In-situ transmission electron microscopy characterizations on dynamical properties of two-dimensional materials

Abstract

During the past decade, two-dimensional (2D) materials are under explosive growth. They are well known for having extraordinary mechanical, chemical, electrical and optical properties owing to their ultrathin structures and outstanding flexibilities. Apart from the most popular 2D materials such as graphene or hexagonal boron nitride, transition metal dichalcogenides (TMDs), as a big 2D family, also have a wide range of applications for high electron mobility and extraordinary mechanical properties. All these physical features are related to bandgaps and directly decided by their atomic structures. Therefore, observation of atomic structures under the effect of strain provides the intrinsic understanding of dynamically related properties in 2D materials. In this thesis, four types of mechanical behaviors in dimensional order are identified with the change of atomic structures in 2D materials: grain boundaries (GBs) (1D), phase transition (2D), fracture (2D) and wrinkling (3D). The pristine 2D materials in experiments include graphene, MoS2 and ReS2. With the help of scanning transmission electron microscopy (STEM), continuous change of crystal lattice can be observed in nanoscale. Electron beam irradiation from (S)TEM provides the energy to introduce strain on monolayer materials under atomic resolution and at the same time, scanning tunneling microscopy (STM) applies direct strain for lower magnification deformation and electrical tests. Related statistics, modeling and DFT calculation are conducted to explain the mechanism behind experimental phenomenon. In monolayer ReS2, a systematic classification of mobile and pinned GBs is clarified. According to the high/low crystal plane index and whether the rhenium atomic arrangements on both sides of GBs are consistent or not, the GBs in ReS2 can be divided into four categories and more detailed types. With similar in situ method, electron beam drives phase transition on ReS2 by creating atomic defects and introducing strain, the atomic structure of secondary phase is clearly observed. Apart from this, fracture process and cracks extension are also observed under electron beam irradiation, while the lattice reconstruction occurs on the tip of the crack under the influence of shear stress. The continuous deformation and van der Waals (vdW) force influence in wrinkling are observed when a monolayer graphene being cracked from its multilayer flake. Thus the critical length of stable wrinkles is calculated from the wrinkling and wrinkle elimination process. In conclusion, in situ (S)TEM is employed as an ideal method to measure the mechanical properties of 2D materials. The present works shed light on the future mechanical engineering, facilitating the high strength and high toughness as well as potential functionalities in 2D materials.