Direct Cation Exchange in Monolayer ${\mathrm{MoS}}_2$ via Recombination-Enhanced Migration

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 ${\mathrm{MoS}}_2$ 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.

Figure 1

Comparison between experimental and simulated STEM-MAADF images. The simulated MAADF image (indicated by the blue rectangle) is superimposed on top of the experimental image. Substitutional Re site, double Mo column, and single S vacancy ($V_{1S}$) are highlighted by red, blue, and orange, respectively, in the simulated image.

Figure 2

Direct Re-Mo exchange in Re-doped monolayer ${\mathrm{MoS}}_2$. (a),(c) STEM-MAADF images showing the position of the Re relative to other Re atoms (a) and (c) the local area of Re-doped ${\mathrm{MoS}}_2$ with the corresponding structural model (e), recorded at time $t_0$ before the exchange event. (b),(d) STEM-MAADF images showing the position of the Re relative to other Re atoms (b) and (d) the local MAADF image with the corresponding structural model (f) recorded after the exchange event at $t_0+3.5\mathrm{s}$. The exchange of Re and Mo atoms is highlighted by blue and red arrows, while sulfur monovacancies are indicated by orange dashed circles and sulfur double vacancies are indicated by yellow dashed circles. Re atoms are in red, Mo in blue, and sulfur in yellow.

Figure 3

Energy barriers for Re-Mo exchange in the presence of sulfur vacancies and defect energy levels of Re-doped ${\mathrm{MoS}}_2$. (a) The calculated Re-Mo exchange migration barriers in the presence of different numbers of sulfur vacancies. (b)–(f) defect energy levels (at $\mathrm{\Gamma }$) in the gap region of Re-doped monolayer ${\mathrm{MoS}}_2$. (b) The energy gap of perfect ${\mathrm{MoS}}_2$. (c) The defect energy level of a single substitutional Re in ${\mathrm{MoS}}_2$. (d) The defect energy level of a Re dopant atom when the atom is displaced by 0.2 Å. (e) Defect energy levels of a Re atom displaced by 0.2 Å with a nearby sulfur vacancy. (f) Defect energy levels of Re atom displaced by 0.6 Å with two nearby sulfur vacancies.

Figure 4

Controllable Re-Mo exchange triggered by controlled creation of sulfur vacancies around the selected Re and Mo atoms. (a),(c) The STEM-MAADF images before and after the controlled scanning. The atomic positions in exchange are highlighted by the yellow and red arrows. (b) Illustrates the region scanned by the electron beam and the sulfur sites where the electron beam was parked. Related sulfur vacancies are highlighted by orange ($V_{1S}$) and yellow ($V_{2S}$) dashed circles in (c).