Hidden polymorphic landscape in two-dimensional ZnO

Abstract

Two-dimensional ZnO has long been described by a graphitic honeycomb model, yet growing experimental evidence suggests a more complex structural landscape. Here, we use physics-constrained random structure search and first-principles calculations to explore the low-energy configurations of monolayer ZnO. We find that the planar graphitic phase is not the ground state. Instead, ZnO favors a double-layer honeycomb structure stabilized by interlayer Zn–O bonds, which appears graphitic in projection but is chemically distinct from a planar sheet. We further uncover a family of metastable triangular–tetrahedral polymorphs located 40–56 meV/atom above the ground state. These phases are dynamically stable and arise from coupled electrostatic and coordination disproportionation, which redistributes electrostatic repulsion and splits distorted tetrahedral sites into trigonal and tetrahedral building blocks. These findings recast polar non-van der Waals semiconductors as competing structural landscapes shaped by polarity, coordination, and electrostatics.