Emergent Above-Gap Photoluminescence in Molecularly Engineered Hybrid Bilayer Crystals.

Abstract

Bilayer crystals, built by stacking two-dimensional (2D) covalent monolayers, give rise to coupled excitonic states whose properties are constrained by fixed lattice symmetry and orientation. Replacing one covalent monolayer with a 2D molecular crystal─held together by noncovalent forces─overcomes this limitation, as molecular functional groups afford tunable in-plane lattice geometry, intermolecular spacing, and interlayer coupling, providing a powerful knob for exciton engineering. Here, we report four-atom-thick hybrid bilayer crystals (HBCs) synthesized by directly growing single-crystalline PDI molecular crystal atop WS₂ monolayers, which exhibit a robust photoluminescence (PL) peak 120 meV above the WS₂ optical band gap alongside a below-gap emission. Both peaks display strong polarization anisotropy─nearing unity for the above-gap emission─and maintain a perfectly linear power-law dependence up to an excitation density of ∼10⁷ mW/cm², indicative of coexisting localized and delocalized excitonic states. Substituting PDI with a PTCDA monolayer on WS₂ fully quenches PL, demonstrating molecular control over excitonic emission. Lattice scale <i>ab initio</i> GW and Bethe-Salpeter equation (BSE) calculations reveal a significantly hybridized bilayer band structure in PDI/WS₂ that supports interlayer excitonic species both above and below the WS₂ gap with strong polarization anisotropy, in excellent agreement with experiment. Our work introduces a molecule-based bilayer platform for the bottom-up design and control of excitonic phenomena in atomically thin optoelectronic and quantum materials.