Imaging Field-Driven Melting of 2D Molecular Solids at the Atomic Scale

Phase transitions are collective phenomena, but originate from swift reconfigurations at the single-particle scale. Visually capturing these dynamics at the atomic-scale remains a challenge. Here we introduce a new technique for controlling and visualizing melting and freezing phase transitions of self-assembled 2D molecular structures on a graphene field-effect transistor using atomically-resolved scanning tunneling microscopy. The tunable back gate on our devices enables us to modify the molecular charge state at the device surface. Consequently, we are able to observe different molecular morphologies arising from electrostatically-induced phase transitions. Transient heating and cooling of our devices allow for the visualization of both equilibrium properties and non-equilibrium melting dynamics. By analyzing the local spectroscopy of each molecular phase, we gain insights into the molecular interactions that cause these phase transitions to occur. Our observed nonequilibrium melting dynamics are successfully modeled using Monte Carlo simulations.