Strain Effects of Vertical Separation and Horizontal Sliding in Commensurate Two-Dimensional Homojunctions

Abstract

Strain, as an economic yet controllable approach for structural modulation, frequently plays a vital role in the preparation and performance optimization of two-dimensional nanomaterials (TNMs). Here, utilizing first-principles simulations, the analysis of energetics shows that the biaxial stretching and compressing could facilitate the vertical separation and horizontal sliding in graphene (Gr/Gr), hexagonal boron nitride (h-BN/h-BN), and molybdenum disulfide (MoS2/MoS2) bilayers. The quantification of electron redistribution between layers confirmed that the shifts of interlayer charge density (ρinter–) and its relative values (Δρ inter–) are responsible for the vertical separation and horizontal sliding facilitated by biaxial strain. More effortless horizontal sliding was enabled by a smoother potential energy surface because a smaller Δρ inter– can be acquired under compression, whereas more effortless vertical separation followed a more vulnerable surface energy because a lower ρinter– occurs under tensile strain. The vertical and horizontal division of strain effect provides a novel idea for further understanding its pivotal roles in strain engineering of commensurate-contact TNMs, such as mechanical exfoliation and solid lubrication.

Superlubricity Enabled by Load‐Driven Redistribution of Electrons

“…The sensitivities of interlayer electron density of top and hollow stackings to normal pressure are dissimilar, and the fluctuation of interlayer electron density Δρ diff is monotonically increasing. [ 11 ] This augmentation of Δρ diff means that the relative potential Δ V , which reflects the strength of interlayer coupling, is monotonic. Hence, accompanied by the monotonic increase of the load‐driven Δρ diff , the relative size of sliding energy barriers Δ V between the top and hollow stackings is reversed as normal pressure increases.…”

“…where ρ Bilayer (x,y, z) indicates the electron density distribution of the bilayers, ρ Top (x,y, z) and ρ Bottom (x,y, z) represent the respective electron density distributions of the isolated top and bottom layers. To further quantify the redistribution of electrons, the distribution curve of charge density perpendicular to the atomic layer ρ(z) is estimated by: [11,30] z ab dy x y z dx b a 1 , ,…”

“…Previous researches have shown that the potential energy determined by the interlayer coupling can be accurately reflected by the electron redistribution. [7,10,11,28] Figure 3 displays the electron redistributions of two stacking states that cause the maximum fluctuation of PES under different pressures in h-BN/h-BN bilayer. The top and hollow stacks correspond to the highest and lowest energies on PES in Figure 1, respectively (see "Supporting Information 5.…”

“…[7][8][9] This approach has been successfully used to reveal the relationship between adsorption and static friction and the one between in-plane strain and friction. [10,11] But, to the best of our knowledge, it has not yet been elaborated and applied to establish a direct and quantitative relationship between friction and electrons redistribution driven by the normal load.Some pioneering works have shown that quasi-zero friction or superlubricity is feasible. [12][13][14][15] At present, there are two main strategies for achieving frictionless sliding between both solids in contact: structural superlubricity (SSL) and pressuredriven superlubricity.…”

“…[7][8][9] This approach has been successfully used to reveal the relationship between adsorption and static friction and the one between in-plane strain and friction. [10,11] But, to the best of our knowledge, it has not yet been elaborated and applied to establish a direct and quantitative relationship between friction and electrons redistribution driven by the normal load.…”