Isomerization of fluorophors on a treated silicon surface

In contrast to the green and familiar orange-red emissions associated with UV excited porous silicon (PS), the origins of the longer wavelength photol uminescence (PL) excited in the visible and near-infrared regions on a PS s urface at 298 degrees K are considered within an alternate molecular model. Ab initio molecular orbital theory is used to suggest that the longest wav elength PL observed as a result of the nitrogen laser pumping of post-etch dye and HCl treated PS samples and that accessed with near-infrared excitat ion sources (PLE) at 298 K arises in large part from singlet-triplet transi tions in silylene-based moieties (:SIRY) attached to the PS surface or pres ent as uncoupled defect sites at or near the surface. This long-wavelength PL is distinct from the ultraviolet light excited PL associated with IFS wh ich has previously been attributed to a silanone-based silicon oxyhydride m oiety attached to the surface. Ab initio molecular theory at the MP2 level with polarized double-basis sets (MP2/DZP) has been used to calculate the s inglet-triplet separation for a number of silylenes with a variety of combi nations of R, Y = H, OH, SiH3, OSiH3, Cl, and NH2 and to evaluate their the rmodynamic stability relative to the silanones. The calculations show that the singlet silylene is always more stable than its triplet with transition wavelengths ranging from 1100 to 420 nm, where the highest transition ener gies are found to correlate with the most electronegative substituents. The silylene isomers are found to be more stable than the corresponding silano nes in most cases. The relative stabilities are strongly coupled to the ele ctronegativity of the substituents and to the formation of an Si-O bond in the silylene as compared to the much weaker Si-Si bond in the silanone.