Thickness-dependent vibrational and ferroelectric properties in CuInp₂S₆

Layered two-dimensional CuInP₂S₆ (CIPS) is a room-temperature ferroelectric material that is promising for a variety of electronics applications. While some previous studies have demonstrated the persistence of ferroelectricity down to few-layer thick flakes, others have reported a critical thickness below which ferroelectricity disappears or is severely diminished. These differences have been attributed to decreasing domain sizes, screening effects and even to a possible thickness-dependent structural phase transition. To uncover the origins of these discrepancies, we conducted a detailed thickness-dependent study on mechanically exfoliated flakes (ranging from 10 - 450 nm thick). We will present Raman spectral data collected at room temperature and across the ferroelectric phase transition temperature (T_C ~315 K in bulk CIPS), probing structural changes (through variations in peak frequencies and widths) with dimensionality as well as T_C. These data are complemented by piezoresponse force microscopy (PFM) measurements to reveal the effect of flake thickness on the ferroelectric domain sizes.