties and atomic structure of La2/3Ca1/3MnO3–YBa2Cu3O7 heterointerfaces

A heterostructure comprised of a 2.7 nm (7 unit cell) thick layer of the metallic ferromagnet La2/3Ca1/3MnO3 and two 50 nm thick layers of the high-temperature superconductor YBa2Cu3O7 epitaxially grown on (100) SrTiO3 by pulsed-laser deposition was characterized by magnetization measurements and spherical-aberration-corrected high-resolution transmission electron microscopy (HRTEM). The saturation magnetization is about half of that in bulk La2/3Ca1/3MnO3. A massive reduction in the magnetization previously inferred from sputter-deposited La2/3Ca1/3MnO3–YBa2Cu3O7 heterostructures can be ruled out. HRTEM image analysis, combined with image simulation and a focus series reconstruction, revealed atomically sharp epitaxial structures with stacking sequences –(La,Ca)O–CuO2– and –BaO–MnO– at the top and bottom interface.

Reaction of CaAl4O7 with (0001)-oriented α-Al2O3

Reactions between thin films of CA2 and (0001)-oriented α-Al2O3 have been studied using a combination of microscopy techniques. Thin films of amorphous CA2 were deposited on sapphire substrates by pulsed-laser deposition at 900 °C in an oxygen ambient atmosphere. After deposition, the reaction couples were heat treated in air for various times either at 1300 or 1400 °C. Atomic-force microscopy was used to monitor changes in the microstructure of the films. Interfaces between the different regions were examined by transmission electron microscopy (TEM) of cross-sectional samples prepared by focused ion-beam milling. The CA2 films had dewetted the substrate surface as a result of the heat treatment. An interfacial reaction layer was observed between the dewetted CA2 droplets and the substrate. The structure of this reaction layer was found to be consistent with γ-Al2O3 by computer analysis of high-resolution TEM images. There is a perfect epitaxy between the interfacial layer and the substrate. For the samples heat treated for longer times, hexagonal features were found on the substrate surface. The presence of these features on (0001)-oriented α-Al2O3 suggests that CA6 platelets form by the transformation of the interfacial reaction layer. The results are discussed in relation to the crystallization behavior of the various calcium aluminate phases and the equilibrium-phase diagram of the CaO–Al2O3 system.

Structure–property relationship in nanostructures of young and mature soot in premixed flames

A structural description of young and mature soot sampled from methane, ethylene, cyclohexane and benzene premixed flames, burning in fuel-rich conditions, was performed by means of high-resolution transmission electron microscopy (HRTEM). HRTEM image analysis was carried out to achieve semi-quantitative information about the soot organization. Different nanostructures were found to develop in the soot, depending on the fuel and on soot aging.The soot characteristics were also investigated by elemental analysis, oxidative thermogravimetry and UV–Vis spectroscopy.Soot nanostructure in terms of the length of the graphitic layers, the size of the coherent domains made of stacked layers was related in some way with UV–Vis spectroscopic characteristics, H/C molar ratio, and oxidation reactivity. This demonstrated that there is a structure–property relationship between soot nanostructure and bulk properties.The different spectral features and chemical properties of soot in dependence on the fuel nature and on aging resulted to be related to a different internal structure of primary particles which in turn derives from a different soot inception mechanism occurring in flames more or less rich in aromatic precursors.

Determining the size of coherent nanoinclusions and the crystal lattice misfit parameter using dark-field transmission electron microscopy images

The task of identifying nanodimensional inclusions with a structure coinciding with that of the surrounding matrix in transmission electron microscopy (TEM) images encounters considerable difficulties. Analysis of high-resolution TEM images requires a large volume of calculations and the exact knowledge of a large number of various parameters. The well-known method of determining coherent inclusion characteristics using dark-field images can be used only for relatively large objects. We describe a new method for determining the size and the crystal lattice misfit parameter using dark-field TEM images, which is applicable to coherent inclusions with dimensions as small as several nanometers.

Structural evolution of nanocrystalline silicon studied by high resolution transmission electron microscopy

Abstract In this work, nanocrystalline silicon (nc‐Si) embedded in amorphous silicon nitride (a‐SixNy) films have been characterized by High Resolution Transmission Electron Microscopy (HRTEM). The films have been grown by remote plasma enhanced chemical vapour deposition (RPECVD) onto silicon substrates using a mixture of SiCl4, NH3 and H2. Subsequently, the sample has been introduced to an annealed process. The nanocrystalline inclusions have been measured from the analysis of HRTEM images. The size of nc‐Si inclusions is modified due to the annealing process. The diameter of nc‐Si inclusions in as‐grown films vary from 1.3 to 3.1 nm. In annealed films, a high density of new clusters with reduction of 1 nm of those diameters is observed, as well as the coalescence phenomenon among some nc‐Si inclusions is observed too. Consequently, the diameters of annealed films nc‐Si inclusions range from 0.7 to 4 nm. The thin films show an excellent photoluminescence emission. The density per unit area and size of the nc‐Si inclusions are correlated with the PL emissions. The influence of the structural changes of the nc‐Si on the PL emissions is discussed. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Investigation of silicon-germanium fins fabricated using germanium condensation on vertical compliant structures

We report the formation of defect-free SiGe vertical heterostructures using Ge condensation on vertical SiGe structures. To evaluate the effectiveness of substrate compliance in vertical structures, SiGe fins of various widths were subjected to Ge condensation. This formed vertical fin heterostructures comprising a SiGe core region sandwiched by Ge-rich regions. Using cross-sectional transmission electron microscopy (TEM), wide fins were found to contain more dislocations than narrower fins, in which we observed few or no dislocations. Lattice strain analysis using high-resolution TEM image analysis was used to confirm that strain relaxation has occurred. In the wide fins (noncompliant substrate), strain relaxation was dislocation mediated. In the narrow fins, substrate compliance enabled strain relaxation in the Ge-rich layer with reduced dislocation formation. Hence, we also demonstrated the formation of a strain-relaxed homogeneous SiGe fin (∼90% Ge concentration) with no observable dislocations.

Structure Determination of Clusters Formed in Ultra-Low Energy High-Dose Implanted Silicon

In this work, we present a detailed structural characterization of the defects formed after 0.5 keV B+ implantation into Si to a dose of 1x1015 ions/cm2 and annealed at 650°C and 750°C during different times up to 160 s. The clusters were characterized by making use of Weak Beam and High Resolution Transmission Electron Microscopy (HRTEM) imaging. They are found to be platelets of several nanometer size with (001) habit plane. Conventional TEM procedure based on defect contrast behavior was applied to determine the directions of their Burger’s vectors. Geometric Phase Analysis of HRTEM images was used to measure the displacement field around these objects and, thus, to unambiguously determine their Burger’s vectors. Finally five types of dislocation loops lying on (001) plane are marked out: with ] 001 [1/3 ≅ b and b ∝ [1 0 1], [-1 0 1], [0 1 1], [0 -1 1].

Influence of support ionicity on the hydrogen chemisorption of Pt particles dispersed in Y zeolite: consequences for Pt particle size determination using the H/M method

Zeolite Y-supported Pt particles were studied by high-resolution transmission electron microscopy (HRTEM), extended X-ray absorption fine structure (EXAFS), and H 2 chemisorption experiments. The ionicity (i.e., electron richness of the zeolite oxygen atoms) was altered by using different cations (NaY, MgY, and LaY) and by steaming and introduction of protons (H-USY). Highly dispersed Pt particles were introduced inside the zeolite pores using a very careful synthesis procedure. Computer analysis of HRTEM images exhibited similar Pt particle size distributions with calculated average Pt particle sizes ranging from 0.98 to 1.26 nm for the various zeolite Y-supported Pt particles. The EXAFS results revealed that the first-shell Pt–Pt coordination number was around 6.5, in good agreement with the observed HRTEM results. H 2 chemisorption experiments found a much higher H/Pt value for Pt/NaY (1.63) than for Pt/H-USY(0.88), implying no relationship with the Pt dispersion as determined by HRTEM. Using the values for the dispersion (Pt s/Pt) as calculated from the HRTEM results, one can obtain a value for the number of chemisorbed hydrogen atoms per surface platinum atom (H/Pt s ratio). This value demonstrates a clearly decreasing trend with decreasing electron richness of the support oxygen atom: 1.96 (Pt/NaY) > 1.74 (Pt/MgY) > 1.51 (Pt/LaY) > 1.11 (Pt/H-USY). These results indicate that the hydrogen coverage on supported Pt particles depends strongly on the support ionicity. This has significant consequences for the catalytic behavior of hydrogenolysis and hydrogenation reactions catalyzed by Pt. These results also indicate that one should be cautious in interpreting H 2 chemisorption results only in terms of Pt dispersion. This approach is valid only when comparing Pt particles dispersed on the same support. Serious deviations in the real Pt dispersions occur if Pt is supported on oxides with different ionicities.

Comparative Characterization Study of Microporous Carbons by HRTEM Image Analysis and Gas Adsorption

The present work presents a useful comparison of micropore size distributions (MPSDs) obtained from gas adsorption and image analysis of high-resolution transmission electron micrographs. It is shown that the MPSD obtained for a chemical activated carbon is concordant with that obtained from CO2 adsorption. In addition, this technique has allowed us to obtain the MPSD of a carbon molecular sieve (CMS) prepared in our laboratory by a copyrolysis process, which could only be characterized by CO2 adsorption at 273 K (not by N2 adsorption at 77 K due to diffusional problems). The MPSD obtained by high-resolution transmission electron microscopy (HRTEM) for the CMS is wider than that obtained by CO2 adsorption, suggesting that HRTEM is detecting the closed porosity existing in this sample, which is not accessible to gas adsorption. The existence of closed porosity in the CMS is explained considering the preparation method used. Thus, HRTEM combined with image analysis seems to be useful for structural analysis of narrow micropores including closed porosity.

Detection of subnanometric layer at the Si∕SiO2 interface and related strain measurements

The structure of the Si∕SiO2 interface was investigated by high-resolution transmission electron microscopy (HRTEM) and electron diffraction, which appears as an efficient tool to detect any organized layer at the interface. Using HRTEM image analysis, a 1-nm-thick distorted Si layer is identified at the interface, the local deformation corresponds to a compressive stress of approximately 2GPa.

Nanocrystalline Ag2S on Polydiacetylene Langmuir Films

Silver sulfide (Ag2S) nanocrystals were prepared in ambient conditions by exposing crystalline polydiacetylene (PDA) Langmuir films at the air−AgNO3 solution interface to H2S gas. The deposited nanocrystals were studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), fast Fourier transform (FFT) analysis of HRTEM images, ultrahigh-resolution scanning electron microscopy (UHR-SEM), and X-ray powder diffraction (XRD). PDA Langmuir films played a dominant role in controlling the size and lateral arrangement of the chemically deposited Ag2S nanocrystals. Two distinct nanocrystal populations were obtained: small nanocrystals of ca. 8 nm in diameter arranged along the strands on the PDA film, and larger Ag2S nanocrystals of ca. 50 nm were formed in the voids between the PDA domains. The concentration of the Ag+ ions in the subphase controlled the nanocrystal film morphology and surface coverage. The linearly arranged nanocrystals were not crystallographically oriented with respect to the PDA film template, as opposed to PbS and CdS (divalent metal ion sulfides) that were previously shown to be oriented along the conjugated direction of the PDA film.

Structural Solution of Mesocaged Material AMS-8

A novel mesoporous material prepared using the recently reported co-assembly of anionic surfactants with organosilane groups has been studied in depth using high-resolution transmission electron microscopy (HRTEM) and electron crystallography. A three-dimensional (3D) structural solution of AMS-8 was obtained by Fourier analysis of HRTEM images, revealing a complex cubic mesocaged solid (space group Fd3̄m) composed of bimodal cages 3D-connected by cage windows. HRTEM images also indicate the presence of structural defects associated with the cubic close packing of spheres, while analysis by nitrogen adsorption−desorption isotherms reveals that accessibility to smaller cages may be restricted in this material. The opened and closed models of this structure derived from a combination of electron crystallography and nitrogen adsorption data are discussed with respect to the arrangement of cages and cage connectivities. Considerations of the nature of the micellar system playing part in this novel templating mechanism are discussed.

Imaging and thickness measurement of amorphous intergranular films using TEM

Fresnel fringe analysis is shown to be unreliable for grain boundaries in yttrium-doped alumina: the determined thicknesses do not agree well with those measured from high resolution transmission electron microscopy (HRTEM), the asymmetry between under- and overfocus is very large, and Fresnel fringes are sometimes shown at boundaries which contain no amorphous film. An alternative approach to the analysis of HRTEM images of grain boundary films is demonstrated: Fourier filtering is used to remove the lattice fringes from the image thereby significantly enhancing the visibility of the intergranular films. The apparent film thickness shows a discrepancy between measurements from the original HRTEM image and the filtered image. It was shown that fringe delocalisation and diffuseness of the amorphous/crystalline interfaces will lead to a significant underestimate of the thickness in unprocessed HRTEM images. In contrast to this, the average thickness can be much more accurately measured from the Fourier-filtered image, provided the boundary is oriented accurately edge-on.