Fig. 3
Examples of possible energy schemes illustrating different types of resonances with van Hove singularities (vHSs) giving rise to 2D Raman band of SWCNTs. a Incoming and outgoing resonances with \( {\mathrm{E}}_{11}^{\mathrm{M}} \) vHSs for armchair (11,11) nanotube at E exc = 1.91 eV. b Incoming resonance with \( {\mathrm{E}}_{33}^{\mathrm{S}} \) vHSs for zigzag (20,0) nanotube at Eexc = 2.18 eV. c Both incoming and outgoing resonances corresponding with \( {\mathrm{E}}_{44}^{\mathrm{S}} \) vHSs and \( {\mathrm{E}}_{33}^{\mathrm{S}} \) vHSs for chiral (15,7) nanotube at Eexc = 2.41 eV. Horizontal lines with arrows show the width of the resonant window [5, 10]