Electronic Tuning of 2D Materials with Zwitterionic and Functional Polymers*

Functional polymers are increasingly recognized for their ability to engineer the electronic properties of 2D materials for device performance enhancement. Although dipole-rich polymer zwitterions have shown significant work function modulation of 2D materials, the contribution of zwitterion structure is not well understood. To this end, a series of zwitterionic sulfobetaine-based random copolymers with varying substituents has been prepared and applied as negative-tone resists on graphene, enabling evaluation of surface potential contrast. The influence of steric footprint on calculated dipole moment and resulting work function measured by ultraviolet photoelectron spectroscopy will be presented. To assess the nature of graphene surface doping by polymer zwitterions, a sample geometry that permits direct access to either the polymer or graphene side is employed using a zwitterionic phosphorylcholine-based polymer coating on graphene. By Kelvin probe and electrostatic force microscopies, a significant upshift in the Fermi level of graphene has been observed from a local field effect rather than pure charge transfer. Overall, understanding how surface dipole orientation impacts the electronics of 2D materials kindles exciting possibilities for precise tuning of device performance.