# Post-synthesis Tellurium Doping Induced Mirror Twin Boundaries in Monolayer Molybdenum Disulfide
## 1 Introduction  
Defects play important roles in tailoring the properties and performance of materials, and their influence may become more deterministic in atomically thin two-dimensional (2D) materials with a reduced dimensionality, such as graphene and transition metal dichalcogenides (TMDs) [1]. Structural defects, including point defects [2; 3], dislocations [4], and grain/phase boundaries [4; 5; 6], are commonly observed and unambiguously confirmed to influence the band gap [7; 8], electrical transport [9; 10], and magnetic [11] and catalytic [12] properties of 2D TMD materials, including the well-investigated MoS\({}_{2}\). Among these defects, mirror twin boundary (MTB), also denoted as inversion domain boundary (IDB), is a special type of one-dimensional (1D) defect that connects two neighboring domains with a misorientation angle of 60\({}^{\circ}\) with respect to each other. Depending on their atomic configurations, MTBs can be classified into two primary types: 4|4P and 4|4E. Both of them consist of successive four-member atomic rings in chains. However, the two neighboring atomic rings share a point at the X\({}_{2}\) site (X = S, Se, Te) in a 4|4P MTB, but they change to edge-sharing at the M-X bond (M = Mo, W) in a 4|4E MTB [13; 14]. With regard to termination, for example, a 4|4P MTB terminated with tellurium can be denoted as 4|4P-Te. Due to the quantum confinement effect and the enhanced electronic correlation arising from the unique structures of MTB defects, i.e., atom width and nanometer-sized length, intriguing 1D physics, including the size-dependent band gap, charge density wave [15], and Tomonaga-Luttinger liquid [16; 17] behaviors have been reported.  
Hence, it is important to practically control and tailor the structures and spatial distribution of MTB defects in the host 2D TMD materials to achieve the desired properties and performance. To date, a few approaches have been proposed for the introduction of MTBs, which can be generallycategorized by three routes: i) direct synthesis approach [18; 19; 20]; ii) post-synthesis excess metal doping approach [21]; and iii) other treatments, such as energetic e-beam irradiation [13]. For example, [PERSON] et al. [18] initially reported the presence of a dense network of MTB defects in a molecular beam epitaxy (MBE)-grown monolayer MoSe\({}_{2}\) and attributed the formation mechanism to growth kinetics. [PERSON] et al. [21] successfully introduced MTB defects into as-grown monolayer MoSe\({}_{2}\) and MoTe\({}_{2}\) via excess metal (such as Mo and Ti) injection. As an analogy to this approach, one may expect proper engineering of chalcogen lattice sites should also be workable to induce MTB. While the validity of this approach remains rarely explored, this method has worked successfully for preparing the alloys of TMD materials [22; 23; 24; 25].  
In this article, we demonstrate that post-synthesis tellurium doping can induce abundant MTB defects into an as-formed MoS\({}_{2}\) monolayer. As revealed by atomic resolution annular dark-field scanning transition electron microscope (ADF-STEM) characterizations, the injected tellurium substituted sulfur and occupied its sublattice sites preferentially around the edge areas of the MoS\({}_{2}\) flakes. As a result, MTB defects with a configuration of 4|4P-Te were formed in these high-tellurium doping regions. Based on the analysis of lattice structures around the MTBs, a model explaining the atomic-scale MTB formation dynamics is proposed.