April 1, 2021
ImagingDynamicalMolecularControlat theSurfaceof aGrapheneFET
Controlling the motion of adsorbates on surfaces is central to a wide range of applications including batteries, supercapacitors, ionic gated devices, and electrochemical sensors. Surface adsorbates can display complex collective behavior such as density fluctuations and electromigration when driven by electric fields in different directions. These phenomena do not only depend on electronic characteristics of the adsorbate system, such as orbital energy alignment relative to the electrode material and charge transfer, but also on inter-particle forces such as Coulomb repulsion and van der Waals attraction. Fuller understanding of such microscopic processes requires the development of new microscopy techniques that can image single adsorbates while simultaneously providing local electronic structure characterization. In this talk, I will present a newly developed technique to construct a gate-tunable array of charged molecules on a graphene field-effect transistor (FET). We have used scanning tunneling microscopy (STM) to achieve all-electrical control over the inter-particle density of F4TCNQ molecular arrays on graphene FETs in ultrahigh vacuum (UHV) at T = 4K. The resulting tunable surface arrays enable controlled charge doping and Fermi-level pinning of the graphene FET and provide a new method for determining molecular energy level alignment based on measuring molecular surface concentration. The gate-tunable molecular density is well-described by a simple capacitor model in which molecules each carry one electron of charge and dynamically rearrange themselves to screen back-gate-induced electric fields and reduce overall electronic energy. The molecules can also be driven directionally by electromigration forces into condensed phases that act as reservoirs to store the molecules, enabling full reversibility of the surface molecule distribution. This technique can be applied to dynamically construct charged impurity arrays on naonoelectronic devices by simple application of external gate voltages, creating new opportunities for exploring superlattice physics in 2D materials.
Franklin‘s work focuses on studying the electronic properties of molecular nanostructures in a field-effect transistor configuration. As the size of electronic devices grow smaller, control over their precise structure becomes ever more important for obtaining consistent electronic behavior. Bottom-up synthesis from well-defined, rigid molecular building blocks offers a way to assemble atomically precise electronic materials. Franklin is using scanning tunneling microscopy and spectroscopy to study bottom-up synthesized molecular nanostructures such as covalent organic frameworks (COFs) and graphene nanoribbons grown within a graphene nanogap, with the goal of correlating the local electronic and atomic structure to global electronic transport properties of the material.