Evolution
of Physical and Electronic Structures of
Bilayer Graphene upon Chemical Functionalization
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Abstract
The
chemical behavior of bilayer graphene under strong covalent
and noncovalent functionalization is relatively unknown compared to
monolayer graphene, which has been far more widely studied. Bilayer
graphene is significantly less chemically reactive than monolayer
graphene, making it more challenging to study its chemistry in detail.
However, bilayer graphene is increasingly attractive for electronic
applications rather than monolayer graphene because of its electric-field-controllable
band gap, and there is a need for a greater understanding of its chemical
functionalization. In this paper, we study the covalent and noncovalent
functionalization of bilayer graphene using an electrochemical process
with aryl diazonium salts in the high conversion regime (D/G ratio
>1), and we use Raman spectroscopic mapping and conductive atomic
force microscopy (cAFM) to study the resulting changes in the physical
and electronic structures. Covalent functionalization at high chemical
conversion induces distinct changes in the Raman spectrum of bilayer
graphene including the broadening and shift in position of the split
G peak. Also, the D peak becomes active with four components. We report
for the first time that the broadening of the 2D<sub>22</sub> and
2D<sub>21</sub> components is a distinct indicator of covalent functionalization,
whereas the decrease in intensity of the 2D<sub>11</sub> and 2D<sub>12</sub> peaks corresponds to doping. Conductive AFM imaging shows
physisorbed species from noncovalent functionalization can be removed
by mechanical and electrical influence from the AFM tip, and that
changes in conductivity due to functionalization are inhomogeneous.
These results allow one to distinguish covalent from noncovalent chemistry
as a guide for further studies of the chemistry of bilayer graphene