Role of Heterointerface in Lithium-Induced Phase Transition in T_d‑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 T_d-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 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. 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.