Tearing behavior induced by van der Waals force at heterogeneous interface during two-dimensional MoS₂ nanoindentation

Abstract

Combining with <i>in situ</i> nanomechanical testing system and video module of scanning electron microscope, the nanoindentation testing is performed to study the peeling-tearing behavior of two-dimensional material van der Waals heterostructures. After two-dimensional MoS₂ nanosheets prepared by chemical vapor deposition are assembled into MoS₂/SiO₂ heterostructures by wet transfer, the nanoindentation is carried out by manipulating the tungsten probe in the<i> in situ</i> nanomechanical testing system. When the tungsten probe is tightly indenting into MoS₂ nanosheets, a new W/MoS₂/SiO₂ heterostructure is assembled. With the tungsten probe retracting, the adhesive effect makes the two-dimensional MoS₂ nanosheet peel off from SiO₂/Si substrate to form a bulge. After reaching a certain height, under the van der Waals adhesion interaction, an incomplete penetration fracture occurs along the arc line contacting the needle. Then cleavage appears and produces two strip cracks and MoS₂/SiO₂ interface separation takes place simultaneously, before a large area of MoS₂ nanosheet is teared. Based on the density functional theory calculation of interface binding energy density of van der Waals heterogeneous interface, the interface binding energy density of MoS₂/W is verified to be larger than that of MoS₂/SiO₂, which explains the adhesion peeling behavior of MoS₂ induced by van der Waals force between heterogeneous interfaces, perfectly. By using the peeling height and tearing length of MoS₂ recorded by video module, the fracture strength of MoS₂ is obtained to be 27.055 GPa and stress-strain relation can be achieved according to the film tearing model. The density functional theory simulation results show that the fracture strength of MoS₂ is in a range of 21.7–32.5 GPa, and the stress-strain relation is consistent with the experimental result measured based on film tearing model. The present work is expected to play an important role in measuring the fracture strengths of two-dimensional materials, the assembly, disassembly manipulation and reliability design of two-dimensional materials and van der Waals heterostructures devices.