Synthesis and Devices*

Copper indium thiophosphate (CuInP₂S₆) is a novel two-dimensional (2D) material with single crystal layered structure and shows decent stability in air. Furthermore, it performs clear switching from para-electricity to ferro-electricity with transition temperature around 320 K. Here, air-stable yellow powder of CuInP₂S₆ with single crystal embedded inside was synthesized. Raman spectroscopy and scanning electronic microscope (SEM) results confirm the high quality of our CuInP₂S₆ samples. Room-temperature ferro-electricity of 2D CuInP₂S₆ is studied by polarization-voltage measurement directly on a fabricated metal/CuInP₂S₆/metal capacitor. The ferroelectric capacitors were fabricated by mechanical exfoliation of thin layered films of CuInP₂S₆ onto Ni/SiO₂/p⁺⁺ Si substrate and deposition of metal electrodes on top of the flakes. The thickness dependent and temperature dependent characteristics of the ferroelectric CuInP₂S₆ devices are systemically studied. The realization of 2D ferroelectric films opens the way to study full 2D negative capacitance field-effect transistors (NC-FETs) or ferro-electric transistors (FE-FETs).