Structural properties of granular PdxC1-x films.

Abstract

We report on detailed and systematic investigations of the structural properties of thin (t32 nm) granular ${\mathrm{Pd}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ films with 0.1x0.5, where x is the metal volume fraction. The films are prepared by coevaporation of palladium (Pd) and carbon (C) onto quartz- and NaCl-crystal substrates at room temperature in an uhv system. Since carbon is known to be insoluble in palladium within the whole composition region, we obtain granular ${\mathrm{Pd}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ films retaining their typical granular structure upon large variations of the metal volume fraction x. As revealed from transmission electron microscopic (TEM) investigations, granular ${\mathrm{Pd}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ films with x0.3 consist of mainly isolated small Pd clusters with mean diameters \ensuremath{\Phi} of a few nanometers embedded in an amorphous carbon matrix. With increasing x clusters progressively coagulate, until at a certain metal volume fraction ${\mathit{x}}_{\mathit{p}}$=0.3\char22{}which is the so-called percolation threshold\char22{}an infinite percolative network exists throughout the entire sample. From the analysis of TEM micrographs, we obtain size distributions for both cluster diameter \ensuremath{\Phi} and cluster separation s for various films with different x, from which we obtain mean values as well as typical values for \ensuremath{\Phi} and s with respect to the metal volume fraction x. This offers a quantitative comparison of the structural properties of a real granular system with that of a simple cubic model, often proposed being appropriate to describe the structural properties of granular systems.