π-electronic and electrical transport properties of conjugated polymer nanocomposites:: Poly(p-phenylenevinylene) with homogeneously dispersed silica nanoparticles

The pi-electronic and hole-transport properties of homogeneous composites of poly(p-phenylenevinylene) (PPV) with 5 nm diam colloidal silica have been characterized. When the interparticle distance becomes comparable to or shorter than the coherence distance of the polymer chains, the intrachain and interchain order in these nanocomposites gets disrupted. This disruption is reflected in a reduction of the mean conjugation length <L-c> and a broadening of the L-c distribution. These parameters may be estimated through a combination of optical absorption, Raman scattering, and fluorescence spectroscopies. The optical [L-c] measured at absorption band maximum decreases from 6.5 repeat units in neat PPV to 4.9 units in the 50 vol % composite. The Raman [L-c] measured with 633 nm excitation correspondingly decreases from 7 units to 5.6 units while the fluorescence [L-c] deduced from the 0-0 molecular transition remains nearly constant at 10-11 units. Therefore the bulk of the L-c distribution shifts by a small fraction toward shorter conjugation while retaining a tail of long conjugation segments thereby causing the distribution width to increase. This indicates PPV has a remarkable propensity to adopt extended conformations around the nanoparticles so that intrachain pi-electron delocalization is only slightly effected. However, the electrical transport characteristics are strongly modified. The zero-field hole mobility is decreased by 1-2 orders of magnitude and its field activation increased by a factor of 2-3, even at 3 vol % particle loading. X-ray photoelectron spectroscopy and infrared spectroscopy rule out any increase in the concentration of chemical defects. Therefore the loss of mobility may be related to roughening of the hopping energy landscape. This reduction in electrical conductivity however can be mitigated through controlled chemical doping of the PPV chains. Interesting properties can thus be obtained by careful design of conjugated polymer-nanoparticle composites. (C) 2002 American Institute of Physics.