Piezoelectric and Ferroelectric Properties of Novel Layered van der Waals Crystals

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 [1]. Ferrielectric CuInP₂S₆ (CIPS) was discovered in the 1980s but only recently it was revealed that this material exhibits a broad spectrum of unusual and even anomalous dielectric properties including negative electrostriction [2]. Through a combination of piezoresponse force microscopy (PFM) and density-functional-theory (DFT) calculations, two co-existing polar phases were discovered [3]. 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 van-der-Waals (vdW) gaps and forms ionic bonds with adjacent layers. These features have implication for material-property tunability and multifunctionality.<br/>I will highlight how PFM can be used to track phase transitions as well as polarization switching in response to external stimuli, including polarization alignment against an external electric field, which is a feature that is unique to van der Waals ferroelectrics [3-5]. 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 which can be compared with structural characterization techniques [6]. This study revealed that the local stress-relaxation behavior of the material is dependent on the orientation of the polar domain. This feature is caused by a surface-driven behavior where it is energetically favored when Cu atoms do not reside on the surface of the material. This behavior is occurring due to the large polar displacements in CIPS. I will discuss at the end how other properties are also affected by this asymmetric behavior including ferroelectric-antiferroelectric phase transitions.<br/><b>References:</b><br/>[1] M. A. Susner, M. Chyasnavichyus, M. A. McGuire, P. Ganesh, P. Maksymovych, “Metal Thio- and Selenophosphates as Multifunctional van Der Waals Layered Materials.” <i>Advanced Materials</i> 29 (2017) 1602852.<br/>[2] S. M. Neumayer, E. A. Eliseev, M. A. Susner, A. Tselev, B. J. Rodriguez, J. A. Brehm, S. T. Pantelides, G. Panchapakesan, S. Jesse, S. V. Kalinin, M. A. McGuire, A. N. Morozovska, P. Maksymovych, and N. Balke, Giant negative electrostriction and dielectric tunability in a van der Waals layered ferroelectric, <i>Phys. Rev. Mater</i>. <b>3</b>, 024401 (2019).<br/>[3] J. A. Brehm, S. Neumayer, L. Tao, A. O’Hara, M. Chyasnavichus, M. A. Susner, M. A. McGuire, S. V. Kalinin, S. Jesse, P. Ganesh, S. T. Pantelides, P. Maksymovych, and N. Balke, Tunable quadruple-well ferroelectric van-der-Waals crystals, <i>Nat. Mater</i>. <b>19</b>, 43 (2020).<br/>[4] S. M. Neumayer, L. Tao, A. O’Hara, J. Brehm, M. Si, P.-Y. Liao, T. Feng, S. V. Kalinin, P. D. Ye, S. T. Pantelides, P. Maksymovych, and N. Balke, Alignment of polarization against an electric field in van der Waals ferroelectrics, <i>Phys. Rev. Applied</i> <b>13</b>, 064063 (2020).<br/>[5] S. M. Neumayer, L. Tao, A. O’Hara, M. A. Susner, M. A. McGuire, P. Maksymovych, S. T. Pantelides, N. Balke, “The Concept of Negative Capacitance in Ionically Conductive van-der-Waals Ferroelectrics”, Advanced Energy Materials, 202001726, 2020. 10.1002/aenm.202001726<br/>[6] S. M. Neumayer, J. A. Brehm, L. Tao, A. O’Hara, P. Ganesh, S. Jesse, M. A. Susner, M. A. McGuire, S. T. Pantelides, P. Maksymovych, and N. Balke, Local strain and polarization mapping in ferrielectric materials, <i>ACS Appl. Mater. Interfaces</i> <b>12</b>, 38546 (2020).