Compositional design rules for tuning functionalities in CuInP2X6 (X = S, Se) van der Waals semiconductor ferroelectrics

Abstract

Two-dimensional van der Waals (2D-vdW) semiconducting ferroelectrics, such as CuInP₂Se₆ (CIPSe) and CuInP₂S₆ (CIPS), offer unique opportunities for lightweight, scalable, low-power nanoscale electronic devices. However, the limited pool of functional 2D-vdW ferroics highlights the need for clear design principles that can be used to guide experiments. Here, we use first-principles density functional theory (DFT) to study how isovalent atomistic substitution at In and P sites modifies structure, polarization, and electronic properties in CIPSe and CIPS. When substituting In with Sb and Bi, and P with As and Sb, we reveal how ionic radius mismatch, electronegativity differences, and stereochemical lone pair activity shape the ferroelectric and semiconducting response in both rigid (S-based) and soft (Se-based) 2D lattices. In CIPSe, Bi doping at In sites widens the band gap to ∼1.07 eV without reducing polarization or switching performance, provided the Bi atoms are placed in a balanced zigzag arrangement that limits local strain. In CIPS, polarization values drop more noticeably than in CIPSe, and dopant location has a stronger effect on switching behavior. P-site substitution leads to stronger distortions in CIPSe, where larger dopants destabilize the P–P dimer network and in many instances, we predict it to be a metal. However, with As doping in CIPS, polarization remains stable, and the band gap decreases without major structural disruption. Our results establish structure–property design rules based on dopant size and location, host lattice stiffness, and chemical compatibility, offering a route to engineer new 2D ferroelectrics in which polarization and band gap can be tuned together through specific compositional changes.