Two-dimensional XC6-enes (X = Ge, Sn, Pb) with moderate band gaps, biaxial negative Poisson's ratios, and high carrier mobility

Graphene-based analogs and derivatives provide numerous routes to achieve unconventional properties and potential applications. Particularly, two-dimensional (2D) binary materials of group-IV elements are drawing increasing interest. In this work, we proposed the design of three 2D graphene-based materials, namely, XC6-enes (X = Ge, Sn, or Pb), based on first-principles calculations. These new materials possess intriguing properties superior to graphene, such as biaxial negative Poisson's ratio (NPR), moderate bandgap, and high carrier mobility. These XC6-enes comprise sp2 carbon and sp3 X (X = Ge, Sn, Pb) atoms with hexagonal and pentagonal units by doping graphene with X atoms. The stability and plausibility of these 2D materials are verified from formation energies, phonon spectra, ab initio molecular dynamic simulations, and elastic constants. The incorporation of X atoms leads to highly anisotropic mechanical properties along with NPR due to the unique tetrahedral structure and hat-shaped configuration. In the equilibrium state, all the XC6-enes are moderate-band-gap semiconductors. The carrier mobilities of the XC6-enes were highly anisotropic (∼104 cm-2 V-1 s-1 along the [010]-direction). Such outstanding properties make the 2D frameworks promising for application in novel electronic and micromechanical devices.