Dark-field Transmission Electron Microscopy Images Research Articles

Rational Solution Growth of α-FeOOH Nanowires Driven by Screw Dislocations and Their Conversion to α-Fe2O3 Nanowires

We report the rational synthesis of α-FeOOH (goethite) nanowires following a dislocation-driven mechanism by utilizing a continuous-flow reactor and chemical equilibria to maintain constant low supersaturations. The existence of axial screw dislocations and the associated Eshelby twist in the nanowire product were confirmed using bright-/dark-field transmission electron microscopy imaging and twist contour analysis. The α-FeOOH nanowires can be readily converted into semiconducting single-crystal but porous α-Fe(2)O(3) (hematite) nanowires via topotactic transformation. Our results indicate that, with proper experimental design, many more useful materials can be grown in one-dimensional morphologies in aqueous solutions via the dislocation-driven mechanism.

Impact of Ni on Accelerated Degradation of 8.5 Mol% Y2O3-Doped Zirconia

This study focuses on the investigation of the influence of solved nickel on the accelerated degradation of the ionic conductivity of 8YDZ (8.5 mol% Y2O3-doped zirconia) under reducing atmosphere. In comparison to pure 8YDZ, the degradation of Ni-containing 8YDZ proceeds 50 times faster and is completed after 100 h at 950 oC. The impact of Ni-incorporation into 8YDZ on microstructure and chemistry was studied by light microscopy (LM) and transmission electron microscopy (TEM). After thermal treatment under reducing atmosphere, a characteristic colour change of 8YDZ is observed by LM that is attributed to the in-diffusion (> 50 µm) of Ni into the 8YDZ substrate. TEM dark-field imaging reveals the coarsening of nano-scaled precipitates of the metastable t'-YDZ in cubic 8YDZ which is correlated with the degradation of the ionic conductivity. Since the decomposition rate of 8YDZ is determined by the mobility of Y and Zr, the dissolved Ni must have a strong impact thereon. An explanation based on the reduction of solved Ni under reducing operation conditions is proposed. It is based on the strain field which are expected to arise in the vicinity of the Ni-atoms after reduction.

Performance Enhancement of SOFC Anode Through Electrochemically Induced Ni/YSZ Nanostructures

The local morphology of the interface electrolyte/anode of solid oxide fuel cells has a strong effect on the electrochemically active triple-phase boundary length. Therefore, the electrical performance is expected to be enhanced by nanostructuring this interface. This study focuses on the anode functional layer (AFL) of Ni/8YSZ (Ni/8.5 mol % yttria-doped zirconia) composite anodes. A nanometer scaled and nanoporous Ni/YSZ interlayer of about 200 nm thickness was formed after applying a short-time reverse current treatment (RCT) at 700°C. Impedance spectroscopy measurements proved a progressive decrease of the overall area specific resistance, when exposing anode supported cells to a series of RCTs. The performance change was attributed purely to the decrease of the polarization resistance in the anode functional layer, which was reduced by 40%. Scanning and transmission electron microscopy (TEM) confirmed the formation of a nanocrystalline, porous YSZ matrix containing finely distributed nanosized Ni grains. Energy-filtered TEM yielded the distributions of the cations and oxygen. Furthermore, high-resolution TEM and dark-field TEM imaging identified mainly cubic YSZ including traces of the metastable t”-phase within the nanostructured YSZ. A reaction scheme for the formation of the highly efficient Ni/YSZ structure at the interface electrolyte/anode is proposed.

Matrix-Mediated Biomineralization in Marine Mollusks: A Combined Transmission Electron Microscopy and Focused Ion Beam Approach

The teeth of the marine mollusk Acanthopleura hirtosa are an excellent example of a complex, organic, matrix-mediated biomineral, with the fully mineralized teeth comprising layers of iron oxide and iron oxyhydroxide minerals around a calcium apatite core. To investigate the relationship between the various mineral layers and the organic matrix fibers on which they grew, sections have been prepared from specific features in the teeth at controlled orientations using focused ion beam processing. Compositional and microstructural details of heterophase interfaces, and the fate of the organic matrix fibers within the mineral layers, can then be analyzed by a range of transmission electron microscopy (TEM) techniques. Energy-filtered TEM highlights the interlocking nature of the various mineral phases, while high-angle annular dark-field scanning TEM imaging demonstrates that the organic matrix continues to exist in the fully mineralized teeth. These new insights into the structure of this complex biomaterial are an important step in understanding the relationship between its structural and physical properties and may help explain its high strength and crack-resistance behavior.

Improvement in Junction Breakdown and GIDL using MFLA in DRAM Product

In this study, we successfully improved junction breakdown using millisecond flash anneal (MFLA) on a dynamic random access memory (DRAM) product. By replacing a rapid thermal annealing (RTA) process with MFLA after contact implantation, improvements in NMOS and PMOS junctions were observed and no degradation was found. Device simulation data show that the PMOS junction electric field (E-field) was reduced with MFLA and that MFLA has an advantage in junction profile tailoring. The behavior of gate-induced drain leakage (GIDL) at different process stages with different annealing temperatures was studied. The preheat and peak temperatures of MFLA were found to be correlated with GIDL improvement. The optimum condition in this study was 1200°C flash anneal with a 750°C preheat temperature. Bright field and weak-beam dark-field transmission electron microscopy images showed perfect dislocation loops and fault dislocation loops remaining in the {113} plane with a size of around 17 × 20 nm in the junction area.

Effect of randomness on ferroelastic transitions: Disorder-induced hysteresis loop rounding in Ti-Nb-O martensitic alloy

We have studied the effect of disorder on the martensitic transformation (MT), especially on the hysteretic behavior of Ti-Nb-O alloy. This system allows arbitrary tuning of the amount of disorder by the introduction of interstitial oxygen atoms into elastically soft host lattice [$\text{Ti-}26\text{Nb}\text{ }(\text{at}\text{.}\text{ }\mathrm{%})$ alloy], which results in the formation of randomly oriented nanometer-sized martensite as confirmed by dark-field transmission electron microscopy images. In the clean system without interstitial oxygen doping, we observe a sharp hysteresis loop with plateau region in both a stress-strain curve and a strain-temperature curve, whereas a slim hysteresis loop without plateau appears by introducing disorder. The rounding of hysteretic behavior can be understood in random-field Ising model regime, providing the possibility of a new physical picture for MT in a dirty system, where the small size of martensite homogeneously nucleates and percolates the whole system.

Grain Boundary Segregation of Carbon and Formation of Nanocrystalline Iron-Carbon Alloys by Ball Milling

Based on a novel defactants (defect acting agents) concept (R. Kirchheim, Acta Materialia 55 (2007) 5129 and 5139), a novel method of understanding and synthesizing NC material was proposed by introducing defactants (segregating solute atoms) into the materials to ease the formation of grain boundaries (GBs) and enhance the formation ability of nanocrystalline (NC) structures. The iron-carbon system was chosen as a model system where carbon acts as the so-called defactant. Iron powders mixed with different amount of graphite were ball milled to prepare NC iron-carbon alloys with different carbon concentrations (C0). After ball milling, the as-milled powder with relatively low carbon concentration was annealed at a certain temperature to achieve saturation of GBs by carbon atoms. The microstructures of the powders were investigated by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods. The mean grain sizes (D) of the powders were determined by analyzing TEM dark field images and X-ray line profiles. The results indicated that once the saturation of GBs is achieved, D of the NC iron-carbon powders will be strongly dependent on C0 and will follow a simple mass balance of carbon in a closed system, i.e. D=3ΓgbVm/(C0-Cg) with Cg the carbon concentration in grains, Γgb the grain boundary excess, and Vm the molar volume of iron. Based on the experimental results, the formation of NC iron-carbon alloys was treated in detail within the framework of the defactant concept. The increase of C0 significantly reduces the formation energy of GBs, leading to a substantial decrease of D.

Hollow-cone dark-field transmission electron microscopy for dislocation density characterization of trimodal Al composites

This paper describes a methodology based on hollow-cone dark-field (HCDF) transmission electron microscopy (TEM) to study dislocation structures in both nano- and micro-crystalline grains. Although the conventional approach based on a two-beam condition has been commonly used to acquire weak-beam dark-field (WBDF) TEM images for dislocation structure characterization, it is very challenging to employ this technique to study nanocrystalline materials, especially when the grains are less than 100 nm in diameter. Compared to the conventional two-beam approach, the method described in this paper is more conducive for obtaining high-quality WBDF-TEM images. Furthermore, the method is suitable for studying samples with both nanocrystalline and coarse-grains. A trimodal Al metal-matrix-composite (MMC) consisting of B 4C particles, a nanocrystalline Al (NC-Al) phase, and a coarse-grained Al (CG-Al) phase has been reported to exhibit an extremely high strength and tailorable ductility. The dislocations in both NC-Al and CG-Al phases of the trimodal Al MMCs at different fabrication stages were examined using the HCDF method described. The influence of the dislocation density in both NC-Al and CG-Al phases on the strength and ductility of the composite is also discussed.

MOCVD growth of GaN layer on InN interlayer and relaxation of residual strain

100 nm InN layer was grown on sapphire c-plane using a metal-organic chemical vapor deposition (MOCVD) system. Low temperature (LT) GaN layer was grown on InN layer to protect InN layer from direct exposure to hydrogen flow during high temperature (HT) GaN growth and/or abrupt decomposition. Subsequently, thick HT GaN layer (2.5 μm thick) was grown at 1000 °C on LT GaN/InN/sapphire template. Microstructure of epilayer–substrate interface was investigated by transmission electron microscopy (TEM). From the high angle annular dark field TEM image, the growth of columnar structured LT GaN and HT GaN with good crystallinity was observed. Though thickness of InN interlayer is assumed to be about 100 nm based on growth rate, it was not clearly shown in TEM image due to the InN decomposition. The lattice parameters of GaN layers were measured by XRD measurement, which shows that InN interlayer reduces the compressive strain in GaN layer. The relaxation of compressive strain in GaN layer was also confirmed by photoluminescence (PL) measurement. As shown in the PL spectra, red shift of GaN band edge peak was observed, which indicates the reduction of compressive strain in GaN epilayer.

Interfacial intermixing in InAs/GaSb short-period-superlattices grown by molecular beam epitaxy

The unique properties of the noncommon-atom InAs/GaSb short-period-superlattices (SPSL) strongly depend on the interface structure. These interfaces are characterized using transmission electron microscopy (TEM). The compositional sharpness is obtained from the comparison of the experimental contrast in g002 two-beam dark-field TEM images with simulated intensity profiles, which are calculated assuming that the element distribution profiles are described by sigmoidal functions. The interfacial intermixing, defined by the chemical width, is obtained for SPSL with different periods and layer thicknesses, even in the extreme case of nominally less than 3 ML thick InAs layers. Nominal 1 ML InSb layers intentionally inserted are also identified.

Decomposition of 8.5 mol.% Y 2O 3-doped zirconia and its contribution to the degradation of ionic conductivity

Transmission electron microscopy (TEM) in combination with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy was applied to analyze the microstructure and the local chemical composition of as-sintered and aged Y 2O 3-doped ZrO 2 (YDZ) thick-film substrates with yttria contents of 8.5 mol.% (8YDZ) and 10 mol.% (10YDZ). Selected-area electron diffraction and dark-field TEM imaging show nanoscale precipitates of the tetragonal YDZ phase, which are coherently embedded in grains consisting of the cubic phase. A distinct coarsening of the tetragonal regions is observed after 2500 h annealing of 8YDZ at 950 °C in an oxidizing atmosphere. This coarsening is accompanied by decomposition on the cationic sublattice on the nanoscale, leading to the depletion of yttrium in the tetragonal regions. The decomposition is discussed in terms of spinodal decomposition which takes place even at a high doping concentration of 8.5 mol.% Y 2O 3.

Transmission electron microscopy and scanning transmission electron microscopy study on B-site cation ordered structures in a (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 single crystal

Transmission electron microscopy and scanning transmission electron microscopy were performed to investigate B-site cation-ordered structures in a (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (x∼0.30) single crystal. Selected-area diffraction patterns showed presence of F-spots corresponding to 1:1 order. Besides, the F-spots were elongated to be “X”-like in shape. Transmission electron microscopy dark-field images revealed that size of ordered domains was about a few nanometers, and they were dispersed within disordered matrix. Scanning transmission electron microscopy indicated the presence of 1:1 ordered domains. Presence of 1:2 ordered structures was also indicated. These are consistent with features of the selected-area diffraction patterns.

Mn Valency at La0.7Sr0.3MnO3/SrTiO3 (0 0 1) Thin Film Interfaces

This article presents a (scanning) transmission electron microscopy (TEM) study of Mn valency and its structural origin at La 0.7 Sr 0.3 MnO 3/SrTiO3(0 0 1) thin film interfaces. Mn valency deviations can lead to a breakdown of ferromagnetic order and thus lower the tunneling magnetoresistance of tunnel junctions. Here, at the interface, a Mn valency reduction of 0.16 +/- 0.10 compared to the film interior and an additional feature at the low energy-loss flank of the Mn-L3 line have been observed. The latter may be attributed to an elongation of the (0 0 1) plane spacing at the interface detected by geometrical phase analysis of high-resolution images. Regarding the interface geometry, high-resolution high-angle annular dark-field scanning TEM images reveal an atomically sharp interface in some regions whereas the transition appears broadened in others. This can be explained by the presence of steps. The performed measurements indicate that, among the various structure-related influences on the valency, the atomic layer termination and the local oxygen content are most important.

Formation of Nanostructures and Solid Solubility Extension in Cryomilled Al-Cu and Al-Si Powders

Nanostructured Al-2024 alloy and Al-12%Si were successfully synthesized via a cryogenic milling process. Equiaxed nanograins of 46 nm and a lattice strain of 0.2% were obtained after 18 hours of milling for the Al-Cu system and grain sizes of 46 nm with a lattice strain of 0.09% were obtained after only 4 hours milling time for the Al-Si system, as measured from the X-ray patterns. Transmission electron microscopy (TEM) characterization was also employed to evaluate the grain size in both systems. For the Al-Cu system, a bimodal grain size distribution (as measured from dark field (DF) TEM images) constituted of nanometric and ultrafine grains was obtained after 8 hours of milling. This bimodal behaviour changes with milling time and after 12 hours results in a unimodal distribution with more than 50% of the grains between 20-40 nm. Also, additional milling time (16 hours) narrows the distribution; and approximately 80% of the grains reach the 20-40 nm range. At this stage, the size of the grains was also verified via direct thickness measurements of selected area diffraction (SAD) rings. The solid solubility extension (SSE) of Cu in Al, as estimated from Vegard's law, in the final stage of milling (18 hours) reached a value of 0.41 atm%. For the Al-Si system the mechanism of particle evolution undergoes similar stages as in the Al-Cu system only in much shorter times such as after only 2 hours of milling the powders have completed the plastic deformation and reached the flattening phase. This was followed by fracturing, reaching an equiaxed morphology after only 4 hours of milling. At this stage the SSE of Si in the Al matrix was found to be 1.72 atm%.

Ordering and grain growth of FePt thin films by annealing

Microstructure variation of FePt thin film upon annealing at elevated temperatures was investigated by transmission electron microscopy (TEM). A special shape aperture was employed to observe the ordered L1 0 phase in the dark-field TEM images. With increasing the annealing temperature, crystal grains formed clusters with gathering of neighboring grains, and crystal grain growth proceeded within the cluster. L1 0 ordered crystal grains were preferentially formed near the grain boundaries, and their sizes grew with increasing the annealing temperature.

Micro‐twin induced structural misalignment in InSb quantum wells

AbstractStructural misalignments in InSb quantum wells (QWs) caused by a micro‐twin (MT) defect have been investigated by transmission electron microscopy (TEM). Dark field (DF) TEM images with the 002 reflection of the parent region show the misalignment clearly, in spite of their low image intensity. Equations that relate the width of a MT and some dimensions regarding the misalignment are derived. These equations make it possible to deduce the dimensions by [$ \bar 1 $10]‐directional 220 DF‐TEM images which have a higher image intensity than 002 DF‐TEM images. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The stability of screw dipoles in Ni 3Al studied by TEM

Transmission electron microscopy (TEM) methods were used to study the dislocation structures in L1 2 ordered binary Ni 3Al deformed at a temperature below and at a temperature above the onset of the anomalous increase of the yield stress (compression axis oriented for single slip). The corresponding change from a dislocation structure dominated by edge dipoles to a structure dominated by locked screw dipoles is caused by the transition from octahedral cross-slip to cube cross-slip. The measurements based on weak-beam dark-field TEM images show that stable configurations of screw dipoles occur above a minimum dipole height and within a certain range of dipole angles. In agreement with the calculations the stability region of the screw dipoles is limited since locked screws can become mobile in the vicinity of a screw of opposite sign. Remobilized screws can annihilate leading to the formation of antiphase boundary tubes or can pass each other generating glissile edge segments that contribute to plastic flow. The screw dipoles showing stable configurations are considered as strong locks and lead to an autocatalytic increase of their density with increasing deformation.

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.

Hydrothermal synthesis of nanocrystalline BaTiO 3 particles and structural characterization by high-resolution transmission electron microscopy

Nanocrystalline BaTiO 3 (BT) particles were synthesized via hydrothermal method, and their microstructure was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution TEM (HRTEM). The results show that the BT nanoparticles remain a metastable cubic structure at room temperature, as revealed by XRD and SAED. Such an abnormal crystallographic phenomenon was caused by the lattice defects such as OH − defects and barium vacancies. The BT nanoparticles exhibit a spherical morphology with an average grain size of 70 nm, and narrow particle size distribution. HRTEM images of individual particles indicate that the BT nanoparticles have a good crystallinity and smooth surfaces. However, dark-field TEM images revealed high strains in BT nanoparticles, which were probably resulted from the surface defects. Anti-phase boundaries were also observed in some BT nanoparticles, which were formed by the intersection of two crystalline parts with a relative displacement from each other by 1 2 d 100 or 1 2 d 111 , as revealed by HRTEM images. Some local dark TEM contrast observed, near the pure edge dislocations with Burgers vector value of d 110, was due to the existence of local strains around the pure edge dislocations.

Characterization of InGaN quantum wells with gross fluctuations in width

Gross well-width fluctuations are observed in InGaN quantum wells (QWs), and have been linked to an overall three-dimensional morphology consisting of a network of interlinking InGaN strips. While these networks may be clearly observed in atomic force microscopy of appropriate uncapped epilayers, the surface morphology of capped (QW) samples reveals little or nothing of the subsurface structure of the InGaN. Hence transmission electron microscopy (TEM) must be used to determine the variations in QW thickness and composition. The network structure may be observed in either bright-field TEM or scanning transmission electron microscope high angle annular dark field images in plan-view studies of single QWs. It is suggested that multi-QWs could be imaged in cross section and the anisotropy of the network structure used as a potential signature of the three-dimensional morphology. The anisotropic nature of the network structure correlates well with the alignment of step edges in the GaN pseudosubstrates, and it is suggested that the network structure forms via the decomposition of In-rich regions arising at or near step edges.