  Tuning the charge carrier density of two-dimensional (2D) materials by
incorporating dopants into the crystal lattice is a challenging task. An
attractive alternative is the surface transfer doping by adsorption of
molecules on 2D crystals, which can lead to ordered molecular arrays. However,
such systems, demonstrated in ultra-high vacuum conditions (UHV), are often
unstable in ambient conditions. Here we show that air-stable doping of
epitaxial graphene on SiC - achieved by spin-coating deposition of
2,3,5,6-tetrafluoro-tetracyano-quino-dimethane (F4TCNQ) incorporated in poly
(methyl-methacrylate) - proceeds via the spontaneous accumulation of dopants at
the graphene-polymer interface and by the formation of a charge-transfer
complex that yields low-disorder, charge-neutral graphene with carrier
mobilities ~70,000 cm2/Vs at cryogenic temperatures. The assembly of dopants on
2D materials assisted by a polymer matrix, demonstrated by spin coating
wafer-scale substrates in ambient conditions, opens up a scalable technological
route towards expanding the functionality of 2D materials.
