Modification in photoluminescence spectra of porous silicon by changing the surrounding dielectric environment

A method to control porous silicon (PS) emission properties by changing the dielectric environment surrounding the Si-crystallites has been demonstrated by undertaking a systematic investigation of photo luminescence (PL) spectra from PS recorded in a chosen set of organic solvents. On increasing the dielectric constant of the embedding medium from 2 to 24, a drastic drop (about four orders of magnitude) in PL intensity(I-PL) accompanied with a blue shift of about 230 nm has been observed. The results have been explained quantitatively in terms of geminate recombination of an isolated electron hole pair applying Onsager's model that predicts variation of Onsager length (l(0)) with dielectric constant of the embedding medium. This approach leads to Onsager length of the order of the PS nanostructure size for dielectric constant greater than 24, beyond which no further quenching was observed. The results suggest use of materials with low dielectric constant for encapsulating porous silicon without compromising its PL properties.