“Film Thickness and Elastic Strain Measurements on Silicon-on-Insulator Thin Films,” I. Cevdet Noyan, Columbia University, hosted by Jorg Maser

Abstract: Silicon-on-insulator (SOI) composites consist of two semiconductor-grade silicon layers bonded to each other via a SiO₂ interface. One of these silicon layers is quite thin; it is possible to get thicknesses between 5 and 150 nm. Since this value is much thinner than the extinction distance of X-rays in silicon for commonly used energies, this layer diffracts in the kinematical mode. The second layer is much thicker, around 700 micrometers, and diffracts in the dynamical mode. Both layers can be considered almost perfect, with negligible mosaic structures and no dislocations.

We have been using SOI composites to test the accuracy and precision of standard X-ray analysis techniques. This talk will describe our experiments in measuring thickness and “strain” from such substrates. The thickness measurements are validated against cross-sectional TEM measurements. Deposition of stressor films and four-point bending were employed to introduce known strain profiles. Our results indicate that, while thickness measurement using diffraction can be quite precise and accurate when performed properly, “strain” measurement requires more care, especially in interpretation of the results.

“All-Conjugated Block Copolymers for High-Performance Polymer Photovoltaics,” Rafael Veruzco, Rice University, hosted by Seth Darling

Abstract: Polymer-based organic photovoltaics (OPVs) have significant potential for providing cheap and scalable solar energy. However, efficiencies of polymer-polymer and polymer-fullerene OPVs are inadequate for widespread use, and a thorough understanding of structure-property relationships associated with polymer OPVs is lacking.

In this work, we have prepared all-conjugated block copolymers with both hole- and electron-conducting polymer blocks to improve performance and address fundamental questions regarding the structure and optoelectronic properties of OPV blends. The synthesis and characterization of two different types of conjugated block copolymers will be presented. First, copper-catalyzed azide-alkyne "click" chemistry is used to synthesize all-conjugated block copolymers with a flexible linker. In a second approach, conjugated polymers are end-capped with a small molecule that can form self-complementary quadrupole hydrogen bonding associations. Physical associations mediated by the endgroups prevent large-scale phase separation in polymer-polymer blends.

The microstructure, including microphase segregation and crystallinity, is quantitatively analyzed by using a combination of X-ray diffraction, grazing-incidence X-ray scattering, and atomic force microscopy. Fluorescence quenching measurements show that energy transfer is more efficient in block copolymers compared with homopolymer blends. This work can lead to improved performance in polymer OPVs and, more importantly, provide quantitative information on the relationship between microstructure and performance in OPVs.

“Tuning the Properties of Individual Dopants in Semiconductors,” Jay Gupta, The Ohio State University, hosted by Jeff Guest

Abstract: The scaling of transistors to nanometer dimensions requires more precise control of individual dopants in semiconductor nanostructures, as statistical fluctuations in dopant distributions can significantly impact device performance. Proposals for next-generation quantum- and spin-based electronics also rely on the tuning of the charge, spin, and interactions of dopant atoms with local electric fields. Using a scanning tunneling microscope (STM), we demonstrate how to control the binding energy and ionization state of individual acceptors in p-GaAs. Charged species, such as native dopants, vacancies, and adatoms, directly influence the acceptor binding energy via the Coulomb interaction. In addition, a combination of defect- and tip-induced band bending can be used to remotely tune the acceptors' ionization state. We find that by applying voltage pulses with the STM tip, charged vacancies and adatoms can be positioned on the surface. These experiments suggest a new and direct method for quantifying the charge of adsorbates (e.g., adatoms or molecules) as well as defects (e.g., vacancies, antisites, interstitials) at semiconductor surfaces.

“Engineering Colloidal Quantum Dot Optoelectronic Devices,” Ted Sargent, University of Toronto, hosted by Yu-Chih Tseng

Abstract: To be efficient, solar photovoltaics must match their absorption spectrum to the sun's spectrum reaching the earth's surface. Our group focuses on using colloidal quantum dots – solution-synthesized, solution-processed, quantum-size-effect-tunable materials – to build low-cost solar cells that offer a route to high efficiencies through spectral utilization matched to the sun. The community has made great progress in recent years, achieving 6% solar power conversion efficiencies half a decade after the first reports of infrared solution-processed photovoltaics. Advances include the realization of densely packed, well-passivated colloidal quantum dot solids based both on short organic and novel small inorganic ligands. I will review the latest advances and discuss the prospects for the field, including the advances in materials chemistry needed to bring CQD PV above 10% solar power conversion efficiencies.

"Anisotropic Semiconductor Nanocrystal Synthesis and Chemical and Biological Functionalization," Preston T. Snee, University of Illinois - Chicago, hosted by Richard Schaller

Abstract: Semiconductor nanocrystals (NCs, or quantum dots) are very bright chromophores that possess significant potential in alternative energy generation and for biological sensing and imaging applications. Our group has made significant advances in the synthesis of rods and multi-pods of near-infrared emitting PbSe NCs through a previously unobserved mechanism. Characterization of anisotropic PbSe NCs show that they have much more robust chemical properties compared to cubic or "dot"-shaped NCs.

Also presented are recent advances on the chemical and biological functionalization of bright NCs. While there are ostensibly many methods for chemical functionalization of water soluble NCs, most of the reagents used in these methods either quench the NCs or have very low reaction yields. We have circumvented these problems by synthesizing polymers which serve as NC functionalization reagents; the polymer-NC-activated intermediate has increased stability and allows us to conjugate chemical and biological vectors to the NCs with 100% reaction yield. We use this method to functionalize NCs with organic fluorophores that can report on the local chemical and biological environment. We have synthesized several ratiometric, or "self-calibrating" sensors, for pH, toxic metals, DNA, and proteins. In our recent work on protein sensing, we have developed an all optical method for sensing un-labeled proteins with a better detection limit than any currently existing technology.

"'Listening' to the Spin Noise of Electrons and Holes in Semiconductors," Scott A. Crooker, Los Alamos National Laboratory, hosted by Matt Pelton

Abstract: Not all noise in experimental measurements is unwelcome. Certain fundamental noise sources contain valuable information about the system itself — a notable example being the inherent voltage fluctuations (Johnson noise) across any resistor, from which temperature can be determined. In magnetic systems, fundamental noise can exist in the form of random spin fluctuations. For example, statistical fluctuations of N paramagnetic spins should generate small noise signals of order sqrt(N) spins, even in zero magnetic field. In accord with the fluctuation-dissipation theorem, the spectrum of these fluctuations — if experimentally measurable — can reveal the dynamical properties of the spins (such as g-factors and spin decoherence times) without ever perturbing the spin ensemble from thermal equilibrium.

This talk describes how we measure electron and hole spin dynamics in semiconductors by passively “listening” to these small spin noise signals. We employ a spin noise spectrometer based on a sensitive optical Faraday rotation magnetometer that is coupled to a digitizer and field-programmable gate array (FPGA), to measure and average noise spectra from 0-1 GHz continuously in real time (no experimental dead time) with picoradian/root-Hz sensitivity. This approach, applied originally to paramagnetic atomic vapors, is now being used to measure spin noise from electron Fermi seas in n-type GaAs and, more recently, from electron and hole spins that are localized in self-assembled InGaAs quantum dot ensembles. Both electron and hole spin fluctuations generate distinct noise peaks, whose shift and broadening with magnetic field directly reveal their g-factors and dephasing rates. These noise signals actually increase as the probed volume shrinks, suggesting possible routes towards non-perturbative, sourceless magnetic resonance of few-spin systems.

Octrober 12, 2011

"Topological Phases of Matter and Why You Should Be Interested," Steven H. Simon, Rudolf Peierls Center for Theoretical Physics, Oxford University, hosted by Daniel Lopez

Abstract: In two-dimensional topological phases of matter, processes depend on gross topology rather than detailed geometry. Thinking in 2+1 dimensions, particle world lines can be interpreted as knots or links, and the amplitude for certain processes becomes a topological invariant of that link. While that sounds rather exotic, we believe that such phases of matter not only exist but have actually been observed in quantum Hall experiments, and could provide a uniquely practical route to building a quantum computer. Possibilities have also been proposed for creating similar physics in systems ranging from superfluid helium to strontium ruthenate to semiconductor-superconductor junctions to spin systems to cold atoms.

September 21, 2011

"Turning a Single Molecule into an Electric Motor," Charles Sykes, Tufts University, hosted by Erin Iski

Abstract: In stark contrast to nature, current manmade devices, with the exception of liquid crystals, make no use of nanoscale molecular motion. In order for molecules to be used as components in molecular machines, methods are required to couple individual molecules to external energy sources and to selectively excite motion in a given direction. Significant progress has been made in the construction of molecular motors powered by light and by chemical reactions, but electrically driven motors have not been demonstrated yet, despite a number of theoretical proposals for such motors. Studying the rotation of molecules bound to surfaces offers the advantage that a single layer can be assembled, monitored, and manipulated by using the tools of surface science. Thioether molecules constitute a simple, robust system with which to study molecular rotation as a function of temperature, electron energy, applied fields, and proximity of neighboring molecules. A butyl methyl sulphide (BuSMe) molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunneling microscope are used to drive directional motion of the BuSMe molecule in a two-terminal set-up. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. The direction and rate of the rotation are related to the chiralities of the molecule and the tip of the microscope (which serves as the electrode), which illustrates the importance of the symmetry of the metal contacts in atomic-scale electrical devices.

August 31, 2011

"Coated Nanoparticles in Solution and at Interfaces," Gary S. Grest, Sandia National Laboratories, hosted by Jeff Greeley

Abstract: Among the most prevalent ways to control the assembly and integration of nanoparticles is to coat them with organic molecules whose specific functionalized groups modifies their interparticle interactions as well as the interaction of nanoparticles with their surrounding, while retaining their inherent properties. While it is often assumed that uniformly coating spherical nanoparticles with short organic molecules will lead to symmetric nanoparticles, I will use explicit-atom molecular dynamics simulations of model nanoparticles to show that the high curvature of small nanoparticles and the relatively short dimensions of the coating can produce highly asymmetric coating arrangements. In solution, geometric properties dictate when a coating’s spherical symmetry will be unstable, and the chain end group and the solvent play a secondary role in determining the properties of surface patterns. At the water-vapor interface, the anisotropic nanoparticle coatings seen in bulk solvents are reinforced by interactions at the interface. The coatings are significantly distorted and oriented by the surface, and they depend strongly on the amount of free volume provided by the geometry, end group, and solvent properties. At an interface, any inhomogeneity or asymmetry tends to orient with the surface so as to minimize free energy. These asymmetric and oriented coatings are expected to have a dramatic effect on the interactions between nanoparticles, and they can influence the structures of aggregated nanoparticles which self-assemble in the bulk and at surfaces.

“Nanoscale Scanning Probe Diffraction Microscopy at the CNM/APS Hard X-ray Nanoprobe Beamline,” Martin Holt, Argonne Center for Nanoscal Materials, hosted by Matthew Pelton

Abstract: The hard X-ray Nanoprobe (HXN) beamline, operated by the Center for Nanoscale Materials in partnership with the Advanced Photon Source, provides the capability for material characterization utilizing hard X-ray microscopy at a landmark ~40-nm spatial resolution. The unique capabilities of hard X-ray microscopy techniques, such as large penetration depths and experimental sensitivity to elemental composition, chemical state, crystallographic phase, and strain, when applied at this length scale, offer new opportunities in many areas of nanomaterials research. Current research will be presented on the use of nanoscale diffraction microscopy as a probe of local structural physics of materials. This will be associated with three areas of study:

1. Unique material behavior of nanoscale objects,
2. Nanoscale critical phenomena of active materials, and
3. Frontier imaging of nanoscale disorder via coherent Bragg diffraction ptychography.

“Manipulating the plasmon resonance of metal oxide nanocrystals for dynamic window coatings,” Delia Dilliron, Molecular Foundry – Lawrence Berkeley National Laboratory, hosted by Elena Shevchenko

Abstract: The Molecular Foundry at Lawrence Berkeley National Laboratory is a sister Facility to the Center for Nanoscale Materials. I will first provide a general introduction to the scope of research underway at the Foundry, particularly highlighting cross-disciplinary, multi-investigator initiatives. I will then spend the remainder of my talk on recent developments in my own research program involving plasmonic features of semiconductor nanocrystals.

Plasmons are light-induced collective oscillations of the free electrons in a metal. In heavily doped metal oxide nanocrystals, they exist as localized surface plasmons that give rise to anoptical absorption feature in the infrared spectral range. Varying the amount of dopant incorporated into the nanocrystals during their chemical synthesis can modify the wavelength of this absorption peak. I will overview our efforts to manipulate such plasmon resonance features, following an innovation cycle of new materials development, investigation of optical andstructural properties, and integration into prototype devices. We have demonstrated that the surface plasmon absorption of a nanocrystal film can be dynamically and reversibly tuned across the near infrared spectrum while maintaining excellent transparency for visible light. These properties are of keen interest for a new breed of carbon-saving, dynamic window coatings that canmodulate solar heating while consistently supplying daylight.

“Graphene Nanoelectronics: Edges, Substrates and Grain Boundaries,” Joseph W. Lyding, University of Illinois at Urbana-Champaign, hosted by Nathan Guisinger

Abstract: We have used ultrahigh-vacuum scanning tunneling microscopy to study the effects of edge structure and substrate interactions on graphene quantum dots (GQDs)]. GQDs on H-Si(100) exhibit the expected size-dependent gapwith the exception of those with predominantly zigzag edges, which are metallic. STM spectroscopy elucidates the predicted zigzag metallic edge state, which has a characteristic decay length of 1 nm. Monolayer graphene deposited in UHV on cleaved GaAs(110) and InAs(110) substrates exhibits an electronicsemitransparency effect in which the substrate electronic structure can be observed ‘through’ the graphene. This effect is observed when the equilibrium graphene-substrate spacing is reduced by about 0.06nm. Strongergraphene-substrate interactions are observed for the case of GQDs on H-Si(100) in which STM electrons are used to remove the hydrogen from beneath GQDs. Simulations indicate that covalent bonds form between graphene and silicon leading to the experimentally observed dimer row ripple in the graphene surface. The predicted changes in GQD electronic structure are also observed via STM spectroscopy. We have also studied grain boundaries in graphene monolayers that have been grown on copper and then transferred to silicon dioxide or mica substrates. STM images show graphene grain misorientation angles and standing wave patterns with ~1-nm decay lengths adjacent to the grain boundaries. For the mica case the graphene exhibits a much smaller rms roughness and there is clear evidence for multiple layers of solid water trapped beneath the graphene. The ability to manipulate this water is also demonstrated.

The second part describes a novel strategy for processing of colloidal nanocrystals into all-inorganic solid films, deployable for photovoltaic applications. The method relies on encapsulation of semiconductor nanocrystal arrays with a matrix of a wide-band-gap inorganic material, which preserves the optoelectronic properties of individual nanoparticles yet renders the nanocrystal film photoconductive. The photovoltaic performance of fabricated nanocrystal solids is demonstrated through the development of prototype solar cells exhibiting stable and efficient operation in ambient conditions.

“Charge Carrier Dynamics in Heterostructured Semiconductor Nanocrystals and Nanocrystal Solids,” Mikhail Zamkov, Bowling Green State University, hosted by Matt Pelton and Chunxing She

Abstract: The first part of the presentation focuses on ultrafast electron processes taking place in heterostructured nanocrystals comprising epitaxially coupled gold nanoparticles and CdS nanorods. The study demonstrates that plasmon oscillations in gold are strongly damped by the presence of the semiconductor domain, which is attributed to mixing of gold and CdS electronic states. We also show that electron transfer from CdS to the gold domain occurs at a rate, which is slower than quenching of FL in the semiconductor component and thus cannot be used to explain the commonly observed suppression of FL emission in Au-CdS nanocomposites. Instead, the measurements indicate that the formation of excitons and corresponding band gap emission in CdS are suppressed as a result of ultrafast carrier trapping by the interfacial states. We propose that charging of gold domains underillumination effectively decreases the quantum confinement of CdS nanorods, which explains previously observed modification of CdS spectra in metal-semiconductor nanocomposites.

The second part describes a novel strategy for processing of colloidal nanocrystals into all-inorganic solid films, deployable for photovoltaic applications. The method relies on encapsulation of semiconductor nanocrystal arrays with a matrix of a wide-band-gap inorganic material, which preserves the optoelectronic properties of individual nanoparticles yet renders the nanocrystal film photoconductive. The photovoltaic performance of fabricated nanocrystal solids is demonstrated through the development of prototype solar cells exhibiting stable and efficient operation in ambient conditions.

"Tailoring The Structure of Chain End-Tethered Nanoparticles in Polymer Hosts," Peter Green, University of Michigan, hosted by Nathan Ramanathan

Abstract: Materials composed of polymers into which organic or inorganic "fillers" of nanoscale dimensions are incorporated, generally identified as polymer nanocomposites (PNCs), constitute a technologically important class of materials. They exhibit diverse functional properties and are used for applications that range from structural and biomedical to electronic and optical. The properties of PNCs are determined, in part, by the chemical composition of the polymer and the type of nanoparticle (graphene, quantum dots, nanorods, clays, fullerenes and metallic nanocrystals), as well as the spatial organization of the nanoparticles within the host.

Fundamentally, one of the most important challenges is to control the spatial organization of the nanoparticles within the polymer host. The strategy of tethering polymer chain ends onto the surfaces of nanoparticles in order to render the nanoparticles miscible with homopolymer hosts is quite promising. The morphological structure of t hese systems is determined by competing interactions between the nanoparticle cores, the free host chains and the grafted chains. In the bulk, when the nanoparticle grafting density is low, the phase behavior is largely determined by a competition between attractive nanoparticle core-core interactions, mediated by the chains grafted to the surface. At high grafting densities, the entropic brush layer/free host chain interactions are dominant, leading to miscibility or to microscopic/macroscopic phase separation. Thin-film mixtures are thermodynamically less stable than their bulk analogs because of the preferential attraction of grafted nanoparticles to the external interfaces. Consequences of entropic and enthalpic interactions on the overall nanoparticle organization in bulk and thin-film polymer-based systems will be discussed.

"Multiblock polymers for Nanoporous Membranes, Monoliths, and Masks," Marc Hillmyer, University of Minnesota, hosted by Seth Darling

Abstract: Block polymers that contain a sacrificial component are finding utility in technologies ranging from liquid purification to nanopatterning. Diblock copolymers that contain a robust matrix segment end-coupled to an etchable segment are the most commonly employed systems and can lead to nanoporous materials with, for example, cylindrical pores hexagonally packed in a continuous matrix. Block polymers that incorporate three or more chemically distinct segments hold tremendous promise for the generation of more exotic and even more promising porous structures. As an example, using ABC triblock terpolymers, nanoporous A matrix materials with specific pore wall functionality B can be generated by selective removal of C. Furthermore, by incorporating other functional attributes into these segments (e.g., etch contrast or ability to crosslink), the applicability of such systems can be tremendously enhanced. Motivated by the tremendous technological potential of nanoporous materials from block polymer precursors, we have explored the incorporation of multiple functional blocks into these hybrid macromolecules that (i) expand the range of accessible nanostructures and (ii) contain the chemical functionality essential for a particular targeted application. These efforts necessitate the controlled synthesis of multiblock polymeric materials from a broad pallet of monomers. In this talk I will discuss our recent efforts in the precision synthesis and detailed characterization of such nanostructured polymeric materials and highlight their usefulness in applications that include magnetic material patterning, water ultrafiltration, and confined crystal growth.

"Collective Plasmon Modes in Nanoparticle Assemblies," Stephan Link, Rice University, hosted by Matt Pelton

Abstract: To incorporate plasmonic nanoparticles into functional devices it is necessary to understand how surface plasmons couple as particles are arranged into ordered structures. Bottom-up assembly of chemically prepared nanoparticles facilitates strong plasmon coupling because of short interparticle distances, but also gives to rise to defects in particle size, shape, and ordering. Single-particle spectroscopy of plasmonic nanoparticle assemblies, especially when correlated with structural characterization by scanning electron microscopy, allows one to gain a detailed understanding about collective plasmon modes. We have used polarization sensitive dark-field scattering spectroscopy covering a broad spectral range from the visible up to 2000 nm and polarization-dependent photothermal imaging to investigate radiative and nonradiative coupling separately in one-dimensional assemblies of plasmonic nanoparticles. For both scattering and absorption, we observed collective plasmon modes that are highly polarized along the main axis of the one-dimensional nanoparticle chain and red-shifted from the plasmon resonance of the individual constituents. These collective plasmon modes are compared with plasmon antenna modes of continuous nanorods with varying length and width. Furthermore, we have developed a fluorescence based method to visualize plasmon propagation in one-dimensional nanostructures.

“Precision Controlled Carbon Nanomaterials for Semiconductor Electronics and Light Harvesting,” Mike Arnold, University of Wisconsin-Madison, hosted by Nathan Guisinger

Abstract: Semiconducting sp2-bonded carbon nanomaterials such as carbon nanotubes and quantum-confined graphene nanostructures have exceptional properties that make them highly attractive for applications in semiconductor electronics and optoelectronics. In this talk, I will detail two recent advances in carbon-based electronic materials that we have realized in my research group.

1. We have pioneered a new class of photovoltaic materials and devices based on structure-controlled and electronic-type controlled semiconducting carbon nanotubes in which we are uniquely employing the nanotubes as the primary optical absorber. We have shown that we can efficiency harvest light by using semiconducting carbon nanotubes and separate the photogenerated charges using all-carbon nanotube/C₆₀ fullerene heterojunctions. The carbon nanotube/carbon fullerene heterostructures are an evolution of polymer photovoltaic systems and exploit carbon nanotubes’ strong near-infrared absorptivity, excellent charge transport characteristics, and chemical stability.
2. I will introduce a new form of semiconducting graphene-based materials that we call nanoperforated graphene. Nanoperforated graphene is created by perforating large-area graphene membranes with nearly close-packed hexagonal arrays of holes with sub-20-nm dimensionality. I will detail how to synthesize nanoperforated graphene by using self-assembling lithographic approaches including block copolymer and nanosphere lithography and will show that it has semiconducting behavior with a band gap inversely proportional to its minimum feature size. Nanoperforated graphene maintains the 2D-form factor of the original graphene, but because it is semiconducting it has potential applications in transparent and flexible electronics and infrared optoelectronics.

“A Tiny Revolution in Biomimicry,” by Gregory Timp, University of Notre Dame, hosted by Daniel Lopez

Absgtract: Using nanotechnology to mimic living tissue, we are striving to harness the basic unit of life, the living cell, for applications in medicine, sensing and computing. In particular, in this talk, we will illustrate the strides we have made towards sequencing DNA using nanometer-diameter pores in nanometer-thick dielectric membranes that resemble ion channels in the plasma membrane of a cell. And we will describe the use of holographic optical traps to manipulate cells with nanometer precision that afford us control of the architecture of synthetic tissue for the study cell-to-cell communication.

“Wash-free multiplex protein assay based on magnetic nanotechnology and its applications in cancer research, " Shan Xiang Wang, Stanford University, hosted by Elena Rozhkova

Abstract: Reproducible and multiplex protein assays are greatly desired by cancer biologists as well as clinical oncologists to rapidly follow numerous proteins in clinical samples. By simply applying patient serum or tissue samples (only 10-50 uL) to the magneto-nano sensor chip developed in our group, one can readily and quantitatively ascertain the presence or absence of a large number of tumor markers, such as those involved in HER-kinase axis pathway, in a multiplex format. This will allow physicians to determine the efficacy of relevant chemotherapy in real time. Combined with a different set of tumor markers, the new protein assays will also allow physicians to detect cancer early (e.g., stage 1 ovarian cancer), so that cancer survival rates can be improved greatly with early intervention. We have now successfully applied magneto-nano biochips based on giant magnetoresistance (GMR) spin valve sensor arrays and magnetic nanoparticle labels (nanotags) to the detection of biological events in the form of multiplex protein assays (4-to 64-plex) with great speed (30 min - 2 hours), sensitivity (1 picogram/milliliter concentration levels or below), selectivity, and economy. More recently, we achieved the first demonstration of a nanolabel-based technology capable of rapidly isolating cross-reactive antibody binding events in a highly multiplex manner. By combining magnetic nanotechnology with immunology, we have devised an easy to use and rapid auto-assembly assay which is ideal for high-density screens of aberrant protein binding events

“Nanophotonics in semiconductor optoelectronics and bioimaging,” Arup Neogi, University of North Texas, hosted by Tijana Rajh and Elena Rozhkova

Abstract: Near-field optical effects induced by metal nanoparticles can significantly influence light emission, modulation, or nonlinear frequency. The role of electrostatics, which has largely been ignored over electrodynamical effects such as surface plasmon effects in metal-semiconductor hybrid light emitters, will be discussed. Electrostatic image charge effects has been used to enhance the efficiency of nanophotonic light emitters. A novel phonon imaging technique using near-field optical spectroscopy has been developed in our laboratory to measure nanoscale strain in optical emitters. A unique nano-material system for nonlinear optical imaging of biological cells will be also presented.

“Direct Synthesis of Nanostructures and Their Self-Assembly,” Elena Shevchenko, CNM, hosted by Tijana Rajh

Abstract :Multicomponent nanostructures can be synthesized directly by a solution-based approach or self-assembled from nanosized building blocks. We will discuss in detail different aspects of the nucleation and growth of single- and multicomponent nanoparticles with different morphologies (e.g., core/shell, dumbbells). We will discuss the possibility of guidance toward a general approach for synthesizing multicomponent nanoparticles and potential applications. We will give examples of periodic and quasicrystalline structures and discuss the key factors that drive self-assembly of nanoparticles into certain periodic lattices, their thermal stability, and mechanical properties.

“Electronic structure of organic-organic heterojunctions: Interface models and implication for organic-based devices,” Antoine Kahn, Princeton University, hosted by Seth Darling

Abstract: Organic-organic heterojunctions (OOHs) are central to the performance of organic devices such as OLEDs or OPV cells. Understanding and controlling their electronic structure is of both fundamental interest and very practical importance. This talk begins with a review of the methodology used to measure molecular level alignment at OOHs and of the extensive set of data collected over the past decade for a wide range of organic pairs. We present one of the proposed models used to explain the observed electronic structures. This simple model, based on the notion of alignment of charge neutrality levels across the organic-organic interface, leads to a surprisingly good qualitative description of the experimental results. We then turn to two specific examples of recent OOH work and their applications. The first concerns a direct determination of the electronic structure of a blend heterojunction, of interest for OPV devices (i.e., P3HT:PCBM), and implications for the understanding of open circuit voltages in OPV cells. The second concerns the use of organic barriers to control the transfer of charges in the channel of a remotely doped OFET.

“Multiexciton Generation at the Nanometer Scale,” Eran Rabani, Tel Aviv University, hosted by Stephen Gray

Abstract: Carrier multiplication is a process in which several charge carriers are generated upon the absorption of a single photon in semiconductors. This process is of great technological ramifications for solar cells and other light harvesting technologies. For example, it is expected that when more charge carriers created shortly after the photon is absorbed, the larger fraction of the photon energy can successfully be converted into electricity, thus increasing the device efficiency. In this talk I will review the current status of the field of multiexciton generation in low-dimensional semiconducting systems, such as quantum dots and nanotubes. Detailed discussion of the relevant length scales, time scales and energy scales will be given.

"Oxides as Energy Materials," Shriram Ramanathan, Harvard University, hosted by Subramanian Sankaranarayanan

Abstract: I will discuss a few examples concerning phase transitions in functional oxides and their applications in solid state devices for energy conversion and electronics. The talk will center on problems concerning ionic-electronic transport, point defect thermodynamics in low-dimensional oxides and experimental methods to study these rigorously. Thin film solid oxide fuel cells as embeddable power sources will be used an example to illustrate broader relevance. Finally, I will point out the inevitable convergence of electrochemistry and solid-state physics towards solving pressing societal problems.

"Anisotropic Semiconductor Nanocrystal Synthesis and Chemical and Biological Functionalization," Preston T. Snee, University of Illinois - Chicago, hosted by Richard Schaller

Abstract: Semiconductor nanocrystals (NCs, or quantum dots) are very bright chromophores that possess significant potential in alternative energy generation and for biological sensing and imaging applications. Our group has made significant advances in the synthesis of rods and multi-pods of near-infrared emitting PbSe NCs through a previously unobserved mechanism. Characterization of anisotropic PbSe NCs show that they have much more robust chemical properties compared to cubic or "dot"-shaped NCs.

Also presented are recent advances on the chemical and biological functionalization of bright NCs. While there are ostensibly many methods for chemical functionalization of water soluble NCs, most of the reagents used in these methods either quench the NCs or have very low reaction yields. We have circumvented these problems by synthesizing polymers which serve as NC functionalization reagents; the polymer-NC-activated intermediate has increased stability and allows us to conjugate chemical and biological vectors to the NCs with 100% reaction yield. We use this method to functionalize NCs with organic fluorophores that can report on the local chemical and biological environment. We have synthesized several ratiometric, or "self-calibrating" sensors, for pH, toxic metals, DNA, and proteins. In our recent work on protein sensing, we have developed an all optical method for sensing un-labeled proteins with a better detection limit than any currently existing technology.

"Synthesis and Applications of FePt and Their Composite Nanoparticles, " Shouheng Sun, Brown University, hosted by Elena Shevchenko

Abstract: I will summarize our recent efforts in synthesizing FePt alloy and its composite nanoparticles with controlled composition and structure for active/durable fuel cell catalysis and effective therapeutic applications.

"Plasmonic Nanostructrures: Artificial Molecules," Peter Norlander, Rice University, hosted by Matt Pelton

Abstract: The recent observation that metallic nanoparticles possess plasmon resonances that depend sensitively on the shape of the nanostructure has led us to a fundamentally new understanding of the plasmon resonances supported by metals of various geometries. This picture, "plasmon hybridization," reveals that the collective electronic resonances in metallic nanostructures are mesoscopic analogs of the wave functions of simple atoms and molecules, interacting in a manner that is analogous to hybridization in molecular orbital theory. The new theoretical insight gained through this approach provides an important conceptual foundation for the development of new plasmonic structures that can serve as substrates for surface enhanced spectroscopies and subwavelength plasmonic waveguiding and other applications. The talk is comprised of general overview material of relevance for chemical applications interspersed with a few more specialized "hot topics" such as plasmonic interference effects, quantum effects, and single-molecule SERS and LSPR sensing.

“Hierarchical Structure Control in Conjugated Polymers,” Rachel Segalman, University of California, Berkeley, hosted by Seth Darling

Abstract: Control over structures on a molecular through nanoscopic lengthscales is vital to optimize polymeric devices for energy generation. For example, while molecular structure affects the electronic properties of semiconducting polymers, the crystal and grain structure greatly affect bulk conductivity, and nanometer lengthscale is vital to charge separation and recombination in photovoltaic and light emitting devices. Careful control over the organic/inorganic interface appears to play a vital role in thermoelectric effects. Block copolymers have been the subject of intense interest for their ability to form intricate, predictable 10nm lengthscale patterns. Unfortunately, this immense background of research has generally focused on classical, flexible polymer molecules. Semiconducting polymers for photovoltaics and other applications are characterized by a rigid, liquid crystalline chain shape and transport properties which rely crucially on crystallinity, which adds complexity to the thermodynamics of self-assembly. In this seminar, I will discuss our work to understand the effect of chain shape on polymer self-assembly and routes to control both self-assembly and crystallization on multiple dimensions, as is required for these applications. The effect of carefully controlled morphology on photovoltaic device performance as well as possible applications in thermoelectrics will be discussed.

“Biomolecules as building blocks for directing the structure and function of new materials, "Nathaniel Rosi, University of Pittsburgh, hosted by Sarah Hurst

Abstract: This talk will detail our work investigating the use of biomolecules as building blocks for constructing new materials. In particular, half of the talk will focus on the use of nucleobases as building blocks for constructing porous materials. Some potential biomedical and environmental applications of these materials will also be presented. The second half of the talk will focus on the development of a new methodology for constructing nanoparticle superstructures that relies on a new class of peptide conjugate molecules which are designed to direct the simultaneous synthesis and assembly of inorganic nanoparticles. Particular focus will be dedicated to a discussion of various nanoparticle superstructures that can be constructed using this methodology and the mechanism of their formation.

"Platforms for the Mechanical, Thermal, and Electrochemical Characterization of Nanoscale Materials," John P. Sullivan, Sandia National Laboratories, hosted by Anirudha Sumant

Abstract: The challenge in characterizing nanoscale materials (e.g., nanowires, nanoparticles) is related to the difficulties in resolving small responses to large external stimuli and to isolating the response of individual particles. In this talk, we describe the integration of individual nanoscale structures with chip-based testing platforms and the use of these platforms to characterize the nanostructure's properties. A suite of these testing platforms has been developed for the mechanical, thermal, and electrochemical characterization of nanoscale materials. We describe the development of microscale tensile test structures for measuring the stress-strain behavior of metallic nanowires, thermal platforms for measuring thermal conductivity and phonon surface scattering in semiconducting nanowires and nanoligaments, and in situ transmission electron microscopy (TEM) platforms for measuring the electrochemical behavior of nanowire anode and cathode materials for use in lithium-ion batteries. The goal of this work is to determine structure-property or size-property relationships that are derived from studies of well-characterized isolated structures.

"Quantum effects in photosynthetic energy transfer," Cesar Rodriguez-Rosario, Harvard University

Abstract: The efficiency of exciton transfer in the photosynthetic Fenna-Matthews-Olson complex is astonishing, 99% of the absorbed photons driving a chemical reaction. Recent experiments have detected long-lived quantum coherences in this complex, but the question remains: are these coherences responsible for the high efficiency?

We develop tools inspired in quantum information and quantum thermodynamic to analyze the efficiency of such systems, towards learning from photosynthesis to designing nanostructures for novel photovoltaic technologies. We study a simple model for exciton transport and show how it behaves like a quantum engine driven by both dephasing and relaxation. We also present how dephasing can assists quantum transport in a network, such as the quantum walk of an exciton through the FMO complex.

This is work done as part of the Center for Excitonics at Harvard/MIT.

In Kyeong Yoo, Samsung

"Macromolecular Surfactants," Frank Bates, University of Minnesota, hosted by Seth Darling

Abstract: Block copolymers belong to a broad class of amphiphilic compounds that includes lipids, soaps, and nonionic surfactants. A macromolecular architecture affords certain unique advantages over conventional low molecular weight amphiphiles in constructing nanoscale objects with prescribed morphologies and desirable physical properties. In this presentation I will describe the structure and dynamics of block copolymers dispersed in aqueous and hydrocarbon media based on small-angle X-ray and neutron scattering and various cryogenic electron microscopy. These fundamental results will be augmented by a discussion of an application to thermoset epoxies, where the dispersion of block copolymer micelles has resulted in extraordinarily tough plastics.

June 16, 2010

"From molecular adsorbates to atomic membranes: How interfacial atomic structure influences nanotribology in carbon-based systems," Robert Carpick, University of Pennsylvania, hosted by Anirudha Sumant

Abstract: Many carbon-based materials, including diamond, carbon nanotubes, diamond-like carbon (DLC), graphite, and graphene, exhibit unusual and extreme material properties. The tribological behavior of these materials is no less interesting: Diamond can function as an abrasive or a solid lubricant; DLC can exhibit extremely low macroscopic friction, yet shows rather high nanoscale friction; DLC has lower friction when dry, while diamond has lower friction when wet. I will briefly review some of the important and at times contradictory behavior that is seen in these materials. I will also mention the application areas where these materials can be applied. I will then focus on the atomistic origins of two particularly interesting effects: (1) the critical environmental sensitivity of friction and wear for diamond interface and (2) the unusual stick-slip friction behavior seen in thin graphene and other atomically thin sheets.

"Dynamics of Nanomechanical Cantilever Devices in Fluid Environments," John E. Sader, University of Melbourne, Australia, hosted by Matt Pelton

Abstract: Nanomechanical sensors are often used to measure environmental changes with extreme sensitivity. Controlling the effects of surfaces and fluid dissipation presents significant challenges to achieving the ultimate sensitivity in these devices. Particularly, the fluid-structure interaction of resonating microcantilevers in fluid has been widely studied and is a cornerstone in nanomechanical sensor development. In this talk, I will give an overview of work being undertaken in our group dedicated to exploring the underlying physical processes in these systems. This will include exploration of recent developments that focus on cantilever sensors with embedded microfluidic fluid channels and examination of the effects of surface stress on the resonant properties of cantilever sensors.

"Metal-insulator transition in vanadium oxide nanobeams," David H. Cobden, University of Washington, hosted by Anand Bhattacharya and Matthias Bode

Abstract: Because of the change in structure, the latent heat, and the need for nucleation, first-order phase transitions involving a solid phase tend to be very sensitive to inhomogeneities and difficult to study in bulk material. For solid-solid transitions the situation is particularly bad, with random domain formation, high strain, and fracture being almost unavoidable. I will discuss how working with nanoscale systems, which are small compared with the scale of inhomogeneities and domains, allows much improved control of such transitions. It also opens up possibilities to investigate phase transitions in reduced dimensionality, where the physics may be qualitatively different from that in three dimensions and may be more theoretically tractable as well. These advantages will be illustrated in two systems: a solid-solid transition driven by electron-electron c correlations in a vanadium dioxide nanobeam; and vapor-liquid-solid transitions occurring in a monolayer of gas atoms adsorbed on a carbon nanotube.

May 26, 2010

“A protein scaffold for nanotechnology and structured enzyme complexes for biofuels,” Chad Paavola, NASA-Ames, hosted by Nathan Ramanathan

Abstract: The thermoacidiphilic archaeon Sulfolobus shibatae produces heat-inducible chaperonins comprised of ~60 kDa protein subunits assembled into 18-mer double rings. These double rings can, in turn, assemble into higher order structures such as two-dimensional crystals or bundled filaments. We have exploited this hierarchical self-assembly to create templates for nanoscale ordered materials incorporating metallic, semiconductor, or magnetic particles. We have also turned this structural scaffold to the purpose of understanding the interactions among enzymes in the bacterial cellulosome, with the goal of producing new systems for deconstruction of plant cell wall biomass. This work has resulted in an enzyme complex that exhibits enhancement of enzymatic activity characteristic of previous work on cellulosomal enzymes. Chaperonin-based complexes are proving useful in examining combinations of cellulosomal enzymes in ratios similar to those observed in nature.

May 16, 2010

"Metal-insulator transition in vanadium oxide nanobeams, " David Cobden, University of Washington

May 12, 2010

"Coherent Excitonic Transport through Disordered Molecular Systems: Extracting Design Principles from Photosynthesis," Greg Engel, University of Chicago

Abstract: Life on earth is effectively solar powered, yet the way in which energy moves through photosynthetic complexes before the biochemical steps of photosynthesis is still not completely understood. Evidence for a purely quantum-mechanical mechanism of energy transfer in photosynthetic complexes was discovered in the Fenna-Matthews-Olson (FMO) complex of Chlorobium tepidum in 2007. The quantum beating phenomenon observed in this complex is now much better understood. Further, data indicate that this mechanism is not specific to FMO, but manifests in reaction centers of purple bacteria and antenna complexes of higher plants. Having observed such a mechanism in disparate photosynthetic complexes, we are exploring what the minimal requirements are to support quantum coherence transfer in a biological environment and how such an environment might be reproduced synthetically using nanomaterials. Emerging details in this story will be presented along with preliminary data from experimental efforts to dissect the details of energy transfer, the basis for the efficiency of the energy transfer process and efforts to isolate signals at room temperature.

April 28, 2010

"Ambipolar Transport in Organic Thin Film Transistors," Cherie Kagan, University of Pennsylvania, hosted by Elena Shevchenko

Abstract: Organic semiconductors have received enormous attention as alternative channel materials for low-cost and flexible thin-film transistors (TFTs). But organic semiconductors are commonly classified as either n-type or p-type as different materials typically show unipolar behavior in TFTs with exclusively electron or hole transpor,t respectively. In many organic thin-film semiconductors that were known to be the best hole conductors, the absence or poor transport of electrons has been attributed to extrinsic factors: (i) high barriers for electron injection at the metal-semiconductor interface; (ii) trap sites for electrons at the dielectric-semiconductor interface; and (iii) the generation of electron traps upon exposure to different environments. Recently, reports have shown that ambipolar transport (showing both n-type and p-type character) is an intrinsic property of organic semiconductors and ambipolar organic TFTs were achieved employing low-workfunction (such as calcium) source and/or drain electrodes, to inject electrons but low workfunction electrodes typically have poor stability . We report ambipolar pentacene TFTs using self-assembled thiolate and carbodithiolate monolayers to enhance charge injection at the metal-organic interface of stable high- workfunction gold source and drain electrodes and polymer gate dielectrics that prevent electron trapping at the organic-dielectric interface in the TFTs. Pentacene is deposited by spin-coating a soluble precursor and thermally retro-converting the precursor thin films at mild temperatures (120-200 ºC) to pentacene. Hole and electron mobilities of 0.1-0.5 cm2/V s and 0.05-0.1 cm2/V s are achieved. We have implemented a wide range of aromatic and aliphatic thiolate and carboditholate chemistries to achieve ambipolar pentacene TFTs. Using these ambipolar FETs, we demonstrate CMOS-like inverters with gains of up to 110. We have further developed processes to integrate the solution-processable precursor route to ambipolar pentacene transistors onto flexible plastic substrates.

April 14, 2010

"Nanoparticles in Biology: Engineering the Interface for Delivery and Sensing," Vince Rotello, University of Massachusetts, hosted by Elena Shevchenko

Abstract: A key issue in the use of nanomaterials is controlling the interfacial interactions of these complex systems. Our research program focuses on the tailoring of interfaces by coupling the atomic-level control provided by organic synthesis with the fundamental principles of supramolecular chemistry. Using these tailored monolayers, we are developing particles for biological applications in particular delivery and sensing. This talk will focus on the interfacing of nanoparticles with biosystems, and will discuss our use of nanoparticles for delivery applications as well as our use of polymer-nanoparticle systems for sensing and identification of proteins, bacteria, and cancer detection/identification of mammalian cells.

March 17, 2010

"Magnetically Responsive Photonic Nanostructures," Yadong Yin, University of California - Riverside, hosted by Gary Wiederrecht and Yugang Sun

Abstract:: This presentation focuses on recent work on magnetically tunable photonic nanostructures. Superparamagnetic iron oxide colloidal particles are synthesized by using a high-temperature hydrolysis reaction and then self-assembled into ordered photonic crystal structures in solution phase using external magnetic fields. The colloids form chain-like structures with regular interparticle spacing of a few hundred nanometers along the direction of the external field so that the system strongly diffracts visible light. The balance between attraction (in this case, magnetic dipole interaction) and repulsion (such as electrostatic and solvation force) dictates interparticle spacing and therefore optical properties. By changing the relative strength of these two forces, we can tune the peak diffraction wavelength over the entire visible spectrum. By controlling the surface properties of the magnetic particles (so that the repulsive forces involved), we have been able to assemble the photonic structures in water, alcohols, and nonpolar solvents. The fast, reversible response and the feasibility for miniaturization impart these photonic materials great potential in applications such as optoelectronic devices, sensors, and color displays. In this presentation, I will also demonstrate high resolution patterning of multiple structural colors by a single "ink" material, the color of which is magnetically tunable and lithographically fixable. The fabrication of color and shape coded micro-objects by using this unique photonic material will also be discussed.

March 11, 2010

"Supramolecular Materials for Separation/Sensing/Release/ Energy Conversion & Hand-Operated Nanotechnology," Katksuhiko Ariga, National Institute for Materials Science (Japan), hosted by Elena Rozhkova

Abstract:: Supramolecular materials have been wisely constructed via bottom-up approaches as seen in preparation of molecular complexes and organized nanostructures. These materials are used for various functions such as one-pot materials separation, selective sensing, auto-modulated drug release, and photo-energy conversions as reported in our recent researches. In addition, we have proposed a novel methodology "hand-operating nanotechnology" where molecular orientation, organization and even functions in nanometer-scale can be operated by our bulk (hand) operation. For example, we successfully manipulated molecules at the air-water interface upon bulk (10-100 cm size) motion of the entire monolayer and realized "capture and release" of aqueous guest molecules using molecular machine as well as mechanically controlled chiral recognition.

"Tuning optical properties of polymer films using nanorods and Janus particles," Russell J. Composto, The University of Pennsylvania, hosted by Seth Darling and Nathan Ramanathan

Abstract: The optical properties of polymer films are tuned using novel nanoparticles. First, nanorods (NRs) of gold are organized and aligned within polymer films. The plasmon adsorption is investigated as a function of nanoparticle concentration as well as matrix type. NR organization and dispersion are compared with a model consisting of noninteracting rods as well as simulations of rod-rod interactions. At NR concentrations ranging from 1 to 15vol%, the NRs disperse very uniformly with a spacing that decreases linearly with the volume fraction. Surprisingly, even at 1 vol%, NRs are locally ordered. and this ordering increases linearly with the volume fraction of rods. Second, multiregion and patchy, opticall -active Janus particles were synthesized via a hierarchical self-assembly process. Gold nanoparticles were assembled on the top surfaces of nano- and sub-micron silica particles, which were selectively protected on their bottom surfaces by covalent attachment to a copolymer film. The m orphologies of the gold particle layer, and the resulting optical properties of the Janus particles, were tuned by changing the surface energy between the silica and gold particles, followed by annealing.

February 24, 2010

"Interfacial Electron Transfer Dynamics from Single Quantum Dots," Shengye Jin,Emory University, hosted by Gary Wiederrecht

Abstract: Understanding the dynamics of charge transfer to and from quantum dots (QDs) is essential to their many applications, ranging from solar cells to light-emitting diodes to biological imaging. Unlike molecular chromophores, previous studies showed that single QDs exhibit dynamic fluctuation of fluorescence intensity (i.e., "blinking"). To understand how interfacial electron transfer can affect and be affected by the blinking dynamics, we investigated electron transfer dynamics from single CdSe core shell QDs to various semiconductor nanomaterials.

Single QDs on TiO2 showed pronounced and correlated fluctuations of fluorescence intensity and lifetime. Compared with QDs on glass, the presence the interparticle ET pathway on TiO2 led to smaller on-state and larger off-state probability densities, as well as a shortened lifetime of the on-state.

Furthermore, the electron transfer dynamics from the same QDs to the (110) surface of a rutile TiO2 single crystal have also been studied as a model system for QD/TiO2 nanoparticles. Atomic force microscopy study of QDs on the TiO2 (110) surface revealed that QDs are preferentially adsorbed on the step edges. The distribution of lifetimes of QDs on the single crystal is much narrower than that on TiO2 nanoparticles, suggesting a considerably less heterogeneous distribution of electron transfer rates on the single crystal surface. However, interfacial electron transfer can be inactivated for QDs on highly n-doped semiconductors. Compared with the QDs on In2O3 and glass, the QDs on ITO show significantly reduced lifetimes and suppressed blinking activity due to the charging of QDs.

Besides semiconductor nanomaterials, interfacial electron transfer dynamics from single QDs to organic dye molecule (Fluorescein 27) were also investigated. A combination of ensemble transient absorption and single QDs measurements revealed a correlated single QD blinking and interfacial electron transfer dynamics. The origins of the intermittent electron transfer dynamics were discussed and an electron transfer and blinking model was suggested.

February 17, 2010

"Spintronics," Sam Bader, Argonne National Laboratory, hosted by Seth Darling

Abstract: Spintronics encompasses the ever-evolving field of magnetic electronics. It is an applied disciple that is so progressive that much of the research that supports it is at the center of basic condensed matter and materials physics. The talk provides a brief perspective on recent developments in switching magnetic moments by spin-polarized currents, electric fields and photonic fields. Developments in the field of spintronics continue to be strongly dependent on the exploration and discovery of novel material systems. An array of novel transport and thermoelectric effects dependent on the interplay between spin and charge currents have been explored theoretically and experimentally in recent years. The talk highlights select areas that hold promise for future investigation, and features some recent work at Argonne.

February 3, 2010

"Materials Synthesis Using Atomic Layer Depositions," Jeffrey Elam, Argonne National Laboratory, hosted by Matthew Pelton

Abstract: Atomic layer deposition (ALD) is a thin-film growth technique that uses alternating, self-limiting chemical reactions to deposit materials in an atomic layer-by-layer fashion. Although this process is already used commercially for microelectronics manufacturing, ALD promises to have a much broader impact extending far beyond microelectronics. In particular, the capability to infiltrate and coat porous substrates coupled with a broad palate of available materials make ALD a versatile technique for synthesizing nanostructured materials. Consequently, we are pursuing a variety of new applications for ALD, including photovoltaics, catalysis, and large-area detectors. A central theme in these efforts is that we use ALD to apply precise, conformal coatings onto nano- or micro-structured scaffolds to impart the desired optical, electrical, or chemical properties to suit the application. In this presentation, I will describe some of our recent research and development work at Argonne to advance these technologies.

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.

November 25, 2009

"Oxide Nanoelectronics," TJeremy Levy, University of Pittsburgh, hosted by Matthew Pelton

Abstract: Electronic confinement at nanoscale dimensions remains a central means of science and technology. In this talk, I will describe a new method for producing extreme nanoscale electronic confinement at the interface between two separately insulating oxides, LaAlO3 and SrTiO3. Using an approach reminiscent of the popular toy "Etch-a-Sketch," we scan an electrically biased probe on the surface of this heterostructure to create nanoscale conducting islands, nanowires, tunnel junctions, and field-effect transistors at the interface. The smallest feature size approaches one nanometer. These structures are created in ambient conditions at room temperature and can be erased and rewritten repeatedly. At low temperatures, a variety of quantum phases have been observed, including integer and fractional quantum Hall states and superconductivity. This new, on-demand nanoelectronics platform has the potential for widespread scientific and technological exploitation.

October 28, 2009

"Directed Self-Oriented Self-Assembly of Block Copolymers: Bottom-Up Meeting Top-Down," Thomas P. Russell, University of Massachusetts, hosted by Seth Darling

Abstract: As the size scale of device features becomes increasingly smaller, conventional lithographic processes are limited. Alternative routes need to be developed to circumvent this hard stop. Ideally, if existing technological processes can be used with novel materials, significant advances can be made. Block copolymers (BCPs), two polymer chains covalently linked together at one end, provide one solution. BCPs self-assemble into a range of highly ordered morphologies, and by controlling the orientation and lateral ordering of the nanoscopic microdomains, numerous applications will emerge. By combining the "bottom-up" self-assembly of BCPs with "top-down" microfabrication processes, faster, better, and cheaper devices can be generated in very simple, yet robust, ways.

September 30, 2009

Abstract: Self-organization phenomena at the nanoscale is a new field of nanotechnology research that brings out new experimental and theoretical results every day. Self-organization processes are also important from the practical perspective because they drastically simplify manufacturing process of nanodevices and nanomaterials with specialized optical/electronic properties. They can involve nearly spherical nanoparticles and monodispersed nanorods. However, more complex one-, two-, and even three-dimensional systems form from nanoparticles with strong anisotropy. Comparison of the processes in solution of CdTe and other nanocolloids reveals a number of surprising similarities with processes in proteins.One can conclude that this is the result of the fundamental analogy in the scales between proteins and nanoparticles. This conclusion will be substantiated by a variety of experimental and theoretical observations and demonstrated for CdTe, CdS, CdSe, Te, Se, and ZnO nanocrystals prepared in the laboratory of the presenter.

I will also address the challenges and opportunities opening for complex three-dimensional assemblies exhibiting dynamic behavior. One of the challenges is the development of tools necessary for the observation of structural transformations of nanoscale structures in liquid phase. One of the potential solutions here is to take the advantage of plasmon-exciton resonances produced when metal and semiconductor particles are combined in a single self-assembled structure. Both emission intensity and wavelength of the resulting new hybrid electronic state is dependent on the distance and special arrangement between the nanocolloids. Besides, providing insight to dynamics of the nanoscale self-assembled structures, modulation of exciton-plasmon interactions can serve as wavelength-based biodetection tool, which can resolve difficulties of quantification of luminescence intensity for complex media and optical pathways.

"Top emerging technologies: Nanogenerators and Nanopiezotronics," Zhong Lin Wang,Georgia Institute of Technology, hosted by Xiao-Min Lin

Abstract: Developing novel technologies for wireless nanodevices and nanosystems is of critical importance for sensing, medical science, defense technology, and even personal electronics. It is highly desirable for wireless devices and even required for implanted biomedical devices to be self-powered, without the use of batteries. Therefore, it is essential to explore innovative nanotechnologies for converting mechanical energy (such as body movement, muscle stretching), vibration energy (such as acoustic/ultrasonic wave), and hydraulic energy (such as body fluid and blood flow) into electric energy that can be used to power nanodevices.

We have demonstrated an innovative approach for converting nanoscale mechanical energy into electric energy by piezoelectric zinc oxide nanowire arrays. The operation mechanism of the electric generator relies on the unique coupling of the piezoelectric and semiconducting dual properties of ZnO as well as the elegant rectifying function of the Schottky barrier formed between the metal tip and the nanowire. Based on this mechanism, we have recently developed a DC nanogenerator driven by ultrasonic waves in bio-fluid. We have also used textile fibers for energy harvesting.

This presentation will introduce the fundamental principle of the nanogenerator and its potential applications. Finally, a new field in nano-piezotronics is introduced that usesthe piezoelectric-semiconducting coupled property for fabricating novel and unique electronic devices and components.

Abstract: Photosynthesis is used by a diversity of plants, algae, and bacteria to convert solar energy into biological fuel. The first steps are somewhat like a highly sophisticated solar cell. Specialized proteins capture the sun's energy and transmit it to reactions centers that secure the energy as electrical potential. In recent work, it has been discovered that quantum-coherence can help to move energy among molecules and thereby assist processes such as light-harvesting in photosynthesis. Quantum-coherence means that light-absorbing molecules capture and funnel energy according to quantum-mechanical probability laws instead of classical laws. The subject has stimulated vigorous cross-disciplinary interest because it was previously thought that long-range quantum-coherence could not be sustained in such complex systems, even at low temperature. I will describe experimental observations of ambient temperature quantum-coherent "wiring" in antenna proteins isolated from photosynthetic marine cryprophyte algae. The results suggest that quantum probability laws may feasibly be employed in light-harvesting function within live cryptophyte marine algae to increase the cross-section for light capture and energy conversion.

Abstract: The cluster expansion methodology has proven to be an invaluable tool for computational materials scientists, enabling the determination of ground state structures and calculation of phase diagrams for multicomponent bulk materials.However researchers are increasingly turning to complex and nano-scale materials to meet the demands of technological progress, and studying such materials using cluster expansions can be prohibitively expensive.I will provide an introduction to cluster expansions and present recent advances in the field, including:

• A Bayesian approach that significantly reduces the computational cost of studying atomic order in nanoparticles and surfaces.
• A method for determining the low-energy orientations of polyatomic ions in ionic materials.
• An alternative to the Wulff construction for the prediction of the shape and energy of small nanoparticles.

I will demonstrate how these methods can be used to study the structure and thermodynamics of metallic nanoparticles as well as sodium alanate nanoparticles and lithium imide, promising hydrogen storage materials.

Abstract: Peptoids are a novel class of non-natural biopolymer based on an N-substituted glycine backbone that are ideally suited for nanomaterials research. This bio-inspired material has many unique properties that bridge the gap between proteins and bulk polymers. Like proteins, they are a sequence-specific heteropolymer, capable of folding into specific shapes and exhibiting potent biological activities; and like bulk polymers, they are chemically and biologically stable and relatively cheap to make. Peptoids are efficiently assembled via automated solid-phase synthesis from hundreds of chemically diverse building blocks allowing the rapid generation of huge combinatorial libraries. This provides a platform to discover nanostructured materials capable of protein-like molecular recognition and function.

"Symmetry, Chaos, and Quantum Localization, " Stephen K. Gray, Group Leader, CNM Theory & Modeling Group, hosted by Matt Pelton

Abstract: While the correspondence principle establishes some links between classical mechanics and quantum mechanics, there can be situations when there are noticeable differences between classical and quantum mechanics. Of course there are some obvious situations (e.g., tunneling through barriers), but sometimes there can be more surprising classical/quantum discrepancies even when relatively high energies and many quantum states, usually associated with the classical limit, are involved. In studying the quantum dynamics of the three-dimensional motion of a lithium atom confined to move inside a C60 molecul ( i.e., the endofullerene Li@C60), a remarkable quantum localization effect was observed. The time average of the wave packet density was found to be very high in certain regions of coordinate space. The corresponding classical dynamics showed no such localization and led to a uniform distribution. This effect was found to be due to (i) the presence of a mirror plane symmetry element in the system coupled with (ii) the corresponding classical motion, for the principle energies contained in the wave packet, being highly chaotic. These two requirements are not particularly severe and many other physical systems should display this quantum localization effect. A second example, corresponding to a simple two-dimensional double well problem that is quite different from Li@C60 is used to illustrate this point.

"Bridging the Environment Gap Using First-Principles-Based Catalyst Modeling," William Schneider, University of Notre Dame, hosted by Jeff Greeley

Abstract: Molecular-scale modeling based on density functional theory (DFT) is a common feature of the catalysis research landscape today. These simulations have contributed significantly to the understanding of catalytic reaction mechanisms, to trends in reactivity amongst metals, and even to the prediction of new catalyst compositions. One of the principal challenges in applying molecular simulation to heterogeneous catalysis is incorporating the often significant effect of the reaction environment (temperature, reactant and product concentrations, catalyst support, poison) on chemical composition and structure and thus on reaction mechanism and activity.

Despite the importance of these environment-induced reaction patterns, molecularly detailed models remain sparse. Here we describe our work over the last several years to combine DFT simulations with first principles thermodynamic and chemical kinetic models to describe the reactivity of metal surfaces and particles as a function of environment.

"Teaching Old Materials New Tricks: Nanopatterning and Localized Properties of Multifunctional Oxides," Vinayak Dravid, Northwestern University, hosted by Xiao-Min Lin

Abstract: The natural evolution of functional materials architecture calls for their confinement in spatial and dimensional modes. Here, spatial confinement refers to inevitable attachment of materials to a substrate or an overlayer, for example. Dimensional constraint arises from the emerging need for materials to be confined to 0 (i.e., nanocrystals), 1 (nanolines) and 2 (i.e., films or membranes) dimensions. Further, by juxtaposing two or more functional materials in close proximity, there are exciting new opportunities for synergistic coupling of disparate phenomena in such hybrid confined materials systems.

In this context, surface patterned nanoscale architecture and colloidal form of nanostructures offer unprecedented opportunities to revisit fundamental materials science phenomena, which flirt with thermodynamics of constrained systems on one hand and dynamics of nanoscale processes on the other.

The presentation will cover synthesis and patterning of oxides down to nanoscale, with an emphasis on multifunctional phenomena. Advanced scanning probe;in situ and ex situ electron, ion, and photon microscopy; spectroscopy; and synchrotron X-ray scattering approaches are being employed to fathom the most intricate details of the internal "microstructure" of nanostructures, coupled with innovative tools to validate their functional identity and localized properties.

The presentation topics will range from soft-eBL nanopatterned ferromagnetics/ferroelectrics for investigating solid-state phenomena to colloidal synthesis of nanostructures for biomedical applications. It will be argued that multifunctional nanostructures go beyond the "hype" and present challenging yet exciting opportunities for synthesis-structure-architecture-form-function-performance relationships in complex oxide systems.

"Carbon-Based Nanoelectronic Devices," Debdeep Jena, University of Notre Dame, hosted by Anirudha Sumant

Abstract: Since the discovery of buckyballs (one-dimensional carbon nanostructures), there has been substantial interest in using carbon nanostructures for electronic and optoelectronic devices. The subsequent discoveries of one-dimensional carbon nanotubes (CNTs) and two-dimensional graphene — all sp2-bonded lattices of carbon — has facilitated electronic devices with remarkable charge transport properties. CNTs and graphene are now being actively considered for electronic devices. In our group, we have studied two-dimensional graphene and graphene nanoribbons and compared their transport and device properties with traditional sp3 bonded semiconductor nanostructures. The picture that emerges is that for novel devices such as tunneling field-effect transistors (FETs), graphene nanoribbons have an advantage due to tunable bandgaps and ease of electrostatic gating. However, for high-performance FETs, an even more attractive option would be sp3-bonded carbon nanowires that potentially offer excellent charge and heat transport properties, and a large bandgap. In the seminar, I will connect our current studies with what can be done with sp3-bonded carbon nanowires and outline various characterization techniques we can apply to these novel nanostructures being studied at CNM.

Abstract: It is known that specific molecules can spontaneously arrange on various surfaces forming two-dimensional polycrystalline monomolecular layers called self-assembled monolayers (SAMs). We will show that when mixed SAMs are formed on surfaces with a radius of curvature smaller than 20 nm, they spontaneously phase-separate in highly ordered phases of unprecedented size. In the specific case of mixed SAMs formed on the surface of gold nanoparticles, the molecular ligands separate into 5-Å-wide phases of alternating composition that encircle or spiral around the particle metallic core. This new family of nanostructured nan-materials shows properties due solely to this unique morphology, both in terms of fundamental properties such as interface energy and in terms of complex interaction with biological materials such as proteins and cells.

Additionally, it will be shown how patterned DNA SAMs can be used as masters for a novel printing technique for organic materials called supramolecular nanostamping (SuNs). This method, like DNA/RNA information transfer, uses the reversible assembly of DNA double strands as a way of transferring patterns from a surface onto another. One of the main advantages of SuNs is that multiple DNA strands (each encoding different information) can be printed at the same time, thus allowing for a complex chemical pattern to be formed, much like Gutenberg movable type.

"Ultrafast photoemission electron microscopy: Imaging light with electrons on the femto/nano scale," Hrvoje Petek, University of Pittsburgh, hosted by Matthew Pelton

Abstract: Light interacting with a metal surface can excite both single-particle (e-h pair) and collective (plasmon) excitations. While most of the incident field will be coherently reflected, a small fraction can be absorbed to excite electron-hole pairs within the skin depth of the metal, or localized and propagating plasmon modes. We investigate electron excitation at clean, single-crystal metal surfaces by ultrafast nonlinear momentum and energy, resolved photoemission spectroscopy and photoemission electron microscopy. We describe the optimal geometry for coupling structures for introducing surface plasmon polaritons into continuous metal films. We discuss the imaging of surface plasmon polariton dynamics on silver surfaces and the function of simple plasmonic optical elements.

April 29, 2009

"Nanostructured Polymer Semiconductors: Charge Transport and Photovoltaic Properties," Samson A. Jenekhe, University of Washington, Seth Darling

Abstract: Advances in the controlled synthesis, processing, and tuning of the properties of conjugated polymer semiconductors promise improvement in the performance of organic semiconductor devices and are accelerating the emerging era of plastic electronics. Our laboratory is exploring a molecular engineering approach to readily processable and robust, high-charge carrier mobility materials needed for developing next-generation high-performance organic light-emitting diodes for displays and solid-state lighting, field-effect transistors, logic circuits, and low-cost solar cells. At the nanoscale, polymer semiconductors are expected to have unique properties resulting from size confinement and restricted dimensionality. We are beginning to test this expectation by investigating the self-assembly, nanoscale morphology, charge transport, and electronic and optical properties of various classes of polymer semiconductor nanostructures. In this talk, I will use examples of nanostructured conjugated polymers, including polymer semiconductor nanowires and assemblies of block copolymers to illustrate some of our recent approaches and efforts in these areas. For example, polymer semiconductor nanowires with widths of 5-30 nm and aspect ratios of up to 1000, that are readily assembled from solution, were found to be promising building blocks for nanoelectronics and solar energy applications. Our results show that polymer semiconductor nanostructures can have better properties and enable high performance devices than simple two-dimensional films.

"The Surface Chemistry of CdSe Quantum Dots, and Its Role in Their Optical Properties," Emily Weiss, Northwestern University, hosted by Gary Wiederrecht

Abstract: Organically passivated colloidal semiconductor quantum dots (QDs) are versatile chemical and biological sensors. The yield and wavelength of photoluminescence (PL) from excited states (called "excitons") of a QD changes in the presence of a variety of analytes, which electrostatically or optically couple with, or exchange electrons with, the QD. The intensity of the PL and photostability of the QDs (‹and, consequently, the sensitivity, specificity, and lifetime of QD-based sensors) is limited, however, by the presence of nonradiative pathways for decay of these excited states. The magnitude of the contribution of nonradiative decay depends on the immediate chemical environment of the fluorescent core of the QD (that is, the structure of its surface atoms, and of the monolayer of coordinated organic molecules that passivate them). This talk will describe methods to link the chemical structure of the surfaces of organically passivated colloidal QDs to their PL quantum yield using a combination of ultrafast transient absorption spectroscopy, structural characterization methods, and ligand exchange studies.

"Efficient Thermionic Energy Conversion Based on Doped Diamond Films," Robert Nemanich, Arizona State University,hosted by Anirudha Sumant

Abstract: The process of thermionic emission of electrons from a surface can be utilized to convert heat directly into electrical energy. High efficiencies are predicted because the process involves transport of electrons. While the advantages (and limitations) of thermionic energy conversion (TEC) systems have been known for many years, the potential development of a system based on doped diamond films could enable TEC devices that operate efficiently at temperatures less than 600C. In this study, the advantages of a TEC based on negative electron affinity surfaces of doped diamond are modeled, and the output power and efficiency are calculated. The potential of nanostructured carbon materials that involve field-dependent effects are also considered. Spectroscopic analysis of the thermionic emission from N-doped diamond is presented and related to the specific materials properties of a multilayered thin-film structure. These results establish that the Fermi level of the material is within a few tenths of eV of the vacuum level, and the barrier to emission is less than 1.4 eV. Results are presented that demonstrate direct energy conversion based on engineered N-doped diamond thin film structures.

March 16, 2009

"Molecular dynamics simulations of complex biomolecular systems," Benoit Roux, University of Chicago, hosted by Jeffrey Greeley

"Nanostructures to examine transport, dissipation, and correlations," Doug Natelson, Rice University, hosted by Jeff Guest

Abstract: Over the last decade, significant advances have been made in the fabrication of nanostructures and their use as tools to examine fundamental issues in condensed matter physics. I will describe two sets of experiments that take advantage of the particular capabilities enabled by the ability to make electrical probes separated on the nanometer scale. In the first, we use such probes to examine electronic transport and Raman response in single molecules. Transport measurements on molecules with unpaired electrons allow the study of the strongly correlated Kondo state driven out of equilibrium, while single-molecule Raman measurements open up the possibility of examining electron-vibrational dissipation with unprecedented precision. In the second set of experiments, we fabricate nanoscale electronic devices that directly incorporate a strongly correlated transition metal oxide, magnetite. In this system at low temperatures we find a nonequilibrium phase transition driven by the application of large electric fields. The small size of the devices allows us to delineate between possible transition mechanisms, since extremely large electric fields may be applied even with very modest voltages. I'll conclude by highlighting exciting possible future experiments made possible by nanostructure techniques.

January 21, 2009

"Electrochemical Interfaces: Structural and Catalytic Properties," Nenad M. Markovic, Material Sciences Division, Argonne National Laboratory, hosted by Jeff Greeley

Abstract: The last decade has witnessed some remarkable advances in the elucidation of microscopic processes at the electrified metal-solution interfaces. One class of electrochemical systems of particular significance, which has turned out to be suitable for characterization by in situ surface sensitive probes, vibrational spectroscopes, and electrochemical methods, is electrocatalysis of fuel cell reactions on single-crystal surfaces. Interest in these systems stems from the opportunity to establish both key structure-composition relationships and a deeper understanding of the activity pattern of metal nanoparticles in the size range of a few nanometers. Although the field is still in its infancy, a great deal has already been learned, and trends are beginning to emerge that give new insight into the relationship between the structure of electrochemical interfaces and catalytic activity/stability of nanoparticles.

To give an overview of the field, this presentation will provide a carefully balanced selection of results for the oxygen reduction reaction and the electrooxidation of CO first on platinum monometallic and bimetallic single-crystal surfaces and then on corresponding real nanoparticles supported on carbon.

December 10, 2008

"Screw Dislocation Driven Nanowire Growth: Nanowire Trees, Helice, and Beyond," Song Jin, University of Wisconsin - Madison, hosted by Matthias Bode

I will discuss a "new" nanowire formation mechanism that is completely different from the well-known metal catalyzed vapor-liquid-solid (VLS) mechanism. The screw component of an axial dislocation provides the self-perpetuating steps to enable 1-dimensional crystal growth, unlike previously understood mechanisms that require metal catalysts. This mechanism was found in hierarchical nanostructures of lead sulfide (PbS) nanowires resembling "pine trees" that were synthesized via chemical vapor deposition. Structural characterization reveals a screw-like dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. The rotating trunks and branches are the consequence of the Eshelby twist of screw dislocations. With the help of X-ray microdiffraction and transmission X-ray microscopy experiments carried out at APS, we have further used dislocation mechanism and elasticity theory to explain the spontaneous formations of nanotubes and helical nanostructures. We suggest that screw dislocation growth is overlooked and underappreciated in modern literature on one-dimensional nanomaterials. The proposed nanowire growth mechanism will be general to many materials and enable more complex nanostructures to be synthesized in the future to enable diverse applications.

"Probing microscopic conduction mechanisms in tunable superconducting thin films," Sambandamurthy Ganapathy, University of Buffalo - SUNY, hosted by Nathan Ramanathan

Abstract: I will present experimental results from our transport measurements on amorphous indium oxide thin films that can be controllably tuned between the insulating and superconducting phases. In particular, the transport behavior of the films at magnetic fields larger than the critical point of the phase transition will be presented. Our results show the emergence of an collective insulating phase that may still sustain superconducting correlations at submicron length scales.

November 12, 2008

"Diamonds Are Forever: Solid-State Quantum Optics Electron-Nuclear Spin Registers," M. V. Gurudev Dutt, University of Pittsburgh, hosted byeffrey Guest

Abstract: Building scalable quantum information systems is a central challenge facing modern science. One promising approach is based on quantum registers composed of several quantum bits that are coupled together via optical channels. I will discuss experiments that demonstrate addressing, preparation, and coherent control of individual nuclear spin qubits in the diamond lattice at room temperature. We have measured spin coherence times exceeding milliseconds, and observed coherent coupling to nearby electronic and nuclear spins. Robust initialization of a two-qubit register and transfer of arbitrary quantum states between electron and nuclear spin qubits has been achieved. Our results show that coherent operations are possible with individual solid-state qubits whose coherence properties approach those for isolated atoms and ions. The resulting electron-nuclear few-qubit registers can potentially serve as small processor nodes in a quantum network where the electron spins are coupled by optical photons to generate entanglement, and the nuclear spins serve as a resource for quantum memory and quantum logic operations. Future prospects along these directions as well as potential applications in nanoscale precision magnetometry and magnetic imaging will be discussed.

November 5, 2008

"Revealing Deterministic Mesoscopic Mechanisms of Polarization Switching: Switching Spectroscopy PFM of Defect-Engineered Structures," Sergei V. Kalinin, Oak Ridge National Laboratory, hosted by Stephen Streiffer

Abstract: Polarization switching in ferroelectric materials is controlled by structural defects that act both as the local nucleation centers and the pinning centers for moving domain walls. Progress in nanoscale ferroelectric device applications ranging from FeRAM to data storage to tunneling barriers necessitates understanding of polarization switching mechanism on a single structural or morphological defect level. In this talk, I will present recent results on local studies of fundamental polarization reversal mechanisms in ferroelectrics]. The direct imaging of a single nucleation center on sub-100-nm level is demonstrated. Using switching spectroscopy piezoresponse force microscopy of the systems with engineered defect structures and phase-field modeling, we demonstrate that deterministic mesoscopic polarization switching mechanism on a single known structural defect can be determined. In particular, the artificial bicrystal grain boundary in (100) BiFeO3 is found to impede ferroelectric switching, but facilitate ferroelastic switching for one of the constituent crystals. The coupling between ferroelastic domain walls and ferroelectric polarization switching is demonstrated and attributed to the kinetic effects. These studies open the pathway for probing kinetics and thermodynamics of local bias-induced phase transitions and dissipation on a single-defect level using field confinement by an SPM tip. The future potential for atomistic studies is discussed. Research was supported by the U.S. Department of Energy Office of Basic Energy Sciences Division of Scientific User Instruments and was performed at Oak Ridge National Laboratory, which is operated by UT-Battelle, LLC.

October 29, 2008

"Mycobacterial Mimics, Microparticle Manipulation, and More," Raghuveer Parthasarathy, University of Oregon, hosted by Xiao-Min Lin

Abstract: Lipid membranes, the underlying architecture of all cellular membranes, are remarkable materials: self-assembled, two-dimensional fluids. Membranes can be constructed on solid supports -- these "supported membranes" enable controlled investigations of a variety of membrane properties as well as new sorts of composite materials. In this talk I'll describe what my group is presently exploring: * Mycobacteria, which include the pathogens that cause tuberculosis and leprosy, have unusual membranes. Building a supported mimic of the mycobacterial envelope, we have discovered that an important mycobacterial lipid has the surprising ability to make membranes resistant to dehydration. * Can we harness interactions between membranes to organize non-biological materials? As "building blocks," we focus on lipid membrane-functionalized silica microparticles. To measure their interaction energies, we have invented a new type of optical trap that sculpts the trapping potential landscape into desired forms. With this approach, we have begun to quantify relationships between interaction energies and membrane properties, such as lipid composition and protein binding.

October 22, 2008

Special Colloquium: "Formation, melting and freezing of Bi nanoparticles embedded in a sodium borate glass" A. F. Craievich, Institute of Physics, University of Sao Paulo, hosted by Brian Stephenson

Abstract: The processes of nucleation and growth of liquid bismuth nanodroplets embedded in a soda-borate glass - submitted to isothermal annealing at different temperatures - were studied by in situ small-angle x-ray scattering (SAXS). Our experimental results indicate that the formation of bismuth droplets occurs in two successive stages. The first stage is characterized by the nucleation and growth of spherical droplets promoted by the diffusion of isolated bismuth atoms through the glass matrix and the second stage by droplet coarsening. The experimental functions describing the time variation of the droplet average radius and density number, at advanced stages of the growth process, agree with those predicted by the classical Lifshitz-Slyozov-Wagner theory for particle coarsening. A combined use of the SAXS and WAXS techniques allowed us to establish that the melting temperature of bismuth nanocrystals strongly decreases for decreasing radius R and is a linear function of 1/R. The freezing temperature of bismuth nanodroplets was determined to be lower than the melting temperature of nanocrystals with the same radius (overcooling effect). The freezing temperature also decreases linearly for increasing values of (1/R), but, in this case, the slope is lower than that determined for the melting temperature. Thus the magnitude of the overcooling progressively decreases for decreasing radius, and it vanishes for a critical radius Rc=1.9A. These mentioned dependences of the melting and freezing temperatures could be explained by a simple model that assumes the nanoparticles being composed of a crystalline core surrounded by a disordered shell. On the other hand, the reduction of the volume of bismuth nanocrystals across the melting transition was determined from the temperature dependence of the integral of the SAXS intensity. This volume reduction - derived from SAXS results - is smaller than that a priori expected for homogeneous bismuth nanocrystals. This finding corroborates the model that suggests the heterogeneous nature of the (crystalline-amorphous) bismuth nanoparticles with R>Rc. Our experimental results also indicate that bismuth nanoparticles with R<Rc are fully amorphous. Thus, for bismuth nanoparticles with subcritical radii, neither liquid-to-crystal nor crystal-to-liquid transitions are expected to occur.

October 15, 2008

"A DIM view of science," Paul Russo, Louisiana State University, hosted by Seth Darling and Nathan Ramanathan

Abstract: Disease-inspired materials science includes two avenues of approach: a) emulating the systems nature has evolved to topple other living systems and b) learning to regulate bio-inspired platforms, possibly for medicinal purposes but also just for the challenge of it. The talk will emphasize systems designed around a fibrillar motif.

October 1, 2008

"Pure Spin Currents," Axel Hoffman, Argonne National Laboratory, hosted by Seth Darling

Abstract: The new development of spintronics aims at utilizing the spin degree of freedom for electronic applications. To this date, in most investigated spintronics systems and devices, the spin and charge currents are generally in parallel and therefore directly coupled. However, using nonlocal geometries allows us to separate spin and charge currents, which enables the investigation of pure spin currents. Because spin states are not necessarily conserved due to spin-flip scattering, they behave differently than charge currents. In particular, this opens up the opportunity to transport spin information via exchange interactions instead of actual spin transport. Thus there is a possibility of significantly reduced dissipation for devices based on pure spin currents. In this talk, I will review our work on pure spin currents as well as alternative approaches to the generation of spin currents, such as spin Hall effects and spin pumping.

September 17, 2008

"Photophysics of nanostructured molecular crystals: from nanorod bending to exciton fission," Christopher Bardeen, University of California Riverside, hosted by Gary Wiederrecht

Abstract: Our research concerns the photophysics of organic molecular crystals, from understanding their basic spectroscopic properties to making new materials. In this talk, we will discuss two interesting properties of molecular crystal nanostructures that motivate our studies. Transforming light into mechanical energy can be done in a controlled way by taking advantage of photochemical reactions in organic crystalline nanorods. Both reversible and irreversible shape changes in these structures can be accomplished using one or two-photon excitation. The ability to make a nanorod bend or expand using light provides a novel approach to making nanoscale photoactuators.

On the other hand, transforming light into electrical energy relies on the production of excitons in a material which can then be ionized into electron-hole pairs. We will discuss our research on exciton fission in organic polyacene structures, where an initially created singlet exciton spontaneously splits into a pair of triplets. This phenomenon provides a possible mechanism for increasing the quantum yield of charge carrier generation above unity.

September 3, 2008

"Arrays of metallic and bimetallic nanoparticles on alumina ultrathin films," Claude Henry, Universite Marseille & CiNaM, hosted by Stefan Vajda

Abstract: Regular arrays of metallic nanoparticles are grown by UHV atomic deposition on in situ prepared ultrathin alumina films. The alumina films (0.5 nm thick) are prepared by oxidation of a Ni3Al (111) surface at high temperature. AFM and STM studies show that the surface of the film is nanostructured, and it presents two interrelated hexagonal lattices of defects with parameters of 2.4 and 4.1 nm. Through the deposition of palladium atoms, palladium clusters are nucleated exclusively on the 4.1-nm lattice of defects and form an ordered array, as seen by STM. By evaporating a second metal, such as gold, only the predeposited palladium clusters grow, forming a perfect lattice of bimetallic clusters. The size (a dozen atoms to about 2 nm) and the composition of the bimetallic clusters can be independently controlled by the amounts of the two deposited metals. If we deposit gold first and then palladium, the result is different because the defects of the films are not perfect sinks for the gold clusters. Both pure palladium and bimetallic PdAu clusters coexist in that case. Kinetic Monte Carlo calculations explain these results.

The growth of the clusters have also been studied by grazing incidence small-angle X-ray scattering. The results show clearly that until coalescence, the lattice of the cluster array is perfect and extends on the whole substrate (1 cm²). The first results on the reactivity of arrays of palladium clusters will be presented. The adsorption of CO has also been studied by pulsed molecular beam techniques.

August 20, 2008

"Modulating the Power Factor with the Morphology of in situ Grown Nanostructured Thermoelectric Films," Clemens Burda, Case Western Reserve University, hosted by Matthew Pelton

Abstract: Thermoelectrics, which convert heat to electricity or vice versa, provide a promising approach to help overcome the current energy challenge by making use of thermal energy, such as waste heat from combustion engines. Recent theoretical and experimental progress on low-dimensional thermoelectric materials demonstrated that the ZT figure of merit could be greatly enhanced through nanoengineering of materials. However, this nanoengineering also increased the cost of thermoelectrics. Therefore, techniques that could compromise both the cost and efficiency requirements are going to be the focus of future studies. This presentation reports the exploration of nanostructured p-type PbSe thin films and fine-tuning of their thermoelectric properties through a cost-efficient wet chemical approach.

August 15, 2008

"Self-Organized Nanoparticle Assemblies: A Panoply of Patterns," Philip John Moriarty, University of Nottingham, hosted by Xiao-Min Lin

Abstract: Nanoparticle-solvent films deposited on solid substrates are associated with a rich dynamic behavior which gives rise to a wide variety of striking self-organized patterns]. Although close-to-equilibrium self-assembly of nanoparticle arrays has been studied in some depth, there has been rather less work on solvent-nanoparticle systems driven far from equilibrium (via, for example, spin coating). In the far-from-equilibrium regime, a remarkably broad array of intricate, spatially correlated patterns form including "foam-like" cellular networks, labyrinthine structures similar to those formed in spinodal decomposition of binary fluids, and well-defined fractal morphologies. I shall focus on our recent results in two areas: (i) "coerced coarsening" of nanoparticle arrays where the system is mechanically driven towards equilibrium], and (ii) the use of scanning probe-defined silicon oxide patterns to direct solvent dewetting and thus control pattern formation in drying nanofluids.

August 6, 2008

"Power Dissipation in Nanoscale CMOS and Carbon Nanotubes," Eric Pop, University of Illinois at Urbana-Champaign, hosted by Matthew Pelton

Abstract: High power densities are considered a major roadblock in the evolution of nanoelectronics. At the device level, such challenges are compounded by reduced thermal conductance and the thermal resistance of material interfaces. This talk will focus on power dissipation in nanoscale CMOS and carbon nanotubes, incorporating interface effects, temperature transients, and hot phonon scattering. Simple experiments are used to gain new insight into the fundamental behavior of nanotube devices. This work suggests much room for the optimization of nanoscale devices and circuits through bottom-up power-aware geometry, interface, and materials design.

July 23, 2008

"Magnetic nanostructures fabricated using block copolymer self-assembly," Caroline Ross, Massachusetts Institute of Technology, hosted by Seth Darling

Abstract: Block copolymers, which microphase-separate into ordered periodic nanoscale structures, provide a path to accomplish large-area patterning of arrays of dots or lines with periodicity on a scale of about 10-100 nm. We will describe the block copolymer lithography process and the magnetic properties of nanostructures made using this method, including CoCrPt dot arrays, Co/Cu/NiFe multilayer antidot arrays, and Co ring structures. We will describe the selection of diblock and triblock copolymer chemistry and processing methods, including substrate functionalization and thermal or solvent annealing, and describe the advantages of silicon-containing block copolymers for lithography. Long-range order can be imposed on the self-assembled block copolymer microdomains using topographical features such as trenches or small pillars made using optical or electron-beam lithography, giving well-ordered arrangements of nanoscale features. For example, in circular pits, concentric ring patterns can form. Of particular interest is the use of "'sparse" templates to produce densely packed block copolymer microdomain arrays with periodicity significantly less than that of the template. Applications in patterned magnetic media and ring-shaped magnetoelectronic memory devices will be discussed.

July 16, 2008

"Graphene-based Functional Materials and Devices," Yong P. Chen, Purdue University, hosted by Derrick Mancini

Abstract: Graphene (two-dimensional carbon) has attracted interest as a novel material promising diverse applications ranging from nanoelectronics to energy conservation. Ongoing research projects on material properties and functional devices of exfoliated and epitaxial graphene will be described. We have characterized epitaxially deposited carbon on metals (then transferred to insulators) and on sapphire and demonstrated their excellent promise as large-area graphene for electronic applications. The fabrication of various nanostructures based on exfoliated graphene using e-beam lithography as well as AFM-tip induced local oxidation and a novel hysteretic field effect found in junctions between graphene with different thickness and its potential application for nonvolatile memory will be discussed. Finally, the prospect of using functional graphene nanostructures to store, convert and manage energy will be discussed.

July 9, 2008

"Three Short Stories about Gold Nanorods," Cathy Murphy, University of South Carolina, hosted by Xiao-Min Lin

Abstract: Our research group has developed synthetic methods to make gold nanorods that are monodisperse in size and shape. The structure-directing agent we use is cetyltrimethylammonium bromide (CTAB) which forms a bilayer on the gold nanorod surface.

• In Short Story 1, "Outside and Inside," I will describe experiments in which we coat the outside of the CTAB bilayer to introduce chemical functionality to the nanorods on the outside, and also use the CTAB bilayer to take up hydrophobic molecules from aqueous solution to the inside of the bilayer, which has interesting environmental implications.
• In Short Story 2, "Cells and Gels," I describe experiments in which we incubate living cells, either in supported on a film or in a collagen gel, with nanorods in order to determine effects on the cells (toxicity, changes in gene expression, etc.).
• In Short Story 3, "The Estuary," I describe experiments in which we introduce gold nanorods into model coastal ecosystems and measure (by ICP-MS) how the gold is distributed in water, sediment, plants and animals.

June 25, 2008

"Advancing the Function of 3D Photonic Crystals through Materials Chemistry," Paul Braun, University of Illinois at Urbana-Champaign, hostd by Yugang Sun

Abstract: Three-dimensional photonic crystals have been of interest for some time. However, simple photonic crystals, such as those formed through colloidal crystallization of polymers or silica microspheres, have generally limited application. Most photonic applications, including solar energy harvesting, lasers, waveguides, and chemical sensors, require the structures to be formed of optically active materials, often materials with high refractive index or metallic materials, and may also require the incorporation of aperiodic defect structures within the periodic structure of the photonic crystal. We have made significant strides in both expanding the materials available for forming photonic crystal through various templating schemes and adding functional defect structures within photonic crystals. Through electrochemical routes, metal inverse opals were formed that may provide the functional element for a new class of solar cells. We found that simple electrochemical infilling of a colloidal crystal followed by removal of the colloidal template was not sufficient to form an optically interesting structure. It was only after the filling fraction of the metal was reduced by a subsequent electropolishing step that optical properties became three-dimensional in nature. High-refractive-index photonic band-gap crystals containing embedded three-dimensional waveguides were formed through a combination of multiphoton polymerization, atomic layer deposition, and chemical vapor deposition. The optical transmission through these waveguides was measured in the near-infrared. Both pH- and glucose-responsive photonic crystal-based sensors were generated through photopolymerization of a chemical responsive hydrogel within a colloidal crystal. In all thes systems, complete optically and structural characterization was performed. Alternative (noncolloidal) routes to photonic crystals, including ink-based direct writing and holographic routes will also be discussed. As well as being interesting means to form photonic crystals, these techniques are also proving powerful in synthesizing nonspherical colloids that may assemble into unique structures.

June 11, 2008

"Semiconductor spintronics (Just the facts)," Nitin Samarth, The Pennsylvania State University, hosted by Anand Bhattacharya

May 28, 2008

"The Spin on Electronics!," Stuart Parkin, IBM Almaden Research Center, hosted by Matthias Bode

Abstract: Today, nearly all microelectronic devices are based on storing or flowing the electron¹s charge. The electron also possesses a quantum mechanical property termed "spin" that gives rise to magnetism. Electrical current is comprised of "spin-up" and "spin-down" electrons, which behave as largely independent spin currents. The flow of these spin currents can be controlled in thin-film structures composed of atomically thin layers of conducting magnetic materials separated by nonmagnetic conducting or insulating layers. The resistance of such devices, so-called spin valves and magnetic tunneling junctions, respectively, can be varied by controlling the relative magnetic orientation of the magnetic layers, giving rise to magnetoresistance tailored for different applications. Recent advances in generating, manipulating, and detecting spin-polarized electrons and electrical current make possible new classes of spin-based sensor, memory, and logic devices, generally referred to as the field of spintronics. In particular, the spin valve is a key component of all magnetic hard-disk drives manufactured today and has enabled their nearly thousandfold increase in capacity over the past eight years. The magnetic tunnel junction allows for a novel, high-performance random-access solid-state memory that maintains its memory in the absence of electrical power. The respective strengths of these two major classes of digital data storage devices, namely the very low cost of disk drives and the high performance and reliability of solid-state memories, may be combined in the future into a single spintronic memory-storage technology, the magnetic Racetrack. The Racetrack is a novel three-dimensional technology that uses nanosecond long pulses of spin polarized current to move a series of magnetic domain walls along magnetic nanowires.

May 19, 2008

"Incorporating Abiological Function in de Novo Designed Proteins," H. Christopher Fry, University of Pennsylvania, hsoted by Tijana Rajh

Abstract: The de novo design of helical bundles capable of binding natural heme complexes is well established. Extending this knowledge to include abiological metalloporphyrin-based chromophores will enhance the utility as well as test the reliability of computationally derived proteins. Success has been found in the design and characterization of a heterotetrameric a-helical bundle (AHis:BThr) that binds the photo-activatable, abiological chromophore, zinc diphenylporphyrin (ZnDPP). In addition, the incorporation of an abiological, nonlinear optic chromophore ((porphinato)zinc-ethyne-(terpyridyl)ruthenium complex, RuPZn) into a computationally designed, asymmetric single-chain helix bundle (SC_RPZ) exploits the natural helical chirality potentially enhancing and macroscopically organizing the complex of interest.

May 14, 2008

"Probing molecular-level organizational structure and electronic properties of weakly surface bound metallic nanoparticles, chiral domains, and single biomolecules," Thomas Pearl, North Carolina State University, hosted by Seth Darling

Abstract: Mechanisms of adsorption and organization of organic molecules on metallic surfaces play a significant role in the growth of chemically and electronically tuned, monolayer thin films. Intercommunication between functional groups for individual adsorbates can serve as the primary driving force for monolayer crystallinity as well as electronic structure especially in the limit of weak interaction between the adsorbate and substrate. In this talk, I will present a series of examples involving weakly bound surface species probed with high-spatial-resolution scanning tunneling microscopy (STM) and spectroscopy. As a first example, data will be discussed regarding spectral diffusion features for ligand encapsulated Au11 nanoparticles supported and isolated on alkanethiolate monolayers. The bulk of the work presented will involve submonolayer ordering of a chiral molecule, tartaric acid (C4H6O6), weakly bound to an achiral metal surface, Ag(111), as studied with low-temperature STM and density functional theory. Molecularly resolved images of enantiomerically pure (R,R)- and (S,S)-tartaric acid domains on Ag(111) will be presented, and the role of intermolecular hydrogen bonding in stereospecific domain and superlattice formation will be addressed. Additionally, we will consider chiral domain formation and phase separation from a racemic mixture of tartaric acid enantiomers. Lastly, we will present differential conductance mapping of tartaric acid molecular domains that highlight an intrinsic decoupling of molecular film electronic states with respect to the metallic lattice. While the chiral expression that drives the formation of enantiomeric domains does not induce stereospecific conductance, we demonstrate electronic differentiation of submonolayer organic domains from the Ag(111) surface. Density functional theory calculations will be discussed as they relate to both the molecular organization as well as the deconvolution of electronic structure between the molecular film and the metallic substrate. Finally, I will also highlight recent work in our group involving the study of functionalized, single- and double-stranded DNA molecules anchored to both metallic and ferroelectric surfaces.

April 30, 2008

"Nanoscale assembly using conditions far from equilibrium," Heinrich Jaeger, University of Chicago, hosted by Seth Darling

Abstract: Far from equilibrium conditions open up a range of new possibilities for forming structures on the nanometer scale. This talk will explore two situations where such conditions can be utilized to self-assemble nanoparticles. The first deals with the selective metal decoration of diblock copolymer scaffolds, while the second concerns drying-mediated nanoparticle assembly. Both situations deal with structures in the range of 5 to 50 nm that is difficult to tackle with conventional approaches.

April 16, 2008

"Ag Nanocrystal Plasmons: Single-Molecule Raman Scattering and Photovoltage," Louis Brus, Columbia University, hosted by Tijana Rajh

Abstract: 30-nm Ag particles act as almost ideal "nano-antennas" for visible light. Two touching 30-nm Ag nanocrystals exhibit a junction "hot spot" in the local electromagnetic field enhancement. If a molecule is chemisorbed in the junction and also electronically resonant with the laser, this enhancement is sufficient to enable single-molecule Raman spectroscopy. The Ag metal "hot hole" coherent polarization also photo-oxidizes adsorbed stabilizing citrate anions, thus charging the nanocrystals. This creates a cathodic photovoltage, which can be observed as open-circuit photovoltage in an electrochemical cell containing nanocrystals adsorbed on a transparent electrode. Photovoltage creates enhanced reduction of Ag+ in solution, and thus, the nanocrystal grows in size. Photovoltage-driven Ostwalt ripening causes growth of aqueous colloidal 70-nm single-nanocrystal prisms from 8-nm Ag seeds in the presence of air. The visible irradiation wavelength controls the lateral size of the prisms.

April 2, 2008

"Towards a Single Quantum Dot Microdisk Laser," Glenn Solomon, National Institute of Standards and Technology (NIST) and University of Maryland, hosted by Matthew Pelton

Abstract: Ultra-low-threshold lasers have applications in low-power communications and emerging areas such as on-chip communications. At these very low thresholds, our common understanding of lasing breakdowns because the lasing occurs via only a few emitters and the gain medium is therefore highly nonuniform. An excellent example is the one studied here, where the gain medium is a very dilute array of semiconductor quantum dots (QDs). In such a case, the turn-on of lasing is not expected to be abrupt; the typically knee in the laser L-I curve, often used to define a threshold, will be transformed into a soft transition.

I will discuss an ultralow, sub-µW threshold microcavity laser. The gain medium is a randomly distributed ensemble of InAs QDs. The cavity is formed in a microdisk of GaAs, with a quality factor of approximately 17000. On average less than one QD spectrally and spatially aligned with a cavity mode.

March 19, 2008

"Nanoscale Spectroscopy with Optical Antennas," Lukas Novotny, University of Rochester, hosted by Gary Wiederrecht

Abstract: Antennas are devices that efficiently convert localized energy to free propagating radiation and vice versa. They are a key enabling technology in the microwave and radiowave regime but their optical counterpart is greatly unexplored.

In order to understand antenna-coupled light emission and absorption, we use a single molecule as an elementary light-emitting device. With an optical antenna in the form of a simple gold particle, we are able to increase the emission efficiency by more than a factor of 10. However, for very short distances between particle and molecule, the fluorescence yield drops drastically because of nonradiative energy transfer. A simple gold particle is not an efficient optical antenna, and it can be expected that favorably designed nanoplasmonic structures will yield much higher enhancement.

Optical antennas can be employed as light sources for high-resolution optical microscopy and spectroscopy. We demonstrate vibrational (Raman scattering) and nonlinear imaging with spatial resolutions down to 10 nm.

March 5, 2008

"Tools for the Synthesis and Characterization of NanoBio Interfaces," Milan Mrksich, University of Chicago

Abstract: This seminar will describe an approach that employs self-assembled monolayers for modifying man-made materials with biological functionality. These surface chemistries permit wide flexibility in patterning the attachment of proteins and cells with nanoscale control. The resulting interfaces are important for studies of cell adhesion, for high throughput assays of biochemical activities, for fundamental studies of enzyme reactions at interfaces and for integrating biological activities with electrical processes.

February 20, 2008

"Novel Carbon Cluster Materials and their Reactivity towards Hydrogen," Manfred Kappes, Institute of Physical Chemistry, University of Karlsruhe and, Institute of Nanotechnology, Forschungszentrum Karlsruhe, hosted by Stefan Vajda

Abstract: SIon beam soft-landing of mass selected fullerene ions has been used to generate multilayer films. We have studied their thermal and electronic properties as well as their reactivity towards atomic hydrogen and alkali atoms.

February 6, 2008

"Mach-Zehnder Interferometry and Microwave- Induced Cooling in Persistent Current Qubits," William D. Oliver, MIT Lincoln Laboratory, hosted by Matthew Pelton

Abstract: Superconducting persistent-current qubits are quantum-coherent artificial atoms with multiple energy levels. In the presence of large-amplitude harmonic excitation, the qubit state can be driven through one or more of the energy-level avoided crossings. The resulting Landau-Zener transitions mediate a rich array of quantum-coherent phenomena as a function of the driving amplitude and frequency.

In this talk, we present three such demonstrations of quantum coherence in a strongly driven niobium persistent-current qubit.

1. The first is Mach-Zehnder-type interferometry, for which we observe quantum interference fringes for 1-50 photon transitions.
2. The second is a new operating regime exhibiting coherent quasi-classical dynamics, for which the MZ quantum interference persists even for driving frequencies smaller than the resonance linewidth.
3. The third is microwave-induced cooling], for which we achieve effective qubit temperatures <3 mK, a factor 10-100x lower than the dilution refrigerator ambient temperature.

These experiments exhibit a remarkable agreement with theory, and are extensible to other solid-state qubit modalities. In addition to our interest in these techniques for fundamental studies of quantum coherence in strongly-driven solid-state systems, we anticipate they will find application to nonadiabatic qubit control and state-preparation methods for quantum information science and technology.

January 23, 2008

"Structure-Property Relationships in Functional Quantum Dots: From Biological Imaging to Solid-State Lighting," Sandra Rosenthal, Vanderbilt University, hosted by Stefan Vajda

December 12, 2007

"Polymer Self-Assembly in Semiconductor Microelectronics," C. T. Black, Center for Functional Nanomaterials, Brookhaven National Laboratory

Abstract: The challenge of defining semiconductor integrated circuit elements at sub-100-nm dimensions has created opportunities for alternative patterning approaches. One attractive nontraditional approach use the phenomenon of self-assembly. Under suitable conditions, certain materials self-organize into patterns offering promise for enabling further advances in semiconductor microelectronics. Diblock copolymers are particularly attractive for this application because — like photoresist materials used for lithography — they can act as sacrificial templates for patterning integrated circuit elements.

Self-assembly has applications in both current and future microelectronics, and materials integration is the critical first step for adoption into high-performance semiconductor technology. I will discuss my prior research program at the IBM T.J. Watson Research Center, which involved demonstrating key applications of self-assembly in high-performance semiconductor device fabrication, including its use in on-chip decoupling capacitors, nanocrystal FLASH memories, and advanced field-effect transistors. The discussion will highlight both the promise and versatility of this technique, as well as some limitations and challenges still to be addressed.

December 10, 2007

“Processes of Ordered Structure Formation iIn Polypeptide Thin-Film Solutions,” Ioan Botiz, Institut de Chimie des Surfaces et Interfaces, CNRS-UHA, hosted by Seth Darling

Abstract: Transforming thin solid films of polystyrene-poly(γ-benzyl-L-glutamate) into solutions, via exposure to solvent vapor, allowed us to nucleate and grow in such solutions ordered ellipsoidal three-dimensional structures containing millions of molecules. These structures were randomly oriented but homogeneously distributed on the film surface and possessed an anisotropic shape that we attributed to asymmetric growth in lateral directions (specific directional interactions act along the various axes of the molecules).

The experiments presented in this work proved that the size of ellipsoidal structures (limited by decrease of supersaturation) was found to be increasing with the decrease of polymer concentration in film solution. Consequently, the ordering of polypeptides in a film solution at two consecutive polymer concentrations (e.g., by performing a two-step experiment) led to a bimodal distribution of ellipsoidal structures on the film surface.

Performing experiments under different environmental conditions allowed us to conclude that polymer solubility could be influenced via surrounding gas-phase humidity variation. Increasing the humidity of the surrounding gas phase led to a decrease of the value of polymer solubility and an increase of interfacial tension between the ordered structures and the solution. Consequently, ordered three-dimensional ellipsoidal structures could be obtained even at very low polymer concentrations.

We propose that complexation of poly(γ-benzyl-L-glutamate) by water, via hydrogen bonding interactions, led to a “new” polymer which causes the formation of solid ordered structures also at low polymer concentrations. We tested this hypothesis by using besides water also other protic non-solvents like methanol or trifluoroacetic acid.

Furthermore, we asked the question if a reversible change of polymer solubility is possible by controlling the amount of protic non-solvent in the surrounding gas-phase. Adequate experiments, in which small amounts of water or methanol molecules were added and then removed from the surrounding gas-phase, proved that the process of structure formation and dissolution was reversible.

“Photo-induced Electron Transfer Processes at Nanostructured Interfaces: Advancing Excitonic Solar Cells,” Garry Rumbles, National Renewable Energy Laboratory, hosted by Tijana Rajh

Abstract: Excitonic solar cells are emerging as viable alternatives to conventional photovoltaic devices. At present, power-conversion efficiencies have only reached 5% and at least a threefold improvement is needed in order to be competitive with the existing technologies.

This presentation will focus on the nanostructured interface that is formed between two chemical systems: one a donor, the other an acceptor. The function of this interface is to dissociate excitons and create free carriers that do not immediately recombine. The technique used to follow this process is time-resolved microwave conductivity (TRMC). This electrode-less technique provides a means of studying the efficiency and kinetics of free carrier production (and loss) and therefore allows us to focus on the interface at the heart of the excitonic solar cell and therefore understand the fundamental processes that are occurring; processes that are key to advancing solar cell conversion efficiencies. Specific donor-acceptor systems that will be discussed includes the conjugated polymer poly(3-hexylthiophene) combined with C60, deposited on zinc oxide and solublized with single-wall carbon nanotubes. Each system will be used to highlight topics such as exciton dissociation, carrier mobility, and exciton annihilation.

November 14, 2007

“Plasmonic Route to Nanophotonics, ” Anatoly Zayats, The Queen’s University of Belfast, hosted by Gary Wiederrecht

Abstract: Recent advances in nanofabrication and subwavelength optical characterisation have led to the development of a new area of nanophotonics concerned with routing and manipulation of optical signals in scalable and integratable devices. In this context, plasmonics that is dealing with surface electromagnetic excitations in metallic structures, may provide a great deal of flexibility in photonic integration in all-optical circuits since with surface plasmons the problem of light manipulation can be reduced from three to two dimensions. Surface plasmon polaritons (SPPs), the electromagnetic excitations coupled to collective motion of conduction electrons near a metal surface, are emerging as a new optical information carrier that enables signal manipulation and processing on the subwavelength scale.

SPPs play a crucial role in determining optical properties of randomly rough and artificially structured metal surfaces and films, such as reflection, transmission, scattering, second-harmonic generation, surface-enhanced Raman scattering, etc. Various elements of two-dimensional optics-based on surface plasmon polaritonic excitations, such as mirrors, lenses, resonators, planar waveguides have been demonstrated. Band-gap effects, enhanced optical transmission through plasmonic crystals, surface plasmon polariton waveguiding along straight and bent line defects in such crystals have also been studied. Hybridization of plasmonic nanostructures with molecular species exhibiting nonlinear optical response allows the development of photonic components with the enhanced nonlinear response due to the electromagnetic field confinement related to surface plasmons.

In this talk the applications of plasmonic nanostructures to light guiding and manipulation in subwavelength photonic elements, the enhanced nonlinear functionalities and dispersion management using metallic nanostructures will be discussed. Surface-plasmon optics provides a basis for the implementation of novel photonic functionalities and development of a new class of active photonic devices for optical signal processing and all-optical integrated circuits. Numerous possible applications of surface plasmon polaritons can be envisaged in nanophotonics, classical and quantum optical information processing and optical communications as well as optical and magneto-optical data storage.

October 24, 2007

"Magnetization Reversals in Cobalt Nanorings and Nanoparticle Rings," Alex Wei, Purdue University

Abstract: Rings of weakly ferromagnetic cobalt nanoparticles (dispersed in toluene by resorcinarene-based surfactants), formed by dipole-directed self-assembly, have been determined to support bistable flux closure (FC) states at room temperature using off-axis electron holography. In many cases the FC polarization can be switched by exposure to a coaxial magnetic pulse, then switched again by applying M(z) in the opposite direction. This physical behavior has no known analogy at the macroscopic level, but the effect can be reproduced by magnetodynamic simulations, which provide some insights into the complex mechanism of field-induced FC reversal.

October 17, 2007

"Nanofluidics: Coulombic Dragging and Mechanical Propelling of Molecules," Petr Kral, University of Illinois at Chicago, hosted by Elena Shevchenko

Abstract: We show by molecular dynamics simulations that polar molecules and ions adsorbed on carbon nanotubes can be dragged by Coulombic scattering with water, individual ions or ionic solutions flowing inside the tubes. The phenomenon is presented on the manipulation of water droplets and inverse micelles in air, and biological molecules in natural solutions.

We also introduce molecular propellers that can pump liquids in the bulk and at the liquid surfaces, in dependence on the chemistry of the liquid-blade interface. The propellers can be attached to biological molecules and cellular components that could be manipulated in vitro and in vivo.

Finally, we model biomolecular docking on predesigned nests formed on doped and ligand-modified material surfaces. We show that the GFP protein can be selectively attached to modified graphene layers. This methodology could be used in the preparation of hybrid bionanostructures.

References

B. Wang and P. Kral, "Dragging of Molecules on Material Surfaces Induced by Flowing Liquids," J. Am. Chem. Soc., 128, 15984 (2006)

B. Wang and P. Kral, "Chemically tunable nanoscale propellers of liquids," Phys. Rev. Lett., 98, 266102 (2007)

B. Wang and P. Kral," Optimal Atomistic Modifications of Material Surfaces: Design of Selective Nesting Sites for Biomolecules," Small, 3, 153110 (2007)

October 3, 2007

"Metal Nanoparticle Plasmonics: Towards Quantum Nanoantennas," Garnett Bryant, National Institute of Standards and Technology, hosted by Matthew Pelton

Abstract: Understanding the nanooptics of metallic nanoparticles is critical for applications in nanometrology, nanosensors, nanoantennas, and nano-optical communication, where large optical response is needed. This understanding becomes even more essential for hybrid structures, which combine semiconductor quantum dots with metallic nanoparticles, where quantum coherent nanooptics is desired.

The response of metallic nanoparticles is provided by the surface plasmons excited in these confined structures. We first discuss the plasmonic excitations of single and coupled metallic nanoparticles based on classical calculations of their electromagnetic response. For isolated particles, the response is dipole-like, similar to a classical radio antenna. We map out the dependence of the plasmon resonance on particle size and shape to highlight critical differences with classical antennas.

In coupled systems, interaction across gaps distorts intraparticle dipolar response, localizes charge at the gaps, significantly red shifts the response and dramatically increases near fields, effects that are important for applications. These effects become singular for nearly touching nanoparticles, dramatically changing the interparticle coupling when particles touch. We discuss this transition in detail to provide a clear picture of coupling effects in these systems.

For nearly touching particles and for small nanoparticles, quantum effects, such as interparticle tunneling, surface scattering and state filling, may play a more important role in determining plasmon response. Density functional theory is being used to study strongly coupled systems.

Preliminary results from these DFT calculations will be discussed. Quantum effects will also play a key role in strongly coupled hybrid structures of metallic nanoparticles and quantum dots. Hybrid structures have been studied with the quantum dots treated quantum mechanically, via a density matrix approach, coupled by dipole interaction to classical metal nanoparticles.

The exciton response in the quantum dot is broadened and shifted by incoherent and coherent interactions with the metal nanoparticles.

Excitation transfer between semiconductor dots in hybrid structures is determined. These results will be discussed to assess prospects for quantum coherent communication and metrology.

September 5, 2007

"Characterization and manipulation of single molecule chemistry on silicon surfaces," Mark Hersam, Northwestern University, hosted by Seth Darling

Abstract: Organic molecules mounted on silicon surfaces present unique opportunities for electronics, photonics, and sensing at the nanometer scale. To help elucidate the potential of organosilicon nanostructures, this talk outlines recent efforts to characterize and manipulate organic chemistry on silicon surfaces down to the single molecule level using ultra-high-vacuum (UHV) scanning tunneling microscopy (STM). Specific topics include templated assembly of heteromolecular organosilicon nanostructures using room-temperature multistep feedback-controlled lithography and molecular resolution characterization of cryogenic UHV STM-driven organosilicon chemical reactions. In an effort to identify the underlying physical mechanisms that control single-molecule chemistry on silicon surfaces, the aforementioned experimental results will be quantitatively compared with density functional theory calculations.

August 22, 2007

"Studies of the Dynamics of Metal Nanoparticles fFrom Electronic Dephasing to Heat Dissipation," Gregory Hartland, University of Notre Dame, hosted by Matthew Pelton

Abstract: I will discuss recent work in my group, where optical spectroscopy has been used to probe the properties of metal particles of different sizes and shapes. Two types of experiments have been performed: ultrafast time-resolved spectroscopy studies and single-particle Rayleigh scattering measurements. The single particle experiments generate information about dephasing of the localized surface plasmon resonance due to electron-surface scattering and radiation damping. Understanding these effects is important for optimizing the size and shape of metal nanostructures for sensing applications. The materials studied include gold nanorods and hollow cubic structured particles made from silver and gold. The results show that both electron-surface scattering and radiation damping are significant effects. In the time-resolved experiments, the pump laser deposits energy into the electrons. Monitoring the response of the sample provides information about electron-phonon coupling and energy exchange with the environment. The laser-induced heating also coherently excites vibrational modes of the particle that correlate with the expansion coordinate. Comparing the measured periods to the results of continuum mechanics calculations gives unique information about the elastic properties of the particles. The vibrational beat measurements also provide a way of measuring the temperature of the particles, and can be used to determine the thresholds for explosive boiling in the solvent and photothermal reshaping of the particles.

August 8, 2007

"Amphiphiles at Bio-Interfaces ­ From Structure Control to Properties," Ka Yee Lee, University of Chicago, hosted by Seth Darling

Abstract: The cell membrane acts as a barrier, controlling the transport of molecules into and out of the cell. When the structural integrity of the membrane is compromised, so does its barrier function. We examine the way the membrane barrier function can be compromised by antimicrobial peptides and how leakage of intracellular materials from a structurally damaged cell membrane can be arrested by triblock copolymers.

Antimicrobial peptides are a class of peptide innate to various organisms that functions as a defense agent against harmful microorganisms by means of membrane disordering. Despite their enormous biomedical potential, progress towards developing these peptides into therapeutic agents has been hampered by a lack of understanding of their mechanism of action. A class triblock copolymer of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)(PEO-PPO-PEO) has been shown to help seal electroporated cell membranes, arresting the leakage of intracellular materials of the damaged cell. However, the interaction mechanism between the cell membrane and poloxamer is still unclear. Using a variety of model systems and biophysical techniques, we have examined the targeting selectivity as well as the membrane disruption mechanism of antimicrobial peptide protegrin-1, and have explored the sealing capability of poloxamer 188.

July 25, 2007

“Mean-Field and Quantum Models for Magnetism in Gold Nanoparticles,” Vladimiro Mujica, Northwestern University, hosted by Tijana Rajh

Abstract: The onset of magnetism in gold nanoparticles is an intriguing phenomenon with a very rich physics. Macroscopic gold is diamagnetic, but bare gold nanoparticles may exhibit ferromagnetism in a certain size range. Chemical coating of nanoparticles may strongly modify their magnetic properties, either quenching or enhancing them.

I will present some recent theoretical work aiming to provide a model for the observed size and chemisorption dependence of magnetism in gold nanoparticles. Spin-polarized density functional calculations support the idea that bare spherical gold nanoparticles exhibiting ferromagnetism can be considered as a core-shell magnetic structure where the core is essentially diamagnetic and the non-zero contribution to the magnetic moment is concentrated on the surface atoms. I will also discuss quantum calculations on gold clusters aiming to assess the influence of different chemical adsorbates particularly in view of some recent experiments that indicate a striking difference between nitrogen and sulfur linkers used to stabilize the nanoparticles,

In addition to the quantum calculations, I will present a mean-field model that exploits the idea of a magnetic gold nanoparticle as a core-shell structure. Our results, regarding the behavior of the size-dependent magnetic moment, agree qualitatively with the experimental observation that the transition from quasi-atomic to bulk-behavior of the magnetic moment exhibits a maximum that is essentially determined by the relative numerical ratio of surface to bulk atoms, which in turn arises from the competition between the contributions to the magnetic moment due to the diamagnetic core and the ferromagnetic surface.