Voordrachten 2013

Vrijdag 13 december 2013, 11.30 u, lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Multimodal imaging of iron oxide particles
Spreker : Dimitri Roose, EMAT, University of Antwerp

Abstract:: In this lecture, two applications of iron oxide particles in preclinical magnetic resonance imaging will be presented. After an introduction to these applications, the developed sample preparation protocol and the obtained results will be discussed.
The first application is the follow-up of cell migration in live animals. The iron oxide particles are loaded in the cell and have to remain inside the cell for a definite time period, but have to be biodegraded in the end. Therefore, the interaction between cells and particles is important.
In another application, the iron oxide particles are injected in the blood stream to detect for example blood-brain barrier leakages. In this example, the particles will enter the brain through these leakages. Several questions arise then: where do these iron oxide particles end up? What happens to these iron oxide particles afterwards?

Donderdag 12 december 2013, 16.00 u., lokaal U.025 (Campus Groenenborger)

Voordracht georganiseerd door TGM
Onderwerp : Dynamical arrest in adhesive hard-sphere dispersions driven by rigidity percolation
Spreker : Prof. Dr. Ramón Castañeda-Priego, Division of Sciences and Engineering, University of Guanajuato, Mexico

Abstract::  One major goal in condensed matter is identifying the physical mechanisms that lead to arrested states of matter, especially gels and glasses. The complex nature and microscopic details of each particular system are relevant. However, from both scientific and technological viewpoints, a general, consistent and unified definition is of paramount importance. Through Monte Carlo computer simulations of states identified in experiments, we demonstrate that dynamical arrest in adhesive hard-sphere dispersions is the result of rigidity percolation with coordination number, $\left< n_b \right>$, equals to $2.4$. This corresponds to an established mechanism leading to mechanical transitions in network-forming materials [Phys. Rev. Lett. {\bf{54}}, 2107 (1985)]. Our findings connect the concept of critical gel formation in colloidal suspensions with short-range attractive interactions to the universal concept of rigidity percolation. Furthermore, the bond, angular and local distributions along the gelation line are explicitly studied in order to determine the topology of the structure at the critical gel state.

Vrijdag 6 december 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Carbon based/Al composites for thermal management applications in the microelectronics industry
Spreker  Gang Ji, EMAT, University of Antwerp

Abstract:: In the microelectronics industry, the ever-increasing power density and continuing miniaturization of semiconductor and optoelectronic devices lead to mounting thermal management challenges. In order to realize high performance, long life and high reliability, it is imperative to develop very efficient electronic packaging materials with high thermal conductivity (TC) and low coefficient of thermal expansion (CTE). In this work, diamond/Al and graphite flakes/Al composites were produced by cost-effective techniques. Microstructure of the composites was characterized at length scales from the macro to the nanoscale in order to investigate detailed interface assembly configurations.

Vrijdag 29 november 2013, 11.30 u., lokaal X.009 (campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Using electron vortex beams to identify the handedness of enanthiomorphic space group pairs
Spreker  Roeland Juchtmans, EMAT, University of Antwerp

Abstract:: An object is said to be chiral if it doesn't possess mirror symmetry. As such, a chiral material always comes in two forms differentiated from one another by a single mirror transformation, which are called enantiomorphs. In crystals chirality manifests itself in the space group possessing solely rotation or screw axes. Distinguishing two enantiomorphic space group pairs (space groups with   screw axes of opposite helicity), seems to be a difficult task.
Experiments using coherent Convergent Beam Electron Diffraction are able to distinguish between certain enantiomorphic space group pairs only when they are combined with twofold rotation axes, e.g. P3121 and P3221. Other space group pairs, e.g. P31 and P32 can only be distinguished by comparing the experimental diffraction pattern with a simulated one, taking multiple scattering into account.
In this lecture we investigate theoretically the effect the chiral character of electron vortices has an on the symmetry of the diffraction pattern, when measured along a screw axis. An experimental setup is proposed to distinguish between the space group P31 and P32 and simulations of the experiment performed on the chiral LaBGeO5 are shown.

Vrijdag 22 november 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Characterisation of epitaxial indium-tin oxide (ITO) nanowires from VLS growth
Spreker : Stuart Turner, EMAT, University of Antwerp

Abstract:: Semiconductor nanowires like Si, GaAs, InGaAs and ITO wires are of huge interest to the scientific community, as they can be applied in many fields like photonics, energy harvesting, electronics and gas sensing.
In order for this type of wires to be incorporated into layered devices, epitaxial growth of semiconductor nanowires on a substrate needs to be achieved in a controlled fashion. Also, an atomic resolution characterization of the nanowire-substrate interface needs to be performed, as the wire-substrate interface structure is of crucial importance for the device properties.
In this presentation, we will present the growth of Vapor-Liquid Synthesis (VLS) Indium-Tin Oxide (ITO) nanowires on various Yttrium-stabilized Zirconia (YSZ) substrates. The ITO-YSZ and Au catalyst-ITO interface structure will be unraveled by high resolution imaging, and the presence, previously unknown indium-tin phase will be evidenced at the nanowire extremities, a result of the VLS wire growth.

Dinsdag 19 november 2013, 16.00 u., lokaal U.024 (Campus Groenenborger)

Voordracht georganiseerd door TGM
Onderwerp : The Wigner Monte Carlo Approach: From Theory to Applications
Spreker : Dr. Jean Michel Sellier, Institute for Parallel Processing, Bulgarian Academy of Sciences

Abstract:: The Wigner-Boltzmann model is a partial integro-differential equation which describes the physics of time dependent, full quantum phenomena in the presence of lattice phonon scattering and general boundary conditions.
This formulation of quantum mechanics is based on a quasi-distribution function defined over a 6-dimensional phase-space. Very recently a Monte Carlo technique exploiting the concept of particle sign has been presented, for the ballistic regime, which allows multi-dimensional, full quantum and time dependent simulations. It has been applied successfully to 1D, 2D and 3D technologically relevant applications. In this talk, we sketch the MC method based on particle sign, include the phonon scattering, and show the similarities with the Boltzmann MC method. We then proceed with validations and applications.

Vrijdag 15 november 2013 , 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Micro- and nano-structural properties second generation High-Tc Superconducting tapes
Spreker : Alexander Meledin, EMAT, University of Antwerp

Abstract:: Superconductivity was discovered more then 100 years ago, and has found a great many applications in the scientific world. High temperature superconductive (HTC) coated conductor tapes are ideal for a wide range of industrial and scientific applications today, but are still open for improvement: HTC tapes should be cheaper, environment friendly and long lasting at a high performance level.  The micro- and nano-structural quality of HTC coated conductors is one of the most important factors that determines the connectivity and the  transport  of current across the entire tape length.
Different advanced transmission electron microscopy techniques were applied to investigate number of various substrates, buffer and superconductive layers. The interfaces, micro- and nano-structure of the layers and inclusions were studied.

Vrijdag 8 november 2013, 11.30 u., lokaal U.024 (Campus Groenenborger)

Voordracht georganiseerd door  EMAT
Onderwerp : Playing Lego with graphene: exfoliation and intercalation strategies to put graphene on surfaces and to produce pillared graphene structures
Spreker : Petra Rudolf, Zernike Institute for Advanced Materials, University of Groningen

Abstract::  Even since the Mayans first used clays to make dyes, mankind has made use of layered materials. Over the past two centuries this effort is mirrored in scientific research leading to experiments to exfoliate into one- or few-atom thick sheets or to intercalate them with atoms and (macro)molecules, as well as to an elucidation of their laminar structure and peculiar properties. Currently we live in an era of enormous interest in the manipulation of two-dimensional materials following the discovery of graphene. In this presentation I shall start by illustrating various aspects of the exfoliation of pure and oxidized graphite. Then I shall discuss the deposition by a modified Langmuir Blodgett technique of graphene oxide and of graphene on a variety of substrates with a coverage that can be varied as desired from isolated sheets to a densely packed 2D arrangement. From there I shall pass on to pillared structures, for example of graphene oxide and carbon nanotubes, build up by the same modified Langmuir Blodgett technique in a layer-by-layer fashion. Finally I shall discuss pillared structures achieved by intercalation and in particular report on experimental and theoretical studies of the intercalation mechanism of polycyclic aromatic molecules into graphite oxide. Two representative molecules of this family, aniline and naphthalene amine behave very differently after intercalation, namely aniline molecules prefer to covalently bind to the graphene oxide matrix via chemical grafting, while napthalene amine molecules bind with the graphene oxide surface through π-π interactions. This new understanding opens new perspectives for the interaction of various aromatic molecules with graphite oxide and the so-called “intercalation chemistry”.

Vrijdag 25 oktober 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Transmission electron microscopy for the study of scheelite related compounds
Spreker: Vladimir Morozov, EMAT, University of Antwerp

Scheelite-type compounds (SRCs) with general formula (A1,A2)n[(B1,B2)O4]m (2/3≤n/m≤3/2) are the subject of large interest owing to their stability, relatively simple preparation and optical properties. SRCs can be prepared with a wide range of vacancies in the cationic subset, giving compositions characterized by a (A’+A’’):(B’O4+B’’O4) ratio different from 1:1. The creation of cation vacancies in the scheelite-type framework and the ordering of A cations and vacancies can be a new factor in controlling the scheelite-type structure and properties. Using of transmission electron microscopy (TEM) allows to gather new information about the real structure of SRCs.

Donderdag 24 oktober 2013, 16.00 u., lokaal U.203 (Campus Groenenborger)

Voordracht georganiseerd door EDF
Onderwerp: Mass? Mass!
Spreker / Speaker: Prof. A. Sevrin, Vrije Universiteit Brussel & UAntwerpen

The 2013 Nobel Prize in Physics was awarded for “the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass …”. As the notion of mass is ubiquitous in physics I will try to situate the “Brout-Englert-Higgs mechanism” within physics as a whole. The original motivation and ideas of Brout and Englert – which turn out to be surprisingly close to our modern point of view – will be explained, our current understanding will be reviewed and important open problems will be highlighted.

Dinsdag 22 oktober 2013, 14.00 u., lokaal N.008 (Campus Drie Eiken)

Voordrachten georganiseerd door VISIE

Onderwerp: What to measure when measuring noise in MRI
Spreker: Prof. Santiago Aja-Fernandez, Universidad de Valladolid, Spain

Noise is one of the main sources of quality deterioration in Magnetic Resonance (MR) data. It is usually modeled attending to the scanner coil architecture and the signal reconstruction method. To establish a level of noise, the variance of the original Gaussian noise in each coil is usually considered. For single coil configurations, where the Rician model is used, this variance is intrinsically related to the distribution. The same case can be considered for non-correlated non-accelerated multiple coil systems. However... is it the case for real-life configurations? Is it always practical to look for the "original" variance of noise? Some practical examples will be discussed.

Onderwerp: Modeling of Brain Microstructure with Diffusional Kurtosis Imaging
Spreker: Prof. Jens H. Jensen, Medical University of South Carolina, USA

Diffusional kurtosis imaging (DKI) is an extension of diffusion tensor imaging that provides metrics for quantifying diffusional non-Gaussianity, in addition to conventional diffusivity and anisotropy measures. Since diffusional non-Gaussianity in brain is a direct consequence of cellular compartments and barriers, the added metrics may be used to constrain models of tissue microstructure. Here these are applied both to white matter fiber tractography and to a specific two-compartment model for the cerebral microenvironment. An advantage of DKI is that whole brain scan times of a few minutes are achievable on standard clinical scanners, facilitating the incorporation of DKI into comprehensive imaging protocols. The potential value of combining DKI with modeling is illustrated with results for stroke and Alzheimer’s disease.

Vrijdag 18 oktober 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Ex situ (TEM) and in situ (EIS) investigation of the electrolyte decomposition layer in silicon electrodes for Li-ion batteries
Spreker: Kristof Van Havenbergh, EMAT, University of Antwerp

It is generally accepted that Li-ion batteries will become the most important energy storage solutions in an energy transition that is currently taking place, where hydrocarbons as privileged fuel are being replaced with sustainable energy sources. To replace the actual graphite based anode, several studies have been initiated over the past 15 years to implement alloy-based systems, but without success. Si has a ten times higher intrinsic capacity than graphite, because of an alloying reaction with lithium rather than an intercalation of lithium between graphene layers. The drawback of an alloying reaction is that it is accompanied by a drastic volume increase up to 300% (10% in graphite)1. Due to this volume change, during every lithiation and delithiation, freshly exposed silicon surfaces will continuously react with the electrolyte and create new decomposition products that accumulate at the surface, resulting in irreversible capacity loss. As these interphase reactions are key points to obtain a good capacity retention, it is essential to monitor and analyze their formation closely in order to be able to propose new strategies.

Vrijdag 11 oktober 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Towards an efficient way of producing electron vortices in TEM
Spreker: Armand Béché, EMAT, University of Antwerp

Since their first creation a few years ago, interest in the production and use of electron vortex beams has grown to reach the ultimate goal of mapping magnetic information down to the atomic scale. However, the creation of an electron vortex beam of good quality, high purity, high intensity and atomic size faces several problems. One of the best solutions seems to rely on the use of the magnetic field produced at the tip of a magnetized needle. Shining an electron plane wave on the spiralling phase shift induced by the magnetic field will generate a vortex whose properties can be changed with the magnetic field. In this lecture, we will introduce electron vortices before reviewing the different ways of creating electron vortex beams. This will lead us to the description of the magnetic needle and its interesting capabilities of producing electron vortex beams.

Vrijdag 4 oktober 2013, , 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Dynamic Motion of Ru-Polyoxometalate Ions (POMs) on Functionalized Few-Layer Graphene
Spreker: Xiaoxing Ke, EMAT, University of Antwerp

The interaction and stability of the tetraruthenate oxo-cluster {Ru4(μ-O)4(μ-OH)2(H2O)4[γ-SiW10O36]2}10-(Ru4POM) on few layer graphene (FLG) functionalized with positively charged ammonium pendants is studied via time-dependent imaging using an aberration-corrected transmission electron microscope (AC-TEM). Under the 80 kV irradiation of the electron beam the Ru4POM demonstrates dynamic motion on the graphene surface. The motion of the Ru4POM is captured as a series of images and is shown to vary as a function of time under certain constraints in the Ru4POM rotation. The frequency and amplitude of rotation is found to be related to the nature of the functional group used, including a polyamine dendron and a N,N,N-trimethyl benzenaminium moiety in our study. Distortions in the Ru4POM structure are revealed as well, suggesting that the ions can stand instantaneous structural changes without losing their integrity. The stability of the Ru4POM-graphene hybrid during the imaging corroborates the long-term robustness of the functional material applied to multielectronic catalytic processes.

Vrijdag 27 september 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT / Seminar organised by EMAT
Onderwerp: First-principles study of layered, van der Waals bonded systems: an efficient search for Bi-Sb-Te-Se groundstate structures and the quasiparticle band structure of SnO
Spreker: Kirsten Govaerts, EMAT, University of Antwerp

Computational modeling at the smallest scale provides us understanding and insight in the atomic origin of the macroscopic behavior of materials. Using ab initio techniques, we studied several properties of layered, van der Waals bonded systems. In a first part of my talk I will demonstrate an efficient search method for finding groundstate structures of binary and ternary Bi-Sb-Te-Se alloys. The second part will tackle the well-known Density Functional Theory problem of underestimating the band gap. Using the GW approximation, we find a band gap of SnO that is in good agreement with experimental results.

 

Vrijdag 14 juni 2013, 15 u., lokaal U.203 (Campus Groenenborger)

 

Voordracht georganiseerd door TGM
Onderwerp: Intermediate State: a new look at an old story*
Spreker: Vladimir Kozhevnikov, Tulsa Community College, U.S.A., and K.U. Leuven

Abstract: Attracted by the rich physics of the intermediate state (IS) and stimulated by revealed inconsistencies between recently measured values of microscopic parameters and those following from historically seminal data on the IS (“played important role in establishing Ginzburg-Landau theory in the first place”, M. Tinkham), we conducted an experimental study of the IS focusing on the equilibrium magnetic flux structure, the flux density and the critical field**. The least expected observation is that the flux density in normal domains can be as small as 40% of the thermodynamic critical field Hc. This fact contradicts and hence overthrows a long established paradigm (Gorter and Casimir, 1934) stating that the normal phase is unstable at the field less than Hc . I will present results of this study and introduce a new theoretical model consistently describing these and all other properties of the IS measured in independent experiments. Moreover, our model, based on rigorous thermodynamics, allows for quantitative determination of the domain-wall parameter and the coherence length. I will also discuss a possible impact of our model on modeling of the magnetic structure of the vortex.
*Supported by NSF
** V. Kozhevnikov, R. J. Wijngaarden, J. de Wit and C. Van Haesendonck, to be published.

Donderdag 13 juni 2013, 15 u., lokaal U.203 (Campus Groenenborger)

Voordracht georganiseerd door TGM
Onderwerp: Defects and grain boundaries in Cu(InGa)Se2
Spreker: Prof. Susanne Siebentritt, University of Luxembourg

Abstract: Cu(InGa)Se2 is used as an absorber in thin film solar cells. Thin film solar cells are considered the next generation of photovoltaics because of their greatly reduced consumption of material and energy. However, these materials are also interesting from a fundamental point of view: their phase behaviour and the properties of their surfaces and grain boundaries are unusual. Many of these properties can be traced back to comparatively low defect formation energies. The talk will summarise the experimental findings on shallow defects and grain boundaries in this material and discuss challenges for theory.

Vrijdag 31 mei 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Development of Dark-Field Electron Holography and propagation of the geometric phase along the electron beam
Spreker: Elsa Javon, EMAT, University of Antwerp

Abstract: Dark-field electron holography (DFEH) is a recently technique developed for measuring strain in nanostructures, in particular over wide fields of view. It has been applied to the study of strained-silicon transistors and epitaxial thin films. The principle is based on measuring the so-called geometric phase carrying the strain information. Currently, it is assumed that the measured geometric phase corresponds to the strain averaged over the sample thickness, projected in the propagation direction.
In this lecture we present 3D projection rules based on dynamical 2-beam theory, developed for DFEH in order to evaluate strain over the thickness of the TEM foil.

Dinsdag 28 mei 2013, 14.30 u., lokaal U.203 (Campus Groenenborger)

Voordracht georganiseerd door EDF
Onderwerp: The Energy Dependence of the Underlying Event in Hadronic Collisions
Spreker: Rick Field, University of Florida

Abstract: Charged particles production (pT > 0.5 GeV/c, |eta| < 0.8) is studied in proton-antiproton collisions at CDF at 300 GeV, 900 GeV, and 1.96 TeV and in proton-proton collisions at CMS at 7 TeV. The 300 GeV and 900 GeV data are a result of the Tevatron Energy Scan which was performed just before the Tevatron was shut down. The direction of the leading charged particle in each event, PTmax, is used to define three regions of eta-phi space; toward, away, and transverse. The transverse region is very sensitive to the underlying event and is separated into a MAX and MIN transverse region, which helps separate the hard component (initial and final-state radiation) from the beam-beam remnant and multiple parton interaction components of the scattering. The data are corrected to the particle level and are compared with the PYTHIA 6.4 Tune Z1 at the particle level (i.e. generator level). The goal is to study the energy dependence of the "underlying event" by examining the behavior of the transverse region as a function of PTmax at four center-of-mass energies.

Vrijdag 24 mei 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Pristine Nanostructures: Make, Shape and Explore
Spreker: Paromita Kundu, EMAT, University of Antwerp

Abstract: Nanoscience and nanotechnology relies on the ability to tune the shape, size and microstructure of the nanomaterials because the properties of the nanostructure are often influenced by its morphology and internal atomic structure. Therefore ability to tune these parameters during synthesis of new materials is imperative and consequently it becomes important to study their structure in detail. This can provide a good understanding of their behaviour which can be exploited for specific applications. Here, a brief overview of the synthesis strategy and a study on ultrathin gold nanowires and Au based nanohybrids will be presented wherein the structural investigation has been carried out using aberration corrected microscopy and tomography techniques.

Vrijdag 17 mei 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Electron vortex beams: total angular momentum, and vortex beam scattering
Spreker: Ruben Van Boxem, EMAT, University of Antwerp

Abstract: Electron vortex beams are novel free space states with a topological charge. Vortex beams also carry orbital angular momentum. After describing how, what and why to use electron vortex beams, relativistic effects of the transverse motion are shown. After this, Born scattering of a vortex beam is calculated, with some interesting novel features that unfortunately may prove to be hard to detect in an electron microscope.

Maandag 13 mei 2013, 14 u., lokaal U.203 (Campus Groenenborger)

Voordracht georganiseerd door EDF
Onderwerp: Improved cross sections using re-summation for squark and gluino production at the LHC
Spreker: Chris Wever (Utrecht)

Abstract: We start by considering the production of strongly interacting, heavy particle pairs at the LHC. When the centre of mass energy is close to the production threshold of the pair, their cross sections receive large higher-loop QCD corrections. These corrections are classified as the so-called soft logarithms and Coulomb singularities and they lead to a break down of the usual perturbation expansion. In this talk I review the origin of these large corrections and explain how they can be directly resummed by using Effective Field theories. Finally, I will present some resummed results for the pair production cross sections of heavy squarks and gluinos. Based on: arXiv:1202.2260 and 1211.3408.

Vrijdag 3 mei 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Nanostructures and Supraparticles from Nanoparticles
Spreker: Marijn van Huis, Debye Institute for Nanomaterials Science, Utrecht University, The Netherlands

Abstract: While many kinds of nanoparticles (NPs) can currently be synthesized, the assembly of these particles into functional three-dimensional nanomaterials is still at an early stage. For the full development of collective nanoparticle properties (catalytic activity, lasing, optical properties), it is required to arrive at binary and ternary superlattices that are in one phase and in well-controlled geometries. In the Soft Condensed Matter group, self-organisation of NPs is initiated in water-in-oil and oil-in-water emulsion droplets, leading to spherical superlattices having e.g., icosahedral and fcc packings. Concurrently, stabilization approaches are developed in order to ensure temporal and thermal stability of NPs and their superlattices. The characterization of the heterogeneous nanoparticles, and of the large assemblies in which they are contained, puts high demands on TEM techniques and methodologies. Several examples will be treated, in particular supraballs of silica-coated Au nanorods, supraballs of FeO/CoFe2O4 core/shell particles, and photocatalytic CdS/Au nanorods.

Vrijdag 26 april 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Recent Methods and Techniques in Optical Microscopy
Spreker: Emeric Mudry, EMAT, University of Antwerp

Abstract: Optical microscopy is a centuries old technique, but has experienced a huge development since the introduction of laser, fluorescent markers and CCD cameras. This lecture aims at presenting some of the recently developed techniques that might be of interest for the electron microscopy community. It mainly focuses on phase measurement techniques, their applications and on different techniques for three-dimensional imaging. Finally some other interesting techniques are quickly reviewed.

Vrijdag 19 april 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Quantitative characterization of microstructure evolution during the fabrication of an Al-Mg-Si-Cu 6111 Alloy
Spreker: Kai Li, EMAT, University of Antwerp

Abstract: Aluminum alloys are important commercial alloys mainly due to their light weight and excellent corrosion resistance. The designing of aluminum alloys with the aid of computational simulations urgently needs exact quantitative experimental data of the microstructure evolution during fabrication as verifications. In this work, three methods such as scanning electron microscopy, convergent beam electron diffraction and 3-dimensional atom probe are applied to quantitatively characterize the microstructures of different stages of the fabrication, and the volume fractions of different types of second phase particles at different size scales have been measured. The advantages and drawbacks of the three methods will be discussed.

Maandag 15 april 2013, 16 u., lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door TQC
Onderwerp : Quantum Gases: Fluctuations & Dynamics
Spreker : Nick Proukakis, Newcastle University & Joint Quantum Centre Durham-Newcastle

Abstract: Ultracold trapped bosonic gases have become a standard test-bed for the study of macroscopic quantum coherence and many-body dynamics under different conditions, due to their excellent experimental control. In addition to the usual Bose-Einstein phenomenon exhibited in three-dimensional geometries, such inhomogeneous systems also enable the controlled study of the enhancement of fluctuations in geometries of reduced dimensionalities. Although different models exist for describing such systems, an established approach that captures most of the relevant physics in the weakly-interacting limit is based on a nonlinear Langevin equation known as the stochastic Gross-Piteavskii equation, which does not rely on the usual picture of symmetry-breaking. In this talk I will show how this method can be used to describe a range of relevant phenomena in one, two and three dimensions, showing clearly its ab initio predictive power by direct comparison to numerous experiments, while also outlining some current limitations in its dynamical implementation.

Vrijdag 12 april 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Producing electron vortex beams by (ab-)using the aberration corrector.
Spreker: Laura Clark, EMAT, University of Antwerp

Abstract: The field of electron vortex beams is very new and developing rapidly. Following the first experimental demonstrations in 2010, many developments have been made in the techniques for electron vortex production. Methods used thus far employ techniques borrowed from the field of singular optics, such as spiral phase plates, holographic masks and distortion of Hermite-Gaussian modes. Here we present an alternative method for producing electron vortex beams, ideally suited for use in an aberration corrected TEM, avoiding some of the side effects of the other techniques. The effectiveness of this method is compared to the published techniques and we discuss the relative merits of the different electron vortex production methods.

Lessenreeks Theoretische Fysica, 6-27 maart 2013 (pdf - 30 kB)

Onderwerp: Methods for modeling quantum gases
Spreker: Prof.dr. Nick Proukakis, (Joint Quantum Centre Durham–Newcastle,School of Mathematics & Statistics, Newcastle University)

 

Vrijdag 29 maart 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Advanced electron tomography at the nano- and atomic scale
Spreker: Bart Goris, EMAT, University of Antwerp

Abstract: To understand the properties of nanostructured materials, knowledge on the three dimensional (3D) structure is indispensable both at the nanometer and the atomic scale. Electron tomography enables us to reconstruct this 3D information out of a series of two dimensional (2D) projection images acquired at the electron microscope. However, due to the limited space between the polepieces of the objective lens, only a limited range of projection angles can be reached during the acquisition, resulting in multiple artefacts in the reconstruction.
Recently, several novel reconstruction algorithms have been developed that exploit some prior knowledge about the sparsity of the reconstructed object during the reconstruction process. At the nanoscale, one can often assume that the reconstructed object has a sparse gradient since many nanoparticles have a uniform density. In this presentations, some applications and advantages of this reconstruction method are investigated.
At the atomic scale, prior knowledge about the sparsity of the object itself can be used. This enables the calculation of a 3D reconstruction of the atomic structure of different nanorods. As a consequence, the influence of the synthesis method on the surface facets composing the final morphology of these Au nanorods can be determined with atomic precision. This information may be crucial for a better understanding of the relationship between the local structure and the properties of the nanostructures.

Vrijdag 22 maart 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Microstructure study of the Coffin-Shaped ZSM-5 Zeolite Crystals Unraveled by FIB-Assisted HRTEM
Spreker : Jiangbo Lu, EMAT, University of Antwerp

Abstract: Zeolite ZSM-5 crystals possessing the MFI (mordenite framework inverted)-type micro-porous structure have drawn much attention because of their industrial applications as a leading shape selective catalyst in the synthesis of dimethylbenzene. Because molecular diffusion barriers inside zeolites have a large impact on their catalytic behavior, and internal intergrowth boundaries might act as such diffusion barriers to the passing molecules, it is essential to know the details of the various intergrowth boundaries and the internal architecture inside the crystal. In this talk, I will show some results about microstructure study of the coffin-shaped ZSM-5 zeolite crystals unraveled by FIB-assisted HRTEM. This talk includes two major parts. In the first part, I will show some results about the intergrowth of 90° rotational components and of ramps. 90° rotational boundaries with local zigzag features between different intergrowth components are observed in the main part of crystal. A new kind of displacement boundary is described. The formation of this displacement boundary at unit cell scale is illustrated, and a multi-site nucleation model of 90° intergrowth components projected along [001] is proposed. Based on lamellas prepared with FIB from different positions of the ramps, the orientation relationships between ramp and main part of the crystal are studied, and the 3D morphology of the ramp is illustrated. In the second part, I will show some results about the internal architecture of the coffin-shaped ZSM-5 crystals in which the hourglass structure was observed by optical microscopy. The debating about the origin of the hourglass structure lasts for years. Our FIB-assisted HRTEM observation revealed the shapes of the interfaces inside the crystal, which suport the so-called 2-component model about the origin of the hourglass structure.

Donderdag 21 maart 2013, 11 u., lokaal U.025 (Campus Groenenborger)

Voordracht georganiseerd door TGM
Onderwerp : A positron view on the electronic structure and surfaces of semiconductor nanocrystals and mesoporous oxide thin films
Spreker: Stephan W.H. Eijt, Delft University of Technology, The Netherlands

Abstract: Positron annihilation spectroscopy stands out as a sensitive technique to probe the electronic structure and open volume defects of solid state materials. A brief introduction to positron annihilation spectroscopy will be presented, and recent advances in the study of semiconductor nanocrystals and mesoporous oxide films will be highlighted.
In particular, we investigated thin films of PbSe colloidal semiconductor nanocrystals with sizes in the range of 2-10 nm as prospective functional elements in highly efficient absorber layers for solar cells. We compare and contrast our findings with previous studies on CdSe nanocrystals (2,3). Evidence was gained from our positron annihilation lifetime spectroscopy (PALS) study using the PLEPS spectrometer at FRM-II, Garching, that 90-95% of the implanted positrons are effectively trapped and are confined at the surfaces of these nanoparticles (1). 2D-ACAR measurements on the same set of films using the intense low-energy positron beam POSH at Delft University of Technology were performed to study the positron-electron momentum densities of the PbSe nanocrystals. Ab-initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both positron lifetime and positron-electron momentum densities are found to correlate with changes in the particle morphology observed in high-resolution TEM characteristic of partial ligand removal.
In order to address the riddle of pronounced positronium annihilation from the center of the Galaxy, laboratory 2D-ACAR experiments on SiO2 mesoporous oxides were performed. A pronounced positronium emission was observed in the high-resolution 2-dimensional images of the momenta of emitted para-positronium from the the surfaces of the mesopores, which form a connected open-porosity network.
Positronium annihilation at the surfaces of oxide and carbon-based nanoparticles in the interstellar medium may form an important source of positron annihilation in the Galaxy. The prospects of future studies on positronium emission from the surfaces of carbon nanoparticles, graphite and graphene are discussed.
References
[1] L. Chai, W. Al-Sawai, Y. Gao, A.J. Houtepen, P.E. Mijnarends, B. Barbiellini, H. Schut, L.C. van Schaarenburg, M.A. van Huis, L. Ravelli, W. Egger, S. Kaprzyk, A. Bansil, and S.W.H. Eijt, submitted.
[2] S.W.H. Eijt, P.E. Mijnarends, L.C. van Schaarenburg, A.J. Houtepen, D. Vanmaekelbergh, B. Barbiellini, and A. Bansil, Appl. Phys. Lett.94, 091908 (2009).
[3] S.W.H. Eijt, A. van Veen, H. Schut, P. E. Mijnarends, A. B. Denison, B. Barbiellini, and A. Bansil, Nature Mater.5, 23 (2006).

Vrijdag 15 maart 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Deposition of metal and metal oxide nanostructures using carbon as a template
Spreker: Mert Kurttepeli, EMAT, University of Antwerp

Abstract: The different allotropes of carbon are ideal templates for the deposition of metal or metal oxide nanostructures such as nanotubes, thin films and nanoparticles. These nanostructures can be used at a wide variety of applications commonly found in everyday life. In this talk, the characteristics and the interactions of the nanostructures with their templates will be discussed, which will enable a better understanding of the growth mechanisms involved. Nanostructured thin films of titanium dioxide and nanotubes of titanium dioxide and platinum processed by atomic layer deposition, as well as platinum/sucrose templated carbon will be analysed with respect to phase content, morphology and growth characteristics.

Dinsdag 12 maart 2013, 16.30 u., lokaal U.203 (Campus Groenenborger)

Voordracht georganiseerd door EDF
Onderwerp : Analysis of the combined HERA data with the impact parameter dependent saturation model
Spreker: Merijn Van der Klundert, EDF, University of Antwerp

Vrijdag 8 maart 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Method and Applications of Model-Based Scanning Transmission Electron Microscopy
Spreker : Annick De Backer , EMAT, University of Antwerp

Abstract: Quantitative electron microscopy allows one to obtain numbers for quantities hidden in experimental high resolution images, such as atomic positions and scattering intensities. In this talk, it will be explained how such quantities can be estimated. An optimized estimation procedure for application to larger nanostructures will be discussed. Furthermore, a methodology to count the number of atoms in monotype nanocrystals from high resolution high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images will be illustrated by means of different experimental examples. The validity of the method will be confirmed using image simulations and it will be shown that the high sensitivity of these measurements enables us to count atoms with trustworthy single atom sensitivity.

Donderdag 7 maart 2013, 16 u., lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door TQC
Onderwerp: Quantum Field Theory of Black Swan Events
Spreker: Prof. Hagen Kleinert, Frei Universität Berlin, Germany

Abstract: Prof. Kleinert, a leading expert in path integration, will describe the link between extreme events, stochastic differential equations, fractional derivatives and quantum field theory.

Vrijdag 1 maart 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : How valid are the Multislice simulations?
Spreker : Ivan Lobato , EMAT, University of Antwerp

Abstract: Multislice image simulation of transmission electron microscopy is a standard tool in the microscopy field. It was originally proposed from physical-optical principles and it has shown to be a good numerical procedure for solving the high-energy Schrödinger equation that describes the electron-specimen interaction. The classical multislice method inherently uses different approximations like forward scattering, not sub-slicing the atomic potential, sharp boundaries, first order multislice expansion, all these approximations are used to speed up calculations. In this presentation, I will discuss these approximations and study they range of validity. We can void almost all these approximations by performing the calculation in real space of the n-moment of the potential distribution (by using the symmetry of the atomic potential, Legendre-Gauss Quadrature, etc.), using high order multislice expansion and proper inclusion of the surface boundary. These approximations might represent an important part of the total intensity for quantitative image simulations, but the proper inclusion of them are computational expensive. Nowadays, modern GPU computing can help us to reduce the computational cost. Therefore, it is important to write a new Multislice code that use GPU computing and the minimum number of approximation to solve the high-energy Schrödinger equation. The new code was written in cuda C++ and compile as a MEX file, in this way it can be called as a normal function from Matlab. In the second part of my talk, I will compare image simulations for TEM, PDE, and STEM carry out our code and the STEMSIM code.

Dinsdag 26 februari 2013, 16.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EDF
Onderwerp : Recent results in high energy factorisation: momentum space equations and matrix elements
Spreker : Dr. K. Kutak (Instytut Fizyki Jądrowej, Kraków)

Abstract: We overview results obtained within high energy factorisation focusing on recently obtained equations and framework for calculations of matrix elements. The recent developement was motivated by the regime of QCD explored nowadays at LHC, where both the total energy of collision and momenta transfers are high. In order to study such effects like parton saturation in final states one is inevitably led to investigate how to combine physics of the Color Glass Condensate with physics of exclusive processes. We propose a method how to combine these two. Also a new framework for obtaining matrix elements with off shell initial state gluons is presented. Such framework is necessary for efficient method of calculation of cross sections in high energy factorisation.

Vrijdag 22 februari 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Quantitative composition determination at the atomic level using model-based high-angle annular dark field scanning transmission electron microscopy
Spreker: Gerardo T Martinez , EMAT, University of Antwerp

Abstract: Recently, quantitative HAADF-STEM (High Angle Annular Dark Field Scanning Transmission Electron Microscopy) has drawn attention because of its capability to extract sample structure parameters with high accuracy and precision, such as composition, number of atoms and atomic column positions. The quantification process relies on a comparison of experimental images with time consuming and computing demanding simulations. In this talk, I will explain an alternative quantification procedure based on statistical parameter estimation theory using an empirical physics-based model. This method allows one to obtain local structural and chemical information that can be used as an input for detailed numerical simulations. This will be illustrated using practical examples. Furthermore, the underlying assumptions of the method will be discussed as well as important considerations when simulating HAADF-STEM images.

Vrijdag 15 februari 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Atomic scale investigation of a PbTiO3/SrRuO3/DyScO3heterostructure
Spreker : Ricardo Juan Egoavil Escobar , EMAT, University of Antwerp

Abstract: Ferroelectric thin films have become an important and exciting field of research within material science because of their many technological applications. An essential component to create a contact to these films is a metallic conductive SrRuO3 oxide layer. Epitaxially grown SrRuO3 thin films were used as an intermediate layer between a ferroelectric PbTiO3 film and a DyScO3 substrate, to pattern the ferroelectric domains walls in the PbTiO3. The aim of this talk is to provide a thorough structural and chemical characterization at the atomic scale, leading to a comprehensive understanding of the detailed growth of SrRuO3 in heterostructures. Direct determination of the exact stacking of the atomic planes at the SrRuO3/DyScO3 as well as PbTiO3/DyScO3 interfaces is provided. A detailed atomic model of the interfaces is proposed which is confirmed by the excellent match between experimental and simulated images based on the proposed structure. In addition to these findings, the atomic structure of the PbTiO3 film has been investigated.

Vrijdag 8 februari 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Correlative characterization by Atom Probe Tomography and Cs-probe corrected STEM
Spreker : Williams Lefebvre, Groupe de Physique des Matériaux, Université de Rouen, France

Abstract: Being able to provide the accurate position of atoms together with their chemical nature is a goal that is partially achieved by the Atom Probe Tomography (APT) technique. The reasons for this partial achievement are numerous and originate either from practical limitations related to the instrument (e.g. detection efficiency) or from the physics of field evaporation (e.g. trajectory aberrations). As a result, lattice resolution is rarely achieved by APT and a part of the original information contained in the specimen is reduced by the loss of ions during the detection. The specimen geometry and the evolution of this geometry during field evaporation also play a major role insofar as they impose reconstruction parameters which might evolve during the APT acquisition. Combined used of Scanning Transmission Electron Microscopy (STEM) or TEM is sometimes helpful for the improvement of reconstructions. However, much more information can be obtained using Cs-probe corrected STEM since this technique already demonstrated the possibility of single atom detection and of 3D atomic imaging. In this seminar, the recent evolution of Atom Probe Tomography techniques will be presented. In particular, the use of sub-picosecond laser pulses to produce field evaporation of atom will be exposed. Then, the possibility of correlative analysis of the same specimen by APT and STEM will be discussed based on HAADF-STEM simulations of TIP shape specimens.

Vrijdag 1 februari 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp : Peptide self-assemblies: from the supramolecular structure and the mechanism of formation to the size control
Spreker: Maité Paternostre, CEA iBiTec-S / SB2SM & UMR CNRS, Gif-sur-Yvette, France

Abstract: Supramolecular self-assembly is an attractive pathway for bottom-up synthesis of novel nanomaterials. In particular, this approach allows the spontaneous formation of structures of well defined shapes and monodisperse characteristic sizes. Since nanotechnology is mainly relying on size-dependent physical phenomena, the control of the monodispersity is required, but the possibility of tuning the size is also essential. For self-assembling systems, shape, size and monodispersity are mainly settled by the chemical structure of the building block. Tentative of changing noticeably the size by chemical modification usually end up with the loss of self-assembly. In this context, Lanreotide is a dicationic octapeptide spontaneously forming long nanotubes (few hundreds of mm) but monodispersed in diameter (24.4nm). Their molecular and supramolecular structure and their mechanism of formation have been recently solved. The most noteworthy feature of these nanotubes is that their walls are curved bi-dimentional crystals and the mechanism of formation of these nanotubes shows that the curvature radius of the nanotube is fixed at a very early stage of the assembly, upholding the idea that molecular determinants are controlling the curvature radius. For diameter tuning, we based our strategy on a structural approach. We modified the size of a precise aromatic amino acid involved in close contacts between peptide within the nanotube walls. We demonstrate that this approach indeed enable the accurate tuning of the diameter of the nanotubes from 9 to 35 nm while keeping a strict monodispersity. We finally build a geometrical model taking into account the close-contact that explains how a modification of a few Å of a single aromatic residue induces a 4-fold increase of nanotube diameter. We further demonstrate the application of such strategy by the formation of composite (silica-peptide) nanotubes of various diameters. We also explored the role of the counterions and repulsive forces in the self-assembly mechanism and by this study we evidence that counterions are tightly bound to the nanotubes and that specific interactions occur.

Vrijdag 25 januari 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Electron Vortex Beams: Behavior in a magnetic field, and applications to plasmonics
Spreker: Giulio Guzzinati, EMAT, University of Antwerp

Abstract: Electron vortex beams are a new and attractive field in electron microscopy, holding significant promise for several applications.
This presentation will be divided into three parts. The first part is a brief introduction on electron vortices, their main properties in contrast to the conventional beams employed in electron Microscopy, and how they are produced in a Transmission Electron Microscope. The second part focuses on the interaction of the magnetic properties of electron vortex beams with external magnetic fields such as those of the lenses of electron microscopes. This interaction determines additional phases in a vortex wavefront and manifests in rich rotational phenomena that have been experimentally studied. These results shed a new light on the common phenomenon of Larmor rotation in the TEM. The third part introduces chiral plasmons, which are developing as promising branch of plasmonics, with applications in biosensing and metamaterials. Our experiments with chiral plasmons and electron vortices will be presented.

Vrijdag 18 januari 2013, 15 u., lokaal U.203 (Campus Groenenborger)

Voordracht georganiseerd door EDF
Onderwerp: An application of the theory of solvable groups to the physics of Higgs bosons
Spreker: Dr. Igor Ivanov (Université de Liège)

Abstract: Symmetries play a pivotal role in physics, and in particular in construction of non-minimal Higgs models in particle physics. One rather challenging problem here is classify finite symmetry groups in these models. I will report our recent solution of this problem for the so-called three-Higgs-doublet model. This work can be viewed as an interesting physics application of the theory of solvable groups and, in particular, of the Burnside's theorem.

Vrijdag 18 januari 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Characterization of nanocrystalline Pd thin films using CTEM, ACOM-TEM and HRTEM
Spreker: Behnam Amin-Ahmadi, EMAT, University of Antwerp

Abstract: The influence of the deposition rate on the formation of growth nanotwins in nanocrystalline Palladium films deposited by electron beam evaporation has been investigated. The statistics results reveal that twin boundary (TB) density, volume fraction of grains containing twins and dislocation density increase with increasing deposition rate. The coherency of the TBs decreased significantly at highest deposition rate. The fraction of high angle grain boundaries (in the range of 55-65˚) continuously increases with increasing the deposition rate. The effect of Hydrogen on the microstructure changes of Pd thin film was also investigated. HRTEM results show that coherency of the TBs decreased after H loading. Surprisingly, the ∑3‹112› incoherent TBs dissociate after hydriding into two phase boundaries bounding a 9R phase. This phase was not expected because of the high stacking fault energy of Pd. This observation is connected to the influence of the Hydrogen on the stacking fault energy of Pd and the high compressive stresses building up during hydriding.

Vrijdag 11 januari 2013, 11.30 u., lokaal X.009 (Campus Groenenborger)

Voordracht georganiseerd door EMAT
Onderwerp: Quantitative structure determination of large three-dimensional nanoparticle assemblies
Spreker: Thomas Altantzis, EMAT, University of Antwerp

Abstract: Nanoparticle assemblies have recently gained increasing interest, since it is an attractive route towards systems in which the properties can be tuned through parameters such as the size of the individual particles and the nature of their 3-dimensional (3D) stacking. In order to investigate these systems in 3 dimensions at the nanoscale, electron tomography can be used. In the case of small assemblies with a size less than 200 nm, electron tomography is performed using HAADF-STEM. For large nanoassemblies however, it is not straightforward to extract quantitative results by applying the conventional approaches of acquisition and reconstruction, due to the appearance of several artefacts, especially in the central part of the volume. We have optimized both the acquisition and reconstruction methodology and this enables us to study large assemblies in a quantitative manner as well.