Search

International audience; To analyze exlended Lime series of high resolulion images, we have employed automated frame-by-frame comparisons that are able to detect dynamic changes in the structure of a grain boundary in Au. Using cumulative averaging of images between events allowed high resolution measurements of the atomic relaxation in the interface with sufficient accuracy for comparison with atomistic models. Cumulative averaging was also used to observe the structural rearrangement of atomic columns at a moving step in the grain boundary. The technique of analyzing changing features in high resolution images by averaging between incidents can be used to deconvolute stochastic events that occur at random intervals and on time scales well beyond that accessible to single-shot imaging. (C) 2014 Elsevier B.V. All rights reserved.

Structural and magnetic properties of granular L10 FePt films are investigated as a function of film thickness in the range 3–11 nm. Scanning transmission electron microscopy observations indicate an increasing contribution of in-plane variants to the L10 FePt crystalline structure with increasing film thickness, which may explain the noticed degradation of the perpendicular anisotropy. The temperature dependence of coercivity suggests a progressive deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm, corresponding to a domain-wall width. The anisotropy field distribution (〈H K〉, ) is estimated using Monte Carlo (MC) simulations of AC transverse susceptibility (ACTS). The average anisotropy field 〈H K〉 displays a similar temperature dependence to (M S)1.1, where M S is the film saturation magnetization, consistent with the theoretical predictions for L10 FePt. The Curie temperature distribution (〈T C〉, ) is evaluated using two methods: fast (ns) thermal erasure (ThEr) of remanent magnetization and MC modeling of the temperature dependence of AC susceptibility (ACS). The values evaluated by the two methods are in agreement, indicating an increase of in the range 2–6% with decreasing film thickness. The observed increase of 〈T C〉 in the range 620–680 K with increasing film thickness is explained using the finite-size scaling law with the critical exponent set to the theoretical value for L10 FePt. A general procedure to extract 〈T C〉 and using MC modeling of ACS, essentially independent of the magnetization reversal mechanism, is proposed. MC simulations of ACS confirm the deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm.

Manipulating free-space electron wave functions with laser fields can bring about new electron-optical elements for transmission electron microscopy (TEM). In particular, a Zernike phase plate would enable high-contrast TEM imaging of soft matter, leading to new opportunities in structural biology and materials science. A Zernike phase plate can be implemented using a tight, intense continuous laser focus that shifts the phase of the electron wave by the ponderomotive potential. Here, we use a near-concentric cavity to focus 7.5 kW of continuous-wave circulating laser power at 1064 nm into a 7 µm mode waist, achieving a record continuous laser intensity of 40 GW/cm2. Such parameters are sufficient to impart a phase shift of 1 rad to a 10 keV electron beam, or 0.16 rad to a 300 keV beam. Our numerical simulations confirm that the standing-wave phase shift profile imprinted on the electron wave by the intra-cavity field can serve as a nearly ideal Zernike phase plate.

The two-dimensional structure of graphene is known to impart high strength, but can be hard to synthesize without grain boundaries. Here, the authors find that strength increases with grain boundary mismatch, which results from low atomic-scale strain in the carbon–carbon bonds at the boundary.

The structure and thermodynamics of interfaces between (111) Pt and the basal plane of α-Al2O3 have been studied through a combination of high-resolution electron microscopy and first-principles calculations. Within the framework of ab initio thermodynamics the structure and excess free energies are calculated as functions of temperature (T) and oxygen partial pressure (PO2), for three competing interface terminations. Comparisons between measurements and calculations establish that the interface is oxygen terminated, and a structural phase transition is predicted in the range of experimentally accessible T and PO2 from the calculated interfacial free energies.

Metallic structural components for micro-electro-mechanical/nano-electro-mechanical systems (MEMS/NEMS) are promising alternatives to silicon-based materials since they are electrically conductive, optically reflective and ductile. Polycrystalline mono-metallic films typically exhibit low strength and hardness, high surface roughness, and significant residual stress, making them unusable for NEMS. In this study we demonstrate how to overcome these limitations by co-sputtering Ni-Mo. Detailed investigation of the Ni-Mo system using transmission electron microscopy and high-resolution transmission electron microscopy (TEM/HRTEM), x-ray diffraction (XRD), nanoindentation, and atomic force microscopy (AFM) reveals the presence of an amorphous-nanocrystalline microstructure which exhibits enhanced hardness, metallic conductivity, and sub-nanometer root mean square (RMS) roughness. Uncurled NEMS cantilevers with MHz resonant frequencies and quality factors ranging from 200-900 are fabricated from amorphous Ni-Mo. Using a sub-regular solution model it is shown that the electrical conductivity of Ni-Mo is in excellent agreement with Bhatia's structural model of electrical resistivity in binary alloys. Using a Langevin-type stochastic rate equation the structural evolution of amorphous Ni-Mo is modeled; it is shown that the growth instability due to the competing processes of surface diffusion and self-shadowing is heavily damped out due to the high thermal energies of sputtering, resulting in extremely smooth films.

In this study we report on the solid-state dewetting of ultrathin Ni films on amorphous ${\text{SiO}}_{2}$. The dewetting process is monitored in situ using time resolved differential reflectometry (TRDR). The time resolved differential reflectivity signal during dewetting is found to exhibit a rich behavior, which is intimately connected the changes in morphology. Finite-difference time-domain simulation is used to explain the observed reflectivity data, where experimentally acquired atomic force microscope heightmaps are used as simulation inputs. From ex situ atomic force microscope heightmaps, the sequential processes of grain growth, grain boundary grooving, hole growth, and particle coarsening are observed. Grain growth of ultrathin films prior to dewetting is critically important in determining the particle density, which has been largely unexplored in previous dewetting studies. Kinetic analysis of the TRDR data revealed two rate-limiting processes, with activation energies of $0.31\ifmmode\pm\else\textpm\fi{}0.04$ and $0.59\ifmmode\pm\else\textpm\fi{}0.06\text{ }\text{eV}$. We hypothesize that these kinetic pathways correspond to Ni grain growth and surface mass self-diffusion on the Ni(111) planes, respectively.

We describe an analytic model for polycrystalline thin film growth based on competition between grains with conic geometries. This model is valid for all film thicknesses and is verified using level set simulations in $2+1$ dimensions. We study the effects of nonuniform initial grain distributions on growth statistics. These results provide a possible explanation for discrepancies between experimentally measured and theoretical scaling laws.

We used neutron reflectometry (NR) to study the structural changes of thin Pd-capped Mg 0.7 Al 0.3 and Mg 0.6 Al 0.4 alloy films after hydrogen absorption and during hydrogen desorption. NR enabled us to determine the hydrogen content and hydrogen distribution in these thin MgAl alloy films along with the structural changes associated with the desorption process. The thin films expand by about 20% during the hydrogen absorption and the hydrogen is stored only in the MgAl layer with no hydrogen content in the Pd layer. The Mg 0.7 Al 0.3 films are fully desorbed at 448 K, whereas for the Mg 0.6 Al 0.4 films a temperature of 473 K is needed to fully desorb the hydrogen. Our NR measurements show that the higher annealing temperature needed to desorb the hydrogen from the Mg 0.6 Al 0.4 films led to an interdiffusion of the Pd layer into the MgAl layer. This Pd interdiffusion was also observed in a Mg 0.7 Al 0.3 film after a 9 h annealing at 473 K. So, the Pd interdiffusion into a MgAl film that has been charged with hydrogen is a common feature of the Pd/Mg 0.7 Al 0.3 and Pd/Mg 0.6 Al 0.4 alloy system. In contrast, for the as-prepared hydrogen-free Pd/Mg 0.7 Al 0.3 film the Pd layer stays intact and only a small interdiffusion zone occurs at the Pd/MgAl interface.

Li- and Mn-rich transition metal oxides have been extensively studied recently for their potential application in Li-ion batteries. These materials have attracted a lot of interest due to the high capacity offered by them. However, the structure of these materials in their pristine state is not clearly understood. Several reports have assigned their structure to be trigonal (R-3m), monoclinic (c2/m), or a combination of both (composite). The present study discusses the structure of Li1.2(Ni0.13Mn0.54Co0.13) O2 prepared with two different morphologies: plates and needles, using the results obtained from aberration corrected scanning transmission electron microscopy (STEM) imaging, electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (XEDS). 3-dimensional information was obtained by imaging the material at different zone axes and it was found that the primary particles are made up of a single phase, consisting of domains that correspond to variants of monoclinic structure, save for rare localized defects and a thin surface layer on certain crystallographic facets. These findings were further confirmed on two commercially available Li-and Mn-rich transition metal oxides, namely TODA HE5050 and Envia HCMR XLE2 . It will be shown how diffraction-based techniques can lead to ambiguous interpretation of structure in this class of materials. The study not only solves the ambiguity in the structure determination of this class of complex oxides but also highlights the importance of atomic resolution imaging performed on multiple zone axes when it is used for similar investigations. Figure 1

We report on the development of a nanometer scale strain mapping technique by means of scanning nano-beam electron diffraction. Only recently possible due to fast acquisition with a direct electron detector, this technique allows for strain mapping with a high precision of 0.1% at a lateral resolution of 1 nm for a large field of view reaching up to 1 μm. We demonstrate its application to a technologically relevant strain-engineered GaAs/GaAsP hetero-structure and show that the method can even be applied to highly defected regions with substantial changes in local crystal orientation. Strain maps derived from atomically resolved scanning transmission electron microscopy images were used to validate the accuracy, precision and resolution of this versatile technique.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Transformations in Li-rich layered oxides have been extensively studied recently for their potential application in Li-ion batteries. These materials have attracted a lot of interest due to the high capacity offered by them. However, the structure of these materials in their pristine state is not clearly understood. Several reports have assigned their structure to be trigonal (R-3m), monoclinic (c2/m), or a combination of both (composite). The present study discusses the structure of Li1.2(Ni0.13Mn0.54Co0.13) O2 prepared with two different morphologies: plates and needles, using the results obtained from aberration corrected (scanning) transmission electron microscopy, electron energy loss spectroscopy (EELS), convergent beam electron diffraction and precession electron diffraction tomography and question the validity of the the claims of them being “composite”. It was found that these materials consist of domains which correspond to variants of monoclinic structure. It will be shown how diffraction-based experiments on such materials can often lead to misleading conclusions, since analysis of diffraction-based techniques inevitably assign them as trigonal, although the present study shows that the three-fold symmetry observed in electron diffraction patterns result from the combination of the variants having monoclinic structure. Furthermore, results from STEM and EELS experiments showed that the pristine materials have several defects consisting of differently ordered structure on their surface, and several Cobalt-rich line defects. These results prescribe that extreme care should be taken while interpreting the electron microscopy results obtained from cycled samples.

Traditional scanning transmission electron microscopy (STEM) detectors are monolithic and integrate a subset of the transmitted electron beam signal scattered from each electron probe position. These convergent beam electron diffraction patterns (CBED) are extremely rich in information, containing localized information on sample structure, composition, phonon spectra, three-dimensional defect crystallography and more. Many new imaging modes become possible if the full CBED pattern is recorded at many probe positions with millisecond dwell times. In this study, we have used a Gatan K2-IS direct electron detection camera installed on an uncorrected FEI Titan-class transmission electron microscope to record 4D-STEM probe diffraction patterns on a variety of samples at up to 1600 frames per second. As an example, a 4D-STEM dataset for a multilayer stack of epitaxial SrTiO3 and mixed LaMnO3-SrTiO3 is plotted in Figure 1. Figure 1A shows a HAADF micrograph of the multilayer along a (001) zone axis. Only the A sites (Sr and La) are visible in this micrograph and the composition can be roughly determined from the relative brightness. One possible 4D-STEM technique is position-averaged convergent beam electron diffraction (PACBED) described by LeBeau et al. [1]. We can easily construct ideal PACBED patterns by averaging the probe images over each unit cell fitted from Figure 1A, which is shown in Figure 1B. By matching these patterns to PACBED images simulated with the multislice method we can precisely determine parameters such as sample thickness and composition, the latter of which is plotted in Figure 1C. For comparison, the composition has also been determined with electron energy loss spectroscopy (EELS) in a separate experiment, shown in Figure 1D. The composition range of 0-85% LaMnO3 measured by PACBED is in good agreement with the EELS measurements. In this talk we will demonstrate several other possible uses for 4D-STEM datasets.

We use transmission electron microscopy to study orientation-patterned GaAs layers very attractive for applications in terahertz and infrared frequency conversion devices. We observe regularly distributed inversion domains separated by inversion boundaries, together with undesirable microtwin defects originating at these boundaries. Atomic resolution aberration-corrected scanning transmission electron microscopy allowed us to resolve the GaAs dumbbells leading to a direct determination of the growth polarity of particular domains and determination of the alternating Ga-Ga and As-As bonds at the {110}-type antiphase boundary planes. We also determined observed microtwins as rotation twins called orthotwins, the defect that can cause optical losses.

A growing number of atomic force microscope (AFM) applications make use of metal-coated probes. Probe metallization can cause adverse side-effects and disadvantages such as stress-induced cantilever bending, thermal expansion mismatch, increased tip radius and limited device lifetime due to coating wear. In this study we demonstrate how to overcome these limitations using microstructural design to create a metallic glass thin film alloy, from which monostructural all-metal AFM cantilevers are fabricated. A detailed compositional study of co-sputtered Cu-Hf films is performed using x-ray diffraction (XRD), nanoindentation, four-point probe and in situ multi-beam optical stress sensing (MOSS). Metallic glass Cu(90)Hf(10) films are found to possess an optimal combination of electrical resistivity (96 microOmega cm), nanoindentation hardness (5.2 GPa), ductility and incremental stress. A continuum model is developed which uses measured MOSS data to predict cantilever warping caused by stress gradients generated during film growth. Subsequently, a microfabrication process is developed to create Cu(90)Hf(10) AFM probes. Uncurled, 1 microm thick cantilevers having lengths of 100-400 microm are fabricated, with tip radii ranging from 10 to 40 nm. As a proof of principle, these all-metal Cu-Hf AFM probes are mounted in a commercial AFM and used to successfully image a known test structure.

We studied the hydrogen absorption and desorption properties of thin Pd-covered Mg1−xAlx alloy films as a function of temperature and alloy composition. Using neutron reflectometry, we were able to determine the hydrogen content and the hydrogen distribution within these MgAl films in situ. For all films, hydrogen was uniformly dispersed within the MgAl film and no hydrogen was observable in the Pd cover layer. The Mg0.7Al0.3 film shows an appreciable 4.1wt% stored hydrogen and improved desorption characteristics with complete desorption at a temperature of 448K.

This study details the resonance properties of 20nm thick nanoelectromechanical system scale cantilevers fabricated from a metallic Al–32at.%Mo nanocomposite. The advantage of the Al–32at.%Mo alloy is that its strength and near-atomic surface smoothness enable fabrication of single-anchored metallic cantilevers with extreme length-to-thickness ratios, as high as 400:1. This yields uniquely compliant structures with exquisite force sensitivity. For example, an 8μm long, 20nm thick Al–32at.%Mo device has a spring constant of K≅280μN∕m. We show through transmission electron microscope analysis and continuum modeling that the relevant damping mechanisms are related to the device microstructure.

A gold–tantalum nanocomposite material synthesized using a co-sputtering technique and designed specifically for nanomechanical systems applications is reported. X-ray diffraction, atomic force microscopy, scanning electron microscopy, nanoindentation and four-point probe contact measurements were performed to characterize the bulk and surface characteristics of the films. The films exhibited an increased reduced modulus and hardness as well as a significant grain size reduction while retaining the face-centered cubic (FCC) structure of pure gold with a very strong texture. In addition, the mechanical characterization of a number of 800 nm wide, 50 nm thick Au–5 at.% Ta cantilevers was performed using optical interferometry. Such devices exhibited resonant frequencies ranging from 371 kHz to 10.9 MHz, with resonant qualities as high as Q = 640.