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We report that recent advances in scanning transmission electron microscopy (STEM) allow the real-time visualization of solid-state transformations in materials, including those induced by an electron beam and temperature, with atomic resolution. However, despite the ever-expanding capabilities for high-resolution data acquisition, the inferred information about kinetics and thermodynamics of the process, and single defect dynamics and interactions is minimal. This is due to the inherent limitations of manual ex situ analysis of the collected volumes of data. To circumvent this problem, we developed a deep-learning framework for dynamic STEM imaging that is trained to find the lattice defects and applymore » it for mapping solid state reactions and transformations in layered WS₂. The trained deep-learning model allows extracting thousands of lattice defects from raw STEM data in a matter of seconds, which are then classified into different categories using unsupervised clustering methods. We further expanded our framework to extract parameters of diffusion for sulfur vacancies and analyzed transition probabilities associated with switching between different configurations of defect complexes consisting of Mo dopant and sulfur vacancy, providing insight into point-defect dynamics and reactions. Finally, this approach is universal and its application to beam-induced reactions allows mapping chemical transformation pathways in solids at the atomic level.« less

Two-dimensional (2D) crystal growth over substrate features is fundamentally guided by the Gauss-Bonnet theorem which mandates that rigid, planar crystals cannot conform to surfaces with nonzero Gaussian curvature. Here we reveal how topographic curvature of lithographically-designed substrate features govern the strain and growth dynamics of triangular WS2 monolayer single crystals. Single crystals grow conformally without strain over deep trenches and other features with zero Gaussian curvature, however features with nonzero Gaussian curvature can easily impart sufficient strain to initiate grain boundaries and fractured growth in different directions. Within a strain tolerant regime, however, triangular single crystals can accommodate considerable (<more » 1.1%) localized strain exerted by surface features that shift the band gap up to 150 meV. Within this regime the crystal growth accelerates in specific directions, which we describe using a growth model. These results present a novel strategy to strain-engineer the growth directions and optoelectronic properties of 2D crystals.« less

The formation of semiconductor heterojunctions and their high density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional (2D) crystalline semiconductors as building blocks in next generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate a process strategy for the formation of lithographically-patterned lateral semiconducting heterojunctions within a single 2D crystal. E-beam lithography is used to pattern MoSe₂ monolayer crystals with SiO₂, and the exposed locations are selectively and totally converted to MoS₂ using pulsed laser deposition (PLD) of sulfur in order to form MoSe₂/MoS₂ heterojunctions in predefined patterns. The junctions and conversionmore » process are characterized by atomically resolved scanning transmission electron microscopy, photoluminescence, and Raman spectroscopy. This demonstration of lateral semiconductor heterojunction arrays within a single 2D crystal is an essential step for the lateral integration of 2D semiconductor building blocks with different electronic and optoelectronic properties for high-density, ultrathin circuitry.« less