# Post-synthesis Tellurium Doping Induced Mirror Twin Boundaries in Monolayer Molybdenum Disulfide
## 3 Results and Discussion
Here, the substitution of sulfur by tellurium might occur following the pathway revealed by nudged elastic band calculations in Reference [33]. Te atoms first absorb on top of S atoms. Then, the Te atoms lift up the neighboring S atoms, and the Te and S atoms are nearly at the same level. Lastly, the Te atom takes the original position of the neighboring S atom, pushing it to the top of the next neighboring S atom, and the S atom possibly evaporates into the atmosphere under the high temperature. Specifically, at MTBs the S\({}_{2}\) sites were mostly replaced with Te\({}_{2}\), apart from minor (Te+S). In the region close to the edge of MoS\({}_{2}\) flakes at X\({}_{2}\) sites were Te\({}_{2}\) and (Te+S). In the inner region distant from the edge at X\({}_{2}\) sites were (Te+S) and S\({}_{2}\), as mapped out for the distribution of doped tellurium in Figure 2c. These findings indicate that the tellurium substitution initiated the MTB from the edge and then extended into the inner part. Comparative experiments conducted at different doping temperatures further revealed the fact that higher doping temperatures led to higher tellurium doping ratios and MTB density as well. For instance, the average doping ratio increased from ~60% at 750 \({}^{\circ}\)C to ~75% at 850 \({}^{\circ}\)C. Such a relationship also suggests that the formation of MTBs could be directly attributed to the tellurium doping. We next tried to determine the mechanism for a Te-substitution-induced MTB in monolayer MoS\({}_{2}\) as illustrated in Figure 3. During the Te doping reaction, some of the sulfur atoms were substituted by tellurium, preferably for those at and around the edge areas. As the doping ratio increased, significant lattice strain built up and accumulated in the tellurized regions due to the atomic size difference between sulfur and tellurium, as indicated by the green arrows in Figure 3a. In turn, these strains led to an upward gliding of the Mo atomic plane with its motion direction marked by black arrows in Figure 3a (the motion direction has three equivalent crystallographic directions with an intersection angle of 120\({}^{\circ}\) with each other, and illustrated in Figure 3a was only one example), followed by the formation of sulfur divacancy lines, similar to previous reports [14; 34]. Notably, the accumulated strain also resulted in the removal of Mo atoms and more S atoms, as illustrated in Figure 3b. Such a process was intrinsically different from the previously proposed mechanism where the as-formed MTBs consisted of identical lattice atoms to those in the host 2D matrix (see the detailed analysis in  
Figure 2: **(a)** A typical atomic resolution ADF-STEM image of the 4|4P-Te MTB with its atomic model shown in (**d**), in which the top image is the top view of the atomic model, and the bottom is the side view. **(b)** The intensity profile along white dashed lines in (**a**). **(c)** The spatial distribution of doped tellurium. Figure S2) [13; 14]. Finally, when more and more sulfur atoms were substituted by tellurium, the mirror twinned domains and 4|4P-Te MTBs were formed, as modeled in Figure 3c. In this case, the number of Mo atoms in lines 1-8 in the high Te doping regions was actually smaller than that in the perfect MoS2, with one Mo atom due to the strain induced by large lattice mismatch. For the loss of Mo atoms and more S atoms shown in Figure 3b, we consider that there was a large mismatch of up to 11.39% between 2H-MoTe2 (aMoTe2 = 3.52 A) and 2H-MoS2 (aMoGa2 = 3.16 A) and the area of a triangular MoS2 domain with an edge length of naMoGa2 (n = 10) corresponded to a triangular MoTe2 domain with an edge length of naMoTe2 (n = 9), as illustrated in Figure S3. If the interior strain inside the high tellurium doping ratio regions was omitted, Mo atoms and more S atoms should be lost during the formation of an inversion domain induced by tellurium doping, and the formed MTBs could be expected to own eight X2 sites (at X2 sites were mostly Te2), as shown in Figure 3c.