Voordrachten 2017

Woensdag 19 juli 2017,  Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Applications of the k.p theory to silicon thin layers
Spreker: Prof. Milan Tadic, University of Belgrade and CMT, University of Antwerp

Abstract:
The talk deals with the 30-band k.p theory and its applications to silicon quantum wells. Numerous spurious solutions are found in the energy spectra of those quantum wells and the algorithm to remove them is proposed. The interband optical absorption in the Si/SiO2 quantum well is calculated as a function of the well width W, and the effective direct band gap is found to agree with the 1/W2 scaling result of the single-band model. Finally, the valley splitting in thin silicon layers is investigated. It was shown that it is necessary to restrict the computation basis to the first Brillouin zone to achieve a correct description of the valley splitting by the 30-band model.

Woensdag 21 juni 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Ab initio calculations of Fluorine dimers and trimers on graphene
Spreker: Jagger Rivera-Julio, CMT, University of Antwerp

Abstract:
First-principles calculations of the absorption and diffusion of small Fluorine clusters on graphene were performed by using Density Functional Theory (DFT). Several different absorption congurations of fluorine dimers were considered on one side of the graphene sheet (cis-clusters) or at both sides (trans-clusters). The energetically most favorable absorption configuration for cis-clusters corresponds to the para configuration, while for trans-clusters, the most favorable is the ortho conguration. The energy barriers for the diffusion of Fluorine atoms were also calculated.

Woensdag 14 juni 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: The quantum canonical ensemble: a projection operator treatment
Spreker: Prof. Wim Magnus, IMEC, Leuven and CMT, University of Antwerp

Abstract:
Knowing the exact number of particles N, and taking this knowledge into account, the quantum canonical ensemble imposes a constraint on the occupation number operators. The constraint particularly hampers the systematic calculation of the partition function and any relevant thermodynamic expectation value for arbitrary but fixed N. On the other hand, fixing only the average number of particles, one may remove the above constraint and simply factorize the traces in Fock space into traces over single-particle states. As is well known, that would be the strategy of the grand-canonical ensemble which, however, comes with an additional Lagrange multiplier to impose the average number of particles. The appearance of this multiplier can be avoided by invoking a projection operator that enables a constraint-free computation of the partition function and its derived quantities in the canonical ensemble, at the price of an angular or contour integration.
Introduced in the recent past to handle various issues related to particle-number projected statistics, the projection operator approach proves beneficial to a wide variety of problems in condensed matter physics for which the canonical ensemble offers a natural and appropriate environment. In this light, we present a systematic treatment of the canonical ensemble that embeds the projection operator into the formalism of second quantization while explicitly fixing N, the very number of particles rather than the average. Being applicable to both bosonic and fermionic systems in arbitrary dimensions, transparent integral representations are provided for the partition function Z_N and the Helmholtz free energy F_N as well as for two- and four-point correlation functions. The chemical potential is not a Lagrange multiplier regulating the average particle number but can be extracted from F_{N+1} - F_N, as illustrated for a two-dimensional fermion gas.  As a particular application to semiconductor device simulations, we quote the use of the canonical treatment to predict a MOSFET threshold voltage, typically characterizing a regime with very low electron concentrations, for which the grand-canonical description fails.

Vrijdag 9 juni 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Crystal structure refinement using electron microscopy and powder X-ray diffraction techniques
Spreker: Anatolii Morozov, MSU, Faculty of Materials Science

Abstract:
The lecture will outline the results obtained during the first 2 months of my internship as a master student at EMAT.
The internship is part of a project to design advanced noble metal-free transition 3d-metal (Mn, Fe,Co and Ni) nano-oxides and hydroxides possessing high activity both in the oxygen reduction (ORR) and theoxygen evolution reactions (OER), in view of their application at positive electrodes of unitized regenerative fuel cells with alkalinemembranes.. However, the structure refinement of such materials is necessary for a correct description of their properties. Combining transmission electron microscopy withpowder X-ray diffraction is a powerful method for reaching this goal.  The lecture will discuss the results of this combination of techniques on several of the candidate materials.

Woensdag 7 juni 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger

Voordracht georganiseerd door TGM

Onderwerp : Theories for electron transport in 2D materials
Spreker: Miša Anđelković, CMT, University of Antwerp

Abstract:
The advance in the semiconductor industry has led to a significant decrease of the characteristic dimensions of the devices in the last decades. Together with the isolation of the first 2D materials, quantum phenomena started to be of great importance, which induced the need for different theories that could properly describe the electronic properties of such structures.
The first part of the talk will be based on the brief review of different theoretical frameworks for electron transport calculations, starting from macroscopic semiclassical Boltzmann theory, to quantum theories such as Kubo linear response and Landauer-Büttiker scattering formalism. We’ll try to emphasize not only their domains of validity and limitations but also their connection.
After the introductory part, we will focus on the numerical effort needed for calculation of transport properties using Kubo formulas in the linear response regime, which is suitable when one is interested in bulk properties of different materials. We will show the application of the method to different structures in the presence of various fields and disorder realisations. Remarkable properties such as preservation of chiral symmetry in the presence of vacancy disorder will be shown in graphene, and it’s breaking in twisted bilayer graphene.

Vrijdag 2 juni 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Quantification of 3D atomic positions for nanoparticles using scanning transmission electron microscopy: statistical parameter estimation, dose-limited precision and optimal experimental design
Spreker: Marcos Alania, EMAT, University of Antwerp

Abstract:
Thanks to their unique properties and numerous applications in a wide range of materials and devices, nanoparticles have attracted enormous attention from the scientific and industrial community during the last years. Therefore, in order to deeply understand their properties, a detailed structural characterization and chemical mapping at atomic level are required. In this talk, we will discuss about a method that allows us to quantify the theoretical limits with which atomic columns or atoms of a nanocluster can be located in 2D and 3D, respectively, from a scanning transmission electron microscopy experiment. The concept of this study is explored from a theoretical point of view. Therefore, the combination of both statistical parameter estimation and image simulations is proposed.

Vrijdag 19 mei 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Quantitative Diffraction and Imaging Based STEM Techniques for Strain & E-Field Mapping
Spreker: Benedikt Haas, INAC, CEA-Grenoble, Grenoble, France

Abstract:
The talk discusses different STEM techniques for quantitative measurements of strain or electric fields. Discussions of the different techniques are each time followed by an example of application to a material system.
In the first part of the presentation three different STEM techniques for strain measurements with increasing strain precision and decreasing spatial resolution are shown: atom position determination using multi-reference rigid-registration (Zorro) and template-matching (TeMA), strain from two-dimensional scanning moiré patterns even for non-square crystal geometry, and nano-beam precession diffraction for strain mapping.
The second part of the talk is discussing the measurement of (mesoscopic) electric fields in STEM. First, DPC is assessed for different convergence angle settings and compared to (off-axis) holography as a more established CTEM based technique. Artifacts in DPC from dynamical diffraction are discussed in terms of convergence angle and sample tilt and also experimentally evidenced in a strained sample by comparison with holography. Afterwards, a new technique to measure electric fields by means of nano-beam precession diffraction is demonstrated. This technique that allows to determine electric fields and strain from the same data set is used to map piezo-electric fields connected to the strain fields of individual dislocations.

Donderdag 18 mei 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Topological band theory
Spreker : Otwin Leenaerts,  CMT, University of Antwerp

Abstract:
I will give an overview of topological band theory through some simple two-band model systems. I will start from diatomic molecules and the Su-Schrieffer-Heeger model for polyacetylene and extrapolate the physics of these models to graphene and 3D Weyl semimetals. 

Vrijdag 12 mei 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Microstructural characterization of an austenitic stainless steel containing TiC nanoprecipitates
Spreker: Niels Cautaerts, SCK•CEN and EMAT, University of Antwerp

Abstract:
For future lead and lead-bismuth cooled fast reactors, stabilized austenitic stainless steels are being considered as fuel cladding material. Materials in the core of such reactors are subject to high temperatures (300-500°C) and high fluxes of fast neutrons. These conditions give rise to the formation of radiation defects, which when accumulated can jeopardize the geometrical stability and mechanical properties of the material. Radiation defects arise from clustering point defects, so mitigation strategies for increasing the lifetime of components in the reactor include optimizing material composition and thermo-mechanical treatments.
Cold worked (15-25%) 15-15Ti steels (steel with 15 wt% of both Ni and Cr and about 0.4 wt% of Ti) are alloys of particular interest for their demonstrated improved radiation resistance as compared to commercial steels. Through heat treatment as well as under certain conditions in the reactor, TiC nanoprecipitates of size 1-10 nm nucleate on dislocations, as well as on twin and grain boundaries. It is believed these semi-coherent precipitates act as point defect traps, which can improve radiation resistance.
This presentation will focus on the efforts that have been made so far to map out the effect of heat treatment on microstructure, with emphasis on the TiC nanoprecipitates and the dislocation structure. TEM DF imaging and BF Moiré fringes (together with CBED and EELS to measure thickness) were used to estimate their size and number density. Dislocation structures were studied with WBDF. EFTEM imaging confirmed the presence of Ti and C in the particles. Mismatch dislocations were directly imaged by HR-STEM. Finally, an outlook is presented on very recent work involving ion irradiation experiments.

Woensdag 10 mei 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Graphene Nanobubbles: MD approach
Spreker : Hossein Ghorbanfekr, CMT, University of Antwerp

Abstract:
In this talk, after a basic review of molecular dynamics (MD) simulations, I will present our recent numerical/experimental study of graphene nanobubbles in collaboration with School of Chemical Engineering and Analytical Science (the Manchester university). This work is accepted and will appear in Nature Communications.
Van der Waals (vdW) interaction between two-dimensional crystals (2D) can trap substances in high pressurized (of order 1 GPa) on nanobubbles. Increasing the adhesion between the 2D crystals further enhances the pressure and can lead to a phase transition of the trapped material. We found that the shape of the nanobubble can depend critically on the properties of the trapped substance. In the absence of any residual strain in the top 2D crystal, flat nanobubbles can be formed by trapped long hydrocarbons (i.e. hexadecane). For large nanobubbles with radius 130 nm, our atomic force microscopy measurements show nanobubbles filled with hydrocarbons (water) have a cylindrical symmetry (asymmetric) shape which is in good agreement with our molecular dynamics simulations. This study provides insights into the effects of the specific material and the vdW pressure on the microscopic details of graphene bubbles.

Vrijdag 5 mei 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Synthesis of MAX Phases in the Zr-Ti-Al-C System
Spreker: Bensu Tunca Altintas, SCK•CEN

Abstract:
MAX phases are nanolaminated carbides or nitrides, which show both metallic properties such as good thermal/electrical conductivity, thermal shock resistance, good machinability, damage tolerance and ceramic properties like good mechanical properties at high temperature, high oxidation and corrosion resistance. Among the MAX phase family, Ti-Al-C based phases have been extensively studied in literature and one of their outstanding properties is their oxidation resistance due to the formation of protective Al2O3 layers. Zr-Al-C based MAX phases: Zr3AlC2 and Zr2AlC are some of the new members of the expanding MAX phase family and their features are yet to be explored. Their particular importance lies in the lower neutron absorption cross-section of Zr, making these MAX phases especially promising for future nuclear applications. To satisfy both the requirements of next generation (GENIII+) nuclear reactor designs and the safety requirements to prevent material failure accidents, new accident tolerant cladding materials should be developed. These materials should have high resistance to oxidation and corrosion and should withstand high temperatures and radiation doses for prolonged exposures. One of the methods to fine-tune the properties of MAX phases is by creating solid solutions with different M, A or X element. Studies have shown that solid solutions can have better properties than their end members. The focus of this PhD to date was on the synthesis and characterization of solid solution MAX phases in the (Zr,Ti)-Al-C system. Further work will focus on improving the phase purity of the obtained ceramics and studying their irradiation, oxidation resistance, coolant interactions and mechanical properties.
Solid solution MAX phases (Zr1-xTix)3AlC2 (312) and (Zr1-xTix)2AlC (211) with x changing from 0 to 1 were synthesized by reactive hot pressing of ZrH2, TiH2, Al and C powders between 1350 and 1700°C. The produced ceramics contained large fractions of 211 and 312 MAX phases, while strong evidence of a 413 stacking was also found. Moreover, (Zr,Ti)C, ZrAl2, ZrAl3, and Zr2Al3 were present as secondary phases. In general, the lattice parameters of the hexagonal 211 and 312 phases followed Vegard's law over the complete Zr-Ti solid solution range, but the 312 phase showed a non-negligible deviation from Vegard's law around the (Zr0.33,Ti0.67)3Al1.2C1.6 stoichiometry. High-resolution scanning transmission electron microscopy combined with X-ray diffraction demonstrated ordering of the Zr and Ti atoms in the 312 phase, whereby Zr atoms occupied preferentially the central position in the close-packed M6X octahedral layers. The same ordering was also observed in 413 stackings present within the 312 phase.

Donderdag 4 mei 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Dynamic transition and quantum bath engineering in driven superconducting flux qubits
Spreker: Prof Daniel Dominguez, Centro Atomico Bariloche (Argentina)

Abstract:
In recent years, the experimental realization of LandauZener-Stuckelberg (LZS) interferometry  in several systems has emerged as a tool to study quantum coherence under strong driving. In two-level systems driven by an ac force, the accumulated phase between periodically repeated LZS tunneling events at avoided crossings gives place to constructive or destructive interferences, depending on the driving amplitude and the detuning from avoided crossing.   
I will discussLZS interferometry in superconducting flux qubits coupled to an Ohmic quantum bath. We have found a dynamic transition manifested by a symmetry change in the structure of the LZS interference pattern, plotted as a function of ac amplitude and dc detuning. The dynamic transition is from an LZS pattern with nearly symmetric multiphoton resonances to antisymmetric multiphoton resonances at long times (above the relaxation time). I will also discuss the effect of the quantum bath spectral structure and the system-bath coupling on the LZS interference pattern and compare with experiments in flux qubits.

Vrijdag 28 april 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Quantitative (S)TEM: towards efficient imaging>
Spreker: Karel van den Bos, EMAT, University of Antwerp

Abstract:
In today’s world, the field of nanotechnology is evolving rapidly, leading to more sophisticated products with improved properties. In this field, advanced material’s characterisation techniques are required since the exact atomic structure of materials determines their physical and chemical properties. A popular method for this task is the transmission electron microscope (TEM). In this presentation, different imaging modes of the TEM are quantitatively evaluated in order to help the microscopy user in deciding which technique is most suitable for his or her purpose.
In the first part, different TEM modes which are capable of visualising light elements are quantitatively compared in terms of accuracy and precision by using statistical parameter estimation theory. Furthermore, the effect of post-processing techniques on scanning TEM (STEM) images is examined. In order to provide an outlook to the future, simulated images, in which the unavoidable presence of Poisson noise is taken into account, are used to determine the ultimate precision.
When visualising atomic structures, the contrast in high angle annular dark field (HAADF) STEM images is sensitive to both thickness and chemical composition. However, when differences in atomic number are negligible or both thickness and chemical information need to be extracted from a single viewing direction, the information given by HAADF STEM images becomes insufficient. Therefore, spectral imaging by using energy dispersive X-rays (EDX) is often essential. In this presentation, the possibility to use these spectral images for atom counting will be explored.
Finally, the opportunities that new direct electron detectors can offer are investigated by using advanced statistical methods. It will be shown how chemical information is distributed in convergent beam electron diffraction (CBED) patterns. This information is crucial for imaging nanomaterials under optimal experimental conditions in the currently available STEM detectors and the ordered direct electron detectors by EMAT.

Vrijdag 21 april 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Measuring band gaps of wide-bandgap semiconductors including diamond with STEM-EELS
Spreker: Svetlana Korneychuk, EMAT, University of Antwerp

Abstract:
Manufactured semiconductor devices generally consist of many layers with different electronic and optical properties and the knowledge about the bandgap of each layer and energy band alignment is very important in order to understand and improve the performance of the device. The conventional methods of band gap measurement, even though they’re well developed and accurate, do not give band gap information at the local scale.
Low loss electron energy loss spectroscopy (EELS) in transmission electron microscopy (TEM) can retrieve the information about dielectric properties of the material including the band gap. However, ambiguity of data interpretation didn’t allow EELS to become a conventional technique for band gap measurements. The retardation losses, including e.g. Cherenkov radiation emission, have an undesirable impact on the low loss signal complicating the retrieval of the band gap signal. Nevertheless, several attempts to overcome the unwanted effect of retardation losses have been shown in EELS community.
In this work we demonstrate a technique to measure the band gap with EELS and highly reduce the influence of unwanted effects. All undesirable losses take place in a certain range of angles which can be shown by simulations using the so-called Kröger formula. This equation allows to calculate the energy losses of fast electrons in thin foils with retardation starting from the dielectric properties. We minimize the impact of undesirable losses by selecting the retardation-free part of the outcoming signal using an annular (Bessel) aperture. A decrease of Cherenkov effect is also achieved by operating at 60 kV - the minimal possible voltage in our microscope. The obtained band gap values are in good agreement with the conventional measurements, with a much improved spatial resolution.

Vrijdag 31 maart 2017 , 11.30 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Quantitative three-dimensional characterization of atomic clusters and nanoparticle assemblies
Spreker: Thomas Altantzis, EMAT, University of Antwerp

Abstract:
Atomic clusters and assemblies of nanoparticles have attracted increasing scientific interest during the last years, thanks to their unique properties and numerous applications in a plethora of scientific fields. It has been proven experimentally that the structure of such nanomaterials is inseparably connected to their characteristic properties. Therefore, in order to deeply understand the structure-to-property relationship, a detailed structural characterization is of utmost importance.
In the first part of my talk, I will show that by the use of advanced Transmission Electron Microscopy (TEM) techniques in combination with X-ray diffraction (XRD) data and a careful image analysis, one is able to unravel the structure of luminescent Ag clusters confined in Faujasite zeolites. Furthermore, approaches for the characterization of free standing atomic clusters will be presented.
The second part of my talk will be dedicated to the results of the structural and morphological characterization of assemblies of nanoparticles with varying shape, size and chemical composition. When investigating such complex structures, the information provided by (S)TEM images is not adequate since they only correspond to 2D projections of a 3D object and very often can be very misleading. For a more reliable structural and morphological characterization in 3D, electron tomography should be used. Since the turn of the century, the technique has been used to investigate the 3D structure of materials at the nanometer scale and below. Furthermore, the need for the optimization of both the acquisition and reconstruction processes will be discussed, in cases where more complex or bigger assemblies are investigated.

Woensdag 29 maart 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp : Excitons and trions in 2D transition metal dichalcogenides
Spreker: Matthias Van der Donck, CMT, University of Antwerp

Abstract:
We study the electronic and structural properties of excitons and trions in 2D transition metal dichalcogenides using two different methods: i) A few-body model in which we construct the Hamiltonian in the basis formed by all the possible products of single-particle states. We decouple the corresponding stationary Schrödinger equation and solve the resulting differential equation self-consistently. ii) The stochastic variational method (SVM) in which a variational wave function is used which is expanded in a basis of a large number of correlated Gaussians. We find good agreement between the results of both methods as well as with other theoretical works. However, the few-body model is a multi band model whereas the SVM is a single band model. This leads to differences between the two methods when (interband) interactions are strong.

Vrijdag 24 maart 2017, 11.30 u., Lokaal U.244 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Ni cluster formation in low temperature annealed Ni50.6Ti49.4
Spreker: Saeid Pourbabak, EMAT, University of Antwerp

Abstract:
The effect of room temperature aging combined with low temperature cycling on the structure of Ni-Ti and martensitic transformation is studied. DSC measurements show that Ms decreases with time, and it decreases even faster when aging is combined with DSC cycling, indicating some microstructural changes in the material. HRTEM and HAADF STEM imaging does not reveal any change in the material structure, however, strong localized and periodic diffuse intensities are observed in electron diffraction patterns. Applying Cluster Model which ascribes the shape and periodicity of the diffuse intensity to that of short ranged microdomains reveals the formation of pure Ni columns along [111]B2 crystallographic directions in the cubic Ni-Ti lattice which can be considered as early stages of precipitates in the low temperature aged samples. Comparing the diffuse intensities of different samples shows enhancement of short-range ordering in system with DSC cycling and aging in the thermomechanical history.

Vrijdag 17 maart 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Investigation of Fatigue Properties and its Governing Mechanisms of Small-scale Nickel Samples by In-situ SEM and In-situ TEM Micro/nano Mechanical Testing
Spreker: Vahid Samaeeaghmiyoni, EMAT, University of Antwerp

Abstract:
The mechanical design of components depends on the mechanical properties of such materials. At the macro-scale, the mechanical properties can be obtained from well-defined mechanical testing methods. However, at the micro/nano-scale, for which there is a dramatic demand of smaller and more durable micro-electro-mechanical systems (MEMS) and electronics, the development of micro/nano-mechanical testing instruments has recently enabled the investigation of the mechanical properties at such scales. In the present work the emphasis is on fatigue as one of the main failure engineering modes of materials. In practice, compressive and tensile fatigue properties of small scale materials are investigated and correlated to the observed mechanical properties and the dislocation structure evolution and mechanisms.
An in-situ SEM micro/nano-compression test was used to investigate the compressive fatigue response of single crystal and bi-crystal Nickel micropillars and their relevant dislocation microstructure were studied by conventional and advanced ex-situ TEM techniques. It has been observed that dislocation propagation and dislocation interaction are the main governing mechanisms in such small-scale samples.
In-situ TEM tensile testing was also used to investigate the tensile fatigue response and to observe directly the governing mechanisms in small-scale samples. The first step, and the main challenge, was to prepare a proper sample, not-affected and not-damaged by used sample preparation methods. Different methods like combination of twin-jet electropolishing, FIB and in-situ TEM have been used to reach this goal. Tensile fatigue tests were carried out on the defect-free Nickel single crystal samples. It was observed that in such scale the governing mechanism is a combination of dislocation propagation and dislocation starvation.

Vrijdag 10 maart 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : New advances in transmission electron microscopy to solve complex problems using neural networks
Spreker : :Ivan Lobato, EMAT, University of Antwerp

Abstract:
Nowadays neural networks remarkably improved the quality of state-of-the-art computer vision, image denoising, semantic segmentation, natural language processing, speech recognition and other techniques. Most of these improvements were realised by using convolutional neural networks (CNN) which is inspired by the organization of the brain visual cortex and dates back decades. Despite the explosive popularity of CNN in different fields, their applications in the field of transmission electron microscopy (TEM) is very limited. In this talk, I will show how CNN can help us to crack several difficult TEM problems including atomic column identification, atom counting, location of impurity atoms, tomography alignment, real-time compressed sensing, and automatic element identification in electron energy loss spectroscopy.

Vrijdag 3 maart 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Modifying the phase of electron beams to probe the real plasmonic potential.µ
Spreker : Giulio Guzzinati, EMAT, University of Antwerp

Abstract:
Plasmonics, the science and technology of the interaction of light with metallic objects, is fundamentally changing the way we can detect, generate and manipulate light at the nanoscale. While the field is progressing swiftly thanks to the availability of nanoscale manufacturing and analysis methods, fundamental properties such as the symmetries of the plasmonic excitations cannot be accessed by direct measurements, leading to a partial and sometimes incorrect understanding of their properties.
In particular, a very important role is that of electron energy loss spectroscopy (EELS), the only method that currently allows to map at the nanoscale the intense electric fields that characterise plasmonic resonances. One of its main drawback is that, while it is sensitive to the field's intensity, it can't detect its orientation, hiding their real symmetry and shape.
We have overcome this limitation by deliberately modifying the wave–function of the electron beam  to match the symmetry of the plasmonic excitations. After briefly introducing electron beams phas manipulation methods, I will show experimentally and theoretically that this new approach allows the selective detection of specific plasmon modes within metallic nanoparticles while filtering out modes with other symmetries.
This method shows some resemblance to the widespread use of polarised light for the selective excitation of plasmon modes but adds the advantage of locally probing the response of individual plasmonic objects and a far wider range of symmetry selection criteria.

Vrijdag 24 februari 2017 , 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Quantitative STEM: Recent developments and applications
Spreker: Annick De Backer, EMAT, University of Antwerp

Abstract:
In this talk, I will show a user-friendly software package, called StatSTEM, which has been released in the summer of 2016. StatSTEM includes well-established image quantification methods using model-based fitting. The advantages of using this model-based approach will be illustrated.
Furthermore, I will discuss a method to create three-dimensional atomic models from a single Z-contrast image. Our new method will be validated against state-of-the-art compressive sensing electron tomography.  This new approach allows for the characterization of beam-sensitive materials, or where the acquisition of a tilt series is impossible. As an example, the utility of this alternative approach is illustrated by the 3D characterization, at the atomic scale, of a nanodumbbell on an in situ heating holder.

Woensdag 22 februari 2017, 16.00 u., Lokaal T.138 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: A micromagnetic description of spin waves in ferromagnetic films
Spreker: Jeroen Mulkers, CMT, University of Antwerp

Abstract:
he theory of micromagnetism, in which the magnetisation is described by a continuous vector field, describes ferromagnetism on the submicron-scale. In this talk, I will give an introduction to spin waves in ferromagnetic films using the micromagnetic framework. Starting from the Landau-Lifshitz-Gilbert equation, which governs the dynamics of the magnetization, it is fairly straightforward to calculate the spin wave dispersion relation of ferromagnetic films. I will discuss how the dispersion relation depends on the exchange interaction strength, the presence of uniaxial anisotropy, and the chiral Dzyaloshinskii-Moriya interaction strength. In the second part of my talk, I will show how one can efficiently compute the eigenmodes of spin structures such as vortices and skyrmions.

Vrijdag 17 februari 2017, 16.00 u., lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door VISIELAB

Onderwerp: Challenges and opportunities in optical tomography
Spreker: Dr. Jeroen Kalkman, Department of Imaging Physics, Faculty of Applied Sciences, Delft University of Technology, The Netherlands

Abstract:
In optical tomography light does not behave in the ideal way that X-rays can do in X-ray computed tomography.  Effects of diffraction, refraction, and scattering can severely affect the image quality in optical tomography when not properly taken into account. In this presentation I will show how these effects can be mitigated by using physical measurement principles and computational image reconstructions algorithms. Results on tomographic reconstructions of zebrafish demonstrate the opportunities this offers for high resolution imaging of small animals.

Maandag 13 februari 2017 , 11.30 u., lokaal U.025 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Inverse problems and object retrieval in (S)TEM
Spreker: Dr. Wouter Van den Broek, Humboldt-Universität zu Berlin, Institut für Physik

Abstract:
Our inverse problem of interest is the retrieval, in two or three dimensions, of an object from a series of (S)TEM images. Some instances with linear image formation are considered, but also cases with multiple scattering. The inclusion of prior knowledge through compressed sensing is treated as well.

Vrijdag 3 februari 2017, 11.30 u., lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Antisite Disorder and Bond Valence Compensation in Li2FePO4F Cathode for Li-Ion Batteries
Spreker: Olesia Karakulina, EMAT, University of Antwerp

Abstract:
The way the crystal structure of a cathode material changes during operation of Li-ion battery has a crucial impact on its performance. Recently, new polymorph of Li2FePO4F was obtained, which can be considered as a promising cathode material for Li-ion batteries. By electrochemical ion-exchange it was prepared from LiNaFePO4F, which has a 3D framework structure formed by FeO4Foctahedra and PO4 tetrahedra. Na resides in the channels of framework and it can reversibly (de)intercalated. We used initial LiNaFePO4F as a cathode in the electrochemical cell and charged it at elevated temperature. As a result Na was fully removed from the channels. Upon discharge of the battery Li was introduced in the framework forming Li2FePO4F.
In this work we determined the crystal structure of Li2FePO4F by means of electron diffraction tomography. Oppositely to our expectations, only 70% of Li occupy former positions of Na in the channels. The remaining 30% Li was found in the Fe position, showing the presence of Li-Fe anti-site defects. To find out the origin of such disorder, we determined the crystal structure of LiFePO4F and Li2FePO4F prepared at room temperature. The anti-site defects were found in both cases, while the initial one does not have them. We also studied a compound with similar composition, LiFePO4. However, after cycling even at elevated temperature such dramatic anti-site disorder as for Li2FePO4F was not found. Therefore, we suppose that the main reason is a specific feature in the crystal structure that is different for Li2FePO4F and LiFePO4: in Li2FePO4F case two oxygens are coordinated by one P and three Li atoms, and two out of these three Li leave structure upon charge. As a result, in charged state these O atoms become underbonded which cannot be compensated by shrinking of the P-O bonds. Therefore, Fe3+ atoms partially migrate into the Li position to eliminate the misbalance. In LiFePO4all O atoms are connected with electrochemically active Li and Fe and thus no such compensation is needed.

Vrijdag 27 januari 2017, 11.30 u., lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Alternative Techniques for Electron Tomography at Atomic Scale
Spreker: Thais Milagres de Oliveira, EMAT, University of Antwerp

Abstract:
Aberration-corrected (Scanning) transmission electron microscopy ((S)TEM) is a very powerful technique to characterize the structure and the composition of materials at the atomic scale. However, one should never forget that (S)TEM images are only two-dimensional (2D) projections of a three-dimensional (3D) object. By using electron tomography one is able to determine the shape and structure of an object in 3D. In this talk, different approaches combined with electron tomography are proposed to recover the structure and composition of nanostructures at the atomic scale.
In the first part of the talk, an approach is proposed where a combination of X-ray energy dispersive spectroscopy (XEDS) in an aberration-corrected scanning transmission electron microscope with electron tomography, which enables a 3D chemical mapping at the atomic scale.
In the second part of the talk, a different approach is discussed where high angle annular dark-field scanning TEM (HAADF-STEM) is combined with exit wave reconstruction (EWR). When using high resolution TEM (HR-TEM) image interpretation and quantification is complex due to the strong radiation-matter interaction and the sample’s exit wave is modified by the microscope. Consequently the contrast observed in the images is an interference effect and is not directly interpretable. By using EWR one is able to retrieve the amplitude and phase of the exit wave from the object. By combining the phase images from different zone axes with HAADF-STEM tomography and by using a compressive sensing based reconstruction algorithm, it is possible to obtain a tomographic reconstruction at the atomic scale.
In the last part of the talk, the combination of low dose imaging with the three-dimensional EWR will be discussed. Such an approach can enable the structural characterization of beam sensitive materials at the atomic scale.

Woensdag 25 januari 2017 , 16.00 u., lokala U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp : Piezoelectricity at Nanoscale
Spreker: Dr Cem Sevik, Department of Mechanical Engineering, Anadolu University, Eskisȩhir, Turkey

Abstract:
Recently, two dimensional materials with noncentrosymmetric structure have received significant interest due to their potential usage in piezoelectric applications. It has been reported by first principles calculations that relaxed-ion piezoelectric strain (d11) and stress (e11) coefficients of some transition metal dichalcogenide monolayers are comparable or even better than that of conventional bulk piezoelectric materials [1]. Furthermore, piezoelectric coefficient of MoS2 has been measured as 2.9×10−10 C/m[2] , which agrees well with the mentioned theoretical calculation. Afterwards, this exceptional potential has been deeply investigated by the calculation of the piezoelectric properties of various single layer structures: two dimensional transition metal dichalcogenides [3] , transition metal oxides [3] , group II oxides [4] , and hexagonal group III-V [4] , IV-VI [5] and II-VI [6] compounds. The reported results have clearly shown that not only the Mo- and W-based transition metal dichalcogenides but also the other materials with Cr, Ti, Zr and Sn exhibit highly promising piezoelectric properties. Moreover, d11 coefficient of some IV-VI and II-VI (see Figure 1) compounds have been predicted as quite larger than that of transition metal dichalcogenides and the bulk materials, α-quartz, w-GaN, and w-AlN which are widely used in current applications. In conclusion, the reported first principle predictions clearly reveal that monolayer semiconductors are strong candidates for future atomically thin piezoelectric applications such as transducers, sensors, and energy harvesting devices.

References
[1] K. A. N. Duerloo, M. T. Ong, and E. J. Reed, J. Phys. Chem. Lett. 3, 2871, (2012)
[2] H. Shu et al. Nat. Nano. 10, 151, (2014)
[3] M. M. Alyörük, Y. Aierken, D. Çakır, F. M. Peeters, and C. Sevik, J. Phys. Chem. C 119, 23231, (2015)
[4] M. N. Blonsky, H. L. Zhuang, A. K. Singh, and R. G. Hennig, Nano Lett. 9, 9885, (2015)
[5] R. Fei, W. Li, J. Li, and L. Yang, Appl. Phys. Lett, 107, 173104, (2015)
[6] C. Sevik, D. Çakır, O. Gülseren, and F. M. Peeters, J. Phys. Chem. C, 120, 13948, (2016)

Vrijdag 20 januari 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Structure characterization of triple perovskites and related systems by transmission electron microscopy
Spreker: Robert Paria Sena, EMAT, University of Antwerp

Abstract:
During my Ph.D. research, I have investigated the structure of specific perovskite based oxides in order to establish the correlation between the structure and magnetic properties. The final goal of our study is to design new compounds with a potential for applicable properties, for instance relaxor ferromagnetism.
The crystal structures have been solved using a combination of transmission electron microscopy techniques, including selected area electron diffraction (SAED) combined with real space imaging using different techniques such as high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), annular bright field scanning transmission electron microscopy (ABF-STEM) and energy dispersive X-ray spectroscopy-STEM (EDX-STEM). Based on these studies, models have been proposed and refined for different triple perovskite compounds. With these models we have tried to explain the variations in the physical properties of the samples and we have compared them to similar perovskites. The disclosed relations have rendered fundamental knowledge, applicable for the optimization of the properties of the investigated materials as well as of related perovskite materials.

Woensdag 18 januari 2017, 16.00 u., lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Different structure models of group IV element monolayer
Spreker: Linyang Li, CMT, University of Antwerp

Abstract:
Since monolayer graphene was successfully realized in 2004, its unusual Dirac cone band structure has attracted great attention. Its honeycomb lattice with perfect hexagonal symmetry plays a crucial role in the formation of the Dirac cone with linear dispersion. Similarly, other group IV element Si (Ge/Sn) monolayer with a hexagonal honeycomb lattice was proposed, called as silicene (germanene/stanene). In experiment, they were successfully synthesized on different substrates. Besides the hexagon structure, another stable structure model, dumbbell structure, was applied on the element Si (Ge/Sn) monolayer, which exhibits a lower energy than that of the hexagon structure.
In this talk, Linyang Li will compare the structure, energy and band structure of the silicene with dumbbell and hexagon structures and some our new results on dumbbell structure will be shown.

Vrijdag 6 januari 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Optimal statistical experiment design for detecting and locating light atoms using quantitative high resolution (scanning) transmission electron microscopy
Spreker: Julie Gonnissen, EMAT, University of Antwerp

Abstract:
We report an innovative method to quantitatively optimise the experiment design for discrete estimation problems in high resolution (scanning) transmission electron microscopy (HR(S)TEM). In a first part of this talk, this quantitative approach is used to investigate the optimal experimental settings for both detecting and locating light elements in HR(S)TEM images. The principles of detection theory are then used to quantify the probability of error as an optimality criterion for the detection of light atoms. To determine the optimal experiment design for locating light atoms, use is made of the so-called Cramér-Rao Lower Bound (CRLB). It is investigated if a single optimal design can be found for both the detection and locating problem of light atoms.
In a second part of this talk, our quantitative approach is also used to optimise the experiment design for nanoparticle atom-counting from both TEM and STEM images. So far, HRSTEM has been shown to be an appropriate method to count the number of atoms in a projected atom column. Recently however, it has been shown that one HRTEM image using negative spherical aberration imaging suffices to count atoms as well. Our quantitative approach based on the principles of detection theory is used, in order to determine the limits to the precision with which the number of atoms in a projected atom column can be estimated. The capabilities of both imaging techniques, HRSTEM and HRTEM, are investigated and compared in terms of atom-counting reliability.
In the last part of this talk, an experimental application is shown where the domain wall in a LiNbO3 crystal is quantified using the principles of statistical parameter estimation theory.