Scanning Transmission Electron Microscopy Analysis Research Articles

Melting kinetics, ultra-drawability and microstructure of nascent ultra-high molecular weight polyethylene powder

For the production of high-performance polyethylene fibers and tapes by ultra drawing, using solid-state processing of disentangled nascent ultra-high molecular weight polyethylene (UHMWPE), the maximum draw ratio is an important design parameter, as it determines the maximum degree of chain alignment and, therewith, the properties of the final product. It would, therefore, be advantageous to have a fast scanning method to estimate the maximum drawability of reactor powders. In the present work, the melting behavior of nascent UHMWPE reactor powder was studied by differential scanning calorimetry, followed by isoconversional analysis to evaluate the apparent activation energy barrier of melting for the different UHMWPE grades of different level of disentanglement. Powder samples were solid-sate processed into tapes at elevated temperature. Ultra drawing of these tapes at the optimum drawing temperature allowed for the evaluation of the maximum draw ratio (λmax) that is determined by the entanglement density. The morphology of the lamellar structure of the nascent powder was studied by small-angle X-ray scattering and by scanning transmission electron microscopy analyses. A correlation between the melting kinetics, the microstructure and the maximum draw ratio was observed for all grades, suggesting that the apparent activation energy of melting can be used as a screening method to estimate the maximum draw ratio.

Biologically reduced graphene oxide as a green and easily available photocatalyst for degradation of organic dyes

The disposal of untreated textile industrial wastewater having unmanageable pollutants is a global issue. Eco-friendly remediation technology is needed for the removal of environmental contaminants. In this study, a simple hydrothermal method is adapted to synthesis reduced graphene oxide (PErGO) using Phyllanthus emblica fruits extract and used as a photocatalyst for the degradation of synthetic toxic dyes. The physicochemical properties of green synthesized PErGO are confirmed using UV–Vis spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction. The ID/IG ratio is found as 1.02 for GO which is improved to 1.15 for PErGO, which confirms the existence of unrepaired defects after the elimination of negatively charged O2 moieties from the surface of GO. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis show well-exploited PErGO morphology. The photocatalytic removal of methylene blue (MB) and methyl orange (MO) dyes is confirmed using UV–vis spectrophotometer. PErGO shows about 92% of MO and 91% of MB degradation within 90 min of sunlight exposure while carried out as a mixed dye degradation. The sustainability of this catalyst is confirmed by testing it for five subsequent degradation cycles and noticed a stable and significant degradation activity. Outcomes from this study suggest that eco-friendly PErGO can be used as an alternate sustainable material to treat a large volume of wastewater from various dye industries.

Effects of La incorporation in catalytic activity of Ag/La-CeO2 catalysts for soot oxidation

Owing to strengthened regulations toward vehicle emissions, the use of diesel particulate filter technology to reduce particulate matter emissions has attracted significant attention. To achieve low temperature oxidation of particulate matter, numerous studies on Ag/CeO2 catalysts for soot oxidation have been reported. Herein, Ag/La-CeO2 catalysts with different La contents are synthesized and compared to analyze the effect of La. Hydrogen temperature programmed reduction analysis confirms that the reducibility increases with an increase in the La content in La-CeO2. X-ray photoelectron spectroscopy and Raman analysis confirm an increase of oxygen vacancies with La doping. Accordingly, the soot oxidation performances estimated by temperature programmed oxidation experiments increase with La doping. However, the catalytic activity of Ag/La-CeO2 exhibits a volcano trend. When an appropriate amount of La is incorporated in Ag/CeO2, peroxide generation and reducibility improve, thereby enhancing the soot oxidation performance. Conversely, the catalytic activities gradually decrease with excess La-doping. Scanning transmission electron microscopy analysis and density functional theory calculations confirm that excess amounts of La induce the sintering of Ag particles, which lead to the degradation of peroxide generation and reducibility of the catalysts. Consequently, an optimal amount of La incorporation on Ag/La-CeO2 results in the best soot oxidation performance.

Photoluminescence study of SBA-15@ZnO/Au nanocomposite for potential use in Staphylococcus aureus detection

Staphylococcus aureus is most common causes of hospital-acquired infections and food-associated disease. In the last years, sensing platform based on fluorescence- nanomaterials and nanocomposites have been attracted a great attention for detection of bacteria and macromolecules. In this study, a new modified mesoporous silica SBA-15 with ZnO and Au nanoparticles (SBA-15@ZnO/Au) was prepared to enhance the photoluminescence emission of SBA-15 matrix and investigate the potential use as a nano probe for detection of S. aureus bacteria. Au nanoparticles were added to the SBA-15 after loading ZnO nanoparticles and the SBA-15@ZnO/Au nanocomposite was successfully fabricated. The Structural, morphological, physicochemical and optical properties of the prepared samples have been studied. The low-angle X-ray diffraction (XRD) pattern showed highly ordered two-dimensional hexagonal lattice of SBA-15 and the wide-angle XRD pattern proved the formation of ZnO and Au nanoparticles. The results of the Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis showed that the prepared nanocomposite have a regular two dimensional hexagonal structure with cylindrical channels even after loading ZnO and Au nanoparticles. According to Nitrogen adsorption–desorption isotherm measurement, the pore diameter and the specific surface area of SBA-15@ZnO/Au nanocomposite have been obtained 3.15 nm and 604.19 m2/g. Photoluminescence (PL) spectra of bare SBA-15@ZnO/Au nanocomposite showed three peaks at central wavelength of 410, 500 and 750 nm arising from electron transfer process and radiative decay of collective plasmonic states. After addition of S. aureus bacteria, the nano probe-target complex was formed and the PL re-emission at 640 nm and quenching of PL emissions at 750 and 410 nm was observed. Furthermore, the new nanocomposite was demonstrated to have the proper sensitivity toward bacteria concentrations. The results can open promising prospect for this nanocomposite as a nano probe for detection of S. aureus in the UV–Visible region of electromagnetic spectrum.

On the Cobalt Carbide Formation in a Co/TiO2 Fischer-Tropsch Synthesis Catalyst as Studied by High-Pressure, Long-Term Operando X-ray Absorption and Diffraction.

Operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) were performed on a Co/TiO2 Fischer–Tropsch synthesis (FTS) catalyst at 16 bar for (at least) 48 h time-on-stream in both a synchrotron facility and a laboratory-based X-ray diffractometer. Cobalt carbide formation was observed earlier during FTS with operando XAS than with XRD. This apparent discrepancy is due to the higher sensitivity of XAS to a short-range order. Interestingly, in both cases, the product formation does not noticeably change when cobalt carbide formation is detected. This suggests that cobalt carbide formation is not a major deactivation mechanism, as is often suggested for FTS. Moreover, no cobalt oxide formation was detected by XAS or XRD. In other words, one of the classical proposals invoked to explain Co/TiO2 catalyst deactivation could not be supported by our operando X-ray characterization data obtained at close to industrially relevant reaction conditions. Furthermore, a bimodal cobalt particle distribution was observed by high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray analysis, while product formation remained relatively stable. The bimodal distribution is most probably due to the mobility and migration of the cobalt nanoparticles during FTS conditions.

Suppressed Stochastic Switching Behavior and Improved Synaptic Functions in an Atomic Switch Embedded with a 2D NbSe2 Material.

We investigated chemical vapor-deposited (CVD) two-dimensional (2D) niobium diselenide (NbSe2) material for the resistive switching and synaptic characteristics. Three different atomic switch devices with Ag/HfO2/Pt, Ag/Ti/HfO2/Pt, and Ag/NbSe2/HfO2/Pt were studied as both memory and neuromorphic devices. Both the inserted Ti and NbSe2 buffer layers effectively control the stochastic Ag-ion diffusion, leading to suppressed variation of switching characteristics, which is a critical issue in an atomic switch device. Especially, the device with the 2D NbSe2 buffer layer strikingly enhanced the device reliability in both endurance and retention. In conjunction with scanning transmission electron microscopy (STEM) and energy-dispersive spectrometry (EDS) analysis of the control of the Ag-ion migration, it was understood that filament connection is interrelated with the SET and RESET processes. Besides resistive behaviors in the memory device, various synapse functions such as spike-rate-dependent plasticity (SRDP), forgetting curve, potentiation, and depression were demonstrated with an atomic switch with the 2D NbSe2 buffer layer. Furthermore, the emulated long-term synaptic property was simulated using the MNIST 28 × 28 pixel database. Using adopting a CVD 2D NbSe2 blocking layer, the stochastic Ag-ion diffusion behavior is well-controlled and therefore stable switching and synapse functions are attained.

Design of halloysite modification for improvement of mechanical properties of the epoxy based nanocomposites

AbstractIn this study, halloysite nanotubes (HNT) were modified by: 3‐glycidyloxypropyltrimethoxysilane (GLYMO), 3‐aminopropyltrimethoxysilane (APTES), and 2,2‐Bis[4‐(glycidyloxy) phenyl] propane (DGEBA), and incorporated in the epoxy resin matrix to enhance its mechanical properties. The HNT/epoxy nanocomposite materials were prepared by mixing different ratios of untreated/treated HNT with neat epoxy resin. Characterization of untreated/treated HNT was performed by Fourier‐transformation infrared (FTIR) spectroscopy, and X‐ray diffraction (XRD). The quantity of grafted molecules and thermal stability of newly synthesized materials were determined by thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis. Tensile properties of newly synthesized materials were compared, and scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis of the fracture surfaces were performed. Incorporation of APTES modified HNT (HNTAPT) and two‐step modification APTES followed by DGEBA (HNTAPTDG) has increased the tensile strength of the nanocomposite materials up to 72% and 61%, and strain at break up to 1082% and 1216%, respectively, compared to neat epoxy. It was concluded that the modification of HNT contributed to the enhancement of the dispersion and the cross‐linking in the epoxy resin matrix.

A simple approach to synthesis Cr2CTx MXene for efficient hydrogen evolution reaction

Two-dimensional chromium carbide-based MXenes are predicted as an efficient catalyst for the hydrogen evolution reaction (HER). However, the lack of an effective approach to fabricate this promising material remains an impediment to practical applications. Herein, a simple strategy is proposed to synthesis Cr2CTx ultrathin structures from Cr2AlC MAX. The high-resolution analytical scanning transmission electron microscopy (STEM) corroborated with X-ray diffraction and scanning electron microscopy demonstrates that the Al atomic layers have been detached from the pristine MAX structure. Further atomic-resolution STEM confirms that the ultrathin Cr2CTx sheets with fluoride-free surface, nanostructured Cr2CO2, and Al2O3 phases are originated as by-products. The electrochemical test shows that the fabricated Cr2CTx composite with minimal Al2O3 have a good performance for hydrogen evolution reaction and exceed even that of Pt sheet catalyst at high current density. Moreover, high stability of more than 160 h without attenuation was observed to be better than the other MXenes catalytic performer to date. This work paves a gentle route to fabricate Cr2CTx MXene and demonstrates the potential application for HER.

Characterizing oxygen atoms in perovskite and pyrochlore oxides using ADF-STEM at a resolution of a few tens of picometers

We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis of perovskite (LaFeO3) and pyrochlore (Yb2Ti2O7 and Pr2Zr2O7) oxides and demonstrate that both the shape and contrast of visible atomic columns in annular dark-field (ADF) images are sensitive to the presence of nearby atoms of low atomic number (e.g. oxygen). We show that point defects (e.g. oxygen vacancies), which are invisible – or difficult to observe due to limited sensitivity – in x-ray and neutron diffraction measurements, are the origin of the complex magnetic ground state of pyrochlore oxides. In addition, we present a method by which light atoms can be resolved in the quantitative ADF-STEM images. Using this method, we resolved oxygen atoms in perovskite and pyrochlore oxides and propose this method to be suitable for other materials containing both light and heavy elements.

Dielectric properties of sub 20 nm homoepitaxial SrTiO3 thin film grown by molecular beam epitaxy using oxygen plasma

Epitaxial SrTiO 3 thin films were grown by molecular beam epitaxy (MBE) using an oxygen plasma source. SrO and TiO2 layers were alternately deposited on 0.5 wt% (001) Nb doped SrTiO3 substrates whose surface were terminated with TiO2. A two-dimensional (2-D) layer-by-layer growth mode was confirmed by the reflection high energy electron diffraction (RHEED) oscillation during the growth and the well-terraced SrTiO3 surface was obtained after growth. X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) analysis showed that the obtained SrTiO3 thin films had epitaxially (001) oriented and stoichiometric crystal structures without any interfacial layer. With simple MIS (Metal-Insulator-Semiconductor) capacitors using Ir top electrodes, the dielectric constants of the SrTiO3 thin film with 10, 15, 20, 100 nm were measured at room temperature, 10 kHz and zero bias.

Phase transformation from PbS to PbSe:S in hot-wall deposition of nanocomposite thin film with evaporation sources of PbS and ZnSe

Simultaneous evaporation of PbS and ZnSe using hot-wall deposition was investigated to prepare nanocomposite thin films. X-ray diffraction patterns indicated that the films formed a phase mixture of ZnSe and PbSe, suggesting that an evaporation source of PbS phase-transformed to PbSe during a film deposition. Wavelength-dispersive spectroscopy indicated that the composite contains a small amount of S below 1 at.%. High-angle annular dark-field scanning transmission electron microscopy and line scan analysis in electron energy-loss spectroscopy indicated that PbSe nanocrystals were dispersed in a ZnSe, while S tended to segregate in ZnSe matrix. Photocurrent spectra indicated that peak position at approximately 460 nm shifted toward a shorter wavelength as Pb concentration increased.

Segregation behavior and phase instability of Eurofer97 after neutron irradiation to 72 dpa

A reduced activation ferritic/martensitic steel, Eurofer97, was neutron irradiated in the vicinity of 300 °C in the High Flux Isotope Reactor (HFIR) up to 72 dpa. Advanced analytical scanning transmission electron microscopy and conventional transmission electron microscopy were applied to investigate the radiation-induced segregation and phase instability behavior after neutron irradiation. Amorphization was observed in M23C6 carbides. Cr-rich clusters were seen within the matrix, near the lath boundaries and close to the M23C6 carbides. Cr enrichment and Fe depletion were detected at both prior austenite grain boundaries and lath boundaries, despite different segregation magnitude. In addition, the enrichment of Ni, the depletion of V, and tiny cavities (presumably helium bubbles) are also found at lath boundaries. This work interrogates the evolution of microstructures after neutron irradiation, which provides detailed understanding on the microstructural aspects controlling the mechanical integrity of Eurofer97 under high-dose neutron damage.

Evolution of microstructure and strain field by precipitation during early ageing of Al–Si–Mg–Cu alloy

ABSTRACT The evolution of microstructure and the variation of strain field induced by precipitation in Al–Si–Mg–Cu alloy with high copper content during early ageing is characterised by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and geometric phase analysis (GPA) calculations. The basic building blocks for GP zones and step-shaped edges around subsequent precipitation are characterised. Incorporation of solute atoms, namely Si, Mg and Cu, accounts for the expansion of GP zones with core–shell structure and further precipitation transformation. The induced strain field rises as the ageing time increases and structural transformation proceeds, which results in a hardening of the sample.

Void engineering in epitaxially regrown GaAs-based photonic crystal surface emitting lasers by grating profile design

We report the engineering of air voids embedded in GaAs-based photonic crystal surface-emitting lasers realized by metalorganic vapor-phase epitaxy regrowth. Two distinct void geometries are obtained by modifying the photonic crystal grating profile within the reactor prior to regrowth. The mechanism of void formation is inferred from scanning transmission electron microscopy analysis, with the evolution of the growth front illustrated though the use of an AlAs/GaAs superlattice structure. Competition between rapid lateral growth of the (100) surface and slow diffusion across higher index planes is exploited in order to increase the void volume, leading to an order of magnitude reduction in threshold current and an increase in output power through an increase in the associated grating coupling strength.

Synthesis and Characterization of Ni-Pt Alloy Thin Films Prepared by Supercritical Fluid Chemical Deposition Technique

Ni-Pt alloy thin films have been successfully synthesized and characterized; the films were prepared by the supercritical fluid chemical deposition (SFCD) technique from Ni(hfac)2·3H2O and Pt(hfac)2 precursors by hydrogen reduction. The results indicated that the deposition rate of the Ni-Pt alloy thin films decreased with increasing Ni content and gradually increased as the precursor concentration was increased. The film peaks determined by X-ray diffraction shifted to lower diffraction angles with decreasing Ni content. The deposited films were single-phase polycrystalline Ni-Pt solid solution and it exhibited smooth, continuous, and uniform distribution on the substrate for all elemental compositions as determined by scanning electron microscopy and scanning transmission electron microscopy analyses. In the X-ray photoelectron spectroscopy (XPS) analysis, the intensity of the Pt 4f peaks of the films decreased as the Ni content increased, and vice versa for the Ni 2p peak intensities. Furthermore, based on the depth profiles determined by XPS, there was no evidence of atomic diffusion between Pt and Ni, which indicated alloy formation in the film. Therefore, Ni-Pt alloy films deposited by the SFCD technique can be used as a suitable model for catalytic reactions due to their high activity and good stability for various reactions.

Cd doped-Alpha-Dimanganese Trioxide Nanoparticles: Synthesis, Structural, Morphological, Optical, Luminescent, Magnetic, Photocatalytic and Antibacterial Characterization

In this paper, we present Cd doped alpha-dimanganese trioxide (Cd-A-DMTO) nanoparticles developed with the particle size of 25-28 nm via the sol-gel technique. The effects of 1-3% Cd concentrations on the crystal structure, size and strain, lattice constant, interplanar distance and unit cell volume values of the alpha-dimanganese trioxide (α-Mn2O3) samples were compared with the powder X-ray diffraction (XRD) results. The crystallite sizes are computed using the Scherrer's formula and the Williamson-Hall (W-H) plot using the XRD data for the Cd-A-DMTO samples, and their values are comparatively discussed. The powder XRD, and Raman spectral studies have shown the formation of the single crystalline cubic bixbyite phase of the prepared Cd-A-DMTO samples. Structural behaviour was investigated for the prepared Cd-A-DMTO samples by the Fourier Transform Infrared (FT-IR). The energy dispersive X-ray (EDX) analysis shows that the Cd has been incorporated into the manganese oxide. The spheres/rods, spindles/spheres and hexagonal/sphere-like particles are found in the 1%, 2% and 3% Cd-A-DMTO samples in the Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses. Superparamagnetic behavior of the Cd-A-DMTO samples was detected by the Vibrating Sample Magnetometer (VSM). The optical band gap increased and the PL emission intensity decreased with an increase in the concentration of the Cd dopant on the manganese oxide samples in the UV absorption and photoluminescence (PL) emission spectra. The photodegradation of the Congo red (CR) dye showed that the 3% Cd-A-DMTO photocatalyst had greater photocatalytic degradation efficiency and a rate constant compared to the 1% Cd-A-DMTO sample. The antibacterial activity of the Cd-A-DMTO samples was examined using selected bacterial pathogens.

Bipyridine-silica nanotubes with high bipyridine contents in the framework

Abstract Bipyridine-silica nanotubes (BPy-NTs) represent a solid chelate ligand for the formation of efficient heterogeneous metal complex catalysts. BPy-NTs are typically synthesized by the co-condensation of bipyridine (BPy)- and benzene (Ph)-bridged organosilane precursors. However, the amount of BPy in the framework has been limited to a maximum of 1.22 mmol g−1 owing to the difficulty in the formation of the NT structure. In this study, BPy-NTs with a large amount of BPy ligands (2.43 mmol g−1) were prepared from a reaction mixture with a high molar ratio of BPy precursor (up to 80 mol%) via the optimization of synthesis conditions. Stable nanotube structures with inner diameters in the range of 6.1–7.0 nm and lengths of tens to hundreds of nanometers were characterized using scanning transmission electron microscopy, N2 adsorption, and 29Si magic angle spinning nuclear magnetic resonance spectroscopy analyses. A large amount of Pt(bpy)Cl2 complexes were homogeneously immobilized on the NT walls (PtCl2@BPy-NTs), which was confirmed by high-angle annular dark-field scanning transmission electron microscopy, UV–vis absorption, and X-ray photoelectron spectroscopy analyses. PtCl2@BPy-NTs exhibited efficient photocatalysis for H2 evolution under visible light irradiation. The photocatalytic activity increased as the amount of Pt complexes loaded on the BPy-NTs increased. A small amount of Pt metal particles was formed on the BPy-NTs during the photoreaction, which promoted the H2 evolution reaction, as a catalyst with higher activity than only the Pt complex.

Optical properties of silicon-implanted polycrystalline diamond membranes

We investigate the optical properties of polycrystalline diamond membranes containing silicon-vacancy (SiV) color centers in combination with other nano-analytical techniques. We analyze the correlation between the Raman signal, the SiV emission, and the background luminescence in the crystalline grains and in the grain boundaries, identifying conditions for the addressability of single SiV centers. Moreover, we perform a scanning transmission electron microscopy (STEM) analysis, which associates the microscopic structure of the membranes and the evolution of the diamond crystals along the growth direction with the photoluminescence properties, as well as a time-of-flight secondary ion mass spectrometry (ToF-SIMS) to address the distribution of Si in implanted and un-implanted membranes. The results of the STEM and ToF-SIMS studies are consistent with the outcome of the optical measurements and provide useful insight into the preparation of polycrystalline samples for quantum nano-optics.

Facile synthesis and thermoluminescence properties of nano bio-ceramic \u03b2-Ca2P2O7:Dy phosphor irradiated with 75\xa0meV C6+ ion beam

Dy3+ doped β-Ca2P2O7 phosphor has been synthesized using wet chemical method. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis confirmed the formation of β-Ca2P2O7:Dy nano-phosphors. However, photoluminescence (PL) study was carried out to confirm the presence of dopant ion in the host matrix of β-Ca2P2O7:Dy material. Thermoluminescence (TL) glow curves of β-Ca2P2O7 were recorded for different concentrations of Dy3+ after exposure to various fluences of C6+ ion beam (75 meV). TL sensitivity of β-Ca2P2O7:Dy3+ (0.1 mol%) phosphor was 3.79 times more than commercially available CaSO4:Dy3+. TRIM code based on the Monte Carlo simulation was used to calculate the absorbed doses, ion range and main energy loss. Glow curve de-convolution (GCD) method was used to determine the number of TL peaks and their trapping parameters. The wide linear response of β-Ca2P2O7 nanoparticles along with high stability of TL glow curve makes this nanomaterial a good candidate for C6+ ion beam dosimetry.

Investigation of the Coherent Strain Evolution of the η' phase in Al–Zn–Mg–Cu alloys via scanning transmission electron microscopy

The coherent strain caused by the different sizes of the nanoscale η' phases in Al–Zn–Mg–Cu alloys was investigated via scanning transmission electron microscopy and geometric phase analysis. It is found that after the η' phase precipitates, a coherent strain field is generated in the matrix and inside the precipitated phase. Such coherent strain field changes with the thickness of the η' phase. The tensile strain is mainly distributed in the 7-layers-thick η' phase, whereas the compressive strain mainly exists in the 11-layers-thick η' phase. The asymmetrically coherent compressive strains are present in the matrices around the 7-layers-thick and 11-layers-thick η' phases, and the corresponding maximum values of the compressive strains are 11.2% and 14.4%, respectively. The quantitative characterization of the coherent strain field induced by the nanoscale η' phase provides a valuable reference for the optimization of the mechanical properties of the Al–Zn–Mg–Cu alloy.