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March 2015
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  1. Center for Functional Nanomaterials Colloquium

    11 am, Bldg 735, Seminar Room 2nd Floor

    Hosted by: Deyu Lu

    Solar energy is the most promising source of renewable, clean energy to replace the current reliance on fossil fuels. Ferroelectric (FE) materials have recently attracted increased attention as a candidate class of materials for use in photovoltaic devices. Their strong inversion symmetry breaking due to spontaneous polarization allows for excited carrier separation by the bulk of the material and voltages higher than the band gap (Eg), which may allow efficiencies beyond the Shockley-Queisser limit. Ferroelectric oxides are also robust and can be fabricated using low cost methods such as sol-gel thin film deposition and sputtering. Recent work has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and FE-electrode interfaces can dramatically increase the current harvested from FE absorber materials. Further improvements have been blocked by the wide band gaps (Eg =2.7-4 eV) of FE oxides, which allow the use of only 8-20% of the solar spectrum and drastically reduce the upper limit of photovoltaic efficiency. In this talk, I will discuss new insight into the bulk photovoltaic effect, and materials design to enhance the photovoltaic efficiency. We calculate from first principles the current arising from the "shift current" mechanism, and demonstrate that it quantitatively explains the observed current. Then, we analyze the electronic features that lead to strong photovoltaic effects. Finally, we present new oxides that are strongly polar yet have band gaps in the visible range, offering prospects for greatly enhanced bulk photovoltaic effects. Please note: If anyone would like to schedule a meeting with Dr. Rappe, please contact Deyu Lu (dlu@bnl.gov)

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  1. Center for Functional Nanomaterials Seminar

    10 am, CFN, Bldg. 735, conference room A, 1st fl.

    Hosted by: Kevin Yager

    Center for Functional Nanomaterials Seminar Wednesday, March 11, 2015 10:00 a.m. - 11:00 a.m. Conference Room A, 1st floor Development and Implementation of Diffraction Imaging Techniques using Coherent Beams Shashidhara Marathe, PhD This presentation describes development of two different diffraction imaging techniques using coherent beams. At first, I will elucidate the Coherent Diffraction Imaging (CDI) technique implemented in reflection geometry for surface image reconstruction. It will be shown that the reflected intensity from the sample surface, measured in the Fraunhofer region, can be used to retrieve the exit wave phase information, quantitatively, without a priori knowledge of the object [1]. For practical applications, where objected to be reconstructed is laying on a substrate, it is much more desirable to use the reflection based CDI rather than the conventional CDI in transmission mode. Efforts in developing CDI techniques using hard x-ray sources at the Advanced Photon Source (APS), USA, and the Pohang Light Source (PLS), South Korea, will also be discussed. Next, I will talk about development and implementation of X-ray Grating Talbot Interferometer (XGTI) at the APS, Argonne National Laboratory (ANL), IL, USA. This is a tri-modal, non-destructive x-ray imaging technique which not only generates the radiograph of the object under investigation but also the differential phase and the dark-field (SAXS) images, simultaneously, from a single interferogram. In particular, I will talk about developing the single-grating x-ray Talbot Interferometer. A single-grating x-ray Talbot interferometer makes full use of the beam coherence available with synchrotron source compared to other laboratory based x-ray sources. This simplifies the setup and the alignment. Moreover, this technique is very useful for in-situ, time-resolved, measur

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  1. Center for Functional Nanomaterials Seminar

    10 am, CFN - Bldg. 735 Second Floor Conference Room B

    Hosted by: Charles Black

    Mixtures of salts and nanostructured block copolymers are promising solid electrolytes for rechargeable batteries with lithium metal anodes. For my doctoral research, I studied the evolution of microstructure in block copolymer/lithium salt mixtures on both nano- and micro- scales under various thermal conditions. My talk concerns the block copolymer/lithium salt mixture, polystyrene-b-polyethylene/lithium bis (trifluoromethane sulphone) imide (SEO/LiTFSI for short), primarily studied through depolarized light scattering (DPLS), to obtain the microstructural information about SEO/LiTFSI mixtures with different concentrations and molecular weights at different temperatures. The results indicated the addition of salt strongly affects the thermodynamics and kinetics of the microstructure of the mixture. We confirmed the presence of a coexistence temperature window for low molecular weight block copolymer/salt mixtures predicted by theory (the coexistence of ordered and disordered phases in equilibrium). For high molecular weight mixtures, a hypothesis has been proposed to explain the discrepancy between the results of SAXS and DPLS on identical mixtures. A high defect density increased the conductivity of block copolymer electrolytes.

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  1. Center for Functional Nanomaterials Seminar

    10 am, CFN - Bldg. 735 - First Floor Conference Room A

    Hosted by: Oleg Gang

    Silicon nanocrystals or quantum dots combine the abundance and nontoxicity of silicon with quantized and size-tunable energy band structure of quantum dots to form a new type of functional material that could find applications in biomedical fluorescence imaging, photodynamic therapy, light-emitting devices, and solar cells. Surface of silicon nanocrystals is a major concern for using them in bio-related applications. Room temperature hydrosilylation is introduced to functionalize silicon nanocrystals in darkness to minimize temperature/photon-induced side reactions which can potentially damage the capping ligands or nanocrystal surface. As a proof of concept, silicon nanocrystals are passivated with styrene at room temperature, without styrene polymerization. Silicon nanocrystals are also conjugated to iron oxide nanocrystals to generate a fluorescent/magnetic cell labeling probe. Thermally-induced thiolation is discovered to generate silicon nanocrystals passivated with silicon-sulfur bonds which are metastable and can be turn to silicon-carbon bonds through ligand exchange. The band gap and emission color of silicon nanocrystals are determined by their sizes. Monodisperse silicon nanocrystals and self-assembly of those nanocrystals are of great importance for their applications in light-emitting devices and solar cells. Silicon nanocrystals are size-selected through a modified size-selective precipitation, in which aggregation and precipitation are allowed to take place simultaneously. Face-centered cubic superlattices are assembled with size-selected silicon nanocrystals, and characterized by grazing incidence small angle X-ray scattering. The structure of silicon nanocrystal superlattice is found to be stable at temperature as high as 375oC. Simple hexagonal AlB2 binary superlattice is also formed with silicon and gold nanocrystals

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  1. Center for Functional Nanomaterials Seminar

    10:30 am, CFN - Bldg. 735 First Floor Conference Room A

    Hosted by: Oleg Gang

    Rational, de novo design of RNA nanostructures can potentially integrate a wide array of structural and functional diversities. Such nanostructures have great promises in biomedical applications. Despite of impressive progress in this field, all RNA nanomotifs (tiles or building blocks) reported so far are not geometrically well-defined. They are generally flexible and can only assemble into a mixture of complexes with different sizes. To achieve defined structures, multiple tiles with different sequences are needed. In our study, we have de nova designed a RNA nanomotif that can homo-oligomerize into a uniform RNA nanostructure. We use PAGE, AFM and Cryo-EM to analyze our data. Based on this work, we further developed single stranded RNA tiles. These artificial single stranded RNA tiles can self-assemble into well-defined 1D, 2D and 3D structures. We believe that development along this line would help RNA nanotechnology to reach the structural control that currently associates with DNA nanotechnology.

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  1. APR

    2

    Thursday

    Center for Functional Nanomaterials Seminar

    "Reconstruction of Hilical Bio-Structure Using X-ray Diffraction Experiment"

    Presented by Dr. Miraj Uddin, University of Wisconsin-Milwaukee, Physics Department

    9:45 am, CFN Bldg. 735 - Second Floor Conference Room B

    Thursday, April 2, 2015, 9:45 am

    Hosted by: Kevin Yager

    Recovery of three-dimensional structure from single particle X-ray scattering of completely randomly oriented diffraction patterns as predicted few decades back has been real due to advent of the new emerging XFEL (X-ray free electron laser) technology. Some of the best-known structure determination of helical objects such as helical viruses or deoxyribonucleic acid has been done by fiber diffraction. Layer line intensities of fiber diffraction pattern as expressed by cylindrical harmonics can be transformed into equivalent spherical harmonic expansion leaving the clue behind that structure of helical objects may be recovered from single particle scattering of randomly oriented helical molecules thus avoiding the tedious challenge of single axis alignment. In this work we have solved the structure of TMV (tobacco mosaic virus) helices from a set of randomly oriented simulated diffraction patterns exploiting symmetry and internal constraint of the diffraction volume thus proving the above claim on step ahead. As the world's first XFEL is in operation starting from June 2009 at SLAC National Lab at Stanford, the very first few experiments being conducted on larger objects such as viruses. We have analyzed a set of experimental diffraction patterns of chlorella virus deposited on cxidb.org and recovered a quadratic coefficients of Fourier shell correlation whose angular momentum selection rule proves that the collected data is primarily from an icosahedral object.

  2. APR

    14

    Tuesday

    Center for Functional Nanomaterials Colloquium

    "The bulk photovoltaic effect in polar oxides for robust and efficient solar energy harvesting"

    Presented by Boris I. Yakobson, Rice University, Houston, Texas, USA

    11 am, Bldg 735, CFN 2nd Floor Seminar Room

    Tuesday, April 14, 2015, 11:00 am

    Hosted by: Dimitri Zakharov

    Connecting the underlying chemical processes with the growth and emergent form remains unsurmountable problem in life sciences [1]. In materials research, the current outlook is more optimistic: establishing such connection, from the basic interatomic forces to growing nanostructure shape and properties becomes a real possibility. We will discuss several important examples, focusing on two recent results. First one concerns the nanotubes, where it took two decades to derive a kinetic formula [2] R ~ sin x (growth rate R, helical angle x). Further analysis of the subtle balance between the kinetic and thermodynamic views reveals sharply peaked abundance distribution A ~ x exp (-x) [3]. This explains the puzzling (n, n-1) types observed in many experiments. In the second example, a combination of DFT and Monte Carlo models explains the low symmetry shapes of graphene on substrates. In equilibrium, edge energy variation dE manifests in slightly distorted hexagons. In growth, it enters as ~exp(-dE/kT), amplifying the symmetry breaking to triangle, ribbon, rhomb [4]. Third example concerns 2D materials of more complex chemistry, h-BN and MX2 among them, and how their defects, dislocations and grain boundaries, predicted from the first principles, find remarkable experimental confirmations [5]. [1] On Growth and Form, by D'Arcy W. Thompson (Cambridge U, 1917). [2] F. Ding et al. PNAS 106, 2506 (2009); R. Rao et al. Nature Mater. 11, 213 (2012). [3] V. Artyukhov - E. Penev et al. Nature Comm. 5, 489 (2014). [4] Y. Liu et al. PRL 105, 235502 (2010); V. Artyukhov et al. PNAS 109, 15136 (2012); Y. Hao et al. Science, 342, 720 (2013); V. Artyukhov et al. PRL 114, 115502 (2015). [5] X. Zou, et al. Nano Lett., 13, 253 (2013); S. Najmaei et al. Nature Materials, 12, 754 (2013); A. Aziz et al. Nature Comm., 5, 4867 (2014). *** Boris I. Yakobson is an expert in theory and computational modeling of materials na

  3. MAY

    31

    Sunday

    CFN Proposal Deadline

    "CFN Proposal Deadline for September-December Cycle 2015"

    11:45 pm, CFN

    Sunday, May 31, 2015, 11:45 pm

  4. SEP

    30

    Wednesday

    CFN Proposal Deadline

    "CFN Proposal Deadline for January-April Cycle 2016"

    11:45 pm, CFN

    Wednesday, September 30, 2015, 11:45 pm