Sponge-like carbon thin films: The dealloying concept applied to copper/carbon nanocomposite

Nanoporous carbon thin films are prepared by means of a selective etching process in nitric acid applied to nanocomposite copper/carbon (nc-Cu/C) thin films grown by magnetron co-sputtering process. Laying on the electrical percolation theory, we demonstrate that to achieve a full etching of the copper phase present within the nc-Cu/C films, the Cu nanoparticles must be percolated. We further show that by adjusting the initial copper content within the nc-Cu/C films (between 61 and 85 at.%), the pore size can be tuned accurately between 2 and 11 nm. Contrary to what one may expect, increasing the pore size from 2 to 11 nm induces an increase in the electrical conductivity of the nanoporous films from 82 to 308 S cm−1. This unexpected electrical behavior is attributed to the structural modification of the carbon skeleton forming the porous material during the etching process. We further show that the transparency of such nanoporous films can be also controlled by tuning the pore size. The fact that the films with the highest electrical conductivity show the lowest optical absorption coefficient makes such material a very promising candidate for transparent electrode applications. This low temperature (less than 100 °C) synthesis approach will pave the way for the direct integration of conductive nanoporous carbon materials in thin film-based flexible electronic devices.