Electro-optical evidence for the chelate effect at semiconductor surfaces

Monoamines and diamines dissolved in cyclohexane solution reversibly enhance the band-edge photoluminescence (PL) intensity of immersed n-type cadmium sulfide (n-CdS) and n-type cadmium selenide (n-CdSe) substrates through adsorption. The magnitude of the PL increase is used to estimate amine-induced contractions in the semiconductor's depletion width, and the dependence of the PL intensity on amine concentration provides an estimate of the adduct formation constant. Two diamines, ethylenediamine and o-phenylenediamine, exhibit unusually low reductions in depletion width and substantially larger adduct equilibrium constants relative to the other amines studied, consistent with chelation to surface Cd²⁺ ions. These studies demonstrate that PL can be used as a contactless, in situ technique for characterizing the steric and electronic landscape of semiconductor surfaces and for correlating molecular and surface chemistry.