Universal moiré buckling of freestanding 2D bilayers.

The physics of membranes, a classic subject, acquires new momentum from two-dimensional (2D) materials multilayers. This work reports the surprising results emerged during a theoretical study of equilibrium geometry of bilayers as freestanding membranes. While ordinary membranes are prone to buckle around compressive impurities, we predict that all 2D material freestanding bilayers universally undergo, even if impurity-free, a spontaneous out-of-plane buckling.

Nonmonotonic Velocity Dependence of Atomic Friction Induced by Multiple Slips.

The transition from single to multiple atomic slips, theoretically expected and important in atomic-scale friction, has never been demonstrated experimentally as a function of velocity. Here we show by high-resolution friction force microscopy on monolayer MoS_{2}/Au(111) that multiple slips leave a unique footprint-a frictional velocity weakening. Specifically, in a wide velocity interval from 10 to 100  nm/s, friction surprisingly decreases.

Frictionless nanohighways on crystalline surfaces.

The understanding of friction at nano-scales, ruled by the regular arrangement of atoms, is surprisingly incomplete. Here we provide a unified understanding by studying the interlocking potential energy of two infinite contacting surfaces with arbitrary lattice symmetries, and extending it to finite contacts. We categorize, based purely on geometrical features, all possible contacts into three different types: a structurally lubric contact where the monolayer can move isotropically without friction, a corrugated and strongly interlocked contact, and a newly discovered directionally structurally lubric contact where the layer can move frictionlessly along one specific direction and retains finite friction along all other directions.

Relation between interfacial shear and friction force in 2D materials.

Understanding the interfacial properties between an atomic layer and its substrate is of key interest at both the fundamental and technological levels. From Fermi level pinning to strain engineering and superlubricity, the interaction between a single atomic layer and its substrate governs electronic, mechanical and chemical properties. Here, we measure the hardly accessible interfacial transverse shear modulus of an atomic layer on a substrate.

Understanding the rheology of nanocontacts.

Mechanical stiffness, as opposed to softness, is a fundamental property of solids. Its persistence or rheological evolution in vibrating solid-solid nanocontacts is important in physics, materials science and technology. A puzzling apparent liquefaction under oscillatory strain, totally unexpected at room temperature, was suggested by recent experiments on solid gold nano-junctions.