[Paper Review] Irida-Graphene: A New 2D Carbon Allotrope

M. L. Pereira Junior, Wiliam Ferreira da Cunha+3 more|arXiv (Cornell University)|Aug 18, 2022

Graphene research and applications21 citations

TL;DR

The paper proposes Irida-Graphene, a 2D all-sp2 carbon allotrope made of fused 3-6-8 rings, and analyzes its structural, mechanical, electronic, and optical properties using DFT and ReaxFF MD.

ABSTRACT

Several 2D carbon-based materials have been computationally designed in the last years due to the success achieved by graphene. Here, we propose a new 2D all-sp$^2$ carbon allotrope, named Irida-Graphene (IG), using a bottom-up approach. IG is composed of fused rings containing 3-6-8 carbon atoms. We employed density functional theory calculations and reactive (ReaxFF) molecular dynamics simulations to examine its mechanical, structural, electronic, and optical properties. Results showed that IG exhibits good dynamical and thermal stabilities. Its estimated elastic modulus varies between 80-113 GPa. IG is a metallic material and presents a Dirac cone above the Fermi level in the center of the band. The intense optical activity of IG is restricted to the infrared and violet regions. IG can act as a violet collector for photon energies of about 3.0 eV since it presents very low reflectivity and refractive index greater than one.

Motivation & Objective

• Motivate the design of new 2D carbon allotropes beyond graphene by exploring non-hexagonal rings and porous structures.
• Introduce Irida-Graphene (IG), a 2D all-sp2 carbon sheet composed of fused 3-6-8 rings, as a bottom-up design candidate.
• Characterize IG’s structural stability, mechanical performance, electronic structure, and optical response to assess potential applications.

Proposed method

• Perform first-principles density functional theory (DFT) calculations with the SIESTA code using GGA-PBE exchange-correlation and norm-conserving pseudopotentials.
• Optimize IG’s geometry with a 2D unit cell (12 carbon atoms) and analyze phonon dispersion for dynamical stability.
• Compute electronic band structure and density of states to identify metallicity and Dirac features, including a Dirac cone above the Fermi level at the Y point.
• Carry out Born-Oppenheimer AIMD simulations at 300 K to test thermal stability and planarity.
• Use reactive (ReaxFF) molecular dynamics to evaluate mechanical properties via uniaxial tension and to study fracture patterns and thermal stability up to 10,000 K.
• Estimate optical coefficients (absorption, refractive index, reflectivity) from the dielectric function under external fields.

Experimental results

Research questions

• RQ1 Is Irida-Graphene dynamical and thermal stable under typical conditions for 2D carbon materials?
• RQ2 What are IG’s mechanical properties and fracture behavior under in-plane loading?
• RQ3 Does IG exhibit metallic electronic structure and where is the Dirac cone located?
• RQ4 What are IG’s optical properties and how do they vary with photon energy and polarization?

Key findings

• IG is dynamically stable with positive phonon frequencies and no imaginary modes; AIMD shows planar, bonded structure at 300 K.
• IG shows in-plane anisotropic elasticity with Young’s moduli YmX = 80 GPa and YmY = 113 GPa; fracture strain 15.4% (x) and 18.0% (y); ultimate strengths 47.4 GPa (x) and 95.1 GPa (y).
• IG is metallic with no band gap; a Dirac cone sits above the Fermi level at the Y point, with valence mainly from 2p_z orbitals.
• IG’s optical activity is strong in infrared and violet regions; absorption peaks near 1.0 eV (IR) and ~3.0 eV (violet); refractive index peaks at ~0.7 eV and reflectivity is low around 3.0 eV but ~5% in violet and ~50% in infrared.
• Melting point of IG is 4176 K, with complete melting by 5500 K; IG has a high thermal stability comparable to graphene in melting behavior.