Abstract
Although it is known that the Raman spectroscopic signature of single-walled carbon nanotubes (SWCNTs) is highly chirality dependent, using Raman spectroscopy with several laser excitations as a tool for quantifying fraction of either metallic or semiconducting nanotubes in a sample has become a widely used analytical method. In this work, using the electron diffraction technique as a basis, we have examined the validity of Raman spectroscopy for quantitative evaluation of metallic fractions (M%) in single-walled carbon nanotube samples. Our results show that quantitative Raman spectroscopic evaluations of M% by using several discrete laser lines, either by using integrated intensities of chirality-associated radial breathing modes (RBMs) or, as has been more commonly utilized in recent studies, by statistically counting the numbers of RBMs can be misrepresentative. Specifically, we have found that the occurrence numbers of certain types of RBMs in Raman spectral mapping depend critically on the diameter distribution, resonant coupling between transition energies and excitation laser energy, and the chirality-dependent Raman scattering cross sections rather than simply on the metallic and semiconducting SWCNT fractions. These dependencies are similar to those observed in the integrated intensities of RBMs. Our findings substantially advance the understanding of the proper use of Raman spectroscopy for carbon nanotube quantification, which is important for carbon nanotube characterization and crucial to guide research in SWCNT growth and their applications.
Highlights
In this work, using the electron diffraction technique as a basis, we have examined the validity of Raman spectroscopy for quantitative evaluation of metallic fractions (M%) in single-walled carbon nanotube samples
Three single-walled carbon nanotubes (SWCNTs) samples were studied in this work, including two SWCNT samples synthesized by the floating catalyst-chemical vapor deposition (FC-CVD) methods,47,48 and a commercial RM8281 SWCNT reference material released by the U.S National Institute of Standards and Technology (NIST)
A commercial purified highpressure CO disproportion (HiPCO) SWCNT powder with a diameter distribution of 0.8−1.2 nm was used as a reference SWCNT sample, with an assumed M% of 36 (±4)%9,18−20,23 to calculate the M% of an unknown SWCNT sample using integrated radial breathing modes (RBMs) intensities
Summary
It is observed that the isolated individual S1 nanotubes or small bundles on substrate exhibit RBM peaks with much narrower peak widths compared to the RBMs measured from the network film sample (Figure 2), while the RBM frequencies are located in a very similar wavenumber range of 170−200 cm−1 due to the same dt distribution This range of RBMs corresponds to M-SWCNTs at 633 nm and SSWCNTs at 488, 514, and 785 nm excitations. To the M% values calculated from integrated RBM intensities, the statistical count of the number of RBM features in the M- and S-SWCNT regions of 256 micro-Raman spectra at each ELaser yields an extremely high M% of 95% at 633 nm but very low values of 3%, 5%, and 1% at 488, 514, and 785 nm lasers (Figure 5f), respectively. In practical measurements, the validation of this strategy for a (n,m)-distribution-unknown sample remains an open question
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