Locally Controlled Cu-Ion Transport in Layered Ferroelectric CuInP₂S₆

Abstract

Metal thiophosphates are attracting growing attention in the context of quasi-two-dimensional van der Waals functional materials. Alkali thiophosphates are investigated as ion conductors for solid electrolytes, and transition-metal thiophosphates are explored as a 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, which 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 90 nm-tall crystallites can be formed and erased reversibly on the surface of this material as a 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 CuInP 2 S 6 to large-scale ionic displacement and Cu vacancies, in part enabled by the metastability of Cu-deficient phases. Furthermore, we have found that the piezoelectric response of CuInP 2 S 6 is enhanced by about 45% when a slight ionic modification is carried out with applied field. This new mode of modifying the lattice of CuInP 2 S 6 , 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.

The Concept of Negative Capacitance in Ionically Conductive Van der Waals Ferroelectrics

“…The four possible polarization states in CIPS, ±LP and ±HP, are indicated. The energy barrier for Cu ions to cross the vdW gap is calculated to be ≈0.6 eV, which fits well in the range of reported activation barriers for Cu-ion conduction [32,35] and can be easily overcome using electric fields. If the Cu ions move beyond the unit cell and across the vdW gap along the z axis under the influence of an electric field, the resulting polarization exhibits unusual behavior.…”

“…[28] In addition, CIPS has a high Cu-ion conductivity that extends into the temperature range of the ferroelectric phase. [30][31][32] It has been shown that in-plane ionic currents can be used to manipulate ferroelectric polarization. [33] These highly unusual properties that are rooted in the vdW structure provide the opportunity for complex polarization switching which might be exploited in transient-NC devices beyond the mechanisms discussed above.…”

Recent Progress in Two‐Dimensional Ferroelectric Materials

“…In 2018, Balke et al studied ionic conduction in CuInP 2 S 6 through local volume changes with the function of temperature and frequency. [96] The displacement induced by the driving field increases with an increasing temperature and drops with an increasing frequency, largely indicating that the ordering of the Cu ions in two off-center sites in the crystal lattice is the most possible explanation for CuInP 2 S 6 ferroelectricity. Although a systematic comparison measurement has been made in this article, the larger electromechanical response and higher contact resonance frequency contribute to a change in the topography, making the experimental results more complex to explain.…”

Ion adsorption-induced reversible polarization switching of a van der Waals layered ferroelectric

“…In contrast, the original downward polarization states remained unchanged. It has been reported that large crystallites can be formed at surface of CIPS when electrically switching ferroelectric domains 32,33 . In this work, we noted that no external electric eld was applied and no surface deformations were observed (see Supplementary Fig.…”

“…These unique characteristics not only bring new insights into the fundamental research 28,29 , but also provide new opportunities for diverse applications of layered ferroelectrics 30,31 . However, when electrically switching ferroelectric domains, the polar structure and ferroelectricity of CIPS can be readily damaged 32,33 due to its high ionic conductivity 34,35 . Because the ionic conductivity and polarization switching have common atomic origin-Cu atoms, it seems di cult to achieve a balance between these two processes.…”