Penta-graphene: A new carbon allotrope

Abstract

A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson's ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics.

Keywords: carbon allotrope; carbon pentagon; electronic structure; negative Poisson's ratio; stability.

Fig. 1.

(A) Crystal structure of T12-carbon viewed from the [100] and [001] directions, respectively. (B) Top and side views of the atomic configuration of penta-graphene. The square marked by red dashed lines denotes a unit cell, and the highlighted balls represent the sp³ hybridized C atoms.

Fig. 2.

(A) Area dependence of total energy per atom for some 2D carbon allotropes. The total energy of the experimentally identified dodecahedral C₂₀ cage is also calculated and plotted here for comparison. (B) Phonon band structures and PhDOS of penta-graphene. (Inset) High-symmetric q-point paths: Γ (0, 0) → X (1/2, 0) → M (1/2, 1/2) → Γ (0, 0).

Fig. 3.

(A) Strain energy with respect to the lateral lattice response when the penta-graphene lattice is under uniaxial strain along the x direction. The arrows indicate the equilibrium magnitude of ε_yy. (B) Stress–strain relationship under equi-biaxial tensile strain. The red arrow denotes the maximum strain. (C) Phonon bands of penta-graphene at the extreme of equi-biaxial strain. (D) Same as C for graphene. Blue lines and red circles represent phonons before and after the failure, respectively. (Insets) The high-symmetry q-point paths in the reciprocal space.

Fig. 4.

(A) Electronic band structure and total and partial DOS of penta-graphene calculated by using HSE06 functional. The Fermi level is shifted to 0.00 eV. Band-decomposed charge density distributions are depicted in B to E: (B) the second highest occupied band, (C) the highest occupied band, (D) the lowest unoccupied band, and (E) the second lowest unoccupied band.

Fig. 5.

(A) Illustration of chiral vectors of penta-tube. Dashed lines with arrows denote the lattice basis vector. (B) Optimized structure of (3, 3) penta-tube from side view, and (C) the corresponding phonon spectra.

Fig. 6.

(A) Crystal structure of AA-T12 carbon. (B) AA-T12 viewed from the [001] and [100] directions, and (C) the corresponding phonon spectra.