Dark-field Transmission Electron Microscopy Research Articles

Polycrystallinity of green rust minerals and their synthetic analogs: Implications for particle formation and reactivity in complex systems

We demonstrate in this study that natural green rust nanoparticles and their synthetic analogs can be complex polycrystalline phases composed of crystallites only a few nanometers in size, and they often include nanometer-sized regions of amorphous material. The natural green rusts are Zn-bearing pseudo-hexagonal platelets previously identified by us in the contaminated mine drainage of the former Ronneburg uranium mine in Germany (Johnson et al. 2014). We also identified Ni- and Cu-bearing green rust platelets in the sediment underlying the drainage outflow 20 m downstream, and, using dark-field transmission electron microscopy (DF-TEM), found that these natural green rusts are not usually structurally coherent single crystals. Synthetic sulfate green rusts are also polycrystalline and composed of crystallites of only a few nanometers in size, though different synthesis conditions produced different patterns of polycrystallinity. While pseudo-hexagonal platelets are the typical morphology of green rust, we also synthesized green rust nanorods, which have not previously been reported. In addition to the known characteristics of green rusts (including a very large aspect ratio and surface area to volume ratio, and the redox properties allowed by the structural mixture of Fe 2+ and Fe 3+ ), these polycrystalline platelets exhibit a high abundance of defect sites and likely a rough surface topography. The combination of these characteristics has important implications for the reactivity of green rust with biogeochemical interfaces in natural and anthropogenic systems.

Grain structures of nitrogen-doped graphene synthesized by solid source-based chemical vapor deposition

Doping of a foreign element in sp2 hybridized graphene lattice is of significant importance to tune the electrical and chemical properties. Here, we report on the grain structures of substitutional nitrogen-doped graphene synthesized by an atmospheric pressure (AP) solid source-based chemical vapor deposition (CVD) technique. Nitrogen-doped graphene was synthesized by mixing solid camphor and melamine as carbon and nitrogen source, respectively. The precursor materials quantity significantly affects the graphene growth on Cu foil and thereby the nitrogen doping and content. Transmission electron microscopy (TEM) analysis was performed to determine the nitrogen substitutional sites in the graphene. Dark-field (DF) TEM analysis was carried out to evaluate the graphene grain structure grown with introduction of nitrogen dopant. We obtained different grain orientations, where an individual grain size is more than 5 μm. Our findings show that graphitic nitrogen defects can be introduced in the large individual graphene grain by the developed solid source-based CVD technique.

Fabrication of hollow platinum–ruthenium core–shell catalysts with nanochannels and enhanced performance for methanol oxidation

This work reports the preparation of hollow PtRu core–shell catalysts with TiO2 as template, in which the Pt nanoparticles (NPs) grow on the exterior surface of Ru layer. The quantity of Pt NPs is easily tailored to control the integrity of Pt shell through varying the concentration of H2PtCl6 solution. Scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and selected-area electron diffraction (SAED) are used to characterize the structure and morphology of H–PtRu. The core–shell structure is confirmed by the high-angle annular dark-field scanning TEM (HAADF-STEM) with energy-disperse X-ray spectroscopy (EDX). The electrochemical performance of H–PtRu is investigated by cyclic voltammetry and chronoamperometry. Results show that the catalytic activity of H–PtRu toward methanol oxidation reaction (MOR) is ∼2.5 times higher than that of Pt/C (JM), and the electrocatalytic stability improves with the increase of Ru content. Furthermore, H–PtRu exhibits better stability for methanol oxidation compared to Pt/C (JM) and PtRu/C (JM).

Rapid, all-optical crystal orientation imaging of two-dimensional transition metal dichalcogenide monolayers

Two-dimensional (2D) atomic materials such as graphene and transition metal dichalcogenides (TMDCs) have attracted significant research and industrial interest for their electronic, optical, mechanical, and thermal properties. While large-area crystal growth techniques such as chemical vapor deposition have been demonstrated, the presence of grain boundaries and orientation of grains arising in such growths substantially affect the physical properties of the materials. There is currently no scalable characterization method for determining these boundaries and orientations over a large sample area. We here present a second-harmonic generation based microscopy technique for rapidly mapping grain orientations and boundaries of 2D TMDCs. We experimentally demonstrate the capability to map large samples to an angular resolution of ±1° with minimal sample preparation and without involved analysis. A direct comparison of the all-optical grain orientation maps against results obtained by diffraction-filtered dark-field transmission electron microscopy plus selected-area electron diffraction on identical TMDC samples is provided. This rapid and accurate tool should enable large-area characterization of TMDC samples for expedited studies of grain boundary effects and the efficient characterization of industrial-scale production techniques.

Characterization of crystallite morphology for doped strontium fluoride nanophosphors by TEM and XRD

Crystallite morphology for Eu-doped and undoped SrF2 nanophosphors have been determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The values for average crystallite size obtained by the application of the Scherrer equation and the full width at half maximum (FWHM) values for XRD peaks are compared to the results obtained using the hollow cone dark field (HCDF) TEM imaging technique. In the case of the TEM analysis, a bimodal crystallite size distribution was revealed with one of the distributions having a measured range of crystallite sizes which was in good agreement with the XRD data. HCDF in combination with FIB specimen preparation was found to be a promising technique for the determination of crystallite size distributions in nanophosphors which might facilitate a better understanding of their scintillation properties.

Mn-doped Zn/Al layered double hydroxides as photocatalysts for the 4-chlorophenol photodegradation

Mn-doped Zn/Al layered double hydroxides (LDH) with Mn 0.5–3.0% mol respect to Zn content with improved photocatalytic degradation of 4-chlorophenol (4Clphenol) were studied. The characterization studies showed the isomorphic incorporation Mn as dopant until 1% mol. The 4Clphenol degradation was proposed as result of a combined effect of oxidation by both hydroxyl radicals (OH) and photogenerated holes (h+). In a proposed mechanism it is suggested that Mn enhances the charge separation acting as electron e− (Mn3+; Mn4+) or hole h+ (Mn2+; Mn3+) traps according to its oxidation state. Exhaustive characterization through EDS, XRD, UV–vis-DRS, TEM-Dark field STEM, fluorescence spectroscopy for OH detection and XPS, has been done denoting the importance of the Mn content and its different oxidation states in the photophysical and photocatalytic properties of the Mn-doped Zn/Al-based layered double hydroxides.

Synthesis and Characterization of Nanostructured Tungsten-30 Atomic % Chromium Produced by Mechanical Attrition and Hydrogen Sintering

Nanostructured W70Cr30 powders were produced by mechanical alloying (MA) of elemental tungsten (W), Chromium (Cr) powders in a high energy planetary ball-mill using tungsten carbide as grinding media and toluene as a process control agent. The crystallite size and lattice strain of the nanostructured powders at different milling time (0 h to 10 h) was calculated from X-ray diffraction patterns (XRD). The crystallite size of W in W70Cr30 powder reduced from 100 μm at 0 h to 32.8 nm at 10 h of milling with increase in lattice strain of 0.43% at 10 h of milling. The lattice parameter of tungsten shows initial expansion of lattice upto 0.56% at 5 h of milling and contraction of lattice upto 0.93% at 10 h of milling. The scanning electron microscopy (SEM) micrograph also revealed mixed morphology of elemental W and Cr powders consist of spherical and elongated particles during mechanical alloying (0 h to 10 h). The dark-field transmission electron microscopy (TEM) observations indicated that the crystallite size (~30 nm) of W in W-Cr alloy in the as-milled powder is in good agreement with calculated crystallite size from XRD. Maximum solid solubility of 4.4 at.% Cr in W was found at 10 h of milling. The dislocation density increases from 6.75 (1016/m2) to 17.56 (1016/m2) with increase in the milling time from 0 h to 20 h. No cracks in the sintered pellets were visible under scanning electron microscope (SEM). Hardness and Elastic Modulus of sintered W70Cr30 alloy determined by nanoindentation test are less compared to pure W.

Analysis of edge threading dislocations b→=12〈110〉 in three dimensional Ge crystals grown on (001)-Si substrates

Threading dislocations (TDs) in germanium (Ge) crystals epitaxially grown on a patterned (001)-silicon (Si) substrate are investigated using transmission electron microscopy (TEM) techniques. Analysis of dislocations performed on the Ge crystals reveals 60° and edge TDs with Burgers vector b→=12〈110〉. High-angle annular dark-field scanning TEM (HAADF-STEM) is used to observe the core of the edge TDs at atomic scale. Pairs of TDs with b→=12〈110〉 are present in the material running parallel at small distances between them (0.5–1.5 nm). The observation of such parallel dislocation pairs in Ge has not been documented before. The interaction between the edge dislocation pairs is obtained experimentally from the high-resolution HAADF-STEM images by applying geometrical phase analysis. The experimental strain maps are compared to analytical calculations based on the anisotropic elastic theory demonstrating a good match between them.

Ag-Decorated ATaO3 (A = K, Na) Nanocube Plasmonic Photocatalysts with Enhanced Photocatalytic Water-Splitting Properties.

Tantalate semiconductor nanocrystals have been at the forefront of the photocatalytic conversion of solar energy to supply hydrogen owing to their favorable and tunable optical and electronic properties as well as advances in their synthesis. However, a narrow band gap is required for response to improve the efficiency of the photocatalysts. Here we propose an efficient enhancement of the H2 generation under simulated sunlight and visible light irradiation by a dispersion of Ag-decorated KTaO3 and NaTaO3 nanocubes. X-ray diffraction and UV-vis diffuse reflectance spectra are used to characterize the products. Transmission electron microscope (TEM) and high-resolution high-angle annular dark-field scanning TEM (HAADF-STEM) images show that the Ag nanoparticles (NPs) are uniformly loaded on the surfaces of KTaO3 and NaTaO3. The photocatalytic water-splitting results over Ag-decorated KTaO3 and NaTaO3 show that the rate for H2 evolution from aqueous CH3OH solutions is up to 185.60 and 3.54 μmol/h·g under simulated sunlight and the rate for H2 evolution is more than 2 times than that of pure NaTaO3 and KTaO3 materials. However, under purely visible light illumination the highest H2 evolution of 25.94 and 0.83 μmol/h·g is observed in the case of Ag-decorated KTaO3 and NaTaO3 nanocubes. To the best of our knowledge, this is the first time that the photocatalytic water-splitting activity of the prepared Ag-decorated KTaO3 and NaTaO3 nanocubes has been reported.

Microstructures of InN film on 4H-SiC (0001) substrate grown by RF-MBE**Project supported by the Thailand Center of Excellence in Physics (ThEP) and the King Mongkut's University of Technology Thonburi under The National Research University Project. One of the authors (S. Sanorpim) was supported by the National Research Council of Thailand (NRCT) and the Thai Government Stimulus Package 2 (TKK2555), under

InN film was grown on 4H-SiC (0001) substrate by RF plasma-assisted molecular beam epitaxy (RF-MBE). Prior to the growth of InN film, an InN buffer layer with a thickness of ∼5.5 nm was grown on the substrate. Surface morphology, microstructure and structural quality of InN film were investigated. Micro-structural defects, such as stacking faults and anti-phase domain in InN film were carefully investigated using transmission electron microscopy (TEM). The results show that a high density of line contrasts, parallel to the growth direction (c-axis), was clearly observed in the grown InN film. Dark field TEM images recorded with diffraction vectors and g = 0002 revealed that such line contrasts evolved from a coalescence of the adjacent misoriented islands during the initial stage of the InN nucleation on the substrate surface. This InN nucleation also led to a generation of anti-phase domains.

Two-dimensional misorientation mapping by rocking dark-field transmission electron microscopy

In this paper we introduce an approach for precise orientation mapping of crystalline specimens by means of transmission electron microscopy. We show that local orientation values can be reconstructed from experimental dark-field image data acquired at different specimen tilts and multiple Bragg reflections. By using the suggested method it is also possible to determine the orientation of the tilt axis with respect to the image or diffraction pattern. The method has been implemented to automatically acquire the necessary data and then map crystal orientation for a given region of interest. We have applied this technique to a specimen prepared from a Ni-based super-alloy CMSX-4. The functionality and limitations of our method are discussed and compared to those of other techniques available.

Effects of Germanium, Copper, and Silver Substitutions on Hardness and Microstructure in Lean Al-Mg-Si Alloys

It is shown that strength loss in a 6060 Al-Mg-Si alloy caused by reduction in solute can be compensated by adding back smaller quantities of Ag, Ge, and Cu. Nine alloys were investigated. Ge was found to be the most effective addition, strongly refining the precipitation. The hardness is discussed in terms of statistics of the precipitates near a T6 condition, as acquired by transmission electron microscopy (TEM). Precipitates in some conditions were also investigated by high-angle annular dark-field scanning TEM. The added elements have strong influence on the main hardening precipitate, β″, changing its structure and promoting disorder.

Quantitative compositional analysis of InxGa1−xN/GaN multiquantum wells in light-emitting diodes

Abstract

Three-dimensional characterisation of the microstructure of an high entropy alloy using STEM/HAADF tomography

The microstructure of a high entropy alloy with composition of Mo0.5Al1Nb1Ta0.5Ti1Zr1 (the digits refer to molar volumes) has been characterised directly in three dimensions using TEM dark field (DF) imaging and by recording tilt pair micrographs using STEM high angle annular DF (HAADF) imaging. The microstructure contains disordered bcc precipitates that appeared as orthogonal stacks of plate-like features. A tapered needle sample was prepared in a focused ion beam/SEM and was used to acquire a 180° tomographic dataset of STEM/HAADF images in 2° increments. The tilt series images were registered, and the algebraic reconstruction technique was used to reconstruct the three-dimensional microstructure. The bcc precipitates were segmented using a combinative approach involving two threshold techniques. The precipitates were then visualised using commercial software, which revealed surprisingly the existence of both cuboidal and plate-like morphologies. Colouring each precipitate according to its morphology (determined using the omega-2 moment invariant) revealed a precipitate arrangement where plate-like features appeared parallel to each cuboid face.

Fluidized bed coupled rotary reactor for nanoparticles coating via atomic layer deposition.

A fluidized bed coupled rotary reactor has been designed for coating on nanoparticles (NPs) via atomic layer deposition. It consists of five major parts: reaction chamber, dosing and fluidizing section, pumping section, rotary manipulator components, as well as a double-layer cartridge for the storage of particles. In the deposition procedure, continuous fluidization of particles enlarges and homogenizes the void fraction in the particle bed, while rotation enhances the gas-solid interactions to stabilize fluidization. The particle cartridge presented here enables both the fluidization and rotation acting on the particle bed, demonstrated by the analysis of pressure drop. Moreover, enlarged interstitials and intense gas-solid contact under sufficient fluidizing velocity and proper rotation speed facilitate the precursor delivery throughout the particle bed and consequently provide a fast coating process. The cartridge can ensure precursors flowing through the particle bed exclusively to achieve high utilization without static exposure operation. By optimizing superficial gas velocities and rotation speeds, minimum pulse time for complete coating has been shortened in experiment, and in situ mass spectrometry showed the precursor usage can reach 90%. Inductively coupled plasma-optical emission spectroscopy results suggested a saturated growth of nanoscale Al2O3 films on spherical SiO2 NPs. Finally, the uniformity and composition of the shells were characterized by high angle annular dark field-transmission electron microscopy and energy dispersive X-ray spectroscopy.

Controlled building of CdSe@ZnS/Au and CdSe@ZnS/Au2S/Au nanohybrids

The addition of Au3+ to spherical amine-capped CdSe@ZnS nanoparticles in toluene at room temperature and under darkness can lead to ternary CdSe@ZnS/Au nanohybrids. We demonstrate that this happens only when the nanoparticles possess a relatively thin ZnS shell, thus showing that thickness plays a key role in gold deposition on the CdSe@ZnS nanoparticle surface. Our hypothesis is that the amine ligand acts as the reductant of Au3+ ions into Au+ ions, whose affinity for sulfur would keep them at the CdSe@ZnS surface. This interaction stabilizes the Au+ ion, making it less prone to reduction than a non-coordinated Au+ ion. In CdSe@ZnS with a thin shell, Au+ ions at the surface of, or most probably within, the ZnS shell cause the transfer of Cd2+ ions into the solution. Subsequently, the core Se2– anion, which is a better reductant than the shell S2–, reduces Au+ ions to Au(0), and large gold nanoparticles (AuNPs) are quickly deposited on the CdSe@ZnS surface in room temperature process, leading to ternary CdSe@ZnS/Au nanohybrids. In solution, these ternary nanohybrids progressively transform into quaternary CdSe@ZnS/Au2S/Au nanohybrids due to the reaction of the shell S2– anion with the remaining Au+ at the CdSe@ZnS surface, thus leading to the growth of Au2S nanoparticles on the CdSe@ZnS surface while Zn concomitantly leaches from the nanohybrid into the solution. Photoirradiation of the heterostructures with visible light enhances their emission efficiency. Comparatively, irradiation of the precursors, i.e., CdSe@ZnS nanoparticles, causes a drastic decrease in their emission accompanied by a blue shift of their emission maximum. The optical properties of these nanohybrids were analyzed by absorption and fluorescence (steady-state and time-resolved) spectroscopy, and the composition of the samples and the chemical states were determined by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS), respectively. Finally, the structural and morphological characterizations of the nanohybrids were performed by bright-field transmission electron microscopy (TEM), dark-field TEM, high-resolution TEM (HRTEM), and selected-area electron diffraction (SAED).

Improved Photocurrents for Water Oxidation by Using Metal–Organic Framework Derived Hybrid Porous Co3O4@Carbon/BiVO4 as a Photoanode

Efforts to improve the overall efficiency of photoelectrochemical water splitting by promoting interfacial charge transfer and suppressing electron-hole recombination remain a challenge. Highly porous Co3 O4 @carbon derived from metal-organic frameworks was successfully embedded on the surface of BiVO4 nanosheets. The Co3 O4 @carbon/BiVO4 composites were fully characterised by a variety of techniques, including powder XRD, diffuse reflectance UV/Vis spectroscopy, BET surface area calculations, SEM, TEM, energy-dispersive X-ray spectroscopy, high-angle annular dark-field scanning TEM, and X-ray photoelectron spectroscopy. The construction of a heterojunction between porous Co3 O4 (p-Co3 O4 ) and BiVO4 significantly promoted charge transfer and suppressed the recombination of holes and electrons. Consequently, the p-Co3 O4 @carbon/BiVO4 photoanode demonstrated approximately tenfold higher photocurrent density relative to bare BiVO4 and bulk Co3 O4 /BiVO4 for water oxidation under the same conditions. Also, this system displayed good durability for water oxidation and could be recycled at least three times without any loss of catalytic activity.

Sensitivity of thermo-electric power measurements to α–α′ phase separation in Cr-rich oxide dispersion strengthened steels

The sensitivity of thermo-electric power (TEP) measurement to detect the undesired α–α′ phase separation occurring in Cr-rich oxide dispersion strengthened steels was investigated. TEP values were found to increase with time of exposure to 475 °C—the maximum temperature of the miscibility gap in Fe–Cr alloys. Exposure to 650 °C did not induce any changes in the TEP value. By examining the bend contours in dark-field TEM images, α′ precipitates were evidenced in aged PM2000. Using the Fe–XCr–YAl model alloys and X-ray photoelectron spectroscopy, the changes in TEP were traced back to the depletion of Cr from the matrix, caused by the formation of the Cr-rich α′ phase. By quantifying the effect of Cr content on the TEP value of model alloys, it was estimated that following 1000 h of aging, the Cr concentration in the α matrix of PM2000 alloy was reduced from 20 to ~13.5 at.%.

Dark field transmission electron microscopy as a tool for identifying inorganic nanoparticles in biological matrices.

Dark field transmission electron microscopy has been applied herein to visualize the interactions of inorganic nanomaterials with biological systems. This new application of a known technique addresses a deficiency in status quo visualization techniques. High resolution and low noise images can be acquired to locate and identify crystalline nanoparticles in complex biological matrices. Moreover, through the composition of multiple images taken at different angular beam tilts, it is possible to image a majority of nanoparticles present at a site in dark field mode. This facilitates clarity regarding the internalization of nanomaterials in cellular systems. In addition, comparing dark field images recorded at different angular tilts yields insight into the character of nanoparticle faceting.

Dark-field Transmission Electron Microscopy Imaging Technique to Visualize the Local Structure of Two-dimensional Material; Graphene

Dark-field Transmission Electron Microscopy Imaging Technique to Visualize the Local Structure of Two-dimensional Material; Graphene