Direct observation of ferroelectricity in two-dimensional MoS2

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.

Automatic 2D Material Detection in Optical Images using Deep Learning Based Computer Vision

“…), a prolific parameter space of systems and structure-property relationships for quantum, optoelectronic, and magnetic technologies exists. [25][26][27] The task of exploring the vast world of 2D materials requires the ability to efficiently and reliable produce single-and few-layer samples of 2D materials. 28,29 However, the most widely-used state-of-the-art method for preparing single-and few-layer samples of 2D materials relies on the manual exfoliation of a large population of smaller crystallites from a bulk crystal.…”

Reducing Threshold of Ferroelectric Domain Switching in Ultrathin Two-Dimensional CuInP2S6 Ferroelectrics via Electrical–Mechanical Coupling

“…Furthermore, according to some recent reports, the tip loading can switch the ferroelectric domains in BaTiO 3 43 and MoS 2 thin films. 44 Inspired by this result, the polarization response of ultrathin CuInP 2 S 6 was further monitored under mechanical loading. The experimental results show that the pre-polarized upward ferroelectric domain (Figure 3a) can also be switched by applying the mechanical loading of 1216 nN onto the ultrathin CuInP 2 S 6 nanoflake (Figure 3b).…”

Automatic detection of multilayer hexagonal boron nitride in optical images using deep learning-based computer vision

“…With over 1000 known van der Waals 2D materials that span all functionalities (i.e., insulators, metals, semiconductors, ferromagnets, ferroelectrics, etc. ), a prolific parameter space of systems and structure-property relationships for quantum, optoelectronic, and magnetic technologies exists 26 – 28 . The task of exploring the vast world of 2D materials requires the ability to efficiently and reliable produce single- and few-layer samples of 2D materials 29 , 30 .…”