Observation of Cu + Displacement Effect in the Ferro-Ionic Order and Disorder of a Multilayered CuInP2Se6

Abstract

Two-dimensional ferro-ionic materials belong to an emerging class of van der Waals systems with promising optoelectronics and memory functionalities. Here we present layered CuInP2Se6, a typical ferro-ionic semiconductor, whose optical transitions and charge transport reflect the structural changes associated with Cu+ ordering and local off-centering. Temperature-dependent Raman spectroscopy reveals pronounced symmetry breaking and in-plane vibrational anisotropy with Cu+ off-centering in a ferroelectric (FE) phase. Microthermoreflectance identifies an optical critical-point transition at 1.59 eV at 300 K, which evolves into resonances at 1.81, 1.91, and 2.05 eV at 20 K, accompanying the development of the low-temperature symmetry-broken state. In contrast, microphotoluminescence is dominated by a Cu+ defect-bound exciton at ∼1.35 eV and a band-edge emission of ∼1.81 eV at 20 K, evidencing a substantial reconstruction of the band edge by Cu+ displacement in a FE state. Electrical measurements exhibit hysteretic current switching behavior between the 120 and 220 K order–disorder crossover window, consistent with polarization-modulated transport in a ferro-ionic semiconductor. First-principles calculations reveal a double-well energy profile along the intralayer Cu-displacement coordinate and a Berry-phase polarization of 2.49 μC cm–2. For the relaxed low-temperature ferrielectric structure, GW+BSE calculations yield a dominant bright excitation at 1.81–1.84 eV, quantitatively supporting the A1 resonance at 1.81 eV, while the structural dependence of the band-edge states and the assignments of A2 and A3 remain qualitative. These findings establish CuInP2Se6 as a prototypical ferro-ionic semiconductor and provide fundamental insights into the interplay between Cu off-centering, band-edge reconstruction, and electronic transport across the order–disorder crossover.