The family of layered thio- and seleno-phosphates has gained attention as potential control dielectrics for the rapidly growing family of two-dimensional and quasi-two-dimensional electronic materials. Here we report a combination of density functional theory calculations, quantum molecular dynamics simulations and variable-temperature, -pressure and -bias piezoresponse force microscopy data to predict and verify the existence of an unusual ferroelectric property—a uniaxial quadruple potential well for Cu displacements—enabled by the van der Waals gap in copper indium thiophosphate (CuInP2S6). The calculated potential energy landscape for Cu displacements is strongly influenced by strain, accounting for the origin of the negative piezoelectric coefficient and rendering CuInP2S6 a rare example of a uniaxial multi-well ferroelectric. Experimental data verify the coexistence of four polarization states and explore the temperature-, pressure- and bias-dependent piezoelectric and ferroelectric properties, which are supported by bias-dependent molecular dynamics simulations. These phenomena offer new opportunities for both fundamental studies and applications in data storage and electronics.
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The experimental and theoretical data presented in this work are available from the corresponding authors upon reasonable request.
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The experimental work, including part of the data analysis and interpretation, was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. Theory was supported by the US Department of Energy (grant no. DE-FG02-09ER46554) and by the McMinn Endowment at Vanderbilt University. The experiments were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility that also provided support with data collection and interpretation. Partial support for sample synthesis, experiments and theory was provided by the Laboratory Directed Research and Development program at the Oak Ridge National Laboratory. Calculations were performed at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the US Department of Energy under contract no. DE-AC02-05CH11231. Manuscript preparation was partially funded by the Air Force Research Laboratory under an Air Force Office of Scientific Research grant (LRIR grant no. 14RQ08COR) and a grant from the National Research Council.
The authors declare no competing interests.
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(a.) Pressure-dependent PFM measurements from a 10x10 μm2 area (Supplementary Fig. 6). (b.) Histograms of measured piezoelectric response of CIPS only. (c.) Pressure-dependent piezoelectric constant extracted for four distinct states and theoretical piezoelectric constant from theory for comparison. Data points are the position of the histogram peaks for each phase and error bars correspond to the peak widths. The d-value is constant or decreasing for LP and increases for HP. The +HP state (light blue) transforms into the -HP state (yellow) indicating a pressure-induced switching event. All changes are reversible. Response on the IPS phase changes little with contact force (see also Supplementary Fig. 7b).
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Brehm, J.A., Neumayer, S.M., Tao, L. et al. Tunable quadruple-well ferroelectric van der Waals crystals. Nat. Mater. (2019) doi:10.1038/s41563-019-0532-z