Imaging gate-induced molecular melting on a graphene field-effect transistor

TL;DR

This study introduces a novel microscopy technique to visualize and control phase transitions of molecular layers on graphene FETs, revealing detailed melting and freezing dynamics at the atomic level.

Contribution

The paper presents a new method combining electrical control and atomically-resolved microscopy to observe phase transition dynamics of molecular layers on graphene.

Findings

Reversible solid-liquid phase transitions induced by back-gate voltages.

Visualization of nonequilibrium melting dynamics via rapid electrical heating.

Development of an analytical model explaining mixed-phase states.

Abstract

Solid-liquid phase transitions are fundamental physical processes, but atomically-resolved microscopy has yet to capture both the solid and liquid dynamics for such a transition. We have developed a new technique for controlling the melting and freezing of 2D molecular layers on a graphene field-effect transistor (FET) that allows us to image phase transition dynamics via atomically-resolved scanning tunneling microscopy. Back-gate voltages applied to a F4TCNQ-decorated graphene FET induce reversible transitions between a charge-neutral solid phase and a negatively charged liquid phase. Nonequilibrium molecular melting dynamics are visualized by rapidly heating the graphene surface with electrical current and imaging the resulting evolution toward new equilibrium states. An analytical model has been developed that explains the observed equilibrium mixed-state phases based on spectroscopic measurement of both solid and liquid molecular energy levels. Observed non-equilibrium melting dynamics are consistent with Monte Carlo simulations.