Direct observation of ferroelectricity in two-dimensional MoS2
Alexey Lipatov, Pradeep Chaudhary, Zhao Guan, Haidong Lu, Gang Li, O. Crégut, Kokou D. Dorkenoo, Roger Proksch, S. Cherifi, Ding‐Fu Shao, Evgeny Y. Tsymbal, Jorge Íñiguez, Alexander Sinitskii, Alexei Gruverman
Abstract
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 (MoS 2 ) 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 MoS 2 , 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 MoS 2 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 MoS 2 belongs to the distorted trigonal structural 1T” phase, where a spontaneous polarization is inferred by its P 3 m 1 space-group symmetry and corroborated by theoretical modeling. Experiments on the flipped flakes reveal that the 1T”-MoS 2 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”-MoS 2 in flexible memory and electromechanical devices.