Stress Modulated Phase Transition in 2D TMDC Materials

Texas A&M Engineering Experiment Station, College Station TX

Investigators

Abstract

Two-dimensional transition metal dichalcogenide (2D TMDC) are atomically thin materials with a generalized formula of MX2, where M is a transition metal atom (Molybdenum or Tungsten) and X is a chalcogen atom (Sulfur, Tellurium, or Selenium). One layer of M atoms is sandwiched between two layers of X atoms. 2D TMDC can exist in two stable structural phases with different atomic arrangements: semiconducting 2H phase and conducting 1T' phase. The dynamic control of transitions between these two phases through applied mechanical stress can lead to revolutionary device applications such as memory devices, reconfigurable circuits and topological transistors at atomically thin limits. This project will provide fundamental knowledge of the role of the stress field on the atomistic mechanism of phase transitions of 2D TMDC, facilitating the application of phase engineering in next generation 2D electronics and optoelectronics, thereby advancing national health, prosperity, and welfare. Educationally, taking advantage of the large Hispanic student population at University of Texas at San Antonio, the major educational goal of the project is to broaden the participation of underrepresented groups in research, and train them through active research engagement. The mechanism of stress dependent phase transition is that the stress field can be applied to change transition energy barriers and pathways. The barriers determine phase transition rates and pathways reveal the atomistic transition process such as new phase nucleation and propagation. So far, the role of stress on transition behaviors of 2D TMDC is not clear. The central objective of this project is to determine transition barriers and pathways of 2D TMDC as a function of applied stress field, in order to build a mechanics foundation for phase engineering of 2D TMDC at the atomic level. A combined computational and theoretical approach will be employed to achieve this objective. However, it is noted that the existing methods cannot be readily used for this study when one considers the finite deformation of 2D materials. Hence, new methods will be developed. A new computation method, called Finite Deformation Nudged Elastic Band method, will be developed by adding nonlinear mechanics to conventional NEB method, for finding transition barriers and pathways under finite deformation. Meanwhile, since it is time consuming to simulate all possible stress states, there is a need to develop a theory that can predict the barriers. Hence, a new theoretical method, called Finite Deformation Bell Theory, will be developed based on the concept of original Bell theory and continuum mechanics. The methodologies developed in this project can be applied to study phase transitions of other crystalline materials, and more broadly to study mechano-chemical problems beyond phase transitions, such as diffusion, dislocation motion, fracture formation and so on, where the rate dependent transition behaviors in materials are coupled with stress and finite deformation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.