Gate-Switchable Molecular Diffusion on a Graphene Field-Effect Transistor

Controlling the surface diffusion of particles on 2D
devices creates
opportunities for advancing microscopic processes such as nanoassembly,
thin-film growth, and catalysis. Here, we demonstrate the ability
to control the diffusion of F4TCNQ molecules at the surface
of clean graphene field-effect transistors (FETs) via electrostatic
gating. Tuning the back-gate voltage (VG) of a graphene FET switches molecular adsorbates between negative
and neutral charge states, leading to dramatic changes in their diffusion
properties. Scanning tunneling microscopy measurements reveal that
the diffusivity of neutral molecules decreases rapidly with a decreasing VG and involves rotational diffusion processes.
The molecular diffusivity of negatively charged molecules, on the
other hand, remains nearly constant over a wide range of applied VG values and is dominated by purely translational
processes. First-principles density functional theory calculations
confirm that the energy landscapes experienced by neutral vs charged
molecules lead to diffusion behavior consistent with experiment. Gate-tunability
of the diffusion barrier for F4TCNQ molecules on graphene
enables graphene FETs to act as diffusion switches.