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

Folds naturally appear on nanometrically thin (also called 2D) materials after exfoliation, eventually creating folded edges across the resulting flakes. In the present work, we investigate the adhesion and flexural properties of single and multilayered 2D materials upon folding. This is accomplished by measuring and modeling mechanical properties of folded edges, which allow the experimental determination of the scaling for the bending stiffness (κ) of a multilayered 2D material with its number of layers (n). In the case of talc, we obtain κ proportional to n3 for n ≥ 5, establishing that there is no interlayer sliding upon folding, at least in this thickness range. Such a result, if applicable to other materials, would imply that layers in folds might be either compressed (at the inner part of the fold) or stretched (at its outer part), leading to changes in their vibrational properties relative to a flat flake. This hypothesis was confirmed by near-field tip-enhanced Raman spectroscopy of a multilayer graphene fold.