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Metallic/intermetalic materials with BCC structures hold an intrinsic instability due to phonon softening along [110] dirrection, causing BCC to lower-symmetry phases transformation when the BCC structures are thermally or mechanically stressed. Fe50Rh50 binary system is one of the exceptional BCC structures (ordered-B2) that has not been yet showing such transformation upon application of thermal stress, although mechanical deformation results in B2 to disordered FCC (gamma) and L10 phases transformation. Here, a comprehensive transmission electron microscopy (TEM) study is conducted on thermally-stressed samples of Fe50Rh50 aged at water and liquid nitrogen from 1150 degree C and 1250 degree C. The results show that, samples quenched from 1150 degree C into water and liquid nitrogen show presence of 1/4{110} and 1/2{110} satellite reflections, the latter of which is expected from phonon dispersion curves obtained by density functional theory calculation. Therefore, it is believed that Fe50Rh50 maintains the B2 structure that is in premartensite state. Once Fe50Rh50 is quenched from 1250 degree C into liquid nitrogen, formation of two short-range ordered tetragonal phases with various c/a ratios (~1.15 and 1.4) is observed in line with phases formed from mechanically deformed (30%) sample. According to our observations, an accurate atomistic shear model ({110}) is presented that describes the martensitic transformation of B2 to these tetragonal phases. These findings offer implications useful for understanding of magnetic and physical characteristics of metallic/intermetallic materials.

Characterisation of the electron beams trajectory in an electron microscope is possible in a few select commercial software packages, but these tools and their source code are not available in a free and accessible manner. This paper introduces the free and open-source software TEMGYM Advanced, which implements ray tracing methods that calculate the path of electrons through a magnetic or electrostatic lens and allow evaluation of the first-order properties and third-order geometric aberrations. Validation of the aberration coefficient calculations is performed by implementing two independent methods – the aberration integral and differential algebra (DA) methods and by comparing the results of each. This paper also demonstrates parallelised electron ray tracing through a series of magnetic components, which enables near real-time generation of a physically accurate beam-spot including aberrations and brings closer the realisation of a digital twin of an electron microscope. TEMGYM Advanced represents a valuable resource for the electron microscopy community, providing an accessible and open source means of characterising electron lenses. This software utilises the Python programming language to complement the growing ecosystem of free and open-source software within the electron microscopy community, and to facilitate the application of machine learning to an electron microscope digital twin for instrument automation. The software is available under GNU Public License number Three (GPL 3).

For energy efficient and fast magnetic memories, switching of perpendicular magnetization by the spin-orbit torque (SOT) appears as a very promising solution, even more using magnetic insulators that suppress electrical shunting. This SOT switching generally requires the assistance of an in-plane magnetic field to break the symmetry. Here, we present experiments demonstrating the field-free SOT switching of perpendicularly magnetized layers of the thulium iron garnet (Tm_{3}Fe_{5}O_{12}) magnetic insulator. The polarity of the switching loops, clockwise (CW) or counter-clockwise (CCW), is determined by the direction of the initial current pulses, in contrast with field-assisted switchings in which this polarity is controlled by the direction of the field. After an independent determination of Dzyaloshinskii-Moriya interaction (DMI), we relate the field free switching to the interplay of SOT and DMI and the polarity of the loops to the imprint of a N\'eel domain wall induced by the first pulse, in agreement with Kerr imaging. Our observation and interpretation of field-free electrical switching of a magnetic insulator is an important milestone for future low power spintronic devices., Comment: 11 pages, 4 figures

Severe plastic deformation of powder blends consisting of SmCo5-Cu results in magnetically hardened nanocomposite bulk materials. The microstructure is continuously refined with increasing torsional deformation, yet, coercivity saturates at a certain level of strain. Transmission electron microscopy (TEM) investigation of the microstructure reveals a partial amorphization of the SmCo5 phase due to high-pressure torsion by 20 applied rotations. In this amorphous matrix nanocrystals are embedded. The effect of these experimentally observed microstructural features on the magnetic properties are investigated by micromagnetic simulations, which show that an increasing volume fraction of nanocrystals is beneficial for higher coercivities. For a fixed volume fraction of nanocrystals the simulations reveal an increasing coercivity with decreasing the size of the nanocrystals due to increasing number of interfaces acting as pinning sites. Furthermore, our micromagnetic simulations disclose the mechanisms of the saturation and decline of magnetic hardening due to the strain induced by high-pressure torsion. The calculated coercivity fits very well to the experimentally observed coercivity of Hc=1.34 T. The knowledge can also be used to develop and provide optimization strategies from the microstructure perspective.

Two-dimensional van der Waals heterostructures are an attractive platform for studying exchange bias due to their defect free and atomically flat interfaces. Chromium thiophosphate (CrPS4), an antiferromagnet, has uncompensated magnetic spins in a single layer that make it an excellent candidate for studying exchange bias. In this study, we examined the exchange bias in CrPS4/Fe3GeTe2 van der Waals heterostructures using anomalous Hall measurements. Our results show that the exchange bias strength is robust for clean interfaces, with a hysteresis loop shift of about 55 mT at 5 K for few-layer Fe3GeTe2, which is larger than that obtained in most van der Waals AFM/FM heterostructures. However, when exposed to air, the ferromagnetic Fe3GeTe2 layer develops a thin surface oxide layer that significantly modulates the exchange bias. Remarkably, this surface oxide layer can induce exchange bias without any field-cooling, but merely by applying a 'preset field' from 5 K up to 140K due to the presence of ferrimagnetic magnetite. We also observed exchange bias beyond the N\'eel temperature of CrPS4, with two local minima due to contributions from CrPS4 and surface oxides. These results illustrate the complex behaviour of exchange bias in van der Waals heterostructures and its potential for tailored control.

We report the use of an electrostatic MEMS-based device to produce high quality electron vortex beams with more than 1000 quanta of orbital angular momentum (OAM). Diffraction and off-axis electron holography experiments are used to show that the diameter of the vortex in the diffraction plane increases linearly with OAM, thereby allowing the angular momentum content of the vortex to be calibrated. The realization of electron vortex beams with even larger values of OAM is currently limited by the breakdown voltage of the device. Potential solutions to overcome this problem are discussed.

MEMS based electron optics are a promising approach for a new generation of experiments in electron microscopy. In this contribution we improve analytically the modelling of the MEMS based phase plates with thin electrodes with accurate account of the fringing fields. As a result we are able to improve the design and the boundary conditions in MEMS-based spiral phase plates acting on the control parameters. We checked the quality of the generated vortex beam.

This deliverable is meant to summarise the final results regarding the plasmon analysis based on OAM sorter. The aim is to prove that we can achieve a substantial scientific advancement using the OAM-sorter that was not possible without it. We document here both the actual scientific results and the partial attempts that could not be brought to conclusions. The deliverable is separated into 2 main chapters. Chapter 1 reports the efforts to produce the Q-SORT setup for different kinds of plasmon observation and explains why the setup has to be different for every specific experiment. Chapter 2 reports the actual scientific results, of high fundamental relevance, that we obtained for volume plasmons. The experimental results are qualitatively and quantitatively compared with simulation. Conclusions are drawn at the end. It is worth highlighting here that the OAM sorter allowed us to better understand the characteristics of plasmon loss excitation and to find a new phenomenon consisting of the ‘spontaneous Emission’ of quanta of OAM in the interaction with plasmon. Surprising theoretical results were also obtained thanks to the definition of coherence through innovative density matrix formalism: we showed that the coherence of the initial OAM state superposition of the electron is partially preserved after the electron-plasmon scattering, which is reflected mathematically by the possibility of expressing the post-scattering electron density matrix as the sum of one fully coherent term containing the initial OAM values and another term which is characterised by full OAM decoherence. The theory was confirmed by experiment in this case as well.

This deliverable reports the comprehensive results and activities aimed to demonstrate Electron Magnetic Circular Dichroism (EMCD). It is difficult to keep the delicate OAM sorter alignment at the large energy-loss of the iron L-edge (about 700 eV). Considering also the slowdowns due to COVID-19 pandemic we opted for an equally and similar problem by looking at the difference in the σ* and π* transition in 2D material in the energy range of 200eV energy loss. The kind of information and the OAM–EELS theory is fundamentally the same as real EMCD but the difficulty in the alignment is reduced. The experiment therefore succeeds in demonstrating the importance of the combination of OAM-EELS experiment also to study atomic transitions. Considering the similarities of the two experiments this work demonstrate that EMCD is feasible and that automatic alignment procedure should soon allow to explore the energy loss range of EMCD. For these reasons, most of this work will be of quite general interest and only a small part will regard the case of h-BN. Given the rising importance of 2D material that are now receiving even more attention than magnetic structure the result has also a large importance in its own right and fully demonstrates the potentialities of a OAM sorter in material science. Chapter 1 will explain all the improvements we carried out in the direction of EMCD and in OAM–EELS experiments. A large space has been dedicated to the automatic control of the microscope by means of a neural network. Given the difficulties to control the many parameters, this will probably be the best way to carry out the most difficult future experiments. Chapter 2 will explain all problems of localization and decoherence. Once again this theoretical framework is valid for all OAM–EELS experiments and is of general interest to understand our data and to design future experiments. Chapter 3 is dedicated to effective manuscript draft with experiments and data analysis. As for the setup also the deconvolution method used in this section can be easily transported to the case of EMCD.

In quantum mechanics, each conserved quantity (e.g., energy, position, linear momentum and angular momentum) is associated with a Hermitian operator. Its expected value can then be determined by performing a measurement on the wavefunction. In modern electron microscopy, one can select the initial and final states of the electron and the measurement basis by performing measurements of scattering processes. For example, the orbital angular momentum (OAM) of an electron can be used to reveal the n-fold symmetry of a wavefunction scattered by a sample. Here, we introduce a new composite planar chirality operator that can be used to measure a spiral-like feature in a sample. This concept develops the concept of chirality to highlight a specific roto-scale symmetry. We show that planar chirality can be characterized using an electron OAM sorter to uncover the atomic structures of biomolecules in cryo electron microscopy, either in a stand-alone analysis for fast identification of protein structures or in the context of conventional cryo electron microscopy to produce faster and more detailed 3D reconstructions by solving upside-down orientation ambiguities., available also at arXiv:2110.03391 This project has received fundings also from the Deutsche Forschungsgemeinschaft (Project-ID 405553726 – TRR 270), from the DARPA TEE program (Grant MIPR# HR0011831554) and from an Ontario Early Researcher Award (ERA) and a Canada Research Chair (CRC).

Beam shaping - the ability to engineer the phase and the amplitude of massive and massless particles - has long interested scientists working on communication, imaging and the foundations of quantum mechanics. In light optics, the shaping of electromagnetic waves (photons) can be achieved using techniques that include, but are not limited to, direct manipulation of the beam source (as in X-ray Free Electron Lasers (XFELs) and Synchrotrons), deformable mirrors, spatial light modulators, mode converters and holograms. The recent introduction of holographic masks for electrons provides new possibilities for electron beam shaping. Their fabrication has been made possible by advances in micrometric and nanometric device production using lithography and focused ion beam patterning. This article provides a tutorial on the generation, production and analysis of synthetic holograms for transmission electron microscopy. It begins with an introduction to synthetic holograms, outlining why they are useful for beam shaping to study material properties. It then focuses on the fabrication of the required devices from theoretical and experimental perspectives, with examples taken from both simulations and experimental results. Applications of synthetic electron holograms as aberration correctors, electron vortex generators and spatial mode sorters are then presented., This project has received funding from the European Union's Horizon 2020 research and innovation programme (Grant No. 766970, project "Q-SORT"; Grant No. 856538, project "3D MAGiC"; Grant No. 823717, project "ESTEEM3"), from the Deutsche Forschungsgemeinschaft (Project-ID 405553726 – TRR 270), from the DARPA TEE program (Grant MIPR# HR0011831554) and from an Ontario Early Researcher Award (ERA) and a Canada Research Chair (CRC).

Nanomaterials : open access journal 11(2), 466 (2021). doi:10.3390/nano11020466 special issue: "Special Issue "Characterization of Quantum Effects in Nanomaterials, Nano-Devices, and Nanophotonics" / Special Issue Editor: Prof. Dr. Jeong Ryeol Cho, Guest Editor", Published by MDPI, Basel

Magnetic high entropy alloys (HEAs) are a new category of high-performance magnetic materials, with multi-component concentrated compositions and complex multi-phase structures. Although there have been numerous reports of their interesting magnetic properties, there is very limited understanding about the interplay between their hierarchical multi-phase structures and their local magnetic structures. By employing high spatial resolution correlative magnetic, structural and chemical studies, we reveal the influence of a hierarchically decomposed B2 + A2 structure in an AlCo0.5Cr0.5FeNi HEA on the formation of magnetic vortex states within individual A2 (disordered BCC) precipitates, which are distributed in an ordered B2 matrix that is weakly ferromagnetic. Non-magnetic or weakly ferromagnetic B2 precipitates in large magnetic domains of the A2 phase, and strongly magnetic Fe-Co-rich interphase A2 regions, are also observed. These results provide important insight into the origin of coercivity in this HEA, which can be attributed to a complex magnetization process that includes the successive reversal of magnetic vortices.

This deliverable reports on the progress that has been achieved in the direction of the dose-effective imaging of organic molecules. We report several essential experimental advancements, including a successful experimental scheme of ghost imaging that could be realised in Modena without the need of travelling. Overall this deliverable includes material for at least 3 papers and a patent. The restriction on travels due to Covid-19 imposed hard constraints on the performance of the intended experiment. The complete experiment on proteins would, in fact, have required the simultaneous presence of three groups at the microscope in Jülich: FZJ personnel for main operation on the microscope, people from MU for the cryomicroscopy part, and people from CNR for the lateral control of software and for the coordination of the experiment. Moreover, the preparation of the experiment required the intervention of FEI personnel. Unfortunately, in spite of the partial release of the restrictions during the extra 6 months allotted to Q-SORT, for the reasons given above these conditions were impossible to meet. This was unforeseeable at the moment the project extension was asked. Given these hard constraints, we chose the combination of experimental activities that allowed us to achieve the greatest possible amount of project goals. In fact, Q-SORT scientists achieved more than what was foreseen; novel results include: 1) the first patentable phase plates of different design ( designed in WP3 D3.4 for protein recognition) able to control up to 24 electrodes by multiplexing the initial 8 inputs. 2) the first computational ghost imaging experiment in microscopy and its optimization for low dose regime 3) a state of the art neural network control for an efficient prediction of the generated caustic pattern. Two approaches to the protein recognition were chosen at the end of WP3: a) the extension of the OAM sorter with an additional programmable phase plate. Such approach is referred to as GOAMS (generalised OAM sorter) in D3.2 as b) the computational ghost imaging with increasingly complex masks. Under the given constraints, it was found that the computational ghost imaging approach (D3.2, task 3.1 & 3.2) was the most appropriate and effective to pursue - enabling the greatest advancement. This development is also consistent with contingency plan #6 “Test random or Hadamard sparse sampling”. The results have been also written as a manuscript that is going to be submitted soon. The hardware for the 3-element GOAMS (i.e. the second approach that was found to be most promising in WP3) was also developed, with the design of an additional element that allowed the recognition of the radial degree of freedom. Such extended “protein specific” phase modulator (D3.4) was also finished and tested as one of the design of the programmable phase plates. [...]

Arrays of interacting 2D nanomagnets display unprecedented electromagnetic properties via collective effects, demonstrated in artificial spin ices and magnonic crystals. Progress toward 3D magnetic metamaterials is hampered by two challenges: fabricating 3D structures near intrinsic magnetic length scales (sub-100 nm) and visualizing their magnetic configurations. Here, we fabricate and measure nanoscale magnetic gyroids, periodic chiral networks comprising nanowire-like struts forming three-connected vertices. Via block copolymer templating, we produce Ni75Fe25 single-gyroid and double-gyroid (an inversion pair of single-gyroids) nanostructures with a 42 nm unit cell and 11 nm diameter struts, comparable to the exchange length in Ni-Fe. We visualize their magnetization distributions via off-axis electron holography with nanometer spatial resolution and interpret the patterns using finite-element micromagnetic simulations. Our results suggest an intricate, frustrated remanent state which is ferromagnetic but without a unique equilibrium configuration, opening new possibilities for collective phenomena in magnetism, including 3D magnonic crystals and unconventional computing.

The Gatan K2 IS direct electron detector (Gatan Inc., 2018), which was introduced in 2014, marked a watershed moment in the development of cameras for transmission electron microscopy (TEM) (Pan & Czarnik, 2016). Its pixel frequency, i.e. the number of data points (pixels) recorded per second, was two orders of magnitude higher than the fastest cameras available only five years before. Starting from 2009, the data rate of TEM cameras has outpaced the development of network, mass storage and memory bandwidth by almost two orders of magnitude. Consequently, solutions based on personal computers (PCs) that were adequate until then are no longer able to handle the resulting data rates. Instead, tailored high-performance setups are necessary. Similar developments have occurred for advanced X-ray sources such as the European XFEL, requiring special information technology (IT) systems for data handling (Sauter, Hattne, Grosse-Kunstleve, & Echols, 2013) (Fangohr, et al., 2018). Information and detector technology are currently under rapid development and involve disruptive technological innovations. This chapter briefly reviews the technological developments of the past 20 years, presents a snapshot of the current situation at the beginning of 2019 with many practical considerations, and looks forward to future developments.

A novel device that can be used as a tunable support-free phase plate for transmission electron microscopy of weakly scattering specimens is described. The device relies on the generation of a controlled phase shift by the magnetic field of a segment of current-carrying wire that is oriented parallel or antiparallel to the electron beam. The validity of the concept is established using both experimental electron holographic measurements and a theoretical model based on Ampere's law. Computer simulations are used to illustrate the resulting contrast enhancement for studies of biological cells and macromolecules.

The use of chiral skyrmions, which are nanoscale vortex-like spin textures, as movable data bit carriers forms the basis of a recently proposed concept for magnetic solid-state memory. In this concept, skyrmions are considered to be unique localized spin textures, which are used to encode data through the quantization of different distances between identical skyrmions on a guiding nanostripe. However, the conservation of distances between highly mobile and interacting skyrmions is difficult to implement in practice. Here, we report the direct observation of another type of theoretically-predicted localized magnetic state, which is referred to as a chiral bobber (ChB), using quantitative off-axis electron holography. We show that ChBs can coexist together with skyrmions. Our results suggest a novel approach for data encoding, whereby a stream of binary data representing a sequence of ones and zeros can be encoded via a sequence of skyrmions and bobbers. The need to maintain defined distances between data bit carriers is then not required. The proposed concept of data encoding promises to expedite the realization of a new generation of magnetic solid-state memory.

Nearly twenty years ago, following a sixty year struggle, scientists succeeded in correcting the bane of electron lenses, spherical aberration, using electromagnetic aberration correction. However, such correctors necessitate re-engineering of the electron column, additional space, a power supply, water cooling, and other requirements. Here, we show how modern nanofabrication techniques can be used to surpass the resolution of an uncorrected scanning transmission electron microscope more simply by sculpting a foil of material into a refractive corrector that negates spherical aberration. This corrector can be fabricated at low cost using a simple process and installed on existing electron microscopes without changing their hardware, thereby providing an immediate upgrade to spatial resolution. Using our corrector, we reveal features of Si and Cu samples that cannot be resolved in the uncorrected microscope., Comment: Roy Shiloh, Roei Remez, and Peng-Han Lu equally contributed to this work

Supplementary material

Quantum complementarity states that particles, e.g. electrons, can exhibit wave-like properties such as diffraction and interference upon propagation. \textit{Electron waves} defined by a helical wavefront are referred to as twisted electrons~\cite{uchida:10,verbeeck:10,mcmorran:11}. These electrons are also characterised by a quantized and unbounded magnetic dipole moment parallel to their propagation direction, as they possess a net charge of $-|e|$~\cite{bliokh:07}. When interacting with magnetic materials, the wavefunctions of twisted electrons are inherently modified~\cite{lloyd:12b,schattschneider:14a,asenjo:14}. Such variations therefore motivate the need to analyze electron wavefunctions, especially their wavefronts, in order to obtain information regarding the material's structure~\cite{harris:15}. Here, we propose, design, and demonstrate the performance of a device for measuring an electron's azimuthal wavefunction, i.e. its orbital angular momentum (OAM) content. Our device consists of nanoscale holograms designed to introduce astigmatism onto the electron wavefunctions and spatially separate its phase components. We sort pure and superposition OAM states of electrons ranging within OAM values of $-10$ and $10$. We employ the device to analyze the OAM spectrum of electrons having been affected by a micron-scale magnetic dipole, thus establishing that, with a midfield optical configuration, our sorter can be an instrument for nano-scale magnetic spectroscopy., Comment: 5 pages, 3 figures, 3 pages of supplementary materials

Trabajo presentado al SPIE Photonics West, celebrado en San Francisco, California (USA) del 13 al 18 de febrero de 2016.-- et al., Complex metal nanoparticles offer a great playground for plasmonic nanoengineering, where it is possible to cover plasmon resonances from ultraviolet to near infrared by modifying the morphologies from solid nanocubes to nanoframes, multiwalled hollow nanoboxes or even nanotubes with hybrid (alternating solid and hollow) structures. We experimentally show that structural modifications, i.e. void size and final morphology, are the dominant determinants for the final plasmonic properties, while compositional variations allow us to get a fine tuning. EELS mappings of localized surface plasmon resonances (LSPRs) reveal an enhanced plasmon field inside the voids of hollow AuAg nanostructures along with a more homogeneous distributions of the plasmon fields around the nanostructures. With the present methodology and the appropriate samples we are able to compare the effects of hybridization at the nanoscale in hollow nanostructures. Boundary element method (BEM) simulations also reveal the effects of structural nanoengineering on plasmonic properties of hollow metal nanostructures. Possibility of tuning the LSPR properties of hollow metal nanostructures in a wide range of energy by modifying the void size/shell thickness is shown by BEM simulations, which reveals that void size is the dominant factor for tuning the LSPRs. As a proof of concept for enhanced plasmonic properties, we show effective label free sensing of bovine serum albumin (BSA) with some of our hollow nanostructures. In addition, the different plasmonic modes observed have also been studied and mapped in 3D.

Using nanometer spatial resolution electron-energy loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDS), and cathodoluminescence (CL) mapping, we demonstrate that Au alloys containing a poor plasmonic metal (Pd) can nevertheless sustain multiple size-dependent localized surface plasmon resonances and observe strong field enhancement at Pd-rich tips, where the composition is in fact least favorable for plasmons. These Au/Pd stellated nanocrystals are also involved in substrate and interparticle coupling, as unraveled by EELS tilt series.

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Entanglement and correlation are at the basis of quantummechanicsand have been used in optics to create a framework for "ghostimaging". We propose that a similar scheme can be used in anelectron microscope to exploit the correlation of electrons with thecoincident detection of collective mode excitations in a sample. Inthis way, an image of the sample can be formed on an electron cameraeven if electrons never illuminated the region of interest directly.This concept, which can be regarded as the inverse of photon-inducednear-field electron microscopy, can be used to probe delicate moleculeswith a resolution that is beyond the wavelength of the collectivemode.

Geochimica Et Cosmochimica Acta

In situ studies of micro- and nano-objects in their characteristic environment have been performed ever since the early days of electron microscopy [1]. Over several decades the in situ observation of the synthesis of filamentous carbon (nanotubes/nanofilaments) during hydrocarbon decomposition has been one of the most popular topics [2] for investigation in the environmental transmission electron microscope (ETEM). In this work we study the growth of carbon nanotubes (CNTs) by the decomposition of acetylene on Co nanoparticles inserted into mesoporous silicas (SBA-15) using both conventional post mortem TEM measurements and real-time in situ ETEM observations.In situ observation of the formation of the carbon nanotubes was performed in an FEI Titan 80-300 ETEM equipped with an objective lens spherical aberration corrector [3]. Prior to acetylene decomposition, the catalyst nanoparticles were reduced in situ in a flow of hydrogen (1 mbar, ~500°C). Electron energy-loss spectra taken before and during reduction showed that the Co oxide nanoparticles were reduced to metallic Co. In situ high resolution TEM images are consistent with cubic Co. A first attempt to study carbon nanotube growth above 600°C in 0.6 mbar of acetylene in situ in the ETEM resulted in complete growth of CNTs within seconds (or faster) which was not consistent with real-time growth observation with the electron microscope. The temperature was therefore decreased to ~ 500°C and acetylene pressure in the 10-3/10-2 mbar range was used to decrease the growth rate to allow real-time observation of the formation of CNTs over several minutes. These conditions also reduced the coking of the nanoparticles and favoured the formation of tubular structures. Two types of CNTs following the tip-growth mechanism with apparently different growth rates were observed : (i) CNTs with diameters of 5 to 10 nm and rather uniform central channels (black arrows in Fig. 1a). These nanotubes grew primarily on small round shaped nanoparticles and (ii) CNTs containing voids and/or non-uniform central channels, with diameters of 15 to 20 nm (white arrows in Fig. 1a). These nanotubes grew on well-faceted nanoparticles that adopted “pyramidal shapes” during growth and were larger than the pores of SBA-15. Real-time morphological changes of the catalyst were observed during growth (Fig. 1b). As reported previously in the literature [2 (fourth and eighth references therein)] the Co nanoparticle pulsated (elongated and contracted) during the growth of the nanotube. This sequential elongation, often associated with the formation of a narrow neck, is clearly responsible for the presence of small nanoparticles within the nanotubes (Fig. 2).Furthermore it was possible to observe severe degradation of the carbon nanotube structure during exposure to the energetic electron beam (Fig. 3) revealing that during real time in situ observation of chemical processes one has to take into account the role of the energetic electron beam or devise ways to minimize its contribution.Ultimately these in situ real-time studies allow measurements of the growth rates that are expected to provide new insights on the catalyst dynamics during growth including the evolution of exposed facets and (ideally) the identification of lattice planes and/or specific sites responsible for preferential carbon expulsion essential to understand the growth mechanisms of the different CNTs.In an more general scope it is clear that ETEM studies of catalytic processes need real-time capability not only in the range of seconds (as it is available in contemporary microscopes) but in much lower timescales (milliseconds to nanoseconds or even bellow) and in the different modes available in the ETEM (high resolution BF and HAADF imaging, diffraction, EELS, tomography…). This can of course be implemented by using the dynamic TEM approach in an ETEM even though some technological difficulties (namely for tomography) have to be overcome at the present time.

Changes in the nanostructure of methylammonium lead iodide (MAPbI(3)) perovskite solar cells are assessed as a function of current-voltage stimulus by biasing thin samples in situ in a transmission electron microscope. Various degradation pathways are identified both in situ and ex situ, predominantly at the positively biased MAPbI(3) interface. Iodide migrates into the positively biased charge transport layer and also volatilizes along with organic species, which triggers the nucleation of PbI2 nanoparticles and voids and hence decreases the cell performance.

Fe-doped GaN semiconductors are of interest for combining the properties of semiconductors and magnetic materials [1]. Depending on the growth temperature used, Fe can either be distributed homogenously in the GaN host lattice or it can accumulate in the form of Fe-N nanocrystals. As a result of the small size of the nanocrystals and the sensitivity of Fe-doped GaN to specimen preparation for electron microscopy, the formation and physical properties of Fe-N nanocrystals in GaN are not yet fully understood. The (Ga,Fe)N samples examined below were grown by metalorganic chemical vapour deposition [1] and studied in cross-sectional geometry using several transmission electron microscopy (TEM) techniques. Great care was taken during TEM specimen preparation to minimize Ar ion-beam induced artefacts. Fe-N nanocrystal formation was observed in samples that had been deposited at temperatures higher than 850°C. Interestingly, most of the Fe-N nanocrystals were found to be associated with closely adjacent void-like features in both TEM and scanning TEM (STEM) images, as shown in Figure 1(a). Here, we use aberration-corrected STEM and electron energy-loss spectroscopy (EELS) to show that these features are bubbles filled with molecular N2. In order to interpret our experimental results, we calculate the N K spectrum for GaN using self-consistent real-space multiple-scattering calculations using FEFF 9.05, which allows to include the experimental conditions. A dedicated STEM EELS measurement was performed across a single nanocrystal (shown in Fig. 1 (b)) embedded in the GaN host. A 100 kV acceleration voltage and a distributed-dose acquisition routine [2] was used to either minimize or control electron beam induced damage during the experiment. Figure 2 (a) shows selected N K edge spectra recorded from the nanocrystal, the adjacent N2-containing region and the GaN host. The N K edge shows a typical three-peaked structure between 400 and 407 eV. Figure 2 (b) shows the experimental spectrum acquired from the bubble alongside with the experimental spectrum of N2 [3] and simulation of a GaN spectrum. Figure 3 shows the result of an experiment that provides direct evidence for the presence of N2 gas by using the electron beam to puncture the bubble adjacent to the Fe-N nanocrystal and to release the gas into the microscope. We used a static sub-Angstrom beam with a current of about 350 pA to make a hole in the specimen exactly at the position of the bubble, while recording EELS spectra every 40 seconds. The intensity of the characteristic first peak of the N K edge at 401 eV was observed to decrease suddenly when the gas was released. The quantitative determination of the pressure of the N2 gas in the bubble from the recorded EELS spectra is in progress.

InAs quantum dots (QDs) grown on InP or InGaAsP are used for optical communication applications operating in the 1.3 – 1.55 μm wavelength range. It is generally understood that the optical properties of such QDs are highly dependent on their three-dimensional structural and chemical profiles. Whilst conventional transmission electron microscopy (TEM) techniques can be used to study capped QDs in plan-view or cross-sectional geometries, the resulting images can provide ambiguous information about their three-dimensional properties. Here, we describe an approach for investigating the applicability of both high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) tomography and atom probe tomography (APT) to the study of surface and buried InAs/InGaAsP QDs grown by metal organic vapour phase epitaxy (MOVPE). Electron tomography was carried out in an FEI Titan TEM instrument operated at 300 kV. TEM specimens were prepared in plan-view geometry using mechanical grinding, polishing and Ar ion milling. Both original HAADF STEM images and final tomographic reconstruction of surface QDs suggest an elongated hexagonal shape for the bases of the QDs (Figure 1). The elongation direction was determined to be [110], using selected area electron diffraction and atomic force microscopy. The HAADF STEM images also suggest that surface QDs have a double-terraced geometry, with steeper facets around their bases and shallower facets close to their tops. This geometry is consistent with a theoretical model of InAs QDs formed on an InGaAs substrate that is lattice matched to InP [1] shown in Figure 1(b). Despite the large inner detector semi-angle used (approximately 50 mrad), strong diffraction effects were present in the original tilt series of HAADF STEM images, resulting in departure from the projection requirement for electron tomography, which states that the recorded intensity should be a monotonic function of a property of the object [2]. These diffraction effects are likely to be associated with diffraction and may lead to artefacts in the tomographic reconstruction. The same tomographic analysis was applied to a buried InAs/InGaAsP QD (Figure 1(d) and (e)). The buried QD appears to be elongated along the [110] direction, although not as strongly as the surface QD. Similarly, the faceting that is clearly visible in both the original HAADF STEM images and the final reconstruction of the surface QD, is not as pronounced for the buried QD. This difference may result from chemical intermixing between the buried QD and the capping material during overgrowth. A limiting constraint in STEM tomography of thin film specimens is the limited tilt range available before the specimen becomes too thick for imaging. This limitation can, in principle, be overcome by fabricating needle-shaped specimens using focused ion beam (FIB) milling, in order to allow unlimited tilting without significant increase in projected specimen thickness. However, FIB milling can introduce considerable damage into III-V semiconductors, including amorphisation and Ga ion implantation [3]. We have fabricated needle-shaped specimens that are 100 nm in diameter, using reactive ion etching, selective wet etching and critical point drying in plan-view geometry (Figure 2). The choice of a plan-view geometry for the needles means that each specimen will contain several QDs. The needles can either be detached from the substrate by cleaving (Figure 2(b)) or lifted out and mounted onto suitable grids using a micro-manipulator in the FIB with minimal additional damage (Figure 2(c)). Significantly, in addition to their suitability for electron tomography, these specimens can be used for APT, for which needle-shaped specimens with sharp tips (narrower than 100 nm) are required. Our ongoing experiments involve the application of both HAADF STEM tomography and APT to the same QD, in order to better understand its morphology and composition. A comparison between reconstructions obtained using both techniques will also assist in the evaluation and mitigation of potential artefacts that are present when using each technique.

As a result of diminishing fossil fuel reserves, there is an increasing need to switch energy dependence to renewable resources such as sunlight. Photocatalysts provide a viable route for converting solar energy into chemical bonds. In order to optimize the performance of such materials, it is necessary to understand the fundamentals of their reaction mechanisms, chemical behavior, structure and morphology before, during and after reaction using in situ investigations. Here, we focus on the in situ characterization of photocatalysts [1] in an environmental transmission electron microscope (ETEM) [2]. Such fundamental insight can be used for further material optimization with respect to performance and stability [3].In this work, we combine conventional TEM analysis of photocatalysts with environmental TEM (ETEM) and photoactivation using light. A novel type of TEM specimen holder that enables in situ illumination is developed to study light-induced phenomena in photoactive materials at the nanoscale under working conditions.Our experiments are aimed at exposing a specimen to light and detecting resulting microstructural and chemical changes using in situ TEM techniques. It is important to investigate photoactive materials under light illumination in order to remove the effects associated with handling of the specimen between ex situ reactions and TEM experiments. Two representative photoinduced phenomena are studied: the photodegredation of Cu2O and the photodepositon of Pt onto a GaN:ZnO photocatalyst (Figure 1).

The latest generation of environmental transmission electron microscopes (ETEMs) incorporates aberration correction and monochromation, allowing studies of chemical reactions and growth processes with improved spatial and spectral resolution. These additions to the columns of commercial ETEMs have improved the point resolution to the sub-Ångström level [1] and reduced image delocalization, allowing images of surface and interface structures to be interpreted more directly [2]. However, when gas is present in the microscope the path of electrons along the column is modified due to gas-electron scattering [3]. In general there are two approaches for performing TEM experiments in the presence of gases. These approaches are based on a differential pumping scheme and the closed cell TEM holder approach and each has its advantages and disadvantages. In the closed cell approach, gas molecules are confined to a thin (typically 50-200 μm thick) slab around the sample, but the electrons interact with the window material (e.g. C, SiN) as well as with the gas and the sample. In addition, the field of view is typically smaller than in a conventional TEM and a limited range of sample geometries can be used. In the differential pumping approach, the gas is confined to the region around the specimen only by pressure-limiting apertures. In order to retain flexibility in the sample region, the pole piece gap and the highest pressure part of the column are relatively large (~7mm). As a result, electron scattering by gas molecules occurs both above and below the sample along a distance that is comparable to the focal length of the objective lens. Gas molecules are also present in the rest of the column, although at a pressure that is several orders of magnitude lower than that around the specimen due to the use of both pressure-limiting apertures and additional pumping capabilities. In order to take full advantage of the aberration corrected objective lens system by retrieving quantitative information from images acquired in the presence of gas, the propagation of the electron wave in the entire microscope column, including electron scattering by gas molecules has to be understood. The most basic effect on images acquired in the presence of gas is a simple decrease in intensity with increasing gas pressure and scattering cross-section of the gas molecules. Figure 1 shows a representative example of intensity profiles determined from bright-field images of a GaN specimen, plotted as a function of Ar pressure in a differential pumped FEI Titan microscope. Significantly, Fig. 1 shows that the details of the image contrast also change with pressure, primarily because the electron wave is modified by scattering from gas molecules. Furthermore, the electrons lose energy when they are scattered by gas molecules leading to a less isochromatic incident electron beam. Figure 2 shows a preliminary result of angle-resolved low-loss EELS acquired in the presence of 980 Pa of O2 in the absence of a specimen. Our on-going work involves the systematic measurements of images, diffraction patterns and energy-loss spectra acquired in the presence of gas, for a variety of different beam current densities, accelerating voltages and choices of specimen.

Semiconductor nanowires have been studied using electron microscopy since the early days of nanowire growth, e.g. [1]. A common approach for analysing nanowires using transmission electron microscopy (TEM) involves removing them from their substrate and subsequently transferring them onto carbon films. This sample preparation method is fast and usually results in little structural change in the nanowires [2]. However, it does not provide information about the interface between the nanowires and the substrate, who’s physical and electrical properties are important for many modern applications of nanowires. In particular, strain and crystallographic defects can have a major influence on the electronic structure of the material. In improved method for the characterization of such interfaces would be valuable for optimizing and understanding the transport properties of devices based on nanowires. Here, we systematically investigate the interface between a nanowire and its substrate using three complementary methods for assessing strain. Results obtained using high resolution TEM for geometric phase analysis (GPA), convergent beam elecron diffraction (CBED) and nanobeam electron diffraction (NBED) are compared and contrasted. GPA measurements were acquired at 300kV in an FEI Titan 89-300 while the two diffraction methods were applied in the same microscope at 120kV. The GPA analysis software developed by C.T. Koch and V.B. Özdöl was used [3]. For samples other than nanowires, previous comparisons of GPA with CBED and NBED [4,5] have shown a high degree of consistency. Strain has previously only been measured in nanowires removed from their substrate [6], or only using GPA [7]. The sample used for the present investigation was an InP nanowire grown on a Si substrate using metal organic vapor phase deposition (MOCVD). Lattice missmatch between Si and InP is 8%. The nanowire had a diameter of approximately 100 nm in the interface area. TEM samples were prepared using a tripod polishing technique, with Ar ion milling used as the final thinning step. The resulting sample was clean and virtually free from defects from sample preparation. Measurements using all three techniques were obtained from the same well defined region of the specimen. Energy dispersive X-ray spectroscopy (EDS) maps were also acquired from the same area. Preliminary results acquired using GPA are shown in Figure 1. Whereas the base of the wire show some strain, little strain is observed in the substrate. The influence of defects, interfacial layers and compositional variations on the GPA will be discussed.

The structural and electronic properties of nonstoichiometric low-temperature grown GaAs nanowire shells have been investigated with scanning tunneling microscopy and spectroscopy, pump probe reflectivity, and cathodoluminescence measurements. The growth of nonstoichiometric GaAs shells is achieved through the formation of As antisite defects, and to a lower extent, after annealing, As precipitates. Because of the high density of atomic steps on the nanowire sidewalls, the Fermi level is pinned midgap, causing the ionization of the subsurface antisites and the formation of depleted regions around the As precipitates. Controlling their incorporation offers a way to obtain unique electronic and optical properties that depart from the ones found in conventional GaAs nanowires.

Photocatalysts are of fundamental interest for sustainable energy research [1]. By means of transmission electron microscopy (TEM) it is possible to obtain insight into the structure, composition and reactivity of photocatalysts. Such insight can be used for their further optimization [2]. We have constructed a specimen holder capable of shining light onto samples inside the TEM. The holder contains a laser diode and an optical system that guides light onto a sample with maximum power transmission. The source can be changed and tuned, in principle spanning the whole visible and UV spectrum. The device can be used inside an environmental TEM (ETEM) allowing specimens to be analyzed during exposure to a controlled gas atmosphere and illumination. The holder is presently being used to study a variety of photoreactive materials and structures, including photocatalysts, photonic devices and solar cells. For example, electron holography is being used to study p-n junctions both in the presence and in the absence of light in order to assess electron beam induced charging and discharging effects during laser light exposure [3]. Here, we present results from ETEM studies of light-induced phenomena that include metal nanoparticle photodeposition, light-driven particle discharging and photodegradation. We concentrate on phase transitions of Cu2O nanocubes under visible light exposure in the presence of water vapor, which we study in situ. Cu2O is an active photocatalyst for water splitting under visible light irradiation, but it undergoes photodegradation in an aqueous environment [4]. [1] Herrmann, J. M., Top. Catal. 2005, 34, (1-4), 49-65. [2] Tsujimoto, M., S. Moriguchi, et al., J. Electron. Microsc. (Tokyo) 1999, 48, (4), 361-366. [3] Shindo, D., K. Takahashi, et al., J Electron Microsc. (Tokyo) 2009, 58, (4), 245-249. [4] Nagasubramanian, G., A. S. Gioda, J. Electrochem. Soc. 1981, 128, (10), 2158-2164.

The development of transportation fuels from sustainable resources requires new and better production paths. One approach involves the use of biogas to synthesize alcohols, such as methanol or higher alcohols for fuel. For the production of methanol a reduction of processing temperature and pressure to lower the process cost and make the product more competitive is desired. Higher alcohols are in general favourable over methanol due to their high energy density and ease of use in current internal combustion engines. However, the yield of higher alcohols in present production routes is poor prompting for the development of better catalysts [1]. The Catalysis for Sustainable Energy Project (CASE) at the Technical University of Denmark aims at discovering new and improved catalysts based on density functional theory and on testing the chemical reactivity of the most promising candidates experimentally. Transmission electron microscopy (TEM) is used for microstructural characterization and provides feedback for both theory and synthesis. TEM is a powerful tool for characterizing of catalysts. However, conventional TEM does not provide dynamic information about catalysts in their working state. We have recently installed an environmental transmission electron microscope (ETEM) equipped with a differential pumping system to confine a controlled flow of gas around the specimen, allowing observation in a gaseous environment (FEI Titan E-cell, monochromated, objective lens aberration corrector, Gatan imaging filter). In combination with a heating holder, this microscope allows catalysts to be studied using a variety of TEM techniques in situ in a reactive environment approaching the working conditions of the catalysts [2,3]. Here, we present recent ETEM studies of newly synthesized catalysts for alcohol synthesis. Using High-Resolution TEM, electron diffraction, energy electron-loss spectroscopy (EELS), Energy Dispersive X-ray spectroscopy (EDX) and High-Angle Annular Dark Field (HAADF) investigations we have studied the structural changes of these catalysts. Complementary obeservations have been done using Extended X-ray Absorption Fine Structure (EXAFS). The experimental findings have been correlated with the observed changes in the chemical activity of the catalysts. Systematic studies involving variation in temperature, gas pressure and composition were performed and related to structural changes in the specimen. Representative TEM images of a CuSn based catalyst for synthesis of higher alcohols are shown in Figure 1. The CuSn particles are observed to sinter during the reduction leading to a decreased activity of the catalyst. Figure 2 shows the distribution of Co and Mo in a Co/MoS2 catalyst during treatment in 1.5 mbar H2 at 600K. The Co phase is seen to segregate from MoS2. The EDX scans in Figure 2 b) and c) indicate this segregation.

The interface of metal-oxide plays pivotal roles in catalytic reactions, but its catalytic function is still not clear. In this study, we report the high activity of nanostructured Ru/ceria (Ru-clusters/ceria) in the ethylene methoxycarbonylation (EMC) reaction in the absence of acid promoter. The catalyst offers 92% yield of MP with TOF of 8666 h–1, which is about 2.5 times of homogeneous Pd catalyst (∼3500 h–1). The interfacial Lewis acid–base pair [Ru-O-Ce-Vo], which consists of acidic Ce-Vo (oxygen vacancy) site and basic interfacial oxygen of Ru-O-Ce linkage, acts as active site for the dissociation of methanol and the subsequent transfer of hydrogen to the activated ethylene, which is the key step in acid-promoter-free EMC reaction. The combination of 1H MAS NMR, pyridine-IR and DFT calculations reveals the hydrogen species derived from methanol contains Bronsted acidity. The EMC reaction mechanism under acid-promoter-free condition over Ru-clusters/ceria catalyst is discussed.