¹³C-NMR Shift of Highly Concentrated Metallic and Semiconducting Single-Walled Carbon Nanotubes

Abstract

Single-walled carbon nanotubes (SWCNTs) have novel electronic properties originating from their unique onedimensional (1D) structures. 1) Their electronic properties are well correlated with the geometrical structure represented by the chiral index ðn; mÞ. Such global features were confirmed using several experimental techniques, such as scanning tunneling microscopy, 2,3) optical absorption, 4) and photoemission spectroscopy. 5) The 13 C-NMR technique has also been applied to reveal the unusual electronic properties of SWCNTs. 6-11) However, to our knowledge, NMR observations have been limited to the SWCNT samples mixed with both metallic and semiconducting SWCNTs. This substantially hampered the study of the individual properties of SWCNTs, which sensitively depend on the detailed electronic structure. One such interesting property is the magnetic response or orbital susceptibility as in other carbon network systems. 12,13) In this connection, we report here the first determination of a 13 C-NMR shift tensor, which is closely related to the orbital susceptibility in metallic and semiconducting SWCNTs.The highly concentrated metallic and semiconducting SWCNTs (more than $95%) with a mean diameter of 1.45 nm were prepared by the method described in Ref. 14; the extractions of metallic and semiconducting SWCNTs from pristine SWCNTs, the Arc-SO SWCNT soot (Meijo Nano-Carbon), were accomplished by the density gradient ultracentrifugation (DGU) technique. Each sample was characterized on the basis of photoabsorption (Shimadzu UV-3600), electrical resistance, 15) and powder X-ray diffraction (XRD) at the BL8B station in the Photon Factory facility, KEK, Japan (see Fig. 1). Note that there is no trace of graphitized carbon and ferromagnetic impurity particles in the XRD profiles. This is consistent with the previous magnetization measurement (see Fig. S.3. in Ref. 15).We observed the 13 C isotope of natural abundance (1.1%). Each sample ($18 mg) was sealed in a quartz tube with helium of $1 atm after sufficient evacuation at 800 K. For semiconducting SWCNTs, two samples of different batches, #1 and #2, were examined. The NMR spectra were obtained by Fourier transformation of the latter half of the spin-echo signal that appeared after the =2-pulse sequence, using a phase-coherent pulsed spectrometer. The =2 pulse width was 6.0 s. The radiated rf field frequency was varied over a range of the NMR spectral width of about 20 kHz to examine spectral distortion effects due to the finite pulse widths. The 13 C-NMR shift was determined with respect to the 13 C resonance in tetramethylsilane (TMS).Figure 2 presents 13 C-NMR spectra of the metallic and semiconducting SWCNTs, along with that of solid C 60 obtained at 4.2 K, where the rotational motion of C 60 completely freezes. Note that the spectrum of solid C 60 is reproduced by a reported shift tensor. 16) Then, we find that the spectra of the SWCNTs have the same characteristic features as those for aromatic carbons having sp 2 hybridization. 7,[17][18][19] The 33 axis sho...

Intertube effects on one-dimensional correlated state of metallic single-wall carbon nanotubes probed by C13 NMR

“…Figure 1 shows a 13 C NMR spectrum of the 13 Cenriched e-DIPS sample, which consists of bundled mixtures of metallic and semiconducting SWCNTs. In contrast to the usual 13 C NMR spectrum observed for our unenriched metallic SWCNT sample, 33 the spectrum in Fig. 1 forms a doublet.…”