The impact of Lewis acid/base exposure on the surface states of luminescent porous silicon

Abstract

Crystalline Si is an indirect gap semiconductor, and thus its luminescence is dipole forbidden. However, if it is possible to modify its electronic structure such that silicon could emit light efficiently, then optical communication devices might be grown directly on a Si chip. The recent discovery that a naturally nanostructured porous Si phase shows room temperature visible luminescence has stimulated extensive research. Increasing evidence points to a modified quantum-confined mechanisms whereby the observed light emission is attributed to the presence of small silicon crystallites (1-5 nm) created during the etching process and embedded in the porous layer, with the corresponding surface states playing a crucial role. The research presented here was focused on the inter-relationship between morphology and composition of a porous Si surface and the ability of its luminescence to be quenched by Lewis acids and bases. In the first project, the effect of porous Si surface oxidation on the ability of a given EPh$\sb3$ Lewis base (E=N,P,As) to quench the visible photo-luminescence of porous silicon was studied. It was demonstrated that for a given type of porous Si surface, the percentage of integrated PS PL which can be quenched for a particular Group 15 derivative follows the gas phase proton affinities of these molecules. The effects of the exposure of selected alkyl amines on the visible photoluminescence (PL) of porous Si, including their intrinsic quenching ability, times scales of restoration, and the additional influence of a weak acid (TFA) on the restoration of PL was also studied. Monitoring the extent of PL restoration over time reveals discriminating behavior between amines. As expected, increasing the porosity of the matrix facilitates the restoration of PL. The introduction of trifluoroacetic acid (TFA) also speeds up the restoration of amine-quenched porous Si PL. In an effort to determine the impact of PS structure on the susceptibility of its photoluminescence (PL) to chemical quenching, the ability of the monodentate Lewis base n-propyl amine and the bidentate ethylene diamine to quench the PL of silicon in three different structural environments was studied. These include: porous silicon fabricated from p-type substrates; porous silicon from n-type substrates; and Si nanocrystallites derived from porous silicon. For these studies, the fractional changes in PL as a consequence of amine addition were measured and fitted to an equilibrium model from which adduct formation constants were calculated. In order to elucidate details of the quenching mechanism of porous Si photoluminescence by amines, time-resolved photoluminescence measurements were carried out. PL quenching can result from dynamic modification of the lifetime of the PS lumophores, or alternatively, a static process between quencher and fluorophore. Average lifetimes and changes in integrated photoluminescence intensity, fit into the Stern-Volmer equation are consistent with a dynamic quenching mechanism. Time-resolved measurements upon the addition of TFA itself to porous Si reveal a static mechanism in terms of changes of porous Si PL. The enhancement in porous Si PL upon TFA addition is probably due to passivation of nonradiative centers present at the surface of porous Si. To analyze the diffusion of Lewis base molecules in the restricted porous matrix independent from solvent effects, the desorption of dephenylamine in vacuum from several types of porous Si matrices at different temperatures was studied. The results suggest that desorption from pores is limited by strong capillary forces and wetting properties of the surface.