Locally Controlled Cu-Ion Transport in Layered Ferroelectric CuInP 2 S 6 .

Metal thiophosphates are attracting growing attention in the context of quasi-2D van-der-Waals functional materials. Alkali thiophosphates are investigated as ion conductors for solid electrolytes and transition metal thiophosphates are explored as new class of ferroelectric materials. For the latter, a representative copper indium thiophosphate is ferrielectric at room temperature, and despite low polarization exhibits giant negative electrostrictive coefficients. Here, we reveal that ionic conductivity in this material enables localized extraction of Cu-ions from the lattice with a biased scanning probe microscopy tip, that is surprisingly reversible. The ionic conduction is tracked through local volume changes with a scanning probe microscopy tip providing a current-free probing technique which can be explored for other thiophosphates of interest. Nearly 90nm tall crystallites can be formed and erased reversibly on the surface of this material as result of ionic motion, the size of which can be sensitively controlled by both magnitude and frequency of the electric field, as well as the ambient temperature. These experimental results and density functional theory calculations point to a remarkable resilience of CuInPS to large scale ionic displacement and Cu-vacancies, in part enabled by metastability of Cu-deficient phases. Furthermore, we have found that piezoelectric response of CuInPS is enhanced by about 45% when a slight ionic modification is carried out with applied field. This new mode of modifying the lattice of CuInPS, and more generally ionically conducting thiophosphates, posits new prospects for their applications in van-der-Waals heterostructures, possibly in the context of catalytic or electronic functionalities.