Adsorption Isotherms of Ag/Si(lll) 7 x 7
Résumé
The equilibrium properties of Ag chemisorbed on a (7 x 7) Si(ll1) 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(ll1) surface at futed temperature in the presence of an incident atomic silver beam of equivalent pressure P. The isotherms so obtained present steps characterizing a fiist-order transition between a diluted phase and a condensed phase. This condensed phase corresponds to the Si(lll)(fi x ~) R 3 0 " Ag structure at T < T, = 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(ll1) 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-') is smaller than that of bulk silver (66.1 kcal.mole-') but its entropy is abnormally high. Data analysis suggests that the Ag/Si(lll) 2D phase has a very weak and soft lateral binding. The vertical binding is typical of chemisorption.