Evolution of Physical and Electronic Structures of Bilayer Graphene upon Chemical Functionalization

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(22) and 2D(21) components is a distinct indicator of covalent functionalization, whereas the decrease in intensity of the 2D(11) and 2D(12) peaks corresponds to doping. Conductive AFM imaging shows physisorbed species from noncovalent functionalization can be removed by mechanical and electrical influence from the MM 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.