Ultrahigh Out-of-Plane Piezoelectricity Meets Giant Rashba Effect in 2D Janus Monolayers and Bilayers of Group IV Transition-Metal Trichalcogenides

Abstract

The simultaneous occurrence of gigantic piezoelectricity and Rashba effect in two-dimensional (2D) materials is unusually scarce. Inversion symmetry occurring in MX3 (M = Ti, Zr, Hf; X = S, Se) monolayers is broken upon constructing their Janus monolayer structures MX2Y (X ≠ Y =S, Se), thereby inducing a large out-of-plane piezoelectric constant d 33 (∼68 pm/V) in them. d 33 can be further enhanced to a super high value of ∼1000 pm/V upon applying vertical compressive strain in the van der Waals bilayers constituted by interfacing these Janus monolayers. Therefore, d 33 in these Janus transition-metal trichalcogenide (TMTC) bilayers reach more than 4-fold times that of bulk ceramic PZT material (∼268 pm/V). The absence of a horizontal mirror symmetry and the presence of strong spin–orbit coupling cause Rashba spin-splitting in electronic bands in these Janus 2D monolayers, which shows up as an ultrahigh Rashba parameter, αR ∼ 1.1 eV Å. It can be raised to 1.41 eV Å via compressive strain. Most of the 2D materials reported to date mainly show in-plane electric polarization, which severely limits their prospects in piezotronic devices. In this present work, the piezoelectricity shown by the Janus monolayers of group IV TMTCs and their bilayers is significantly higher than the ones generally utilized in the form of three-dimensional bulk piezoelectric solids, for example, α-quartz (d 11 = 2.3 pm/V), wurtzite-GaN (d 33 = 3.1 pm/V), and wurtzite-AlN (d 33 = 5.6 pm/V). It is exceedingly higher than that in Janus multilayer/bulk structures of Mo- and W-based transition-metal dichalcogenides, for example, MoSTe (d 33 ∼ 10 pm/V). The 2D Janus TMTC monolayers and their bilayers reported herewith straddle giant Rashba spin-splitting and ultrahigh piezoelectricity, thereby making them immensely promising candidates in the next-generation electronics, piezotronics, spintronics, flexible electronics, and piezoelectric devices.

Piezotronics in two‐dimensional materials

“…The entrance of piezotronics from 2D materials was opened thoroughly since the first experimental confirmation from 2D MoS 2 by Hone and Wang groups in 2014, where piezo‐voltage and output current were detected only from MoS 2 flakes with odd number of atomic layers 31 . Then, piezotronics based on 2D materials has been grown into forest since the expansion from black phosphorus (BP), 55–59 hexagonal boron nitride (h‐BN), 60–63 transition metal dichalcogenide (TMDCs) and Janus TMDCs, 63–72 II‐VI semiconductors, 73–79 III‐V, 80–88 Group‐III monochalcogenides (III‐VI), 89,90 and Janus III‐VI, 91,92 Group‐IV monochalcogenides (IV‐VI), 93–97 Group‐V (V‐V), 98 and so on 99–109 . At the same time, the variety of applications of piezotronics from 2D materials were achieved on flexible optoelectronics, piezotronic catalysis, biological medicine, information storage, and so forth.…”

Type‐II van der Waals Heterostructures Based on AsP and Transition Metal Dichalcogenides: Great Promise for Applications in Solar Cell

“…Their sizes of the built‐in electric fields are 0.688, 0.869, 0.735, and 0.956 eV Å −1 , determined by the slope of the plane average electrostatic potential between the minima of the contact layer edge (see the solid red line in Figure 6). [ 61–63 ] For the WSe 2 /AsP AA’ stacking heterostructure in Figure 6g, the size of the build‐in electric field is 0.832 eV Å −1 from AsP to WSe 2 layer. It is beneficial for photogenerated electron–hole pair separation.…”

Bridge role of weak chemical bonding in photocatalytic performance of asymmetric 2H-MoS2/BiOCl Janus heterostructure

“…There has been a large number of previous research on electrons and various excellent properties in 2D Janus materials [13][14][15][16][17][18][19][20][21][22][23]. Electric field and strain are considered as two effective ways to realize versatile physical properties in Janus XSn 2 Y monolayers [13].…”