Electronic properties of a π-conjugated Cairo pentagonal lattice: Direct band gap, ultrahigh carrier mobility, and slanted Dirac cones

摘要

Two-dimensional (2D) lattices composed exclusively of pentagons represent an exceptional structure of materials correlated to the famous pentagonal tiling problem in mathematics, but their π conjugation and the related electronic properties have never been reported. Here, we propose a tight-binding (TB) model for a 2D Cairo pentagonal lattice and demonstrate that p-d π conjugation in the unique framework leads to intriguing properties, such as an intrinsic direct band gap, ultrahigh carrier mobility, and even slant Dirac cones. On the basis of first-principles calculations, we predict a candidate material. 2D penta-NiP_2 monolayer, derivated from bulk NiP_2 crystal, to realize the predictions of the TB model. It has ultrahigh carrier mobility (～10~5-10~6 cm~2 V~(-1) s~(-1)) comparable to that of graphene and an intrinsic direct band gap of 0.818 eV, properties which have long been desired for high-speed electronic devices. The stability and possible synthetic routes of penta-NiP_2 monolayer are also discussed.