Stress Modulated Phase Transition in 2D TMDC Materials

二维过渡金属二卤化物(2D TMDC)是一种原子薄的材料，通式为MX2，其中M是过渡金属原子(钼或钨)，X是硫族原子(硫、碲或硒)。一层M原子被夹在两层X原子之间。二维TMDC可以存在于两种不同原子排列的稳定结构相中：半导体2H相和导电1T‘相。通过施加机械应力来动态控制这两个阶段之间的转变，可以带来革命性的器件应用，如存储器件、可重构电路和原子薄限制的拓扑晶体管。该项目将提供应力场对二维TMDC相变原子学机制的作用的基础知识，促进相工程在下一代二维电子学和光电子学中的应用，从而促进国家的健康、繁荣和福祉。在教育方面，利用德克萨斯大学圣安东尼奥分校的大量拉美裔学生，该项目的主要教育目标是扩大代表不足的群体在研究中的参与，并通过积极的研究参与对他们进行培训。应力相关相变的机制是通过施加应力场来改变相变的能垒和途径。势垒决定了相变率，路径揭示了新相形核和传播等原子化相变过程。到目前为止，应力对二维TMDC相变行为的影响还不清楚。本项目的中心目标是确定二维TMDC的相变势垒和路径作为外加应力场的函数，从而为在原子水平上进行二维TMDC的相工程奠定力学基础。将采用计算和理论相结合的方法来实现这一目标。然而，值得注意的是，当人们考虑二维材料的有限变形时，现有的方法不能很容易地用于这一研究。因此，将开发新的方法。在传统的NEB方法的基础上加入了非线性力学，发展了一种新的计算方法，称为有限变形推挤弹性带方法，用于寻找有限变形下的过渡势垒和路径。同时，由于模拟所有可能的应力状态都很耗时，因此需要开发一种能够预测障碍的理论。因此，在原有贝尔理论和连续介质力学概念的基础上，发展了一种新的理论方法--有限变形贝尔理论。该项目开发的方法可用于研究其他晶体材料的相变，以及更广泛地研究相变以外的机械力化学问题，如扩散、位错运动、断裂形成等，其中材料中依赖速率的转变行为与应力和有限变形相结合。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.