THE COORDINATION OF MONOPHOSPHINES 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. © 1990.