Dynamic Negative Compressibility of Few-Layer Graphene, h-BN, and MoS₂

Abstract

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%. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS(2)), but it is absent in single-layer graphene and in few-layer mica and Bi(2)Se(3). We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed.

Friction of low-dimensional nanomaterial systems

“…The interpretation of the observation of Lee et al was supported by the Brownian dynamics simulations of Smolyanitsky et al [59] and by the molecular dynamics simulations of Ye et al [60]. The tip-induced out-of-plane deformation of 2D materials was also experimentally confirmed and systematically studied by Barboza et al [61]. They referred to this effect as "dynamic negative compressibility": When the probe tip slides on these 2D materials, a vertical expansion proportional to the applied normal load appears for dynamical wrinkling of the upper material layers induced by the simultaneous compression and shear from the tip.…”

Recent development in 2D materials beyond graphene

“…The thin nanosheets showed greater frictions because of improved susceptibility to outside of plane deformation. Very recently, researchers have reported that the friction force might accelerate flexible, dynamic creasing on the top most layer for different types of nanosheets including graphene, BNNSs and most of TMD layers [438].…”

Potential energy surface for graphene on graphene:Ab initioderivation, analytical description, and microscopic interpretation

“…The effects of shear forces constitute, thus, an important aspect of FLG physics, which has been recently investigated by different experiments. [1][2][3][4][5][6][7][8][9] Moreover, it has been demonstrated that the electronic properties in FLG can be tuned by changing the layer stacking. A spontaneous gap opening has been detected in three layer graphene (3LG) when passing from Bernal (ABA) to rhombohedral (ABC) stacking.…”