Understanding the bottom-up synthesis of atomically skinny two-dimensional (2D) crystals and heterostructures is necessary for the event of recent processing methods to assemble 2D heterostructures with desired practical properties. Right here, we make the most of in situ laser-heating inside a transmission electron microscope (TEM) to grasp the phases of crystallization and coalescence of amorphous precursors deposited by pulsed laser deposition (PLD) as they’re guided by 2D crystalline substrates into van der Waals (vdW) epitaxial heterostructures. Amorphous clusters of tungsten selenide have been deposited by PLD at room temperature onto graphene or MoSe2 monolayer crystals that have been suspended on TEM grids. The precursors have been then stepwise developed into 2D heterostructures with pulsed laser heating therapies inside the TEM. The lattice-matching supplied by the MoSe2 substrate is proven to information the formation of large-domain, heteroepitaxial vdW WSe2/MoSe2 bilayers each in the course of the crystallization course of by way of direct templating and after crystallization by aiding the coalescence of nanosized domains by means of nonclassical particle attachment processes together with area rotation and grain boundary migration. The favorable energetics for area rotation induced by lattice matching with the substrate have been understood from first-principles calculations. These in situ TEM research of pulsed laser-driven nonequilibrium crystallization phenomena symbolize a transformational device for the fast exploration of synthesis and processing pathways which will happen on extraordinarily completely different size and time scales and lend perception into the expansion of 2D crystals by PLD and laser crystallization.