Exchange of Re and Mo atoms in MoS 2 driven by Scanning Transmission Electron Microscopy

Abstract

Two-dimensional materials exhibit unique properties that can be used for novel applications. These properties are adversely affected or positively promoted by different types of defects. " Defect engineering " is thus employed as a tool to control material functionality. For example, Lin et al. used a STEM's electron beam to sculpt and simultaneously image nanowires that are only three-atoms thick in transition-metal dichalcogenide monolayers[1]. Ishikawa et al. reported the direct observation of the migration of Ce and Mn dopant-atoms in a bulk AlN crystal[2]. Scanning transmission electron microscopy has proven to be an effective tool to study defect structures. We study the dynamic structure evolution of Re dopants in MoS 2 through atomic level scanning transmission electron microscopy. In our experiments, samples were grown by chemical vapor deposition. The as-grown sample was transferred onto a TEM grid by the solution method, and afterwards the sample was baked at 160 0 C for eight hours. Imaging was performed with an aberration-corrected scanning transmission electron microscope (Nion UltraSTEM 100) operated at 60 kV using medium angle annular dark field (MAADF, inner angle 50 mrad) with a beam current of 9 pA. Exchange between Re and Mo atoms was observed by electron microscopy. The direct exchange migration process is shown in Figure 1, where nearby Mo and Re atoms were found to exchange their positions. The images in Fig. 1a and 1b are the sum of twelve consecutive images with single frame time of 1.18 second. It was found that the exchange event occurred within two frames (1.18 second). No cation vacancies were found to be involved in the process and thus it is concluded that the migration process is through direct exchange. Further experiments were carried out to account for the poor time resolution (1.18 second). Migration processes mediated by cation vacancies were observed and recorded. It was found that the cation vacancy is highly mobile under similar imaging conditions. Although surface cation atoms moved around, no surface cations were observed to move to the cation vacancy site and fill the vacancy. This behavior is further support for the conclusion that Re and Mo exchange their positions.