Tempering is widely applied to make carbon atoms beneficially rearrange in high strength steel microstructures after quenching; though the nano-scale interaction of carbon atoms with crystallographic defects is hard to experimentally observe. To improve, we investigate the redistribution of carbon atoms along martensite grain boundaries in a quenched and tempered low carbon steel. We observe the tempering-induced microstructural evolution by in-situ heating in a transmission electron microscope (TEM) and by compositional analysis through atom probe tomography (APT). Probe volumes for APT originate from a single martensite packet but in different tempering conditions, which is achieved via a sequential lift-out with in-between tempering treatments. The complementary use of TEM and APT provides crystallographic as well as chemical information on carbon segregation and subsequent carbide precipitation at martensite grain boundaries. The results show that the amount of carbon segregation to martensite grain boundaries is influenced by the boundary type, e.g. low-angle lath or high-angle block boundaries. Also, the growth behavior of cementite precipitates from grain boundary nucleation sites into neighboring martensite grains differs at low- and high-angle grain boundaries. This is due to the crystallographic constraints arising from the semi-coherent orientation relationship between cementite and adjacent martensite. We also show that slower quenching stabilizes thin retained austenite films between martensite grains because of enhanced carbon segregation during cooling. Finally, we demonstrate the effect of carbon redistribution along martensite grain boundaries on the mechanical properties. Here, we compare micro-scale Vickers hardness results from boundary-containing probe volumes to nanoindentation results from pure bulk martensite (boundary-free) probe volumes.

Crystallization of metallic glasses (MGs) is a complex dynamic process, driven by thermodynamics and limited by kinetics, which often involves phase transformation from the metastable amorphous state, via intermediates, to the final stable crystalline states. The intermediate structural state remains mysterious at present but crucial for a deeper understanding of the physics and mechanisms of the crystallization process. Detailed structural characterization of the complex intermediate crystalline phases using transmission electron microscopy (TEM) provides a unique platform to study such issues. Here, we monitor the evolution of the crystallization process for Ni65Zr35 (at%) MG ribbon with structural heterogeneities. Direct visualization combined with compositional analysis reveal that the intermediate phase with Zr concentration higher than that of the MG consists of stacked nanometer-sized layers of Ni-rich units (Ni at% > 67%) and Ni10Zr7-like units, where the thin Ni-rich single layer gradually disappears with increasing annealing temperature. Our findings provide insight into the key role of Ni in the structural transition process, improving the understanding of the atomic diffusion-dominated crystallization in MGs.

Graphite composite electrodes mixed with silicon are proposed as next generation anode material for high energy and high power applications. In order to overcome drawbacks caused by volume changes of silicon particles during electrochemical cycling and to maintain high specific capacities at enhanced C-rates, free-standing structures are generated on silicon/graphite electrodes by applying ultrafast laser ablation. Electrochemical properties are systematically investigated by means of galvanostatic measurements, cyclic voltammetry, and electrochemical impedance spectroscopy. Cells with structured electrodes exhibit improved battery performances and lithium-ion transport kinetics in comparison to cells with unstructured electrodes. Furthermore, the cells with structured electrodes exhibit a lower impedance at fully lithiated state. After cycling, post-mortem analysis is performed revealing that the mechanical stress within the electrodes and current collector can be significantly reduced due to laser generated free-standing structures.

Synthesis of hard/soft magnetically exchange-coupled heterostructures is one promising way to design energy-efficient rare-earth-free artificial magnetic materials for application as permanent magnets and in spintronics. As a model system, we experimentally investigate MnGa/FeCo bilayers and simulate their physical behavior in a combined density functional theory and micromagnetic approach. Using high-quality L1(0)-Mn1.5Ga thin films with bulklike magnetic properties, we show that optimal coherent exchange coupling is obtained below a critical soft magnetic layer thickness that depends on the interface structure and composition. In particular, for atomically smooth and matched epitaxial interfaces of L1(0)-Mn1.5Ga to a Co-terminated and Co-rich FeCo layer, coherent exchange coupling is observed for FeCo thicknesses below 2 nm. In optimized bilayers, the magnetic coercivity of MnGa (approximately 6 kOe) can be fully conserved while the overall saturation magnetization is increased beyond 1000 emu/cm(3). Our model correlates interface structure and magnetic exchange coupling, providing guidelines to engineer high-performance exchange-coupled heterostructures for permanent magnets or spintronic devices.

Clarifying the crystallization path, from the thermodynamic point of view, is of importance for the structure and properties of metallic glasses (MGs). The influence of oxygen on the crystallization pathway of Ni65Zr35 MG was investigated by in-situ heating transmission electron microscopy (TEM). As temperature increases, crystalline Ni7Zr2 and t-ZrO2 phases are formed first rather than the Ni10Zr7 and Ni21Zr8 phases, which are the main crystallization products following the phase diagram. Oxygen changes the crystallization pathway by varying the Ni/Zr ratio, indicating that the effects of oxygen on the crystallization in nanometer-sized MG cannot be neglected even in high vacuum TEMs.

Model Fe-Cr alloys with and without W were nanostructured by severe plastic deformation at 350 degrees C. Transmission electron microscopy observations of the microstructure indicate that the addition of 1 wt % of W led to a reduction of the mean grain size from 129 to 110 nm. Atom probe analyses show that only Cr and Si segregate at grain boundaries during deformation while W remains homogeneously distributed. This solid solution leads to higher mechanical strength and plasticity as compared to Fe-Cr. The origin of the beneficial effect of W on the mechanical performance is discussed. (C) 2018 Elsevier B.V. All rights reserved.

The demand for compact and highly integrated power electronic devices is growing continuously. However, the increase in power density leads to higher operation temperatures of the power modules causing premature device failures. One of the major failure sources is the standard SAC solder to attach the die. Novel low temperature silver sinter die attachment materials are much more reliable. Most commercially available sinter materials have to be applied on noble surfaces like gold or silver. Therefore, the chip is metalized with silver and the DCB is coated by a layer of electroless nickel and immersion gold (ENIG). However, the sinter process quality on the ENIG layer is not always constant. Shear tests on various ENIG layers have been conducted exposing quality variations covering a range from 5 N/mm(exp 2) to more than 90 N/mm(exp 2). Wet chemical analysis as well as FIB-SEM and XPS-investigations have been performed to identify the root cause. We observed that test specimens showing lower shear values exhibit a higher copper and nickel concentration on the surface. These non-noble metals influence the formation of the intermetallic phase of the sinter layer and the substrate. Consequently, the mechanical stability and thermal resilience is reduced.

Non-percolating and percolating silicon quantum dot (QD) networks were investigated by plane-view energy filtered transmission electron microscopy (EF-TEM). The Si QD networks were prepared by plasma enhanced chemical vapor deposition on free standing 5 nm Si3N4 membranes, followed by high temperature annealing. The percolation threshold from non-percolating to percolating networks is found to be in between a SiOx stoichiometry of SiO0.5 up to SiO0.7. Using the EF-TEM images, key structural parameters of the Si QD ensemble were extracted and compared, i.e., their size distribution, nearest neighbor distance, and circularity. Increasing the silicon excess within the SiOx layer results in an ensemble of closer spaced, less size-controlled, and less circular Si QDs that give rise to coupling effects. Furthermore, the influence of the structural parameters on the optical and electrical Si QD ensemble properties is discussed.

A meticulous examination of transient stages of dynamic processes at the micrometer, nanometer or atomic scale is the aim in the field of in situ transmission electron microscopy. Studying a specimen in a state close to its native environment without drying possibly leading to structural alterations, triggers research in the field of in situ liquid TEM. However, for any in situ experiment, it is pivotal to accurately consider if the observed dynamic processes are a result of electron–matter interaction or are resulting from the applied stimuli. Therefore, critical aspects for in situ liquid TEM experiments include electron beam stability of the liquid medium, concentration and composition of the liquids in addition to damage free sample preparation on SiN membranes and alignment of the thin observation windows. Efforts towards identifying the critical aspects are discussed. Furthermore, an ergonomic sample preparation setup is pivotal for an easier precise handling of the fragile specimens. An example for such a setup involving micro-manipulators and grippers is shown here. Introduction A large number of groups worldwide are working to understand the basic processes that occur at the nanoscale to improve their materials, interfaces, to understand the underlying phenomena or to optimize devices. For this purpose, different characterizing tools are used under in situ and in operando conditions providing key information to understand the aforementioned purposes. The aim is a meticulous examination of transient stages of dynamic processes at the micrometer, nanometer or atomic scale. In situ transmission electron microscopy (TEM) is at the forefront of such dynamic studies with high spatial resolution [1-4], but still provides significant challenges for research. Applying stimuli such as external fields or forces to TEM specimens turns the TEM specimen chamber into a micro-laboratory in which reactions [5-7], structures [8], or physical properties [9-13] can be initiated, modified or changed at nanometer scale. Ideally, all of these processes are monitored at high spatial and temporal resolutions. In situ TEM comprises complex sample environments, requiring careful planning of all experimental aspects and the development of precise solutions to correlate with the true operating conditions [14]. For this purpose, different variations of in situ TEM sample holders have been developed, providing thermal (heating, cooling), mechanical (straining, compression), electrical (biasing), atmosphere (liquid, gas, light), ion irradiation, or combinatory stimuli. These facilitate a plethora of in situ TEM capabilities [5] illustrated in figure 1. In situ TEM has two important components: stimulus and real-time observation. Therefore, a specially designed TEM specimen holder with an optimum specimen and a TEM with a fast image recording system are the important requirements to apply an external stimulus to the specimen and simultaneously perform TEM observations. One of the major limitations in electron microscopy is that the specimen is exposed to high vacuum leading to the study of completely dried and therefore potentially modified or destroyed specimens. Studying a specimen in its native medium triggered research in the field of liquid TEM [15-20]. This resulted in the necessity of enclosed sample chambers in the TEM. The enclosed chamber should be permeable to electrons and have a low background scattering for good imaging capabilities in addition to optimum mechanical strength to withstand the vacuum, besides being inert to water vapor or other solvents and media. The first concept for an environmental cell was already developed in 1934 to observe biological specimens in a hydrated state inside the TEM by Marton [21,22]. In 1944, Abrams and McBain constructed the first enclosed wet cell [21]. Difficulties in financial support, instrumentation and resolution resulted in a slow growth of the field. However, recently, liquid TEM has seen a rapid development triggered by the increasing demand in both physical and biological sciences to understand reactions and transitions in materials and complete systems. Thus dynamic in situ electron microscopy is emerging as a tool to meet the challenges of the nanoworld. Correspondingly, sample holder development for liquid and gas phase electrochemistry has increased in the present decade [15] stimulating further research activities. From the materials perspective, in situ liquid TEM studies were triggered by a variety of research aspects ranging from electrochemical processes in batteries and fuel cells over electrodeposition/stripping, alloying, electrocatalysis to the interaction of biological species with nanomaterials and sensors involving chemical, morphological and structural changes. The wish of any experimentalist would be to mimic the exact conditions of an ex situ system, in situ inside the TEM. For example, for electrochemists working on battery systems, observation of interfaces, electrode dissolution / redeposition, solid-electrolyte interphase (SEI) formation, understanding the components of the SEI and their modification during the cycling stages, capacity decay are critically needed to improve battery performance [23]. In this work, we discuss some of the challenges involved in carrying out in situ liquid TEM experiments and a setup for ergonomic sample mounting of a liquid system.

Nanoglasses are noncrystalline solids with a granular nano-/microstructure. In contrast to their nanocrystalline analogs, typically constituted of grains and grain boundaries, nanoglasses consist of glassy regions with a structure corresponding to melt-quenched glasses and amorphous interfaces characterized by a reduced density. Their unique properties can be controlled by modifying size and chemical composition of the granular and interfacial regions. Ni50Ti45Cu5 amorphous films were obtained by magnetron sputtering and analyzed to determine their nanoscale morphology and the formation mechanisms. The nanoglasses were noted to have a hierarchical nano-columnar structure with the smallest Ni-rich (Ni:Ti ratio of ca. 5:3) amorphous columns with diameters of about 8 nm and Ti-rich glassy interfacial regions with a substantially lower density. The results were obtained utilizing X-ray diffraction and different microscopic methods, e.g., atomic force microscopy and transmission electron microscopy. A detailed analysis indicates the complexity of the formation mechanisms of topologically and chemically distinguishable structural units with curvature driven surface diffusion, surface mobility, self-shadowing and internal stresses as the most important parameters. Common and simple synthesis method and the possibility for easy modification of the morphology and, consequently, the physical properties offer an opportunity for intensive studies of this new class of materials, opening the way towards possible applications.

This paper reports the growth of silicon nanocrystals (SiNCs) from SiH4–O2 plasma chemistry. The formation of an oxynitride was avoided by using O2 instead of the widely used N2O as precursor. X-ray photoelectron spectroscopy is used to prove the absence of nitrogen in the layers and determine the film stoichiometry. It is shown that the Si rich film growth is achieved via non-equilibrium deposition that resembles a interphase clusters mixture model. Photoluminescence and Fourier transformed infrared spectroscopy are used to monitor the formation process of the SiNCs, to reveal that the phase separation is completed at lower temperatures as for SiNCs based on oxynitrides. Additionally, transmission electron microscopy proves that the SiNC sizes are well controllable by superlattice configuration, and as a result, the optical emission band of the Si nanocrystal can be tuned over a wide range.

PbTe thin films on silicon substrates were prepared by an atomic layer deposition (ALD) for the first time, using lead (II) bis (2,2,6,6‐tetramethyl‐3,5‐heptanedionato) and (trimethylsilyl) tellurid as ALD precursors, at deposition temperature as low as 170 °C. The formation of a PbTe thin film on the Si substrates was strongly dependent on the growth temperature. X‐ray diffraction measurement indicated that thin films were polycrystalline and have characteristic face‐centered cubic rock salt structure with a preferential (200) orientation. Scanning electron microscopy showed PbTe thin films were grown in the Volmer–Weber island mode.

In the present paper, the decomposition and the crystallization behaviour of amorphous Si2C films, which were deposited by r.f. magnetron co-sputtering on Si wafer substrates, are investigated. For analysis, the following methods were used: x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), grazing incidence x-ray diffractometry (GIXRD), atomic force microscopy and scanning electron microscopy. After deposition, the films exhibited a homogenous amorphous structure with a variety of bonding states reaching from homonuclear silicon-like Si-Si bonds over mixed Si-Si-C bonds to heteronuclear Si-C bonds. Annealing at 1200 °C for 2 h leads to the crystallization of silicon and silicon carbide with grain diameters of several nanometers within the amorphous matrix, as evidenced by GIXRD and TEM. With XPS also a distinct change of the bonding states is detected. After 2 h of annealing, only Si-Si and Si-C bonds are detectable. After prolonged annealing at 1200 °C for 20 h, XPS shows only Si-C bonding states but no more Si-Si bonding. In addition, GIXRD verifies the absence of any polycrystalline silicon in the film. The microstructure of the film changed dramatically towards a jagged and porous structure. The vanishing of silicon during isothermal annealing is explained on base of in situ and ex situ TEM measurements, and a possible model for decomposition is suggested.

Miscible (Al/Nb) and immiscible (Cu/Nb) nanometallic multilayer systems were characterized by means of transmission electron microscopy techniques, primarily by automated crystallographic orientation mapping, which allows for the resolution of crystal structures and orientations at the nanoscale. By using this technique, distinctive Nb orientations in relation to the crystallographic state of the Al and Cu layer structures can be observed. Specifically, the Al and Cu layers were found to consist of amorphous, semi-amorphous, and crystalline regions, which affect the overall multilayer microstructure.

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

The increasing need for long-life lithium-ion batteries requires the further development of electrode materials. Especially on the cathode side new materials or material composites are needed to increase the cycle lifetime. On the one hand, spinel-type lithium manganese oxide is a promising candidate to be used as cathode material due to its non-toxicity, low cost and good thermal stability. On the other hand, the spinel structure suffers from change in the oxidation state of manganese during cycling which is also accompanied by loss of active material into the liquid electrolyte. The general trend is to enhance the active surface area of the cathode in order to increase lithium-ion mobility through the electrode/electrolyte interface, while an enhanced surface area will also promote chemical degradation.In this work, laser microstructuring of lithium manganese oxide thin films was applied in a first step to increase the active surface area. This was done by using 248 nm excimer laser radiation and chromium/quartz mask imaging techniques. In a second step, high power diode laser-annealing operating at a wavelength of 940 nm was used for forming a cubic spinel-like battery phase. This was verified by means of Raman spectroscopy and cyclic voltammetric measurements. In a last step, the laser patterned thin films were coated with indium tin oxide (ITO) layers with a thickness of 10 nm to 50 nm. The influence of the 3D surface topography as well as the ITO thickness on the electrochemical performance was studied by cyclic voltammetry. Post-mortem studies were carried out by using scanning electron microscopy and focused ion beam analysis.

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

We report site-specific energy loss magnetic dichroism measurements of the technologically interesting Heusler alloy Ni2MnSn. In addition, we confirm the theoretical prediction that under certain conditions, two different atoms on inequivalent lattice sites give dichroic signals with opposite signs. With this, it is possible to distinguish the magnetic moments of atomic columns that are merely 1.5Å apart using a conventional transmission electron microscope without the need for aberration corrections.

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