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

Abstract

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 \ensuremath{\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 \ensuremath{\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-$\mathrm{Ni}{\mathrm{P}}_{2}$ monolayer, derivated from bulk $\mathrm{Ni}{\mathrm{P}}_{2}$ crystal, to realize the predictions of the TB model. It has ultrahigh carrier mobility ($\ensuremath{\sim}{10}^{5}\ensuremath{-}{10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) 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-$\mathrm{Ni}{\mathrm{P}}_{2}$ monolayer are also discussed.