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

We report on structural and electrical properties of thin granular Pdx Cl-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 PdxC l-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 infin ite percolative network within the amorphous-C matrix. The whole netwo rk is progressively disrupted with decreasing x. The overall conductiv ity behavior of the films is metallic, strongly influenced by electron localization and electron-electron interaction effects. The temperatu re dependence of the dc conductivity follows sigma(T) proportional to T-1/2 over a large range in temperature at elevated temperatures, simi lar to what is expected for three-dimensional (3D) homogeneous systems . However, below distinct low temperatures T delta we observe characte ristic 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 dimensio nal (3D-->2D) crossover with respect to localization and electron-elec tron 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 leve l separation within the small metallic clusters due to quantum size ef fects (QSE). These are known to be of significance in the transport pr operties of insulating granular films. With this paper we propose that likewise QSE are of importance for granular metallic films.