Layered polysilane: thermolysis and photoluminescence

Layered polysilane (Si6H6) and its thermolysis have been studied using FTIR spectroscopy, thermogravimetry/differential thermal analysis, mass spectrometry, electron paramagnetic resonance, Si K-edge absorption and photoluminescence spectroscopy. It is found that cross-linking between (-Si6H6-)(n) layers occurs through dehydrocoupling reactions when the layered polysilane is heated under vacuum or an inert atmosphere at temperatures of 100-200 degrees C. Structural changes in the silicon network are evident during thermolysis: the layered structure of polysilane starts to collapse at 200 degrees C and is transformed to amorphous hydrogenated silicon and subsequently to crystalline silicon (c-Si) at temperatures higher than 450 degrees C. This process is accompanied by the evolution of H-2 and SiH4 gases. The resulting layered polysilane exhibits strong room temperature photoluminescence at 560 nm (ca. 2.2 eV) and a blue-shift of Si K-edge absorption (0.6 eV) relative to c-Si. Annealing the layered polysilane results in red-shifts of luminescence peak energy with the increase of annealing temperature, consistent with the trend observed in the Si K-edge absorption measurement. These results are interpreted in terms of the growth of silicon network dimension during the thermolysis. The reduction in visible luminescence intensity for the annealed product at 300 degrees C (or higher) is further attributed to the creation of defects, e.g., silicon dangling bonds (g = 2.0047) which provide pathways for non-radiative recombination. The relationship between layered polysilane (as well as its annealed products) and porous Si is discussed.