Postdoctoral Candidate, Franklin Liou: Controlling and Imaging Molecular Motion on the Surface of a Gate Tunable Graphene Device

The Department of Physics is pleased to welcome Dr. Franklin Liou for an in-person talk on Friday, February 17, 2023. Liou is a candidate who will be visiting Syracuse University as a candidate for one of the vacant postdoctoral researcher positions in the Physics Department. Having completed his B.S. in Physics at Stanford in 2015, Liou is a 2022 Physics PhD recipient of UC Berkeley. Franklin is an experimental condensed matter physicist with a knack for uncovering simple patterns in complex data, working in Mike Crommie’s group at UC Berkeley.

Abstract 

The ability to control nanoscale molecular motion with device-scale electric fields opens many exciting possibilities for nanotechnology. Collective motion of molecules can be used to assemble new nanostructures, induce mass transport, and transform device properties by surface modifications. A full understanding of such microscopic processes requires the development of new microscopy techniques that can image single adsorbate motion while simultaneously providing local electronic structure characterization. In this talk, I will present a recently developed technique to dynamically tune the surface density of single molecular adsorbates on a graphene field-effect transistor (FET) while imaging the surface with a scanning tunneling microscope. Application of a gate electric field causes F₄TCNQ molecules deposited on the device surface to reversibly transform between a charge-neutral self-assembled phase and a negatively-charged liquid phase. The changing surface molecular configuration also causes the conductivity of the device to exhibit Fermi level-pinning by molecular orbitals. In addition, stop-motion movies of the surface molecular configuration were created by applying short source-drain current pulses through the graphene FET, allowing diffusion and electromigration processes of individual molecules to be tracked. These phenomena reveal insights about how nanoscale molecular motion can be controlled by external electric fields, and how force and momentum are transmitted between electrons and adsorbates under non-equilibrium conditions.