High-temperature evolution of linear carbon chains inside multiwalled nanotubes E. Cazzanelli, 1,2, * M. Castriota, 1,2 L. S. Caputi, 1,3 A. Cupolillo, 1,3 C. Giallombardo, 1,3 and L. Papagno 1,3 1 Dipartimento di Fisica, Università della Calabria, 87036-Arcavacata di Rende, Cosenza, Italy 2 LYCRIL (CNR-INFM) and CEMIF.CAL, 87036-Arcavacata di Rende, Cosenza, Italy 3 INFN, Gruppo collegato di Cosenza, 87036-Arcavacata di Rende, Cosenza, Italy Received 8 February 2007; published 23 March 2007 Multiwalled carbon nanotubes have been deposited on graphite cathodes by the arc discharge technique in a He atmosphere. A micro-Raman mapping, performed after scanning electron microscopy, reveals, in some regions of the sample, strong bands in the 1780– 1860 cm -1 range due to linear carbon chains inside the nanotubes. A temperature-dependent Raman study, up to about 1000 K, shows a strong but reversible decrease in their intensity. This phase transition behavior is explained as a temperature-driven reduction of the Peierls distortion of the linear carbon chain. DOI: 10.1103/PhysRevB.75.121405 PACS numbers: 81.07.De, 63.22.+m, 68.37.Hk A linear carbon chain with pure sp hybridization carbyne molecule can be considered another allotropic form of car- bon. Two possible bond configurations 1 have been proposed: cumulene only double bonds and polyyne alternating single and triple bonds, but their real existence has been debated. 2–7 Recently, Raman bands in the range 1900–2200 cm -1 , assigned to carbon chains, 8 have been ob- served in carbonaceous materials grown inside an UHV clus- ter beam apparatus, but their time evolution under exposure to various gases indicates short lifetimes order of hours. 9 A greater stability for carbon linear chains can be ex- pected inside a protected environment, like a carbon nano- tube, single-walled SWCNT or multiwalled MWCNT. The existence of such a system is demonstrated by images obtained by high-resolution transmission electron micros- copy of a black line inside the innermost tube of a MWCNT. 10 Other evidence is provided by the remarkable Raman bands in the wave number range 1780– 1860 cm -1 , found for a MWCNT obtained by arc discharge in hydrogen. 11–14 These peculiar bands labeled L bands in the following, not assignable to any sp 2 or sp 3 carbon form, have been associated with the vibrations of sp linear chains of carbon atoms inside the MWCNT C@MWCNT in the following. The stability of such complexes is very high, in fact the L band Raman signal remains almost unchanged even after heating up to 1400 ° C. 15 An extraordinary average length of the chains, about 100 atoms, has been postulated 11–14 to explain the frequency downshift of L bands with respect to those of the usual carbyne. 8,9 It is consistent with a rough extrapolation to high carbon numbers for the vibrational frequencies of oligoynes, 16 as well as for those of polyyne isolated chains. 17 A different interpretation of the L Raman bands, based on first-principles calculations, has been recently proposed: 18 short transverse carbon chains join con- centric nanotubes in MWCNTs or connect two inner tubes surrounded by binocularlike outer carbon tubes, but it seems unsupported by strong experimental evidence. In this work we present additional evidence of L Raman bands for MWCNT samples grown in a He atmosphere and report the temperature dependence of such bands up to about 1000 K revealing a reversible phase transition behavior. A simple carbon arc method was used to produce MWCNT samples. The experimental chamber was filled with high-purity helium gas at 500 mbar pressure. Pure graphite electrodes with 10 mm cathode and 3 mm anode diameter were used. The cathode temperature was stabilized by a liquid nitrogen flux, while a direct current of 60 A, driven by 25 V, was used to create a high-temperature dis- charge. The discharge resulted in the formation of a small rod-shaped deposit on the cathode as shown in scanning electron microscope SEM images Figs. 1 and 2 obtained by a Cambridge Instruments SEM Stereoscan 360. Raman measurements were carried out by a microprobe setup Horiba-Jobin-Yvon, model Labram equipped with a charge-coupled device detector and a He-Ne laser 632.8 nm emission, corresponding to 1.96 eV. The final spectral reso- lution is about 1 cm -1 . The laser power out of the objective, a 50 Olympus, was about 5 mW, while the diameter of the focused laser spot was about 3 – 4 m, giving an estimated irradiance of the order of 50 kW/cm 2 . By using neutral fil- ters of different optical density OD, this irradiance can be reduced: to 50% for the OD0.3 filter, 25% for OD0.6, 10% for OD1, and 1% for OD2. In the temperature-dependent Raman measurements, sequences of spectra were collected FIG. 1. SEM total view of MWCNT deposited on graphite cath- ode by arc discharge. The diameter of the circular deposited zone is about 3 mm. PHYSICAL REVIEW B 75, 121405R2007 RAPID COMMUNICATIONS 1098-0121/2007/7512/1214054 ©2007 The American Physical Society 121405-1