Charge transfer and ZnO defect state mediated FRET dependent luminescence in nanocomposites based on PPy nanowires and ZnO microflowers

Abstract

The role of Forster resonance energy transfer (FRET) and charge transfer on controlling the overall luminescence of the nanocomposites based on PPy and ZnO is discussed here. Polypyrrole-ZnO (PPy-ZnO) nanocomposites have been synthesised with the variation of ZnO content (10 wt %, 30 wt % and 50 wt %) in the nanocomposite through chemical oxidative polymerization method. Nanowires of PPy and PPy-ZnO nanocomposites are observed from SEM and TEM images. The diameters of nanowires in PPy-ZnO nanocomposites are found to vary with ZnO content. Ordering in the polymer chain with the incorporation of ZnO is observed from XRD spectra. Red shift of the absorption peak of PPy in PPy-ZnO nanocomposites gives the evidence of increase in order of the polymer chain which in turn increases the singlet exciton diffusion through the polymer chain. Two photophysical phenomena i.e. FRET and charge transfer have significant impact on controlling the overall luminescence of the composites depending upon the ZnO content in the composite. At lower amount of ZnO (10 wt %), FRET occurs predominantly from defect states of ZnO to PPy and overall enhancement of luminescence is noticed. But no significant enhanced luminescence is observed when higher amount of ZnO is added to the composite. This happens due to dominant charge transfer between PPy and ZnO since they have type II band alignment. When the amount of ZnO is high in the composite, the interfacial area between PPy and ZnO get increased. XPS spectra reveals that ratio of polaron to neutral nitrogen in PPy chain also increases in the composite with higher amount of ZnO which results in the modification of band structure of PPy. Thus high interfacial area between PPy and ZnO as well as modification of band structure in PPy increases the rate of charge transfer between PPy and ZnO. Therefore, inspite of possibility of FRET, charge transfer occurs more efficiently in the nanocomposite when the ZnO content is high in the composite. This ultimately prohibits the radiative recombination of singlet exciton inside the composite with higher amount of ZnO. Hence the Photoluminescence (PL) intensity quenches considerably. Thus optimizing the amount of ZnO in the composite, the photophysical phenomena as well as luminescence of the composite can be controlled for the potential application of different optoelectronic devices like solar cell.