U. Dahmen

Structure and Magnetism of Co and CoAg Nanocrystals

Monodisperse, air-stable Co/CoO and CoAg55 nanoparticles with a mean diameter of about 11 nm have been synthesized using methods of colloidal chemistry. High resolution transmission electron microscopy (TEM) and Electron Energy-Loss Spectroscopy (EELS) element-specific TEM images reveal a multiply-twinned fcc Co metallic core covered with a 2-2.5 nm thick CoO shell. The lattice parameters are in agreement with those of bulk Co and CoO. A shift of the hysteresis loop of 0.4 T, induced by field CoOling of the Co/CoO particles, indicates a strong unidirectional exchange anisotropy due to the interaction between the ferromagnetic Co core and the antiferromagnetic CoO shell. CoAg55 composite particles consist of grains of fcc Co and fcc Ag. No evidence for alloy formation was observed. Electron energy-loss and X-ray microanalysis indicate that Co is predominantly found in the surface region of the particles. SQUID magnetometry shows that at room temperature the CoAg55 particles are superparamagnetic while at 90 K a hysteresis loop was detected with a coercive field of 0.07 T and a remanent magnetization of 32 % of the saturation value.

In Situ TEM Investigation of Diffusion of Nano-Scale Liquid Pb Inclusions on Dislocations and in Bulk Aluminum

Diffusion of nano-sized liquid Pb inclusions in thin aluminum foils is investigated using in-situ transmission electron microscopy (TEM). Free diffusion of the inclusions in the bulk and diffusion constrained by dislocations trapping is studied. The motion of trapped Pb inclusions is spatially confined in close proximity to the dislocations. The diffusion coefficients of free motion of the inclusions are determined using Einsteins equation. The diffusion coefficients of trapped inclusions were obtained using an equation based on Smoluchowskis analysis of the Brownian motion of particle in a harmonic potential. The agreement of the diffusion coefficients of free and trapped inclusions indicates the same underlying microscopic mechanism, and no strong influence from dislocations. The microscopic mechanism controlling the mobility is discussed.

Introduction: The Otto Scherzer special issue on aberration-corrected electron microscopy.

The year 2009 marked the centenary of the birth of Otto Scherzer, one of the early pioneers of electron microscopy. Scherzer was the originator of the famous microscopy theorem that the spherical and chromatic aberrations of rotationally symmetric electron lenses were unavoidable. In honor of this centennial occasion, we organized a special memorial symposium during Microscopy & Microanalysis 2009, which was held in Richmond, Virginia, in late July. The introductory talks of the symposium presented a fascinating mix of first-hand accounts about working with Scherzer in Darmstadt and descriptions of the correction concepts and the early corrector prototypes that emerged from his group. Placed in this historical context, the latest advances in aberration correction for scanning and fixed-beam instruments that were presented in this symposium were all the more impressive and conveyed a vivid sense of history in the making. Applications of aberration correction to a broad range of materials were also highlighted in platform and poster presentations. This special issue of Microscopy and Microanalysis contains refereed contributions from the work presented at the symposium and thus provides a representative overview of the recent emergence of aberration-corrected electron microscopy (ACEM) and some of the prospects and challenges for this burgeoning field.

An update on the TEAM project — first results from the TEAM 0.5 microscope, and its future development

Recent advances in aberration-correcting electron optics have led to increased resolution, sensitivity and signal to noise in atomic resolution microscopy. Building on these developments, the TEAM project was designed to optimize the electron microscope around aberration-corrected electron optics and to further advance the limits of the instrument and the technique [1]. The vision for the TEAM project is the idea of providing a sample space for electron scattering experiments in a tunable electron optical environment by removing some of the constraints that have limited electron microscopy until now. The resulting improvements in resolution, the increased space around the sample, and the possibility of exotic electron-optical settings will enable new types of experiments. The TEAM microscope will feature unique corrector elements for spherical and chromatic aberrations, a novel AFM-inspired specimen stage, a high-brightness gun and numerous other innovations that will extend resolution down to the half-Angstrom level.

Low-Voltage Atomic-Resolution Off-Axis Holography on Hexagonal Boron Nitride

Off-axis electron holography is a powerful method that can access the phase shift of the electron wave from only one single electron micrograph [1]. Its capabilities have been proven in numerous applications [e.g. 2]. After hardware CS-correction has found to be very beneficial for holography to improve the signal resolution in the reconstructed phase shift [3], the chromatic aberration CC remains the limiting factor for low-voltage high-resolution holography. Recently, off-axis holography had been set up in the TEAM I microscope to allow recording holograms in a CC-CS-corrected TEM [4]. Since no monochromator is required to reduce the resolution-limiting focus spread due to chromatic aberration, the full beam current is available and high-quality holograms with atomic resolution can be recorded at low voltages such as 80 kV. In general, low-voltage imaging capabilities have attained a lot of attention since the lower electron energy avoids knock-on damage, which is especially important for light elements. Here, we exemplify the capabilities of CC-corrected low-voltage off-axis holography on hexagonal boron nitride (h-BN). Figure 1a shows an 80 kV high-resolution hologram of h-BN, which is only one single member of a time series of 31 frames. Since the underlying phase shifts are so extremely small, virtually no bending in the interference fringes can be observed. Due to the great instrumental stability, exposure times of eight seconds can reliably be used without significant concessions to hologram fringe contrast and lateral resolution. Consequently, the image wave can be reconstructed with very little noise. Figure 1c shows the reconstructed image wave, which is still affected by residual aberrations. After numerical correction of defocus, twoand three-fold astigmatism, second order coma and residual spherical aberration, the pure object phase shift (figure 1d) can be observed to investigate such details as defects and the phase shift behavior at the edge of individual layers. Since the material only degrades slowly under the electron beam, holographic time series can easily be used to cover a decent amount of time. In this particular time series, the same h-BN area was observed for about 20 minutes. The TEAM I microscope is part of a user facility. Therefore, the holographic setup is available to the electron microscopy community and a wide range of researchers can benefit from this instrumentation to solve materials science problems. [5]

In-Situ TEM Observations of Colloidal Particles in Liquids

One of the most challenging tasks in electron microscopy is the observation of mechanisms and dynamics of processes in materials. The ability to adapt sample geometries to the microscale has made it possible to investigate fundamental features such as crystal growth, phase transformations or plastic deformation of materials in real time. This field of research has expanded tremendously over the past several years, largely because MEMS and AFM technology can now readily be adapted to electron microscope stages and samples.

Correlation between Atomic Structure and Superglide of an Incommensurate Grain Boundary in Au

Grain boundaries play a central role in the deformation of materials, especially in nanocrystalline materials where normal dislocation mechanisms are inhibited by the small grain size. In this work, we are investigating the behavior of an incommensurate grain boundary in gold by atomic resolution imaging, atomistic simulation and in-situ deformation experiments [1]. Simulations predict certain incommensurate boundaries to deform by a process of frictionless gliding, or superglide. To test this prediction experimentally, we used a bicrystal with a misorientation of 90° around a common <110> axis. In cubic materials, such a bicrystal makes a good model for incommensurate boundaries because at the grain boundaries {h h k} crystal planes of one grain align with {k k 2h} planes in the other grain. The ratio of plane spacings ( ) is irrational and hence incommensurate. Figure 1 shows a high resolution micrograph of such a √ 2 incommensurate boundary where (200) planes of the lower grain are aligned with (110) planes of the upper grain. The structure of this boundary was computed by EAM calculations and tested against experimental images using image simulation, template matching and pattern recognition techniques. At the intersection with free surfaces, this boundary decomposed into a stable triangular Chevron-shaped defect about 1-2nm in size, whose existence range depended on the orientation of the free surface relative to the grain boundary. For the surface orientation in Fig. 1, this instability is seen as a kink or facet in the last nanometer of the boundary.

K-space Navigation for Accurate High-angle Tilting and Control of the TEAM Sample Stage

In this talk we present a new software tool that links the coordinates of double-tilt or tilt-rotation stages to crystallographic coordinates of crystalline samples. The tool has been developed as part of the TEAM stage control program, but its use is more general – it can be run in a stand-alone mode and matched to any goniometer by means of linearizing lookup tables and configurable parameters. The software enables sample tilt via a control sphere that can be moved with mouse drags. The control sphere has the same constraints as the actual stage, and can be set to be of tilt-rotation or double tilt type. The stage limits can be configured and displayed on the control sphere, along with the sample’s crystallographic coordinates for a simple visual display of all crystal orientations accessible for a given experiment. Sample and crystallographic information is fed into the program in CIF (crystallographic information file) format. CIF files for all crystal systems can be downloaded from numerous databases to save the effort of assembling atomic position coordinate files. Once the CIF loader has extracted all atom and symmetry data, the resulting unit cell can be viewed, and remains synchronized with the stage control sphere. The crystallographic coordinates can be displayed on the control sphere as well, while the corresponding simulated kinematic diffraction and Kikuchi line patterns can be invoked in another window. To align the crystal model with the actual sample, an algorithm has been developed that calibrates the major tilt axis in a few easy steps. Once a sample zone axis and azimuth have been aligned, the tilts for any other crystallographic orientation can be calculated with great accuracy, and the resulting tilt coordinates can directly drive the TEAM stage. Structure factor and diffraction contrast visibility add a quantitative aspect, and this software can also be used for manual diffraction pattern fingerprinting. Figure 1 demonstrates the possibilities of this software with recent results from the newly developed TEAM stage, using an array of Pt-Pd cubes with a size of around 30 nm [1]. This and other examples will be used to illustrate the capabilities of the software and the stage for a number of demanding experiments in electron microscopy.

Statistical Tomography of 3D Thin Film Structure using Transmission Electron Microscopy

Thin films play a crucial role in many modern technologies, like microelectronics, solar cells, sensors, and coatings. A key to understanding and controlling thin film growth processes and properties is the knowledge of the variation in structure with distance from the substrate. Therefore experimental techniques for quantitative characterization of the three-dimensional (3D) structure of thin films are highly demanded. Transmission electron microscopy (TEM) is well established as a powerful tool for investigation of thin film structures down to the nanometer scale. However, cross-section and plan-view geometries commonly used in TEM studies of thin films are not suited for quantitative evaluation of 3D data since these geometries represent only two particular 2D sections through the film. On the other hand most film structures are much too complicated for application of standard 3D tomography techniques.

Effect of Morphology on the Mobility of Nanosized Liquid Pb Inclusions in Solid Al

Diffusion of nanosized liquid Pb inclusions attached to dislocations in thin aluminum foils was investigated in a wide temperature range using in-situ transmission electron microscopy. Trajectories of motion of the inclusions along the dislocations were used to determine their diffusion coefficients. The temperature and size dependences of diffusion coefficients of the inclusions were obtained. They indicate that (i) studied inclusions hold {111} facets on their surface in the studied temperature range; (ii) the mobility of the inclusions is controlled by step nucleation at the {111} facets.