Emergent Above-Gap Photoluminescence in Molecularly Engineered Hybrid Bilayer Crystals
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 crystalheld together by noncovalent forcesovercomes t...
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2025
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| Čoahkkáigeassu: | 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 crystalheld together by noncovalent forcesovercomes 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 WS<sub>2</sub> monolayers, which exhibit a robust photoluminescence (PL) peak 120 meV above the WS<sub>2</sub> optical band gap alongside a below-gap emission. Both peaks display strong polarization anisotropynearing unity for the above-gap emissionand maintain a perfectly linear power-law dependence up to an excitation density of ∼10<sup>7</sup> mW/cm<sup>2</sup>, indicative of coexisting localized and delocalized excitonic states. Substituting PDI with a PTCDA monolayer on WS<sub>2</sub> 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/WS<sub>2</sub> that supports interlayer excitonic species both above and below the WS<sub>2</sub> 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. |
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