Highly Diameter Selective C Enrichment in Carbon Nanotubes

We performed vapor phase filling of SWCNTs with various kinds of fullerenes. C60, C70, C enriched C60 and mixtures of these were used. The resulting peapods as well as the DWCNTs obtained by annealing were characterized with multi frequency Raman spectroscopy. Simultaneous filling with C60 and C70 was found to take place with the same efficiency. When using the C enriched C60 for DWCNT synthesis, mode softening for all inner shell lines was observed, while the outer shell response remained unchanged. The yield of inner shell CNTs is almost the same for C60 and C70 except that C70 is very inefficient in forming mid diameter inner CNTs with RBM frequencies close to 365 cm. If inner CNTs are grown from mixed peapods of C70 and C enriched C60 these particular CNTs are mainly built from C60. This is confirmed by the observation of distinct line shifts in the inner shell RBM. INTRODUCTION Raman spectroscopy is a convenient and widely used technique in the investigation of different nano structured phases of carbon. Especially multi frequency Raman spectroscopy was demonstrated to be applicable for the observation of distinct inner shell CNTs in DWCNTs [1]. So far this was demonstrated for the strongly diameter dependent RBM, whereas the high frequency modes are on top of each other. By using C enriched fullerenes and turning the obtained peapods into DWCNTs all inner shell modes are down shifted. This technique allows a clear separation of the outer and inner shell response throughout the whole spectrum. Furthermore, C marked fullerenes in combination with Raman spectroscopy can be used to monitor the diameter dependency of the filling efficiency for different fullerenes in a mixture of these. This contribution is focused on an observed unusual behavior of the diameter dependence of the filling efficiency for C70. This anomaly can be exploited to prepare very special DWCNT samples. The outer shell CNTs as well as any other carbonic impurities have the natural abundance (~1%) of C. Only the inner shell CNTs are C enriched. This enrichment is the same for almost all inner tubes, but in a very narrow diameter range corresponding to 1 365 − = cm RBM ν it is twice as high compared to all other very thin CNTs. The general spectroscopic benefit of C enriched inner shell CNTs in the high frequency domain (D,G and D* Band) is reported in detail later [2] CP723, Electronic Properties of Synthetic Nanostructures, edited by H. Kuzmany et al. © 2004 American Institute of Physics 0-7354-0204-3/04/$22.00 251 Downloaded 28 Nov 2007 to 131.130.1.18. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp EXPERIMENTAL Peapods were obtained through the vapor phase filling method. Fullerenes and SWCNTs material purified by oxidation and prepared as bucky paper were sealed in an evacuated quartz ampoule and heated at 650 °C for 2h. Excess fullerenes were removed by heating the peapods at 800 °C for 30 min in dynamic vacuum. Transformation to DWCNTs was performed by 1 h annealing at 1270 °C in dynamic vacuum. Raman spectra were recorded with a xy triple monochromator spectrometer from Dilor. The 180° backscattered light was analyzed. An ArKr laser was used for excitation at different laser lines. RESULTS AND DISCUSSION The above described vapor filling method for preparation of peapods works for either C60 or C70. Figure 1 depicts Raman spectra of C60 and C70 peapods at 488 nm. In both cases the characteristic lines of the fullerenes are observed together with the Dband of the hosting SWCNTs. If a 50%:50% mass ratio mixture of C60 and C70 is subject to peapod preparation a superposition of the Raman spectra from Fig. 1 is observed, as shown in Fig. 2a. As expected this spectrum can be simulated by superimposing the two spectra of Fig. 1. Best agreement for this simulation is obtained for a 45% to 55% ratio of C70 and C60. The resulting spectrum is shown in Fig. 2b. 115