2, Vanderbilt University, Nashville, Tennessee, United States
3, University College Dublin, Dublin, , Ireland
Van-der-Waals crystals of metal thiophosphates can be seen as derivatives of transition metal dichalcogenides where 1/3 of metal atoms is replaced with diphosphorous, thereby stabilizing the remaining 2/3 of metal ions in low oxidation states. As a result, thiophosphates develop a panoply of desired properties, such as magnetic, dipolar and correlated electron orderings, all of which are rare or non-existent in the dichalcogenide family. Thiophosphates therefore enable ultrathin magnetic, ferroelectric and Mott insulating materials, while also presenting new opportunities for multifunctional interfaces with electronic 2d materials. We recently established giant out-of-plane piezoelectric coefficients, ferroelectric switching, and dielectric tunability in copper indium thiophosphate (CuInP2S6) . Here, we reveal that CIPS additionally exhibits ionic conductivity that enables localized extraction of Cu ions from the lattice above the Curie temperature upon application of electric fields via a conductive scanning-probe-microscopy tip. Surprisingly, the extraction is fully reversible, dependent on the polarity of the applied electric field. Cu crystallites of up to 90 nm in height can be formed and erased on the surface utilizing ionic motion in a process that can be precisely controlled by the amplitude of the applied voltage, frequency, temperature, and position of the tip. The underlying resilience of CIPS to large-scale ionic displacements and Cu vacancies is further corroborated by density-functional-theory calculations. At room temperature, newly created vacancy-rich areas exhibit even higher electromechanical response than pristine CIPS areas, making it possible to tailor piezoelectric properties through ion transport. The tunable surface deformation provides interesting opportunities for applications as sensors and actuators, and control of van-der-Waals heterostructures.
Research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. The experimental work was supported by the Division of Materials Sciences and Engineering, Basic Energy Sciences, Department of Energy.
 Neumayer et al, ”Giant negative electrostriction and dielectric tunability in a van der Waals layered ferroelectric”, arXiv:1803.08142 (2018)