On folded graphene and its properties : a preliminary study

Abstract

Graphene is one of the newly discovered forms of carbon and exists as a single sheet, made from sp2-hybridised carbon atoms, which is flexible enough to be able to adopt a variety of conformations. This material, or its variants, may exhibit wide-ranging properties, including a negative Poisson’s ratio (auxeticity). This property may be imparted to graphene through a process which involves modification through the use of patterned or randomly-placed defects which guide graphene to adopt particular three-dimensional conformations that are amenable to exhibiting negative Poisson’s ratios. This work re-examines and extends recent work which has shown how graphene with defects inserted along equally spaced lines may exhibit giant negative Poisson’s ratios as a result of the corrugated-sheet-like conformation it adopts, which defect lines act as crease lines. It is shown, through a combination of static force-field based simulations using the polymer consistent force-field (PCFF) and more computationally intensive NPT molecular dynamics simulations using the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) force-field that the corrugated forms of graphene which had previously been reported are neither the most stable nor the most auxetic forms that these systems may adopt. In fact, it is shown that unless constrained, the low-density corrugated forms of graphene are likely to fold and adopt much denser forms with graphite-like features that impart added stability, which may or may not be auxetic. It was also shown that other stable variants of the less dense form of the corrugated graphene may exist which may exhibit an even higher extent of auxeticity than what was reported so far. Other anomalous properties, such as negative tangential stiffness or zero Poisson’s ratios for some of these forms are also identified and discussed.