# Post-synthesis Tellurium Doping Induced Mirror Twin Boundaries in Monolayer Molybdenum Disulfide
## 3 Results and Discussion  
The sample morphology of the tellurized MoS\({}_{2}\) films was outlined by the low-mag ADF-STEM image in Figure 1a, from which a striking feature was found over all the tellurized MoS\({}_{2}\) flakes: all their edges appeared in brighter contrast (arrowed in pink), indicating possible tellurium enrichment around the edges as predicted by the Z-contrast mechanism [3]. A close-up view of the edge area illustrated in Figure 1b confirmed the presence of enriched tellurium dopants around the edge, with their ratio reaching as high as 80% in atomic percentage among all chalcogen lattices. The average lattice parameter of the tellurium-enriched region was determined to be 0.346 nm from direct imaging and the corresponding fast Fourier transform spectrum, a value very close to that of the 2H-MoTe\({}_{2}\) of 0.352 nm, indicating that at high tellurium doping ratios up to 80%, quasi-pure 2H-MoTe\({}_{2}\) regions formed locally. In contrast, in the interior regions further from the edge of the same MoS\({}_{2}\) flakes, tellurium substitutions were rarely observed, further confirming that the tellurium substitution was initiated from the edges. Here we should note that the preferred substitution of doped tellurium at the edges was not due to the phase separation arising from a miscibility gap. As reported in Reference [29], the calculated critical temperature for complete miscibility of MoS\({}_{2(1\times 1)}\)Te\({}_{2\times 2}\) alloy is 493K, and the doping temperature for tellurium in our experiments was 750 \({}^{\circ}\)C or 850 \({}^{\circ}\)C, namely 1023K or 1123K, and was sufficient for the formation of completely miscible MoS\({}_{2(1\times 1)}\)Te\({}_{2\times 2}\) alloy. The preferred substitution of tellurium at the edges can be attributed to the higher reactivity of the edges compared to that of the basal plane of MoS\({}_{2}\)[30; 31; 32]. Moreover, as reported in Reference [33], the density functional calculations indicated that the absorption energy and substitution barrier at the edges were lower than that in the inner region, which resulted in the preferred substitution at the edges. Then, due to the large lattice mismatch between 2H-MoTe\({}_{2}\) and 2H-MoS\({}_{2}\), tensile strain was formed in the substituted regions. The substitution barrier further decreased with increasing occupation, indicating a continually enhanced substitution rate (self-promoted) and eventually resulting in the preferred substitution of tellurium at the edges. More importantly, MTBs (as outlined by the yellow dashed rectangles) were observed in these high-level Te doping edge areas in Figure 1b. Further analysis revealed that these MTBs possessed an average length of \(\sim\)2 nm, an intersection angle of 60\({}^{\circ}\) (or 120\({}^{\circ}\)) with each other, and showed a tendency to form a closed triangular loop. The atomic resolution ADF-STEM image and the atomic model showing the atomic structure of a representative MTB are given in Figure 2a,d, from which  
Figure 1: **(a)** A low-mag annular dark-field scanning transition electron microscope (ADF-STEM) image showing the morphology of the tellurized MoS\({}_{2}\) sample. All the edge areas (arrowed in pink) appear brighter than their interior parts. **(b)** A magnified ADF-STEM image showing the atomic structure of a typical edge area after Te doping. The 44P-Te mirror twin boundaries (MTBs) are mapped out in the yellow dashed rectangles. **(c)** An image of the corresponding fast Fourier transformation spectrum (FFT) of **(b)**, where the first-order diffraction spectra of 2H-MoTe\({}_{2}\) and 2H-MoS\({}_{2}\) are marked with blue and yellow dashed lines, respectively. the configuration of the MTB was assigned to 4|4P. Nevertheless, as shown in Figure 2b, the intensity profile along the white dashed lines in Figure 2a confirmed that the doped tellurium replaced and occupied either one or both of the top and bottom sulfur sublattices, i.e., forming substitutional point defects of (Te+S) or Te\({}_{2}\) in the host MoS\({}_{2}\) crystal.