MSE Colloquium Series
Spring Quarter 2012
Fridays at 3:15 to 4:45 in Skilling Auditorium
Date |
Speakers |
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Apr. 6 |
Professor Seiji Takeda Operand Structural Study of Metal Nanoparticle Catalysts by High Spatial Resolution Environmental TEMIn-situ observation at the atomic scale becomes extremely important in materials science and technology. To examine the processes for synthesising materials and fabricating devices, in-situ environmental transmission electron microscopy (ETEM) is now regarded as a promising tool. After describing the fundamentals of in-situ ETEM, we present recent applications of high spatial resolution ETEM on the operand studies of working catalysts such as supported metal nanoparticles for the oxidation of CO gas and the growth of carbon nanostructures. As is well known, chemically inactive solid gold shows remarkable catalytic activity for CO oxidation even below room temperature when it is supported on crystalline metal oxides such as TiO2 and CeO2 in the form of nanoparticles. The mechanism of catalysis by gold nanoparticles (GNPs) has recently aroused much attention. The systematic and quantitative analyses by ETEM on the working catalyst showed that the change in morphology of GNPs correlates with the catalytic activity [1]. Using an aberration corrected high spatial resolution ETEM, we found that CO gas makes the {100} facets of a GNP reconstructed at reaction conditions [2]. CO molecules are adsorbed at the on-top sites of gold atoms in the reconstructed surface. The stability of the adsorbate-induced structure was confirmed by ab initio calculations. Both the temperature and pressure gaps that were general issues in applying microscopy to real catalysts could be overcome in this study. Therefore, the atomic-scale visualizing method combined with the systematic and statistical analyses of observation data can be applied to the elucidation of the mechanism of various heterogeneous catalysis. In recent years, various growth methods of carbon nanotubes (CNTs) have been developed and the growth conditions have been investigated carefully. Consequently, high purity CNTs can now be grown in large quantities. It is known that nanoparticles act as catalyst for the growth of CNTs. We investigated Fe-catalyzed growth of CNTs by ETEM. Though only Fe was deposited on the substrate, Fe3C (cementite) nanoparticles act as catalysts for the growth of CNTs [3]. In Fe-Mo catalyzed CVD conditions, CNTs are grown from nanoparticles of (Fe,Mo)23C6 besides Fe3C. Carbide nanoparticles fluctuate structurally in the CVD processes. Because of experimental difficulties in the past, not much systematic atomic scale information has been obtained by ETEM. However, recently developed ETEMs are well-designed and robust when one operates them with standard cares on gases. Therefore, ETEM now becomes a standard analytical tool for various processes in materials science.
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Apr. 13 |
Spring MRS Meeting - No Colloquium *********** |
Apr. 20 |
Professor Eva Olsson Dynamics of charges and matter at interfaces ************ |
Apr. 27 |
Professor Xiaoling Zheng Bridging Combustion and NanotechnologyIntersection between combustion and nanotechnology offers exciting opportunities to provide mutual benefits for both areas. Previous combustion research related to nanotechnology has primarily focused on the synthesis of nanoparticles (NPs), combustion of Al NPs and soot formation. Nevertheless, nanotechnology, in the past decade, has achieved significant progress in the area of one-dimensional (1-D) nanomaterials, such as nanowires (NWs) and nanotubes (NTs), and the high aspect ratios of these 1-D nanomaterials offer additional benefits of isotropic properties in comparison to NPs. 1-D nanomaterials have already made great impact on many areas, ranging from energy conversion systems, electronic and optical devices, to biological sensing and health monitoring systems, but, to a much less degree, on combustion. This talk will present two examples of our efforts in bridging combustion and 1-D nanomaterials. First, we developed several flame synthesis methods to synthesize, decorate or dope 1-D metal oxide nanomaterials and these methods exhibit advantages of fast growth rates, high purity and crystallinity materials, versatility, scalability and low-cost. Second, we applied 1-D transition metal oxides to catalyze the oxidation reactions of hydrocarbons. These 1-D nanostructured catalysts compared to the supported NPs, exhibit comparable or even better catalytic activity and stability, great flexibility in increasing the catalyst loading, and convenience in tuning the surface chemistry. Finally, we demonstrated a distributed optical ignition method that uses a camera flash to ignite Al NPs, resulting in the ignition of solid phase energetic materials, and liquid and gaseous fuels. The flash ignition occurs when the Al NPs have suitable diameters and sufficient packing density to cause a temperature rise above their ignition temperatures. Importantly, transmission electron microscopy analysis reveals that the Al NPs are oxidized via the melt-dispersion mechanism, providing the first direct experimental evidence thereof. Biographical Sketch Xiaolin Zheng is an Assistant Professor of Mechanical Engineering at Stanford University. She received her Ph.D. in Mechanical & Aerospace Engineering from Princeton University (2006), B.S. in Thermal Engineering from Tsinghua University (2000). Prior to joining Stanford in 2007, she did her postdoctoral work in the Department of Chemistry and Chemical Biology at Harvard University. Her research interests lie at the interfacial science involving chemistry, nanomaterials, thermofluidics and electronic devices. She is a member of MRS, ACS and combustion institute. She received the Presidential Early Career Award (PECASE) from the white house (2009), Young Investigator Awards from the ONR (2008), DARPA (2008), Terman Fellowship from Stanford (2007), and Bernard Lewis Fellowship from the Combustion Institute (2004). For additional information: http://www.stanford.edu/group/zheng ************ |
May 4 |
Dr. Alex Hamza Please note the Colloquium will start 10 minutes later (3:25pm). Materials Science for Targets for Inertial Confinement FusionBringing “star power to earth” is the goal of controlled inertial fusion energy. Inertial confinement fusion (ICF) experiments rely heavily on precision fabrication of small, complex targets. The fabrication of these targets in turn relies heavily on advanced material science. This talk will describe the target designs, their complexities, and how material science is being used to enable fabrication of these complex structures. ************ |
May 11 |
Professor Dieter Neher Chain aggregation and Function of Polymer-based Organic Solar CellsThe performance of polymer-based organic solar cells has dramatically improved during the last years, now reaching power conversion efficiencies larger than 9 %. This development has been promoted by the improved understanding of the elementary steps governing the generation und extraction of carriers in these devices. Evidently, these processes are largely determined by the morphology of the samples, particularly the conformation and the packing of the polymer chains. In this talk, various aspects of chain aggregation in blends of conjugated donor polymers with small molecules or polymeric acceptors are discussed. Optical spectroscopy is used to determine the degree of crystallinity and to extract information on the size and perfection of polymer aggregates in such blends.[1] Special emphasis will be put on the correlation between thermal properties, layer morphology and the dynamics of photogenerated charge carriers. For blends of P3HT with PCBM, we find that the degree of crystallinity is not a key parameter determining the solar cell performance, while the probability to extract photogenerated holes from the blend layer is highly correlated with the energetic order in the P3HT crystals. In such blends, the photogenerated current can be described by considering only the sweep-out of carriers out of the device by the internal electric field and losses due to non-geminate recombination of the remaining charge.[2] In contrast, blends of the low bandgap polymer PCPDTBT with PCBM exhibit a pronounced field-dependence of charge generation, suggesting efficient geminate recombination.[4] We show that the efficiency of geminate and non-geminate recombination in these blends is connected correlated to the blend morphology, and that both decay channels are strongly reduced in blends with extensive interchain order.[4] [1] S.T. Turner, P. Pingel, R. Steyrleuthner, E.J.W. Crossland, S. Ludwigs, D. Neher, „Quantitative analysis of bulk heterojunction films using linear absorption spectroscopy and solar cell performance, Adv. Funct. Mater. 2011, 22, 4640. [2] J. Kniepert, M. Schubert, J.C. Blakesley, D. Neher, “Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments”, J. Phys. Chem. Lett. 2011, 2, 700. [3] S. Albrecht, W. Schindler, J. Kurpiers, J. Kniepert, J.C. Blakesley, I. Dumsch, S. Allard, K. Fostiropoulos, U. Scherf., D. Neher, “ On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM: influence of solvent additives”, J. Phys. Chem. Lett. 2012, 3, 640. [4] S. Albrecht et al, ”Fluorinated PCPDTBT with Enhanced Open Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells” submitted to JACS. ************ |
May 18 |
Professor Nadya Mason Using graphene to study superconductivity (new tricks for an old dog)Superconductors are materials that can have zero electrical resistance. They are thus of great interest for applications such as power transmission and energy storage. While the fundamental physics of standard superconductors has now been understood for over 50 years, questions remain about what happens when superconductors are coupled to other materials. For example, it was known that superconducting carriers could be transmitted through a normal metal, but the spectroscopy of the individual modes had not been measured. Graphene, a single atomic layer of carbon which has only recently been isolated for electrical measurements, is also of great interest for fundamental studies and applications. In this talk, I will discuss the new properties that emerge when “superlative” materials such as superconductors and graphene are put together in hybrid structures. In particular, I will show how the creation of a superconductor-nanoparticle-graphene sandwich structure enables a direct measurement of individual superconducting modes. Our measurements show that the spectra of these modes are sharp and tunable. I will also discuss the materials issues involved in coupling graphene to nanoparticles and superconductors; for example, we found that consistent tunnel barriers could be formed at lead-graphene interfaces. ************ |
May 25 (Fri) |
Professor Andrew Rappe TBD ************ |
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As material synthesis and nanofabrication methods are refined and the control of material structure reaches beyond the nanoscale the role of individual interfaces, defects and atoms is pronounced and can dominate the properties. The strong influence of local atomic structure offers the possibility of designing new components with tailored and unique properties. Electron microscopes offer the possibility of correlating the structure to transport properties with a spatial resolution that reaches the atomic scale. A knowledge platform of how to design new materials by combining experiments and theory can thus be established. In addition, inserting a scanning tunnelling or an atomic force microscope in the electron microscope enables studies of dynamic events. In this talk, observations of, for example, Al/AlOx/Al tunnel junctions, perovskite oxide interfaces and contact points of soft matter and water will be discussed.