Voordrachten 2016

Vrijdag 9 december 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: A new method for quantitative XEDS tomography of complex hetero-nanostructures
Spreker: Daniele Zanaga, EMAT, University of Antwerp

Abstract:
Over the last decades, electron tomography based on HAADF-STEM has evolved into a standard technique to investigate the morphology and inner structure of nanomaterials. The HAADF-STEM intensity depends on sample thickness but also scales with the atomic number Z and therefore, chemical compositions can be studied from these three-dimensional (3D) reconstructions. Nevertheless, it is not straightforward to interpret the gray levels in a 3D HAADF-STEM reconstruction, when mixing of elements is expected or elements with atomic number Z close to each other are present.
In an increasing number of recent studies, X-ray Energy Dispersive Spectroscopy (XEDS) has been combined with tomography to understand complex nanostructure morphology and composition in 3D.
Here, we propose an alternative approach to optimize the reconstruction of an XEDS tomography series by minimizing the impact of shadowing effects and improving the spatial resolution. The method is based on the synergistic combination of HAADF-STEM tomography and XEDS quantitative mapping. HAADF-STEM yields a relatively high signal-to-noise ratio, enabling an accurate reconstruction of the morphology. XEDS, on the other hand, yields chemical information, but the limited amount of data that can be usually collected, hampers a good morphological reconstruction. As a proof of principle, we apply our methodology to a nanostructure containing a mix of Au, Ag and Pt atoms. It should be mentioned that the approach we propose here enables quantitative 3D chemical characterization of a broad variety of nanostructures.

Woensdag 7 december 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Tuning quantum non-local effects in graphene plasmonics
Spreker: Ben Van Duppen, CMT, University of Antwerp

Abstract:
The response of an electron system to short range electromagnetic fields is strongly dependent on quantum electronic properties, even under ambient conditions, but difficult to access experimentally. We use high-quality graphene plasmons, together with an engineered dielectric–metallic environment, to access this regime of short-ranged response, where non-local effects due to quantum properties become significant. We clearly identify three types of quantum effects as keys to understanding the experimental response of graphene to short-ranged terahertz electric fields. The first type is of single-particle nature and is related to shape deformations of the Fermi surface during a plasmon oscillation. The second and third types are a quantum many-body effect controlled by the inertia and compressibility of the interacting electron liquid in graphene. We demonstrate how, in principle, experiments can determine the full spatiotemporal response of an electron system.

Vrijdag 2 december 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: TEM analysis of the thin film solar cells based on Cu(In,Ga)Se2 and Cu2ZnSnSe4absorbers
Spreker: Maria Batuk, EMAT, University of Antwerp

Abstract:
Solar cells based on chalcogenide absorbers such as Cu(In,Ga)Se2 (CIGSe) and Cu2ZnSnSe4(CZTSe) are a good alternative to silicon-based photovoltaics. CIGSe solar cells applied in industry reach efficiency of up to 20% but are manufactured using an expensive vacuum-based co-evaporation process. To minimize the production costs, other methods are widely investigated. The solar cells produced by a solution-based method at IMEC (within the framework of the SOPPOM project) showed efficiency around 5%. Different modifications of the procedure were performed (extra annealing; variation of binder content) to increase the efficiency. Although the morphology of the materials improved, the efficiency dropped down to 0.5-1%. Using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX), we proved that the deterioration is due to the high amount of secondary phases formed during the treatments.
Another group of the materials under investigation is CZTSe-based solar cells. In my talk I will present the TEM analysis of the solar cells with an efficiency of 9.7% obtained in IMEC. Influence of other treatments (KCN etching; extra annealing; TiO2 layer formation) on the microstructure and properties will be also discussed.

Vrijdag 25 november 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Atomic resolution tomography and dynamics of nano-objects
Spreker: Dirk Van Dyck, EMAT/VISION LAB, University of Antwerp

Abstract:
The latest generation of aberration-corrected Transmission Electron Microscopes (TEM) have a resolution and sensitivity that is sufficient to detect even single light atoms from the periodic table of elements and to pinpoint their position with a lateral precision that reaches the wavelength of the imaging electrons. However the depth (z) information remains less certain. For the study of beam-sensitive crystalline nanoparticles such as catalysts there is a need for a tomographic method for fast characterization of the shape of pristine particles at atomic resolution.
In this work we describe a quantitative parameterless 3D reconstruction method that uses the exit wave obtained from only one viewing direction parallel to the atomic columns.
In this configuration the strong dynamical scattering yields a signal that is stronger than the incoherent signal in HAADF STEM which allows to minimize the exposure of the object to the incident beam. The method is based on the “channeling” theory which has all the ingredients for a full 3D quantification of the atomic structure since it is not influenced by channeling in neighboring columns up to thicknesses of tens of nm, so that the exit wave can be analyzed column by column. Furthermore the atoms of a column act as weak lenses, which focus the electron wave periodically with depth so that the exit wave of a column is a very sensitive peaked fingerprint of the “weigth” of the column. Every pixel in the exit wave function is a complex number.
The theory of channeling is simple and provides a way to interpret the exit wave, which can be visualized graphically by plotting the complex values of the pixels in complex 2D space.
From the Argand plot of a column we can deduce the position of the column, the defocus distance (with sub-Angstrom precision), the total mass of the column and the residual aberrations [4].
By combining this information we can then reconstruct the object in 3D including profile of top and bottom surface with single atom sensitivity. We have applied this successfully to nanoparticles of Ge, MgO, Au. We also developed a fast method to visualize the vertical position of atoms in a thin sheet and applied it to study the dynamics of thin graphene sheet in real time.

Woensdag 23 november 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp : Stroboscopic phenomena in superconductors with dynamic pinning landscape
Spreker: Zeljko Jelic, CMT, University of Antwerp

Abstract:
Introducing artificial pinning centers is a well-established strategy to trap quantum vortices and increase the maximal magnetic field and applied electric current that a superconductor can sustain without dissipation. In case of spatially periodic pinning, a clear enhancement of the superconducting critical current arises when commensurability between the vortex configurations and the pinning landscape occurs. With recent achievements in (ultra-fast) optics and nano-engineered plasmonics it has become possible to exploit the interaction of light with superconductivity and create not only spatially periodic imprints on the superconducting condensate, but also temporally periodic ones.
Here we show that in the latter case, temporal matching phenomena develop, caused by stroboscopic commensurability between the characteristic frequency of the vortex motion under applied current and the frequency of the dynamic pinning. The matching resonances persist in a broad parameter space, including magnetic field, driving current, or material purity, giving rise to unusual features such as externally variable resistance/impedance and Shapiro steps in current-voltage characteristics. As one of the possible applications for the stroboscopic effect, we devised a method combining conventional transport measurements and a frequency-tuned flashing pinning potential to obtain reliable estimates of the vortex velocity. We discuss the sensitivity of the proposed technique on applied current, temperature and heat diffusion, as well as the vortex core deformations during fast motion.

Vrijdag 18 november 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Implementation of compressed sensing in STEM: towards efficient dose reduction
Spreker : Armand Béché, EMAT, University of Antwerp

Abstract:
Transmission electron microscopy (TEM) is a very powerful technique to investigate materials down to their atomic components. Its versatility allows quantifying samples from their shape to the nature of their constituents and surroundings. However, the strong interaction of the electron beam with matter potentially induces damages in samples under investigation, especially for those composed of soft matter such as zeolites, metal organic frameworks (MOFs) and most life science samples. Current workarounds involve the reduction of the beam intensity, such as in the so-called low dose imaging, or increase of the detector performances as provided e.g. by direct electron detectors.

Recently, improvement in signal processing lead to the development of compressed sensing, a numerical algorithm based on the assumption that real life images are sparse in some particular well-chosen basis. Images can then be expressed with much less components than what required by the Nyquist sampling theorem. By extension, not all pixels in a given image are necessary and only a random selection of them is sufficient to retrieve the original image with fidelity. By definition, compressed sensing is a very dose efficient technique as only parts of the sample need to be exposed to the electron beam to reconstruct a faithful image.
Very recently, we demonstrated the first physical implementation of compressed sensing in a Scanning TEM (STEM) based on the use of a solenoid as a fast beam blanker. The solenoid is placed in the condenser plane of a STEM in a specially designed condenser aperture holder with feedthrough electrical contacts. By synchronizing the STEM signal with the current source driving the solenoid, we successfully acquired compressed images by shifting the beam away from the region of interest on blanked pixels. The case of both medium scale imaging and high resolution imaging was investigated and reconstructed using the SPGL1 algorithm with a signal compression of up to 80%.
The results on more sensitive materials such as MOF (MIL 101) will be presented together with 3D tomographic reconstruction of gold nanorods. The improvement in terms of electron dose will be discussed and compared to uncompressed images acquired with a similar total electron dose.

Woensdag 16 november 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: :  Geometrical structure of 2D materials
Spreker : Ortwin Leenaerts, CMT, University of Antwerp

Abstract:
n the literature on two-dimensional materials, there sometimes appear bizarre suggestions for novel 2D material structures. These appear to result from a lack of understanding  of the underlying chemical bonding picture and other, more physical, notions.  In this seminar, Dr Ortwin Leenaerts will discuss some chemical and physical properties that are important for understanding the geometrical structure of some common 2D crystals. 

Woensdag 9 november 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp:  Thermo-mechanical properties of carbon nanostructures: MD approach
Spreker: Hossein Ghorbanfekr, CMT, University of Antwerp

Abstract:
Van der Waals (vdW) interactions between two-dimensional (2D) nano-crystals (e.g. graphene) exert an enormous pressure (~1 GPa) on the substance trapped inside a encapsulated region known as the nanobubble. These nanobubbles are experimentally investigated with ample applications due to their extremely high-pressure and nano-confined environment. In this talk, after reviewing the importance of molecular dynamics (MD) approach, we will discuss mainly the geometrical and the structural properties of different nanobubbles using the equilibrium MD simulation. For a few nano-size bubble, we demonstrated that theory of elasticity could fail to give correct physical properties (i.e. pressure and shape) of any type  of nanobubbles. For large nanobubble, we showed that the atomic force microscopy (AFM) experiment is perfectly consistent with our MD data. Then, as a second case study, the mechanical properties of nitrogen-doped graphene (NG) will be investigated which is an effective way to intrinsically modify properties of carbon-based materials. Based on our MD approach, we found that Young’s modulus, tensile strength, and intrinsic strain decrease with the number of dopants. Furthermore, the ripples -induced by the dopants- change the roughness of NG which depends on the number of dopants and their local arrangement.

Vrijdag 4 november 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Engineering the morphology and organization of gold nanostructures for SERS detection
Spreker: Andrea La Porta, EMAT, University of Antwerp

Abstract:
Since its discovery, Surface-enhanced Raman Scattering (SERS) has become one of the most powerful and intensively studied spectroscopic analytical techniques. The electric near-field enhancement created by illumination of metallic nanostructures provides SERS with the ability to overcome the main drawback of standard Raman scattering spectroscopy, namely its low sensitivity. Many efforts are therefore currently devoted toward the fabrication of high-performance, homogeneous and reproducible SERS substrates by means of the most advanced methods, both top-down and bottom-up. Metallic nanoparticles represent an attractive route to the design of SERS supports with suitable properties.
In this work, different approaches in the fabrication of SERS substrates have been studied. Among all the available metals and related alloys, gold and silver are the principal materials of choice because of their special interaction with light. Applications of SERS spectroscopy are foreseen in a wide variety of fields like medicine, biology, forensic science, archaeology, pharmacy and others.

Donderdag 3 november 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: :  Hybrid monolayer-bilayer graphene quantum dots
Spreker : Mohamad Mirzakhani, CMT, University of Antwerp

Abstract:
Often real samples of graphene consist of islands of both monolayer and bilayer graphene. Bound states in such hybrid quantum dots are investigated for (i) a circular single-layer graphene quantum dot surrounded by an infinite bilayer graphene sheet and (ii) a circular bilayer graphene quantum dot surrounded by an infinite single-layer graphene. Using the continuum model and applying zigzag boundary conditions at the single-layer–bilayer graphene interface, we obtain analytical results for the energy levels and the corresponding wave spinors. Their dependence on perpendicular magnetic and electric fields are studied for both types of quantum dots. The energy levels exhibit characteristics of interface states, and we find anticrossings and closing of the energy gap in the presence of a bias potential. 

Vrijdag 28 oktober 2016, 11.30 u, Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Investigation of the liquid metal corrosion mechanisms of the DIN 1.4970 austenitic stainless steel fuel cladding for MYRRHA
Spreker: Evangelia Charalampopoulou, EMAT, University of Antwerp

Abstract:
MYRRHA is an experimental liquid lead bismuth eutectic (LBE)-cooled fast research reactor currently under development at SCK•CEN. It is anticipated that the candidate MYRRHA driver fuel (mixed plutonium-uranium oxide fuel pellets) will be encapsulated in cladding tubes made of austenitic stainless steel DIN 1.4970. The fuel cladding is the first barrier separating the radioactive fuel and fission products from the rest of the reactor system. The fuel cladding must maintain leak tightness and effective containment of fuel pellets and fission products during the fuel residence in the reactor; hence, its failure must be avoided. Therefore, it is important to understand the resistance of the fuel cladding material to liquid metal corrosion caused by its contact with liquid LBE. Equilibrium thermodynamics as well as reaction kinetics will determine the threshold temperature for a given concentration of oxygen dissolved in liquid LBE, below which the fuel cladding can be reliably used. The main liquid metal corrosion (LMC) mechanisms are oxidation, dissolution and erosion. For this work, the focus is on the study of oxidation and dissolution corrosion in static LBE, as erosion implies the presence of LBE flow. All corrosion phenomena are promoted during the exposure of the steel cladding to high temperatures in the liquid LBE, irrespective of the LBE oxygen concentration. Oxygen-poor liquid LBE, in particular, favours dissolution corrosion, as it suppresses the formation of protective oxide scales. Logically, oxidation is promoted by high amounts of dissolved oxygen in the liquid LBE.
The objective of this study is to characterize the effects of LMC on the surface of the DIN 1.4970 steel after its exposure to static LBE and to correlate them to the specific exposure conditions. The specimens exposed to LBE were DIN 1.4970 cladding tubes 24% cold worked (reference cladding material for MYRRHA). The temperature and LBE oxygen concentration during the performed exposures were monitored by thermocouples and electrochemical oxygen sensors (air/Lanthanum Strontium Manganese oxide (LSM) reference electrode), respectively. An automatic oxygen control system, developed at SCK•CEN, maintained the amount of dissolved oxygen in the liquid LBE constant throughout the experiment. This work presents results from the steel characterization by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM).

Vrijdag 21 oktober 2016, 11.30 u, Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Bi3n+1Ti7Fe3n-3O9n+11 Perovskite-based Homologous Series
Spreker: Dmitry Batuk, EMAT, University of Antwerp

Abstract:
Although the perovskite structure (ABO3) is one of the simplest structure types of complex oxides, perovskite-based compounds can demonstrate extreme structural complexity. Structural and electronic instabilities, ordering of point vacancies and planar defects, formation of intergrowths and stacking faults and, more importantly, combinations of these factors results in extraordinary diversity of the perovskite crystal chemistry. The perovskite-based homologous series have been studied for decades, but apparently there is still room for exploring new materials constructed of 2D perovskite modules separated by interfaces of various types. A new Bi3n+1Ti7Fe3n-3O9n+11 perovskite-based homologous series will be presented in the lecture.
In this series perovskite blocks are separated by translational interfaces, which are based on anatase-like chains of double edge-sharing (Ti,Fe)O6 octahedra. The chains are connected to the octahedra of the perovskite blocks by sharing edges and corners. This configuration shifts the adjacent perovskite blocks relative to each other over a vector 1/2[110]p and creates S-shaped structure tunnels along the [010] direction. The tunnels accommodate double columns of Bi3+ cations, which stabilize the interfaces owing to their stereochemical activity.
Detailed structure analysis of the n = 3-6 members of the series using a combination of transmission electron microscopy and powder diffraction demonstrates that the stereochemical activity of the Bi3+ also plays an important role in the deformation of the perovskite blocks induced by the interfaces.

Vrijdag 14 oktober 2016, 11.30 u, Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: First-principles analysis of the spectroscopic limited maximum efficiency of photovoltaic absorber layers for CuAu-like chalcogenides and silicon​
Spreker: Marnik Bercx, EMAT, University of Antwerp

Abstract:
Chalcopyrite semiconductors are of considerable interest for application as absorber layers in thin-film photovoltaic cells. When growing films of these compounds, however, they are often found to contain CuAu-like domains, a metastable phase of chalcopyrite. It has been reported that for CuInS2, the presence of the CuAu-like phase improves the short circuit current of the chalcopyrite-based photovoltaic cell. We investigate the thermodynamic stability of both phases for a selected list of I-III-VI2 materials using a first-principles density functional theory approach. For the CuIn-VI2 compounds, the difference in formation energy between the chalcopyrite and CuAu-like phase is found to be close to 2 meV per atom, indicating a high likelihood of the presence of CuAu-like domains.
Next, we calculate the spectroscopic limited maximum efficiency (SLME) of the CuAu-like phase and compare the results with those of the corresponding chalcopyrite phase. We identify several candidates with a high efficiency, such as CuAu-like CuInS2, for which we obtain an SLME of 29% at a thickness of 500 nm. We observe that the SLME can have values above the Shockley-Queisser (SQ) limit, and show that this can occur because the SQ limit assumes the absorptivity to be a step function, thus overestimating the radiative recombination in the detailed balance approach. This means that it is possible to find higher theoretical efficiencies within this framework simply by calculating the J-V characteristic with an absorption spectrum.
Finally, we expand our SLME analysis to indirect band gap absorbers by studying silicon, and find that the SLME quickly overestimates the reverse saturation current of indirect band gap materials, drastically lowering their calculated efficiency.

Vrijdag 7 oktober 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Real-Time 3D Tomography
Spreker: Prof. Joost Batenburg, Mathematical Institute, Universiteit Leiden, The Netherlands

Abstract:
This lecture deals with the development of very fast algorithms for 3D image reconstruction in tomography.  Recent experimental developments in both X-ray and electron tomography demonstrate that it is feasible to acquire tomographic datasets in the order of a second, opening up fascinating new possibilities for imaging of dynamic processes. What is still lacking, however, is the computational ability to create large-scale 3D images at the same speed as the image acquisition. Even more challenging is the combination of “limited data” (limited number of views, limited angular range) with “big data” (high resolution 3D imaging), for which advanced reconstruction techniques may take days of computing time.
I will discuss the various challenges involved in speeding up the tomography pipeline towards real-time tomography, where the object can already be analyzed during the scan, which requires  new types of algorithms that are capable of dealing with limited data, yet fast enough to reconstruct and analyze a volume of 10243 voxels in one second.
Successful realization of such a pipeline can dramatically change the way we work with tomographic scanners, as we will create the ability to perform in-situ experiments, immediately observing  the results in 3D, and adapting the experimental control to the observations.

Woensdag 5 oktober 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp : Engineering and probing topological properties of Dirac semimetal films by asymmetric charge transfer
Spreker: Prof. Kyungwha Park, Virginia Tech University, U.S.A.

Abstract:
Dirac semimetals (DSMs) have topologically robust three-dimensional (doubled Weyl) nodes with Fermi-arc states. In heterostructures involving DSMs, charge transfer may occur at the interfaces, which can be used to probe and control their bulk and surface topological properties through surface-bulk connectivity. We demonstrate that despite a band gap in DSM films, asymmetric charge transfer at the surface enables one to accurately identify locations of the Dirac-node projections from gapless band crossings and to examine and engineer properties of the topological Fermi-arc surface states connecting the projections, by simulating adatom-adsorbed DSM films using a first-principles method with an effective model. The positions of the Dirac-node projections are insensitive to charge transfer amount or slab thickness except for extremely thin films. By varying the amount of charge transfer, unique spin textures near the projections and a separation between the Fermi-arc states change, which can be observed by gating without adatoms.

Maandag 3 oktober 2016, 16.00 u., Lokaal N0.08 (Campus Drie Eiken)

Voordracht georganiseerd door EGM

Onderwerp: Graphene nanoribbon formation inside carbon nanotubes
Spreker: Prof. Katalin Kamaris, Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest, Hungary

Abstract:
Graphene nanoribbons of controlled width can be formed by encapsulation into carbon nanotubes and subsequent polymerization reactions. Encapsulation by sublimation often results in unwanted side reactions as degradation of the molecule or adsorption of oligomers on the outer nanotube surface. We used nanoextraction by supercritical carbon dioxide to fill various planar molecules into carbon nanotubes [1] and followed their transformation to nanoribbons and double-walled nanotubes by Raman and photoluminescence spectroscopy and electron microscopy. The transformation can take place either thermally or by energy transfer from an electron beam. We compare the reactions starting from coronene and tetrathiotetracene and find that the latter forms well-defined nanoribbons under milder conditions. This observation is in accordance with earlier results [2] on the beneficial role of heteroatoms in nanoribbon formation.

[1] B. Botka, M.E. Füstös, H.M. Tóháti, K. Németh, G. Klupp, Zs. Szekrényes, D. Kocsis, M. Utczás, E. Székely, T. Váczi, G. Tarczay, R. Hackl, T.W. Chamberlain, A.N. Khlobystov, K. Kamarás: Small 10, 1369-1378 (2014)
[2] T.W. Chamberlain, J. Biskupek, G.A. Rance, A. Chuvilin, T.J. Alexander, E. Bichoutskaia, U.  Kaiser, A.N. Khlobystov: ACS Nano 6, 3943-3953 (2012)

Vrijdag 30 september2016, 12.00 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Polymer matrix composites: An advanced nanoscale characterization​
Spreker : Hamed Heidari, EMAT, University of Antwerp

Abstract:
Polymer matrix composites are a developing group of functional materials that due to their specific properties are commonly used in designing and manufacturing of new devices. However, a major concern about the design of these materials is to correlate the properties at the macroscale and the physical mechanisms and properties originating from interfaces at the micro- and nanoscale. Advanced electron microscopy characterization is part of a multiscale approach towards intelligent design of these materials. 
The HINTS InterPoCo project is trying to investigate the components and the interfaces of several model composite systems. In the lecture, the latest development on electron microscopy of two model composite systems will be presented; Self-healing polymer composites and Carbon-fiber reinforced polymer composites.

Vrijdag 23 september 2016, 16.00 u., Lokaal N0.08 (Campus Drie Eiken)

Voordracht georganiseerd door TQC

Onderwerp:  On the dynamical instability of the exciton-polariton condensates
Spreker : Nataliya Bobrovska, Institute of Physics, Polish Academy of Sciences, Warsaw

Abstract:
The phenomenon of the polariton condensate dynamical instability induced by the interactions of condensate polariton with the exciton reservoir was predicted by the phenomenological open-dissipative Gross-Pitaevskii equation (ODGPE) [1]. However, since to date there was no experimental evidence of this phenomenon. Here, based on ODGPE model, we demonstrate the presence of the condensate instability induced by the reservoir and confirm it by the single-shot realizations of the polariton condensate emission from an organic semiconductor  microcavity [2], see Fig. 1.  An excellent agreement between experimental data and the numerical simulations was obtained without using of  any free parameters. We observe the transition from stable to unstable condensate induced by the particle flows as well as  the specific form of reservoir-mediated instability. The lifetime of the reservoir places the system in the unstable and strongly nonadiabatic regime, where the simplified complex Ginzburg-Landau description becomes invalid [3].
[1] M. Wouters and I. Carusotto, Phys. Rev.Lett.99, 140402 (2007).
[2] K. S. Daskalakis, S. A.  Maier, and S. Kena-Cohen,  Nat. Mater. 13, 271–278 (2014); J. D. Plumhof, T. Stoferle, L. Mai, U. Scherf, and R. Mahrt, Nat. Mater. 13, 247–252 (2014); K. S. Daskalakis, S. A. Maier, and S. Kéna-Cohen,Phys. Rev. Lett.115, 035301 (2015).
[3] N. Bobrovska and M. Matuszewski, Phys. Rev. B 92, 035311 (2015).

 Vrijdag 16 september 2016, 14.00 u., Lokaal N0.08 (Campus Drie Eiken)

Voordracht georganiseerd door VISIE

Onderwerp: Vision based Advanced Driver Assistance Systems
Spreker: Prof. Philomina Simon, University of Kerala, India

Abstract:
Traffic safety is an important concern now days as the road accidents make nearly 10 million causalities whole over the world. Advanced Driver Assistance Systems are the global technologies that help to improve the awareness of vehicle drivers and they assist the driver in making decisions, provide signals in possibly dangerous driving situations, and execute counteractive measures. Lot of research is going on in the area of automated driver assistance systems. Scientific community and automotive industry is constantly trying to improve the safety mechanisms in vehicles. There are several real time applications in ADAS such as Pedestrian Detection, Vehicle Detection and Classification, Lane Departure Warning System, Driver Drowsiness detection, Traffic Signal Recognition and so on. ADAS can save human lives.

The outline of talk deal with presenting a generic idea about the Pedestrian Detection System, Lane detection Warning system and some approaches for developing such a system. Pedestrians are often seriously injured in traffic accidents, particularly at night. Data show that nearly 70% of pedestrian fatalities happen at night, while night time driving is only 20% of the total traffic. Pedestrian detection at night time in ADAS aims to detect these potentially dangerous situations in advance, to warn the driver. A real-time pedestrian detection at night time is a system based on ROI generation and Object Classification. The Lane Departure Warning (LDW) system warns the driver when the vehicle moves out of the lane unintentionally. This system has a camera placed behind the windshield. The lines on the road are interpreted and based on which the warning are given. The lane detection is performed using template matching and hough transform, lane tracking is done using Kalman filtering and the lane departure warning is based on the position of vanishing point.

Vrijdag 15 juli 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Study of local order in topological insulators using HR-STEM and EDX
Spreker: Carolien Callaert, EMAT, University of Antwerp

Abstract:
Recently, the established division between insulators and conductors was torn down by the remarkable discovery of topological insulators. These materials are bulk insulating, but conducting at their surfaces. Their exotic physics and interesting practical applications in spintronics (which uses the spin of the electron as data elements, in addition to the charge) and quantum computing and the creation of Majorana fermions (fermions that are their own antiparticles) in these materials makes it a hot topic. To make applications feasible, the surface Dirac spectrum should ideally be located in the middle of the largest possible bulk band gap and its position should be robust against perturbations. However, most topological “insulators” are actually slightly bulk conducting. More insight in the physics of these materials is needed. For this, as a first topic, the phase transition from a topological insulator to a trivial insulator was studied in (Bi1-xInx)2Se3. The average structure is known, but we studied the local order using state-of-the-art electron microscopy. Secondly, the topological insulator Bi2Te3 doped with Fe is studied because in contradiction to current belief, the surface states remain intact and gapless. The characteristics of topological insulators cause them to also be good thermoelectrics. On the other hand, excellent thermoelectrics are not necessarily topological insulators. For example GeTe; its defect structure is the third topic of this lecture. GeTe has good thermoelectric properties but is not a topological insulator. 

Donderdag 7 juli 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp : Andreev-reflection-enhanced conductance of semiconductor-superconductor hybrid nanodevices
Spreker : Dr. Michal  Nowak, TU Delft Theoretical Physics

Abstract:
Andreev reflection at a normal (N) – superconductor (S) interface corresponds to transfer of 2e charge through the junction. As a consequence, the conductance of a quantum point contact realized in the normal part of N-S junction is predicted to be quantized in twice the conductance quantum. However, the recent experimental realization of such a structure reveal an unexpected feature: despite a nearly perfect conductance quantization for electrons with energies above the superconducting gap, the conductance governed by Andreev reflection drops after the first enhanced plateau [1,2]. We investigate N-S junction defined in a proximitized nanowire and explain, that the lack of the conductance doubling is a result of mode mixing induced by residual disorder in the semiconductor. Furthermore, in a biased S-N-S junction, multiple conversions of Cooper pairs to electrons and vice-versa imprint the conductance with sub-gap features. Taking as an example Josephson junction defined in a 2DEG we demonstrate how the multiple Andreev reflections can be further exploited to infer transparency of the junction and to provide an estimate of the superconducting gap induced in the semiconductor [3].
[1] H. Zhang, Ö. Gül, S. Conesa-Boj, K. Zuo, V. Mourik, F. K. de Vries, J. van Veen, D. J. van Woerkom, M. P. Nowak, M. Wimmer, D. Car, S. Plissard, E. P. A. M. Bakkers, M. Quintero-Pérez, S. Goswami, K. Watanabe, T. Taniguchi, L. P. Kouwenhoven, arXiv:1603.04069 (2016).
[2] M. Kjaergaard, F. Nichele, H. J. Suominen, M. P. Nowak, M. Wimmer, A. R. Akhmerov, J. A. Folk, K. Flensberg, J. Shabani, C. J. Palmstrøm, C. M. Marcus, arXiv:1603.01852 (2016).
[3] M. Kjaergaard, H. J. Suominen, M. P. Nowak, A. R. Akhmerov, J. Shabani, C. J. Palmstrøm, F. Nichele, C. M. Marcus (to be submitted).

Dinsdag 5 juli 2016, 11.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Tomonaga-Luttinger liquid and beyond: the intriguing case of lithium molybdenum purple bronze
Spreker: Dr. P.M. Chudzinski,  University of Utrecht

Abstract:
We study the low energy physics of a quasi-1D material - lithium molybdenum purple bronze (LMO), Li$_0.9$Mo$_{6}$O$_{17}$, which undergoes a mysterious phase transition at T$^*=28K$ to later become superconductor at 1.9K. Based on band structure results we derive an effective low energy theory within the Tomonaga-Luttinger liquid framework. We estimate the TLL parameters and strength of possible instabilities. Our aim here is to understand these experimental findings that are certainly lying within the 1D regime. In the second part we move beyond the standard 1D theory, we investigate the role of inter-orbital fluctuations. We make a conjecture that the physics around T$^*$ is dominated by multi-orbital excitons. They couple with 1D fermions and properties of such system can be captured using a polaronic picture. Using this model we compute fermionic Green's function to find that the spectral function is broadened with a Gaussian and its temperature dependence acquires an extra T$^1$ factor. Both effects are in perfect agreement with experimental findings. We also compute the resistivity for temperatures above and below the critical temperature T$^*$ which allows us to explain an upturn of the resistivity and interpret the suppression of this extra component when a magnetic field is applied along the conducting axis.

Vrijdag 1 juli 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Momentum-resolved STEM: Measurement of atomic electric fields and angular multi-range analysis
Spreker : Dr. Knut Müller-Caspary, Institute for solid state physics - Department for electron microscopy, University of Bremen, Germany

Abstract:
The detection and quantification of electric fields by STEM faces 2 main challenges: Firstly the angular deflection of the STEM probe must be measured reliably. To this end, we calculate the centre-of-gravity in each diffraction pattern which equals the expectation value for the quantum mechanical momentum transfer. Secondly a robust relation to the local electric field is needed, which we derive from Ehrenfest's theorem for thin specimen. The method is worked out by detailed simulations and applied to the mapping of momentum transfer, electric field and charge densities in strontium titanate and defects in 2D-molybdenum disulfide, partly employing the ultrafast pnCCD camera in STEM.
In the second part we report the setup and application for materials analysis based on the evaluation of multiple scattering angles. We use a motorised iris-type aperture, allowing for a dedicated setting of the ADF detector acceptance angles. The method is applied to measure both specimen thickness and composition independently, to visualise angular contrast behaviour as to strain and composition in MOSFETs, and to compare with the angular dependence predicted from theories.

Donderdag 23 juni 2016, 11.00 u., Lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door VISIELAB

Onderwerp: Acquisition and reconstruction in fetal imaging: Building the University of Washington Fetal Brain MRI Database
Spreker : Dr. Colin Studholme, Biomedical Image Computing Group, Department of Pediatrics, University of Washington

Abstract:
This talk will cover techniques used in creating an MRI database of fetal brain structure, micro-structure and functional connectivity. The database covers normal fetal brain development between 18-36 gestational weeks, together with abnormal development in a range of clinical studies. Methods for acquisition and between slice motion correction aimed at dealing with unconstrained fetal head motion will be described, along with techniques for robust 3D and 4D image reconstruction from scattered slice data.

Donderdag 23 juni 2016, 11.30 u., Lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door VISIELAB

Onderwerp: Quantitative imaging of carotid atherosclerotic plaque: the value of complex fitting
Spreker : Dr. Dirk Poot, Erasmus MC - Biomedical Imaging Group Rotterdam, Department of Radiology and Medical informatics

Woensdag 22 juni 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp : Quantum transport in graphene Hall bars - effects of vacancy disorder
Spreker : Marko Petrovic, CMT, University of Antwerp

Abstract:
Using the tight-binding model, we numerically study the influence of vacancy disorder on electron transport in graphene Hall bars in high magnetic fields.  Disorder, induced by random distributions of single vacancies, breaks the  graphene sublattice symmetry and creates vacancy localized states. These  states are observable in the bend resistance, as well as in the total DOS.  Their energy is proportional to the square root of the magnetic field, while their localization length is proportional to the cyclotron radius. We further  study how do these localized states change with the vacancy concentration and next nearest neighbor hopping.

Vrijdag 17 juni 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Growth, Structure, Stability and Properties of Nanostructures and Hybrids: Insights using Electron Microscopy
Spreker: Prof. N. Ravishankar, Materials Research Centre Indian Institute of Science, Bangalore, India

Abstract:
Nucleation and growth processes play a key role in controlling the structure, microstructure and chemistry and consequently every conceivable property of advanced functional materials.  Our group has been working on wet-chemical methods for the synthesis of nanostructures and hybrids. While these methods are simple and undoubtedly very powerful, the mechanisms of nucleation and growth are poorly understood.  In particular, there is an over-emphasis on the role of specific reagents rather than broad principles that are applicable for a wide variety of systems.
My talk will focus on three specific issues. In the first part, I will discuss some general principles of morphology evolution during wet chemical synthesis. In particular, the formation of anisotropic structures of high symmetry materials and the associated symmetry breaking mechanisms will be discussed. Specific examples include the growth of ultrathin single crystalline Au nanowires and the formation of plate-shaped structures. I will present some of the newer results on the intriguing structure and properties of the ultrathin metal nanowires.
In the second part, I will discuss a general method for the synthesis of nanoporous materials and discuss some of their applications. Some unexpected and interesting results on the stability of these nanoporous systems will be presented.
In the third part, I will discuss about the role of heterogeneous nucleation for controlled synthesis of nanoscale hybrids for a variety of applications including catalysis and photovoltaic applications.
The key tool that we use to delineate the mechanisms is electron microscopy.  The overall emphasis will be on illustrating general principles that we have been able to extract based on our research over the past few years and also some thoughts on future directions, applications and possible collaborations.

Woensdag 15 juni 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Ab initio and atomistic simulations of steel-polymer interface
Spreker: Samira Dabaghmanesh, PLASMANT & CMT, University of Antwerp

Abstract:
The first part of my talk is about generating a high accuracy force field and atomistic model for steel-polymer interactions. In order to generate a force field potential for our system first, we performed a large number of ab initio calculations on relevant geometries and crystal energies within the Density Functional Theory (DFT). Then the potential parameters are fitted to these DFT data, using a one-parameter search technique. In the second part of my talk, I focus on the adsorption of small fragments of polymer on steel surface and discuss the role of Van der Waals interactions in the adsorption of molecules on top of steel surface.​

Vrijdag 10 juni 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: The investigation of the stability of defects in hydrided/dehydrided nanocrystalline Palladium films using in-situ HRTEM techniques
Spreker: Gunnar Lumbeeck, EMAT, University of Antwerp

Abstract:
Palladium (Pd) has been known as an enabling material for future hydrogen technology such as hydrogen purification and sensing applications. However, the mechanical stability and response to hydrogen pressure of Pd thin films is still insufficiently understood. It is already reported that in nanocrystalline (nc) Pd films, intrinsic or extrinsic stacking faults (SFs), Shockley partial loops and 9R phase are formed after hydriding to β-phase indicating a clear effect of hydrogen on the behaviour of high stacking fault energy (SFE) of Pd.
The stability of glissile intrinsic SF loops in nc Pd films after dehydriding was also attributed to the presence of large internal stress heterogeneities typical of nc materials. In the present research, the effect of these defects on mechanical properties and corresponding nanoplasticity mechanisms is investigated. In-situ HRTEM nanomechanical testing was performed on the Pd film containing SF loops and 9R phase (hydrided to β phase). The results shows the shrinkage of 9R phase, loss of coherency of twin boundaries, formation of multiple SFs and a change in the width of the SFs and faulted loops.
Moreover, the stability of these defects was also investigated using in-situ HRTEM heating measurements. The critical temperature for removing these unstable SFs in the hydrided Pd film was also determined. 

Donderdag 9 juni 2016, 14.00 u., Lokaal U.244 (Campus Groenenborger)

Voordracht georganiseerd door CMT

Onderwerp: Enhanced electron-hole superfluidity in graphene systems
Spreker: Dr. Mohammad Zarenia, CMT, University of Antwerp

Abstract:
Superfluidity of spatially separated electrons and holes was predicted nearly half of century ago. Despite long standing theoretical predictions and considerable experimental efforts such electron-hole superfluidity in double layered systems has not yet been observed in zero magnetic field. In my talk, I will first discuss on two coupled electron-hole sheets of few-layer graphene as a new nanostructure to observe superfluidity at enhanced densities and enhanced transition temperatures, i.e. about 40K in double trilayer and quad-layer graphene. Next, I will present our recent results for two coupled electron-hole armchair graphene nanoribbons where we predicted an enhanced superfluidity thanks to the strong coulomb interaction brought by the quantum confinement in quasi-one-dimensional graphene nanoribbons.

Maandag 6 juni 2016, 16.00 u., Lokaal U.203 (Campus Groenenborger)

Voordracht georganiseerd door EDF

Onderwerp: High-energy QCD and rapidity evolution of Wilson lines
Spreker: Prof. Ian Balitsky, Thomas Jefferson Laboratory/Old Dominion University

Abstract:
I review the approach to the high-energy behaviour of QCD amplitudes based on the rapidity evolution of Wilson-line operators. After a brief introduction to BFKL physics, in the first part of the talk I discuss the BK evolution of color dipoles in high-energy QCD. In the second part, I present the evolution equation for the gluon transverse-momentum dependent distributions which interpolates between the linear DGLAP and Sudakov limit at moderate xB and the non-linear BK equation at small xB.

Vrijdag 3 juni 2016, 11.30 u., lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Advanced characterization of the growth mechanism of metallic nanoparticles by 2D and 3D electron microscopy
Spreker: Naomi Winckelmans,, EMAT, University of Antwerp

Abstract:
Due to the unique optical properties of gold bipyramids, these nanostructures are promising for a variety of applications such as surface-enhanced Raman scattering, plasmonics and biosensing. The size and shape of these nano-objects plays an essential role in their optical response. Therefore, it is important to achieve precise control of all the parameters involved in the synthesis of such objects. Within this respect, it is of key importance to understand the growth mechanism from seed to bipyramid. Here, the growth is investigated using two dimensional and three dimensional electron microscopy.
In a second study, supermagnetic ironoxide octapods are investigated. Such structures have potential as T2 negative contrast agents for magnetic resonance imaging. The oxidation type of iron has an important influence on the magnetization. Here, we investigate the oxidation by using EELS in three dimensions for a full characterization. Furthermore, the size of the octapods as well as their tip length plays an essential role in their transverse relaxivity value. During synthesis, different parameters were tested, which influences the growth of the tips. This tip growth is investigated using high resolution HAADF STEM and HAADF STEM tomography.

Vrijdag 27 mei 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Bayesian model-order selection in electron microscopy to detect atomic columns in noisy images
Spreker: Jarmo Fatermans, EMAT, University of Antwerp

Abstract:
A common way to measure unknown structure parameters from electron microscopy data is to perform a best fit approach to determine the optimal parameters of a physics-based model describing the underlying data. A danger in using this statistical parameter estimation based approach is that of over-or underfitting, in which the choice for a specific model excludes the determination of an unexpected structure, which cannot be described by the chosen model. An alternative is the use of the maximum entropy method which makes use of some prior knowledge. However, it is questionable whether this method is able to extract precise structure information from the data. Therefore, a generalized form of model fitting is proposed in this presentation, which is appropriate when the exact number of parameters to be optimized is unknown.
It will be shown how such a generalized form of model fitting, which is known as Bayesian model-order selection, can be applied to scanning transmission electron microscopy images to find the number of atomic columns there is most evidence for in the image. It is especially useful to quantitatively detect the presence of light atomic columns in the proximity of heavier columns or when the signal-to-noise ratio is very low. The theoretical framework will be outlined and demonstrated using simulated and experimental images.

Vrijdag 20mei 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Optimization of Automated Crystal Orientation and Phase Mapping in TEM Applied to Ni-Ti All Round Shape Memory Alloy
Spreker: Xiayang Yao, EMAT, University of Antwerp

Abstract:
A new technique called automated crystal orientation and phase mapping in TEM is applied to the investigation of Ni-Ti shape memory alloys which will be used for the implantation of artificial sphincter due to the presence of all round shape memory effect.
Since it is the first use of such technique to the investigation of Ni-Ti alloys, and due to the weak contrast in the identification of matrix and precipitate phases, an optimization process was carried out to adjust all parameters in this technique to obtain more reliable data.
Results indicate that precession is not necessary in detecting precipitates and tilting condition also plays important role. Parameters in template generation are optimized as well.

Vrijdag 13 mei 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : How to image nanoparticles and their functional coatings by advanced TEM
Spreker: Nathalie Claes, EMAT, University of Antwerp

Abstract:
The properties of nanoparticles can be drastically modified by applying functional coatings. An increase in the flowability, encouragement of self-assembly and counteraction of oxidation are only a few examples of the functionalities that can be tuned. To understand the structure-property connection, it is important to visualize and characterize the coating at the nanometer scale and below.
Image contrast in electron microscopy is produced through electron scattering by the atomic nuclei of the sample. For organic polymers the scattering is weak, which hampers the investigation of encapsulated nanoparticles and requires dedicated sample preparation. By applying optimized staining procedures, the Janus character for gold nanoparticles coated by two different polymers was demonstrated through the use of electron tomography.
Also the use of energy dispersive X-ray spectroscopy (EDX) is of great potential.  For example, the surface modification of plasma treated alumina particles was studied by 3D EDX. Special attention was given to the chemical composition and the homogeneity of coatings.
Finally, the use of exit wave reconstruction to investigate the effect of surface ligands on the morphology of nanoparticles will be discussed.

Vrijdag 29 april 2016, 16.00 u., Lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door VISIE

Onderwerp: Diffusion MRI fiber tractography of the brain
Spreker: Ben Jeurissen, Vision Lab, University of Antwerp 

Abstract:
In the last decade, fiber tracking has become the method of choice to investigate quantitative MRI parameters in specific bundles of white matter for a wide range of diseases. For the neurosurgeons, fiber tractography is quickly becoming an invaluable tool for the planning of surgery, allowing for visualization and localization of important white matter pathways before and even during surgery. Fiber tracking has also claimed a central role in the field of ‘connectomics’, a technique that builds and studies comprehensive maps of the complex network of connections within the brain, and to which significant resources are allocated worldwide.
Despite its unique abilities and exciting applications, diffusion MRI fiber tracking is not without controversy. On one hand are the enthusiasts that are eager to apply this appealing technique to better understand the human brain. On the other hand, there are the so-called cynics that feel that fiber tracking is frequently abused or that fiber tractography based results are often misinterpreted. In light of this ongoing controversy, this paper will provide an overview of the key concepts of tractography, the technical considerations at play, the different types of tractography, as well as the common misconceptions and mistakes that surround it.

Vrijdag 29 april 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Artifact reduction methods in electron tomography by means of inpainting
Spreker: Kadir Sentosun, EMAT, University of Antwerp

Abstract:
Characterization of core-shell type nanoparticles in 3 dimensions by transmission electron microscopy (TEM) can be very challenging. Especially, when low and high atomic number elements co-exist within the same nanostructure. In such cases, artifacts in the 3D reconstruction are often present. Both the experimental parameters of the electron tomography experiments and the reconstruction procedures of the acquired tomography series have to be optimized in order to obtain reliable reconstruction. The flexibility of the modern TEMs was used for obtaining tomography tilt series from mesoporous SiO2- Au nanoparticles consisting of both low and high atomic number elements by simultaneously utilizing the ADF and HAADF detectors. Due to the artifacts that appear in the reconstruction of the ADF-STEM tilt series; the reconstruction procedure was further optimized. First, we removed the complete Au nanoparticle from the ADF-STEM projection images. Next, a technique known as inpainting was applied which replaces the removed information by a continuation of the texture of the surrounding area. In this manner, we were able to characterize the structure of the mesoporous SiO2- Au nanoparticles.
Reconstruction algorithms such as filtered back projection and simultaneous iterative reconstruction technique suffer from artifacts caused by the missing wedge. This missing information in a tilt series is mostly inevitable due to the shadowing from the single tilt holder. In the second part of the talk, the inpainting approach is used for retrieving the missing wedge information. During the talk, compensating for missing wedge information by means of sinogram inpainting will be discussed. First, the concept of sinogram will be explained and subsequently how it can be used to reduce the missing wedge artifacts. The results of the procedures will be discussed both on calculated phantoms and experimental data.

Vrijdag 22 april 2016, 16.00 uL, Lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door VISIE

Onderwerp: X-ray physics matters
Spreker: Wim Van Aarle, Vision Lab, University of Antwerp

Abstract:
Tomographic reconstruction relies on a mathematical model in which the physical properties of X-ray generation, interaction and detection are very much simplified.  This leads to an elegant - and efficiently solvable - model that most of the times results in accurate reconstructions.  In some extreme cases, however, the link between this simplified model and the reality of the scan can break down and all subsequent attempts at reconstruction are doomed to fail.  In this presentation, a short overview is given of the physical properties of X-rays that are typically ignored, and a case is presented demonstrating the resulting reconstruction artefacts.  Some attempts at correcting these model inconsistencies by exploiting prior knowledge are also presented.

Vrijdag 22 april 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Detecting and positioning light elements and counting the number of atoms in HR TEM and STEM: possibilities and limitations of both imaging methods
Spreker: Julie Gonnissen, EMAT, University of Antwerp

Abstract:
In the first part of this talk, we report an innovative method to quantitatively compare the precision for atom-counting from both TEM and STEM images. So far High Resolution Scanning Transmission Electron Microscopy (HR STEM) has been shown to be an appropriate method to count the number of atoms in a projected atomic column. Recently however, it has been shown that one HR TEM image using negative spherical aberration imaging suffices to count atoms as well. In this work, a 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 atomic column can be estimated. The capabilities of both imaging techniques, HR STEM and HR TEM, are investigated and compared in terms of atom-counting reliability.
In a second part of this talk, this quantitative approach is also used to investigate the optimal experiment design for both the detection and positioning of light elements from HR STEM images. The principles of detection theory are then used to quantify the probability of error for the detection of light atoms from HR STEM images. To determine the optimal experiment design for positioning light atoms, use is made of the so-called Cramèr-Rao Lower Bound (CRLB). It is investigated if a single optimal design can be found for both the detection and positioning problem of light atoms.

Vrijdag 15 april 2016, 16.00 u., Lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door VISIE

Onderwerp : Direct estimation of 3D atom positions of simulated Au nanoparticles in HAADF STEM
Spreker: Jan De Beenhouwer, Vision Lab, University of Antwerp

Abstract:
Before a reconstruction algorithm can be applied to compute a 3D image from a set of 2D projections in electron tomography, alignment must first be carried out to correct for geometrical changes that may have occurred during the acquisition. However, in many cases, even a perfectly aligned set of projections will not allow to fully resolve the atomic structure in 3D due to both noise and the limited amount of projection data that is available. In this talk, a new reconstruction method will be presented that compensates for the lack of information, by using a 3D atomic model as prior knowledge within the reconstruction and by a gradually refined estimation of the atomic positions both in image and projection space.

Vrijdag 15 april 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Advanced characterization of colloidal semiconductor heteronanocrystals by 2D and 3D electron microscopy
Spreker: Eva Bladt, EMAT, University of Antwerp

Abstract:
Due to the specific size-dependent photoluminescence spectra of semiconductor nanocrystals, their use is promising as building blocks for new electronic and optical nanodevices such as light-emitting diodes, solar cells, lasers and biological sensors. The growth of a shell of a higher band gap semiconductor around a semiconductor core has proven to increase the photoluminescence quantum yield and the photochemical stability of the nanocrystal.
In order to design these hetero-nanocrystals with tailored properties for specific applications, a high level of control over their synthesis is of key importance. Therefore, both the three dimensional structure as the position of the core inside the shell need to be characterized. Here, we investigate the cation exchange reaction from CdSe/CdS dot core/rod shell nanorods to CuInSe2 / CuInS2 dot core/rod shell nanorods and a CdSe core/CdS giant shell heteronanocrystal by the use of two dimensional and three dimensional electron microscopy techniques.

Maandag 11 april 2016, 16.00 u., lokaal N0.08 (Campus Drie Eiken)

Voordracht georganiseerd door TQC

Onderwerp: Dynamics of correlations in long-range quantum systems following a quantum quench
Spreker: Lorenzo Cevolani, Université Paris-Sud 11, Palaiseau, France

Abstract:
We study the out-of-equilibrium dynamics of quantum systems with long-range interactions[1]. We study how (and how fast) correlations can spread in a quantum system abruptly driven out of equilibrium by a quantum quench. This protocol can be experimentally realized with ultra-cold atoms, which allow to address fundamental questions concerning the quasi-locality principle in isolated quantum systems with both short- [2,3] and long-range interactions [4]. We focus on two different models describing, respectively, lattice bosons, and spins. Our study is based on a combined approach, based on one hand on accurate many-body numerical calculations [5,6], and on the other hand on a quasi-particle microscopic theory [7]. We find that, for sufficiently fast decaying interaction potential, the long-range version of the Lieb-Robinson theorem [8], is never attained and the propagation is ballistic, as predicted from the standard short-range quasi-particle point of view [9]. When the interactions are really long range the scenario is completely different in the two cases. In the bosonic system the locality is preserved and a ballistic propagation is still present while in the spin system an instantaneous propagation of correlations completely destroys locality [10]. Using the microscopic point of view we can provide a justification of all the different regimes studied in the two model and we can understand how locality is protected in the bosonic model.
[1] L. Cevolani, G. Carleo, and L. Sanchez-Palencia, arXiv:1503.01786 (2015). 
[2] M. Cheneau et al., Nature 481, 484 (2012). 
[3] T. Langen et al., Nat. Phys. 9, 640 (2013). 
[4] P. Richerme et al., Nature 511, 198 (2014). 
[5] G. Carleo, F. Becca, M. Schiro, and M. Fabrizio, (Nature) Sci. Rep. 2, 243 (2012). 
[6] G. Carleo, F. Becca, L. Sanchez-Palencia, S. Sorella, and M. Fabrizio, Phys. Rev. A 89, 031602(R).
[7] Stefan S. Natu and Erich J. Mueller Phys. Rev. A 87, 053607 (2013).
[8] M. B. Hastings and T. Koma, Commun. Math. Phys. 265 , 781 (2006). 
[9] Pasquale Calabrese and John Cardy Phys. Rev. Lett. 96, 136801 (2006). 
[10] P. Hauke and L. Tagliacozzo Phys. Rev. Lett. 111, 207202 (2013).

Vrijdag 8 april 2016, 16.00 u., Lokaal N0.08 (Campus Drie Eiken)

Voordracht georganiseerd door EGM

Onderwerp: Efforts Towards Total Control of Single-Wall Carbon Nanotube Structure and Endohedral Environment
Spreke: Jeffrey A. Fagan, National Institute of Standards and Technology (NIST), Gaithersburg, USA

Abstract:
Single-wall carbon nanotube (SWCNT) populations from synthesis typically contain substantial distributions of SWCNT species (chiralities), enantiomers (i.e. left- vs. right-handed), lengths, modifications, and defectiveness. A driving challenge for the field has been in purifying these initially polydisperse materials into homogenous populations by one or more of these factors. Multiple strategies for chirality separation in particular, have been developed over the last decade including density gradient ultracentrifugation, chromatographic methods, selective dispersion strategies, and recently aqueous two-phase extraction (ATPE).
In my lab we are demonstrating the power of the ATPE method to resolve SWCNTs by the metallic/semiconducting nature, diameter and even their enantiomeric handedness. This technique can be readily applied to process bench scale quantities of well specified SWCNTs by hand, with implementation of automation strategies promising even further improvement.
Separately we are also developing methods for controlling modifying parameters to SWCNT properties. Best known of these are the property differences between closed-ended (empty) and open-ended (solvent-filled) nanotubes, in which solvent ingestion, typically water, alters and generally negatively, the optical, electronic and density properties of the nanotubes. Recently we determined that simple exposure of most SWCNT soots to liquid phase organic compounds would result in their ingestion and long term capture, even after sonication based dispersion in aqueous media. This ingestion can be utilized to greatly enhance many of the optical properties of the nanotubes, as well as to provide leverage for further development of other separation technologies and applications.
This talk will describe progress in the development of these methods, including use of multistage ATPE to separate individual species of metallic and semiconducting nanotubes < 1.8 nm thick and the filling of open ended nanotubes to control their endohedral environment post dispersion, towards the goal of generating fully homogenous SWCNT populations.

Vrijdag 25 maart 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Quantitative STEM: new developments and applications for analysing complex nanostructures
Spreker: Karel van den Bos, EMAT, University of Antwerp

Abstract:
A popular material’s characterisation technique is scanning transmission electron microscopy (STEM) because of its ability to create images revealing structural information at the atomic level. Statistical parameter estimation theory offers an excellent tool to quantitatively extract unknown structure parameters from these images. A new program, StatSTEM, is developed in which this framework is implemented for public use. A live demonstration will be given on how this program can help quantifying your STEM images.
In high angle annular dark field (HAADF) STEM, the extracted parameters can be linked to the number of atoms in each atomic column. The combination of counting results from different viewing directions has proven to be a powerful technique to retrieve the 3D structure of homogeneous materials. In order to extend the atom counting technique to heterogeneous materials, a new atomic lensing model will be presented facilitating both atom counting and 3D compositional determination in such materials.
Finally, the precision with which atomic column positions can be measured from a new imaging technique called imaging STEM (ISTEM) will be explored. It will be shown that both light and heavy atomic column positions can be estimated with a precision in the picometer range.

Vrijdag 18 maart 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: TEM characterization of new triple perovskites
Spreker : Robert Paria Sena, EMAT, University of Antwerp

Abstract:
Recently, it was proposed that La3Ni2SbO9 is a relaxor ferromagnet. A relaxor ferromagnet would be, in analogy with relaxor ferroelectrics, magnetically ordered in small domains. This compound is a triple perovskite with order between Ni and Sb.
In a search for other so called relaxor ferromagnets, we have investigated several other triple perovskites, CaLa2Ni2WO9, La3Ni2TaO9, La3Ni2NbO9, La3Ni2Nb0.5Sb0.5O9 and Sr3Fe2TeO9. The compounds show a variety of magnetic properties, and therefore a difference in their structure could be expected. The structures were determined using different TEM techniques as will be presented in the lecture.

Maandag 14 maart 2016,16.30 u., Lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door TQC

Onderwerp: Thermal blurring of a pair-condensed Fermi gas
Spreker: Dr. Hadrien Kurkjian (Laboratoire Kastler-Brossel, ENS, Paris)

Abstract:
It is generally assumed that a condensate of paired fermions at equilibrium is characterized by a macroscopic wavefunction with a well-defined, immutable phase. In reality, all systems have a finite size and are prepared at non-zero temperature; the condensate then has a finite coherence time, even when the system is isolated in its evolution and the particle number $N$ is fixed. The loss of phase memory is due to interactions of the condensate with the excited modes that constitute a dephasing environment. This fundamental effect, crucial for applications using the condensate of pairs macroscopic coherence, was scarcely studied. In my presentation, I will link the coherence time to the condensate phase dynamics, and show with a microscopic theory that the time derivative of the condensate phase operator $hat{ heta}_0$ is proportional to a chemical potential operator $hat{mu}$ that I will construct including both the pair-breaking and pair-motion excitation branches. In a single realization of energy $E$, $hat{ heta}_0$ evolves at long times as $-2mu_{ m mc}(E)t/hbar$ where $mu_{ m mc}(E)$ is the microcanonical chemical potential; energy fluctuations from one realization to the other then lead to a ballistic spreading of the phase and to a Gaussian decay of the temporal coherence function with a characteristic time $propto N^{1/2}$. In the absence of energy fluctuations, the coherence time scales as $N$ due to the diffusive motion of $hat{ heta}_0$. I will also propose a method to measure the coherence time with ultracold atoms, which we predict to be tens of milliseconds for the canonical ensemble unitary Fermi gas.

Vrijdag 11 maart 2016, 16.00 u., Lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door VISIE

Onderwerp: Super-resolution estimation of quantitative MRI parameters
Spreker: Gwendolyn Van Steenkiste, Vision Lab, University of Antwerp

Abstract:
Quantitative T1 mapping is a magnetic resonance imaging technique in which the spin-lattice relaxation time of tissues is estimated. Even though T1 mapping has a broad range of potential applications, it is not routinely used in clinical practice as accurate and precise high resolution T1 mapping requires unfeasibly long acquisition times. In this presentation we propose to improve the trade-off between the acquisition time, SNR and spatial resolution of T1 mapping by estimating a high resolution T1 map using super-resolution. The proposed  technique enables high resolution 1x1x1mm3 whole-brain T1 mapping, which was previously unfeasible due to scanner limitations.

Woensdag 9 maart 2016, 16.00 u., Lokaal U.203 (Campus Groenenborger)

Voordracht georganiseerd door EDF

Onderwerp : Factorization framework for semi-inclusive processes: recent results
Spreker: Igor Anikin, BLTP JINR, Dubna

Abstract:
We discuss the Drell-Yan process and the direct photon production in two hadron collisions where one hadron is transversely polarized. We present the hadron tensors for these processes and study the effects which lead to the soft breaking of the factorization (or the universality breaking) through the QED and QCD gauge invariance. The special role is played by the contour gauge for gluon fields. The latter allows us to find a new type of contributions to the hadron tensor. We show that the new ("non-standard") terms do contribute to the hadron tensors exactly as the "standard" terms known previously. 

Vrijdag 4 maart 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: Advanced TEM characterization of 2G HTS tapes​
Spreker: Alexander Meledin, EMAT, University of Antwerp

Abstract:
Since the discovery of high-temperature superconducting (HTS) materials great attention has been paid to their unique properties for industrial applications. However, at the moment the field is still open for improvement.
The optimization of substrates, buffer layers, growth process as well as the consequent superconducting (Y,Gd)BCO layer deposition conditions is extremely important for the production of long length (>500 meter) superconducting wires with optimized current-carrying capability. The introduction of nanoparticles into the (Y,Gd)BCO layer helps to increase the critical current performance in high magnetic fields. A wide range of (S)TEM techniques has been applied to investigate various substrates, buffers and superconducting layers. Interfaces, diffusion processes, nanoinclusions were studied. Electron microscopy is a key characterisation technique to the guide the preparation processes and to obtained the desired micro- and nano-structures.

Vrijdag 29 januari 2016, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: A twist on Phase Contrast in (S)TEM
Spreker: Laura Clark, EMAT, University of Antwerp

Abstract:
Transmission electron microscopy enables the highest resolution imaging possible, for many materials, down to the sub-Angstrom scale. Biological materials however, suffer acutely from beam damage, which limits the dose permitted before the sample is damaged. The requirement then, is to optimise image resolution and contrast at the lowest possible dose. This leads to a low signal to noise ratio, subsequently limiting the achievable resolution.

In recent years, phase plates have been introduced to the TEM community, to enable in-focus contrast of weak-phase objects (biological materials, with low-Z, scatter the beam electrons much less than other types of sample). There is a veritable zoo of TEM phase plate designs now, with different structures leading to different intensity contrasts in the detection plane - each with different local SNR behaviour.

Here, we systematically study the noise behaviour of each option, to find the phase plate enabling the most optimal imaging of electron-dose-sensitive samples. A particular form of spiral phase plate is found to be an especially promising candidate, with minimisation of the background intensity, and strong contrast in the detection plane.

Vrijdag 22 januari 2016 , 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp: FAULTS, a powerful tool for analysing X-ray, Neutron and Electron diffraction patterns of materials with planar defects
Spreker : Marine Reynaud, EMAT, University of Antwerp

Abstract:
Layered systems include a large number of mineral families and synthetic compounds of great technological importance with, for instance, applications in the field of electrochemical energy storage. Their physical-chemical properties being directly related to their structural features, the microstructural characterization of these materials is of high importance and includes the determination of different kinds of defects, their amount and their location.
So far, a widely used tool to interpret the diffraction data of one-dimensionally disordered systems was the program DIFFaX [1], which permits to simulate X-ray (XRD) and Neutron (NPD) powder diffraction patterns as well as Electron (ED) diffraction patterns. In order to overtake the limitations of simple simulations, we have developed the FAULTS program [2-3], based on the DIFFaX code, which enables to refine experimental XRD and NPD powder patterns of crystal systems with any type of planar defects, such as twins and stacking faults. An improved version of this program is now available within the FullProf suite of programs [4] or as an independent program [5].
In FAULTS, the structure is described in terms of layers of atoms which are interconnected via stacking operations that occur with a certain probability. Among the major features of FAULTS is the implementation of a more adequate isotropic size broadening treatment which takes into account the Gaussian (HG) and Lorentzian (HL) contributions to the FWHM in addition to the consideration of a finite number of layers per crystallite. This treatment allows a successful description of the separate contributions to line broadening of instrumental features, the finite crystallite size and planar defects.<
The presentation will show the structure and operation of the program FAULTS, and some examples will be given.

Woensdag 20 januari 2016, 16.00 u., Lokaal N1.08 (Campus Drie Eiken)

Voordracht georganiseerd door TQC

Onderwerp: A First-order Phase Transition to Metallic Hydrogen<
Spreker: Prof. Isaac Silvera, Harvard University

Abstract:
The insulator-metal transition in hydrogen is one of the most outstanding problems in condensed matter physics. The high-pressure metallic phase is now predicted to be liquid atomic from T=0 K to very high temperatures. We have conducted measurements of optical properties of hot dense hydrogen created in a Diamond Anvil Cell in the pressure region 1.1 to 1.7 Mbar and temperatures up to 2200 K. We present evidence supportive of a first-order phase transition accompanied by changes in transmittance and reflectance characteristic of a metal. The phase line of this transition has a negative slope in agreement with theories of the so-called plasma phase transition from liquid molecular to atomic liquid hydrogen.

Dinsdag 19 januari 2016, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Electron-phonon and spin-orbit coupling in MoS2
Spreker: Prof. Ludger Wirtz, Physics and Materials Science Research Unit, University of Luxembourg

Abstract:
Recently, many research groups have been performing detailed spectroscopy experiments on single and multi-layer transition metal dichalcogenides (TMDs). A precise quantitative understanding of the electronic structure and the vibrational modes is necessary for a correct interpretation of the experiments. I will give an overview from the theoretical perspective. [1]
We summarize the ample literature on quasi-particle band-structures of semiconducting TMDs where spin-orbit coupling plays an important role. Calculations of optical absorption spectra on the level of the Bethe-Salpeter equation display a variety of excitonic effects (enhanced by the 2D nature of the system and influenced by the substrate): a series of bound excitons as well as higher-lying excitons in resonance with the continuum of electron-hole pair transitions. Including electron-phonon coupling in order to simulate spectra at finite temperature, we find that bound excitons (A and B peaks) and resonant excitons (C peak) exhibit different behavior with temperature, displaying different non-radiative lifetimes. We conclude that the inhomogeneous broadening of the absorption spectra is mainly due to electron-phonon scattering mechanisms. Our calculations explain the shortcomings of previous (zero-temperature) theoretical spectra and match well with the experimental spectra acquired at room temperature.
Finally, I will give an outlook on the calculation of resonant Raman spectra of single and multi-layer MoS2 via a finite-difference calculation of the polarizability of the system including excitonic effects on the level of the Bethe-Salpeter equation.
[1] A. Molina-Sánchez, K. Hummer, L. Wirtz, Surface Science Reports 70, 554 (2015).