Nanomechanics of few-layer materials: do individual layers slide upon folding?

Abstract

Folds naturally appear on nanometrically thin materials, also called “2D materials”, after exfoliation, eventually creating folded‎edges across the resulting flakes. We investigate the adhesion and flexural properties of single-layered and multilayered 2D materials‎upon folding in the present work. This is accomplished by measuring and modeling mechanical properties of folded edges,‎which allows for the experimental determination of the bending stiffness (κ) of multilayered 2D materials as a function of the number‎of layers (n). In the case of talc, we obtain κ ∝ n3 for n ≥ 5, indicating no interlayer sliding upon folding, at least in this thickness‎range. In contrast, tip-enhanced Raman spectroscopy measurements on edges in folded graphene flakes, 14 layers thick, show‎no significant strain. This indicates that layers in graphene flakes, up to 5 nm thick, can still slip to relieve stress, showing the richness‎of the effect in 2D systems. The obtained interlayer adhesion energy for graphene (0.25 N/m) and talc (0.62 N/m) is in good‎agreement with recent experimental results and theoretical predictions. The obtained value for the adhesion energy of graphene on a‎silicon substrate is also in agreement with previous results.