46693-GB10
Charge Transfer and Charge Transport in Nanofibers of Conjugated Polymer and ZnO Nanoparticles
Hybrid organic semiconductor/inorganic nanomaterials are of great interest not only as potential materials for photovoltaic applications but also as model systems for studying the fundamental physical properties and interactions at the nanometer scale. In recent years, progresses have been made in understanding the mechanism of photoinduced charge transfer process and the effect of the morphology on the charge transfer properties. However, the effects of polymer chain conformation and order/disorder in the nanocomposites on the exciton formation and dissociation, charge transfer, and charge transport properties have not been well addressed.
Our work investigates the optical and charge transport properties of nanocomposites of conjugated polymers and ZnO nanoparticles prepared through an electrospinning process in order to understand the effects of morphology, polymer chain conformation, and confinement on the physical properties of the electrospun nanocomposite materials. In the first year of the project, we have prepared the conjugated polymer poly[2-methoxy-5-(2'-ethyl- hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) in poly(ethylene oxide) (PEO) matrix with spincoated thin film or electrospun nanofiber morphologies, and we have studied the fluorescence emission spectra of these composites with various MEH-PPV/PEO compositions. A typical fluorescence microscope image of the MEH-PPV/PEO nanofibers is shown in the middle figure below, indicating that the nanofibers are high fluorescent with fairly uniform diameters. SEM images of the fibers show that the diameters of the nanofibers are around ~500 nm. The fluorescence emission spectrum of MEH-PPV at ~595 nm and ~630 nm are blue-shifted with decreasing MEH-PPV concentration for both thin films and nanofibers. Compared to those of MEH-PPV/PEO thin films with the same composition, the emission spectra of the electrospun fibers are blue-shifted. The spectrum can be de-convoluted into two emission peaks originating from the exciton species related to the amorphous and ordered regions of the MEH-PPV in the aggregate states (left figure). We have observed a large blueshift of the low energy emission from the ordered region of MEH-PPV with decreasing the concentration of MEH-PPV in the nanofibers (right figure), suggesting that the MEH-PPV polymer chains are less ordered and less aggregated in the electrospun nanofibers. We have measured the fluorescent spectra of single MEH-PPV nanofiber using the Near-field scanning optical microscope and the confocal fluorescence microscope at the Center of Nanoscale Materials in Argonne National Lab (user proposal CNM 205), and the data are being analyzed. A manuscript based on these results has been submitted to Synthetic Metals, and is currently under review.
We have also been working on the surface modification of ZnO nanoparticles in order to prepare well-dispersed ZnO nanoparticle/MEH-PPV blends. ZnO nanoparticles are synthesized by hydrolysis and condensation of zinc acetate dihydrate by potassium oxide in methanol/chloroform solution according to the literature. Addition of polymer PVP during sol-gel process produces ZnO nanoparticles with a polymer capping layer. We have found that the ZnO nanoparticles capped with PVP have different emission spectrum compared with the ZnO nanoparticles with no capping, presumably due to the passivation of the surface trapping states of the nanoparticles. Studies of the fluorescence properties of the MEH-PPV/ZnO composites are under way.
We are planning to prepare the MEH-PPV nanofibers in more dilute concentrations so that individual polymer chain can be imaged and measured by the facilities in Argonne National Lab. Future work also includes incorporation of ZnO nanoparticles in the electrospun conjugated polymer nanofiber and studies of the charge transfer process in the composite fibers.
