#pi#-electronic nd electrical transport properties of conjugated polymer nanocomposites:poly(p-phenylenevinylene)with homogeneously dispersed silica nanoparticles

摘要

The #pi#-electronic and hole-transport properties of homogeneous composites of poly(p-phenylenevinylene)(PPV) with 5nm diam colloidal silica have been characterized.When the interparticle distance becomes comparable to or shorter than the coherence distance of the polymer chains,the interachaim 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 conbinationof optical absorption,Raman scattering,and fluorescence sopectroscopies.The optical measured at absorptiion band maximum decreases from 6.5 repeat units in neat PPV to 4.9 units in the 50vol precent composite.The Raman measured with 633nm excitation correspondingly decreases from 7 units to 5.6 units while the fluorescence 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 fractioin toward shorter conjugation while retaining a tail of long conjugation segments thereby causing the distribution width to increase.This indicates PPV has a remarkable propersity to adopt extended conformations around the nanoparticles so that intrachain #pi#-electron delocalization is only silghtly effected.However,the electrical transport characteristics are strongly modified.The xero-field hole mobility is decreased by 1-2orders of magnitude and its field activation increased by a factor of 2-3,even at 3 vol precent 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 otained by carful design of conjugated polymer-nanoparticle composites.