Appl Phys A (2011) 102: 309–317 DOI 10.1007/s00339-010-5997-1 Vibrational mode assignments for bundled single-wall carbon nanotubes using Raman spectroscopy at different excitation energies Qiaohuan Cheng · Sourabhi Debnath · Elizabeth Gregan · Hugh J. Byrne Received: 11 January 2010 / Accepted: 27 July 2010 / Published online: 19 August 2010 © Springer-Verlag 2010 Abstract This study establishes a generic fitting approach to assignment of nanotube chiralities based on radial breath- ing mode frequencies (ω RBM ) of SWCNTs in as-produced bundles. Four laser lines with energies of 2.62 eV, 2.33 eV, 1.88 eV and 1.58 eV were employed. The observed RBM frequencies, ω RBM , were plotted as a function of the pos- sible diameters, d , as identified from the so-called Kataura plot and reported values of the parameters A and B , where ω RBM = A/d + B , assuming that SWCNTs resonant at the respective laser frequencies dominate the spectrum. The re- fined values of A and B , obtained by the best fit of a linear regression between ω RBM and 1/d , were found to vary sig- nificantly for different laser frequencies. This variation is in- terpreted in terms of the differences in electronic properties of SWCNTs resonant at different frequencies. The assigned nanotubes match well with those identified in the Kataura plot, falling within a resonant line width of ±0.2 eV of the respective laser lines. 1 Introduction Since the first publication on Raman scattering in single- walled carbon nanotubes (SWCNTs) in 1997 [1], resonance Q. Cheng ( ) · S. Debnath · E. Gregan · H.J. Byrne FOCAS Research Institute/School of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland e-mail: qiaohuan.cheng@dit.ie S. Debnath e-mail: sourabhi.debnath@dit.ie E. Gregan e-mail: elizabeth.gregan@dit.ie H.J. Byrne e-mail: hugh.byrne@dit.ie Raman spectroscopy has became one of the most impor- tant techniques in the characterisation of SWCNTs and their composites [2–6]. Compared to other techniques for SW- CNT structural assignment [6–10], Raman spectroscopy is the most convenient and rapid technique to determine the structural indices (n, m) of both semiconducting and metal- lic nanotubes [2, 11–13]. The electronic properties of a SWCNT are theoretically predicted to depend on its structural indices (n, m) [14]. When (n − m) mod 3 = 0, the tubes are metallic, otherwise semiconducting when (n − m) mod 3 = 1 or 2 [14]. The electronic character of a SWCNT can be identified by the lineshape of the tangential carbon stretching modes which constitute the G-band, the dominant feature in the Raman spectrum of a SWCNT, appearing at ∼1500–1600 cm −1 . Normally the G-band shows two features, the lower fre- quency component (G − ), and the higher frequency com- ponent (G + )[15]. The G + features of metallic and semi- conducting tubes are found to show no significant differ- ence in the frequency and width. But the line shape of the G − feature, while Lorentzian for semiconducting tubes, is found to be broadened and down shifted for metallic tubes and to have a characteristic Breit–Wigner–Fano (BWF) line- shape [15, 16]. Semiconducting and metallic nanotubes can also be distinguished by the intensity variation of their radial breathing modes (RBM) upon chemical or electrochemical doping [17, 18]. RBM features, arising from scattering by the radial breathing modes, are observed in the low frequency range ∼100–300 cm −1 . The frequency of a RBM (ω RBM ) sensi- tively depends on the diameter of the tube d [1] according to the following expression [3]: ω RBM = A/d + B, (1)