Strain-induced hierarchical ripples in MoS2 layers investigated by atomic force microscopy

Abstract

Strain engineering plays a vital role in controlling the physical properties of two-dimensional (2D) materials. However, the nanomechanical behavior of atomically thin 2D crystals under strain has not been completely understood. Here, strain-induced hierarchical ripple nanostructures in triangular MoS2 flakes were investigated by advanced atomic force microscopy and optical spectral measurements. The hierarchical nanoripples exhibited a threefold radial pattern, and their mechanical, electronic, and optical spectra characteristics were significantly modified due to the suffering from large tensile strain. Structure evolution of these hierarchical nanoripples was further discussed based on the geometry and thickness of MoS2 flakes, and we attributed the curtain effect at the limit of a single atomic layer. Our study will be beneficial in designing nanomechanical structures and prototype electromechanical devices with 2D materials.

Strain-Engineered Rippling and Manipulation of Single-Layer WS2 by Atomic Force Microscopy

“…In addition to the compressive strain, the tensile strain can also induce a ripple structure within the thin 2D films. 14,19,22 The thermal tensile strain-induced hierarchical ripples in MoS 2 monolayer flakes have been observed in our previous work. 14 At here, the in-plane stretch/tension at the rear area of the sliding tip will also induce the ripple reformed along the direction of the tip sliding (Figure 5h).…”

“…14,19,22 The thermal tensile strain-induced hierarchical ripples in MoS 2 monolayer flakes have been observed in our previous work. 14 At here, the in-plane stretch/tension at the rear area of the sliding tip will also induce the ripple reformed along the direction of the tip sliding (Figure 5h). By the simultaneous "pushing" and "stretching" mechanism, the new rippling domain was formed via the reorientation of rippling lines.…”

“…Monolayer transition metal dichalcogenides (TMDs) have attracted the intensive research attention due to their excellent mechanical, electrical, and optical properties. − Furthermore, their exceptional mechanical properties facilitate the strain engineering as a controllable way for modifying their electrical and optical properties, indicating the great promise of their applications in strain-related electronics and optoelectronics . Strain in two-dimensional (2D) materials can be applied through strain engineering, such as bending of TMD layers on elastic substrates − and so on. ,− In addition, thermal strain engineering, by utilizing the thermal coefficient of expansion (TCE) mismatch between the TMDs and the growth substrate, can also apply built-in strain in TMD layers. − …”

Toplayer-dependent crystallographic orientation imaging in the bilayer two-dimensional materials with transverse shear microscopy

“…Following the discovery of graphene prepared by the micromechanical exfoliation technique, 1 much research intension has been paid on the two-dimensional (2D) [2][3][4][5][6][7] material due to its superior characteristic properties, including excellent physical, 8,9 mechanical, 10,11 chemical, 12,13 electronic and optoelectronic properties. [14][15][16][17] One of the intriguing advantages of the 2D layered material is that their isotropic properties can be converted to be anisotropic, which results from the breaking symmetry owing to their intrinsic puckering behaviors.…”

“…Nanomechanical deformation has been extensively investigated by the atomic force microscopy (AFM) indentation experiments on suspended films. [21][22][23] Based on AFM platform, [24][25][26][27] friction force microscopy (FFM) and transverse shear microscopy (TSM), as the derivative mode of contact AFM, [28][29][30][31][32] now have been widely used to study the elastic deformation, 19,31 crystal structure, 28 surficial ripples of 2D materials 8,34,35 and so on. Such studies have revealed an important phenomenon in which nanomechanical contact behavior could be dependent on the number of layers and substrates.…”