SURFACE-BOUND CARBONYL-COMPOUNDS AS LEWIS-ACIDS - PHOTOLUMINESCENCE AS A PROBE FOR THE BINDING OF KETONES AND ALDEHYDES TO CADMIUM-SULFIDE AND CADMIUM SELENIDE SURFACES

The band gap photoluminescence (PL) intensity of n-CdSe and Te-doped n-CdS (CdS:Te) single crystals is reversibly quenched by adsorption of more than two dozen aldehydes and ketones from cyclohexane solution onto the (0001) surfaces of these CdS(e) substrates. The adsorbate-induced quenching of PL intensity is consistent with the adsorbates acting as Lewis acids toward these surfaces. For a representative selection of aldehydes and ketones, the decrease in PL intensity is well fit by a dead-layer model for adsorption onto CdSe, allowing estimation of the adduct-induced expansion in depletion width ranging from several hundred to as much as similar to 1000 Angstrom. Temporal PL results suggest that the change in surface recombination velocity accompanying adsorption is modest and a less important contributor to steady-state PL quenching than the band-bending increase inferred from the fit to the dead-layer model. Adduct formation constants, estimated from fits of the concentration-dependent PL changes to the Langmuir adsorption isotherm model, vary over 4 orders of magnitude, from similar to 10(1) to 10(5) M(-1). Hammett plots of adduct formation constants of several families of derivatives show that the formation of CdS:Te surface adducts can be stabilized by electron-withdrawing substituents, consistent with a Lewis acidic interaction with the surface. Adduct stability is enhanced by resonance effects: Substitution of phenyl substitutents for methyl substituents can lead to order-of-magnitude increases in binding constants. alpha-Diketones and quinones exhibit the largest binding constants, presumably due to the ability of the second carbonyl group to stabilize the negative charge transferred from the semiconductor. A model of orbital interactions for the carbonyl functional group with the CdS(e) surface is proposed. Evidence that steric factors contribute to surface coverage and thus to the magnitude of PL quenching is presented.