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 T-d phase to the new lithiated T-d ' 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 5layered 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. 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.