Photoluminescence properties of heat-treated silicon-based polymers: promising materials for LED applications.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2010)
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|Item Type:||Ph.D. Thesis|
|Title:||Photoluminescence properties of heat-treated silicon-based polymers: promising materials for LED applications|
For the development of photoluminescent materials for LEDs, mouldability, insensitivity to air and moisture and resistance to high temperatures are of primary importance, besides improved optical properties. In the present work, a new approach to obtain homogeneous, thermally stable and formable photoluminescent materials starting from silicon-based polymers through heat-treatment at low temperatures (200-700 °C) is presented. These commercially available or laboratory synthesized polymers are usually employed in the PDC route and annealed at temperatures beyond 1000 °C. In the present work, the polysiloxane Wacker- Besil®PMS MK, the polysilazanes KiON Ceraset (PUVMS), KiON VL20 and KiON S, four polysilylcarbodiimides and a copolymer polysilazane-polysilylcarbodiimide were heat-treated up to 700 °C for 2 h under Ar atmosphere. The structural rearrangements during annealing lead to materials with tunable photoluminescence properties. The maximum emission spectra of many of the polymers investigated show a bathochromic shift as the annealing temperature increases, indicating the development of new luminescent centers with smaller band gap. For MK, Ceraset and VL20, UV emission is detected after heat-treatment up to 400 °C, and it was observed to be strictly related to the crosslinking extent of the polymers. For heat-treatments from 500 °C, the materials emit in the visible range and this was related to the formation of aromatic agglomerations, precursors of the free carbon phase. At 700 °C for MK and at 600 °C for Ceraset and VL20, the emission range is further red-shifted and the intensity is decreased, due to the concentration quenching related to the development of free carbon in higher amounts. In the dried KiON S, a visible emission is obtained after annealing at a temperature as low as 300 °C, but the aromatic solvent, still present after drying, has an influence on the fluorescence properties. All four polysilylcarbodiimides show luminescence properties in the polymer state, due to the presence of phenyl groups. As the treatment temperature increases, red-shifted emission ranges are analyzed for the two polymers which can be crosslinked, while no red-shift was observed for the ones which cannot. After annealing at 500 °C, the fluorescence is quenched in all polymers due to the development of free carbon in high amounts. The copolymer polysilazane-polysilylcarbodiimide is very interesting because it shows the maximum emission intensity among all heat-treated polymers analyzed, in the visible range and after heat-treatment at a temperature of just 200 °C. A new luminescent center arises after annealing, formed via crosslinking of the polymer. At 600 °C the luminescence is quenched due to the formation of free carbon. Therefore, our studies have clearly shown that the luminescence properties are related to structural rearrangements occurring during the polymer to the ceramic transformation, namely crosslinking and formation of sp2 carbon. The formation of dangling bonds was also considered as a possible source of luminescence, but was demonstrated not to have an influence on the photoluminescent properties.
|Place of Publication:||Darmstadt|
|Uncontrolled Keywords:||silicon polymer, luminescence, fluorescence, dangling bonds, sp2 carbon, free carbon, aromatic aggomerations, polysiloxane, polysilazane, polysilylcarbodiimide, heat-treatment, LED, Raman polymer derived ceramics|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 540 Chemie
500 Naturwissenschaften und Mathematik > 530 Physik
600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften
|Divisions:||11 Department of Materials and Earth Sciences > Material Science|
|Date Deposited:||23 Mar 2010 12:24|
|Last Modified:||07 Dec 2012 11:56|
|Referees:||Riedel, Prof. Dr. Ralf and Sorarù, Prof. Dr. Gian Domenico|
|Refereed:||10 March 2010|