Suppression of Magnetoresistance in Thin WTe₂ Flakes by Surface Oxidation

Recent renewed interest in layered transition metal dichalcogenides stems from the exotic electronic phases predicted and observed in the single- and few-layer limit. Realizing these electronic phases requires preserving the desired transport properties down to a monolayer, which is challenging. Surface oxides are known to impart Fermi level pinning or degrade the mobility on a number of different systems, including transition metal dichalcogenides and black phosphorus. Semimetallic WTe₂ exhibits large magnetoresistance due to electron-hole compensation; thus, Fermi level pinning in thin WTe₂ flakes could break the electron-hole balance and suppress the large magnetoresistance. We show that WTe₂ develops an ∼2 nm thick amorphous surface oxide, which shifts the Fermi level by ∼300 meV at the WTe₂ surface. We also observe a dramatic suppression of the magnetoresistance for thin flakes. However, due to the semimetallic nature of WTe₂, the effects of Fermi level pinning are well screened and are not the dominant cause for the suppression of magnetoresistance, supported by fitting a two-band model to the transport data, which showed the electron and hole carrier densities are balanced down to ∼13 nm. However, the fitting shows a significant decrease of the mobilities of both electrons and holes. We attribute this to the disorder introduced by the amorphous surface oxide layer. Thus, the decrease of mobility is the dominant factor in the suppression of magnetoresistance for thin WTe₂ flakes. Our study highlights the critical need to investigate often unanticipated and sometimes unavoidable extrinsic surface effects on the transport properties of layered dichalcogenides and other 2D materials.