Computational and Theoretical Methods for Stress Modulated Phase Transition in Solid State Materials with Applications to Two Dimensional MoTe2
Phase transition of solid state materials is important to many new technology developments. In particular, recently, the phase transition of two dimensional (2D) transition metal dichalcogenides (TMD) materials (between semiconducting and metallic phases) has shown potential revolutionary applications such as memory devices, reconfigurable circuits and topological transistors at atomically thin limits. 2D MoTe2, as a member in TMD material family, is investigated in this study. In addition, silicon (Si) is used as a model material to test the new methods and tools developed in this study. Understanding and manipulating the phase transitions of Si can open up new optoelectronic applications, advance the ductile machining of Si, and improve the mechanical reliability of Si based devices. Stress can be employed as a useful tool to control phase transition process. For example, the high stretchability of 2D TMD materials made it possible to use stress and deformation to dynamically modulate their phase transitions in a controlled manner. In addition, under hydrostatic pressure, bulk Si experiences a series of sequential phase transitions. Recent experiments have shown many unique stress-dependent features of phase transition in nanostructured Si that are different from the bulk Si. The mechanism of stress dependent phase transition is that applied stress can change transition barriers and pathways. Transition barriers determine phase transition rates. The lower the barrier, the higher the rate and more likely transition occurs. Transition pathways reveal the atomistic process of phase transition such as new phase nucleation and propagation. We noticed the following two knowledge gaps in studying phase transitions. First, previous computation method leads to inaccurate phase transition barriers and deviated transition pathways, due to an inaccurate evaluation on the external work contributions in enthalpies and barriers. As a result, previous methods are only accurate for phase transitions that undergo infinitesimal or small deformation. However, phase change materials can experience finite deformation during phase transition. Therefore, a new computation method is needed for determining minimum energy path when material is subject to finite deformation. Second, to determine the phase transition barriers under many possible external stresses, a large number of simulations must be performed. To avoid such high computational cost, the approximate theory to estimate stress dependent barriers is needed. Unfortunately, there is no existing theory readily to be applied. We proposed a new computation method, called Finite Deformation NEB (FD-NEB) method, which can be used to determine the minimum energy path of a transition process inside a solid state material that is subject to stress and finite deformation. In addition, we proposed a finite deformation Bell theory (FD-BT) developed based on the concept of original Bell theory and continuum mechanics, for predicting transition barriers as a function of the applied stress. These new methods were applied to investigate the atomistic mechanism of phase transitions in 2D MoTe2 (a model TMD material) and silicon. Finally, we developed a new computational tool that integrates FD-NEB method with concurrent atomistic-to-continuum multiscale modeling. This tool can be used to identify the atomistic mechanism of a long-time scale transition event in a coupled atomistic-continuum material model.