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Bi-Weekly Seminar

Nanoscale Self-Assembly: A Theoretical Analysis

 Gemunu  Gunaratne

by: Gemunu Gunaratne

Date: Friday November 10, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We use linear and nonlinear stability analysis on a paradigmatic model to extract general characteristics of nano-scale self-assembly. In particular, we identify the order of occurrence of hexagonal and striped arrays, and show that square arrays cannot form when the elastic forces between the substrate and the monolayer are isotropic. In addition, we introduce a method that can be used to estimate hard-to-extract material properties of the monolayer using characteristics of the self-assembled patterns. Finally, we will discuss a technique that can be used to help generate patterns with long-range order.

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Bi-Weekly Seminar

Lattice-strained Nanoparticle Electrocatalysts for PEMFC Cathodes — From Combinatorial Discovery to Structure-property Relationships

by: Dr. Peter Strasser

Date: Friday September 29, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The cell voltage and performance of Polymer-Electrolyte-Membrane Fuel Cells (PEMFCs) deviate strongly from their theoretical values due to severe kinetic overpotentials at the cathode where oxygen is electroreduced to water. The overpotentials are a manifestation of the sluggish rate of adsorption and reaction of molecular oxygen on Pt cathode electrocatalysts. The identification of more active, cost-effective and corrosion stable electrocatalysts for the oxygen reduction reaction (ORR) therefore continues to be a scientific priority in Fuel Cell catalysis research.

We report the combinatorial discovery, bulk synthesis and physico-chemical characterization of a new structural class of Pt electrocatalysts for use for the ORR in PEM fuel cell cathodes. The catalysts exhibit outstanding performance characteristics in terms of their Pt mass based as well as their Pt surface specific activity for the ORR, meeting Department of Energy performance targets for 2010.

Electrochemical Rotating Disk Electrode (RDE) measurements and physico-chemical characterization - including synchrotron X-ray diffraction (XRD) and synchrotron Small Angle X-ray Scattering (SAXS) - show that rapid de-alloying and corrosion processes of base metal rich alloy nanoparticles of a catalyst precursor compound result in the formation of Pt particle lattices with unusually high lattice strain. The data suggests that the formation of strained Pt lattices is correlated with the favorable catalytic activity. SAXS results further show how the electrochemical treatment affects the particle size and metal composition distributions of the catalytic particles inside their ionomer-carbon matrix. Our synchrotron studies allow us to formulate relationships between synthetic conditions, structural characteristics and electrochemical activity. Experimental observations are compared to DFT computational predictions as to the impact of strain on the ORR activity of Pt lattices.

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Bi-Weekly Seminar

Strain and n-Type Doping Effects on Colossal Magnetoresistance Films

by: Prof. Hsiung Chou

Date: Friday August 18, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The mechanism of strain effect and the achievement of n-type doping on colossal magnetoresistance (CMR) films has been debated and tried for the past decade. It has been believed, but there has been a lack of direct evidence to support, that the distorted MnO6 octahedron due to in-plane strain effect causes the change of transport and magnetic properties. To investigate the origin of the strain effect, La0.7Ca0.3MnO3 and La0.8Ba0.2MnO3 films with various thicknesses grown on SrTiO3 substrates were examined by Near Edge X-ray Absorption Spectroscopy (NEXAS). This study finds that the strain doesn’t affect the MnO6 octahedron significantly, but weakens substantially the La-O and Ca-O (or Ba-O) hybridization, which is responsible for the reduction and the enhancement of TC in La0.7Ca0.3MnO3 and La0.8Ba0.2MnO3 strain films, respectively. For the n-type CMR issue, it has been believed that an n-type CMR can be realized by partially substituting tetravalent ions on trivalent La3+ sites. By investigating the La0.7(Ce or Te)0.3MnO3 bulks with SEM and EDS, it is found that the compound decomposed into La0.9-&epsilonCe&epsilonMnO3+&epsilon, Mn-O, and CeO2, none of which contained original stochiomatry. The n-type compound cannot be formed in thermal equilibrium process, such as post annealing. Only those under metastable processing such as in-situ epitaxial films can possibly assist in forming n-type CMR.

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Bi-Weekly Seminar

Neutron Reflectivity from NbTi/Nb Multilayers

Dr. Wolfgang  Donner

by: Dr. Wolfgang Donner

Date: Friday August 04, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Thin film multilayers of superconducting materials display unusually high pinning forces compared to their bulk counterparts. One such system is NbTi/Nb multilayers: here it was found that the critical current density of such multilayers sensitively depends on the multilayer periodicity. It has been speculated that the vortex lattice in those multilayers would “match” the multilayer period under an applied external field of sufficient magnitude.

The talk presents results on the growth, x-ray and neutron characterization of NbTi/Nb multilayers in an effort to test the hypothesis of vortex lattice “matching.”

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Bi-Weekly Seminar

Does DNA act as it's own sunscreen?

Prof. Eric  Bittner

by: Prof. Eric Bittner

Date: Friday July 07, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

DNA is a surprisingly robust molecular system in spite of its rather large UV absorption cross-section in the 310-290 nm range. Part of this robustness comes from DNA's ability to rapidly dissipate the electronic photoexcitation energy into heat, thus preventing to some extent photochemical processes that can lead to mutation. One of the key questions is whether or not this dissipation is due to base-pairing and hydrogen transfer in localized excited states or if it is due to base-stacking effects. In this talk, I shall give an overview of our two-band lattice model for the excited states of DNA double helices. Our theoretical calculations corroborates recent ultrafast experimental results that indicate that base-stacking dictates the fate of an excitation in A-T DNA. Moreover, our work suggest that in AT DNA, excitonic dynamics along the A chain is dramatically different than along the T chain. Finally, we speculate that these processes may have played a crucial role in the evolutionary selection of DNA.

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