Borophene bottom-up syntheses: a critical review

Abstract The ever-expanding portfolio of two-dimensional (2D) materials now also includes metallic boron sheets, known as borophene. Initially predicted by simulation in 1997, borophene was first synthesized in 2015. This 2D form of boron exists under various polymorphic structures, the most observed being β_12 and χ_3. Borophene fills a crucial gap in the 2D material repertoire thanks to its intrinsic metallicity and relative stability in air. However, in view of practical applications including borophene, it is crucial to master its synthesis to enable controlling its structure and thus tuning its properties. This review focuses on its current bottom-up syntheses, i.e., physical vapor deposition and chemical vapor deposition. Indeed, borophene’s structure is different from the bulk boron one, which excludes its top-down synthesis. The impact of substrate characteristics, temperature, pressure conditions, and precursor molecules on borophene's phase and domain size is analyzed, along with key characterization techniques. The review extends to hydrogenated borophene and bilayer borophene for a comprehensive understanding of boron bonding and substrate interactions. Given the importance of ambient stability and transferability of 2D boron layers for device applications, a thorough comprehension of borophene oxidation is imperative. The review thus also discusses potential solutions to mitigate oxidation-related challenges.