Dark-field Transmission Electron Microscopy Research Articles

Strain-dependent shear-band structure in a Zr-based bulk metallic glass

This work presents strong evidence for structural damage accumulation as a function of shear strain admitted by shear bands in a Zr-based bulk metallic glass. Analyzing the shear-band structure of shear-band segments that experienced shear strains covering four orders of magnitude with high-angle annular dark field transmission electron microscopy (HAADF-STEM) reveals strongly scattered data with on overall trend of increasing local volume dilatation with increasing shear strain. Locally, however, a variety of trends is observed, which underlines the strong heterogeneity of structural damage in shear bands in metallic glasses.

Manganese–Fluorouracil Metallodrug Nanotheranostic for MRI-Correlated Drug Release and Enhanced Chemoradiotherapy

For cancer therapy, drug delivery systems are often limited by insufficient drug loading capacity, which usually results in systemic toxicity and heavy metabolic burden to excrete the carriers. Her...

Direct observation and catalytic role of mediator atom in 2D materials.

The structural transformations of graphene defects have been extensively researched through aberration-corrected transmission electron microscopy (AC-TEM) and theoretical calculations. For a long time, a core concept in understanding the structural evolution of graphene defects has been the Stone-Thrower-Wales (STW)-type bond rotation. In this study, we show that undercoordinated atoms induce bond formation and breaking, with much lower energy barriers than the STW-type bond rotation. We refer to them as mediator atoms due to their mediating role in the breaking and forming of bonds. Here, we report the direct observation of mediator atoms in graphene defect structures using AC-TEM and annular dark-field scanning TEM (ADF-STEM) and explain their catalytic role by tight-binding molecular dynamics (TBMD) simulations and image simulations based on density functional theory (DFT) calculations. The study of mediator atoms will pave a new way for understanding not only defect transformation but also the growth mechanisms in two-dimensional materials.

Bi-Magnetic Core-Shell CoFe2O4@MnFe2O4 Nanoparticles for In Vivo Theranostics.

In this work, we report the synthesis and characterization of three magnetic nanosystems, CoFe2O4, CoFe2O4@ZnFe2O4, and CoFe2O4@MnFe2O4, which were developed as potential theranostic agents for magnetic hyperthermia and magnetic resonance imaging (MRI). These nanosystems have been thoroughly characterized by X-ray Diffraction (XRD), Transmission Electron Miscroscopy (TEM), Dark Field-TEM (DF-TEM), Vibrating Sample Magnetometry (VSM), and inductive heating, in order to elucidate their structure, morphology, and magnetic properties. The bi-magnetic CoFe2O4@ZnFe2O4 and CoFe2O4@MnFe2O4 nanoparticles (NPs) exhibited a core-shell structure with a mean average particle size of 11.2 ± 1.4 nm and 14.4 ± 2.4 nm, respectively. The CoFe2O4@MnFe2O4 NPs showed the highest specific absorption rate (SAR) values (210–320 W/g) upon exposure to an external magnetic field, along with the highest saturation magnetization (Ms). Therefore, they were selected for functionalization with the PEGylated ligand to make them stable in aqueous media. After the functionalization process, the NPs showed high magnetic relaxivity values and very low cytotoxicity, demonstrating that CoFe2O4@MnFe2O4 is a good candidate for in vivo applications. Finally, in vivo MRI experiments showed that PEGylated CoFe2O4@MnFe2O4 NPs produce high T2 contrast and exhibit very good stealth properties, leading to the efficient evasion of the mononuclear phagocyte system. Thus, these bi-magnetic core-shell NPs show great potential as theranostic agents for in vivo applications, combining magnetic hyperthermia capabilities with high MRI contrast.

Ion channelling effect and damage accumulation in yttria-stabilized zirconia implanted with Ag ions

Yttria stabilized zirconia (YSZ) is well known as a radiation-resistant material. In this study, we present results from 400 keV Ag+ implantations of the (1 0 0) YSZ single crystals to fluences ranging from 5 × 1015 to 5 × 1016 cm−2. The damage depth profiling and accumulation were probed using Rutherford backscattering spectrometry in the channelling mode (RBS-C), Transmission electron microscopy (TEM) and X-ray diffraction (XRD). The axial channelling effect of 2 MeV He+ ions in the implanted YSZ was studied. RBS-C provides us with detailed information about the displaced atoms density depth profiles progressing into greater depths, especially in the case of higher fluence. TEM was utilized to characterize the microstructure evolution and damage accumulation in the buried layer after the implantation. At the highest fluence (5 × 1016 cm−2), Ag depth profile in the depth of 30–130 nm was identified in TEM bright and dark field images as well as in the electron diffraction patterns. Ag depth profiles are in agreement with depth profiles determined by RBS which show maximum Ag concentration in the depth of 94 nm. The reason for the decrease of the deformation identified by XRD in the vertical direction is the defect formation.

Template-free Synthesis of Mesoporous and Crystalline Transition Metal Oxide Nanoplates with Abundant Surface Defects

Summary Mesoporous transition metal oxides with crystalline walls show promising applications in catalysis and energy storage. Syntheses of those crystalline mesoporous oxides, however, currently remain as an unmet challenge because of the large volume shrinkage during crystallization in a sol-gel process. Here, we demonstrate a facile and general template-free method to prepare six mesoporous transition metal oxides, e.g., CuO, CoO, and spinel MCo2O4 (M = Co, Cu, Mn, Zn), with highly crystalline walls and continuous mesopores in the forms of two-dimensional nanoplates or nanosheets. The method is based on the thermal crystalline transformation from basic carbonates to mesoporous metal oxides. The crystal interconversion not only endows the crystallinity and mesoporosity of oxides but also generates abundant surface-step defects that show remarkable enhancement of the catalytic activity of porous oxides. Our method likely provides an alternative paradigm for cost-effective and universal synthesis of crystalline mesoporous oxides beyond the traditional templating methods.

Trimetallic Ru@AuPt core-shell nanostructures: The effect of microstrain on CO adsorption and electrocatalytic activity of formic acid oxidation

It is desirable to unravel the correlation between the geometric and electronic structures and the activity and further prepare high-performance electrocatalysts. Here in this paper, trimetallic Ru@Au-Pt core-shell nanoparticles were prepared by sequential ethanol reduction method, and further subject to characterization of X-ray diffraction, high angle annular dark field transmission electron microscopy, X-ray photoelectron spectroscopy and electrochemical CO stripping. Further analysis based on Williamson-Hall method revealed that the Au/Pt atomic ratio and shell thickness result in apparent variation of micro-strain and CO binding energy of Ru@AuPt nanoparticles, where the CO oxidation peak potential showed an inverted volcano-shape dependence on the microstrain of the metal nanoparticles while the catalytic activity towards electrooxidation of formic acid is linearly dependent on the micro-strain. The best Ru@Au-Pt catalyst delivers a specific activity of 4.14 mA cm−2, which is 52 times that of Pt/C, respectively. This study indicated that the microstrain and stacking fault of metal nanoparticles might be a good descriptor for the catalytic activity and may shed light the rational design, synthesis and surface engineering towards the high-performance electrocatalyst.

In Situ TEM Crystallization of Amorphous Iron Particles

Even though sub-micron and nano-sized iron particles generally display single or polycrystalline structures, a growing interest has also been dedicated to the class of amorphous ones, whose absence of a crystal structure is capable of modifying their physical properties. Among the several routes so far described to prepare amorphous iron particles, we report here about the crystallization of those prepared by chemical reduction of Fe3+ ions using NaBH4, with sizes ranging between 80 and 200 nm and showing a high stability against oxidation. Their crystallization was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and in situ heating transmission electron microscopy (TEM). The latter technique was performed by the combined use of electron diffraction of a selected sample area, and bright and dark field TEM imaging, and allowed determining that the crystallization turns the starting amorphous particles into polycrystalline α-Fe ones. Also, under the high vacuum of the TEM column, the crystallization temperature of the particles shifted to 550 °C from the 465 °C, previously observed by DSC and XRD under 105 Pa of Ar. This indicates the pivotal role of the external pressure in influencing the starting point of phase transition. Conversely, upon both the DSC/XRD pressure and the TEM vacuum conditions, the mean size of the crystal domains increases as a consequence of further thermal increase, even if with some pressure-related differences.

Method combining BAC film and positive staining for the characterization of DNA intermediates by dark-field electron microscopy

DNA intermediate structures are formed in all major pathways of DNA metabolism. Transmission electron microscopy (TEM) is a tool of choice to study their choreography and has led to major advances in the understanding of these mechanisms, particularly those of homologous recombination (HR) and replication. In this article, we describe specific TEM procedures dedicated to the structural characterization of DNA intermediates formed during these processes. These particular DNA species contain single-stranded DNA regions and/or branched structures, which require controlling both the DNA molecules spreading and their staining for subsequent visualization using dark-field imaging mode. Combining BAC (benzyl dimethyl alkyl ammonium chloride) film hyperphase with positive staining and dark-field TEM allows characterizing synthetic DNA substrates, joint molecules formed during not only in vitro assays mimicking HR, but also in vivo DNA intermediates.

Microstructure investigation of r.f. current annealed FINEMET cold drawn microwires by UHR-TEM

The microstructure of FINEMET (Fe73.5Nb3Cu1Si13.5B9, at %) ferromagnetic microwires with diameters of 30 µm obtained by cold drawn of 120 µm thicker conventional amorphous microwires followed by radio frequency current annealing has been investigated by ultra-high resolution transmission electron (UHR-TEM) microscopy. The cold drawn microwires (CDWs) were annealed by radio frequency current of 375 mA@10MHz and 750 mA@10MHz, respectively, for 10 minutes to achieve cold drawn microwires with nano-crystalline structure. The specimens for UHR-TEM investigation were prepared in the shape of lamella using Focus Ion Beam microscope. For each annealed CDNWs were obtained two lamella one from the edge of microwire and another one from the center of the wire. Each specimen has been examined by transmission electron microscopy (TEM) using a Libra200MC microscope operated at 200kV through the bright-field and dark-field TEM, HR-TEM, HAADF STEM and EDX method. Electron diffraction patterns are interpreted using the computer programs SingleCrystal® and JEMS®. Mean grain size is measured using ImageJ software and making a geometrical correction for film thickness. The cold drawn microwires annealed at 350mA are largely amorphous, although containing occasional Fe3Si regions of short-range order as presented in Figure 1 (a). The cold drawn microwires annealed at 750mA present a nano-crystalline structures, with all nano-grains appearing to be of the bcc α-Fe(Si) structure as shown in Figure 1 (b). The estimated number density of nano-grains is 4.8 x 1023, which corresponds to an estimated phase fraction of 25 vol.% for a nano-grain radius of 5 nm. Line-scan EDX shows no particularly striking deviation from a homogenous distribution of elements for nanocrystalline sample, while a clustering behavior of elements for amorphous sample was observed.

Tailoring commensurability of hBN/graphene heterostructures using substrate morphology and epitaxial growth conditions

Hexagonal boron nitride (hBN) thin films were grown by plasma-enhanced chemical beam epitaxy (PE-CBE) on epitaxial graphene (EG) on macrostepped 4°-offcut 4H-SiC(0001) substrates. The choice of growth conditions in this system allowed for two prominent in-plane hBN/EG rotational alignments: a direct alignment of the hBN and EG lattices or a 30° in-plane rotational twist such that the ⟨112¯0⟩hBN and ⟨101¯0⟩EG directions are parallel. The use of nitrogen plasma in conjunction with borazine at growth temperatures of 1450 °C increased the crystallinity of the few-monolayer-thick films relative to films grown by CBE without plasma exposure. In vacuo x-ray photoelectron spectroscopy showed that films grown with nitrogen plasma exposure were stoichiometric to nitrogen-rich, depending on growth conditions, and exhibited no bonding indicative of additional phase formation. This PE-CBE process was shown to produce films with atomically abrupt interfaces between the hBN and EG lattices, as determined by cross-sectional transmission electron microscopy (TEM). Annular dark field and bright field scanning TEM paired with energy dispersive x-ray spectroscopy confirmed that the EG persisted throughout this deposition and no intercalative growth of hBN under the EG was detected. Higher PE-CBE growth rates produced hBN domains that nucleated uniformly across the substrate with little preferred orientation of their edges. In comparison, lower growth rates appeared to cause preferential nucleation on the macrostep edges with a 30° in-plane rotation relative to the EG, as confirmed by cross-sectional TEM. By correlating the hBN nuclei shape in AFM to the atomic registry of the hBN to the substrate, it was found that the triangular, macrostep-edge nuclei were arm-chair edge terminated. The ability to select different rotational alignments by changing epitaxial growth conditions may be used in future wafer-scale growth of hBN/graphene heterostructures to achieve varying degrees of graphene band structure modulation.

Radiation-induced precipitation with concurrent bubbles formation in an austenitic stainless steel (AISI-348)

Radiation-induced precipitation has been examined in an Nb-stabilised austenitic stainless steel (AISI-348) during heavy ion irradiation in situ within a transmission electron microscope (TEM) at 1073 K. Selected-area electron diffraction (SAED), bright- and dark-field TEM were used to investigate the nature of the precipitates within the austenite phase (γ-Fe). The crystal structure of the precipitates was investigated using TEM and matched with reference data for the Cr23C6 phase. The results herein reported indicate that the concurrent formation of inert gas bubbles may accelerate clustering and precipitation kinetics in the austenite phase during irradiation.

Crystallographic ordering in a series of Al-containing refractory high entropy alloys Ta–Nb–Mo–Cr–Ti–Al

High entropy alloys based on the Ta–Nb–Mo–Cr–Ti–Al system are expected to possess high creep and oxidation resistance as well as outstanding specific mechanical properties due to presumed high melting points and low densities. However, we recently reported that arc-melted and subsequently homogenized alloys within this system exhibit a lack of ductility up to 600 °C [H. Chen et al. in Metall. Mater. Trans. A 49 (2018) 772–781 and J. Alloys Cmpd. 661 (2016) 206–215]. Thermodynamic calculations suggest the formation of a B2-type ordered phase below the homogenization temperature. In the present article, we provide results of a detailed microstructural characterization of a series of Ta–Nb–Mo–Cr–Ti–Al derivatives and evaluate if B2-type ordering could be the origin for the observed lack of ductility. Backscatter electron (BSE) imaging, energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT) were used to verify uniform elemental distribution after homogenization. X-ray diffraction (XRD) patterns indicate both, A2 or B2-type crystal structure, whereas transmission electron microscopy (TEM) diffraction experiments unambiguously confirm B2-type order in the as-homogenized state of all investigated alloys. In MoCrTiAl, planar defects that show antiphase boundary contrast with a {100}-type habit plane were detected by TEM dark field (DF) imaging. They are wetted by a Cr-enriched and Ti-depleted layer as confirmed by scanning transmission electron microscopy (STEM)-EDX line scans as well as APT analyses. The planar defects arise from a disorder-order solid-state phase transformation during cooling, as indicated by differential scanning calorimetry (DSC).

Particle-Mediated Nucleation Pathways Are Imprinted in the Internal Structure of Calcium Sulfate Single Crystals

Calcium sulfate minerals are found under the form of three crystalline phases: gypsum (CaSO4.2H2O), bassanite (CaSO4.0.5H2O), and anhydrite (CaSO4). Due to its relevance in natural and industrial processes, the formation pathways of these calcium sulphate phases from aqueous solution have been the subject of intensive research. There is a growing body of literature, that calcium sulfate forms essentially through a non-classical nanoparticle- or cluster-mediated crystallisation process. At the early stages of precipitation calcium sulfate crystals grow through the reorganization and coalescence of aggregates rather than through classical unit addition. Here, we show by using low-dose dark field (DF) transmission electron microscopy (TEM) and electron diffraction, that these re-structuring processes by no means continue until a near-perfectly homogeneous single crystal is obtained. Instead the growth process yields a final imperfect mesocrystal of the overall morphology resembling that of a single crystal, yet composed of smaller nano-domains. Hence, our data indicate that organic-free mesocrystal grown by a particle mediated-pathway may preserve a 'memory or 'imprint' of the non-classical growth process in the final crystal structure, something that has been largely overlooked until now. Furthermore, the nano-scale misalignment of the structural sub-units within crystals might propagate through the length-scales, and potentially is expressed macroscopically as misaligned zones/domains in large single crystals, e.g. as those observed in some of the giant crystals from the Naica Mine, Chihuahua, Mexico.

Spatially Resolved Chemical Analysis of Geobacter sulfurreducens Cell Surface.

Geobacter sulfurreducens is of interest for the highest efficiency of power generation and extremely long extracellular electron transfer (EET) between the bacterium and electrodes. Despite more than 15 years of intensive molecular biological research, there is still no clear answer which molecules are responsible for these processes. In the present work, we look at the problem from another (atomic) perspective and identify the location and shape of the compounds that are known to be conductive, particularly those containing Fe atoms. By using highly sophisticated energy dispersive X-ray spectroscopy combined with high-angle annular dark-field transmission electron microscopy enabling detection, identification, and localization of chemical compounds on the surface at nearly atomic spatial resolution, we analyze Fe spatial distribution within the G. sulfurreducens community. We discover the presence of small Fe-containing particles on the surface of the bacterium cells. The size of the particles (diameter 5.6 nm) is highly reproducible and comparable with the size of a single protein. The particles cover about 2% of the cell surface, which is similar to that expected for molecular conductors responsible for electron transfer through the bacterium cell wall. We find that G. sulfurreducens filaments ("bacterial molecular wires") also contain Fe atoms in their bundles. We observe that the bacterium enable changing the distance between the Fe-containing bundles in the filaments from separated to attached (the latter is needed for the efficient electron transfer between the Fe-containing particles), depending on the bacterium metabolic activity and attachment to extracellular substrates. These results are consistent with the recently published research about the role of Fe atoms in protein molecular conductance ( Phys. Chem. Chem. Phys. , 2018 , 20 , 14072 - 14081 ) and show what type of Fe-containing particles are involved in the bacterial extracellular communication. They can be used for the design and construction of artificial biomolecular wires and bioinorganic interfaces.

Double-Spiral Hexagonal Boron Nitride and Shear Strained Coalescence Boundary.

Among the different growth mechanisms for two-dimensional (2D) hexagonal boron nitride (h-BN) synthesized using chemical vapor deposition, spiraling growth of h-BN has not been reported. Here we report the formation of intertwined double-spiral few-layer h-BN that is driven by screw dislocations located at the antiphase boundaries of monolayer domains. The microstructure and stacking configurations were studied using a combination of dark-field and atomic resolution transmission electron microscopy. Distinct from other 2D materials with single-spiral structures, the double-spiral structure enables the intertwined h-BN layers to preserve the most stable AA' stacking configuration. We also found that the occurrence of shear strains at the boundaries of merged spiral islands is dependent on the propagation directions of encountering screw dislocations and presented the strained features by density functional theory calculations and atomic image simulations. This study unveils the double-spiral growth of 2D h-BN multilayers and the creation of a shear strain band at the coalescence boundary of two h-BN spiral clusters.

Radiation-Induced Precipitation with Concurrent Bubbles Formation in an Austenitic Stainless Steel (Aisi-348)

Radiation-induced precipitation has been examined in an Nb-stabilised austenitic stainless steel (AISI-348) during heavy ion irradiation in situ within a transmission electron microscope (TEM) at 1073 K. Selected-area electron diffraction (SAED), bright- and dark-field TEM were used to investigate the nature of the precipitates within the austenite phase (γ-Fe). The precipitates were confirmed to be of Cr23C6 crystal structure. The results herein reported indicate that the concurrent formation of inert gas bubbles may accelerate clustering and precipitation kinetics in the austenite phase during irradiation.

TEM-EELS analyses of protactinium

A fragment of metallic protactinium (Pa) has been studied using a transmission electron microscope (TEM) equipped with an electron energy loss spectroscopy (EELS) detector. Bright and dark field TEM images have been acquired, and selective area electron diffraction (SAED) has been used to study the crystal structures. The results showed the presence of domains both of metallic and of oxidized material that likely occurred during sample preparation. EELS edges have been collected for the first time for the oxide compounds and compared with those computed using many-electron atomic spectral methods.

Determination of the interface parameter in terahertz quantum-cascade laser structures based on transmission electron microscopy

We have determined the interface profiles of GaAs/Al0.25Ga0.75As and GaAs/AlAs terahertz quantum-cascade laser structures using transmission electron microscopy (TEM). The experimental composition profiles are directly extracted from the analysis of the diffracted intensity distribution of g002 dark-field TEM images. Simulated composition profiles, which have been obtained using a Fourier-transform-based model, are in remarkably good agreement with the experimental results so that the determined interface parameter can be used for designing terahertz quantum-cascade lasers. Due to the interface grading, these actual profiles reveal a significantly lower aluminum content of the Al0.25Ga0.75As and AlAs barriers than the nominal values.

Polarity Control within One Monolayer at ZnO/GaN Heterointerface: (0001) Plane Inversion Domain Boundary.

In semiconductor heterojunction, polarity critically governs the physical properties, with an impact on electronic or optoelectronic devices through the presence of pyroelectric and piezoelectric fields at the active heteropolar interface. In the present work, the abrupt O-polar ZnO/Ga-polar GaN heterointerface was successfully achieved by using high O/Zn ratio flux during the ZnO nucleation growth. Atomic-resolution high-angle annular dark-field and bright-field transmission electron microscopy observation revealed that this polarity inversion confines within one monolayer by forming the (0001) plane inversion domain boundary (IDB) at the ZnO/GaN heterointerface. Through theoretical calculation and topology analysis, the geometry of this IDB was determined to possess an octahedral Ga atomic layer in the interface, with one O/N layer symmetrically bonded at the tetrahedral site. The computed electronic structure of all considered IDBs revealed a metallic character at the heterointerface. More interestingly, the presence of two-dimensional (2D) hole gas (2DHG) or 2D electron gas (2DEG) is uncovered by investigating the chemical bonding and charge transfer at the heterointerface. This work not only clarifies the polarity control and interfacial configuration of the O-polar ZnO/Ga-polar GaN heterojunction but, more importantly, also gives insight into their further application on heterojunction field-effect transistors as well as hybrid ZnO/GaN optoelectronic devices. Moreover, such polarity control at the monolayer scale might have practical implications for heterojunction devices based on other polar semiconductors.