Optical Characteristics of Ultrathin Oligosilane Films Prepared by Molecular Beam Deposition Method

In order to investigate the correlation between the molecular orientation and the optical characteristics of ultrathin films of permethyl-dodecasilane, CH3{Si(CH3)2}12CH3 (DM12), we grew the films on quartz, KBr and KCl by molecular beam deposition, and characterized them by optical absorption, photoluminescence (PL), and photoexcitation spectroscopy and atomic force microscopy. We found that DM12 molecules formed a monolayer on quartz with their molecular chains perpendicular to the surface, while DM12 films grew on KBr and KCl with their molecular axis oriented parallel to the surfaces. These films exhibited quite different absorption spectra as a result of their molecular orientation and the interaction between molecules. The PL spectra obtained for the films on quartz and KBr were, however, almost the same, indicating that the origin of the luminescence was the localized states such as defects on the chains.