title: Second-order overtone and combination Raman modes of graphene layers in the range of 1690-2150 cm(-1).
abstract: Though graphene has been intensively studied by Raman spectroscopy, in this letter, we report a study of the second-order overtone and combination Raman modes in a mostly unexplored frequency range of 1690-2150 cm(-1) in nonsuspended commensurate (AB-stacked), incommensurate (folded) and suspended graphene layers. On the basis of the double resonance theory, four dominant modes in this range have been assigned to (i) the second order out-of-plane transverse mode (2oTO or M band), (ii) the combinational modes of in-plane transverse acoustic mode and longitudinal optical mode (iTA+LO), (iii) in-plane transverse optical mode and longitudinal acoustic mode (iTO+LA), and (iv) longitudinal optical mode and longitudinal acoustic mode (LO+LA). Differing from AB-stacked bilayer graphene or few layer graphene, single layer graphene shows the disappearance of the M band. Systematic analysis reveals that interlayer interaction is essential for the presence (or absence) of the M band, whereas the substrate has no effect on the presence (or absence) of the M band. Dispersive behaviors of these "new" Raman modes in graphene have been probed by laser excitation energy-dependent Raman spectroscopy. It is found that the appearance of the M band strictly depends on the AB stacking, which could be used as a fingerprint for AB-stacked bilayer graphene. This work expands upon the unique and powerful abilities of Raman spectroscopy to study graphene and provides another effective way to probe phonon dispersion, electron-phonon coupling, and to exploit the electronic band structure of graphene layers.
authors.affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore.
authors.forename: Chunxiao
authors.initials: C
authors.lastname: Cong
authors.forename: Ting
authors.initials: T
authors.lastname: Yu
authors.forename: Riichiro
authors.initials: R
authors.lastname: Saito
authors.forename: Gene F
authors.initials: GF
authors.lastname: Dresselhaus
authors.forename: Mildred S
authors.initials: MS
authors.lastname: Dresselhaus
mesh_terms: D006108:Graphite / Q000737:chemistry*; D046911:Macromolecular Substances / Q000737:chemistry; D008422:Materials Testing; D008968:Molecular Conformation; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure*; D010316:Particle Size; D013059:Spectrum Analysis, Raman / Q000379:methods*; D013499:Surface Properties
chemical_list: D046911:Macromolecular Substances; D006108:Graphite