Abstract
Recent theoretical predictions of ferroelectricity in two-dimensional (2D) van der Waals materials reveal exciting possibilities for their use in scalable low-power electronic devices with polarization-dependent functionalities. These prospects have been further invigorated by the experimental evidence of the polarization response in some transition metal chalcogenides (TMCs)—a group of narrow-band semiconductors and semimetals with a wealth of application potential. Among the TMCs, molybdenum disulfide (MoS2) is known as one of the most promising and robust 2D electronic materials. However, in spite of theoretical predictions, no ferroelectricity has been experimentally detected in MoS2, while the emergence of this property could enhance its potential for electronics applications. Here, we report the experimental observation of a stable room-temperature out-of-plane polarization ordering in 2D MoS2 layers, where polarization switching is realized by mechanical pressure induced by a tip of a scanning probe microscope. Using this approach, we create the bi-domain polarization states, which exhibit different piezoelectric activity, second harmonic generation, surface potential, and conductivity. Ferroelectric MoS2 belongs to the distorted trigonal structural 1T” phase, where a spontaneous polarization is inferred by its P3m1 space-group symmetry and corroborated by theoretical modeling. Experiments on the flipped flakes reveal that the 1T”-MoS2 samples consist of the monolayers with randomly alternating polarization orientation, which form stable but switchable “antipolar” head-to-head or tail-to-tail dipole configurations. Mechanically written domains are remarkably stable facilitating the application of 1T”-MoS2 in flexible memory and electromechanical devices.
Highlights
The emergence of ferroelectricity in two-dimensional (2D) materials is ascribed to the loss of a center of symmetry in the intrinsically centrosymmetric structures
To perform highresolution structural characterization, the freshly exfoliated and lithiated MoS2 flakes were transferred on a transmission electron microscopy (TEM) grid
While the 2H-MoS2 flakes exhibit a hexagonal arrangement of bright reflection spots in the selected area electron diffraction (SAED) pattern (Fig. 1b), the emergence of the superstructure spots can be seen in SAED of the lithiated MoS2 flakes, which corresponds to the doubling of the unit cell parameters (Fig. 1c)
Summary
The emergence of ferroelectricity in two-dimensional (2D) materials is ascribed to the loss of a center of symmetry in the intrinsically centrosymmetric structures. It was predicted that the in-plane trimerization in monolayer 1T-MoS2 could break the inversion symmetry and introduce the out-of-plane polarization[2]. Stacking of structurally symmetric nonpolar monolayers can break the inversion symmetry and introduce ferroelectricity in the bulk structure as was observed experimentally for Td-WTe23. Another mechanism is related to the intrinsic polar symmetry, such as in In2Se3, where the ferroelectricity was predicted to arise due to the vertical displacement of Se atoms[4]. The distorted trigonal 1T”-MoS2 phase belongs to the space group P3m1, which has two topologically equivalent crystal structures related to each other by the inversion symmetry operation, designating its ferroelectric nature. First-principles calculations confirm the presence of spontaneous polarization in 1T”-MoS2 as well as the existence of stable head-tohead (H-H) and tail-to-tail (T-T) dipole configurations, which have been experimentally revealed by the experiments involving the flipped MoS2 flakes
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