30th October 2008 – 15:00
Prof. Franco Cacialli
Conjugated molecular and supramolecular materials now provide a class of semiconductors in its own right, with potential application to devices such as light-emitting diodes, LEDs, displays, transistors, and solar cells, now mature for both large-scale industrial take-up and commercial exploitation. After a brief introduction to conjugated polymers, I will present an electro-optical technique for the non-invasive probing of internal built-in fields in sandwich devices (LEDs or solar cells) and, in combination with data from ultraviolet photoelectron spectroscopy (UPS) and Kelvin Probe characterisation, for the analysis of the energy level line-up at organic semiconductors/electrodes interfaces. I will also present an approach to high-resolution lateral patterning of an electroluminescent conjugated polymer, based on near-field lithography with apertured probes. The technique is based on the spatially selective inhibition of the solubility of the polymer precursor by exposure to the UV field present at the apex of double-tapered, gold-coated probes [aperture diameters between 40 and 80 nm (±5 nm)]. After development in methanol and thermal conversion under vacuum we obtain features with a minimum dimension of 50 nm. I will also report results on heating of the SNOM apertured probes resulting from the combined effect of significant absorption in the metallic coating and small optical throughput of the probes. Implications for microspectroscopy and lithography will also be discussed. Insulated molecular wires made with conjugated-polymers-based polyrotaxanes offer an example of an alternative, bottom-up approach to electroluminescent nanostructures. An attractive feature here is that this class of materials is engineered at a supramolecular level by threading a conjugated macromolecule, such as poly(para-phenylene), poly(4,4-diphenylene vinylene) or poly(9,9-fluorene) through a- or b-cyclodextrin rings, so as to reduce intermolecular interactions and solid-state packing effects, that red-shift and partially quench the luminescence. Such a supramolecular approach preserves the fundamental semiconducting properties of the conjugated wires, and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still charge-transport. We used the polymers to prepare a range of LEDs and light-emitting electrochemical cells (LECs). The reduced tendency for polymer chains to aggregate shows in both solid-state films, as well as in solution (as clearly demonstrated by time-resolved fluorescence studies) and allows solution-processing of individual polyrotaxane wires onto substrates, as revealed by scanning-force microscopy.