  Unconventional magneto-transport fingerprints in the quantum Hall regime
(with applied magnetic field from one to several tens of Tesla) in chemically
functionalized graphene are reported. Upon chemical adsorption of monoatomic
oxygen (from 0.5% to few percents), the electron-hole symmetry of Landau levels
is broken, while a double-peaked conductivity develops at low-energy, resulting
from the formation of critical states conveyed by the random network of
defects-induced impurity states. Scaling analysis hints towards the existence
of an additional zero-energy quantized Hall conductance plateau, which is here
not connected to degeneracy lifting of Landau levels by sublattice symmetry
breakage. This singularly contrasts with usual interpretation, and unveils a
new playground for tailoring the fundamental characteristics of the quantum
Hall effect.
