Yang Yida, Hirsinger Laurent, Devel Michel
FEMTO-ST Institute, UBFC, CNRS, ENSMM, 15B Avenue des Montboucons, 25030 Besançon CEDEX, France.
J Chem Phys. 2022 May 7;156(17):174104. doi: 10.1063/5.0088972.
Flexoelectricity is an electromechanical coupling phenomenon that can generate noticeable electric polarization in dielectric materials for nanoscale strain gradients. It is gaining increasing attention because of its potential applications and the fact that experimental results were initially an order of magnitude higher than initial theoretical predictions. This stimulated intense experimental and theoretical research to investigate flexoelectric coefficients in dielectric materials such as two-dimensional materials. In this study, we concentrate on the calculation of the flexoelectric coefficients of 2D-MoS due to a model using self-consistently determined charges and dipoles on the atoms. More specifically, we study the importance of two contributions that were neglected/omitted in previous papers using this model, namely, the charge term in the total polarization and the conservation of electric charge through a Lagrange multiplier. Our calculations demonstrate that the results for flexoelectric coefficients computed with this improved definition of polarization agree better with experimental measurements, provided that consistent definitions for signs are used. Additionally, we show how two physical contributions with opposite signs compete to give net values of flexoelectric coefficients that can be either positive or negative depending on their relative importance and give net values for the case of MoS.
挠曲电是一种机电耦合现象，对于纳米级应变梯度，它能在介电材料中产生显著的电极化。由于其潜在应用以及实验结果最初比初始理论预测高一个数量级这一事实，它正受到越来越多的关注。这激发了对二维材料等介电材料中挠曲电系数进行研究的大量实验和理论工作。在本研究中，我们专注于基于一个在原子上自洽确定电荷和偶极子的模型来计算二维二硫化钼（2D-MoS）的挠曲电系数。更具体地说，我们研究了此前使用该模型的论文中被忽略/遗漏的两个贡献的重要性，即总极化中的电荷项以及通过拉格朗日乘子实现的电荷守恒。我们的计算表明，用这种改进的极化定义计算出的挠曲电系数结果与实验测量结果吻合得更好，前提是使用一致的符号定义。此外，我们展示了具有相反符号的两种物理贡献如何相互竞争，以给出挠曲电系数的净值，该净值可能为正或负，具体取决于它们的相对重要性，并给出了二硫化钼情况下的净值。
