Thermal Conductivity of Novel 2D Carbon Allotropes with 558 Rings: Insights from Molecular Dynamics

Abstract

This study investigates the thermal transport of two newly proposed two‑dimensional carbon allotropes—octagon‑pentagon graphene‑line (OPG‑L) and octagon‑pentagon graphene‑zigzag (OPG‑Z)—using reverse and equilibrium molecular dynamics simulations. The thermal conductivity (TC) grows steadily with sample size; extrapolation to infinite length yields values of 310–332 W m⁻¹ K⁻¹ for OPG‑L and 247–228 W m⁻¹ K⁻¹ for OPG‑Z. Compared with graphene (≈ 3000–5000 W m⁻¹ K⁻¹), the lower TC originates from reduced phonon group velocities and mean free paths. Temperature and tensile strain systematically suppress TC, offering tunability for thermal‑management applications.

Introduction

Carbon allotropes such as diamond, carbon nanotubes, and graphene are celebrated for their exceptional thermal conductivity, with single‑walled nanotubes and graphene reaching several thousand W m⁻¹ K⁻¹. Recent theoretical work has unveiled a host of 2D carbon networks—graphyne, octagraphene, T‑graphene, and H‑net—each with distinct ring motifs and promising electronic or mechanical properties. Su et al. introduced two energetically favorable, kinetically stable 2D sheets composed of octagons and pentagons (558 rings). These structures, OPG‑L and OPG‑Z, possess formation energies of 0.31 and 0.34 eV atom⁻¹, respectively, substantially lower than graphyne (0.76 eV atom⁻¹) and exhibit anisotropic electronic behavior. However, their thermal dissipation characteristics remain unexplored, motivating the present investigation.

Model and Methods

The atomic models of OPG‑L (Fig. 1a) and OPG‑Z (Fig. 1b) are built from repeating 558 ribbons, with chirality defined analogously to graphene (armchair and zigzag). LAMMPS was employed with a modified optimized Tersoff potential to capture C–C interactions accurately. The equilibrium lattice constants obtained (OA = 3.63 Å, OB = 9.38 Å for OPG‑L; OA = 6.78 Å, OB = 5.04 Å for OPG‑Z) agree with prior first‑principles results.

Reverse non‑equilibrium molecular dynamics (rNEMD) was used to calculate TC. After 0.25 ns of Nosé–Hoover equilibration at 300 K, the sample was partitioned into 50 slabs along the heat‑transfer direction. The hottest atom in the cold slab and the coldest atom in the hot slab exchanged kinetic energy, generating a steady heat flux. The resulting temperature gradient yields TC via Fourier’s law: \[ \kappa = \frac{J}{2A\,\partial T/\partial L} \] where the factor 2 accounts for bi‑directional heat flow and the thickness is taken as 0.34 nm, the graphene interlayer spacing.

Equilibrium MD (EMD) calculations were performed using the Green‑Kubo formalism. Running TC was obtained from the heat‑flux autocorrelation function, and infinite‑size values were extracted by averaging over 1–2 ns of correlation time.

Results and Discussions

Size dependence: TC increases monotonically with effective sample length (L_eff = L/2). For 50‑nm OPG‑L (zigzag) and OPG‑Z (zigzag), TC values are 125 and 94 W m⁻¹ K⁻¹, respectively; at 1000 nm they rise to 296 and 236 W m⁻¹ K⁻¹. Linear fits of κ⁻¹ versus L_eff⁻¹ provide extrapolated infinite‑size TC: 310–332 W m⁻¹ K⁻¹ for OPG‑L and 247–228 W m⁻¹ K⁻¹ for OPG‑Z, in excellent agreement with EMD results (313–344 W m⁻¹ K⁻¹ for OPG‑L, 261–233 W m⁻¹ K⁻¹ for OPG‑Z).

Temperature effect: Across 200–500 K, TC decreases due to enhanced Umklapp scattering. From 300 K to 500 K, TC falls by 36–42 %. The relative conductivities follow the trend observed in graphene, confirming the universality of phonon‑phonon scattering in 2D lattices.

Strain effect: Uniaxial tensile strain applied along the heat‑transfer direction monotonically suppresses TC in both allotropes. At 10 % strain, TC reductions reach 49 % (OPG‑L zigzag) and 31 % (OPG‑Z armchair). Vibrational density‑of‑states analysis reveals pronounced phonon softening in the in‑plane modes, accounting for the diminished thermal transport.

Mechanical properties: Under uniaxial tension, OPG‑L and OPG‑Z exhibit Young’s moduli of 538–648 GPa and 492–550 GPa, respectively, comparable to graphyne but lower than pristine graphene. Ultimate strains reach 17.2 % (OPG‑L zigzag) and 8.7 % (OPG‑L armchair), indicating robust yet strain‑sensitive behavior.

Conclusions

Through comprehensive rNEMD and EMD simulations, we have quantified the size, temperature, and strain dependence of TC in OPG‑L and OPG‑Z. Their conductivities are markedly lower than graphene’s due to reduced phonon velocities and mean free paths, yet remain tunable via strain engineering. These insights establish OPG‑L and OPG‑Z as promising candidates for tailored thermal management in micro‑ and nanoscale devices.

Abbreviations

558

Five‑five‑eight-membered rings

OPG‑L

Octagon and pentagon graphene‑line

OPG‑Z

Octagon and pentagon graphene‑zigzag

rNEMD

Reverse non‑equilibrium molecular dynamics

TC

Thermal conductivity

VDOS

Vibrational density of states