  Phase transitions of two-dimensional materials and their heterostructures
enable many applications including electrochemical energy storage, catalysis,
and memory; however, the nucleation pathways by which these transitions proceed
remain underexplored, prohibiting engineering control for these applications.
Here, we demonstrate that the lithium intercalation-induced 2H-1T' phase
transition in MoS2 proceeds via nucleation of the 1T' phase at a
heterointerface by monitoring the phase transition of MoS2/graphene and
MoS2/hexagonal boron nitride (hBN) heterostructures with Raman spectroscopy in
situ during intercalation. We observe that graphene-MoS2 heterointerfaces
require an increase of 0.8 V in applied electrochemical potential to nucleate
the 1T' phase in MoS2 compared to hBN-MoS2 heterointerfaces. The increased
nucleation barrier at graphene-MoS2 heterointerfaces is due to the reduced
charge transfer from lithium to MoS2 at the heterointerface as lithium also
dopes graphene based on ab initio calculations. Further, we show that the
growth of the 1T' domain propagates along the heterointerface, rather than
through the interior of MoS2. Our results provide the first experimental
observations of the heterogeneous nucleation and growth of
intercalation-induced phase transitions in two-dimensional materials and
heterointerface effects on their phase transition.
