  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.
