Annular Dark Field STEM Imaging Research Articles

Constrained patterning of orientated metal chalcogenide nanowires and their growth mechanism

One-dimensional metallic transition-metal chalcogenide nanowires (TMC-NWs) hold promise for interconnecting devices built on two-dimensional (2D) transition-metal dichalcogenides, but only isotropic growth has so far been demonstrated. Here we show the direct patterning of highly oriented Mo6Te6 NWs in 2D molybdenum ditelluride (MoTe2) using graphite as confined encapsulation layers under external stimuli. The atomic structural transition is studied through in-situ electrical biasing the fabricated heterostructure in a scanning transmission electron microscope. Atomic resolution high-angle annular dark-field STEM images reveal that the conversion of Mo6Te6 NWs from MoTe2 occurs only along specific directions. Combined with first-principles calculations, we attribute the oriented growth to the local Joule-heating induced by electrical bias near the interface of the graphite-MoTe2 heterostructure and the confinement effect generated by graphite. Using the same strategy, we fabricate oriented NWs confined in graphite as lateral contact electrodes in the 2H-MoTe2 FET, achieving a low Schottky barrier of 11.5 meV, and low contact resistance of 43.7 Ω µm at the metal-NW interface. Our work introduces possible approaches to fabricate oriented NWs for interconnections in flexible 2D nanoelectronics through direct metal phase patterning.

Effects of shear strain on HZO ferroelectric orthorhombic phases

The stabilities of hafnium and zirconium oxide ferroelectric orthorhombic phases, oIII-phase (Pca21) and oIV-phase (Pmn21), under shear strain are investigated theoretically by atomic modeling with density functional theory calculations. The results indicate that oIV-phase serves as a buffer state preventing oIII-phase from transforming into m-phase (P21/c) under structural distortion caused by shear strain and meanwhile preserves the remanent polarization to some certain degree. Shear strain will also induce the reduction in coercive field of the HZO film due to the distortion of oIII-phase or phase transition into oIV-phase. It is very possible to identify oIV-phase with careful analysis of high-resolution transmission electron microscopy and high-angle annular dark-field STEM images at the region having a crystal tilt angle smaller than 86°.

Negative-Pressure-Induced Large Polarization in Nanosized PbTiO3.

Ferroelectric materials usually undergo decay with particle size decreasing into the nanoscale. At the critical value, the crystal structure undergoes a transition from the ferroelectric to paraelectric phase and the ferroelectricity vanishes. It is a big issue to sufficiently maintain strong ferroelectricity at the nanoscale. Herein, it is reported that synthesized 0D freestanding PbTiO3 nanoparticles (NPs) present negative pressure along the c axis (Δc/cbulk × 100% =-2.406), inducing large spontaneous polarization PS (71.2 µC cm-2 in 12nm). Further local structural studies by atomic pair distribution functions and extended X-ray absorption fine structure indicate the structural evolution of nanosized PbTiO3 . High-angle annular dark-field STEM images reveal the existence of preponderant PbO-terminations on the surface of the PbTiO3 NPs. Ab initio calculation reveals the enhanced hybridization between Pb and O ions, which gives rise to the negative pressure and tensile stress to stabilize the high tetragonality and large polarization. The present work demonstrates an untraditional route to enhance the ferroelectricity and related properties in functional nanostructured materials, being of significance to nanodevices.

Lattice distortion and atomic ordering of the sigma precipitates in CoCrFeNiMo high-entropy alloy

This study aims to understand the critical lattice distortion effect of σ-phase precipitation in CoCrFeNiMo high-entropy alloys (HEAs) using various quantitative methods. The high-angle annular dark-field (HAADF) STEM image illustrates the actual atomic arrangement of materials. From the σ-phase atom image, we can distinctly observe the atomic distortion and realize that the lattice constant coefficient of variation is approximately 2.9%, which is similar to the difference in the lattice constant calculated using XRD (ε∼2.74%). The values 2.9% and ∼2.74% represent the values of lattice distortion from the microscopic and macroscopic perspectives, respectively; however, they are significantly different from the lattice distortion value (∼9.9%) obtained from atomic size difference. Therefore, it is speculated that factors such as atomic bonding and electronic structure can significantly affect the lattice distortion effects. In addition, we use EELS elemental mappings at the atomic scale for understanding compositional distributions in the σ phase. We found that 8j sites have Mo, Fe, Co, and Cr signals; however, they do not contain Ni signal. Based on these findings, it can be explained that the ordering degree of the σ phase is neither entirely ordered nor disordered. This study not only provides the most direct evidence of the critical lattice distortion effect of high-entropy alloys but also proves that the σ phase is not entirely disordered, unlike the conventional HEAs.

Identification of interdendritic phases in Ni[sbnd]30Cr weld metal with additions of tantalum and molybdenum using electron diffraction pattern and high-resolution scanning transmission electron microscopy image analysis

Ta-rich γ/MC eutectic is the predominant interdendritic constituent that forms upon solidification in Ni30Cr weld metal with Ta and Mo additions. In addition, a very small amount of sigma phase (σ) forms on weld solidification. These terminal solidification constituents were investigated in detail by a combination of various methods in transmission electron microscopy (TEM): selected area electron diffraction (SAED) analysis and associated SAED pattern simulation, high-angle annular dark-field (HAADF) STEM imaging and associated image simulation, as well as energy-dispersive X-ray spectroscopy (STEM-EDS). Ta-rich γ/MC and tetragonal sigma (σ) phase were observed based on acquisition, analysis and simulation of six SAED patterns by specimen tilt into six defined zone axis orientations. The presence of sigma (σ) phase was further validated and its atomic configuration was revealed by high-resolution HAADF-STEM imaging in combination with HAADF-STEM image simulation experiments.

Study on the surface segregation evolution of Ag-TiOx nanocomposite coatings

The objective of this study is to systematically investigate the evolution of silver surface segregation of silver/titanium oxide nanocomposite coatings produced by reactive sputtering process. Different thickness barrier layers were deposited on the surface of Ag-TiOx two-dimensional (2D) and three-dimensional (3D) composite coatings at different deposition rates. In 2D mode, the SEM images and XPS patterns clearly show that the still strong silver segregation occurred even 50 nm thickness barrier coated with very low deposition rate (3 nm/min), while a sharp decrease of silver surface segregation can be achieved when high barrier deposition rate (20 nm/min) was applied. The high angle annular dark field (HAADF) STEM images reveal that the high silver mobility can be effectively suppressed under high barrier deposition rate. Similar phenomenon can be observed in 3D Ag-TiOx composite coatings. Moreover, the comparable study of the metallic titanium barrier layers show that a 30 nm barrier layer is enough to cover all the silver NPs, and silver atoms diffusion onto the surface was stopped under pure Ar atmosphere. The silver surface segregation evolution indicates that silver segregation could be the result of the combined effect of sputtering plasma and activated oxygen environment.

Functionalized MoS2 supported core-shell Ag@Au nanoclusters for managing electronic processes in photocatalysis

Metal nanoclusters (MNCs) based on gold as core and silver as shell, abbreviated as Ag@AuNCs, were electrostatically associated with functionalized semiconducting MoS2 nanosheets, abbreviated as f-MoS2. The realization of the Ag@AuNCs/f-MoS2 ensemble was manifested by UV–vis and photoluminescence titration assays, while its morphology was evidenced by high-angle annular dark-field STEM imaging coupled with EDX analyses. Photoinduced intra-ensemble charge-transfer phenomena were witnessed and attributed to the suppression and concomitant shift of Ag@AuNC photoluminescence by incremental additions of f-MoS2. The Ag@AuNCs/f-MoS2 was employed as RhB photodegradation catalyst for wastewater purification from organic pollutant dyes. The production of highly oxidant radicals from the photogenerated electron-hole pairs within Ag@AuNCs/f-MoS2 were found to be the main reactive species, while the holes left on Ag@AuNCs attacked RhB, as evidenced by the disappearance of the characteristic absorption peak. The photocatalytic reaction by the ensemble followed fast kinetics as compared with bare Ag@AuNCs or individual AuNCs and AgNCs.

Characterization of thin film displacements in the electron microscope

It is important to characterize behavior along the normal (z) direction to the plane of a thin film to enable three-dimensional reconstruction at atomic-resolution. In this study, we have investigated displacements of a specimen along the z direction using dark field high angle annular dark field STEM images of single atoms as a function of specimen tilt. These showed an elongation perpendicular to the tilt axis. Experimental measurements of the contrast of single atoms indicate fluctuations in the film along the z direction with a displacement amplitude of 0.6 nm.

StatSTEM: An efficient program for accurate and precise model-based quantification of atomic resolution electron microscopy images

An efficient model-based estimation algorithm is introduced in order to quantify the atomic column positions and intensities from atomic resolution (scanning) transmission electron microscopy ((S)TEM) images. This algorithm uses the least squares estimator on image segments containing individual columns fully accounting for the overlap between neighbouring columns, enabling the analysis of a large field of view. For this algorithm, the accuracy and precision with which measurements for the atomic column positions and scattering cross-sections from annular dark field (ADF) STEM images can be estimated, is investigated. The highest attainable precision is reached even for low dose images. Furthermore, advantages of the model-based approach taking into account overlap between neighbouring columns are highlighted. To provide end-users this well-established quantification method, a user friendly program, StatSTEM, is developed which is freely available under a GNU public license.

The Role of Gas in Determining Image Quality and Resolution During In Situ Scanning Transmission Electron Microscopy Experiments

AbstractAs gas–solid heterogeneous catalytic reactions are molecular in nature, a full mechanistic understanding of the process requires atomic‐scale characterization under realistic operating conditions. Although atomic resolution imaging has become routine in modern high‐vacuum (scanning) transmission electron microscopy ((S)TEM), both image quality and resolution nominally degrade on introduction of the reaction gases. In this work, we systematically assess the effects of different gases at various pressures on the quality and resolution of images obtained at room temperature in the annular dark field STEM imaging mode by using a differentially pumped (DP) gas cell. This imaging mode is largely free from inelastic scattering effects induced by the presence of gases and retains good imaging properties over a wide range of gas mass/pressures. We demonstrate the application of ESTEM with atomic resolution images for a complex oxide alkane oxidation catalyst MoVNbTeOx (M1) immersed in light and heavy gas environments.

Anodic Activity of Pt and PtRu Catalysts on Ta-Doped SnO2 Supports for Direct-Fuel Fuel Cells Using a New Energy Carrier

Polymer electrolyte fuel cells (PEFCs) directly fueled liquid fuels are suitable power source for portable energy applications. Liquid alcohol fuels such as methanol and ethanol are attractive energy carriers with good energy density and low cost. However, the problems of their low flash points (below room temperature) and/or toxicity should be overcome for the safe application of alcohol-fueled PEFCs. The liquid oligomers of poly oxymethylene dimethyl ether (POMMn: CH3-O-(CH2-O)n-CH3, n = 3~8) have been proposed as alternative fuels, due to their advantages, including their higher energy density, lower toxicity and higher flash point1-3). In this study, we synthesized Pt and PtRu catalysts supported on Ta-SnO2 (Pt/Ta-SnO2, PtRu/Ta-SnO2) for the direct oxidation of POMM and evaluated their performance in a PEFC. The Ta-SnO2 support4) was synthesized by the flame spray synthesis method. The oxide supports obtained were nanometer-sized particles with a carbon-like fused-aggregate structure. Pt and PtRu catalysts were loaded on the Ta-SnO2 by the colloidal method. The Pt/Ta-SnO2 (Pt loading amount, 14.0 wt%; particle size, 6.3 nm) and PtRu/Ta-SnO2 (Pt/Ru loading amount, 6.3/3.5 wt%; particle size, 2.2 nm) were obtained after heat treatment. The Pt and PtRu particles were highly dispersed on the Ta-SnO2 support. As shown in the high-angle annular dark-field STEM (HAADF-STEM) image and STEM-EDX line profile, Sn metal diffused into the Pt catalysts with the sintering process. X-ray photoemission spectroscopic (XPS) Sn 3d5/2 spectra for each catalyst indicated that part of the Sn was converted from Sn4+ to Sn0. We concluded that the catalysts formed binary Pt-Sn and ternary Pt-Ru-Sn alloys during the sintering procedure (Fig. 1). The onset potential for POMM2 oxidation on PtRu/Ta-SnO2, which was measured by a half-cell with a rotating disk electrode (RDE), was 0.3 V less positive than that of a commercial Pt2Ru3/CB catalyst (Fig. 2). The mass activity for POMM2 oxidation of PtRu/Ta-SnO2 at 0.5 V was also 3.5 times higher than that for a commercial Pt2Ru3/CB catalyst. The anodic activity of Pt/Ta-SnO2 was also higher than that of the commercial catalyst, by a factor of 2. The electrochemical performances of these anode catalysts were also evaluated by the use of membrane-electrode assemblies (MEAs, NRE117 membrane, commercial Pt/CB(TEC10E50E) 0.5 ± 0.025 mgPt cm2 cathode, Pt/Ta-SnO2 or PtRu/Ta-SnO2 0.25 or 0.27 mgPt cm2 anode, Fig. 3). The current densities of the MEAs using the Pt/Ta-SnO2 and PtRu/Ta-SnO2 catalysts as the anodes and Pt/CB as the cathode (POMM2 simulated fuel, O2 oxidant, 80oC operation temperature) were more than 10 times higher than that using the commercial Pt2Ru3/CB anode. The current density at 0.6 V for formaldehyde fuel supplied to an MEA applying the Pt/Ta-SnO2anode was 15 times higher than that for the commercial anode, even though the performance for methanol remained at the same level. We consider that the Sn on the surface of the Pt catalyst may suppress CO poisoning by weakening the adsorption strength of CO formed during the fuel oxidation processes, even at such low potentials as 0.2 to 0.4 V, and directly accelerate the oxidation of carbon monoxide at more positive potentials by a bifunctional mechanism, similar to that for Pt-Ru catalysts. Acknowledgements This work was partially supported by funds for the A-STEP project from the Japan Science and Technology Agency (JST), JSPS KAKENHI (B24350093) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and the SPer-FC project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References 1) D. Devaux, H. Yano, H. Uchida, J.L. Dubois, M. Watanabe, Electrochim. Acta 56 (2011) 1460. 2) S. Baranton, H. Uchida, D.A. Tryk, J.L. Dubois, M. Watanabe, Electrochim. Acta 108 (2013) 350. 3) K. Kakinuma, I.T. Kim, Y. Senoo, H. Yano, M. Watanabe, and M. Uchida, ACS Appl. Mater. Interfaces, 6 (2014) 22138. 4) Y. Senoo, K. Taniguchi, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, Electrochem. Commun., 51 (2015) 37. Figure 1

StatSTEM: An efficient approach for accurate and precise model-based quantification of atomic resolution electron microscopy images

An efficient model-based estimation algorithm is introduced to quantify the atomic column positions and intensities from atomic resolution (scanning) transmission electron microscopy ((S)TEM) images. This algorithm uses the least squares estimator on image segments containing individual columns fully accounting for overlap between neighbouring columns, enabling the analysis of a large field of view. For this algorithm, the accuracy and precision with which measurements for the atomic column positions and scattering cross-sections from annular dark field (ADF) STEM images can be estimated, has been investigated. The highest attainable precision is reached even for low dose images. Furthermore, the advantages of the model-based approach taking into account overlap between neighbouring columns are highlighted. This is done for the estimation of the distance between two neighbouring columns as a function of their distance and for the estimation of the scattering cross-section which is compared to the integrated intensity from a Voronoi cell. To provide end-users this well-established quantification method, a user friendly program, StatSTEM, is developed which is freely available under a GNU public license.

Tracking Electrochemical Processes at the Nanoscale: Operando Transmission Electron Microscopy of Lithium Dendrite Formation

While lithium metal possesses one of the highest specific energy storage densities for secondary batteries at 3.86 A-hr g-1, its use is problematic. Li metal anodes are known to form dendrites during charging cycles and cause device failure. If the dendrite extends to the cathode it shorts and heats up the battery to the flashpoint of the solvent and creates a pressure build up in the casing. The device becomes ripened for fire and explosion. Here we focus on the direct imaging of first the formation of the solid-electrolyte interphase (SEI) on a glassy carbon electrode using in situ electrochemical scanning transmission electron microscopy (ec-STEM) [1]. After SEI formation we follow the early stages of Li dendrite nucleation and growth and speculate on the role that the SEI has upon Li electrodeposition and dendrite growth. We expand the used of Z-contrast scanning TEM imaging to estimate the density of the SEI and correlate the amount of lithium deposited onto the electrode in view with the current measured during the cyclic voltammogram. We are able to record high angle annular dark field (HAADF) STEM images of SEI formation and Li deposition by utilizing a microfluidic electrochemical cell that is placed within a tip of a TEM holder. We will briefly discuss the small-scale STEM microfluidic electrochemical liquid cell platform which is formed by compressing two silicon microchip devices together. Each microchip supports a 50 nm thick electron-transparent silicon nitride membrane with a rectangular viewing area of 50 x 200 μm. Three platinum electrodes is patterned one microchip's surface with the central electrode having glassy carbon deposited on it to be used as a working electrode. The lower microchip has a layer of SU-8 that acts as a spacer and flow channel (400 μm wide, 800 μm in the vacuum TEM environment) for transport of electrolyte between the two microchips when inserted into the TEM holder. Battery-grade 1.2 M LiPF6EC:DMC electrolyte was delivered to the microfluidic electrochemical cell by a syringe pump. In this work, HAADF STEM imaging was used to facilitate Z-contrast conditions, which allows for quantitative image analysis, to estimate thickness and density of the SEI layer and Li deposits. It also allows for providing definitive proof of Li electrodeposition though contrast analysis. The electrochemical window of the electrolyte is generally taken as 1.2 to 4 V vs Li/Li+. We show that during a thin film developed on the glassy carbon's edge during potential cycling more negative than 1.5 V due to the reduction of the ethylene carbonate. EC reduces to carbonate, ethylene gas, and intermediate radical species that can polymerize. The dynamics of the SEI in response to the change in electric field is directly imaged; the SEI swells and becomes denser with each potential cycle. We also show how Li dendrites seem to grow from defects in the SEI that form about the deposits which suggests that there is a competition between EC reduction to Li2CO3(or some other passivating species) and Li electrodeposition. Research supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Department of Energy’s Office of Basic Energy Sciences Division and by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. [1] Sacci, R. L., Black, J. M., Balke, N., Dudney, N. J., More, K. L., & Unocic, R. R. (2015). Nanoscale imaging of fundamental li battery chemistry: solid-electrolyte interphase formation and preferential growth of lithium metal nanoclusters. Nano Letters, 15(3), 2011–2018.

Linear chemically sensitive electron tomography using DualEELS and compressed sensing

Electron tomography (ET) is now increasingly important for recovering the three-dimensional (3D) morphology of nanostructured materials in the physical and life sciences. ET typically involves the acquisition of a set of two-dimensional projection images at different tilts using (scanning) transmission electron microscopy ([S]TEM), followed by alignment and reconstruction using established algorithms to reconstruct a 3D volume that represent the physical morphology or 3D distribution of some other property of the specimen under investigation. In principle, the methodology is independent of the nature of the images and is applicable to any imaging technique that fulfils the projection requirement [1] such that the signal should change monotonically with the physical property of the sample. This condition is approximately fulfilled for mass-thickness contrast in bright field TEM of amorphous biological specimens, and high angle annular dark field (HAADF) STEM imaging of thin specimens. Consequently, both imaging techniques have been widely used in ET.

Nanoscale imaging of fundamental li battery chemistry: solid-electrolyte interphase formation and preferential growth of lithium metal nanoclusters.

The performance characteristics of Li-ion batteries are intrinsically linked to evolving nanoscale interfacial electrochemical reactions. To probe the mechanisms of solid electrolyte interphase (SEI) formation and to track Li nucleation and growth mechanisms from a standard organic battery electrolyte (LiPF6 in EC:DMC), we used in situ electrochemical scanning transmission electron microscopy (ec-S/TEM) to perform controlled electrochemical potential sweep measurements while simultaneously imaging site-specific structures resulting from electrochemical reactions. A combined quantitative electrochemical measurement and STEM imaging approach is used to demonstrate that chemically sensitive annular dark field STEM imaging can be used to estimate the density of the evolving SEI and to identify Li-containing phases formed in the liquid cell. We report that the SEI is approximately twice as dense as the electrolyte as determined from imaging and electron scattering theory. We also observe site-specific locations where Li nucleates and grows on the surface and edge of the glassy carbon electrode. Lastly, this report demonstrates the investigative power of quantitative nanoscale imaging combined with electrochemical measurements for studying fluid-solid interfaces and their evolving chemistries.

Morphological study of Insoluble Organic Matter from carbonaceous chondrites: Correlation with petrologic grade

The major form of organic material delivered to Earth from an extraterrestrial origin is Insoluble Organic Matter (IOM). A morphological study of IOM in the CR (Renazzo-type) and CM (Mighei-type) carbonaceous chondrites was performed in order to constrain its origins and processing history. IOM residues from the following CR chondrites: GRO 95577 (CR1), Al Rais (CR1/2), EET 92042 (CR2), QUE 99177 (CR3) and the CM chondrites: MET 01070 (CM2.2), Cold Bokkeveld (CM2.3), Murchison (CM2.4) and QUE 97990 (CM2.5) were studied using Annular Dark Field STEM imaging. Characteristic features of the IOM, organic nanoglobules, were manually identified and measured for their abundances and size distributions. The IOM residues were also compared holistically for their degree of average ‘roughness’ or ‘coarsening’ using fractal image analysis. Manually identified nanoglobules have abundances making up less than 10% of the total IOM, which is consistent with previous studies. Their measured abundances do not correlate with petrologic grade. Thus parent body processing did not systematically deplete their abundances. The IOM is however on average ‘smoother’ or ‘coarser’ in the more altered chondrites, demonstrated by a lower fractal dimension using fractal box counting (DB). The DB values for the IOM in the CR chondrites are distinctive: QUE 99177 has the largest DB value (average=1.54±0.004) and GRO 99577 has the lowest (average=1.45±0.011). Al Rais and EET 92042 have IOM with average DB values within this range (average, 1.46±0.009 and 1.50±0.006). The CMs record a similar but less distinctive trend in DB, with QUE 97990 having the largest value (1.52±0.004), MET 01070 the lowest (1.45±0.019), and Cold Bokkeveld (1.50±0.011) and Murchison (1.49±0.017) equivalent to one another within error. The identified nanoglobules in the IOM of the CM chondrites are on average larger than those in the CR chondrites. The ‘coarsening’ or ‘smoother’ texture of the IOM (lower DB) in the more altered chondrites coupled with a tentative increase in the size of large features (identified nanoglobules) demonstrates that the aqueous processes leading to the lower petrologic types also formed the overall IOM morphology. In addition, observations of fluid-like textures more frequently found in the more altered carbonaceous chondrite residues suggests that organic and aqueous fluids determined at least some of these morphologies. The polymerization of organic solutions is consistent with these morphologies. Their formation conditions are more favorable under the containment of carbonaceous chondrite parent bodies.

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

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

A STEM study of twin defects in Fe3O4(111)/YZO(111)

We observe twin defects on both the (111) and (11-2) planes of thin film Fe3O4 using atomically resolved high angle annular dark field (HAADF) STEM imaging. These defects are significant for understanding the previously reported anomalous properties of Fe3O4 films as they generate non-bulk bonding configurations leading to non-bulk superexchange interactions in these regions.

Aberration-corrected STEM/TEM imaging at 15kV.

The performance of aberration-corrected (scanning) transmission electron microscopy (S/TEM) at an accelerating voltage of 15kV was evaluated in a low-voltage microscope equipped with a cold-field emission gun and a higher-order aberration corrector. Aberrations up to the fifth order were corrected by the aberration measurement and auto-correction system using the diffractogram tableau method in TEM and Ronchigram analysis in STEM. TEM observation of nanometer-sized particles demonstrated that aberrations up to an angle of 50mrad were compensated. A TEM image of Si[110] exhibited lattice fringes with a spacing of 0.192nm, and the power spectrum of the image showed spots corresponding to distances of 0.111nm. An annular dark-field STEM image of Si[110] showed lattice fringes of (111) and (22¯0) planes corresponding to lattice distances of 0.314nm and 0.192nm, respectively. At an accelerating voltage of 15kV, the developed low-voltage microscope achieved atomic-resolution imaging with a small chromatic aberration and a large uniform phase.

The development of a 200kV monochromated field emission electron source

We report the development of a monochromator for an intermediate-voltage aberration-corrected electron microscope suitable for operation in both STEM and TEM imaging modes. The monochromator consists of two Wien filters with a variable energy selecting slit located between them and is located prior to the accelerator. The second filter cancels the energy dispersion produced by the first filter and after energy selection forms a round monochromated, achromatic probe at the specimen plane. The ultimate achievable energy resolution has been measured as 36 meV at 200 kV and 26 meV at 80 kV. High-resolution Annular Dark Field STEM images recorded using a monochromated probe resolve Si-Si spacings of 135.8 pm using energy spreads of 218 meV at 200 kV and 217 meV at 80 kV respectively. In TEM mode an improvement in non-linear spatial resolution to 64 pm due to the reduction in the effects of partial temporal coherence has been demonstrated using broad beam illumination with an energy spread of 134 meV at 200 kV.