Bilayer borophene prevails over monolayer counterpart

Boron (B), the lightest element among the metalloids, shares similar characteristics with carbon in planar and cage molecules. Intensive theoretical studies have predicted a variety of mono- and bi-layer polymorphs of B with intriguing properties. Single-layer B sheets (borophene) have been experimentally realized on different metal substrates but show poor ambient stability [1,2]. The bilayer (BL) counterpart [3,4], featuring excellent thermal stability and resistance to oxidation, however, remains difficult to prepare. Here, in this Opinion, we highlight recent achievements in the epitaxial growth of BL borophene on noble metal surfaces, and illustrate the crucial roles of the substrate and thermodynamic growth conditions. The superior properties of BL over single-layer B sheets are addressed. Experimental realization of BL borophene calls for further exploration of other substrates that may enable phase engineering.