  Understanding the growth mechanisms of two-dimensional (2D) van der Waals
(vdW) heterostructures is of great importance in exploring their
functionalities and device applications. A custom-built system integrating
physical vapor deposition and optical microscopy/Raman spectroscopy was
employed to study the dynamic growth processes of 2D vdW heterostructures in
situ. This allows us to identify a new growth mode with a distinctly different
growth rate and morphology from those of the conventional linear growth mode.
We propose a model that explains the difference in morphologies and quantifies
the growth rates of the two modes by taking the role of surface diffusion into
account. We have systematically investigated a range of material combinations
including CdI2/WS2, CdI2/MoS2, CdI2/WSe2, PbI2/WS2, PbI2/MoS2, PbI2/WSe2 and
Bi2Se3/WS2. These findings may be generalized to the synthesis of many other 2D
heterostructures with controlled morphologies and physical properties,
benefiting future device applications.
