Reactivity of the Bi2Sr2CaCu2O8 and Bi1.7Pb0.3Sr2CaCu2O8 surfaces for d-metal overlayers

X-ray photoelectron spectroscopy has been used to investigate the chemical stability of the cleaved surface of Bi2Sr2CaCu2O8 and Bi1.7Pb0.3Sr2CaCu2O8 single crystals interacting with overlayers of various transition and noble metals (Fe, Ti, Cr, Cu, Pd, Au). The largest reactivity is found for metals with an incomplete d-shell as Ti, Cr and Fe, while Cu and Pd are much less reactive and Au is inert. Chemical shifts and lineshape modifications of the core and valence levels result from the disruption of the outermost Bi-O and Cu-O layers of the superconductor by the reactive metals. Energy considerations based on the difference in the heats of formation of the oxides and on surface free energies of the metals suffice to explain the different reactivity of most interfaces with the exception of Cu. The growth mode of an unreactive interface at room temperature is of the Volmer-Weber type with formation of clusters of increasing thickness and size.