Understanding Substrate-Guided Assembly in van der Waals Epitaxy by in Situ Laser Crystallization within a Transmission Electron Microscope

Understanding the bottom-up synthesis of atomically thin two-dimensional (2D) crystals and heterostructures is important for the development of new processing strategies to assemble 2D heterostructures with desired functional properties. Here, we utilize in situ laser-heating within a transmission electron microscope (TEM) to understand the stages of crystallization and coalescence of amorphous precursors deposited by pulsed laser deposition (PLD) as they are guided by 2D crystalline substrates into van der Waals (vdW) epitaxial heterostructures. Amorphous clusters of tungsten selenide were deposited by PLD at room temperature onto graphene or MoSe₂ monolayer crystals that were suspended on TEM grids. The precursors were then stepwise evolved into 2D heterostructures with pulsed laser heating treatments within the TEM. The lattice-matching provided by the MoSe₂ substrate is shown to guide the formation of large-domain, heteroepitaxial vdW WSe₂/MoSe₂ bilayers both during the crystallization process via direct templating and after crystallization by assisting the coalescence of nanosized domains through nonclassical particle attachment processes including domain rotation and grain boundary migration. The favorable energetics for domain rotation induced by lattice matching with the substrate were understood from first-principles calculations. These in situ TEM studies of pulsed laser-driven nonequilibrium crystallization phenomena represent a transformational tool for the rapid exploration of synthesis and processing pathways that may occur on extremely different length and time scales and lend insight into the growth of 2D crystals by PLD and laser crystallization.