Interfacial stiffening of Cu, Ni, and Ag thin films on the Ru(0001) surface

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Abstract
We have studied the vibrational behavior of overlayer films of Cu, Ni, and Ag grown on the Ru(0001) surface as a function of film thickness. Specifically, we have measured the thermal mean-square vibrational amplitude, through a Debye-Waller analysis of low-energy electron diffraction, for films having thicknesses ranging between 1 and 10 monolayers. The analysis indicates that the strong interfacial interaction, which gives rise to a pseudomorphic structure for 1 monolayer Cu and Ni films, also results in a reduction in the overlayer vibrational amplitude (i.e., the films are stiffened by the interaction). In addition, we find that this interfacial stiffening is long ranged and requires overlayer-film thicknesses in excess of about 10 monolayers to heal to bulklike values. In contrast, Ag is found not to grow pseudomorphically at the 1-monolayer level, and the vibrational behavior shows a lesser degree of stiffening and a much more rapid healing to bulklike values. These findings are discussed in terms of the instrumental aspects of the Debye-Waller analysis, and in terms of the I-V character of the diffraction process, and are contrasted with similar studies of the structural and electronic properties of film growth for these overlayer systems.