Degradation of ultra-thin boron films in air

Corrosion of B films in air can limit their practical applications. Here, we study the evolution of the elemental composition, thickness, and morphology of 10-100-nm-thick amorphous B films sputter-deposited onto glassy carbon substrates and stored under different conditions. Results show that films with thicknesses of ≳55 nm have expected excellent corrosion resistance during storage in laboratory air at room temperature over several months. In contrast, ≲45-nm-thick films exhibit pronounced degradation upon air exposure, starting with a change in the composition to ∼30 and ∼50 at.% of O and H, respectively. After such an O and H uptake, the degradation proceeds via mass loss with a characteristic time constant of ∼5 days in air at room temperature. A post-deposition annealing at 1000 °C in an inert atmosphere makes all the films corrosion resistant.