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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

First principles calculations for vacancies and antisites in PbSe and PbTe: bulk and nanowire.

The energetic stability and the electronic properties of vacancies (VX) and antisites (XY) in PbSe and PbTe are investigated. PbSe and PbTe are narrow band gap semiconductors and have the potential to be used in infrared detectors, laser, and diodes. They are also of special interest for thermoelectric devices (TE). The calculations are based in the Density Functional Theory (DFT) and the General Gradient Approximation (GGA) for the exchange-correlation term, as implemented in the VASP code. The core and valence electrons are described by the Projected Augmented Wave (PAW) and the Plane Wave (PW) methods, respectively. The defects are studied in the bulk and nanowire (NW) system. Our results show that intrinsec defects (vacancies and antisites) in PbTe have lower formation energies in the NW as compared to the bulk and present a trend in migrate to the surface of the NW. For the PbSe we obtain similar results when compare the formation energy for the bulk and NW. However, the Pb vacancy and the antisites are more stable in the core of the NW. The intrinsec defects are shallow defects for the bulk system. For both PbSe and PbTe VPb is a shallow acceptor defect and VSe and VT e are shallow donor defects for the PbSe and PbTe, respectively. Similar electronic properties are observed for the antisites. For the Pb in the anion site we obtain an n-type semiconductor for both PbSe and PbTe, SeP b is a p-type for the PbSe, and T eP b is a n-type for PbTe. Due the quantum con¯nement effects present in the NW (the band gap open), these defects have different electronic properties for the NW as compared to the bulk. Now these defects give rise to electronic levels in the band gap of the PbTe NW and the VT e present a metallic character. For the PbSe NW a p-type and a n-type semiconductor is obtained for the VP b and P bSe, respectively. On the other hand, deep electronic levels are present in the band gap for the VSe and SePb. These results show that due an enhanced in the electronic density of states (DOS) near the Fermi energy, the defective PbSe and PbTe are candidates for efficient TE devices.

Einfluss des Aktin-Zytoskeletts und intrazellulärer Signalkaskaden auf die postsynaptische Lokalisation von Gephyrin

Neuronen haben für die Informationsübertragung untereinander spezielle Strukturen entwickelt, welche als Synapsen bezeichnet werden. Um eine schnelle und präzise synaptische Signalübertragung zu gewährleisten, ist eine hohe Konzentration von Neurotransmitter-regulierten Ionenkanälen in der postsynaptischen Plasmamembran notwendig. Die spezifische Verankerung der Rezeptoren wird durch intrazelluläre Proteine der Postsynapse vermittelt. Das periphere Membranprotein Gephyrin spielt eine essentielle Rolle in der synaptischen Lokalisation von Glyzin- und GABAA-Rezeptoren an inhibitorischen Synapsen. Um das postsynaptische Netzwerk zu stabilisieren, ist eine Interaktion von Gephyrin mit Proteinen der Mikrofilamente und der Mikrotubuli nötig. In der vorliegenden Arbeit sollte analysiert werden, wie Gephyrin mit dem Aktin-Zytoskelett interagiert, und ob die Größe und Stabilität neuronaler Gephyrincluster durch das Aktin-Zytoskelett reguliert wird. Dies wurde mittels Expression von GFP-Gephyrin-Konstrukten in HEK293T-Zellen und Aktin-depolymerisierende Alkaloidbehandlung von hippokampalen Primärkulturzellen untersucht. Der Einfluss unterschiedlicher Signalkaskaden auf die Lokalisation und Funktionalität von GABAA- oder Glyzin-Rezeptoren wurde bereits intensiv untersucht, jedoch für Rezeptor-assoziierte Proteine wie Gephyrin existierten nur wenig relevante Daten. Ein weiteres Ziel war daher, durch pharmakologische Beeinflussung von Schlüsselenzymen in hippokampalen Primärkulturen jene Signalwege zu identifizieren, die am Transport von Gephyrin, seiner Stabilisierung im postsynaptischen Netzwerk und seinem Abbau beteiligt sind. Im Verlauf der Arbeit konnte belegt werden, dass das Aktin-regulierende Phosphoprotein ena/VASP als Adapter die Interaktion von Gephyrin mit F-Aktin vermittelt, und dass diese Bindung ausreicht, um Gephyrin an das Aktin-Zytoskelett zu rekrutieren. Entgegen früheren Veröffentlichungen konnte die Bindung von ena/VASP im Bereich der sog. Linkerregion von Gephyrin nachgewiesen werden. Aktin-depolymerisierende Alkaloidbehandlungen von hippokampalen Neuronen bestätigten, das die Lokalisation von Gephyrin an sich entwickelnden inhibitorischen Kontakten von einem intakten Mikrofilamentsystem abhängig zu sein scheint. Das Aktin-Zytoskelett könnte somit eine transiente Rolle in der Ausbildung und Stabilisierung des Gephyrin-Netzwerkes in der frühen Entwicklung von inhibitorischen GABAergen Synapsen haben, während die Abhängigkeit der synaptisch-lokalisierten Gephyrincluster vom Aktin-Zytoskelett mit steigender neuronaler Differenzierung abnimmt. Zusätzlich konnte erstmals der Einfluss einzelner Signaltransduktionskaskaden auf die synaptische Lokalisation von Gephyrin nachgewiesen werden. Dabei hatte die Inhibition der Protein- Phosphatasen 1 und 2A eine Destabilisierung synaptisch-lokalisierter Gephyrincluster bei gleichzeitigem Anstieg der zytoplasmatischen Immunreaktivität zur Folge. Dies ist möglicherweise auf eine Hyperphosphorylierung wichtiger Sequenzabschnitten von Gephyrin zurückzuführen, wobei Änderungen im Phosphorylierungsstatus der Linkerregion von Gephyrin unter anderem die Assoziation mit dem Zytoskelett beeinträchtigen oder lösen könnten. Umgekehrt könnte eine Dephosphorylierung möglicherweise die Stabilität der Vernetzung erhöht. Kopräzipitationsstudien konnten zusätzlich nachweisen, dass Gephyrin, PP1 und PP2A nicht nur gemeinsam an inhibitorischen Synapsen vorliegen, sondern dass eine direkte Interaktion besteht. Dabei handelt es sich um den ersten Nachweis einer direkten Bindung von Gephyrin an Ser/Thr-Phosphatasen. Die Komplexbildung von PP1 und PP2A mit Gephyrin könnten der Regulation des Phosphorylierungsgrades dienen. Der genaue Mechanismus wird jedoch in weiteren Experimenten zu untersuchen sein.

Estudio a escala laboratorio y planta piloto, de la adsorción de NO y SO2, sobre CR2O3/AL2O3 a altas temperaturas, provenientes de fuentes fijas

Resumen: Se planificaron las experiencias con el objeto de analizar el comportamiento del catalizador en la columna metálica de mayor diámetro. Se modificaron las masas usadas para verificar la eficiencia de retención respecto de la masa. Se realizaron ciclos de adsorción, desorción y readsorción sobre una misma muestra para determinar variaciones en la eficiencia del catalizador. En otra fase, en colaboración con el Dr. V. A. Ranea y el Prof. E. E. Mola (INIFTA, UNLP), se desarrolló el estudio teórico de la adsorción de moléculas de SO2, CH4, CO2, O2 y CO sobre Cr2O3(0001) mediante Teoría del Funcional Densidad (programa VASP, Vienna Ab-initio Simulation Package), y el estudio de la cinética de la reacción entre CH4, SO2 y el O2 junto con la presencia de especies sulfito y sulfato. Este estudio permitió hallar los sitios preferenciales de adsorción de Sº y la posible competencia con SO2 experimentalmente y por cálculos teóricos. Dentro del marco de la presente línea de investigación, la Ing. Sabrina Hernández Guiance continúa realizando experiencias en el marco del proyecto conjunto con el INIFTA, las cuales forman parte del desarrollo de su tesis doctoral. Experimentalmente, se observa que la eficiencia de adsorción del catalizador respecto al SO2 es cercana al 100%. Se observa un pico de termodesorción a 1120 K. Luego, se estudió la oxidación de CH4 con SO2. Se observa que hay producción de CO2 desde temperatura inicial, seguida de un aumento significativo en la formación de CO2 hasta 330-340 K. Luego, la producción de CO2 se mantiene aproximadamente constante. Mediante el empleo de la ecuación de Arrhenius y resultados experimentales, se obtuvo la energía de activación de la reacción global, de 7 Kcal/mol. Mediante estudios teóricos, se determinó que la energía de quimisorción del SO2 sobre el Cr2O3 es de -3.09 eV para la configuración más estable, una energía de adsorción de O2 en estado disociativo de -1.567 eV, una energía para CH4 sobre O2 adsorbido previamente de -0.335 eV, y - 0.812 eV para la configuración más estable de CO2 sobre el sustrato.

Ab-Initio calculations including Van der Waals interactions: the SnS2 layered material

Tin disulfide SnS2 was recently proposed as a high efficiency solar cell precursor [1]. The aim of this work is a deep study of the structural disposition of the most important polytipes of this layered material, not only describing the electronic correlation but also the interatomic Van der Waals interactions that is present between the layers. The two recent implementations to take Van der Waals interactions into account in the VASP code are the self-consistent Dion et al. [2] functional optimized for solids by Michaelides et al [3] and the Grimme [4] dispersion correction that is applied after each autoconsistent PBE electronic calculation. In this work these two methods are compared with DFT PBE functional. The results we will presented at this Conference, demonstrates the enhancement of the geometric parameters by the use of the Van der Waals interactions in agreement with the experimental values.

Involvement of unique leucine-zipper motif of PSD-Zip45 (Homer 1c/vesl-1L) in group 1 metabotropic glutamate receptor clustering

Several scaffold proteins for neurotransmitter receptors have been identified as candidates for receptor targeting. However, the molecular mechanism underlying such receptor clustering and targeting to postsynaptic specializations remains unknown. PSD-Zip45 (also named Homer 1c/vesl-1L) consists of the NH2 terminus containing the enabled/VASP homology 1 domain and the COOH terminus containing the leucine zipper. Here, we demonstrate immunohistochemically that metabotropic glutamate receptor 1α (mGluR1α) and PSD-Zip45/Homer 1c are colocalized to synapses in the cerebellar molecular layer but not in the hippocampus. In cultured hippocampal neurons, PSD-Zip45/Homer1c and N-methyl-d-aspartate receptors are preferentially colocalized to dendritic spines. Cotransfection of mGluR1α or mGluR5 and PSD-Zip45/Homer 1c into COS-7 cells results in mGluR clustering induced by PSD-Zip45/Homer 1c. An in vitro multimerization assay shows that the extreme COOH-terminal leucine zipper is involved in self-multimerization of PSD-Zip45/Homer 1c. A clustering assay of mGluRs in COS-7 cells also reveals a critical role of this leucine-zipper motif of PSD-Zip45/Homer 1c in mGluR clustering. These results suggest that the leucine zipper of subsynaptic scaffold protein is a candidate motif involved in neurotransmitter receptor clustering at the central synapse.

Vaccinia locomotion in host cells: Evidence for the universal involvement of actin-based motility sequences ABM-1 and ABM-2

Vaccinia uses actin-based motility for virion movement in host cells, but the specific protein components have yet to be defined. A cardinal feature of Listeria and Shigella actin-based motility is the involvement of vasodilator-stimulated phosphoprotein (VASP). This essential adapter recognizes and binds to actin-based motility 1 (ABM-1) consensus sequences [(D/E)FPPPPX(D/E), X = P or T] contained in Listeria ActA and in the p90 host-cell vinculin fragment generated by Shigella infection. VASP, in turn, provides the ABM-2 sequences [XPPPPP, X = G, P, L, S, A] for binding profilin, an actin-regulatory protein that stimulates actin filament assembly. Immunolocalization using rabbit anti-VASP antibody revealed that VASP concentrates behind motile virions in HeLa cells. Profilin was also present in these actin-rich rocket tails, and microinjection of 10 μM (intracellular) ABM-2 peptide (GPPPPP)3 blocked vaccinia actin-based motility. Vinculin did not colocalize with VASP on motile virions and remained in focal adhesion contacts; however, another ABM-1-containing host protein, zyxin, was concentrated at the rear of motile virions. We also examined time-dependent changes in the location of these cytoskeletal proteins during vaccinia infection. VASP and zyxin were redistributed dramatically several hours before the formation of actin rocket tails, concentrating in the viral factories of the perinuclear cytoplasm. Our findings underscore the universal involvement of ABM-1 and ABM-2 docking sites in actin-based motility of Listeria, Shigella, and now vaccinia.

Profilin I is essential for cell survival and cell division in early mouse development

Profilins are thought to play a central role in the regulation of de novo actin assembly by preventing spontaneous actin polymerization through the binding of actin monomers, and the adding of monomeric actin to the barbed actin-filament ends. Other cellular functions of profilin in membrane trafficking and lipid based signaling are also likely. Binding of profilins to signaling molecules such as Arp2/3 complex, Mena, VASP, N-WASP, dynamin I, and others, further implicates profilin and actin as regulators of diverse motile activities. In mouse, two profilins are expressed from two distinct genes. Profilin I is expressed at high levels in all tissues and throughout development, whereas profilin II is expressed in neuronal cells. To examine the function of profilin I in vivo, we generated a null profilin I (pfn1ko) allele in mice. Homozygous pfn1ko/ko mice are not viable. Pfn1ko/ko embryos died as early as the two-cell stage, and no pfn1ko/ko blastocysts were detectable. Adult pfn1ko/wt mice show a 50% reduction in profilin I expression with no apparent impairment of cell function. However, pfn1ko/wt embryos have reduced survival during embryogenesis compared with wild type. Although weakly expressed in early embryos, profilin II cannot compensate for lack of profilin I. Our results indicate that mouse profilin I is an essential protein that has dosage-dependent effects on cell division and survival during embryogenesis.

Excitonic properties of transition metal oxide perovskites and workflow automatization of GW schemes

The Many-Body-Perturbation Theory approach is among the most successful theoretical frameworks for the study of excited state properties. It allows to describe the excitonic interactions, which play a fundamental role in the optical response of insulators and semiconductors. The first part of the thesis focuses on the study of the quasiparticle, optical and excitonic properties of \textit{bulk} Transition Metal Oxide (TMO) perovskites using a G$_0$W$_0$+Bethe Salpeter Equation (BSE) approach. A representative set of 14 compounds has been selected, including 3d, 4d and 5d perovskites. An approximation of the BSE scheme, based on an analytic diagonal expression for the inverse dielectric function, is used to compute the exciton binding energies and is carefully bench-marked against the standard BSE results. In 2019 an important breakthrough has been achieved with the synthesis of ultrathin SrTiO3 films down to the monolayer limit. This allows us to explore how the quasiparticle and optical properties of SrTiO3 evolve from the bulk to the two-dimensional limit. The electronic structure is computed with G0W0 approach: we prove that the inclusion of the off-diagonal self-energy terms is required to avoid non-physical band dispersions. The excitonic properties are investigated beyond the optical limit at finite momenta. Lastly a study of the under pressure optical response of the topological nodal line semimetal ZrSiS is presented, in conjunction with the experimental results from the group of Prof. Dr. Kuntscher of the Augsburg University. The second part of the thesis discusses the implementation of a workflow to automate G$_0$W$_0$ and BSE calculations with the VASP software. The workflow adopts a convergence scheme based on an explicit basis-extrapolation approach [J. Klimeš \textit{et al.}, Phys. Rev.B 90, 075125 (2014)] which allows to reduce the number of intermediate calculations required to reach convergence and to explicit estimate the error associated to the basis-set truncation.

Analytical and numerical study of polarons

The aim of this thesis is to introduce the polaron concept and to perform a DFT numerical calculation of a small polaron in the rutile phase of TiO2. In the first chapters, we present an analytical study of small and large polarons, based on the Holstein and Fröhlich Hamiltonians. The necessary mathematical formalism and physics fundamentals are briefly reviewed in the first chapter. In the second part of the thesis, Density Functional Theory (DFT) is introduced together with the DFT+U correction and its implementation in the Vienna Ab-Initio Simulation Package (VASP). The calculation of a small polaron in rutile is then described and discussed at a qualitative level. The polaronic solution is compared with the one of a delocalized electron. The calculation showed how the polaron creates a new energy level 0.70 eV below the conduction band. The energy level is visible both in the band structure diagram and in the density of states diagram. The electron is localized on a titanium atom, distorting the surrounding lattice. In particular, the four oxygen atoms closer to the titanium atom are displaced by 0.085 Å outwards, whereas the two further oxygen atoms by 0.023 Å. The results are compatible, at a qualitative level, with the literature. Further developments of this work may try to improve the precision of the results and to quantitatively compare them with the literature.

Theoretical models and ab initio calculations of phonons

Questa tesi intende approfondire da un punto di vista, sia teorico sia computazionale, le proprietà fondamentali dei fononi. A tal fine, sono presentati i modelli quantistici di Einstein e di Debye che permettono la derivazione analitica degli osservabili macroscopici principali di un solido, come l’energia media e la capacità termica. Ciò è possibile tramite una trattazione meccano-statistica basata sull’approssimazione armonica dei modi normali di vibrazione degli ioni reticolari. Quindi, all’inizio si mostrano brevemente i risultati principali riguardanti l’oscillatore armonico quantistico. Successivamente, si approfondiscono i temi della dispersione fononica e della densità degli stati vibrazionali per reticoli cristallini 1D e 3D. Si ottiene che la prima non può essere considerata lineare se non nel limite di alte lunghezze d’onda, e che la seconda può presentare punti di singolarità correlati alla forma della relazione di dispersione. Infine, sono state svolte alcune analisi computazionali ab initio relative alla dispersione fononica, la densità degli stati vibrazionali e la frequenza di Debye del Carbonio (diamante) tramite i programmi VASP e Phonopy, confrontando i risultati con dati sperimentali presenti in letteratura.

Transizioni di fase: da Ising alla ferroelettricità

La ferroelettricità è la proprietà di alcuni materiali solidi di presentare una polarizzazione elettrica in assenza di campo elettrico. Tutti i materiali ferroelettrici sin'ora studiati esibiscono anche proprietà piezoelettriche, ossia si deformano in maniera elastica quando sottoposti ad un campo elettrico e, viceversa, si polarizzano se soggetti a deformazioni meccaniche. Questa sensibilità a stimoli elettrici e meccanici rende questi materiali particolarmente interessanti da un punto di vista pratico: applicazioni importanti si trovano nella nanotecnologia (e.g. memory devices), nella sensoristica e nell'energy harvesting. Lo scopo dell'elaborato è fornire un'introduzione allo studio delle transizioni ferroelettriche. Inizialmente il fenomeno delle transizioni di fase viene affrontato utilizzando come riferimento il caso standard dei materiali ferromagnetici: a tal riguardo viene presentato il modello di Ising, che rappresenta il paradigma per la descrizione di fenomeni collettivi in numerosi ambiti. In seguito viene presentata la teoria fenomenologica di Landau, dove si interpreta il fenomeno sulla base delle simmetrie del sistema, utilizzando un approccio di campo medio. Successivamente viene introdotto il tema centrale dell'elaborato, ossia le transizioni ferroelettriche, analizzando similitudini e differenze dal caso ferromagnetico; in particolare si presenta un' applicazione della teoria fenomenologica di Landau-Devonshire allo studio della transizione ferroelettrica del BaTiO3, un cristallo del gruppo delle perovskiti. L'ultima parte dell'elaborato ha lo scopo di introdurre un approccio più moderno allo stesso fenomeno, che utilizza la teoria quantistica del funzionale densità (DFT): utilizzando il pacchetto software VASP viene esposto un semplice calcolo a primi principi dell'energia del sistema in funzione degli spostamenti atomici, mettendone in luce la centralità nella transizione in esame.