The coordination of mono- and diphosphines to the surface of cadmium selenide

Band edge photoluminescence (PL) of single-crystal n-CdSe provides evidence for adduct formation between the solid's surface and various mono- and diphosphines present in toluene solution. Relative to a toluene ambient, solutions of triphenylphosphine (PPh₃), ethyldiphenylphosphine (PEtPh₂), bis(diphenylphosphino)methane (dppm) and bis(diphenylphosphino)ethane (dppe) in toluene enhance the semiconductor's PL intensity. For all of the phosphines, the increases in PL intensity can be fit to a dead-layer model, allowing the estimation of the reduction in depletion width resulting from phosphine exposure; reductions in dead-layer widths are about 300-400 Å for all phosphines except PEtPh₂, which yields a reduction roughly twice as large. Fits of the PL changes to the Langmuir adsorption isotherm model yield formation constants for the phosphine/CdSe adducts of 30±7 M^-1 for PPh₃ and 220± 30 M^-1 for PEtPh₂; the diphosphines, which do not fit this simple model as well, give adsorption equilibrium constants of ∼ 100-200 M^-1. The results suggest that the diphosphines do not coordinate to the surface in a chelating fashion, but in some combination of bridging and monodentate bonding modes at lower effective surface coverages than the monophosphines.