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2D materials and van der Waals heterostructures with atomic-scale thickness provide enormous potential for advanced science and technology. However, insufficient knowledge of compatible synthesis impedes wafer-scale production. PdSe 2 and Pd 2 Se 3 are two of the noble transition-metal chalcogenides with excellent physical properties that have recently emerged as promising materials for electronics, optoelectronics, catalyst, and sensor. This research presents a feasible approach to synthesize PdSe 2 and Pd 2 Se 3 with inherently asymmetric-structure on honeycomb lattice 2D monolayer substrates of graphene and MoS 2 . We directly deposit a molecular transition-metal precursor complex on the surface of the 2D substrates, followed by lowtemperature selenization by chemical vapor flow. Parameter control leads to tuning of the material from monolayer nanocrystals with Pd 2 Se 3 phase, to continuous few-layer PdSe 2 films.Annular dark-field scanning transmission electron microscopy (ADF-STEM) reveals the structure, phase variations and heteroepitaxy at the atomic level. PdSe 2 with unconventional interlayer stacking shifts appeared as the kinetic product, whereas the bilayer PdSe 2 and monolayer Pd 2 Se 3 are the thermodynamic product. The epitaxial alignment of interlayer rotation and translation between the PdSe 2 and underlying 2D substrate were also revealed by ADF-STEM. These results offer both nanoscale and atomic-level insights into direct growth of van der Waals heterostructures, as well as an innovative method for 2D synthesis by predetermined nucleation.

Transition metal dichalcogenides (TMDs) have emerged as promising 2D materials that can be atomically thin semiconductors, semimetals, metals, or even superconductors. These materials consist of transition metal atoms sandwiched between two layers of chalcogen atoms. Their innate layered structure stems from the existence of strong intralayer bonding forces and weak interlayer van der Waals interactions. [1] Recently, studies have uncovered a Layered MoS 2 is a prospective candidate for use in energy harvesting, valleytronics, and nanoelectronics. Its properties strongly related to its stacking configuration and the number of layers. Due to its atomically thin nature, understanding the atomic-level and structural modifications of 2D transition metal dichalcogenides is still underdeveloped, particularly the spatial control and selective precision. Therefore, the development of nanofabrication techniques is essential. Here, an atomic-scale approach used to sculpt 2D few-layer MoS 2 into lateral heterojunctions via in situ scanning/transmission electron microscopy (STEM/TEM) is developed. The dynamic evolution is tracked using ultrafast and high-resolution filming equipment. The assembly behaviors inherent to few-layer 2D-materials are observed during the process and included the following: scrolling, folding, etching, and restructuring. Atomic resolution STEM is employed to identify the layer variation and stacking sequence for this new 2D-architecture. Subsequent energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy analyses are performed to corroborate the elemental distribution. This sculpting technique that is established allows for the formation of sub-10 nm features, produces diverse nanostructures, and preserves the crystallinity of the material. The lateral heterointerfaces created in this study also pave the way for the design of quantumrelevant geometries, flexible optoelectronics, and energy storage devices.

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