Luminescence of heat-treated silicon-based polymers: promising materials for LED applications

A new strategy to obtain transparent, thermally stable, and formable photoluminescent materials for LED applications is presented. Starting from commercially available silicon-based polymers, luminescence properties are developed by means of simple heat treatment. Solid polymethylsilsesquioxane MK (Wacker-Besil^®PMS MK) and liquid poly(ureamethylvinyl)silazane Ceraset (Kion Ceraset^® PUVMS) were thermally treated between 200 and 700 °C for 2 h under Ar atmosphere. Photoluminescence properties were observed in all the samples. The structural rearrangements during thermal annealing were effective in order to red-shift the emission spectra of the untreated polymers to the visible range. The formation of dangling bonds and carbon sp², associated with the annealing procedure and confirmed by means of Electron Paramagnetic Resonance (EPR) spectroscopy and solid state Magic Angle Spinning NMR (MAS-NMR) contribute to the red-shift of the photoluminescence emissions of the polymers. After heat treatment at low temperatures (200, 300, and 400 °C), both the polymers show fluorescence in the UV range. While the polysiloxane reveals white luminescence after annealing at 500 and 600 °C, the polysilazane heat-treated at 500 °C exhibits emission in the blue-green range and is transparent. At higher temperatures the presence of free carbon counteracts the luminescence properties.