Co-sputtering C-Cu thin film synthesis: Microstructural study of copper precipitates encapsulated into a carbon matrix

Abstract

Co-sputtered C-Cu thin film depositions have been performed in the temperature range 80-873 K, the atomic carbon concentration varying from 16% to 96%. To characterize the microstructure of the C-Cu thin films, transmission electron microscopy, extended X-ray absorption fine structure and grazing incidence small angle X-ray scattering experiments have been used. During the deposition process, a demixing occurs of the carbon and copper species due to their very low solubilities that leads to the formation of nanometric copper precipitates homogeneously distributed in a more or less graphitic matrix. These precipitates have an elongated shape in the direction of the thin film growth. When the deposition was performed at 273 K for copper atomic concentrations C_Cu > 55%, as well as for all thin films synthesized at 573 K whatever the C_Cu value, the formation of graphene layers parallel to the surface of the copper precipitates was observed so that an encapsulation of the Cu aggregates in carbon cages occurs. We propose that surface diffusion of the different species occurring during the deposition process leads to the demixing of carbon and copper. Thus, we suggest that the copper acts as a catalyst for graphitization of carbon to explain the formation of such structures at temperatures as low as those used in these experiments.