Room-temperature ferroelectricity in CuInP2S6 ultrathin flakes

Results

Characterization of CuInP2S6

CIPS is one of the few layered compounds which exhibits room-temperature ferroelectricity11. The atomic structure of CIPS contains of a sulfur framework with the octahedral voids filled by the Cu, In and P–P triangular patterns. Bulk crystals are composed of vertically stacked, weakly interacting layers packed by van der Waals interactions (Fig. 1a,b)12. Owing to the site exchange between Cu and P–P pair from one layer to another, a complete unit cell consists of two adjacent monolayers to fully describe the material’s symmetry. It is a collinear two-sublattice ferrielectric system with Tc of about 315 K (ref. 11). When the temperature drops below Tc, due to the off-centre ordering in the Cu sublattice and the displacement of cations from the centrosymmetric positions in the In sublattice (symmetry changes from C2/c to Cc), spontaneous polarization emerges in the ferrielectric phase with polar axis normal to the layer plane13. For simplicity, CIPS will be referred as ferroelectric because ferrielectric materials exhibit the same macroscopic properties as ferroelectrics: namely, a spontaneous and switchable net electric polarization. To verify the ferroelectricity of the bulk sample, polarization versus out-of-plane electric field curve was measured on a 4-μm-thick CIPS flake using a commercial ferroelectric analyzer, where the clear hysteresis loop is direct evidence of ferroelectricity13. Details of the ferroelectric and dielectric measurements as well as the temperature dependence are shown inSupplementary Note 1 and Supplementary Figs 1 and 2.

Figure 1: Crystal structure and characterization of CIPS.

(a,b) The side view (a) and side view (b) for the crystal structure of CIPS with vdW gap between the layers. Within a layer, the Cu, In and P–P form separate triangular networks. The polarization direction is indicated in by the arrow. (b) The ferroelectric hysteresis loop of a 4-μm-thick CIPS flake. (c) AFM image of the CIPS flakes with different thicknesses. Scale bar, 2 μm. (d) The height profile along the line shown in c. Clear step height of 0.7 nm corresponding to single layer thickness of CIPS can be observed. L, Layers.