2D Molecular Crystals for Quantum Solids

Bilayer crystals, built by stacking monolayers of van der Waals (VdW) crystals, generate interlayer potentials that govern excitonic and quantum phenomena but are constrained by fixed covalent lattices and orientations. Replacing one layer with an atomically thin molecular crystal overcomes this limitation, as diverse functional groups enable tunable molecular lattices and interlayer potentials, tailoring a wide range of physical properties. In this talk, I will discuss recent progresses in the synthesis and novel properties of four-atom-thick hybrid bilayer crystals (HBCs), that are directly synthesized with single-crystalline perylene molecular crystal atop monolayers transition metal dichalcogenide (TMD) crystals [1]. These HBCs show, as demonstrated by our studies, remarkable properties, including fully polarized above gap photoluminescence and room-temperature charge localization.

First, the excitons in HBCs arise from a hybridized bilayer band structure, revealed by lattice-scale first-principles calculations, inheriting properties from both monolayers. This allows us to design a specific HBC that exhibits bright photoluminescence with near-unity polarization above and below the TMD bandgap, along with spectral signatures of exciton delocalization [2]. Second, we discovered room-temperature, switchable charge localization in high-quality HBC transistors. By using an ion gate, we selectively populated either localized molecular states or semiconductor band states, achieving complete localization from mobile charges at densities up to 3 × 10¹³ per square centimeter. This transition was energetically stabilized by the formation of coupled electron-ion dipoles [3]. Our work introduces a molecule-based 2D quantum materials platform for bottom-up design and control of optoelectronic and correlated electronic properties.