Dirac States versus Nearly-Free-Electron States in Ternary C2As6(1-x)P6x Monolayers - A Density Functional Theory Study

Using density functional theory-based calculations, we study the stability, geometric, and electronic properties of ternary C2As6(1-x)P6x monolayers in a buckled honeycomb lattice. As the composition of P atoms (x) varies from 0 to 1 in steps of 1/6, we generated a total of 13 different possible geometric configurations. Binding energy calculations and ab initio molecular dynamics simulations under ambient conditions predict the stability of the ternary monolayers. Their electronic band structure and density of states calculations reveal a rich variety of electronic properties, ranging from semimetal to semiconducting or metallic, depending on the value of x and the crystal symmetry. The ternary C2As6(1-x)P6x monolayers possess two distinct electronic dispersions near the Fermi level: a linear dispersion (Dirac cone) around the high-symmetry K point and a nearly-free-electron (NFE)-like parabolic dispersion around the high-symmetry Gamma point in the Brillouin zone. Isoelectronic substitution breaks some spatial symmetries, causing the Dirac cones to become anisotropic and unpinned from the K point. It also leads to the opening of a small energy gap between the Dirac cone states. In contrast, the nature of the NFE-like parabolic dispersion remains nearly unaltered by the crystal symmetry. Notably, the minimum energy of the NFE band and the effective mass of the electron can be tuned by the composition of the ternary monolayers. Detailed orbital analysis reveals that the contributions of s and p z orbitals of C atoms play a crucial role in tuning the NFE states. Overall, we reveal the novel and versatile electronic properties of the ternary monolayers and demonstrate their tunability via composition.