Stress Modulated Phase Transition in 2D TMDC Materials

二维过渡金属二硫族化物（2D TMDC）是一种原子薄的材料，其广义公式为MX2，其中M为过渡金属原子（钼或钨），X为硫原子（硫、碲或硒）。一层M原子夹在两层X原子之间。二维TMDC可以以半导体2H相和导电1T′相两种不同原子排列的稳定结构相存在。通过施加机械应力来动态控制这两个阶段之间的过渡可以导致革命性的器件应用，如存储器件、可重构电路和原子级薄极限的拓扑晶体管。本项目将提供应力场在二维TMDC相变原子机制中的作用的基础知识，促进相位工程在下一代二维电子和光电子领域的应用，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庞大的西班牙裔学生群体，该项目的主要教育目标是扩大代表性不足的群体对研究的参与，并通过积极的研究参与来培养他们。应力相关相变的机理是应力场可以改变相变能垒和途径。势垒决定了相变速率，途径揭示了新相成核和扩展等原子转变过程。到目前为止，应力对二维TMDC相变行为的作用尚不清楚。本项目的中心目标是确定二维TMDC的过渡障碍和路径作为应力场的函数，以便在原子水平上为二维TMDC的相位工程奠定力学基础。将采用计算和理论相结合的方法来实现这一目标。然而，需要指出的是，当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本研究。因此，新的方法将被开发出来。在传统NEB方法的基础上加入非线性力学，提出了一种新的计算方法——有限变形微推弹性带法，用于寻找有限变形下的过渡屏障和路径。同时，由于模拟所有可能的应力状态是耗时的，因此有必要发展一种可以预测障碍的理论。因此，一种新的理论方法，称为有限变形钟理论，将发展基于原来的贝尔理论和连续介质力学的概念。本项目开发的方法可以应用于研究其他晶体材料的相变，更广泛地应用于研究相变以外的机械化学问题，如扩散、位错运动、断裂形成等，其中材料中的速率依赖的转变行为与应力和有限变形相结合。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.