• Morais Faustino, Bruno Miguel

Abstract

This thesis reports investigations into some photophysical aspects of luminescent nanocomposites in a number of different systems.<br/>The preparation of Ln3+ doped rutile SnO2 (Ln = Sm, Eu, Tb, Ho, Nd) by a sol-gel method is described. Average particle size was found to be 23nm by x-ray powder diffraction and electron microscopy. Factors affecting growth kinetics are also discussed.<br/>Successful sensitisation of Ln3+ emission via SnO2 host energy transfer is reported. Optimum Ln3+ concentrations were investigated and a proposed mechanism of Dexter type energy transfer based on SnO2 lattice defects acting as sensitising centres is presented. The relationship between the Ln ionic radius and the dopant concentration required to maximise its emission is also explored. It was found that the larger the ionic radii, the less Ln3+ was required to maximise its emission. Investigations into the distance dependence of the energy transfer between Ln3+ and Sn4+ as well as between neighbouring Ln3+ ions have been undertaken – e.g. it was concluded that maximum Sm3+ sensitisation occurred when every fourth Sn ion was substituted by Sm.<br/>Poly(p-phenylene vinylene) (PPV) was synthesised via the standard thermal conversion route proposed by Wessling and Zimmerman. Monomers, precursor polymer and polymer were fully characterized by means of infra-red, UV-Vis absorption and photoluminescence spectroscopy. By-products were also characterised by 1H and 13C-NMR.<br/>Incorporation of the SnO2:Ln3+ nanocomposites into PPV was followed by extensive photoluminescent studies. It was found that sensitisation of both polymer and lanthanide was dependent on the system composition and concentration ratios. There was clear evidence to support successful harvesting of PPV non-emissive triplet state energy into useful Ln3+ photon emission. To better benefit from triplet state population, a poly(vinyl alcohol) (PVA) two-dimensional hydrogen-bonded cage was used to reduce the triplet quenching. A proposed mechanism of energy transfer is described for each system.<br/>i<br/>Finally, poly(2,5-bis(3-sulfonatopropoxy)-1,4-phenylene, disodium salt-alt-1,4-phenylene) (PPP-OPSO3) was used as a donor, to systematically compare the effect of SnO2:Ln3+ nanoparticles on the luminescence of a polymer with a larger band gap and higher triplet state energy. It was established that the high triplet state of PPP-OPSO3 matched the emissive levels of most lanthanides, and a clear indication of triplet harvesting by Eu3+ was obtained under oxygen-free conditions in a PVA matrix. Exchange of the polymer’s Na+ cation by Eu3+ brought both donor and acceptor into close proximity. Dexter type energy transfer was seen from both singlet and triplet states, with Eu3+ emission lifetimes as long as 400ms in phosphorescence measurements.

Topics

• nanoparticle
• nanocomposite
• impedance spectroscopy
• photoluminescence
• polymer
• Oxygen
• laser emission spectroscopy
• Hydrogen
• defect
• two-dimensional
• electron microscopy
• Nuclear Magnetic Resonance spectroscopy
• alcohol
• Lanthanide
• quenching
• phosphorescence