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Issue Date: 13-Mar-2017
Authors: Vitale, Stefania
Title: Surface engineering of oxide systems for energy and molecular electronics applications
Abstract: This thesis work was focused on oxide surfaces modification through the anchoring of functional molecular systems, for perspective applications in the fields of molecular electronics and energy. The main goal was the development of an effective experimental strategy for the robust anchoring of functional molecules onto oxide substrates of technological interest, such as transparent (semi)conducting oxides (TCOs). The strategy exploited for the modification involved a preliminary stepwise surface priming based on the chemistry of zirconium phosphates/phosphonates (ZP-priming), consisting in the deposition on a zirconium-phosphate layer directly at the oxide surface. Experimental evidences are presented that this preliminary surface treatment provides a stable and robust platform for the subsequent anchoring of functional molecules, bearing a phosphonic group, onto oxides of different chemical nature and morphology. The ZP-priming strategy was successfully applied to the treatment of nanostructured, micrometres-thick TiO2 and SnO2 substrates, and its efficacy in providing a uniform priming along the entire nanostructured layer was studied and assessed by ToF-SIMS depth profiling measurements. ZP-TiO2 and ZP-SnO2 were subsequently used as substrate for the preparation of photoactive electrodes with perspective application in dye-sensitised cells for solar energy conversion. It was demonstrated that the preliminary ZP treatment provides a suitable anchoring platform for the sensitisation with photoactive dye RuP (a ruthenium tepryridyl-triazin complex bearing a phosphonic group). ToF-SIMS depth profiling showed that the sensitisation involved the whole thickness of the oxides layers, and by means of UV-Vis spectroscopy it was possible to demonstrate that the preliminary ZP-priming has several benefits on the substrate sensitisation, namely an improvement of the binding stability and a greater amount of dye molecules adsorption. Photoelectrochemical measurements (JV characteristics upon irradiation) showed that the presence of the zirconium-phosphate layer at the interface between oxide and dye molecule does not negatively affect the dye-to-oxide charge injection, and, as a consequence, the functionality of the electrode. The use of ZP-primed TCOs was also explored in the preparation of conductive molecular wires of metal-polypyridinic molecules through stepwise metal-coordination reactions directly at the oxide surface. A surface functionalised with a layer of terpyridinic units was obtained by anchoring onto a ZP-FTO substrate the ditopic molecule PPTP (terpyridyl-benzenephosphonic acid); the free terpyridinic groups at the surface were afterwards used as anchoring sites for the integration of ditopic RuDT2 molecules (ruthenium(II) phenylene bis-terpyridine complex) through direct metal-coordination by iron(II) of the terpyridine groups at the surface and one of the two free tepryridine units of RuDT2. By the stepwise iteration of the metal-coordination reaction, molecular wires were prepared with different numbers of Fe-RuDT2 units, as confirmed by ToF-SIMS measurements and UV-Vis. Data on the electrical behaviour of the wires were obtained by liquid-metal EGaIn junction technique, with results that show a good electrical conduction along the wires, similar to that of similar wires assembled on gold surfaces. The feasibility of the integration of these metal-polypyridinic conductive wires at the surface of a transparent and conductive oxide by means of the ZP-PPTP platform, and the evidence for good electrical conduction could allow perspective applications within the field of nanoscale molecular electronics, with possible extension to photoresponsive molecular electronics, given the presence of metal-terpyridinic centres (which are well-known photoactive sites).
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