Thickness dependent studies of ferroelectricity and crystal structure of CuInP₂S₆

Van der Waals material CuInP₂S₆ (CIPS) has attracted great attention due to its ferroelectricity at room temperature and the potential applications in data storage devices and photovoltaic effect. Its ferroelectricity is due to the off-center ordering in the Cu sublattice and the displacement of cations from the centrosymmetric positions in the In sublattice, therefore, the competition between the surface energy, depolarization field, and interfacial bonding in thin-film CIPS may lead to modification of ferroelectricity and crystal structure. However, the change of topography and interface condition in thin-film samples make it difficult to interpret the experimental results. In this work, we investigate the thickness dependence of ferroelectricity and crystal structure in CIPS using nonlinear optics and low-energy electron microscopy. Our results demonstrate a structural phase transition when the film thickness reaches below ~100 nm.