Binding of fullerenes to cadmium sulfide and cadmium selenide surfaces, photoluminescence as a probe of strong, Lewis acidity-driven, surface adduct formation

Abstract

The C{sub 60} and C{sub 70} fullerenes can be adsorbed from toluene solution onto the surfaces of etched, single-crystal n-CdS and n-CdSe [n-CdS(e)] semiconductors. These fullerene adsorbates act as Lewis acids toward the CdS(e) surface, causing quenching of the solids` band-edge photoluminescence (PL) intensity relative to the intensity in a reference ambient of pure toluene. For C{sub 60} adsorbed onto CdSe, the quenching of PL intensity is well fit by a dead-layer model that permits estimation of the adduct-induced expansion in depletion width as being as large as approximately 300 A. The degree of quenching is somewhat larger for C{sub 70} at a wavelength where the two fullerenes can be directly compared. PL quenching by both fullerenes is concentration dependent and can be fit to the Langmuir adsorption isotherm model to yield large equilibrium binding constants in the range of 10{sup 5} to 10{sup 6} M{sup -1}; the fullerenes can be detected by this PL method at submicromolar concentrations. Use of the polar Cd-rich (0001) and Se-rich (0001O) faces of a n-CdSe sample reveals similar binding constants for C{sub 60} and C{sub 70} on the two faces but larger expansions of the dead-layer thickness from adsorption of either fullerene onmore » the Cd-rich face. 15 refs., 7 figs.« less