DOI: 10.1002/adma. 201504779 actuators,[20] sensors,[21] and microwave circuits.[22] It is, therefore, desirable that these properties missing in TMDs be complemented by interfacing the 2D TMDs with such functional materials, forming a synergistic system to offer new functionalities that neither of the two components alone can do. For example, the ferroelectric field-effect transistor (FeFET), in which the gate dielectric in a conventional FET is replaced with a FE film, has been considered as a potential candidate for future memory.[17, 18, 23–25] The advantage of this FET-type FeRAM cells is that their readout operation is nondestructive, unlike the capacitor-type FeRAMs that are destructive reads and thus need to be re-written after every readout process. However, memory stability and integration density of FeFETs need to be much improved to become a competitive memory technology. The memory instability is caused by depolarization field weakening the FE polarization, as well as interdiffusion of ions and chemical reaction across the semiconductor/FE interface that occurs when the FE film undergoes high-temperature annealing. To circumvent the chemical instability issue, dielectric buffer layers such as SiO 2 and HfO 2 are generally inserted into the interface, but they bring about additional issues such as deterioration of electrical breakdown strength and memory retention. In addition to expected benefits in terms of size scaling, the 2D/FeFET structure where a 2D TMD layer is employed as a semiconductor channel in FeFETs have two major advantages over existing thin-film FeFET memories that use conventional semiconductor channels. First, the van der Waals (vdW) interface in the TMD/FE heterostructures largely eliminates the interfacial issues. Secondly, the ultra-thin thickness of the 2D TMD channel would facilitate accumulation of compensating charges to screen the polarization in the FE, effectively suppressing the depolarization field.[26, 27] Previously, 2D/FeFETs have been demonstrated based on 2D MoS 2 as well as black phosphorous coated with organic FE films, respectively, and further CMOS memory circuits have been also constructed combining the two elements by Lee et al.[28, 29] Using the similar structures of few-layer MoS 2/organic FE, high performance photodetectors have been lately developed by Wang et al.[30] However, the applicability of organic FeFET is hampered by slow dipole dynamics, high operation voltage, and low mechanical and thermal durability in comparison to its inorganic counterparts.[28, 29, 31] Recently, a couple of groups have investigated inorganic FeFETs interfacing 2D MoS 2 n-type channels with Pb [Zr xTi 1− x] O 3 films, but the device performance still falls short of those of thin-film-based FeFETs, especially in terms of switching speed, and the underlying mechanisms of memory operation across 2D/FE interfaces were poorly understood.[32, 33]

Synergy between materials of dissimilar dimensionalities and functionalities may lead to new properties and device applications. By virtue of extremely high area-to-volume ratio and susceptibility to external stimulus, atomically thin, nearly 2D materials are suitable for constituting planar heterostructures by coupling with other 2D counterparts [1–5] or different dimensional structures including 1D nanowires [6] and 3D, thick films.[7] The recent boost of interest in semiconducting layered transitionmetal dichalcogenides (TMDs) originates from their exotic characteristics in the monolayer limit, such as indirect-to-direct band gap transition,[8] valley-dependent polarization,[9] tightly bound excitonic states,[10] and piezoelectricity.[11] Besides fundamental …