We demonstrate room-temperature ferroelectric field-effect transistors (Fe-FETs) with MoS and CuInPS two-dimensional (2D) van der Waals heterostructure. The ferroelectric CuInPS is a 2D ferroelectric insulator, integrated on top of MoS channel providing a 2D/2D semiconductor/insulator interface without dangling bonds. The MoS- and CuInPS-based 2D van der Waals heterostructure Fe-FETs exhibit a clear counterclockwise hysteresis loop in transfer characteristics, demonstrating their ferroelectric properties. This stable nonvolatile memory property can also be modulated by the back-gate bias of the MoS transistors because of the tuning of capacitance matching between the MoS channel and the ferroelectric CuInPS, leading to the enhancement of the on/off current ratio. Meanwhile, the CuInPS thin film also shows resistive switching characteristics with more than four orders of on/off ratio between low- and high-resistance states, which is also promising for resistive random-access memory applications.

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A material with reversible temperature change capability under an external electric field, known as the electrocaloric effect (ECE), has long been considered as a promising solid-state cooling solution. However, electrocaloric (EC) performance of EC materials generally is not sufficiently high for real cooling applications. As a result, exploring EC materials with high performance is of great interest and importance. Here, we report on the ECE of ferroelectric materials with van der Waals layered structure (CuInP2S6 or CIPS in this work in particular).Over 60% polarization charge change is observed within a temperature change of only 10 K at Curie temperature. Large adiabatic temperature change (|ΔT|) of 3.3 K, isothermal entropy change (|ΔS|) of 5.8 J kg -1 K -1 at |ΔE|=142.0 kV cm -1 at 315 K (above and near room temperature) are achieved, with a large EC strength (|ΔT|/|ΔE|) of 29.5 mK cm kV -1 . The ECE of CIPS is also investigated theoretically by numerical simulation and a further EC performance projection is provided.Electrocaloric refrigerators using electrocaloric materials are low noise, environmentfriendly and can be scaled down to small dimensions, compared to the common vaporcompression refrigerators. 1-13 Electrocaloric cooling is also much easier and lower cost to realize compared to other field induced cooling techniques such as magnetocaloric and mechanocaloric cooling, because the electric field is easily to be realized and accessible. Thus, electrocaloric effect is promising for future cooling applications, especially in micro-or nano-scale such as onchip cooling. Electrocaloric effect in ferroelectric materials is of special interest because of the large polarization change near the ferroelectric-paraelectric (FE-PE) phase transition temperature 21 TOC.

The van der Waals layered material CuInP 2 S 6 features interesting functional behavior, including the existence of four uniaxial polarization states, polarization reversal against the electric field through Cu ion migration, a negative-capacitance regime, and reversible extraction of Cu ions. At the heart of these characteristics lies the high mobility of Cu ions, which also determines the spontaneous polarization. Therefore, Cu migration across the lattice results in unusual ferroelectric behavior. Here, we demonstrate how the interplay of polar and ionic properties provides a path to ionically controlled ferroelectric behavior, achieved by applying selected DC voltage pulses and subsequently probing ferroelectric switching during fast triangular voltage sweeps. Using current measurements and theoretical calculations, we observe that increasing DC pulse duration results in higher ionic currents, the buildup of an internal electric field that shifts polarization loops, and an increase in total switchable polarization by ∼50% due to the existence of a high polarization phase which is stabilized by the internal electric field. Apart from tuning ferroelectric behavior by selected square pulses, hysteretic polarization switching can even be entirely deactivated and reactivated, resulting in three-state systems where polarization switching is either inhibited or can be performed in two different directions.