Oxidation of aluminum coating on ultrathin films of carbon nanotubes as filters for high-energy astrophysics space missions

All high-energy missions require filters to reject unwanted radiation and to prevent contamination from molecules and particles onto sensitive devices. Aluminum-coated carbon nanotube pellicles represent an intriguing option for such missions due to their good X-ray and UV transmittance alongside mechanical robustness. Moreover, it has been demonstrated that these pellicles can be fabricated on a large scale even when their thickness reaches a few tens of nanometers. At a microscopic level, these pellicles consist of a network of carbon nanotube bundles resembling a sponge. This structure, when observed via SEM technique, reveals a multitude of holes, creating a large specific area that may enhance the oxidation of the metal coating when compared to deposition on thin plastic membranes, such as polyimide, already widely used in X-ray missions. Oxidation of aluminum spontaneously occurs when air-exposed and causes changes in the spectroscopic properties, especially the loss in the infrared, visible, and ultraviolet rejection. For this reason, a significant effort to protect aluminum by passivation has been made. In this study, we present the characterization of the surface of aluminum-coated carbon nanotube pellicles showing the results of an X-ray photoelectron spectroscopy experiment performed at the BACH beamline of Elettra, the Italian synchrotron. Our analysis indicates the presence of 5.4-5.7 nm of surface aluminum oxide for all the investigated samples but the aluminum nitride-passivated ones that present an aluminum oxide layer less then 1 nm, thus witnessing the efficiency of the nitride passivation to protect Al-coated Single Walled Carbon Nanotubes pellicles from the oxidation.