Argonne National Laboratory Center for Nanoscale Materials U.S. Department of Energy

Archive: Seminars 2010

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December 17, 2010

"Microsystems for Control of Light-Matter Interaction at the Nanoscale," Il Woong Jung, hosted by Daniel Lopez

Abstract: It has been shown that electromagnetic fields incident upon metal particles can be confined over dimensions on the order of or smaller than the wavelength of light. This strong light-matter interaction leads to an enhanced optical near-field at metallic interfaces or in small metallic nanostructures. These highly enhanced fields can be used to probe materials with subwavelength resolution (near-field scanning optical microscopy) and allow single-molecule surface-enhanced Raman spectroscopy detection. The ability to control these interactions (i.e., tune the plasmonic resonance) is highly desirable for developing ultrasensitive Raman and fluorescence detectors, highly efficient optical antennas, tunable single-emitters, nano-patterning, and novel nanophotonic devices.

In contrast to the strong light-matter interaction observed for metallic structures, photonic crystals are periodic dielectric structures that support many weak interactions to combine coherently to a strong interaction. By controlling the structure and geometrical dimensions, photonic crystal slabs can support guided resonances that couple to external radiation in ways that profoundly change its optical properties. By using nanofabrication tools such as focused ion beam, these nanophotonic elements can be integrated with micromechanical devices for added functionality to control the electromagnetic radiation.

In this talk, I will give an introduction to photonic crystal integrated microsystems and then present current research efforts focused on using micromechanical devices to actively control the optical properties of plasmonic nanostructures. In addition, just as microsystems have advanced the capability to manipulate light at the visible to infrared wavelengths, they can vary the focus, scan, modulate, and dynamically adjust the beam profile for X-rays, thereby enhancing the capability to probe materials at the nanoscale.

December 15, 2010

"Nanotechnology Enabled Research: from Superconductivity to Nanocomposites," Dmitriy Dikin, Northwestern University, hosted by Daniel Lopez

I will talk about the importance of nanotechnology from different aspects of my past, present, and wishful research. Superconductivity, nanomechanics, graphene-based materials, and many other interesting research areas always require precise control of the sample fabrication, inspection, and in some cases nanomanipulation for measurements, an adequate understanding and correct interpretation of new results. Quite often in my practice a microscope was not only the tool for taking images (which is very important in itself), but a research method and a laboratory space. I am going to describe (i) nanomechanical and other experiments performed inside of the scanning electron microscope, (ii) some perspectives and technologies based on the graphene sheets, (iii) some examples of new nanotools for research and development.

December 7, 2010

"Heusler compounds: Multifunctional materials for spintronics," Claudia Felser, Institute of Inorganic Chemistry, Johannes Gutenberg University Mainz, hosted by Amanda Petford-Long

Abstract: In 1905, Fritz Heusler discovered that the compound Cu2MnAl is ferromagnetic, even though none of its elemental constituents are themselves magnetic. This remarkable material and its cousins, a vast collection of more than 800 compounds, are now known as Heusler materials. Surprisingly, the properties of many of the Heuslers can be forecast simply by counting the number of their valence electrons. For example, Co2-Heusler compounds with more than 24 valence electrons are half-metallic ferromagnetics: electrons of one spin orientation are semiconducting. whereas electrons with the opposite spin orientation are metallic. Such compounds display nearly fully spin-polarized conduction electrons. making them very useful for spintronic applications. Mn2-Heusler compounds have attracted interest because their excellent performance for spin torque transfer applications.

Another subclass of more than 250 Heusler compounds are semiconductors. Their band gaps can readily be tuned from zero to ~4 eV by changing their chemical composition. These materials have thus attracted attention as potential candidates for both solar cell and thermoelectric applications. Indeed, excellent thermoelectric properties have recently been demonstrated. Finally, we discuss a third class of Heuslers that have been predicted, on the basis of their calculated electronic band structures, to be topological insulators, a new state of matter in which spin-polarized edge and surface states are topologically protected from impurity scattering.

December 6, 2010

"Scanning tunneling microscopy investigation of the heteroepitaxial interfacial electronic properties," Ya-Ping Chiu, National Sun Yat-Sen University, hosted by Te-Yu Chien

Abstract: Motivated by the importance of critical nanoscale interfacial science, the objective of our studies is the interfacial characteristics of heteroepitaxial structures and the fundamental mechanisms that pertain in these systems.

Cross-sectional scanning tunneling microscopy (XSTM) is employed to observe directly the epitaxial interfacial structures and the local electronic properties with atomic-level insight. Measurements of the local density of states (DOS) characteristics with local atomic precision enable us to directly demonstrate the natural evolution of the electronic properties across the heteroepitaxial interfacial properties. In this talk, topics to be discussed include the high-*'s/III-V's system (Gd2O3/GaAs) and multiferroic materials (BiFeO3). The significant heteroepitaxial interfacial electronic properties of the systems will be discussed.

December 3, 2010

"High-Spatial-Resolution Chemical Imaging: Biomedical Applications for Pathology and in vivo Imaging,"Carol Hirschmugl, University of Wisconsin, Milwaukee, hosted by Leonidas Ocola

Abstract: Chemical imaging concurrently provides morphological and chemical information about a sample by combining microscopy and spectroscopy. An emerging technology for this is synchrotron based wide-field Fourier Transform infrared (FTIR) imaging, which uses a chemically sensitive infrared microscope illuminated with a stable, broadband source and equipped with multiple, parallel detection channels. This instrument will greatly expand the ability to examine biological structures, and to track their chemical changes. The probe is nondestructive, making it feasible to study living cells, and one obtains chemical images with diffraction-limited resolution in minutes. In comparison, more commonly used visible light microscopy reveals the morphology of a sample through colour and contrast from labels such as fluorescent proteins or other sample preparation techniques that are frequently chemically specific. Importantly, infrared imaging provides new opportunities, since it delivers both morphological and chemical information without the need for stains or labels. The design of this facility and initial applications of biomedical interest will be presented.

High-resolution synchrotron imaging has the potential to be very useful for biomedical research. Whilst the major cell types within human tissue; epithelial and stromal cells, may be resolved using conventional FTIR imaging, there are a number of very important cell types that cannot be easily identified including basal cell, endothelial cells and immune cells. The identification of cell types within tissue using IR imaging is critical for understanding cell and tissue systems and is the key approach to diagnosis of diseases within tissue. The work demonstrated here shows a significant improvement in spatial resolution, potentially allowing for the identification of very small cell types in prostate and breast tissues.

Another example of the research enabled by this system includes subcellular spatially resolved spectra of molecular changes in brain tissue induced by Alzheimer disease (AD). The hallmark characteristics of AD include formation of extraneuronal plaques (aggregated -amyloid protein) and intraneuronal neurofibrillary tangles (NFT, fibrillar hyperphosphorylated tau protein). Numerous models for their biogenesis have been proposed but the etiology and pathogenesis are still not well understood. Our work is based on two mouse lines that express two familial AD mutations, K670N/M671L and V717F mutant form of human APP695, and a different, triply mutant mouse.

November 22, 2010

"A New Portable Nucleic Acid Impedance Detection Platform based on Electrokinetic Nanosensors," Chia Chang, University of Notre Dame, hosted by Leonidas Ocola

Abstract: We report a new label- and PCR-free portable DNA/RNA detection platform based on nanomembrane sensors embedded in continuous flow microfluidic biochips. The nanomembranes are synthesized or fabricated on the sides of the microflow channel to allow a large flow throughput, and are functionalized with oligo probes. Fabricated microelectrodes sustain a dc or ac electric field across the membrane to create an ion depleted zone on the probe-covered membrane surface: to remove inhibitors and contaminants but also to effect a dielectrophoretic trapping action to rapidly transport the target molecules from the flowing solution to the probe. The same electrodes are used to measure both impedance and conductance signals of the hybridized molecules on the membrane surface, with the conductance signals amplified by charge inversion effects and the impedance Warburg signals by the ion depletion dynamics. The shear force due to the high throughput flow minimizes nonspecific binding and yields highly selective hybridization. Preliminary data with dengue virus, Carcinus maneas, green crab and Daphinia magna-water flea show picoM detection limit, large dynamic range, single-mismatch (SNP) discrimination in a 26 docking sequence of a kb target and 1-minute assay time without pretreatment and with a single turnkey operation. RNA detection with this rapid portable platform is particularly attractive, as most organisms produce many (ten thousand) more copies of mRNA than DNA but the RNAs degrade within 30 minutes without preservation. Plasmonic sensors are being integrated into this platform to allow multitarget detection.

November 19, 2010

Research and Developments on Spintronics at AIST,” Shinji Yuasa, National Institute of Advanced Industrial Science and Technology, hosted by Tiffany Santos

Abstract: AIST is one of the biggest national research institutes in Japan. The Spintronics Research Center at AIST was established in 2010 to promote research and development in spintronics. We are working on various topics ranging from basic studies of semiconductor spintronics to device applications such as magnetoresistive random access memory (MRAM). In this seminar, I will talk about our research activities based on magnetic tunnel junctions such as spin-transfer-torque MRAM, spin-torque microwave oscillator, and novel physical random number generator.

October 20, 2010 Hybrid porous materials for responsive nanomachine and catalyst,” Rui Liu, University of California, Riverside, hosted by Elena Rozhkova

Abstract: A series of hybrid porous materials have been synthesized by combining the unique features of mesoporous materials and functional molecules. Several responsive nanomachines have been developed by using mesoporous materials as reservoir and functional polymers or nanoparticles as valves to perform the controlled responsive functionalities. Sulfonated mesoporous carbons have been synthesized and show stable and highly efficient catalytic performance in biodiesel production.

October 20, 2010

"What in the World is M2D2?", John Mitchell, Materials Science Division, Argonne National Laboratory, hosted by Amanda Petford-Long

Abstract: Materials and molecular fesign and fiscovery (M2D2) is one of Argonne's major initiatives that will cross-cut divisions, disciplines, and programs in the coming years. But who and what exactly is M2D2? What are we trying to accomplish, what scientific and technological directions do we plan to follow, how are we organized, where are we headed? And most importantly, how does the CNM fit in? I will present the M2D2 vision as it stands today, tell you how we have seeded an initial program, and briefly highlight our path forward. I hope this will be a very interactive discussion and I welcome your questions, comments and thoughts on how to help define the CNM's role in M2D2.

October 18, 2010 Engineering Single Nanostructures as Plasmonic Antennas for Giant Raman Enhancements,” Matthew Rycenga, Washington University, hosted by Elena Rozhkova

Abstract: Antennas, like radio antennas, can collect radio waves and convert them into an electric current. Similarly, metal nanostructures can serve as antennas to convert light into surface plasmons, which are electromagnetic waves coupled to the collective oscillations of free electrons in a metal nanostructure. By tailoring the size, shape, and environment of a metal nanostructure, light can be manipulated in remarkable ways far below the diffraction limit. For example, light can be concentrated into tiny volumes with specially designed nanostructures, creating giant electromagnetic field (E-field) enhancements. These types of nanostructures are ideal for surface-enhanced Raman scattering (SERS), whereby the enhanced E-fields of a nanostructure can increase the Raman scattering of molecules in their vicinity by factors up to 1012. This talk will discuss the methods we have developed to study and characterize the SERS of single nanostructures, one at a time. This allows for an accurate comparison between a nanoparticle’s structure to its SERS. Such close scrutiny can reveal variables connected to SERS that have largely gone unnoticed. In particular, we have developed a new strategy for creating giant Raman enhancements by using the substrate supporting a nanoparticle. This allows for a single silver nanocube to approach single molecule detection with SERS. This simple technique will be discussed in context to our previous work using individual silver nanostructures for SERS.

October 8, 2010

Material properties of self-assembled actin bundles," Alec Robertson, Massachusetts Institute of Technology, hosted by Amanda Petford-Long

Abstract: Actin is an ubiquitous structural protein fundamental to such biological processes as cell motility and muscle contraction. Our model system is the acrosome of the Limulus sperm, which extends a 60-m-long actin bundle during fertilization. It is an example of a biological spring where the force of elongation derives from twist energy stored within the bundle during spermatogenesis. In addition to actin, the acrosome comprises only one other protein: scruin, an actin-binding protein specific to Limulus that decorates and cross-links actin filaments into a crystalline bundle. This talk will present the self-assembly of acrosome structure using these two proteins and their characterization by optical means, in order to elucidate the role of cross-linking in actin bundle formation and mechanics.

A multiscale approach is adopted wherein the bending rigidity of actin bundles and their constituent filaments are probed individually, then interrelated by simple mechanical models. Material properties of filaments and bundles are measured using a combination of optical tweezers, electron and fluorescence microscopy. Actin-scruin bundles assembled off acrosome fragments are found to display an ordered structure and a bending rigidity orders of magnitude higher than their individual filaments, while actin-only bundles formed by osmotic pressure exhibit similar properties, suggesting an intrinsic regime of crosslinked actin bundle formation. A second regime also emerges, spanning several orders of magnitude in bending rigidity and mediated by scruin conformation and bundle crystallinity. These highlight a cell’s ability to tailor the rigidity of its cytoskeleton to meet specific mechanical requirements, either by varying stiffness over orders of magnitude through conformation changes to a single actin-binding-protein, or through the use of different cross-linking mechanisms to achieve comparable rigidities.

September 14, 2010

Spin Currents: New Opportunities at the Nanoscales,” IAxel Hoffman, Argonne National Laboratory, Materials Science Division, hosted by Amanda Petford-Long

Abstract: The recent development of spintronics aims at utilizing the spin degree of freedom for electronic applications. To date, in most investigated spintronics systems and devices, spin currents are mainly considered as spin-polarized charge currents, and as a result, the spin and charge currents are in parallel and directly coupled. However, using nonlocal geometries allows us to separate spin and charge currents, which in turn enables the investigation of pure spin currents This approach opens up new opportunities to study spin-dependent physics and gives rise to novel approaches for generating and controlling angular momentum flow.

In this talk, I will review our work on pure spin currents generated in various different ways. First, I will discuss electrical injection coupled with nonlocal spin-dependent potentiometric detection, from which we can directly determine spin-diffusion lengths and spin relaxation times. In particular, by investigating the temperature dependence of spin and charge relaxation times, we can identify different spin relaxation mechanisms. In mesoscopic silver wires, we observed that the spin flip probability for surface scattering is significantly larger than for the bulk.

Second, I will discuss the practical usefulness of using spin Hall effects due to spin-dependent electron scattering, which has been suggested as a pathway to pure spin currents without the need to employ ferromagnetic materials. However, in our measurements, we can exclude any large spin Hall effects in gold, which nevertheless has relatively strong spin-orbit coupling. In fact, we found an upper limit for the spin Hall angle of 0.022 at 4.5 K and 0.027 at 300 K.

Lastly, I will discuss spin pumping, which allows the generation of pure spin currents by exciting ferromagnetic resonance in an adjacent ferromagnet. This later approach is attractive, since it allows the macroscopic generation of pure spin currents without the need of directly applied charge currents. These measurements allow us to uniquely quantify spin Hall effects in a wide variety of materials, such as platinum, palladiu, gold, and molybdenum.

August 26, 2010

Perspectives on Resistance Change Random Access Memory (ReRAM) Technologies,” In Kyeong Yoo, Samsung Electronics, hosted by Amanda Petford-Long

Abstract: Oxide materials have attracted much attention for their memory switching properties and electron mobility, which are higher than those for amorphous silicon. Some applications such as ReRAM for nonvolatile memory and oxide TFT for display were proposed, and excellent progress has been demonstrated. However, it is not clear yet whether or not new services will require oxide devices integrated into IT devices, because of uncertainty in technology trends and directions. For example, ferroelectric random access memory (FRAM) was one of the promising candidates for nonvolatile RAM in 1996; however, NAND Flash is replacing hard disk drives (HDDs) with a form of solid-state disks (SSDs), and dynamic random access memory (DRAM) is still a core device in the computer system. This is because new product concepts are developed by utilizing existing feasible technologies, components, and devices. It is thought that new technology alone cannot meet the product trends, so that there are gaps between service patterns and technology evolution. It is suggested that the gap stems from the differences in the way of reasoning in service concept and technology prediction. Evolution pattern in products and technologies are analyzed in order to understand the reasoning differences. Approaches on how to connect product concept with technology forecast will be proposed with an example. Finally, the mechanisms and issues in ReRAM will be reviewed in the viewpoint of possible new products.

August 4, 2010

Radiation-Enhanced Transverse and Lateral Diffusion in Nanoscale Structures,” Bhupendra N. Dev, Indian Association for the Cultivation of Science, hosted by Jin Wang and Xiao-Min Lin

Abstract: Ion irradiation of a material usually gives rise to enhanced diffusion. To use it or to avoid unwanted diffusion, the diffusion enhancement process needs to be understood. We follow ion-beam-induced migration of iron impurity atoms in a Pt/C multilayer, where each layer is about 2 nm thick, by the X-ray standing wave (XSW) technique. We find that iron atoms migrate from C- to Pt-layers, forming FePt ferromagnetic nanoparticles. As a consequence, an ion-beam-induced nonmagnetic-to-ferromagnetic transformation is revealed by magnetooptical Kerr effect measurements.

For nanoelectronic applications, p-type doped nanoscale structures can be fabricated by implanting gallium ions from a focused ion beam source into an n-type silicon substrate. However, how closely these structures can be fabricated would be governed by the lateral diffusion coefficient of the implanted atoms. Determination of lateral diffusion coefficient is difficult. We demonstrate a method for the determination of lateral diffusion coefficient based on photoemission electron microscopy and discuss the extent of diffusion in the light of nanoscale fabrication.

July 19, 2010

Domain-wall dynamics in ferromagnetic nanowires with spiral order and minimization of Ohmic losses,” Oleg Tretiakov, Texas A&M University, hosted by Amanda Petford-Long

Abstract: I will talk about current-induced magnetization dynamics in thin ferromagnetic wires. It is known that there are two regimes of the domain wall motion in the wire, depending on applied current. Below the critical value of current, the magnetization in the domain wall does not rotate, whereas above the critical current domain wall both moves and rotates. We determine the domain wall dynamics as a function of applied current and calculate the drift velocity of the domain wall along the wire. First, I will show that the critical current is suppressed in the presence of Dzyaloshinskii-Moriya interaction (DMI). This suppression is exponential at large DMI. Second, I will propose a novel way to move domain walls with a resonant time-dependent current.

This method dramatically decreases the Ohmic losses in the wire and allows to drive the domain wall with higher speed without burning the wire. For any domain wall velocity, we find the time dependence of the current needed to minimize the Ohmic losses. Furthermore, I will identify the wire parameters for which the losses reduction from its dc value is the most dramatic. I will show that our approach gives a dramatic power reduction even in the least favorable cases of the systems with very weak or very strong nonadiabatic spin transfer torque (conventionally described by the parameter beta), thus opening new doors for using materials with much wider range of beta for spintronic devices.

June 14, 2010

STM Manipulation of Charge, Spin, and Conformation of Atoms and Molecules,” Saw-Wai Hla, Ohio University, hosted by Axel Hoffmann

Abstract: Scanning tunneling microscope (STM) manipulation of single atoms and molecules on surfaces allows construction of artificial structures on an atom-by-atom basis and demonstration of single-molecule devices one molecule at a time. STM is not only an instrument used to "see" individual atoms by means of imaging, but also a tool used to "touch" and "take" atoms and molecules or to "hear" their vibration by manipulations. Therefore, STM can be considered as the "eyes, hands, and ears" of the scientists connecting our macroscopic world to the exciting world of atoms and molecules.

In our research projects, we combine a variety of STM manipulation schemes with tunneling spectroscopy techniques to investigate properties of atoms and molecules on surfaces. This talk will include our recent achievements: In spintronic research, we will show imaging and manipulation of spin directions in individual atoms. In superconductivity, we will present the smallest molecular superconductor ever studied to date. Here, the finding of superconductivity in just four pairs of (BETS)2-GaCl4 molecules not only provides the possibility of investigating this phenomenon locally, but also opens the potential for applications in nanoelectronics. In molecular devices, single-molecule switches and molecular rotors operated by injecting tunneling electrons from an STM tip will be presented. These innovative experiments are tailored to address several critical issues covering both basic science and demonstration of novel atomic and molecular devices.

June 10, 2010

Polarization Dynamics and Ionic Currents on the Nanoscale: New Applications of Piezoresponse Force Microscopy and Spectroscopy,” Sergei V. Kalinin,Oak Ridge Natiional Laboratory, hosted by Axel Hoffmann

Abstract: Coupling between the electric fields and strains is ubiquitous on the nanoscale, ranging from piezoelectricity and electrostriction in ferroelectrics to complex phenomena mediated by the ionic currents in energy storage and conversion materials and correlated oxides. In this presentation, I will summarize recent advances in piezoresponse force microscopy applied for studies of bias-induced phase transitions in ferroelectrics and multiferroics, and demonstrate potential for mapping polarization switching on a single-defect level. Phase-field modeling allows the corresponding mesoscopic mechanisms to be deciphered and further suggests strategies for (symmetry forbidden) manipulation of in-plane polarization component. Controlled creation of ferroelectric closure domains is demonstrated.

In the second part of the talk, I will demonstrate how scanning probe microscopy can be use to probe electrochemical behavior of nanoscale volumes based on the strong strain-bias coupling inherent for lithium-ion and oxygen conductive materials. The imaging capability, as well as time and voltage spectroscopies analogous to traditional current-based electrochemical characterization methods are developed. The reversible intercalation of lithium and mapping electrochemical activity in LiCoO2 is demonstrated, illustrating higher lithium diffusivity at non-basal planes and grain boundaries. In silicon anode device structure, the direct mapping of lithium diffusion at extended defects and evolution of lihtium activity with charge state is explored. The electrical field-dependence of lihtium mobility is studied to determine the critical bias required for the onset of electrochemical transformation, potentially allowing reaction and diffusion processes in the battery system to be separated at each location. The scanning probe microscopy measurements are compared with classical characterization methods such as cyclic voltammetry and electrochemical impedance spectroscopy. The prospects for using scanning probe microscopy for battery and fuel cell characterization are discussed.

June 3, 2010

Biological and electrochemical approaches towards tunable synthesis and assembly of nanostructures,” Joun Lee, University of California, RIverside, hosted by Elena Rozhkova

Abstract: Noble metal and semiconducting nanostructures have received much attention for their exciting optical, catalytic, and electrical properties, which can be tuned further by changing size, shape, and composition. A number of conventional synthesis and assembly methods have been studied by taking chemical and physical approaches. However, environmental concerns and the pursuit of highly selective self-assembly have motivated scientists to explore alternative methods for the synthesis and assembly of nanostructures, in particular using biological entities. In this talk, tunable synthesis and assembly techniques using amino acids and DNA will be presented with a combination of electrochemical methods. The research outlined in this talk will describe recent advances on (1) environmentally benign processes to synthesize gold nanostructures in controlled shape and size by use of amino acids, (2) spatially controlled assembly of nanowires via DNA hybridization, and (3) growth of tellurium and palladium on the surfaces of silicon using galvanic displacement.

May 28, 2010

Measurements and Coupling of Electrical and Electromechanical Characteristics of Piezoelectric Thin Films for piezo-MEMS Applications,” Andreas Roelofs, aixACCT Systems Inc., hosted by Amanda Petford-Long

Abstract: Micro-electromechanical systems (MEMS) based on piezoelectric thin films are leading candidates for a broad range of new applications (e.g., energy harvester for autonomous devices and self-controlled sensors for space applications, piezoelectric printer heads and tilted mirror arrays). The characterization of the piezoelectric film properties is essential for device design as well as device simulation and critical for process qualification. Especially process qualification requires characterization of the piezoelectric films after steps such as etching and annealing as the piezoelectric properties can be altered by thermal cycles and exposure to different chemical environments. Therefore, comprehensive electrical and electromechanical characterization of the integrated piezoelectric material is necessary.

Different measurement methods for the investigation of the piezoelectric thin film properties are discussed. The first method presented focuses on dedicated test samples adapted to a four-point bending set-up in which a new characterization method is presented that combines the measurement of the effective longitudinal (d33,f) and the transverse piezoelectric coefficient (e31,f). This new set-up allows fast and accurate measurements of both coefficients on the same sample under well-defined homogeneous mechanical strain. Stress and corresponding strain distributions in the film were verified by finite element simulations.

To ramp up to production, the testing cannot be carried out solely on dedicated test samples; the only feasible method is wafer level testing. This is necessary to capture distributions across the wafer and variations in the manufacturing process. However, in wafer level testing only d33,f can be accessed directly but not e31,f, which is the relevant coefficient for piezo-MEMS functionality.

This issue will be addressed in the second part of the paper where we present a method to determine the correlation between e31,fand d33,f, resolving the wafer level testing challenge.

May 27, 2010

 "Quantum Optical Applications in Spectroscopy: Investigation of Entangled Two-Photon Absorption in Organic Dendritic Systems," Özgün Süzer, University of Michigan - Ann Arbor, hosted by Jeffrey Guest

Abstract: Entangled states of light have received great interest in recent years for the purposes of spectroscopy as well as for applications such as quantum information manipulation, quantum communication, and investigations of the foundations of quantum mechanics. It has been theoretically shown that the interaction of entangled light with matter will exhibit interesting non-classical effects, which have been demonstrated in experiments carried out in our laboratory. This presentation will discuss the application of entangled states of light toward spectroscopy wherein entangled pairs of photons are utilized to excite two-photon transitions in various organic molecules under extremely low excitation flux levels. The entangled-pair flux utilized to excite two-photon transitions in our experiments constitutes approximately 10 orders of magnitude fewer photons than any classical counterpart requires. Further, collection of fluorescent light from an organic dendrimer subsequent to two-photon excitation by entangled pairs of photons is presented. A novel, high geometric efficiency, spherically-enclosed optical collection system for collection of the resulting fluorescence photons is introduced, which is utilized to circumvent any drawbacks related to the low flux conditions under which experiments are carried out and the weak quantum yield of the organic materials. These novel results have widespread impact in applications ranging from spectroscopy to chemical and biological sensing, imaging, and microscopy.

May 26, 2010

"Quantum Optical Applications in Spectroscopy: Investigation of Entangled Two-Photon Absorption in Organic Dendritic Systems," Özgün Süzer, University of Michigan - Ann Arbor, hosted by Jeffrey Guest

Abstract: Entangled states of light have received great interest in recent years for the purposes of spectroscopy as well as for applications such as quantum information manipulation, quantum communication, and investigations of the foundations of quantum mechanics. It has been theoretically shown that the interaction of entangled light with matter will exhibit interesting nonclassical effects, which have been demonstrated in experiments carried out in our laboratory. This presentation will discuss the application of entangled states of light toward spectroscopy wherein entangled pairs of photons are utilized to excite two-photon transitions in various organic molecules under extremely low excitation flux levels. The entangled-pair flux used to excite two-photon transitions in our experiments constitutes approximately 10 orders of magnitude fewer photons than any classical counterpart requires. Further, collection of fluorescent light from an organic dendrimer subsequent to two-photon excitation by entangled pairs of photons is presented. A novel, high-geometric-efficiency, spherically enclosed optical collection system for collection of the resulting fluorescence photons is introduced, which is used to circumvent any drawbacks related to the low flux conditions under which experiments are carried out and the weak quantum yield of the organic materials. These novel results have widespread impact in applications ranging from spectroscopy to chemical and biological sensing, imaging, and microscopy.

May 26, 2010

"Oxygen Reduction Reaction on Core-Shell Metal Alloy Catalysts: Density functional Theory Studies," Pussana Hirunsit, Texas A&M University, hosted by Jeffrey Greeley

Abstract: Platinum-based alloys are used to catalyze the molecular oxygen decomposition and its subsequent reduction to water on fuel cell electrodes. In particular, core-shell structures with a monolayer of platinum on the surface have been proposed and are being intensively tested to determine activity and durability in acid medium. In this presentation, I will report results of surface segregation and other factors that influence oxygen reduction reaction (ORR) activity and surface stability against dissolution. Density functional theory (DFT) analysis of calculated surface Pourbaix diagrams and reaction mechanisms provides useful predictions on onset water oxidation potential, surface atomic distribution, most stable coverage of oxygenated species, and activity. I will first discuss structures proposed by Adzic and collaborators, consisting of a surface platinum-skin monolayer over an IrCo or Ir3Co core, with or without a palladium interlayer between the platinum surface and the Ir-Co core. I will emphasize the role of the palladium interlayer in the core-shell structures that have experimentally shown higher ORR activity relative to Pt(111) surfaces. Second, I will analyze a new core-anchor-shell material that, based on DFT calculations, is predicted to have improved stability and maintain relatively high ORR activity. Here I will focus on the stability and activity enhancement of Pt/Fe-C/core, Pt/Co-C/core and Pt/Ni-C/core demonstrated with core materials of Ir, Pd3Co, Ir3Co, IrCo and IrNi.

May 24, 2010

"Ubiquinone analogs and their reactivity with the bc1 complex and a cytochrome b mutant," Diana Cedeno, University of Arizona, hosted by Elena Rozhkova

Abstract: Cytochrome bc1 (Complex III) is an important enzyme that takes part in the respiratory electron transport chain in vertebrates, yeast, and many bacteria. The complex exists as a dimmer and each monomer contains three catalytic subunits: cytochrome c1, cytochrome b and the Rieske iron-sulfur protein or ISP. Within the inner mitochondrial membranes of eukaryotes, Complex III catalyzes the transfer of two electrons from ubiquinol (UQH2) to cytochrome c, a water-soluble protein. At a separate site of the same complex, two electrons are sequentially transferred from a ubiquinone molecule (UQ) to form UQH2. These redox processes and the associated proton transfers ultimately establish a proton gradient across the membrane that is used to drive ATP synthesis. This process is known as the Q-cycle mechanism. Under very specific conditions, avoidance of the Q-cycle mechanism leads to bypass reactions including the formation of superoxide and reactive oxygen species. The structure of UQ consists of a p-quinone head group and a hydrophobic isoprene unit (tail) that can vary in length depending on the species in which it is found. The present work highlights modifications to the substituent groups attached to the quinone head and to the length of the isoprene unit. Since the midpoint potentials of these molecules are pH dependent, cyclic voltammetry and spectroelectrochemistry studies in buffered aqueous solutions have been carried out on these molecules (analogs of UQ). Modifications of the substituent groups attached to the quinone head gave the molecules a different ability to either donate or receive electrons, while modifications to the length of the tail either increased or decreased the solubility of these molecules inside the phospholipid membrane. We examined the normal activity and the production of superoxide in wild-type and a cytochrome b mutant (T61V) of bacterial Rhodobacter sphaeroides in the presence of these analogs. We confirmed that, to prevent damaging side reactions, normal operation of the Q-cycle requires a fairly narrow window of reduction potentials with respect to the ubiquinol substrate.

May 13, 2010

"Stability in a Turbulent (Fermi) Sea: The Ever More Remarkable High-Temperature Superconductors ," Eric Hudson, Massachusetts Institute of Technology, hosted by Axel Hoffman

Abstract: For over two decades high-temperature superconductivity has captured the attention of scientists the world round. However, rather than finding a simple explanation for the properties of these materials, as was done for their low-temperature cousins half a century ago, intensive research has instead led to an increasingly complex picture of materials characterized by an intricate phase diagram, full of competing or coexisting states, yet still dominated by a superconducting state that persists, at least in some materials, almost halfway to room temperature.

In this talk I will describe nanoscale investigations of the electronic structure of high-temperature superconductors using scanning tunneling microscopy. We have recently found that a still-not-understood high-temperature phase in these materials, the pseudogap, is characterized by strong charge inhomogeneity. Surprisingly, although this disorder persists into the superconducting state, it does not seem to perturb coexisting homogeneous superconductivity. The resolution of this apparent contradiction gives new insight into the onset of superconductivity and its relationship with the pseudogap phase.

May 4, 2010

"Uncovering the Atomic-Scale Physics of Graphene," Gregory M. Rutter, National Institute of Standards and Technology, hosted by Nathan Guisinger

Abstract: Graphene, a two-dimensional honeycomb lattice of sp2-bonded carbon atoms, has received considerable attention in the scientific community due to its unique electronic and mechanical characteristics. Distinct symmetries of the graphene wave functions lead to unique properties, such as reduced back scattering and a half-integer quantum Hall effect. These quantum properties combined with its atomic thickness make graphene enticing for post-CMOS electronic applications. However, before graphene can be used as a new paradigm for nanoelectronics, a complete understanding of its electronic properties at the atomic scale is vital.

In this presentation, I will illustrate how scanning tunneling spectroscopy (STS) can be effectively used to investigate the atomic scale properties of graphene. First, I show how point defects can be used as local probes of the electronic structure via images of the energy-dependent standing-wave patterns that appear in maps of the differential conductance. Simple Fourier transform analysis can extract scattering wave vectors, leading to a local measurement of the energy-momentum dispersion, E(k). Then, I will discuss how STS can be used to uncover the unique magnetic quantization of graphene, the hallmark of the half-integer quantum Hall effect. In addition, I will show how such observations can be used to judge the quality of the graphene. Finally, recent STS data of exfoliated graphene on SiO2 in high magnetic field will be discussed. I will show clear evidence of single-electron charging and spatial mapping of localized states in the quantum Hal l regime.

May 3, 2010

"Advanced Confocal Microscopy," Wibool Piyawattanametha, National Electronics and Computer Technology Center (Thailand), hosted by Daniel Lopez

Abstract: Biomedical research needs new advances in imaging. Existing modalities of in vivo imaging, such as magnetic resonance imaging (MRI) or ultrasound, lack the spatiotemporal resolution required to image the fundamental building block of living tissue, namely ,the cells. By contrast, existing high-resolution techniques for imaging cells and their subcellular features are technologies that are best suited for in vitro experiments in tissue slices. Yet, the ability to make direct connections between human pathological symptoms and behavior and the underlying cells and molecules responsible for such behavior requires in vivo techniques that can image cellular constituents.

My research aim is to develop novel high-resolution (submicron to 5-micron) optical endoscopes to satisfy unmet needs in the clinical environment. These differ from medical endoscopes, which are generally larger and designed to image macroscopic abnormalities. Most importantly, this novel optical endoscopic imaging might suggest new approaches to disease diagnoses and treatment. This talk will be focused on the development of a novel confocal imaging modality integrated with microelectromechanical systems (MEMS) technology and their imaging applications.

Confocal microscopy is an attractive tool for three-dimensional imaging because of its optical sectioning property. Conventional single-axis confocal microscopes have a trade-off among resolution, field of view, and objective lens size, since a high numerical aperture lens is needed for sufficient resolution, and a long focal length is needed for a large field of view and working distance. A dual-axis confocal (DAC) microscope architecture has been proposed that uses two overlapping low numerical aperture beams and effectively decouples these trade-offs.

The DAC architecture offers several advantages over the traditional single-axis confocal architecture, such as simplicity in miniaturization from deploying low numerical aperture lenses and aberration-free beam scanning from post-objective scanning configuration. Another important advantage is the ability to achieve a much superior optical sectioning. The microscopes are miniaturized into form factors (5- and 10-mm diameter). The imaging demonstrations of the probes will be on both ex vivo and in vivo samples from mice and human for cancer oncology and genetic research.

April 29 2010

"Nanostructuring Thin Films for Optical Applications," Rosalia Serna, Instituto de Optica (Spain), hosted by Amanda Petford-Long

Abstract: The development of thin films with functionalized optical response is relevant in technological areas such as photonics and photovoltaics. Optical doping by incorporating rare-earth ions and/or nanoparticles is a particularly useful method to produce active materials with novel and improved optical response, including light emission, optical gain, and enhanced nonlinear optical properties. This talk will show how nanostructured optically doped dielectric thin films are prepared by pulsed laser deposition with a control within the nanometer scale on the dopant location, and how such a nanometric control is a powerful tool to provide a new insight in the understanding of ion-ion and nanoparticle-ion energy transfer processes. Special consideration will be given to recent results on the enhancement of Er3+ infrared emission by codoping with silicon nanoparticles. The erbium-silicon nanoparticle codoped films benefit from the broad absorption band (ultraviolet-visible) of the silicon nanoparticles and their ability to transfer the absorbed energy to a nearby Er3+ ion. However, for an efficient performance this system requires the formation of silicon nanoparticles with an appropriate size and density, and an erbium spatial distribution that maximizes the number of ions separated from a silicon nanoparticle at a distance below the interaction length (≤ 1 nm). The design of nanostructured doped films allows successfully meeting these demands and photoluminescence measurements show that this method allows to obtain the larger fraction of erbium ions efficiently excited through silicon nanoparticles ever reported.

April 26, 2010

"Ultrafast Spectroscopy Study of Hot Carrier Dynamics and Coherent Controlled Photocurrent Generation in Epitaxial Graphene," Dong Sun, University of Michigan–Ann Arbor, hosted by Jeff Guest

Abstract: We have performed ultrafast pump-probe experiment in a wide range of wavelength on epitaxial graphene to study the ultrafast relaxation of hot Dirac fermionic quasiparticles. Our infrared DT spectra are well described by interband transitions with no electron-hole interaction. Our experiment time resolves the optical phonon and acoustic phonon scattering process. We also observed thermal coupling of hot carriers between graphene layers in epitaxial graphene and thermal coupling to the SiC substrate, both coupling are strong in the first few picosecond at high carrier temperature and significant slows down when electrons cools down. This indicates a high energy optical phonon related thermal coupling at initial cooling stage. Another work we can do with ultrafast spectroscopy is spectrally resolving the precise doping profile of heavily doped layers. We determined the screening length to be 1 layer in carbon face grown epitaxial graphene using ultrafast 800 nm pump, mid-infrared probe spectroscopy.

On the other hand, we have generated coherently controlled electrical currents in epitaxial graphene using both 3.2 μm/1.6 μm and 4.8μm/2.4 μm, 280fs pulses. These ballistic currents depend on relative phases between pulses. By pre-injection background hot carriers in the system and cool down to low temperature with a novel near field THz detection technique, we have studied effect of hot carriers in the coherent controlled photocurrent generation.

April 21, 2010

"Nanostructured Organic Semiconductor Solar Cells," Charles T. Black, Brookhaven National Laboratory, hosted by Amanda Petford-Long

Abstract: Solar cells based on organic semiconductors inspire a long-term vision of electrical power generation from low-cost materials. A reason for optimism is that these materials can convert incident photons to electrons with high quantum efficiencies. Devices based on organic semiconductors operate with maximum overall power conversion efficiencies of ~5-7%, half of their potential and a quarter of the thermodynamic efficiency limit for a single-junction solar cell.High-performing organic bulk heterojunction active layers form via a self-assembly process of phase separation of blended donor and acceptor materials. Optimizing the device performance is a delicate balance of trapping the blended material in a nonequilibrium configuration.

I will discuss examples of our experimental efforts to improve organic bulk heterojunction solar cell performance by modifying the internal device architecture. In one case, we have incorporated nanostructured electrodes into the bulk heterojunction active layer to improve photovoltaic performance by shortening the travel distance for dissociated free charge carriers. Nonplanar electrodes permit a 20% increase in the blend layer thickness for optimal device performance, enhancing photocurrent output and associated photovoltaic power conversion efficiency by as much as ~10%. We are also pursuing a self-assembly-based approach to confining both organic semiconductors and semiconductor blends within nanometer-scale volumes for better control of material phase separation and understanding the effects of geometry on material structure, electronic properties, and photovoltaic performance. Changes in polymer chain alignment within the confined volumes lead to electronic mobility enhancements of more than two orders of magnitude, with associated improvements in solar cell efficiency.

April 13, 2010

"Basic characterization of Nanoparticles by X-ray absorption based-techniques: effect of environment on their fundamental properties," Felix G. Requejo, National University of La Plata, Argentina, hosted by Elena Shevchenko

Abstract: Basic properties of nanoparticles depend on (i) morphology and size , (II) crystal structure, and (iii) composition. Nanoparticles are usually too small to have bulk-like properties and, because of surface effects, they avoid interaction with the substrate, support, capping (from the synthesis process), or solvent media where they are immersed. A comprehensive characterization (with local-order sensibility, chemical selectivity, suitable to be performed in situ conditions) is mandatory to understand the correlation between "structure" and "property," and consequentially controlled and intelligent design in nanomaterials with specific aims.

In this work, we discuss the analysis of the distortion of homogenously dispersed and shaped colloidal platinum nanoparticles of about 2-nm diameter by their interaction with SBA-15 mesoporous silica and the effect of PVP capping on the thermodynamics of platinum nanoparticles, and the effect on synthesis route of the final atomic distribution in PtCo nanoparticles. Our independent characterization tools include those based on synchrotron radiation sources such as small-angle X-ray scattering (SAXS) and X-ray Absorption fine structure (XAFS).

April 7, 2010

"Ultrafast Dynamics of Photoexcited Bismuth Films," Yu-Miin Sheu, University of Michigan, hosted by Jeffrey Guest

Abstract: The carrier and lattice relaxation processes following photoexcitation in solids occur over time scales ranging from femtoseconds to nanoseconds. The eventual conversion of the light to lattice heating involves carrier-carrier, carrier-phonon, and phonon-phonon interactions. More fundamental understandings of these processes may lead to advances in thermoelectrics, photovoltaics, and other technologically important materials. Even for bismuth, a well-studied thermoelectric material, detailed information on these processes is still unavailable. I will present ultrafast optical andXx-ray studies of photoexcited carrier diffusion and recombination, acoustic phonon generation and propagation, and lattice heating and diffsion in thin bismuth films. The combination of laser and x-ray experiments confims that carriers relax by rapidly heating the lattice before diffusing and ultimately recombining, leaving an inhomogeneous temperature profile near the surface. We observe a temperature di scontinuity across Bismuth/sapphire interface and derive Kapitza conductance by depth- and time-resolved X-ray diffraction. Comparing counter-propagating and conventional pump-probe measurements at low excitation, we find that the carrier density is not determined by the electron-hole plasma temperature after a few picoseconds.

April 6, 2010

"Fabrication and characterization of plasmonic thin films and  fluorescence spectroscopy based on an integrated optofluidic chip," Aiqing Chen,University of California, Santa Cruz, hosted by Jeffrey Guest

Abstract:: I will present my doctoral research on the fabrication and optical characterization of nanostructured ordered and disordered plasmonic thin films, specifically (a) polarization-sensitive optical response from circular and elliptical cross-section gold nanopillar arrays, ordered nanostructures prepared by electron beam lithography and (b) design and fabrication of large, broadband asymmetric mirrors through disordered silver nanoparticle coatings based on Tollen's reaction. Such nanoparticle coatings are extended to decorate the surface of silica microspheres (nanoshells) and self-assembly photonic crystals. Potential application such as increasing light trappings for thin-film solar cells with disordered silver nanoparticle coatings will be addressed.

I will also introduce my current research on fluorescence resonance energy transfer (FRET) and fluorescence correlation spectroscopy (FCS) based on an integrated optofluidic chip. FRET and FCS are two useful techniques to study the dynamics of biomolecules. Here we demonstrate a novel way of measuring FRET from oligonucleotides using an integrated optofluidic chip containing a planar liquid-core waveguide that can guide liquid and light simultaneously. By photobleaching the acceptors and manipulating the fluidic flow, we also demonstrate controllable FRET events: an increase in donor signal, a decrease in acceptor signal and the recovery of FRET due to the influx of new FRET pairs. I will also introduce the first implementation of two-color fluorescence cross correlation spectroscopy in liquid-core waveguide and apply the technique to discriminate between singly and doubly labeled fluorescent nanobeads.

March 10, 2010

"Molecular Architectures based on Heteroborane Clusters: Electronic Structure and Applications in Nanosciences," Josep M. Oliva, Institute of Chemical-Physics (Spain), hosted by Stephen Gray

Abstract: In the last ten years, we have been interested in determining interesting properties derived from borane and heteroborane clusters, isolated and connected among them in different dimensions. Since their first syntheses in the 1960s, heteroborane clusters may become interesting tools for applications in recently developed nanosciences, such as ion/spin transport within molecular networks.

March 5, 2010

"Tuning between the Ferromagnetic and Antiferromagnetic Phases of La(1-x)Sr(x)MnO3 by Digital Synthesis,” Tiffany Santos, Distinguished Postdoctoral Fellow, Center for Nanoscale Materials, Argonne National Laboratory, hosted by Matthias Bode

Abstract: The perovskite manganite La(1-x)Sr(x)MnO3 has a rich magnetic and electronic phase diagram, exhibiting ferromagnetism for La-rich compositions and antiferromagnetism for those that are Sr-rich. Our interest lies in the region near the x=0.5 doping level containing the F-AF phase transition, particularly the role of disorder and strain in nucleating the F or AF state. Using ozone-assisted molecular beam epitaxy, we have synthesized fully-epitaxial superlattices of LaMnO3 and SrMnO3, designed to be equivalent in composition to random alloys of La(1-x)Sr(x)MnO3 in the vicinity of x=0.5. In our digital synthesis method, whereby we interleave single unit-cell layers of undoped LaMnO3 and SrMnO3 layers, we are able to tune between the ferromagnetic and antiferromagnetic metallic states by inserting an extra LaMnO3 or SrMnO3 layer, respectively. We have achieved atomic layer precision in the synthesis of these superlattices, as confirmed by our structural characterization.

Using neutron diffraction we have verified the A-type antiferromagnetic spin structure and measured a Néel temperature of 300 K, which is an enormous enhancement of 160 K over the value for bulk (unstrained crystal) La(0.5)Sr(0.5)MnO3. A large enhancement was also found for x=0.55. However, this material is on the verge of ferromagnetism. We find that inserting an additional single unit cell layer of LaMnO3 into the superlattice, and thereby delta-doping a layer of electrons, causes a significant increase of the net magnetic moment while still retaining the A-type spin structure. Our polarized neutron reflectometry experiments revealed a highly modulated moment commensurate with the structural periodicity of the superlattices, with higher moment in the region of the extra LaMnO3 layer. Thus, introducing a single La dopant layer results in a localized enhancement of ferromagnetic double exchange along the c-axis and a canted moment in an otherwise antiferromagnetic structure. The polarized neutron reflectometry measurements determined the length scale over which these delta-doped charges extend normal to the interfaces.

March 4, 2010

"Electrical and magnetic properties of epitaxial (SrMnO3)n/(LaMnO3)2 superlattices," Carolina Adamo, Cornell University, hosted by Matthias Bode

Abstract:: A variety of interesting and unexpected electronic and magnetic phenomena have been observed at the interfaces between different oxides and the role of the charge transfer across those interfaces has been actively studied in the recent years. In this context, (SrMnO3)n/(LaMnO3)2n superlattices, consisting of n planes of SrMnO3 alternated by 2n planes of LaMnO3, show a remarkable interfacial modification of the bulk properties of the two constituent materials: Although SrMnO3 and LaMnO3 are insulators, (SrMnO3)n/(LaMnO3)2n superlattices can become metallic. Dynamical mean field theory calculations have suggested that the metallic behavior nucleates from an electronic reconstruction at the interface and that the charge transfer effects extend deeply in the layers away from the interfaces, thus leading to original electronic and magnetic properties of the superalttices as a whole. In particular, (SrMnO3)n/(LaMnO3)2n superlattices are particularly interesting for having the same La/Sr stoichiometry ratio (i.e., Mn3+/Mn4+) as La2/3Sr1/3MnO3 (LSMO), the optimally doped Sr-based manganite.

However, in bulk LSMO La/Sr A-sites are randomly populated resulting in a disordered Coulomb trapping potential, whereas in SMOn/LMO2n superlattices an ordered sequence of SrMnO3 and LaMnO3 planes occurs. As a consequence the Mn3+/Mn4+ mixed valence arises only on the interfacial MnO2 planes, which are thus ferromagnetic (FM) and metallic (M) as opposite to the individual constituent blocks, where the MnO2 planes are antiferromagnetic (AF) and insulating (I). In fact LaMnO3 is a Mott insulator and SrMnO3 can be considered as a high spin "band" insulator. As a result, along the MnO2 plane sequence one should expect a region of transition from pure FM to pure AF ordering, possibly characterized by a spin canting regime.

The resistivity temperature dependence has been investigated by varying n from 1 to 8,. It can be observed that the (SrMnO3)n/(LaMnO3)2n superlattices with n ? 2 behave like the FM conductor LSMO. As n increases, themagnetic properties become dominated by the LaMnO3 layers, but the electronic transport properties continue to be controlled by the interfaces.

March 3, 2010

"Engineered Magneto-electric Heterostructures," Jaydip Das, Virginia Tech, hosted by Matthias Bode

Abstract: Engineered magneto-electric (ME) monolithic heterostructures are an extremely attractive option for high-performance, low -ost, and energy-efficient logic devices, sensors, communications, and RF applications. Such layered and composite structures can offer two kinds of cross-tuning: 1) electric field control of the magnetic response and 2) magnetic field control of the electric response. I will show evidence of both mechanisms for controlling thin-film ME heterostructures. Electric field control of the magnetic response in monolithic structures allows one to achieve a tuning of the ferromagnetic resonance frequency at 10- to 25-V applied voltages in the 10- to 60-GHz frequency range. All-oriented multilayered structures with low-loss yttrium iron garnet or barium hexaferrite and barium strontium titanate show frequency shifts of 2 MHz/V and 3.5 MHz/V at 10 and 60 GHz, respectively. These structures indicate a factor-of-10 improvement over the previous works on non-m onolithic structures and demonstrate substantial hybrid mode ME coupling between the magnetic and electric layers.

Further, our recent results for an ordered cobalt ferrite-bismuth iron oxide nano-array structure demonstrate a local control in the magnetic responses with electric fields. The tuning of electric response with magnetic fields allows one to use ME structures as magnetic field sensors. I will briefly present our approaches to develop highly sensitive ME laminate sensors at Virginia Tech. These optimized structures show magnetic field sensitivity as low as 0.3 nT and better object sensing capacity than some of the presently available sensors. Moreover, this work will present initial attempts to make semi-monolithic ME heterostructures that shows a control in the electric response with magnetic fields.

February 11, 2010

"Diruthenium Alkynyl Compounds and Their Role in Molecular Electronics," Tong Ren, Purdue University, hosted by Al Sattelberger and Derrick C. Mancini

Abstract:Our long-term goal is to achieve both molecular wires and devices based on Ru2-alkynyl species, for which facile charge transfer across the conjugated backbone of Ru2-alkynyl species is the key. Charge transfer properties have been carefully examined in Ru2- alkynyl compounds of extended carbon bridges and the voltammetric and spectroelectrochemical measurements revealed the facile electron transfer across both the carbon-rich bridges and the Ru2 fragment. Scanning tunneling microscopy study of Au-bound Ru2(ap)4((C≡CC6H4)2S)- indicated that the intrinsic conductance was significantly improved over the pure organic species of comparable lengths. The current-voltage characteristics of related compounds trans-(S(C6H4C≡C)k)Ru2(ap)4((C≡CC6H4)kS)- was also investigated using electromigration break junction technique, and highly reproducible conductance switching were uncovered at low potential bias.

February 5, 2010

"Nanostructured Semiconducting, Ferroelectric, and Multiferroic Crystals: Synthesis, Characterization and Energy Application," Jun Wang, Iowa State University, hosted by Elena Shevchenko

Abstract: Dye-sensitized solar cells (DSSCs), a photo-electrochemical technology, are one of the most promising of several alternative cost-effective concepts for solar-to-electric energy conversion that have been offered to challenge conventional silicon solar cells over the past decade. Compared with sintered TiO2 nanoparticle films, highly ordered TiO2 nanotube arrays possess excellent electron percolation pathway for vectorial charge transfer. Functional and multifunctional nanocrystals have attracted tremendous interest in various fields, such as energy conversion, bio-imaging, and drug delivery. Much attention has thus been given to the controlled synthesis of nanocrystals and their surface functionalization. In this presentation, our efforts on the synthesis and energy applications of various functional nanostructured materials will be presented. First I will introduce the electrochemical synthesis of nanoporous metal oxide thin films (i.e., porous alumina membrane and TiO2 nanotube arrays) and application of highly ordered TiO2 nanotube arrays for dye-sensitized solar cells where markedly improved photovoltaic performance is induced by rational surface engineering on TiO2 nanotubes. After that, I will show our recent progress on functional and multifunctional nanocrystals, including CdSe quantum dots, Cu2ZnSnS4 (CZTS) nanoparticles, TiO2 nanoparticles, magnetic nanoparticles, BaTiO3, and BiFeO3. These nanocrystals possess potential applications in solar cells, light emitting diodes, biosensors, thin-film capacitors, transducers, actuators, and magnetically recorded ferroelectric memory.

January 27, 2010

"Phosphorylated Peptide Enrichment Based on Nanoporous Metal Oxides," Dong-Keun Lee, Michigan State University, hosted by Elena Shevchenko

Abstract: Reversible protein phosphorylation is a key regulatory process in controlling many cellular events, such as cell cycle, cell growth, cell differentiation, and metabolism. To achieve detailed insights into the regulation of these reversible phosphorylation processes, it is often necessary to characterize the phosphorylation sites of specific proteins. However, the identification of phosphopeptides including phosphorylation sites by analytical technique, such as mass spectrometric methods, remains challenging. Therefore, specific isolation and enrichment of phosphorylated peptides are needed prior to the analytical process. So far the most common enrichment method is immobilized metal affinity chromatography (IMAC). However, because of the less selective interaction between phosphorylated proteins and the separation media, this technique still has limitations.

In the present work, mesoporous metal oxide films were designed and synthesized for the enrichment and separation of peptides down to pico- or femto-mole level. These functionalized media have rigid framework structures, regular mesoscopic pores (2.0 – 50 nm), and high surface areas (500 – 1200 m2g-1), which are a great advantage for protein separation. To increase the selectivity toward phosphopeptides, the mesoporous phases have been organically modified with diazo groups for coupling to the phosporylated proteins. In addition, other high-surface-area metal oxides films (functionalized silica, titania, and alumina) were synthesized, which also improves the selectivity and sensitivity toward phosphate enrichments in small volume fractions. Esterified and nonesterified tryptic digests of ovalbumin and β-casein were used to demonstrate phosphopeptide enrichment. The properties of the porous media and the methods used to achieve surface functionalization and phosphorylated peptides enrichment will be discussed.

January 20, 2010

"A Discussion of the Utility of Dealloyed Nanoporous Metals for Electrocatalysis," Jonah Erlebacher, Johns Hopkins University, hosted by Jeffrey Greeley

Abstract: Dealloying refers to the electrochemical dissolution of a majority component from a uniform multicomponent alloy. In certain controlled cases, the remaining components diffuse along the metal/electrolyte interface to restructure each grain of the polycrystalline alloy to possess high surface area and open porosity. Depending on the system of interest, one may find extremely small pores and extremely high surface areas, rivaling nanoparticles but with the added advantage of good electrical contact to all surfaces (e.g., ~2-nm ligaments and pores with ~50 m^2/g in dealloyed Ni/Pt). Dealloyed metals are also quite beautiful porous materials where the underlying crystallography is strikingly apparent, such as in dealloyed Ag/Au alloys. In this talk, I will discuss our current understanding of the physics and chemistry controlling the competition between dissolution and surface diffusion that leads to porosity evolution, as well as electrochemical methods to control pore and ligament size and the relative core/shell compositions of the dealloyed materials.

Dealloyed nanoporous materials naturally find utility in electrochemical catalysis, and we will also discuss their activity toward electrochemical oxygen reduction in aqueous solution. Oxygen reduction is famous for its inefficiency in hydrogen/oxygen fuel cells and the dealloyed Ni/Pt system is quite interesting for this application. Large roughness factors, well over 100, are easily fabricated, and we have measured half-waves for oxygen reduction in rotating disk electrode experiments over 0.98 V vs. RHE, a less-than 250 mV over potential. The origin of this effect is related to the high surface area, but not in a simple way, and we will argue that the effective "active area" of the porous metal is itself dependent on the overpotential. Finally, we will discuss composite nanoporous metals that further improve oxygen reduction activity.

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