Challenges in piezoelectric force microscopy characterization of sliding ferroelectric WTe₂

Sliding ferroelectricity, an emerging ferroelectric mechanism in 2D systems whose polarization arises from interlayer charge transfer, has attracted considerable attention due to its unique switching behavior, such as superior endurance and high switching speed. However, its experimental characterization remains challenging due to the low polarizability and technical difficulties. Using piezoelectric force microscopy (PFM), we investigate T_d-WTe₂ as a model sliding ferroelectric system, aiming to fill the gap in the characterization of sliding ferroelectricity from a microscopic perspective. We observed ferroelectric-like PFM phase contrast and amplitude hysteresis in few-layer (1–4 L) WTe₂ flakes under ambient conditions, in contrast to the theoretical predictions for flakes thicker than 2 L. Surface oxidation of WTe₂ has been identified by Raman and other characterization techniques. Therefore, similar PFM characterization has been carried out on the flakes covered by h-BN, which prevented WTe₂ from oxidation, and no ferroelectric behavior could be found. Consequently, the ferroelectric-like PFM phenomena are attributed to oxidation rather than intrinsic ferroelectricity in WTe₂. Combined with analyses of PFM testing principles and the mechanism of sliding ferroelectricity, we identify the limitations and challenges of PFM in characterizing sliding ferroelectricity. This work provides valuable insights for the subsequent characterization and application of related sliding ferroelectrics.