Dynamic negative compressibility of few-layer graphene, h-BN, and MoS2.

We report a novel mechanical response of few-layer graphene, h-BN, and MoS(2) to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150%.

Raman signature of graphene superlattices.

When two identical two-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly observed in graphite, where the rotation between graphene layers generates Moiré patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented.

Raman study of ion-induced defects in N-layer graphene.

Raman scattering is used to study the effect of low energy (90 eV) Ar(+) ion bombardment in graphene samples as a function of the number of layers N. The evolution of the intensity ratio between the G band (1585 cm(-1)) and the disorder-induced D band (1345 cm(-1)) with ion fluence is determined for mono-, bi-, tri- and ∼50-layer graphene samples, providing a spectroscopy-based method to study the penetration of these low energy Ar(+) ions in AB Bernal stacked graphite, and how they affect the graphene sheets. The results clearly depend on the number of layers.