Proceedings of nanoGe Fall Meeting 2021 (NFM21)
DOI: https://doi.org/10.29363/nanoge.nfm.2021.247
Publication date: 23rd September 2021
The family of layered thio- and seleno-phosphates has gained attention as possible control dielectrics for the rapidly growing family of 2D and quasi-2D electronic materials. Ferrielectric CuInP2S6 has been discovered in the 80’s but only recently it was revealed that this material exhibits a broad spectrum of unusual and even anomalous dielectric properties including negative electrostriction. Here we report a combination of density-functional-theory (DFT) calculations, DFT-based molecular-dynamics (MD) simulations, and variable-temperature, -pressure, and -bias piezoresponse force microscopy (PFM) 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 (vdW) gap in ferrielectric CuInP2S6. Dependent on the position of Cu atoms, the polarization is either ±5 µC/cm2 when Cu resides within the layers or ±11 µC/cm2 when Cu displaces into the vdW gaps and forms ionic bonds with adjacent layers. Theoretical calculations provide values for the longitudinal piezoelectric coefficient of each of those four polarization states. Properly calibrated PFM amplitude and phase response can be related to the piezoelectric coefficient, which allows to identify these states. Consequently, we are able to track phase transitions as well as polarization switching in response to external stimuli. The calculated potential-energy landscape for Cu displacements is strongly influenced by strain, accounting for the origin of the giant negative piezoelectric coefficient. The combined theory-experiment approach also allows to map strain and stress in the material on the length scales of 10’s of nm using PFM. These phenomena offer new opportunities for both fundamental studies and applications in data storage and electronics.
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