Argonne National Laboratory

2011 Colloquium Archive

Date Title
December 21, 2011

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.

December 14, 2011

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.

December 7, 2011

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.

November 16, 2011

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.

November 10, 2010

"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.

October 26, 2011

"'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.

July 20, 2011

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.
July 6, 2011

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.

June 22, 2011

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.

June 8, 2011

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.

May 11, 2011

"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.

April 27, 2011

"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.

April 13, 2011

"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.

March 30, 2011 Shan Wang, Stanford University, hosted by Elena Rozhkova
March 16, 2011

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.
March 2, 2011

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.

February 23, 2011

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

February 16, 2011

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. 

February 9, 2011

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.

January 19, 2011

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.

January 12, 2011

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.

January 5, 2011

"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.