Thickness-dependent in-plane polarization and structural phase transition in van der Waals Ferroelectric CuInP2S6

TL;DR

This study reveals that in CuInP2S6 van der Waals ferroelectric crystals, in-plane polarization diminishes below a critical thickness due to a structural phase transition, impacting their physical properties and potential applications.

Contribution

It uncovers the thickness-dependent in-plane polarization and structural phase transition in CuInP2S6, combining experimental and theoretical methods to elucidate the underlying mechanisms.

Findings

In-plane polarization disappears below 90-100 nm thickness.

Structural phase transition from monoclinic to trigonal occurs at critical thickness.

Young's modulus shows an abrupt change at the critical thickness.

Abstract

Van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intra-layer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel physical properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high-density storage devices. As the thickness going thinner, the competition between the surface energy, depolarization field and interfacial chemical bonds may give rise to the modification of ferroelectricity and crystalline structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, we report the existence of the intrinsic in-plane polarization in vdW ferroelectrics CuInP2S6 (CIPS) single crystals, whereas below a critical thickness between 90-100 nm, the in-plane polarization disappears. The Young's modulus also shows an abrupt stiffness at the critical thickness. Based on the density functional theory calculations, we ascribe these behaviors to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. Taken together, these findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelectrics.