SIGNIFICANCE OF QUANTUM-SIZE EFFECTS IN THE CONDUCTIVITY OF GRANULAR PDXC1-X FILMS

We report on structural and electrical properties of thin granular PdxCl-x films with palladium (Pd) metal volume fractions 0. 3 < x < 0. 34, approaching the percolation threshold (x(p)=0. 3) from the metallic side. As revealed from transmission-electron microscopy, granular PdxCl-x, films consist of small spherical Pd clusters with mean diameters Phi ranging between 3 nm < Phi < 4 nm, embedded in an amorphous carbon (C) matrix. The Pd clusters are only weakly coupled, forming an infinite percolative network within the amorphous-C matrix. The whole network is progressively disrupted with decreasing x. The overall conductivity behavior of the films is metallic, strongly influenced by electron localization and electron-electron interaction effects. The temperature dependence of the dc conductivity follows sigma(T) proportional to T-1/2 over a large range in temperature at elevated temperatures, similar to what is expected for three-dimensional (3D) homogeneous systems. However, below distinct low temperatures T delta we observe characteristic deviations from the sigma(T) proportional to T-1/2 law towards a stronger than logarithmic temperature dependence of sigma(T), and a saturation of sigma(T) for T-->O. This does not result from a dimensional (3D-->2D) crossover with respect to localization and electron-electron interaction, but is discussed as resulting from the influence of the granular film structure on electronic transport, since T delta is for all films related to the mean cluster diameter Phi via k(B)T(delta) = [N(E(F))Phi(3)](-1). Here, k(B)T(delta) is the average energy level separation within the small metallic clusters due to quantum size effects (QSE). These are known to be of significance in the transport properties of insulating granular films. With this paper we propose that likewise QSE are of importance for granular metallic films.