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 of N,N'-ethylenebis(3-methoxysalicylideneiminato)cobalt(II), Co(3-MeO-salen), enhances the band gap photoluminescence (PL) intensity of etched, single-crystal n-CdSe relative to its intensity in methylene chloride alone, 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-induced 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 model in this concentration regime, yielding an equilibrium constant K of similar to 10(4)-10(5) M(-1). At higher concentration, between similar to 50 and 100 mu M, there is an irreversible, concentration-independent (to similar to 1000 mu M) inversion of the PL response to a net quenching, which is consistent with multilayer film formation. A thin layer of Co(3-MeO-salen) adsorbed onto CdSe from methylene chloride solution acts as a transducer: Exposure of the coated semiconductor to gaseous dioxygen quenches the band gap PL intensity relative to its intensity in a nitrogen atmosphere, while negligible PL intensity changes are induced by dioxygen in the absence of the film. The dioxygen-induced 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 quenching of PL intensity that increases with decreasing incident intensity. Implications for on-line sensor design using this transduction strategy are discussed.