Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G24: Polymer Nanomaterials I |
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Sponsoring Units: DPOLY Chair: Kookheon Char, Seoul National University Room: Baltimore Convention Center 321 |
Tuesday, March 14, 2006 8:00AM - 8:36AM |
G24.00001: BREAK
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Tuesday, March 14, 2006 8:36AM - 8:48AM |
G24.00002: Marker Motion Measurements of Nanoparticles in Polymer Matrices Aleta Hagman, Kenneth R. Shull, Jin Wang, Xuefa Li, Dong Ryeol Lee, Suresh Narayanan Marker motion can be used to infer the ordering kinetics and nanoparticle dynamics in model polymer/metal nanocomposite thin films. In current experiments, the thermally evaporated gold particles (few nanometers in diameter) act as a marker layer between two polymer layers with different mobilities. The disparity in the mobility causes a marker movement towards the layer with the higher mobility. X-ray standing waves (XSWs), generated by total external reflection above a mirror surface, are used to monitor the time evolution of the gold nanoparticle distribution as the nanocomposite ultrathin films are heated above the polymer glass transition temperatures. Diffusion coefficients are obtained from the time-dependence of the nanoparticle distribution evolution. Polymer/gold interactions has shown to be system dependent, with pol(tert-butyl acrylate) (PtBa) showing weak interactions while poly(2-vinyl pyridine) (PVP) shows strong interactions as expected. Free surface effects relating to mobility will also be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G24.00003: Effect of Areal Density of Polymer Chains on Gold Nanoparticles on Nanoparticle Location in a Block Copolymer Template B.J. Kim, J. Bang, C.J. Hawker, E.J. Kramer It is well established that one block of a copolymer can interact preferentially with an inorganic substrate to produce wetting and domain orientation. We take advantage of this preferential interaction to control the location of 2.5 nm diameter Au nanoparticles coated with short thiol-terminated polystyrene (M$_{n}$=3.4 kg/mol) chains (PS-SH) in a symmetric poly(styrene-$b$-2 vinyl-pyridine) (PS-b-P2VP) diblock copolymer (M$_{n}$=196 kg/mol) by changing the areal density $\Sigma $ of the PS-SH on the Au. If $\Sigma \quad \ge $ 1.6 chains/nm$^{2}$, the preferential interaction between the P2VP of the PS-b-P2VP and the Au surface is screened and the Au localizes in the center of the PS domains. If $\Sigma \quad \le $ 1.4 chains/nm$^{2}$ , the Au particles are localized at the PS-P2VP interface. Au nanoparticles coated with thiol terminated P2VP (M$_{n}$=3 kg/mol) localize in the center of the P2VP domain of the PS-P2VP over the entire range of $\Sigma $, demonstrating the localization of the PS coated Au nanoparticles at the interface at low values of $\Sigma $ is due to the unscreened Au-P2VP interaction. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G24.00004: Nanocomposite Microdomain Inversion in Au Nanoparticle/ PS-b-PFOMA Block Copolymer Films Luciana Meli, Yuan Li, Keith P Johnston, Peter F Green We report that Au nanocrystals sequestered within the microdomains of self-assembled aggregates of polystyrene-b- poly(1,1',2,2'-tetrahydroperflurooctyl methacrylate) (PS-b- PFOMA) thin films are capable of following the structure-guiding matrix in a morphological transition from spherical PFOMA aggregates in a PS matrix to the inverse morphology upon supercritical CO2 (scCO2) sorption. Furthermore, the domain size of the aggregates can be finely tuned by adjusting the scCO2 density. By pretreating the particle´s surface with a host of ligands with different chemical affinities for the constituent blocks, we observe that the Au/diblock co-assembly process is dependent not only on the ligand size and ligand- matrix compatibility, but also on the interaction of the ligands with the interfaces that confine the film. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G24.00005: Dielectrophoretic alignment of montmorillonite nanoplatelet suspensions in an organic matrix Evangelos Manias, Georgios Polizos, Hilmar Koerner, Richard Vaia High orientational alignment of pseudo-two-dimensional inorganic platelets in an organic matrix is achieved by external AC electric fields ($\sim$0.5V/$\mu$m, rms). Namely, montmorillonite alumino-silicate platelets are organically modified by alkyl-ammonium surfactants and dispersed in an uncrosslinked epoxy. Orientation is quantified through wide angle 2D X-Ray diffraction under an AC electric field ($\sim$0.05-4V/$\mu$m), following the reorientation of inorganic stacks (tactoids), resulting in Hermans orientation factors of 0.7--0.9 even at moderate field strengths. The degree of orientation dependence on the electric field frequency and strength is presented. The electrophoretic motion of the cationic surfactants as a possible mechanism to produce alignment is delineated via broadband (10$^{-2}$-10$^7$ Hz) dielectric relaxation spectroscopy, and dipole moment theoretical analyses. The cationic electrophoretic motion does not have any major contribution for the platelet alignment, suggesting that the primary cause is due to induced dipoles (image charges) on the dielectric inorganic platelets. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G24.00006: Surface morphology in dewetting polystyrene/polyhedral oligomeric silsesquioxane (POSS) thin-film bilayers Rituparna Paul, Michael C. Swift, John R. Hottle, Alan R. Esker Polymer/polymer and polymer/nanoparticle bilayers on solid substrates are common in technological applications. Dewetting or the spontaneous formation of holes in polymer films above their glass transition temperature, poses a serious problem in the nanofabrication of polymeric multilayers. This study utilizes bilayer films of polystyrene (PS) and trisilanolphenyl- POSS (TPP) as a model to investigate the morphological evolution of polymer/nanoparticle bilayers upon annealing. X-ray photoelectron spectroscopy, and optical and atomic force microscopies have been used to study dewetting as a function of annealing time and temperature. Upon annealing the PS/TPP bilayers at 200 C, the upper TPP layer undergoes instantaneous cracking and these cracks serve as nucleation sites for dewetting of the TPP and PS layers for annealing times up to 90 minutes. For annealing times longer than 90 minutes, the dewetting of TPP and PS layers is complete resulting in the formation of TPP encapsulated PS droplets. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G24.00007: Block Copolymer Nanocomposites for RF Magneto-dielectric Applications Ta-i Yang, Peter Kofinas The aim of this research is to develop novel block copolymer nanocomposites for radio frequency applications. Block copolymers (BCP) act as templates to enhance the ordering of nanoparticles within the polymer matrix and tailor the desired electromagnetic properties such as permeability ($\mu )$ and permittivity ($\varepsilon )$. We incorporated iron and strontium or titanium organometallic precursors into three different polymer matrices (styrene-$b$-isoprene-$b$-styrene (SIS), styrene-b-(ethylene-ran-butylene)-b-styrene (SEBS), and sulfonated styrene-b-(ethylene-ran-butylene)-b-styrene (S-SEBS)) and then formed mixed metal oxide nanoparticles within the BCP. Preliminary experimental results demonstrated we can obtain $\mu =\varepsilon $ near 2.7 with low loss tangent (tan $\delta \quad <$0.01) through the 1 M to 1G Hz frequency range by utilizing SIS or SEBS with templated Fe$_{2}$0$_{3}$/SrO nanoparticles. For the S-SEBS copolymer composites, most mixed metal oxide nanoparticles were successfully templated into BCP self-assembled patterns confirmed by TEM, and the loss tangent could be significantly reduced by heat treatment. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G24.00008: Effect of confinement on the polymer segmental motion and on ion mobility in PEO/layered-silicate nanocomposites Spiros H. Anastasiadis, Kiriaki Chrissopoulou, Athanasios Afratis, M.M. Elmahdy, George Floudas, Bernhard Frick The effect of confinement on the local, the segmental and the ionic processes of poly(ethylene oxide)/montmorillonite (PEO/MMT) intercalates are investigated. X-ray diffraction, DSC and polarizing optical microscopy data show that the intercalated PEO chains remain amorphous and it is only the excess PEO chains outside the galleries that crystallize. This is also verified by the behavior of the elastic intensity of quasi-elastic neutron scattering (QENS) experiments on the same hybrids. Dielectric relaxation spectroscopy reveals that confinement results in an acceleration of the PEO segmental dynamics that display an Arrhenius temperature dependence with very low activation energy, whereas the local beta-process remains unaffected. Likewise, ionic mobility is enhanced in the nanohybrids suggesting that ion mobility is assisted by the faster PEO segmental motion. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G24.00009: Photovoltaic Responses of Solar Cells Based on Lead Selenide Quantum Dots and Conducting Polymers Xiaomei Jiang, Sergey Lee, Anvar Zakhidov, Richard D. Schaller, J.M. Pietryga, Victor Klimov We report on hybrid solar cells based on nanocomposites of conjugated polymers (polythiophene and polyphenylvinylene derivatives) and IR-sensitive PbSe QDs that have a size-tunable energy gap between 0.3 and 1 eV. Thin film cells show very good diode characteristics and sizable photovoltaic response. The good performance of our devices in both photovoltaic and photodiode regimes indicates quite efficient charge separation between the polymer and QD components. To elucidate the mechanism for charge separation in these composite structures, we analyze conduction and valence-band energy offsets derived from cyclic voltammetry measurements. We find the significant increase of photocurrent due to more efficient charge separation when the surface of the quantum dots was treated with pyridine to remove the surface ligand. The analysis of a spectrally resolved photocurrent measurements reveals some evidence of carrier multiplication-enhanced photocurrent in our devices. Specifically, we observe indications of a rapid increase in the photocurrent at spectral energies in correlation with the size-dependent energy gap of the QD component. The onset of this increase correlates with what is expected for the onset of carrier multiplication in PbSe QDs. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G24.00010: Electric Field Effect on Wannier- Frenkel Hybrid Exciton in a Quantum Dot Coated by an Organic Shell Que Huong Nguyen, Joseph L. Birman The effect of external electric field on the Wannier Mott-Frenkel hybrid excitons in the configuration of a spherical semiconductor quantum dot coated by an organic shell has been studied. The electric field reduces the exciton binding energy and shifted the resonance region where the Wannier and Frenkel exciton are strongly mixed. The quadratic Stark dependence of the hybrid exciton energy on the electric field was found for the confined Stark effect for the hybrid exciton. Upon the application of field the coupling term between the two kinds of excitons increases. An important result is the possibility of tuning the Wannier-Frenkel exciton resonance by applied electric field [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G24.00011: Transport measurements in semiconductor nanocrystals capped with electroactive ligands Claudia Querner, Peter Reiss, Michael D. Fischbein, Patrice Rannou, Joel Bleuse, Adam Pron, Marija Drndic Organic-inorganic hybrid materials based on semiconductor nanocrystals and $\pi $-conjugated oligomers or polymers have seen an increasing interest over the past years mainly because of their possible applications in optoelectronic devices, such as solar cells. We report here the preparation of organic-inorganic hybrids by chemical grafting of tetraaniline onto the surface of semiconductor nanocrystals, such as CdSe and PbSe. We proved by spectroscopic and spectroelectrochemical studies that the grafting reaction does not influence in a significant way the electroactivity of both systems nor does it modify the doping mechanism of the grafted ligands. By cyclic voltammetry we were able to estimate the position of the energy levels in the hybrid, which are aligned such that a charge separation can occur at the organic-inorganic interface after photo-excitation. Highly efficient photoluminescence quenching confirmed such a process. Finally, we studied photoconductivity in these materials. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G24.00012: Photooxidation of sugar and alcohol on TiO$_{2}$ surfaces: A first-pirnciples study Mao-Hua Du, Jun Feng, Shengbai Zhang First-principles studies are carried out on TiO$_{2}$/sugar (alcohol) interfaces under UV illumination. A rapid charge separation takes place at the interfaces through a two-step process: (1) trapping of photo-generated carriers at the gap levels induced by the chemisorbed molecules, and (2) upon the carrier trapping, a structural transformation of the adsorbed molecules, which in turn eliminates the gap levels. The second-step here is necessary to avoid carrier recombination, and hence results in an irreversible charge separation. Such a charge transfer across the semiconductor/molecule interface leads to various oxidation and reduction processes with structural reconfigurations (bond- forming and breaking) of the surface molecules. These results reveal the underlying microscopic mechanism of photo-catalytic reactions on the TiO$_{2}$ surfaces. The mechanism for the observed self-assembly of TiO2/cyclodextrin wires under UV illumination will also be discussed. This work was supported by the U. S. Department of Energy, BES and EERE, under Contract No. DE-AC39-98-GO10337. [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G24.00013: Directly Patterned Mesoporous Carbon and Metal-Doped Carbon Films Prepared using Block Copolymer Templates in Supercritical CO2 Gaurav Bhatnagar, James Watkins Porous carbon and metal-doped porous carbon films have wide applications in separations, sensors, catalysis and magnetic storage applications. Conventionally prepared carbon films have micropores in the size range of 2 nm. Recent techniques for the synthesize of mesoporous carbon with 10-50 nm pores using sacrificial silica templates have limitations with respect to pore connectivity and the preparation of patterned and metal-doped carbon films. Here we report supercritical fluid-assisted infusion and phase selective reaction within pre-organized block copolymer and block copolymer/homopolymer templates to synthesize mesoporous carbon and metal doped carbon films. Well-defined mesoporous carbon films patterned at the device level with good mechanical properties and metal doped carbon films by pre-infusion of block copolymer templates with metal precursors in supercritical carbon dioxide were synthesized. Platinum doped carbon films for applications in fuel cells having good electrochemical response were produced. The preparation of cobalt containing carbon films with potential use in magnetic storage applications demonstrates the general applicability of supercritical infusion technique to produce metal cluster containing films. [Preview Abstract] |
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