PHOTOLUMINESCENT PROPERTIES OF CADMIUM SELENIDE COATED WITH A PHOTOACTIVE COBALT COORDINATION COMPLEX - A DIOXYGEN-DRIVEN TRANSDUCER

In methylene chloride solution; in the absence of dioxygen, addition o f hylenebis(3-methoxysalicylideneiminato)cobalt(II), Co(3-MeO-salen), enhances the band gap photoluminescence (PL) intensity of etched, sing le-crystal n-CdSe relative to its intensity in methylene chloride alon e, suggesting that the complex acts as a Lewis base toward the solid. Use of a dead-layer model permits an estimate of the maximum adduct-in duced reduction in depletion width of similar to 130 Angstrom. The PL enhancement is concentration dependent, saturating at similar to 40 mu M, and can be well fit by the simple Langmuir adsorption isotherm mod el in this concentration regime, yielding an equilibrium constant K of similar to 10(4)-10(5) M(-1). At higher concentration, between simila r to 50 and 100 mu M, there is an irreversible, concentration-independ ent (to similar to 1000 mu M) inversion of the PL response to a net qu enching, which is consistent with multilayer film formation. A thin la yer of Co(3-MeO-salen) adsorbed onto CdSe from methylene chloride solu tion acts as a transducer: Exposure of the coated semiconductor to gas eous dioxygen quenches the band gap PL intensity relative to its inten sity in a nitrogen atmosphere, while negligible PL intensity changes a re induced by dioxygen in the absence of the film. The dioxygen-induce d PL quenching is pressure-dependent; a good fit to the Langmuir model yields an equilibrium constant K of similar to 10 to 20 atm(-1). The film is observed to be photoactive: Excitation at wavelengths shorter than similar to 600 nm, which are absorbed by the oxygenated complex, induces the loss of dioxygen from the film, giving fractional quenchin g of PL intensity that increases with decreasing incident intensity. I mplications for on-line sensor design using this transduction strategy are discussed.