  The environmental aging effect of doped graphene is investigated as a
function of the organic doping species, humidity, and the number of graphene
layers adjacent to the dopant by studies of the Raman spectroscopy, x-ray and
ultraviolet photoelectron spectroscopy, scanning electron microscopy, infrared
spectroscopy, and electrical transport measurements. It is found that higher
humidity and structural defects induce faster degradation in doped graphene.
Detailed analysis of the spectroscopic data suggest that the physical origin of
the aging effect is associated with the continuing reaction of H2O molecules
with the hygroscopic organic dopants, which leads to formation of excess
chemical bonds, reduction in the doped graphene carrier density, and
proliferation of damages from the graphene grain boundaries. These
environmental aging effects are further shown to be significantly mitigated by
added graphene layers.
