Adsorption and adhesion energies of Pb on(1×1)−Mo2C/Mo(100)by calorimetry

Abstract

This paper reports calorimetric measurements of the adsorption energy of Pb onto an ordered $1\ifmmode\times\else\texttimes\fi{}1$ Mo carbide thin film on Mo(100) at 300 K, as well as measurements of the sticking probability and film growth mode of this system. The heat of adsorption is constant up to a Pb coverage of 0.5 ML, with a value of $244.5\ifmmode\pm\else\textpm\fi{}2\mathrm{kJ}/\mathrm{mol}.$ Between 0.5 and 0.9 ML the heat decreases linearly to a value close to the heat of sublimation for bulk Pb, where it remains at higher coverages. The sticking probability increases linearly for submonolayer coverages from an initial value of about 0.97. At coverages above 0.8--0.9 ML it stays constant at 0.994. The growth mode is Stranski-Krastanov, with two-dimensional (2D) growth up to 0.9 ML and 3D island growth thereafter. A growth model, consisting of Pb adatoms not filling nearest neighbor sites up to 0.5 ML and random filling of nearest neighbor sites at higher coverages, provides a very good fit to the calorimetry data. The model involves a 14-kJ/mol Pb-Pb pairwise repulsion at nearest neighbor sites, as well as a 14-kJ/mol relaxation of the repulsion by slight relaxations from the site centers, when not constrained by other Pb nearest neighbors. The adhesion energy, from the integral heat of adsorption for multilayer Pb, is $194\ifmmode\pm\else\textpm\fi{}10\ensuremath{\mu}\mathrm{J}/{\mathrm{cm}}^{2}.$