Adsorption-Induced Ferroelectric Symmetry Breaking in Two-Dimensional CuInP₂S₆

Two-dimensional (2D) ferroelectric (FE) materials offer unique opportunities for molecular sensing because their switchable polarization strongly couples surface chemistry with electronic response. Here, we use first-principles calculations to investigate the adsorption of representative organic molecules on monolayer CuInP₂S₆ (CIPS) and demonstrate how molecular interactions modulate FE polarization and near-surface electronic structure in monolayer CIPS. All investigated molecules exhibit thermodynamically favorable adsorption, revealing a robust molecule-surface interaction across diverse chemical functionalities. Adsorption-induced coordination, particularly through O-Cu and N-Cu interactions, drives local Cu displacement and breaks the intrinsic symmetry between FE states, generating pronounced molecule-dependent FE energy asymmetry of up to 282 meV together with out-of-plane polarization asymmetry reaching 0.54 μC/cm², as confirmed by Berry-phase polarization calculations. This asymmetry persists in the presence of a static interfacial water layer, indicating that adsorption-polarization coupling remains effective under realistic environmental conditions. In all investigated systems, the adsorption complexes remain semiconducting, indicating that molecular adsorption does not suppress the intrinsic semiconducting character of monolayer CIPS. Electronic structure analysis reveals FE-state-dependent electronic asymmetry and characteristic projected density of states (PDOS) signatures arising from Cu-molecule hybridization, providing experimentally accessible spectroscopic fingerprints. Relative energetics of representative Cu-displacement configurations suggest that polarization evolution proceeds through intermediate ferrielectric (FiE) and antiferroelectric-like (AFE-like) states without requiring a paraelectric (PE) intermediate, even under molecular adsorption. These results demonstrate that molecular adsorption can serve as an effective route for tuning ferroelectric polarization and near-surface electronic structure in 2D ferroic materials. Based on these findings, we propose a monolayer CIPS-based ferroelectric field-effect transistor (FET) architecture in which adsorption-induced polarization asymmetry may influence the local electronic response of the CIPS channel. This work establishes an atomistic framework for understanding adsorption-induced polarization asymmetry in 2D ferroelectrics and suggests potential implications for future ferroelectric sensing architectures.