Two-dimensional diboron trioxide crystal composed by boroxol groups

Diboron trioxide (B 2 O 3 ) represents an unusual case among polymorphic oxides, because its vitrified state features superstructural units—planar boroxol groups—that are never observed in its three-dimensional crystalline polymorphs. Crystalline polymorphs that incorporate boroxol groups have only been predicted theoretically, although their formation is crucial to rationalize the ability of B 2 O 3 to vitrify. Here we present the synthesis of a two-dimensional crystalline B 2 O 3 polymorph constituted by boroxol groups arranged in an atomically thin honeycomb lattice. By combining surface science experimental techniques with ab initio calculations, we characterize the structural and electronic properties of this B 2 O 3 polymorph down to the atomic level. This discovery enlarges the family of two-dimensional materials and enables the atomic tracking of individual structural units in trioxides.