Imaging Reconfigurable Molecular Concentration on a Graphene Field-Effect Transistor

• Published in: Nano letters Vol. 21; no. 20; pp. 8770 - 8776
• Main Authors: Liou, Franklin, Tsai, Hsin-Zon, Aikawa, Andrew S, Natividad, Kyler C, Tang, Eric, Ha, Ethan, Riss, Alexander, Watanabe, Kenji, Taniguchi, Takashi, Lischner, Johannes, Zettl, Alex, Crommie, Michael F
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 27-10-2021
• Subjects: gate-tunable adsorbates; molecular electronics; Fermi level pinning; graphene field-effect device
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/acs.nanolett.1c03039

The spatial arrangement of adsorbates deposited onto a clean surface under vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors (FETs) exhibit reversible, electrically tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F4TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables the precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. Gate-tunable molecular concentration is explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. The molecular surface concentration is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.