Gate-Switchable\nMolecular Diffusion on a Graphene\nField-Effect Transistor

Controlling the surface diffusion of particles on 2D\ndevices creates\nopportunities for advancing microscopic processes such as nanoassembly,\nthin-film growth, and catalysis. Here, we demonstrate the ability\nto control the diffusion of F<sub>4</sub>TCNQ molecules at the surface\nof clean graphene field-effect transistors (FETs) via electrostatic\ngating. Tuning the back-gate voltage (<i>V</i><sub>G</sub>) of a graphene FET switches molecular adsorbates between negative\nand neutral charge states, leading to dramatic changes in their diffusion\nproperties. Scanning tunneling microscopy measurements reveal that\nthe diffusivity of neutral molecules decreases rapidly with a decreasing <i>V</i><sub>G</sub> and involves rotational diffusion processes.\nThe molecular diffusivity of negatively charged molecules, on the\nother hand, remains nearly constant over a wide range of applied <i>V</i><sub>G</sub> values and is dominated by purely translational\nprocesses. First-principles density functional theory calculations\nconfirm that the energy landscapes experienced by neutral vs charged\nmolecules lead to diffusion behavior consistent with experiment. Gate-tunability\nof the diffusion barrier for F<sub>4</sub>TCNQ molecules on graphene\nenables graphene FETs to act as diffusion switches.