Achieving Ambient Stability of Borophene via Ultrahigh Vacuum Alumina Encapsulation.

Aklile, Eden B; Liu, Xiaolong; Goronzy, Dominic P; Rahn, Matthew S; Li, Qiucheng; Lamarca, Cataldo W; Hersam, Mark C · Nano Lett · 2025

basic_science · Level V

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Abstract

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<sub><i>x</i></sub>) prevents oxidation of borophene in ambient conditions. This protection of borophene from chemical degradation is achieved with UHV-deposited AlO<sub><i>x</i></sub> 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.