Z-contrast Imaging Research Articles

Rapidly Solidified Microstructure in Laser Alloyed Ni–Al Layer by TEM, STEM z-contrast and HRTEM Techniques

In the present study, laser alloying of electroless Ni–P coating on aluminum substrate was conducted using Nd:YAG pulsed laser under the condition of 5.36 × 109 W/m2 in power density and 3.0 mm/s in scanning speed. The rapidly solidified microstructure in the alloyed layer was studied. The results showed that the alloying element distributed in the alloyed layer is inhomogeneous. The dendrite containing relatively high Ni was identified as Al3Ni phase and the areas between the dendrites are rich in Al content. Featureless with cell structure in Al-rich areas was firstly displayed by z-contrast image. Amorphous structure was revealed to exist in Al-rich areas.

Vacancy-mediated ω-assisted α-phase formation mechanism in titanium–molybdenum alloy

This study investigates the mechanism of the ω-to-α phase transformation in a titanium–molybdenum alloy. Using aberration-corrected high annular angle dark-field scanning transmission electron microscopy and density functional theory (DFT), the authors demonstrate that the ω-to-α phase transformation is mediated by vacancy ordering. Atomic resolution Z-contrast images display high- and low-contrast regions in a single ω precipitate, and it was found that the low Z-contrast regions are composed of an unknown structure, which differs significantly from the ω-phase crystal structure. DFT calculations show that the vacancies could be stabilized by Mo and should preferably be assembled with ordering. The relaxed ω-phase structure with ordered defects explains the intricate atomic images in the low Z-contrast regions of the ω phase. It was found that this distorted ω phase with defects is similar to the hexagonal α-phase structure with vacancies. It was confirmed that these vacancies could easily be occupied by Ti atoms and could form a perfect α phase.

Si-magnetite nano-precipitates in silician magnetite from banded iron formation: Z-contrast imaging and ab initio study

Si-bearing magnetite or silician magnetite is common in low- and high-temperature rocks. However, details about possible Fe-silicate or Si-Fe-oxide discrete phases/nano-precipitates were not available due to the limitations of conventional high-resolution TEM. Combining Z-contrast imaging and ab initio calculation using density functional theory (DFT) method, we have derived both composition and crystals structure of the discrete nano-precipitates within host magnetite. The nano-precipitates of Si-magnetite with composition of [¨0.5Fe2+ 0.5]VI[Fe3+]VISiIVO4 or g-Fe1.5SiO4 occur in silician magnetite from a banded iron formation from Western Australia. In the Si-magnetite precipitates, Si replaces Fe3+ in tetrahedral sites of the magnetite structure and vacancies are introduced in the octahedral Fe2+ sites. The Si-magnetite precipitates distribute along {111} of the host magnetite. Widths of the precipitates are even multiples of d111 of magnetite, such as 2d111, 4d111, and 6d111. Ordering of the vacancies in the Si-magnetite will result in symmetry of P4332, which is a subgroup of Fd3m for magnetite. Stacking of Si-magnetite and magnetite (111) layers along the [111] direction also occur in magnetite. The nanoprecipitates result from exsolution of Si-magnetite from the host silician magnetite at low temperature. The occurrence of the thin nano-precipitates within the magnetite host results from the minimization of interfacial energy between the precipitate and the host magnetite. Relatively high concentrations of aqueous silica and Fe-silicate complex species in pore fluid might enhance the incorporation of Si into the silician magnetite during crystallization of the magnetite.

S–Te Interdiffusion within Grains and Grain Boundaries in CdTe Solar Cells

At the CdTe/CdS interface, a significant Te–S interdiffusion has been found a few nanometers into the CdTe grain interiors with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy. This interdiffusion happens on both as-grown and CdCl2-treated CdTe. S substitution at Te sites has been directly resolved in CdTe with STEM Z-contrast images, which further confirms the S diffusion into CdTe grain interiors. Moreover, when a sufficient amount of S substitutes for Te, a structural transformation from zinc-blende to wurtzite has been observed. In the CdCl2-treated CdTe, Cl segregation has also been found at the interface. STEM electron-beam-induced current shows that the p-n junction occurs a few namometers into the CdTe grains, which is consistent with the S diffusion range we observe. The shift of the p-n junction suggests a buried homojunction which would help reduce nonradiative recombination at the junction. Meanwhile, long-range S diffusion in CdTe grain boundaries (GBs) has been detected, as has Te and Cl diffusion in CdS GBs.

Preferential sites for InAsP/InP quantum wire nucleation using molecular dynamics

In this paper, stress fields at the surface of the capping layer of self-assembled InAsP quantum wires grown on an InP (001) substrate have been determined from atomistic models using molecular dynamics and Stillinger-Weber potentials. To carry out these calculations, the quantum wire compositional distribution was extracted from previous works, where the As and P distributions were determined by electron energy loss spectroscopy and high-resolution aberration-corrected Z-contrast imaging. Preferential sites for the nucleation of wires on the surface of the capping layer were studied and compared with (i) previous simulations using finite element analysis to solve anisotropic elastic theory equations and (ii) experimentally measured locations of stacked wires. Preferential nucleation sites of stacked wires were determined by the maximum stress location at the MD model surface in good agreement with experimental results and those derived from finite element analysis. This indicates that MD simulations based on empirical potentials provide a suitable and flexible tool to study strain dependent atom processes.

Roughness of the SiC/SiO2 vicinal interface and atomic structure of the transition layers

The SiC/SiO2 interface is generally considered to be the cause for the reduced electron mobility of SiC power devices. Previous studies have shown a correlation between the mobility and the transition layer width at the SiC/SiO2 interface. The authors investigated this interface with atomic resolution Z-contrast imaging and electron energy-loss spectroscopy, and discovered that this transition region was due to the roughness of the vicinal interface. The roughness of a vicinal interface consisted of atomic steps and facets deviating from the ideal off-axis cut plane. The authors conclude that this roughness is limiting the mobility in the channels of SiC MOSFETs.

Determination of interfacial atomic structure, misfits and energetics of Ω phase in Al–Cu–Mg–Ag alloy

The interfacial atomic structure and misfits of Ω precipitates formed in the face-centered cubic Al in the Al–Cu–Mg–Ag alloy have been determined by combining scanning transmission electron microscopy (STEM) Z-contrast imaging with chemical analysis and ab initio density functional theory (DFT) calculations. Precipitates of several thicknesses formed after heat treatment at 250°C, starting from 0 to 2 unit cells of Ω phase, were examined by STEM in four different projections. The results show that a remarkably stable double-layered interface is formed at all observed thicknesses, which separates the Ω phase from the Al matrix. The outermost interfacial layer next to Al is composed of Ag atoms in a hexagonal structure and Mg or Cu atoms below the center of the hexagon. Structural models constructed based on the experimental data were relaxed using DFT-based molecular dynamics calculations. The results show that interfacial Mg atoms, together with Ag atoms, greatly stabilize the interface structure and consequently the Ω phase on the Al {111} habit planes. Comparison between the measured and calculated precipitate misfit along the thickness direction suggests that atomic substitutions of light atoms, Al and Mg, at the interface mediate the misfit strain and free energy, which is further supported by experimental evidence obtained from STEM. Thus, we have identified here: (i) the driving force for the Ag and Mg segregation in the formation of the Ω phase; (ii) the precipitation sequence characterized by a stable interfacial double-layer; and (iii) an interfacial substitution mechanism for misfit accommodation.

Crystal structure of Guinier-Preston zones in orthopyroxene: Z-contrast imaging and ab inito study

Nano-precipitates of Guinier-Preston zones (or G.P. zones) occur in slowly cooled orthopyroxenes (OPX). Due to their nanometer sizes and intergrowth with host OPX, both X-ray diffraction and high-resolution TEM imaging cannot provide a consistent structure model for the G.P. zone precipitates. Combining Z-contrast imaging and density functional theory (DFT) methods, a correct crystal structure for the G.P. zone precipitates can be obtained, because Z-contrast imaging can reveal positions and occupancies of atoms directly. The crystal structure for the G.P. zone has P 21/ c symmetry with 4 types of tetrahedral chains (OA1, OA2, OB, OC). The structure can be considered as periodic stacking of half unit cells of enstatite sub-layers and of diopside sub-layers in a twinning-like relationship along a -axis. The G.P. zones that precipitate out from their host Ca-bearing OPX at low temperature are metastable with respect to end-members of enstatite and diopside.

Intergrowth structure of α-phase in β-type TmAlB4 compound studied by high-angle annular detector dark-field scanning transmission electron microscopy

Nanostructure of a ThMoB4-type (β-type) TmAlB4 compound, in which YCrB4-type (α-type) domains are locally intergrown, is studied by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Z-contrast images by HAADF-STEM directly represent the arrangements of Tm atoms located at centers of heptagonal atomic columns of B atoms as bright dots, and give us detailed information of intergrowth of type domains in the matrix of the β-type phase, which coherently occurs. Structural and bonding analyses for β-TmAlB4 point out the closeness in atomic interactions and energy of the α- and β-type structures which support the easy formation of such nanostructure intergrowths. From combination between HAADF-STEM and electronic structure calculation, a detailed local crystal structure with intrinsic building defects is effectively revealed.

The Impact of Pretreatments and Conditioning on the Structure and Performance of NSTF ORR Alloy Catalysts

Pt-alloy nanoparticles and extended structures are under development for use as the oxygen reduction reaction (ORR) catalyst in polymer electrolyte membrane (PEM) fuel cell cathodes [1,2]. The addition of transition metals such as Co, Fe, or Ni can result in surface structures with enhanced specific activity [3,4], and in some cases increased specific area (Pt3Ni7/NSTF, Pt1Ni3 foils and NPs), which in combination can lead to overall increase in mass activity. Although highly active, alloy catalysts can pose serious durability issues due to rapid dealloying of the transition metal during fuel cell operation. Thus, ongoing development of these alloy catalysts aims not only at meeting critical cost and activity milestones, but also improving durability.A variety of advanced scanning transmission electron microscopy (STEM) techniques have been employed to elucidate the complex structural-performance relationship of Pt3Ni7 nanostructured thin film (NSTF) catalysts after various dealloying and thermal treatments. These STEM methods include aberration-corrected Z-contrast imaging, energy dispersive X-ray spectroscopy, and electron tomography, which have been utilized to explore the impact of various alloy catalyst pretreatments (dealloying, annealing, etc.) on morphology and composition, and, consequently, performance and durability. Electrode conditioning on either rotating disk electrodes or in membrane electrode assemblies (MEA) has a significant impact on alloy catalyst morphology and composition. An example of this is presented in Figure 1. While a particular pretreatment results in the catalyst morphology presented in Figure 1a, the conditioned catalyst, Fig. 1b, has a very different morphology and composition. In this case, the dissolution of Ni during conditioning results in the formation of nanopores in the NSTF catalyst and a new catalyst composition (Pt7Ni3). Since the catalytic metrics of the cathode (mass activity, specific activity, specific area, etc.) are measured after conditioning, it is the post-conditioned catalyst that must be characterized in order to establish accurate structure-performance relationships. Electron tomography is a powerful tool for visualizing nanoscale materials in three dimensions, as shown for a Pt3Ni7 NSTF catalyst before and after drive cycle testing in Figure 2. In this work, electron tomography will not only be used to visualize the porous network formed during catalyst conditioning and cycling, but also to quantify changes in surface area, pore density, and pore volume resulting from different catalyst pre-treatments. ReferencesM. K. Debe, Nature, 486, 43 (2012).S. M. Alia et al., ACS Catalysis, 4, 1114 (2014).V. R. Stamenkovic et al., Nat. Mater., 6, 241 (2007).J. Snyder et al., J. Am. Chem. Soc., 124, 8633 (2012). Acknowledgements This work was supported by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy under Award Number DE-EE0005667 and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Flexible Metallic Nanowires with Self-Adaptive Contacts to Semiconducting Transition-Metal Dichalcogenide Monolayers

In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques.

In Situ HAADF-STEM Imaging and Tomography of AuIr Bimetallic Catalysts

High-angle annular dark field (HAADF) STEM is an indispensable technique for analyzing heterogeneous catalysts, in particular those comprising high atomic number (Z) metallic nanoparticles (NPs) dispersed on low-Z supports. Readily interpretable atomic-scale Z-contrast imaging with high spatial resolution spectroscopy, including X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), allows researchers to investigate structural information such as dimensions, morphologies, and size distribution of catalytic NPs as well as their material chemistry (e.g., chemical composition, bonding of catalytic particles with supports at interface, etc.).

Study of Au/Pd and Au/Co Bimetallic Nanoparticles Using Aberration Corrected STEM

The synergetic combination of two metals in bimetallic (BM) nanoparticle (NP) significantly changes the optical, catalytic, electronic and magnetic properties. The possible reason for synergetic effects is the heterometallic bond between two metals, dilution of active site, the combination of two kinds of metals and their fine structures. The enhanced catalytic properties of BM nanoparticles are widely used in several industrial reactions. For practical applications and better understanding of this system, there should be well-controlled synthesis of shape and size selected structures, powerful structural characterization tools including advanced in situ atomic-level analytical techniques, modern modeling tools with supercomputers, and in-depth understanding of structure-property relationship. In this paper, we are presenting the use of Z-contrast imaging in aberration-corrected HAADF STEM images in the study of atomic ordering in nanoalloys and core/shell and also investigating the growth mechanism in Au-Pd and the interface region between Au and Pd.

Chemical composition study of high-k La-silicate gate stacks at sub-nanometer scale

La-based high-k oxides as gate dielectrics are promising for the complementary metal–oxide–semiconductor (CMOS) devices. We investigated the chemical compositions across a TaN/LaSiO/Si gate stack at sub-nanometer scale with electron energy loss spectroscopy (EELS) mapping, Z-contrast imaging, high-resolution transmission electron microscopy (HRTEM) imaging, and other tools. The interfaces in the gate stack were shown to maintain their integrity even after post annealing. The alloying between La2O3 and SiO2 to form LaSiO depended on the annealing conditions. To the as-deposited sample, a La–O super atomic layer structure was observed and La was in lower oxidation states as it diffused deeper into SiO2. The LaSiO thickness only increased slightly after post annealing and the gate stack showed the required thermal stability. All the results make TaN/LaSiO/Si a promising choice for transistor gate stacks. The combination of EELS mapping and other techniques is very useful in the chemical composition study of gate stacks at ultra-high spatial resolution.

Z-contrast imaging and ab initio study on "d" superstructure in sedimentary dolomite

Nano-precipitates with tripled periodicity along the c-axis are observed in a Ca-rich dolomite sample from Proterozoic carbonate rocks with “molar tooth” structure. This observation is consistent with previous description of d reflections. High-angle annular dark-field STEM imaging (or Z-contrast imaging) that avoids dynamic diffraction as seen in electron diffraction and high-resolution TEM imaging modes, confirms that d reflections correspond to nanoscale precipitates aligned parallel to (001) of the host dolomite. The lamellae precipitates have a cation ordering sequence of Ca-Ca-Mg-Ca-Ca- Mg along the c direction resulting in a chemical composition of Ca0.67Mg0.33CO3. This superstructure is attributed to the extra or d reflections, thus is referred to as the d superstructure in this study. The structure can be simply described as interstratified calcite/dolomite. The crystal structure of the d superstructure calculated from density functional theory (DFT) has a space group of P31c and has a and c unit-cell parameters of 4.879 and 16.260 Å, respectively, values between those of dolomite and calcite. The detailed structural characteristics and parameters obtained from ab initio calculations are also reported in this paper. The method of combining Z-contrast imaging and ab initio calculations can be used for solving structures of other nano-precipitates and nano-phases.

Analytical transmission electron microscopy in the third dimension

For researchers in metallurgy, energy storage or functional nanomaterials, where elemental distribution is crucial to performance, full characterization is more difficult by decreasing feature sizes and increasingly complex architectures. Two-dimensional imaging and analytical techniques often cannot characterize nanostructures completely. A new three-dimensional (3D) tomography technique for STEM XEDS, namely ChemiSTEMTM Technology [1], reveals the true microstructure of materials offering high X-ray collection efficiency via its windowless detector design, and fast XEDS mapping with tilt response at all angles. ChemiSTEM features the X-FEG and Super X, containing four symmetric detectors for 3D chemical imaging at 0 degree tilt, with significant EDX signal collected under all tilt conditions. The EDS signal of STEM XEDS image tilt series can be processed like normal z-contrast images, because of the monotonic relationship between the EDS signal and the thickness of the TEM foil. Four symmetrically arranged silicon drift detectors (SDD) provide extreme high output count rates (300 kcps) required for acquisition of EDX maps of thick samples, especially on 3 mm disk samples for 3D tomography studies. Examples of 3D chemical mapping using XEDS are given on (InGa)N Nanopyramid LEDs, Ni based super alloy material for turbine blades, dielectric transistor, and catalytic particles. In lithium ion batteries, the elemental segregation of manganese and nickel, which is responsible for the aging of electrode materials made from lithium-nickel-manganese oxide layered nanoparticles was analyzed in 3D XEDS mapping [2].

90° Domain Structure of (La,Li)TiO3 and Its Influence on Li Conduction Properties

The influence of 90° domain boundaries in (La,Li)TiO3 (LLTO) on the Li conduction mechanism has been examined by a combination of state-of-the-art experiments and first-principles calculations. The atomistic structure of 90° domain boundaries in LLTO has been determined using HAADF-STEM observations. Using HAADF-STEM observations, we can obtain Z-contrast image. As shown in Fig.1, bright spots correspond to heavier La column. Important finding is that at 90° domain boundary, each cells connected by La layer. Based on this structural information, first-principles calculations were carried out. In this study, we use projector augmented wave (PAW) method based on the density functional theory (DFT). Li conduction path and its activation energy were determined using nudged elastic band (NEB) method. Phonon dispersion relations were determined by frozen phonon method. Firstly we performed bulk crystal calculations using tetragonal (SG: P4/mmm) (La0.5,Li0.5)TiO3 assuming alternative stacking La layer and Li layer along c-direction. The calculated phonon dispersion for bulk crystal shows soft-mode in the entire wave vector space. This indicate lattice instability mainly TiO6-octahedoron rotation. Because of this TiO6-octahedoron rotation instability, calculated Li conduction activation energy for LLTO bulk shows exceptionally small value (E a = 0.2 eV). However, this theoretical E a was quite small to compare with experimentally reported value (E a = 0.4 eV). To reveal the origin of this E a discrepancy between theory and experiments, secondary, we examined Li-conduction energies at 90° domain boundary. We assumed two type of Li conduction: a) Li conduction through La layer, b)Li conduction through La vacancy at La layer. a) Li conduction through La layer shows very high E a of 3 eV. This indicate La layer acting blocking layer for Li conduction. However, if we assume the La vacancy at La layer, conduction energy significantly decreased to 0.6 eV. This conduction energy well corresponds to experimental value. Our calculations reveal that Li conduction in LLTO should be strongly influenced by 90° domain boundaries in this system.

Nanocrystallization in Ni60Ta40 and Ni60Nb40 metallic glasses below calorimetric glass transition

Crystallization of Ni60Ta40 and Ni60Nb40 metallic glasses at temperatures well below their respective calorimetric glass transition temperatures has been studied by differential scanning calorimetry, nanodiffraction, and electron microscopies. Nanometer-scaled metastable FCC phases were found to grow out from the glass structures. The Z-contrast images reveal that the nanocrystals are enriched in Ta or Nb in comparison with the surrounded amorphous structures. The experimental evidence is suggestive of the decoupled cooperative diffusion of Ni atoms in these fragile glasses.

Compositional variations in In0.5Ga0.5N nanorods grown by molecular beam epitaxy

The composition of InxGa1 − xN nanorods grown by molecular beam epitaxy with nominal x = 0.5 has been mapped by electron microscopy using Z-contrast imaging and x-ray microanalysis. This shows a coherent and highly strained core-shell structure with a near-atomically sharp boundary between a Ga-rich shell (x ∼ 0.3) and an In-rich core (x ∼ 0.7), which itself has In- and Ga-rich platelets alternating along the growth axis. It is proposed that the shell and core regions are lateral and vertical growth sectors, with the core structure determined by spinodal decomposition.

Crystal structures of laihunite and intermediate phases between laihunite-1M and fayalite: Z-contrast imaging and ab initio study

Crystals of laihunite from Xiaolaihe of Liaoning Province, northeast China, were studied using selected-area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), and Z-contrast imaging. Z-contrast images directly reveal ordered vacancies in M1 sites. The results confirm early structural models for 1-layer laihunite (or laihunite-1M) with ideal stoichiometry of □ 0.5 Fe 2+ 0.5 Fe 3+ SiO 4 . 2-layer laihunite and 3-layer laihunite are found to be chemically different from laihunite-1M. The 2-layer laihunite can be viewed as a periodic intergrowth of laihunite and fayalite in the 1:1 ratio. The 3-layer laihunite can be considered to be a periodic intergrowth of laihunite and fayalite in the 1:0.5 ratio along the c -axis. Ideal stoichiometries for the 2-layer structure and the 3-layer structure are □ 0.5 Fe 2+ 2.5 Fe 3+ [SiO 4 ] 2 and □ 1.0 Fe 2+ 3.0 Fe 3+ 2.0 [SiO 4 ]3, respectively. The structural intergrowth of the 3-layer laihunite and the 1-layer lahunite results in chemical compositions falling within the range between the two aforementioned structures, such as the chemical formula of □ 0.4 Fe 2+ 0.8 Fe 3+ 0.8 SiO 4 , reported earlier in the literature. The crystal structures of the 1-layer laihunite (1M), the 2-layer laihunite (2M), and the 3-layer laihunite (3Or) determined from Z-contrast images and ab initio calculations using the density functional theory (DFT) have space groups of P21/b, P21/b, and Pbnm, respectively. The previously reported monoclinic symmetry for the 3-layer laihunite may be an artifact due to overlapping diffraction spots from both, the laihunite-3Or and the laihunite-1M. Our study demonstrates that the method of combining Z-contrast imaging and ab initio calculation can be effectively used for identifying structures of nano-phases in host crystals. Perhaps more importantly, Z-contrast imaging provides a powerful means for direct observation of vacancies and other defects, and may be utilized to map vacancies in Fe 3+ -bearing olivines, the alignments of which can greatly affect anisotropic diffusion in such structures.