Continuous Two-Dimensional Phase Transition of F₄TCNQ Molecules on Graphene

Two-dimensional melting has long been explored in colloidal systems, where direct imaging allows testing of the Kosterlitz–Thouless–Halperin–Nelson–Young (KTHNY) theory. Here we demonstrate an analogous, gate-tunable phase transition in a 2D device system: F₄TCNQ molecules adsorbed onto graphene. Using low-temperature scanning tunneling microscopy, we directly observe a continuous evolution from a disordered molecular liquid into a hexatic phase with quasi-long-range orientational order (no crystalline phase has yet been reached). By tuning the molecular density via the gate voltage of the graphene field-effect device substrate, we are able to reversibly transition between the liquid and hexatic phases. Kinetic Monte Carlo simulations reproduce the experimental trends and confirm that a screened Coulomb potential on graphene naturally yields a continuous liquid-to-hexatic transition consistent with KTHNY predictions. Our results establish charged molecular layers on graphene as a new, tunable platform for exploring Coulomb crystallization and defect-mediated phase transitions.