We demonstrate that poly(vinylidene fluoride) (PVDF)-based percolative composites using two-dimensional (2D) MXene nanosheets as fillers exhibit significantly enhanced dielectric permittivity. The poly(vinylidene fluoride-trifluoro-ethylene-chlorofluoroehylene) (P[VDF-TrFE-CFE]) polymer embedded with 2D TiCT nanosheets reaches a dielectric permittivity as high as 10 near the percolation limit of about 15.0 wt % MXene loading, which surpasses all previously reported composites made of carbon-based fillers in the same polymer. With up to 10 wt % MXene loading, the dielectric loss of the MXene/P(VDF-TrFE-CFE) composite indicates only an approximately 5-fold increase (from 0.06 to 0.35), while the dielectric constant increased by 25 times over the same composition range. Furthermore, the ratio of permittivity to loss factor of the MXene-polymer composite is superior to that of all previously reported fillers in this same polymer. The dielectric constant enhancement effect is demonstrated to exist in other polymers as well when loaded with MXene. We show that the dielectric constant enhancement is largely due to the charge accumulation caused by the formation of microscopic dipoles at the surfaces between the MXene sheets and the polymer matrix under an external applied electric field.

2D ferroelectric material has emerged as an attractive building block for highdensity data storage nanodevices. Although monolayer van der Waals ferroelectrics have been theoretically predicted, a key experimental breakthrough for such calculations is still not realized. Here, hexagonally stacking α-In 2 Se 3 nanoflake, a rarely studied van der Waals polymorph, is reported to exhibit out-of-plane (OOP) and in-plane (IP) ferroelectricity at room temperature. Ferroelectric multidomain states in a hexagonal α-In 2 Se 3 nanoflake with uniform thickness can survive to 6 nm. Most strikingly, the electric-field-induced polarization switching and hysteresis loop are, respectively, observed down to the bilayer and monolayer (≈1.2 nm) thicknesses, which designates it as the thinnest layered ferroelectric and verifies the corresponding theoretical calculation. In addition, two types of ferroelectric nanodevices employing the OOP and IP polarizations in 2H α-In 2 Se 3 are developed, which are applicable for nonvolatile memories and heterostructure-based nanoelectronics/ optoelectronics.

We report a strong effect of MXene flake size and surface termination on the dielectric permittivity of MXene polymer composites. Specifically, poly(vinylidene fluoride-trifluoroethylene-chlorofluoroehylene) or P(VDF-TrFE-CFE) polymer embedded with large (ca. 4.5 μm) Ti3C2Tx flakes reaches a dielectric permittivity as high as 10 5 near the percolation limit of 15.3 wt % MXene loading. In comparison, the dielectric permittivity of MXene/P(VDF-TrFE-CFE) using small (ca. 1.5 μm) Ti3C2Tx flakes (S-MXene) reaches a dielectric permittivity of 10 4 near the percolation limit of 16.8 wt%. Meanwhile, increasing the concentration of surface functional groups on MXene surface (-O,-F,-OH) by extending the etching time, gives a dielectric constant of 2204 near the percolation limit of 15.7 wt%. The ratio of permittivity to loss factor of our large flake composite is superior to that of small flake composite, and to all previously reported carbon-based fillers in P(VDF-TrFE-CFE). We show that the dielectric permittivity enhancement is strongly related to the charge accumulation at the surfaces between the two dimensional (2D) MXene flakes and the polymer matrix under external applied electric field.

This study demonstrates the first synthesis of MXene‐derived ferroelectric crystals. Specifically, high‐aspect‐ratio potassium niobate (KNbO3) ferroelectric crystals is successfully synthesized using 2D Nb2C, MXene, and potassium hydroxide (KOH) as the niobium and potassium source, respectively. Material analysis confirms that a KNbO3 orthorhombic phase with Amm2 symmetry is obtained. Additionally, ferroelectricity in KNbO3 is confirmed using standard ferroelectric, dielectric, and piezoresponse force microscopy measurements. The KNbO3 crystals exhibit a saturated polarization of ≈21 µC cm−2, a remnant polarization of ≈17 µC cm−2, and a coercive field of ≈50 kV cm−1. This discovery illustrates that the 2D nature of MXenes can be exploited to grow ferroelectric crystals.

No abstract