Departamento de Física

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El Dpto. de Física anuncia la disertación del Dr. Flavio Pardo quien se referirá al desarrollo de un sistema de OCT (Optical Coherence Tomography)  1000 veces más chico que los sistemas actuales usando tecnologías de microfabricación en silicio y sílice, trabajo que se desarrolla dentro del área de Silica Lightwave Models en Nokia Bell Labs, Murray Hill, NJ, USA. La presentación titulada "Micro-Máquinas y Micro-Circuitos Ópticos para imágenes médicas no invasivas" tendrá lugar en la sala de conferencias de esa Unidad Académica (Ex 23H, de Av. Alem 1253)

HOY lunes 7 de Octubre de a las 18hs.

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Aula 23H. Dpto. Física-UNS. Viernes 26/10. 17:30hs.

Dr. Gábor Vári
Department of Applied
and Environmental Chemistry.
University of Szeged
Szeged, Hungary

Interaction of metals with the hexagonal boron nitride monolayer studied on Rh(111)
Two dimensional monolayers (MLs) of hexagonal boron nitride (h-BN) are promising insulator components for nanoelectronics. The h-BN monolayer has a similar structure and lattice constants as those of graphene. On Rh(111) surface, monolayer h-BN forms a periodically corrugated surface structure, called “nanomesh”. This phenomenon allows its application as a nanotemplate.
Here we report on the growth of gold and rhodium on the h-BN/Rh(111) surface and on subsequent thermal effects studied by STM, XPS and LEIS. The latter technique reveals the elementary composition of the outermost atomic layer. The h-BN monolayer was formed on Rh(111) by the decomposition of borazine (BAz) at high temperatures (1000 K). Gold forms 1-2 atomic layer thick nearly 2D nanoparticles, when it is evaporated in small amounts (~0.15 ML) on the nanomesh at 300 K. At higher coverages, the growth is strongly 3D. The gold peak was observed at a rather low position (83.7 eV), indicating significant electronic interaction either with h-BN. Indeed, previous density function theory (DFT) calculations indicated an electron transfer from boron nitride to gold. The intercalation of gold is the dominant process upon stepwise thermal annealing to 1050 K, but agglomeration and evaporation also occur to a limited extent. Interestingly, though gold and rhodium form a surface alloy after intercalation, the presence of ~0.15-0.50 ML of Au below the h-BN layer does not significantly influence the nanomesh structure. At higher gold doses a partial or full flattening of the nanomesh was observed. Surface alloying of gold and rhodium and the interaction of these metals with the h-BN layer are addressed with DFT calculations as well.
We also investigated the growth of h-BN on Au-Rh alloyed surfaces varying the gold content until 4 ML. In these measurements gold was evaporated on Rh(111) at 500 K, followed by annealing at 1000 K for 5 min. Subsequently, the surface was gradually exposed to BAz at 1000 K. The exposure was increased until the whole metal surface was covered by h-BN, as shown by LEIS, but not above 260 L. Decomposition of BAz on the alloyed surface led to the attenuation of both Rh and Au LEIS (normalized) intensities and Au signal slowed down with increasing Au content. It was shown by STM measurements that up to a gold coverage of 0.5 ML, the nanomesh structure is only slightly disturbed, but larger parts are flattened at higher Au doses. LEIS studies on the growth of rhodium on h-BN/Rh(111) indicated a predominantly 3D growth, similar to the gold case. When small amounts of rhodium (up to 1 ML) were deposited on h-BN/Rh(111), intercalation was nearly complete upon annealing to ~900 K, while dewetting of the h-BN layer set in at ~1050 K. At higher rhodium doses, complete intercalation could not be reached at any temperatures.

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Aula 23H. Dpto. Física-UNS. Viernes 26/10. 16:30hs.

Dr. Laszlo Deák

MTA-SZTE

Reaction Kinetics and Surface
Chemistry Research Group.
Szeged, Hungary

Activation of small molecules on oxides and oxide-metal interfaces
The catalytic activation of small molecules, like CO, H2O and H2 is of importance in different processes, such as hydrogenation of CO and water gas shift reaction, for which metal oxide supported rhodium catalysts are widely applied. The critical role of metal particle-oxide interface in activating small molecules was established under high pressure conditions , but have rarely been addressed by UHV model studies. We have studied the impact of rhodium-titania and Rh-molybdena interfaces on the CO and H2O decomposition by AES, ISS, STM, XPS, TPD and sensitive temperature-programmed work function (WF) measurements.
The TiOx overlayers were formed by annealing the TiO2(110) supported rhodium, while the atomically thin MoOx deposits by the oxidation of Mo. Both oxides exerted inhibition effect on the molecular bonding of CO, which, however, was accompanied by considerable promotion of CO dissociation, maximized at 0.2-0.3 ML oxide coverages. The MoOx overlayer, having much lower surface free energy than the atomically thin TiOx film, completely covered the Rh particles by annealing to 650 K and eliminated the CO adsorption capability of the surface. On the contrary, the titania supported Rh particles showed the maximum CO dissociation propensity after annealing to 700 K. The maximization of CO decomposition is straightforwardly associated with the active role of metal particle-oxide interfaces. Desorption peak temperature for the associative CO desorption on the MoOx modified Rh particles (Tp=700 K) suggests a lower activation energy in the recombination reaction of Oa and Ca atoms, than that for the TiOx covered surfaces (Tp=800 K) which allows higher reaction rates for these intermediates in the former case. Our UHV study is in harmony with the results of high pressure experiments, concerning the formation of carbon intermediate from CO dissociation during CO methanation.
The reactions of water showed variety with the extent of reduction of titania support. Strongly reduced surfaces produced H2O and H2 desorption states with Tp=370 and 470 K, corresponding to recombinative and dissociative reaction paths of the surface OH moieties. The interaction of water with the Rh-TiOx interface was characterized with a H2 desorption state at 580 K, arising from OH groups exhibiting maximal population at intermediate TiOx coverages. The concomitant operation of inhibition and promotional effects of metal oxide overlayers on the adsorption properties of a metal could be rationalized with a simple island model. Some extreme hydrogen dissolution properties of a strongly reduced, black titania crystal have also been revealed.

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Sala de conferencias. Departamento de Ing. Eléctrica y de Computadoras.
Viernes 19/10. 16:00hs.

Dr. Juan Sebastián Ardenghi
Universidad Nacional del Sur
IFISUR-CONICET

Diagramas de Feynman en Teoría Cuántica de Campos y materia condensada
Los diagramas de Feynman revolucionaron la forma de calcular secciones eficaces en Teoría Cuántica de Campos. Mediante estos diagramas fue posible organizar y visualizar la serie perturbativa de los procesos de la electrodinámica cuántica. Mediante un conjunto de reglas sencillas, que asignan factores algebraicos a cada elemento del diagrama, fue posible reinterpretar la física involucrada y poder hacer predicciones con alto grado de exactitud. En esta charla se contará como se construyen estos diagramas, que son las antipartículas, que son las partículas virtuales y como se aplican estas técnicas a la física del estado sólido.

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commInteracción metal-Ceria y su actividad catalítica.
Una perspectiva teórica

Dra. M. Verónica Ganduglia-Pirovano
Instituto de Catálisis y Petroleoquímica-CSIC
Madrid, España

Ceria (CeO2) es uno de los más importantes óxidos de los elementos de tierras raras con usos en la catálisis industrial debido a sus condiciones de reducibilidad. La complejidad de los catalizadores en polvo reales dificulta en entendimiento básico de como es su comportamiento. Una forma de revelar este comportamiento es mediante la preparación de catalizadores modelo o modelos teóricos. En esta charla se presentaran resultados recientes de catalizadores modelo Ni, Co y Cu soportados sobre ceria como ejemplos de la catálisis del reformado en seco de metano (DRM: CH4+CO2 -> 2H2+2CO). Además, el sistema Ni/ceria se considerará para la producción de hidrógeno. El enfoque teórico se orienta a escribir a ceria como material soporte. La habilidad de ceria para estabilizar especies oxidadas (MOx: Co2+ y Ni2+) sobre superficies estequiométricas de CeO2 mediante una relocalización de estados f y estados metálicos (M0: Co0, Ni0) sobre el soporte reducido, CeO2-x, resulta esencial para la actividad catalítica en la reacción DRM.
La disociación de metano (CH4 -> CH3 -> CH2 -> CH -> C) ocurre a temperatura ambiente sobre MOx/CeO2, mientras que la disociación de CO2 ocurre sobre centros metálicos M0/CeO2-x en vacancias de oxígeno formadas a altas temperaturas (C + Osurf -> COgas + Vac). El sistema Co/ceria es el más activo con una barrera para la disociación de metano que se torna despreciable en la medida que la transformación MOx/CeO2 -> M0/CeO2-x se desarrolla con el incremento de la temperatura.

Jueves 09/11. 16hs.
Sala de conferencias
(Av. Alem 1253 - Cuerpo B - Subsuelo)

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commStructure and relative probabilites of formation of chlorobenzene dications induced by electron, proton and photon impact

Dra. Wania Wolff
Instituto de Física
Universidade Federal do Rio de Janeiro


Monochlorobenzene(C6H5Cl) is proposed as a prototypical molecule for studying substituent effects on the benzene ring, demonstrating the advantages of combining theoretical and experimental techniques to probe the molecular structure and formation. A literature survey has shown limited insights on mono substituted benzene derivatives, especially on the formation of multiply-charged parent ions. In this talk we present the structure and relative stabilities of chlorobenzene dications generated by the multiple ionization of C6H5Cl induced by electron, proton and photon impact. The most stable structure and low-lying isomers of multiply-charged parent ions (dications and trications) are analysed by Generalized Valence Bond and Density Functional Theory calculations, which revealed unusual carbon-chlorine bonding patterns. Experimentally, direct evidence of the formation of stable doubly-charged molecular parent ions observed by time-of-flight mass spectroscopy. Mass spectra and ion yields give the measure of the dications formation and their stability against dissociation is analysed through the ratios of dication to the molecular ion. Both aspects were investigated using keV energetic photons around the chlorine K-edge, electrons with energies from the double ionization threshold to 2000eV and low-energy protons in the 50-155 keV range. From the chemist's point of view, the authors hope that the results presented herein will encourage experimentalists to synthesize novel chlorine-containing species with unusual bonding pattern. Similarly, from the physicist’s point of view it is expected that these results will enable investigation into the competition between direct double ionization and auger emission channels in the formation of stable multiply-charged parent ions.

Viernes 10/11. 15hs.
Sala de conferencias
(Av. Alem 1253 - Cuerpo B - Subsuelo)

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Departamento de Física, Universidad Nacional del Sur
 Avenida Alem 1253 - Bahía Blanca, Buenos Aires, Argentina
 Tel: 54-(0291)-4595141
 Fax: 54-(0291)-4595142
2010 - 2020
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