Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking

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Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking

Jun Xiao, Hanyu Zhu, Ying Wang, Wei Feng, Yunxia Hu, Arvind Dasgupta, Yimo Han, Yuan Wang, David A. Muller, Lane W. Martin, PingAn Hu, and Xiang Zhang

Phys. Rev. Lett. 120, 227601 – Published 31 May 2018

Abstract

Out-of-plane ferroelectricity with a high transition temperature in ultrathin films is important for the exploration of new domain physics and scaling down of memory devices. However, depolarizing electrostatic fields and interfacial chemical bonds can destroy this long-range polar order at two-dimensional (2D) limit. Here we report the experimental discovery of the locking between out-of-plane dipoles and in-plane lattice asymmetry in atomically thin ${In}_{2} {Se}_{3}$ crystals, a new stabilization mechanism leading to our observation of intrinsic 2D out-of-plane ferroelectricity. Through second harmonic generation spectroscopy and piezoresponse force microscopy, we found switching of out-of-plane electric polarization requires a flip of nonlinear optical polarization that corresponds to the inversion of in-plane lattice orientation. The polar order shows a very high transition temperature ( $\sim 700 K$ ) without the assistance of extrinsic screening. This finding of intrinsic 2D ferroelectricity resulting from dipole locking opens up possibilities to explore 2D multiferroic physics and develop ultrahigh density memory devices.

Received 3 October 2017
Revised 10 April 2018

DOI:https://doi.org/10.1103/PhysRevLett.120.227601

Physics Subject Headings (PhySH)

Ferroelectricity Phase transitions

2-dimensional systems

Atomic force microscopy Confocal imaging High-resolution transmission electron microscopy Optical second-harmonic generation Raman spectroscopy Scanning transmission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jun Xiao¹, Hanyu Zhu¹, Ying Wang¹, Wei Feng², Yunxia Hu², Arvind Dasgupta³, Yimo Han⁴, Yuan Wang¹, David A. Muller^4,5, Lane W. Martin^3,6, PingAn Hu², and Xiang Zhang^1,6,*

¹NSF Nanoscale Science and Engineering Center (NSEC), University of California, Berkeley, California 94720, USA
²School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, People’s Republic of China
³Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
⁴Department of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
⁵Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA
⁶Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA

^*Corresponding author. xiang@berkeley.edu

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Issue

Vol. 120, Iss. 22 — 1 June 2018

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Images

Figure 1
Characterization of 2D ${In}_{2} {Se}_{3}$ . (a), (b) Side and top view of two energy-degenerate ferroelectric ${In}_{2} {Se}_{3}$ structures. Single quintuple layer consists of covalently bonded indium and selenium triangular lattices. The crystal belongs to R3m space group (No. 160). The polarization flipping requires locked out-of-plane and in-plane motion of middle Se atom (purple ball) accompanied by covalent bond breaking and formation, and reverses both the out-of-plane polarization and the in-plane lattice orientation with nonlinear optical polarization. (c) SHG polarization pattern of a trilayer triangular ${In}_{2} {Se}_{3}$ on mica substrate under normal incidence as a function of crystal angle. The threefold rotational symmetry manifests through the sixfold SHG intensity pattern (black square) when the polarization of the optical excitation and detection are rotated collinearly. Red curve shows the fitting by $I = I_{0} \cos^{2} (3 θ + θ_{0})$ , $θ$ is crystal angle. (d) Out-of-plane dipole probed by angle resolved polarization-selective SHG on the same trilayer sample. Angle-dependent SHG intensity ratio $I_{p} / I_{s}$ increases symmetrically with tilted incidence (black triangular) and agrees well with the model of response from out-of-plane dipole to vertical electrical oscillation field, indicating the ratio ( $13.6 : 1$ ) between in-plane and out-of-plane second-order susceptibility at 1080 nm pump (red curve, see Supplemental Material [24], Sec. III for model details).
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Figure 2
Visualization of domain structure in 2D ${In}_{2} {Se}_{3}$ by SHG and piezoresponse mapping. (a) SHG mapping of the 3-nm ${In}_{2} {Se}_{3}$ crystal, showing intensity contrast in different regions corresponding to domains with different dipole strength that match well with the piezoresponse mapping in (b). The inset is the optical image of the same sample. SHG intensity dark lines are observed within the region of nearly uniform optical contrast and SHG intensity, that also match the boundaries between domains with 180° piezoresponse phase contrast in (c) (blue curves). They originate from the destructive interference from the oppositely polarized domains.
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Figure 3
Electrically switching the out-of-plane ferroelectric polarization and corresponding in-plane atomic configuration through dipole locking. (a) The hysteresis of remnant out-of-plane polarization of a 3-nm-thick ${In}_{2} {Se}_{3}$ crystal on conductive ${SrRuO}_{3}$ , as a function of perpendicular poling voltage. Black, red, blue curves represent the normalized piezoresponse measured with $V_{ac} = 0.5$ , 1, and 1.5 V, respectively. The collapse of the hysteresis loop agrees very well with the behavior of the conventional field-switchable ferroelectrics and is in contrast to the charging artifact of dielectrics. (b) Polarized domain patterned by electrically biased scanning probe and measured by PFM. The inner box corresponds to positive applied voltage ( $+ 6 V$ ) and positive piezoresponse while the outer box to negative voltage ( $- 6 V$ ) and negative piezoresponse. (c) SHG intensity mapping on another trilayer ${In}_{2} {Se}_{3}$ sample before PFM reversed poling. The area enclosed by dashed line was then scanning by a negatively biased AFM tip. The color bar is in linear scale with arbitrary unit. (d) SHG mapping after the electrical reversed poling. It shows dark lines at the boundary of the patterned area resulting from destructive interference, which indicates the reversal of in-plane crystal orientation and corresponding nonlinear optical polarization after reversed electrical poling.
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Figure 4
Observation of temperature-dependent ferroelectric to centrosymmetric phase transition. The SHG intensity and the corresponding ferroelectric ordering remains robust as a four-layer ${In}_{2} {Se}_{3}$ sample on ${SiO}_{2} / Si$ substrate was heated from room temperature to 600 K. It then dropped sharply to about one-sixth of the initial value when the temperature further increased to 700 K, indicating the disappearance of spontaneous polarization and the structural transition to a centrosymmetric phase.
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