O-2 Adsorption On Non-Uniform Fractal Surfaces Of Copper Films

Oxygen adsorption on copper thin films was investigated in a homebuilt high-vacuum setup equipped with turbomolecular and ionic pumps. Prior to each experiment (copper films deposition or oxygen admission), the pressure inside the working cell was about 5x10(-8) Torr. Copper films were deposited by evaporation on polycrystalline W substrate. Oxygen adsorption on copper films was investigated at room temperature (300 K) at doses between 10 and 10(4) Langmuir (L).Modeling of the adsorption data with Langmuir isotherm indicates the non-uniformity of the copper film surfaces. The isotherms corresponding to the exponential energy distribution (Boltzmann or Fermi-Dirac) of the adsorption centers match better the experimental data, from the point of view of correlation, standard errors, and attribution of the fitting parameters. The energetically non-uniform character of the adsorption sites could result from two processes: the thermal treatment of the as-deposited Cu films (biographic nonuniformity) and the rapid surface reconstruction, following the admission of the first oxygen doses (induced nonuniformity).This is in line with fractal analysis performed by Dubinin approach, which emphasized that all analyzed surfaces exhibit fractal behavior from lower fractal dimensions (2.27-2.29), indicating smooth surfaces, to higher ones (2.70-2.80) corresponding to highly corrugated surfaces.