Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
Department of Physics and Astronomy and Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, USA.
Phys Rev Lett. 2019 Mar 15;122(10):106101. doi: 10.1103/PhysRevLett.122.106101.
In addition to their unique optical and electronic properties, two-dimensional materials provide opportunities to directly observe atomic-scale defect dynamics. Here we use scanning transmission electron microscopy to observe substitutional Re impurities in monolayer MoS_{2} undergo direct exchanges with neighboring Mo atoms in the lattice. Density-functional-theory calculations find that the energy barrier for direct exchange, a process that has only been studied as a diffusion mechanism in bulk materials, is too large for either thermal activation or energy directly transferred from the electron beam. The presence of multiple sulfur vacancies next to the exchanged Re-Mo pair, as observed by electron microscopy, does not lower the energy barrier sufficiently to account for the observed atomic exchange. Instead, the calculations find that a Re dopant and surrounding sulfur vacancies introduce an ever-changing set of deep levels in the energy gap. We propose that these levels mediate an "explosive" recombination-enhanced migration via multiple electron-hole recombination events. As a proof of concept, we also show that Re-Mo direct exchange can be triggered via controlled creation of sulfur vacancies. The present experimental and theoretical findings lay a fundamental framework towards manipulating single substitutional dopants in two-dimensional materials.
除了独特的光学和电子特性外，二维材料还为直接观察原子尺度的缺陷动力学提供了机会。在这里，我们使用扫描透射电子显微镜观察到单层 MoS_{2}中的取代 Re 杂质与晶格中的相邻 Mo 原子直接交换。密度泛函理论计算发现，直接交换的能量势垒对于热激活或直接从电子束转移的能量来说太大了，而直接交换只是在体材料中作为扩散机制进行了研究。正如电子显微镜所观察到的，在交换的 Re-Mo 对旁边存在多个硫空位，但它们并没有降低能量势垒，不足以解释观察到的原子交换。相反，计算发现，Re 掺杂剂和周围的硫空位在能隙中引入了一组不断变化的深能级。我们提出，这些能级通过多次电子-空穴复合事件来介导“爆炸”重组增强迁移。作为概念验证，我们还表明，通过控制硫空位的形成可以触发 Re-Mo 直接交换。本实验和理论研究结果为在二维材料中操纵单个取代掺杂剂奠定了基础。
