First-principles prediction of two-dimensional copper borides

Ordered borides of group IB and IIB metals (Cu, Ag, Au, Zn, Cd, Hg), as the well-known immiscible materials, are practically unknown due to the small electronegativity difference and large mismatch in their atomic sizes. Nevertheless, such rule may be broken under extreme conditions, i.e., high pressure or low dimensionality. Here two-dimensional (2D) copper borides were predicted from ab initio evolutionary searches, identifying that two structures are metallic whereas another one is strikingly a nodal line semimetal. These results challenge the long lasting puzzle in the immiscible systems and add new members to the 2D materials.