Role of Heterointerface in Lithium-Induced Phase Transition in Td-WTe2 Nanoflakes

A new polytype of WTe2 with a bandgap has been recently discovered through the intercalation of lithium into the van der Waals gaps of Td-WTe2. Here, we report the effects of reduced thicknesses and heterointerfaces on the intercalation-induced phase transition in WTe2. Using in situ Raman spectroscopy during the electrochemical lithiation of WTe2 flakes as a function of flake thickness, we observe that additional electrochemical energy is required for the phase transition of WTe2 from the Td phase to the new lithiated Td' phase, going from 0.8 V of the applied electrochemical voltage for a thick flake to 0.5 V and 0.3 V for 7- and 5-layered samples, respectively. We ascribe this suppression of the phase transition to the interfacial interaction between the nanoflake and SiO2/Si substrate, which plays an increasing role as the sample thickness is reduced. The suppressed kinetics of the phase transition can be mitigated by placing the WTe2 flake on a hexagonal boron nitride (hBN) flake, which facilitates the release of the in-plane strain induced by the phase transition. Importantly, our study underscores the significance of interfacial effects in modulating phase transitions in two-dimensional (2D) materials, suggesting heterogeneous transition pathways, as well as interfacial engineering to control these phase transitions.