The mechanisms of van der Waals (vdW) epitaxial growth of monolayer two-dimensional (2D) crystals from amorphous precursors were revealed by in situ pulsed laser heating within a TEM and first-principles calculations. This work demonstrates a transformational in situ approach to induce and study the atomistic origins of non-equilibrium crystallization processes.
Pulsed laser heating within a TEM revealed non-classical crystallization pathways in van der Waals epitaxy wherein predeposited amorphous precursors of tungsten selenide were transformed in stepwise fashion into atomically-thin, two-dimensional van der Waals heterostructures. Lattice matching between WSe2 and MoSe2 was found to form well-aligned 2D WSe2/MoSe2 heterostructures, while the poor lattice match between WSe2 and graphene (Gr) produced polycrystalline, misaligned WSe2/Gr heterostructures. First, crystallization of WSe2 nanoflakes was observed to proceed through a series of changes in metastable phases and stoichiometry, which are consistent with Ostwald’s rule of stages. Then the in situ transmission electron microscopy measurements revealed how the nanoflakes rotate and migrate under the influence of van der Waals forces to attach and coalesce into larger domains. Domain matching with the underlying lattice was shown in first-principles calculations to provide the energetic advantage for the epitaxial alignment of the WSe2/MoSe2 heterostructures on the MoSe2 substrates. Understanding such crystallization mechanisms involved in laser processing within the TEM should enable the rapid exploration of transformational processing opportunities for additive manufacturing and integration of 2D materials at the macroscale.
