Measurements on electric polarization of 2D crystals of ferroelectric layered transition metal chalcogenides using graphene as charge sensor

2D crystals of several layered semiconducting transition metal chalcogenides (TMCs) have been reported to be ferroelectric in recent years, based mostly on piezoelectric force microscopy (PFM) measurements. These TMCs may be used as the channel material in a field effect transistor (FET) that may in turn be used as a nonvolatile memory. The determination of certain basic ferroelectric properties of these materials, such as electric polarization, will be required for the successful use of these ferroelectric TMCs for such a purpose. However, these parameters are for the most part yet to be measured. Conventional dielectric or P-E loop measurements are usually not possible because of the large leakage current stemming from the semiconducting nature of the material, especially the TMC under study is atomically thin. We have developed an alternative approach to the problem which involves combining a capacitor and a back gated FET, using high-mobility, monolayer graphene as a sensor for the surface charge induced by an out-of-plane polarization, which can often be reoriented even when the leakage current is large. We will report our results obtained in atomically thin 2D crystals of CuInP₂S₆ and α-In₂Se₃.