  Electronic devices using epitaxial graphene on Silicon Carbide require
encapsulation to avoid uncontrolled doping by impurities deposited in ambient
conditions. Additionally, interaction of the graphene monolayer with the
substrate causes relatively high level of electron doping in this material,
which is rather difficult to change by electrostatic gating alone.
  Here we describe one solution to these problems, allowing both encapsulation
and control of the carrier concentration in a wide range. We describe a novel
heterostructure based on epitaxial graphene grown on silicon carbide combined
with two polymers: a neutral spacer and a photoactive layer that provides
potent electron acceptors under UV light exposure. Unexposed, the same double
layer of polymers works well as capping material, improving the temporal
stability and uniformity of the doping level of the sample. By UV exposure of
this heterostructure we controlled electrical parameters of graphene in a
non-invasive, non-volatile, and reversible way, changing the carrier
concentration by a factor of 50. The electronic properties of the exposed SiC/
graphene/polymer heterostructures remained stable over many days at room
temperature, but heating the polymers above the glass transition reversed the
effect of light.
  The newly developed photochemical gating has already helped us to improve the
robustness (large range of quantizing magnetic field, substantially higher
opera- tion temperature and significantly enhanced signal-to-noise ratio due to
significantly increased breakdown current) of a graphene resistance standard to
such a level that it starts to compete favorably with mature semiconductor
heterostructure standards. [2,3]
