PHOTOLUMINESCENCE STUDIES OF SILVER-EXCHANGED CADMIUM SELENIDE CRYSTALS - MODIFICATION OF A CHEMICAL SENSOR FOR ANILINE DERIVATIVES BY HETEROJUNCTION FORMATION

Single-crystal n-CdSe surfaces have been modified by an exchange reaction with aqueous Ag+ that converts part of the surface to Ag2Se and releases Cd2+ to solution. Formation of Ag2Se is established by X-ray powder diffraction. ESCA and Auger spectroscopy are consistent with the formation of Ag2Se islands having thicknesses of approximately 20-500 angstrom, depending on reaction conditions. Steady-state photoluminescence (PL) experiments show that Ag+-exchanged CdSe (CdSe/Ag+) can be superior to CdSe as an aniline sensor: ring-substituted aniline derivatives in toluene solution cause enhancements of the CdSe band edge PL in CdSe/Ag+ samples relative to a toluene ambient, with the effect about twice as large as for unexchanged CdSe. The variations in PL intensity of CdSe/Ag+ are well fit by a dead-layer model, allowing estimation of the adduct-induced change in depletion width. The magnitude of these variations in depletion width for CdSe/Ag+ can approach 1000 angstrom. The affinity of the aniline derivatives for the CdSe/Ag+ surfaces, as estimated from the fit of concentration-dependent PL changes to the Langmuir adsorption isotherm model, is also about twice that of unexchanged CdSe: equilibrium constants are about 60-300 M-1 for the binding of several aniline derivatives to CdSe/Ag+. Time-resolved PL decay curves of the CdSe edge emission of CdSe/Ag+ samples demonstrate that the rate of carrier recombination increases with the extent of the exchange reaction, eventually saturating. PL decay curves of CdSe/Ag+ are insensitive to adduct formation for p-OCH3 and p-CH3 derivatives, suggesting that the surface recombination velocity is not greatly affected by aniline adsorption. An isotype heterojunction model for the CdSe/Ag2Se interface is proposed to rationalize these observations.