When is 6 less than 5? Penta- to hexa-graphene transition

Abstract

Penta-graphene is a recently proposed graphene allotrope consisting of repeating pentagons. Here, we characterize the small-strain mechanical properties of finite penta-graphene using full atomistic molecular dynamics (MD). Using tensile tests, the uniaxial yield strength (12.7 N/m) and stiffness (approximately 378.3 N/m) are characterized. Extreme strain is observed at ultimate failure (> 50%), with an associated elastic toughness on the order of 5.4 J/m2. In addition, the bond structure of penta-graphene enables a structural transition from a repeating penta- to hexa-form, e.g., the common hexagonal-based sp2-bonding structure of graphene. We show that penta-graphene can be transitioned to a hexagonal energy-reducing graphene form, via applied strain or temperature. The pentagon-based structure begins to degenerate to a graphene-like hexagonal structure at a critical strain of 5.1%, as the system transitions to a lower energy structure. Beyond this critical strain, the system behaves plastically, and does not revert to initial conditions upon stress removal. The hexagonal form (6-ring) represents a lower energy state than the pentagonal form (5-ring). We further observe conformational changes occur due to thermal energy, leading to a transition at T > 600 K. Post-transition, the system behavior reflects that of graphene with lattice defects. This represents a new paradigm of nanomaterials, whereby one material (graphene) can emerge from another metastable form (penta-graphene).