Adsorption Isotherms of Ag/Si(111) 7 × 7

Abstract

The equilibrium properties of Ag chemisorbed on a (7 × 7) Si(111) surface in the 560-650 °C temperature range were obtained by measuring adsorption isotherms. To this purpose one measures, by using a quadrupole mass spectrometer, the silver flux leaving a Si(111) surface at fixed temperature in the presence of an incident atomic silver beam of equivalent pressure P. The isotherms so obtained present steps characterizing a first-order transition between a diluted phase and a condensed phase. This condensed phase corresponds to the Si(111)(√3 × √3) R30° Ag structure at T < Tc = 600 °C well known from LEED or RHEED. The coverage at completion of this phase is under equilibrium condition of one silver atom for one silicon atom of a non-reconstructed and perfect Si(111) surface. Thermodynamic parameters of the surface sites have been determined by fitting experimental isotherms by Frumkin-Fowler and Fowler-Guggenheim expressions. In the domain of temperature under study data analysis shows that the formation of the two-dimension phase is entropic-driven, since its sublimation enthalpy (61.4 kcal · mole-1) is smaller than that of bulk silver (66.1 kcal · mole-1) but its entropy is abnormally high. Data analysis suggests that the Ag/Si(111) 2D phase has a very weak and soft lateral binding. The vertical binding is typical of chemisorption.