title: Effects of layer stacking on the combination Raman modes in graphene.
abstract: We have observed new combination modes in the range from 1650 to 2300 cm(-1) in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. A peak at ∼1860 cm(-1) (iTALO-) is observed due to a combination of the in-plane transverse acoustic (iTA) and the longitudinal optical (LO) phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent for bulk graphite. The overtone of the out-of-plane transverse optical (oTO) phonon at ∼1750 cm(-1), also called the M band, is suppressed for both SLG and IBLG. In addition, two previously unidentified modes at ∼2200 and ∼1880 cm(-1) are observed in SLG. The 2220 cm(-1) (1880 cm(-1)) mode is tentatively assigned to the combination mode of in-plane transverse optical (iTO) and TA phonons (oTO+LO phonons) around the K point in the graphene Brillouin zone. Finally, the peak frequency of the 1880 (2220) cm(-1) mode is observed to increase (decrease) linearly with increasing graphene layers.
authors.affiliation: Materials and Manufacturing Directorate, Air Force Research Laboratory, WPAFB, Ohio 45433, United States. rahul.rao@wpafb.af.mil
authors.forename: Rahul
authors.initials: R
authors.lastname: Rao
authors.forename: Ramakrishna
authors.initials: R
authors.lastname: Podila
authors.forename: Ryuichi
authors.initials: R
authors.lastname: Tsuchikawa
authors.forename: Jyoti
authors.initials: J
authors.lastname: Katoch
authors.forename: Derek
authors.initials: D
authors.lastname: Tishler
authors.forename: Apparao M
authors.initials: AM
authors.lastname: Rao
authors.forename: Masa
authors.initials: M
authors.lastname: Ishigami
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; D012822:Silicon Dioxide / Q000737:chemistry*; D013059:Spectrum Analysis, Raman / Q000379:methods*; D013499:Surface Properties
chemical_list: D046911:Macromolecular Substances; D012822:Silicon Dioxide; D006108:Graphite