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

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 pi 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 pi 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-NiP2 monolayer, derivated from bulk NiP2 crystal, to realize the predictions of the TB model. It has ultrahigh carrier mobility (similar to 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-NiP2 monolayer are also discussed.