Two-dimensional diboron trioxide crystal composed by boroxol groups.

Diboron trioxide (B2O3) 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 B2O3 to vitrify. Here we present the synthesis of a two-dimensional crystalline B2O3 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 B2O3 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. Editor's summary: Macroscopic crystallization of boria (B2O3) occurs only under applied pressure. However, Zio et al. showed that a two-dimensional crystalline B2O3 polymorph can grow on a platinum surface through elemental deposition. Large-scale scanning tunneling microscopy revealed that the B2O3 monolayer had a honeycomb structure with high degree of crystallinity. This layer formed from boroxol (B3O3) groups and had minimal interaction with the substrate, as confirmed by density functional theory. Boroxol groups are not constituents of the two known macroscopic crystalline phases, although their presence as nanodomains in boria glass has been inferred. —Phil Szuromi [ABSTRACT FROM AUTHOR]