Direct measurement of ferroelectric polarization in a tunable semimetal

Abstract

Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. Recent measurements, however, demonstrated signatures of ferroelectric switching in the electrical conductance of bilayers and trilayers of WTe2, a semimetallic transition metal dichalcogenide with broken inversion symmetry. An especially promising aspect of this system is that the density of electrons and holes can be continuously tuned by an external gate voltage. This degree of freedom enables measurement of the spontaneous polarization as free carriers are added to the system. Here we employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe2 to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. We compare our results to a low-energy model for the electronic bands and identify the layer-polarized states that contribute to transport and polarization simultaneously. Bilayer WTe2 is thus shown to be a fully tunable ferroelectric metal and an ideal platform for exploring polar ordering, ferroelectric transitions, and applications in the presence of free carriers.

Progress on Emerging Ferroelectric Materials for Energy Harvesting, Storage and Conversion

“…[248] Microscopically, the polarization reversal can be attributed to re-distribution of electrons and holes and even interlayer sliding. [69,249] The switchable polarization states are further confirmed by PFM experiments carried out on the 1T′-WTe 2 thin films [250] (Figure 8f,g). Distinct from cycle-dependent activity decay in ordinary catalysts, [251] the HER performance of 1T′-MoTe 2 is dramatically improved as the electrode is held at a cathodic bias, [252] where its over-potential decreases from 320 to 178 mV at a current density of 10 mA cm −2 (Figure 8h).…”

Identifying the Transition Order in an Artificial Ferroelectric van der Waals Heterostructure

“…In recent years, novel atomically thin ferroelectric semiconducting materials have been synthesized and are generically known as group-IV monochalcogenide monolayers, which exhibit stable ferroelectrics at the monolayer level. − However, these materials usually suffer from lack of chemical stability, making their potential applications in memory challenges. Unlike conventional epitaxial growth, van der Waals (vdW) integration offers an alternative strategy to tailor and engineer crystal symmetry by simply stacking and rotating different building blocks, − as exemplified by two-dimensional (2D) twist electronics. − Recently, artificial ferroelectrics created by tearing-and-stacking bipartite honeycomb 2D materials have been theoretically predicted and experimentally realized in the parallel hexagonal boron nitride (hBN) bilayer system − and very recently in semiconducting transition metal dichalcogenides (s-TMDs). , For these materials, the ferroelectric phase transitions have not yet been carefully studied, and the phase transition order has not been identified experimentally.…”

Ferroelectrics-Integrated Two-Dimensional Devices toward Next-Generation Electronics

“…Yasuda et al reported that parallel-stacked bilayer h-BN exhibits OOP ferroelectricity that reverses depending on the stacking order . Similar phenomenon was found in multilayer TMDs (d1T-MoTe 2 , 1T’-ReS 2 , WTe 2 , ), which exhibit OOP ferroelectricity arising from in-plane slipping. For example, in bilayer ReS 2 , two energy-degenerate ferroelectric structures can be obtained by moving the upper layer with a sliding distance from the high-symmetry nonpolar structure, and the nonequivalence of the top and bottom layers leads to an uncompensated charge transfer between them and results in the vertical polarization .…”

“…Although 2D ferroelectricity has been theoretically predicted for a wide range of 2D layered materials, only a few 2D vdW ferroelectrics have been experimentally achieved and explored, including bilayer h-BN, TMDs, ,,− group-IV metal chalcogenides, − ,, CIPS, ,,− and α- and β-In 2 Se 3 . − Owing to the different ferroelectricity between IP and OOP directions depending on the crystal structure, 2D ferroelectrics provide a promising platform to explore domain physics. In particular, IP ferroelectricity is confined within the 2D plane by the interlayer vdW gap, thus stabilizing it against OOP perturbation.…”

“…The discovery of 2D layered ferroelectric materials with weak interlayer vdW interaction promotes the development of 2D ferroelectricity for their basic properties such as the dangling-bond-free surface and excellent durability against strain. Intrinsic ferroelectricity in 2D materials, such as TMDs, − group IV metal chalcogenides, − CuInP 2 S 6 (CIPS), − and indium selenide (In 2 Se 3 ), − have been theoretically or experimentally demonstrated, while extrinsic ferroelectricity can be achieved through the defect, strain, or surface engineering of particular 2D materials with no ferroelectricity . In addition, 2D ferroelectrics have been reported to possess properties rarely found in the bulk counterpart, e.g., the interrelated coupling effect of out-of-plane (OOP) and in-plane (IP) ferroelectric polarizations in In 2 Se 3 . ,, As a result, 2D vdW ferroelectrics can be intuitively integrated with 2D semiconductors to form vdW heterostructures regardless of lattice mismatch, which probably leads to sharp band edges and minimal interfacial traps for realizing intriguing properties and versatile device functionalities.…”