Strain-Driven and Layer-Number-Dependent Crossover of Growth Mode in van der Waals Heterostructures: 2D/2D Layer-By-Layer Horizontal Epitaxy to 2D/3D Vertical Reorientation

Heterogeneously integrated 2D van der Waals (vdW) solids composed of compositionally distinct atomic layers are envisioned to exhibit exotic electrical/optical properties unattainable with their monocomponent counterparts. However, the underlying principle for their morphology-controlled chemical vapor deposition (CVD) growth and its associated growth variables have not been clarified, leaving their projected technological opportunities far from being realized. Herein, by employing tungsten trioxide (WO3) nanowires as a model system that uniquely enables the detailed atomic-scale inspections of 2D/2D interfaces, the CVD growth mechanism of 2D molybdenum/tungsten disulfide vdW vertical stacks is studied. By employing extensive transmission electron microscopy (TEM) characterization, an intriguing growth mode transition is identified in these materials, i.e., 2D/2D layer-by-layer horizontal epitaxy to 2D/3D vertical layer reorientation, and it is confirmed that it is driven by varying 2D layer numbers. Corroborating molecular dynamics simulations clarify that the internal strain accumulated during the course of 2D layers growth dictates the final growth mode, further supported by TEM strain map analysis. This study not only sheds a new insight on better understanding the growth principles for 2D vdW heterostructures but also offers important technical guidance on tailoring their functionalities toward exploring 2D/2D heterojunction devices.