Photoluminescence-Based Correlation of Semiconductor Electric Field Thickness with Adsorbate Hammett Substituent Constants. Adsorption of Aniline Derivatives onto Cadmium Selenide

Adsorption of ring-substituted aniline derivatives, presumably through the amino group, onto the (0001) face of single-crystal n-CdSe or n-CdS [CdS(e)] profoundly affects the semiconductor's photoluminescence (PL) by effecting charge transfer between surface states and the bulk semiconductor. The variations in PL intensity of etched samples are well fit by a dead-layer model, allowing estimation of the adduct-induced changes in depletion width. The magnitude of these changes can be molecularly tuned over nearly 1000 Å for moderately doped samples by the control of electron density at the coordination site, a parameter characterized by the Hammett substituent constant σ. In contrast, the affinity of the aniline derivatives for the CdS(e) surface, as estimated from the fit of concentration-dependent PL changes to the Langmuir adsorption isotherm model, is relatively insensitive to aniline substituent; equilibrium constants are ∼ 10² M^-1. From variable-temperature experiments, the binding reaction for the p-OMe derivative of aniline is approximately thermoneutral and appears to be entropy driven. Temporal PL results support the presence of a broad distribution of recombination sites. PL decay curves are insensitive to adduct formation for the p-OMe and p-Me derivatives, suggesting that surface recombination velocity is not greatly affected by aniline adsorption. Variable incident light intensity studies, using the p-OMe derivative, are consistent with the notion of depletion width-driven PL changes. A model of orbital interactions occurring at the CdSe-aniline interface is proposed, in which interactions of the semiconductor surface with the aniline derivatives alter the occupancy of surface states by shifting their energy distribution relative to the band edges.