The 40 kV high-resolution transmission electron microscopy (TEM) experiments are performed to understand defect formation and evolution of their atomic structure in single-layer 2H MoTe2 under electron beam irradiation. We show that Te vacancies can agglomerate either in single Te vacancy lines or in extended defects composed of column Te vacancies, including rotational trefoil-like defects, with some of them being never reported before. The formation of inversion domains with mirror twin boundaries of different types, along with the islands of the metallic T′ phase was also observed. Our first-principles calculations provide insights into the energetics of the transformations as well as the electronic structure of the system with defects and point out that some of the observed defects have localized magnetic moments. Our results indicate that various nanoscale structures, including metallic quantum dots consisting of T′ phase islands and one-dimensional metallic quantum systems such as vacancy lines and mirror twin boundaries embedded into a semiconducting host material can be realized in single-layer 2H MoTe2, and defect-associated magnetism can also be added, which may allow prospective control of optical and electronic properties of two-dimensional materials.

Atom migrations in single-layer 1H-MoTe2 are studied with Cc/Cs-corrected highresolution transmission electron microscopy (TEM) at an electron energy of 40 keV using the electron beam simultaneously for material modification and imaging. After creating tellurium point defects and single Te vacancy lines, we observe their migration pathways across the lattice. Furthermore, we analyze local strain-dependent phase transformations from the 1H-to the 1T'-phase associated with single Te vacancy lines. Combining the experimental data with the results of first-principles calculations, we explain energetics and driving forces of point and line defect migration and the phase transformations due to an interplay of electron-beam-induced energy input, atom ejection, and strain spread. Our results enhance the understanding of defect dynamics in 2D transition metal dichalcogenides, which facilitates tailoring their local optical and electronic properties.

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Transition metal phosphorous trisulfides (TMPTs) are inorganic materials with inherent magnetic properties. Due to their layered structure, they can be exfoliated into ultra-thin sheets, which show properties different from their bulk counterparts. Herein, we present a detailed analysis of the interaction of the electron beam (30−80 kV) in a transmission electron microscope with freestanding few-layer TMPTs, with the aim of tailoring their properties. The irradiation-induced structure modifications were systematically investigated by various transmission electron microscopy methods on FePS 3 , MnPS 3 , and NiPS 3 , and the results are rationalized with the help of ab initio calculations, which predict that the knock-on threshold for removing sulfur is significantly lower than that for phosphorus. Therefore, a targeted removal of sulfur is feasible. Eventually, our experiments confirm the dose-dependent, predominant removal of sulfur by the impinging electrons, thus showing the possibility of tuning the sulfur concentration. Using ab initio calculations, we analyze the electronic structure of the TMPTs with single vacancies and oxygen impurities and predict distinct electronic properties depending on the type of defect. Therefore, our study shows the possibility of tuning the properties of ultra-thin freestanding TMPTs by controlling their stoichiometry.

Quasi-two-dimensional (2D) manganese phosphorus trisulfide, MnPS 3 , which exhibits antiferromagnetic ordering, is a particularly interesting material in the context of magnetism in a system with reduced dimensionality and its potential technological applications. Here, we present an experimental and theoretical study on modifying the properties of freestanding MnPS 3 by local structural transformations via electron irradiation in a transmission electron microscope and by thermal annealing under vacuum. In both cases we find that MnS 1−x P x phases (0 ≤ x < 1) form in a crystal structure different from that of the host material, namely that of the αor γ-MnS type. These phase transformations can both be locally controlled by the size of the electron beam as well as by the total applied electron dose and simultaneously imaged at the atomic scale. For the MnS structures generated in this process, our ab initio calculations indicate that their electronic and magnetic properties strongly depend on both in-plane crystallite orientation and thickness. Moreover, the electronic properties of the MnS phases can be further tuned by alloying with phosphorus. Therefore, our results show that electron beam irradiation and thermal annealing can be utilized to grow phases with distinct properties starting from freestanding quasi-2D MnPS 3 .