Alignment of Polarization against an Electric Field in van der Waals Ferroelectrics

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Alignment of Polarization against an Electric Field in van der Waals Ferroelectrics

Sabine M. Neumayer, Lei Tao, Andrew O'Hara, John Brehm, Mengwei Si, Pai-Ying Liao, Tianli Feng, Sergei V. Kalinin, Peide D. Ye, Sokrates T. Pantelides, Petro Maksymovych, and Nina Balke

Phys. Rev. Applied 13, 064063 – Published 26 June 2020

Abstract

Polarization in ferroelectrics can be switched in the direction of an applied electric field by dipole reorientation, enabling numerous applications and fundamental phenomena. Here, we demonstrate that, in the van der Waals (vdW) layered ferrielectric ${Cu In P}_{2} S_{6}$ , a unique mechanism exists where polarization aligns against the direction of the applied electric field, seemingly in violation of the fundamental properties of a dipolar solid. The mechanism is the result of the electric field driving the $Cu$ atoms unidirectionally across the vdW gaps, which is distinctively different from dipole reorientation. The crossing of $Cu$ atoms is the fundamental process of ionic conductivity, yet it is compatible with the existence of polarization. These phenomena are confirmed by nanoscale imaging and spectroscopy of ferroelectric capacitors, coupled with dynamic density-functional-theory simulations. The symbiotic relationship of ferroelectric and ionic phenomena enables alternative approaches to control polarization and necessitates a change in perspective on nucleation, domain-wall dynamics, and other ferroelectric and electromechanical characteristics in material systems where ionic and ferroelectric phenomena manifest.

Received 22 January 2020
Revised 27 May 2020
Accepted 29 May 2020
Corrected 21 October 2020

DOI:https://doi.org/10.1103/PhysRevApplied.13.064063

Physics Subject Headings (PhySH)

Electric polarization Ferroelectricity Ionic transport Phase transitions Piezoelectricity

Devices Ferroelectrics Van der Waals systems

Condensed Matter, Materials & Applied Physics

Corrections

21 October 2020

Correction: An affiliation indicator for the seventh author was incorrect and has been fixed.

Authors & Affiliations

Sabine M. Neumayer ¹, Lei Tao^2,3, Andrew O'Hara ², John Brehm², Mengwei Si ^4,5, Pai-Ying Liao ^4,5, Tianli Feng², Sergei V. Kalinin¹, Peide D. Ye^4,5, Sokrates T. Pantelides ^2,6,†, Petro Maksymovych^1,‡, and Nina Balke ^1,*

¹Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA
²Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
³University of Chinese Academy of Sciences & Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁴School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA
⁵Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA
⁶Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, USA

^*balken@ornl.gov
^†pantelides@vanderbilt.edu
^‡maksymovychp@ornl.gov

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Vol. 13, Iss. 6 — June 2020

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Images

Figure 1
Electromechanical response as a function of V_dc pulse duration. (a) Structural phases for each of the four polarization states in ${Cu In P}_{2} S_{6}$ : two upward-oriented polarization states (+HP and +LP) and two downward-oriented polarization states (−LP and −HP). (b) Three-dimensional topography image of $Ni$ electrode on layered ${Cu In P}_{2} S_{6}$ and schematic depiction of the experimental setup. Sequence of applied voltage pulses; green arrows indicate when the PFM data are acquired. Three-dimensional histogram of measured d values as a function of pulse duration time. Distribution of counts is indicated by the height. (c) PFM maps of the $Ni$ / ${Cu In P}_{2} S_{6}$ / $Ni$ capacitor structure acquired after applying V_dc pulses, showing spatial variation in piezoresponse. Plot titles indicate duration of V_dc pulses; a 0.1 s pulse is omitted, as it does not show any differences from that of 0 s.
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Figure 2
Analysis of polarization-state transitions. Sankey diagram of polarization-state transitions dependent on voltage pulse calculated from transitions observed in the PFM maps shown in Fig. 1. Widths of the lines are proportional to the number of pixels that transition between two particular states for a given voltage step. White dashed line indicates the switching path discussed in detail in Fig. 3 and the text.
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Figure 3
Trajectory of $Cu$ during switching experiment. (a) Polarization as a function of pulse duration for the path outlined by the dashed line in Fig. 2. Direction of the applied electric field is indicated by black arrow. (b) Structural phases involved in the switching pathway shown in (a). The red circle indicates one specific atom in the structure, which can be moved by the applied electric field. Direction of the applied electric field and individual polarization states are indicated by black arrows. (c) Relative $Cu$ displacement as function of pulse duration for the path outlined in (a).
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Figure 4
Simulation of ferroelectric switching in ${Cu In P}_{2} S_{6}$ . (a) Trajectory of $Cu$ ions in two individual layers with excess $Cu$ under application of an external electric field. Excess $Cu$ is marked by dark blue. Two arrows indicate times when all $Cu$ move across the layer and one $Cu$ atom moves across the vdW gap, respectively. (b) Configurations tracking $Cu$ migration in the presence of an electric field. Upper panel (outlined in black) in the presence of a vacancy identified by a dashed black circle, and lower panel (outlined in red) in the presence of excess $Cu$ identified by a dashed red circle: initial +HP state (a), intermediate –HP state (b), and final +HP state (c). (c) Energy barriers for a $Cu$ atom migrating from the +HP state of one layer to the −HP state of the adjacent layer corresponding to (a). Solid black curve and solid red curve are the energy profiles with a vacancy and excess $Cu$ , respectively. When an external electric field (−1.0 V/Å) is applied along the z direction, energy profiles are shown as dashed curves. Solid blue curve is the pathway for a $Cu$ atom jumping across the vdW gap in stoichiometric four-layer ${Cu In P}_{2} S_{6}$ .
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