Stress Modulated Phase Transition in 2D TMDC Materials

二维过渡金属二硫属化物（2D TMDC）是具有广义式MX2的原子薄材料，其中M是过渡金属原子（钼或钨），X是硫属原子（硫、碲或硒）。一层M原子夹在两层X原子之间。二维TMDC可以存在两种不同原子排列的稳定结构相：半导体2 H相和导电1 T '相。通过施加的机械应力对这两个相之间的转变的动态控制可以导致革命性的器件应用，例如存储器器件、可重构电路和原子薄极限的拓扑晶体管。该项目将提供应力场对2D TMDC相变原子机制的作用的基础知识，促进相位工程在下一代2D电子学和光电子学中的应用，从而促进国民健康，繁荣和福利。在教育方面，利用德克萨斯大学圣安东尼奥分校大量的西班牙裔学生，该项目的主要教育目标是扩大代表性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施加应力场可以改变相变能垒和相变途径。势垒决定了相变速率，而路径则揭示了新相的成核和增长等原子级相变过程。到目前为止，应力对二维TMDC相变行为的影响尚不清楚。本项目的主要目的是确定二维TMDC的相变势垒和路径随外加应力场的变化关系，为二维TMDC在原子水平上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，现有的方法不能很容易地用于这项研究时，考虑有限变形的二维材料。因此，将开发新的方法。在传统的NEB方法基础上，加入非线性力学，发展了一种新的计算方法--有限变形轻推弹性带方法，用于寻找有限变形下的相变势垒和通道。同时，由于模拟所有可能的应力状态是耗时的，因此需要开发一种可以预测障碍的理论。因此，在原有Bell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Bell理论。本计画所发展之方法可应用于其他结晶材料之相变研究，并可更广泛地应用于研究相变以外之机械化学问题，如扩散、位错运动、断裂形成等，该奖项反映了NSF的法定使命，并被认为是值得支持的，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。

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.