Achieving Ambient Stability of Borophene via Ultrahigh Vacuum Alumina Encapsulation

In recent years, a range of two-dimensional boron polymorphs, collectively referred to as borophene, have been experimentally realized on a diverse set of metallic substrates by bottom-up synthesis in ultrahigh vacuum (UHV). However, since borophene is highly reactive chemically and rapidly oxidizes in ambient conditions, robust encapsulation methods are needed to ensure the long-term stability of borophene outside of UHV environments. Here, we demonstrate that encapsulation using UHV electron-beam evaporation of alumina (AlO_x) prevents oxidation of borophene in ambient conditions. This protection of borophene from chemical degradation is achieved with UHV-deposited AlO_x encapsulated layers as thin as 3.7 nm. X-ray photoelectron spectroscopy and scanning probe microscopy confirm that this encapsulation scheme preserves the integrity of borophene for at least 12 months in ambient conditions. This long-term stability addresses a critical hurdle in the processing and integration of borophene into practical device architectures.