Mechanical Behavior and Strain Engineering of 2D Transition Metal Dichalcogenide (MoTe2) in the Presence of Defects and Twist Angle

Shoieb Chowdhury, PhD Qualifying Exam, Advised by Professor Hesam Askari

Monday, January 10, 2022

Two dimensional (2D) materials are a class of layered material with merely a single to few atomic layers thickness. Due to the ultrathin structure and high surface-to-volume ratio, 2D materials have favorable mechanical properties such as large in-plane stiffness, high strength, and low flexural rigidity. Their weak interlayer interactions allow fine control over stacking order and orientations of individual layers. Besides, they offer tunable properties ranging from metallic to semiconducting and intensive optical emission at a broad wavelength from ultraviolet to near-infrared. These properties generally arise from confinement of electron waves to single-layer 2D wave form that is extremely sensitive to atomic configurations. Virtually, any atomic reconfiguration can alter electron signatures in these materials. Due to the strong relationship between mechanical and opto-electronic properties, understanding the role of various types of crystalline defects and stacking designs on mechanical properties can offer much-needed insights into the behavior and performance of 2D materials.

Mechanical response and strain distribution extensively depend on the presence, evolution, and interaction of defects during deformation. Although most 2D materials are theoretically predicted to withstand large deformation, many experimental works report failure initiation at a small 1-5% strain level which has been attributed to preexisting defects. Quantitative knowledge on the change of local structural configurations, interlayer strain transfer, and fracture behavior due to the presence of different types and amount of intrinsic defects are largely unknown. Strain distribution also strongly correlates with the efficacy of interlayer bonding. Controlling the relative rotation angle between two layers known as “Twist Angle” results in interesting properties such as lower temperature superconductivity. In twisted bilayer structure, local patterns of deformation known as “Moire´ Pattern” are observed due to the resultant lattice mismatch. When twisted bilayers are combined with strain engineering techniques such as heterogenous straining between two layers, the resultant Moire´ pattern is expected to be distorted. Calculation of local deformation within the distorted lattice and reconstruction of the Moire´ pattern in a heterogeneous twisted bilayer is yet to be performed.

We have developed an atomistic model that can accurately predict elastic, failure, and strain transfer behavior of pristine 2D Molybdenum Ditelluride (MoTe2). In this study, we aim to further extend the work to defected structure and twisted bilayer. The primary objective is to establish a computational framework to explore the effects of structural features of MoTe2 on mechanical and strain-induced properties. These attributes include vacancy and grain boundary defects, and twist angle. We seek to establish the relationship between mechanical response variations and defects, particularly, how vacancy and grain boundary can impact failure properties and structural phase changes. Different defect types, concentrations, and distributions will be considered in our atomistic models and the most effective routes for facilitating structural changes and enhancement of interlayer strain transfer will be identified. For twisted bilayer, Moire´ Pattern evolution as a function of twist angle and applied strain will be explored. Changes in deformation magnitude and local stacking will be calculated using an improved interatomic potential. The outcome of this work will lead to more controllable strain engineering of 2D materials with many potential electrical, optical, and thermal applications.