  Not only the apparent atomic arrangement but the charge distribution also
defines the crystalline symmetry that dictates the electronic and vibrational
structures. In this work, we report reversible and direction-controlled
chemical doping that modifies the inversion symmetry of AB-bilayer and
ABCtrilayer graphene. For the top-down and bottom-up hole injection into
graphene sheets, we employed molecular adsorption of electronegative I2 and
annealing-induced interfacial hole doping, respectively. The chemical breakdown
of the inversion symmetry led to the mixing of the G phonons, Raman active Eg
and Raman-inactive Eu modes, which was manifested as the two split G peaks, G-
and G+. The broken inversion symmetry could be recovered by removing the hole
dopants by simple rinsing or interfacial molecular replacement. Alternatively,
the symmetry could be regained by double-side charge injection, which
eliminated G- and formed an additional peak, Go, originating from the barely
doped interior layer. Chemical modification of crystalline symmetry as
demonstrated in the current study can be applied to other low dimensional
crystals in tuning their various material properties.
