Scanning Transmission Electron Microcopy Research Articles

Chemistry, Local Molybdenum Clustering, and Electrochemistry in the Li2+xMo1-xO3 Solid Solutions.

A broad range of cationic nonstoichiometry has been demonstrated for the Li-rich layered rock-salt-type oxide Li2MoO3, which has generally been considered as a phase with a well-defined chemical composition. Li2+xMo1-xO3 (-0.037 ≤ x ≤ 0.124) solid solutions were synthesized via hydrogen reduction of Li2MoO4 in the temperature range of 650-1100 °C, with x decreasing with the increase of the reduction temperature. The solid solutions adopt a monoclinically distorted O3-type layered average structure and demonstrate a robust local ordering of the Li cations and Mo3 triangular clusters within the mixed Li/Mo cationic layers. The local structure was scrutinized in detail by electron diffraction and aberration-corrected scanning transmission electron microcopy (STEM), resulting in an ordering model comprising a uniform distribution of the Mo3 clusters compatible with local electroneutrality and chemical composition. The geometry of the triangular clusters with their oxygen environment (Mo3O13 groups) has been directly visualized using differential phase contrast STEM imaging. The established local structure was used as input for density functional theory (DFT)-based calculations; they support the proposed atomic arrangement and provide a plausible explanation for the staircase galvanostatic charge profiles upon electrochemical Li+ extraction from Li2+xMo1-xO3 in Li cells. According to DFT, all electrochemical capacity in Li2+xMo1-xO3 solely originates from the cationic Mo redox process, which proceeds via oxidation of the Mo3 triangular clusters into bent Mo3 chains where the electronic capacity of the clusters depends on the initial chemical composition and Mo oxidation state defining the width of the first charge low-voltage plateau. Further oxidation at the high-voltage plateau proceeds through decomposition of the Mo3 chains into Mo2 dimers and further into individual Mo6+ cations.

Simultaneous doping of sulfur and chloride ions into ZnO nanorods for improved photocatalytic properties towards degradation of methylene blue

In this study, we have successfully doped sulfide (S−2) and chloride (Cl−1) simultaneously into ZnO by hydrothermal method. The structure, morphology, composition, and optical properties of the prepared S–Cl doped ZnO nanostructures were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microcopy (STEM), energy dispersive spectroscopy (EDX) and UV–visible (UV–Vis) spectroscopy techniques. After, doping of both S and Cl into ZnO no significant alteration in the nanorod morphology was noticed. The wurtzite crystal structure of ZnO was retained with a red shift in the 2Ɵ towards high angle. The XRD results are supported by the high-resolution transmission electron microscopy. The elemental analysis revealed a successful doping of S and Cl into ZnO structure. Importantly, the optical band gap of ZnO was decreased 2.9 eV upon doping of S and Cl ions. Thus, the nonmetal doping via S and Cl has significantly enhanced the photocatalytic properties of ZnO towards methylene blue (MB) due to high density of active sites and decreased charge recombination rate of electron-hole pairs. The photo-degradation efficiency towards MB reached 99.64% upon using S and Cl doped ZnO photocatalyst using optimum dose of 15 mg and initial dye concentration of 1.87 × 10−6 M. The use of simultaneous nonmetal doping concept could of great importance for the preparation of other nanostructured materials and their use in potential applications like solar cells, and photoelectrochemical water splitting.

Microstructure engineering of yttrium-doped barium zirconate thin films via seed layer technique

Proton conducting yttrium-doped barium zirconate (BZY) thin films have been successfully synthesized by chemical solution deposition (CSD). The seed layer technique is developed to obtain BZY films with different growth orientations, in combination with the layer-by-layer approach. The concentration of the precursor solution for the deposition of the seed layer is the decisive parameter to the final orientations and microstructures of BZY films. In particular, BZY thin films with the preferred (111) and (100) orientations have been fabricated on platinized silicon wafers, which are rarely reported in previous literature results. Meanwhile, the average grains size can be about 230 nm in the plane of the interface. In addition, these films with certain orientations and larger grains are obtained at a, compared to bulk BZY processing, relatively low annealing temperature (950 °C). The different nucleation behavior during the deposition of seed layers has been discussed to explain this relationship between the fabricating processes and final microstructures. For the BZY thin film with (111) orientation, annular dark field imaging in scanning transmission electron microcopy (HADDF-STEM) has been employed to reveal its porous and layered internal microstructure. In addition, proton conductivity of these thin films has been evaluated by electrochemical impedance spectroscopy (EIS) at intermediate temperatures (350–600 °C).

Synthesis of Au@Pt Core-Shell Nanoparticles as Efficient Electrocatalyst for Methanol Electro-Oxidation.

Bimetallic Au@Pt nanoparticles (NPs) with Pt monolayer shell are of much interest for applications in heterogeneous catalysts because of enhanced catalytic activity and very low Pt-utilization. However, precisely controlled synthesis with uniform Pt-monolayers and stability on the AuNPs seeds remain elusive. Herein, we report the controlled deposition of Pt-monolayer onto uniform AuNPs seeds to obtain Au@Pt core–shell NPs and their Pt-coverage dependent electrocatalytic activity for methanol electro-oxidation. The atomic ratio between Au/Pt was effectively tuned by varying the precursor solution ratio in the reaction solution. The morphology and atomic structure of the Au@Pt NPs were analyzed by high-resolution scanning transmission electron microcopy (HR-STEM) and X-ray diffraction (XRD) techniques. The results demonstrated that the Au@Pt core–shell NPs with Pt-shell thickness (atomic ratio 1:2) exhibit higher electrocatalytic activity for methanol electro-oxidation reaction, whereas higher and lower Pt ratios showed less overall catalytic performance. Such higher catalytic performance of Au@Pt NPs (1:2) can be attributed to the weakened CO binding on the Pt/monolayers surface. Our present synthesis strategy and optimization of the catalytic activity of Au@Pt core–shell NPs catalysts provide promising approach to rationally design highly active catalysts with less Pt-usage for high performance electrocatalysts for applications in fuel cells.

Development of Fe/N-Doped Carbon Nanotubes As a Stable Non-Precious Electrocatalyst for Oxygen Reduction Reaction

Heteroatom-doped carbon materials, particularly carbons co-doped with single atomic transition metals (e.g. Fe) and nitrogen, have emerged as one of the most competitive non-precious electrocatalysts for the cathodic oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells to replace precious platinum (Pt) catalyst. Most carbon-based non-precious electrocatalysts, however, were built on conventional carbon materials such as activated carbon and more recently metal-organic-framework-derived carbon, both of which feature essential drawbacks of carbon corrosion and low electron conductivity due to their low extent of graphitization.1-2 We report here the direct synthesis of well-defined CNTs co-doped with a high density of Fe-Nx active sites (denoted as Fe-N/CNT) through a solid-phase catalytic growth approach, where Fe precursor catalyzed the growth of CNTs from melamine sponge precursor under NH3 atmosphere and simultaneously doped in-situ into the CNT walls with nitrogen. The atomic structure and electrocatalytic properties of the Fe-N/CNT electrocatalysts were characterized by aberration-corrected scanning transmission electron microcopy, Mößbauer spectroscopy, electrochemical analysis and density functional theory calculations. We further design and synthesis a hierarchical Fe-N/CNT electrocatalyst where primary Fe-N/CNTs are enwrapped with secondary Fe-N/CNTs in order to increase the proportion of the Fe-N/CNT active site. The optimized Fe-N/CNT catalyst featured with a high degree of graphitization and high density of Fe-Nx catalytic sites, demonstrating both high ORR activity and remarkable stability in both half-cell test and a single hydrogen fuel cell membrane-electrode-assembly test. Reference: [1] Lefevre M, Proietti E., Jaouen F., Dodelet J. Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science, 324, 71-74 (2009). [2] Chen, YJ. Ji SF., Wang D.S., Li Y.D., Angew Chem Int Ed, 56, 6937-6941 (2017) [3] Xia D.S., Wang R.Z., Wei Y.P., Gan L., Kang F.Y. Melamine-sponge-derived non-precious fuel cell electrocatalyst with hierarchical pores and tunable nitrogen chemical states for exceptional oxygen reduction reaction activity. Materials Today Energy, 9, 271-278 (2018)

Silver Modified Cathodes for Solid Oxide Fuel Cells

La1-x-ySrxAgyMnO3-δ was synthesized via modified Pechini method. Silver (Ag) was incorporated into La1-x-ySrxAgyMnO3-δ cathode material for solid oxide fuel cells as a catalyst precursor for oxygen reduction reaction. The La1-x-ySrxAgyMnO3-δ perovskite single phase formed at 800°C, and the solubility of Ag in the crystal lattice of the perovskite was up to 5 mol%. The single-phase materials revealed thermal stability in different oxidizing atmospheres up to sintering temperature of the cathode at 1050°C. The exsolution of the metallic Ag nanoparticles was performed at 350–600°C in reducing atmosphere. Scanning electron and scanning transmission electron microcopy revealed a good mechanical contact of the Ag nanoparticles to the surface of the perovskite after reducing conditions. The electrochemical tests of the materials showed a good electrocatalytic effect of nanosized Ag toward oxygen reduction reaction. The electrochemical performance of the cathodes revealed the dependence on electrolyte material and exsolution time.

The role of oxygen doping on elemental intermixing at the PVD‐CdS/Cu (InGa)Se2 heterojunction

AbstractElemental intermixing at the CdS/CuIn1−xGaxSe2 (CIGS) heterojunction in thin‐film photovoltaic devices plays a crucial role in carrier separation and thus device efficiency. Using scanning transmission electron microcopy in combination with energy dispersive X‐ray mapping, we find that by controlling the oxygen in the sputtering gas during physical vapor deposition (PVD) of the CdS, we can tailor the degree of elemental intermixing. More oxygen suppresses Cu migration from the CIGS into the CdS, while facilitating Zn doping in the CdS from the ZnO transparent contact. Very high oxygen levels induce nanocrystallinity in the CdS, while moderate or no oxygen content can promote complete CdS epitaxy on the CIGS grains. Regions of cubic Cu2S phase were observed in the Cu‐rich CdCuS when no oxygen is included in the CdS deposition process. In the process‐of‐record sample (moderate O2) that exhibits the highest solar conversion efficiency, we observe a ~26‐nm‐thick Cu‐deficient CIGS surface counter‐doped with the highest Cd concentration among all of the samples. Cd movement into the CIGS was found to be less than 10 nm deep for samples with either high or zero O2. The results are consistent with the expectation that Cd doping of the CIGS surface and lack of Zn diffusion into the buffer both enhance device performance.

Characterisation of surface recrystallisation in a grit-blasted single-crystal superalloy

ABSTRACTIn this work, detailed experimental insights regarding the microstructural features on surface recrystallisation of a grit-blasted single crystal superalloy DD6 have been obtained by transmission electron microcopy (TEM), scanning transmission electron microcopy (STEM), scanning electron microscopy and electron backscattered diffraction (EBSD) techniques. The broadened diffraction ring arcs typical for micro-textures revealed the fragmented finer grains after surface grit-blasting. Energy dispersive spectroscopy analysis and STEM mapping confirmed the characteristic elements for differentiation of γ′ and γ phase, and the precipitated secondary or tertiary γ′ in the recrystallised region was characterised by dark field TEM. Moreover, the recrystallisation depth was feasibly revealed by ion-etching and further used for EBSD imaging without polishing. The resulted EBSD pole figures and misorientation profiles confirmed that the recrystallised region exhibited a rather random orientation, destroying the original single crystal type.

Subsurface Distribution of Antisite Defects in LiMnPO4: Direct Comparison with LiFePO4

We demonstrate a thermodynamically stable distribution of antisite exchange defects within a single LiMnPO4 crystal through a combination of atomic-resolution scanning transmission electron microcopy and ab initio calculations. In contrast to the strong segregation of exchange defects in subsurface layers with selective orientations in LiFePO4, the highly ordered arrangement between Li and Mn in the bulk is preserved near the surface of LiMnPO4 crystals, showing no preferential aggregation of defects irrespective of the location in a particle. On the basis of the identical crystal structure and the analogous compositional basis of the two olivine phosphates, the distinct distribution characteristics of the antisite exchange defects are notably unexpected findings, highlighting the value of direct visualization at an atomic scale.

Distortion Correction in Scanning Transmission Electron Microcopy with Controllable Scanning Pathways

Journal Article Distortion Correction in Scanning Transmission Electron Microcopy with Controllable Scanning Pathways Get access Xiahan Sang, Xiahan Sang Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TNInstitute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Andrew R Lupini, Andrew R Lupini Institute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TNMaterials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Raymond R Unocic, Raymond R Unocic Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TNInstitute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Tricia L Meyer, Tricia L Meyer Materials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Thomas Z Ward, Thomas Z Ward Materials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Ho Nyung Lee, Ho Nyung Lee Materials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Eirik Endeve, Eirik Endeve Institute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TNComputer Science and Mathematics, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Richard K Archibald, Richard K Archibald Institute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TNComputer Science and Mathematics, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Albina Y Borisevich, Albina Y Borisevich Institute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TNMaterials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Sergei V Kalinin, Sergei V Kalinin Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TNInstitute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar ... Show more Stephen Jesse Stephen Jesse Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TNInstitute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TN Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 22, Issue S3, 1 July 2016, Pages 900–901, https://doi.org/10.1017/S1431927616005341 Published: 25 July 2016

Sub-solvus Cellular Recrystallization and P Phase Formation in a Single-Crystal Superalloy Containing Re

Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated cellular recrystallization accompanied with formation of twin structure and TCP phase of P during annealing at sub-solvus temperature of 1,100 °C. The precipitation of P phase is considered to be attributed to the coarsening of γ′ phase in the recrystallized aggregates which lower the activation energy for atomic migration.

Dealloying of Noble-Metal Alloy Nanoparticles

Dealloying is currently used to tailor the morphology and composition of nanoparticles and bulk solids for a variety of applications including catalysis, energy storage, sensing, actuation, supercapacitors, and radiation damage resistant materials. The known morphologies, which evolve on dealloying of nanoparticles, include core-shell, hollow core-shell, and porous nanoparticles. Here we present results examining the fixed voltage dealloying of AgAu alloy particles in the size range of 2-6 and 20-55 nm. High-angle annular dark-field scanning transmission electron microcopy, energy dispersive, and electron energy loss spectroscopy are used to characterize the size, morphology, and composition of the dealloyed nanoparticles. Our results demonstrate that above the potential corresponding to Ag(+)/Ag equilibrium only core-shell structures evolve in the 2-6 nm diameter particles. Dealloying of the 20-55 nm particles results and in the formation of porous structures analogous to the behavior observed for the corresponding bulk alloy. A statistical analysis that includes the composition and particle size distributions characterizing the larger particles demonstrates that the formation of porous nanoparticles occurs at a well-defined thermodynamic critical potential.

Polyelectrolyte–Surfactant Complexes of Poly[3,5-bis(dimethylaminomethyl)-4-hydroxystyrene]-block-poly(ethylene oxide) and Sodium Dodecyl Sulfate: Anomalous Self-Assembly Behavior

Polyelectrolyte-surfactant complexes (PE-S) formed by double hydrophilic cationic polyelectrolyte poly[3,5-bis(dimethylaminomethyl)-4-hydroxystyrene]-block-poly(ethylene oxide) (NPHOS-PEO) and anionic surfactant sodium dodecyl sulfate (SDS) in acidic aqueous solutions were studied by light scattering, SAXS, and scanning transmission electron microcopy in the environmental mode (wet-STEM) for various stoichiometric ratios between the numbers of SDS anions and dimethylaminomethyl groups of NPHOS in the complex. The obtained results show that the NPHOS-PEO/SDS system behaves differently from other systems of double hydrophilic block polyelectrolyte and oppositely charged ionic surfactant because it forms water-insoluble PE-S for compositions close to the zero net charge of the complex. This phase separation occurs, instead of the PE-S rearrangement to core-shell particles, which is hindered due to conformational rigidity of the NPHOS blocks. For the surfactant amounts below and above the precipitation region, large spherical aggregates and their clusters are present in the solution. SAXS measurements indicate that although the NPHOS-PEO/SDS system does not form the core-shell particles with the NPHOS/SDS core and the PEO shell as other PE-S of double hydrophilic polyelectrolytes, the aggregates contain domains of closely packed surfactant micelles which bind to both NPHOS polyelectrolyte blocks and PEO blocks.

A review of MBE grown 0D, 1D and 2D quantum structures in a nanowire

We review different strategies to achieve a three-dimensional energy bandgap modulation in a nanowire (NW) by the introduction of self-assembled 0D, 1D and 2D quantum structures, quantum dots (QDs), quantum wires (QWRs) and quantum wells (QWs). Starting with the well-known axial, radial (coaxial/prismatic) or polytypic quantum wells in GaN/AlN, GaAs/AlAs or wurtzite/zinc-blende systems, respectively, we move to more sophisticated structures by lowering their dimensionality. New recent approaches developed for the self-assembly of GaN quantum wires and InAs or AlGaAs quantum dots on single nanowire templates are reported and discussed. Aberration corrected scanning transmission electron microcopy is presented as a powerful tool to determine the structure and morphology at the atomic scale allowing for the creation of 3D atomic models that can help us to understand the enhanced optical properties of these advanced quantum structures.

Single shot damage mechanism of Mo/Si multilayer optics under intense pulsed XUV-exposure

We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference-polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources.

Structural Characterization and Luminescence of Porous Single Crystalline ZnO Nanodisks with Sponge-like Morphology

We report the synthesis of porous single crystalline ZnO nanodisks with sponge-like morphology through a wet chemical approach. To our best knowledge, this is the first report about highly porous single crystalline nanodisks of ZnO with an average diameter of ∼100 nm. The ZnO nanodisks exhibit strong visible (blue-green) light emission on UV excitation. Scanning Transmission Electron Microcopy (STEM), High-Resolution Transmission Electron Microscopy (HRTEM), and Selected Area Electron Diffraction (SAED) were performed to confirm that the nanodisks are single crystalline and porous in nature. The porosity of the nanodisks gives them the sponge-like appearance. Energy Dispersive X-ray Spectrometry (EDS) and Electron Energy Loss Spectrometry (EELS) analysis of the nanodisks together with high-resolution electron microscopy and photoluminescence measurements were used to determine the cause of the visible emission and its relation to the sponge-like morphology and growth mechanism. The larger surface area to volume ratio of these sponge-like nanostructures makes them very attractive for applications like biochemical sensors and solar cells.

Quantitative STEM and HRTEM Studies on Au Metallic Nano-Catalysts

AbstractNano-catalysts, Au nano-particles on TiO2 (anatase), were studied by means of quantitative scanning transmission electron microcopy (Q-STEM) and high-resolution transmission electron microcopy (HRTEM). All the Au nano-catalysts were produced from an Au13 precursor, Au13[PPh3]4[S(CH2)11CH3]4, TiO2 supports, with three treatments: (1) thermal heating in the air at 400°C for 2 hours, (2) exposure to ozone (O3) at room temperature, and (3) exposure to atomic oxygen (AO, or O) with a AO dose of 7.3 ×1018 atom/cm2 at room temperature. Both reactive oxygen species O3 and AO produced significantly small sizes of Au particles as compared to those from the heating treatment in the air (2.7 ± 0.6 nm, 324 b 264 atoms). Ozone produced the smallest (1.2 ± 0.5 nm, 40 ± 49 atoms), whereas AO produced smaller (2.1 ± 0.7 nm, 72 ± 98 atoms), with a broad size distribution and a variety of shapes. HRTEM studies on the AO treated Au/TiO2 samples found that there could exist relationship between the particles size and their shapes which also affected by the interaction with TiO2 supports. The support effect of TiO2 to the shapes of Au particles was also studied.