Resonance Raman study of linear carbon chains formed by the heat treatment of double-wall carbon nanotubes

Abstract

We study the resonance behavior of the unusual Raman feature known as the coalescence-inducing mode ͑CIM͒, observed at ϳ1850 cm −1 , in samples of double-wall carbon nanotubes annealed at high temperatures. Resonance Raman spectra taken with different laser energies show that the intensity of the CIM band exhibits a maximum around 2.20 eV. By comparing the resonance Raman experimental results with first-principles calculations for the vibrational frequency and the energy gap, we propose that the CIM feature is associated with short carbon chains with an odd number of atoms, interconnecting the nanotube surfaces. DOI: 10.1103/PhysRevB.73.193408 PACS number͑s͒: 78.30.Na, 63.22.ϩm, 78.20.Bh, 78.66.Tr The Raman spectra of carbon nanotubes exhibit weak features in the spectral range between 1600 and 2000 cm −1 that are ascribed to a second-order Raman process involving the combination of two phonons.1,2 However, the observation of unusual and strong spectral features around 1850 cm −1 have been reported recently in the Raman spectra of carbon nanotube systems, [3][4][5] and have been ascribed to the vibration of one-dimensional ͑1D͒ chains of carbon atoms. In particular, a strong and sharp feature at ϳ1850 cm −1 is observed in the Raman spectra of double-wall nanotube ͑DWNT͒ samples heat treated at high temperatures and, since it appears at specific annealing temperatures ͑T htt ͒ that occur just below the T htt needed for full coalescence of DWNTs, it is named the coalescence-inducing mode ͑CIM͒.To characterize this unusual Raman feature at ϳ1850 cm −1 , we report a complete resonance Raman study of this phonon band, using many different laser excitation energies ͑E laser ͒. The intensity dependence of the CIM band on E laser is here determined, and is shown to exhibit a maximum at around 2.2 eV. The experimental results are compared with theoretical calculations, using either density functional theory ͑DFT͒ or Hartree-Fock ͑HF͒ approaches, for finite carbon chains containing a small number of carbon atoms for which the normal mode frequencies and energy gaps were obtained. According to the calculations presented here, the vibrational frequencies and the resonance behavior obtained experimentally are in close agreement with the formation of short linear carbon chains during the coalescence process, with the chains having an odd number of carbon atoms fixed to the carbon nanotube walls under different environments ͑deformed and twisted chains͒.Room-temperature Raman spectra were recorded in the backscattering configuration using a Dilor XY triple monochromator, equipped with a cooled charge-coupled device ͑CCD͒ and using several different laser line excitations from an Ar-Kr ion laser and a dye laser in the range 1.9-2.7 eV. A laser power Ͻ1 mW was focused on a ϳ2 m 2 spot during the measurements.The samples studied here consist of highly purified DWNT bundles synthesized by a catalytic chemical vapor deposition method. 6,7 The diameter distribution of the sample is 0.77Յ d t Յ 0.90 nm for the inner tubes and 1.4...

Resonance Raman spectroscopy in one-dimensional carbon materials

“…The existence of a stable linear chain with more than 8 atoms seems unlikely for DWNTs during coalescence. Calculated results for the vibrational normal mode frequencies and energy gaps of linear chains with a small number of carbon atoms, with fixed and free ends have also been performed (Fantini et al 2006). Qualitative agreement with experimental results is obtained for C5 and C7 chains with fixed ends.…”

“…5. Figure 6(a) shows a detailed analysis of the CIM Raman spectra obtained with E laser = 2.20 eV (where the CIM feature shows its maximum intensity), from four different sample locations of the laser spot on the undoped specimen (T htt = 1500 • C) (Fantini et al 2006). We observed that the detailed line shape and the mean frequency of the CIM band is somewhat dependent on the position of the laser spot on the sample.…”

“…It is known that an infinite linear chain with one atom per unit cell (= C = C = C =) does not have a Raman active mode. Molecular dynamics calculations show that carbon chains containing 3, 4 and 5 atoms covalently linking the inner and the outer tubes can be formed (Fantini et al 2006). The existence of a stable linear chain with more than 8 atoms seems unlikely for DWNTs during coalescence.…”

“…Qualitative agreement with experimental results is obtained for C5 and C7 chains with fixed ends. Furthermore, small twists and bending of these small chains can change their vibrational frequencies by values up to 100-150 cm −1 (Fantini et al 2006). Based on these arguments, we propose that the CIM feature should be related to linear carbon chains with a small and odd number of carbon atoms.…”

“…To characterize this unusual Raman feature at ∼ 1850 cm −1 , we performed a detailed resonance Raman study of this phonon band, using many different laser excitation energies (E laser ) (Fantini et al 2006). Room temperature Raman spectra were recorded in the backscattering configuration using the Dilor XY triple monochromator, using several different laser line excitations from an Ar-Kr ion laser, and a dye laser in the range 1.9-2.7 eV.…”