Effects of Layer Stacking on the Combination Raman Modes in Graphene

We have observed <i>new</i> combination modes in the range from 1650 to 2300 cm<sup>−1</sup> in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. A peak at ∼1860 cm<sup>−1</sup> (iTALO<sup>−</sup>) 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<sup>−1</sup>, also called the M band, is suppressed for both SLG and IBLG. In addition, two previously unidentified modes at ∼2200 and ∼1880 cm<sup>−1</sup> are observed in SLG. The 2220 cm<sup>−1</sup> (1880 cm<sup>−1</sup>) 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<sup>−1</sup> mode is observed to increase (decrease) linearly with increasing graphene layers.