[seminar] IF

[Ticijana Ban]

Poštovani

Obavještavam Vas o održavanju niza seminara na Institutu za fiziku, Bijenička cesta 46, predavaonica u zgradi Mladena Paića.

U srijedu 16.04.2008. u 15:00 sati seminar pod naslovom Chemisorption of molecular hydrogen on carbon nanotubes: a route to the effective hydrogen storage? održat će dr. Ante Bilić, Institute of High Performance Computing, Singapore.

U četvrtak, 17.04.2008. u 15:00 sati seminar pod naslovom Dynamics of electrons and holes in bulk metals and at surfaces: mechanisms of decay održat će prof. E.V.Chulkov,  Departamento de Física de Materiales, Facultad de Ciencias Químicas and Donostia International Physics Center, San Sebastian/Donostia, Spain.

U petak, 18.04.2008. u 15:00 sati seminar pod naslovom Ordered phases of soft colloids održat će prof. Primož Ziherl, Fakultet matematike i fizike, Jožef Stefan institut, Ljubljana, Slovenija.

Sažetke svih predavanja možete pronaći u nastavku poruke.
Srdačan pozdrav.

Ticijana Ban
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Chemisorption of molecular hydrogen on carbon nanotubes: a route to the
effective hydrogen storage?

 

Ante Bilić

 

Institute of High Performance Computing

1 Science Park Road, #01-01 The Capricorn,

Singapore Science

Park II, Singapore 117528

 

 

            The energetics of the chemisorption of molecular hydrogen on small-diameter armchair carbon nanotubes has been investigated using first principles density functional theory (DFT). The adsorption of hydrogen has been examined at a range of occupations, from low to full monolayer coverage. Several pathways for hydrogenation have been investigated and those that could lead to energetically favorable, stable structures of fully saturated nanotubes have been identified. For these routes the calculations indicate that the addition of hydrogen, apart from at the very onset, is exothermic and it also becomes increasingly more favorable with the degree of protonation. Carbon nanotubes, of sufficiently small diameter, are shown to have the capacity to store a full monolayer of hydrogen effectively via chemisorption. In addition, kinetic barriers for the dissociative chemisorption of H2 and thermal equilibration of the system have been considered. These have been found to be quite large for admolecules on an otherwise clean nanotube, but drop substantially in the vicinity of preadsorbed hydrogen, i.e., the adsorbed hydrogen acts as an autocatalyst for further hydrogenation. On the basis of these findings chemical reaction of hydrogen with carbon nanotubes is expected to become increasingly exothermic and also to proceed more rapidly at higher occupations.

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Dynamics of electrons and holes in bulk metals and at surfaces: mechanisms of decay

 

E.V.Chulkov

 

Departamento de Física de Materiales, Facultad de Ciencias Químicas,

UPV/EHU, Apdo.1072, 20080 San Sebastián/Donostia, Basque Country, Spain

and Donostia International Physics Center, 20018 San Sebastian/Donostia,

Basque Country, Spain

waptctce at sq.ehu.es

 

 

       Interaction between lattice, electron, and spin subsystems as well as interaction within each of these subsystems is crucial to understand mechanism of single-particle excitation dynamics, i.e. lifetime of excitations. The lifetime sets the duration of excitation and in combination with the velocity determines the mean free path, a measure of influence of the excitation. Interest to the study of excited particles dynamics is motivated by an important  role that excited electrons and holes play in many processes, e.g. in energy, charge, and spin transport in bulk materials, at surfaces, across interfaces, and at nanosystems. 

      In this presentation theoretical and experimental results on electron and hole dynamics are discussed in terms of different decay mechanisms and different kinds of interactions for bulk electronic states in paramagnetic and ferromagnetic metals, for surface and image states on metal surfaces, for excited states at single adatoms on these surfaces and for quantum well states in adlayers on metals. Of different interactions elastic and inelastic electron-electron (e-e) interaction as well as electron-phonon (e-ph) interaction are discussed. E-ph decay channel is shown to be important for all systems considered being especially important for low dimensional systems (nanosystems in 1-, 2-, or 3-dimensions). In the e-e decay channel the electron (hole) decay can be realized via creation of electron-hole pairs or plasmon excitation. In ferromagnetic systems the electron (hole) decay via the Stoner pair  excitation or/and excitation of spin waves is made possible. Dimensionality effects in the lifetime of electrons and holes on metal surfaces and the role of screening and intra- (inter-) band transitions are also discussed. 

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Ordered phases of soft colloids

 

Primož Ziherl

 

Faculty of mathematics and physics, University of Ljubljana, Slovenia

Jožef Stefan Institute, Slovenia

 

 

            In many colloidal systems, the pair interaction can be tuned so as to stabilize several condensed phases on top of the most usual close-packed lattice. Many of these phases, such as the A15 and the ? lattices observed in self-assembled dendrimer micelles, are characterized by anisotropic coordination of lattice sites. Their stability can be understood in terms of a phenomenological model based on the analogy of the interpenetrating soft colloids and the soap froth, and is thus related to the Kelvin problem of partitioning the space into cells of equal volume such that the total area of the partition is minimal. Another useful theoretical concept associated with the open crystal phases of soft colloids is the hard-core/soft-shoulder repulsive pair interaction. In the limit of large shoulder width to core size, i.e., well-pronounced penetrability of the colloids, the morphology of the condensed phases is universal and consists of lattices whose building blocks are clusters of colloids rather than colloids themselves. In 2D, the cluster mesophases include micellar, lamellar, and inverted micellar lattice. We will discuss the phase diagram of such systems and the main structural features of the mesophases. Surprisingly, the phase sequence and aggregate size adjusts so as to keep almost constant inter-aggregate separation.

 

 



 
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