Browsing by Author "Marciu, D."

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    Creation of a gradient polymer-fullerene interface in photovoltaic devices by thermally controlled interdiffusion
    Drees, M.; Premaratne, K.; Graupner, W.; Heflin, James R.; Davis, Ruth M.; Marciu, D.; Miller, M. (AIP Publishing, 2002-12-01)
    Efficient polymer-fullerene photovoltaic devices require close proximity of the two materials to ensure photoexcited electron transfer from the semiconducting polymer to the fullerene acceptor. We describe studies in which a bilayer system consisting of spin-cast 2-methoxy-5-(2(')-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and sublimed C-60 is heated above the MEH-PPV glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created bringing the fullerene molecules within the exciton diffusion radius of the MEH-PPV throughout the film to achieve highly efficient charge separation. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface. (C) 2002 American Institute of Physics.
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    Enhanced nonlinear optical response of an endohedral metallofullerene through metal-to-cage charge transfer
    Heflin, James R.; Marciu, D.; Figura, C.; Wang, S.; Burbank, P.; Stevenson, Steven A.; Dom, H. C. (AIP Publishing, 1998-06)
    A new mechanism for increasing the third-order nonlinear optical susceptibility, X-(3), is described for endohedral metallofullerenes. A two to three orders of magnitude increase in the nonlinear response is reported for degenerate four-wave mixing experiments conducted with solutions of Er-2@C-82 (isomer III) relative to empty-cage fullerenes. A value of - 8.7x 10(-32) esu is found for the molecular susceptibility, gamma(xyyx), of Er-2@C-82 compared to previously reported values of gamma(xxxx) = 3 x 10(-34) esu and gamma(xyyx) = 4 x 10(-35) esu for C-60. The results confirm the importance of the metal-to-cage charge-transfer mechanism for enhancing the nonlinear optical response in endohedral metallofullerenes. (C) 1998 American Institute of Physics.
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    Thickness dependence of second-harmonic generation in thin films fabricated from ionically self-assembled monolayers
    Heflin, James R.; Figura, C.; Marciu, D.; Liu, Y.; Claus, Richard O. (AIP Publishing, 1999-01-01)
    An ionically self-assembled monolayer (ISAM) technique for thin-film deposition has been employed to fabricate materials possessing the noncentrosymmetry that is requisite for a second-order, chi((2)), nonlinear optical response. As a result of the ionic attraction between successive layers, the ISAM chi((2)) films self-assemble into a noncentrosymmetric structure that has exhibited no measurable decay of chi((2)) at room temperature over a period of more than one year. The second-harmonic intensity of the films exhibits the expected quadratic dependence on film thickness up to at least 100 bilayers, corresponding to a film thickness of 120 nm. The polarization dependence of the second-harmonic generation yields a value of 35 degrees for the average tilt angle of the nonlinear optical chromophores away from the surface normal. (C) 1999 American Institute of Physics; [S0003-6951(99)02904-6].