Annealing effect on carbon nanotubes. An ESR study

3 February 1995

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical PhysicsLetters 233 (1995) 47-51

Annealing effect on carbon nanotubes. An ESR study Mayumi Kosaka a, Thomas W. Ebbesen a,b, Hidefumi Hiura a, Katsumi Tanigaki a a Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305, Japan b NEC Research Institute, 4 Independence way, Princeton, NJ 08540, USA

Received 31 October 1994

Abstract The effect of annealing on the electronic properties ofnanotubes has been studied. The conduction ESR intensity is temperatureindependent for both annealed and non-annealed nanotubes. However, the g-value of the nanotubes and its temperature dependence change significantly after annealing. These results suggest that some types of defects are present in the nanotubes which are then removed by the annealing process. The ESR studies of the annealed nanotubes indicate that the intrinsic electronic properties of nanotubes are quite different from those of graphite.

1. Introduction We recently reported the observation of conduction electron spin resonance, CESR, in carbon nanotubes [1] using samples which were purified by the solidgas phase oxidation method [ 2]. The CESR is evidence of the existence of metallic and/or narrow gap semiconducting nanotubes [ 1 ] as predicted by theory [ 3 8]. Furthermore, the ESR results for the nanotubes were similar to those reported for graphite. Although we did not have definite evidence for the presence of defects, the observed temperature dependence of the gvalue, which is similar to that of graphite, is difficult to interpret if the nanotubes are defect-free [ 1 ]. The possibility that nanotubes may have some kind of defect was pointed out from recent pressure and intercalation experiments [9]. It was suggested that nanotubes are not formed of closed shells but instead composed of pieces of graphitic domains stuck together in a paper-mache model. While this suggestion is reasonable considering the reported observations, it is in contradiction with oxidation results for nanotubes [ 10,11]. The latter results show that nanotubes are 0009-2614/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSD10009-2614(94)01416-7

preferentially oxidized from the tip inwards, layer by layer. If the paper-mache model is correct, one would expect oxidation to start at the edges of the graphitic domains along the tube. The comparison of results between different groups is further complicated by the fact that the quality of nanotube samples varies significantly from one carbon arc machine to another depending on their design [ 12]. It is well known that carbon materials can be graphitized and the amount of defects can be decreased by annealing at around 2800°C [ 13]. In this Letter, we report the effect of such annealing on nanotubes which clearly suggest the presence of defects. Various possibilities are discussed which would be consistent with the different experimental results.

2. Experimental Crude carbon nanotubes were produced by the arcdischarge method [ 14]. To purify the nanotubes, we oxidized the crude nanotubes in pure oxygen until 0.25% of the weight remained. Since the oxidation

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M. Kosaka et al. / Chemical Physics Letters 233 (1995) 47-51

reaction rate of nanoparticles is faster than that of nanotubes, finally only open nanotubes remain [2]. The purified nanotubes and the crude nanotubes were annealed at 2850°C in an evacuated furnace for 30 rain. ESR was measured with a JEOL JES-RE2X electron spin resonance spectrometer operating in the X-band from 4 to 300 K. Mn 2+ was used as a reference marker of ESR intensity, linewidth and g-values. The TEMPOL standard was used to determine the spin concentration.

3. Results and discussion

ESR spectra for the annealed purified carbon nanotubes and the annealed crude nanotubes-nanoparticles samples were measured both at room temperature (296 K) and at low temperature (4 K), as shown in Fig. 1. For the annealed purified tubes, only one peak, A*, was observed from 4 to 296 K (Fig. la). The g-value of this peak is 2.0022 at 296 K. In the annealed crude tubes, only peak A* was observed at room temperature, while two peaks, A* and D, were observed at low temperature (Fig. lb). The g-value of peak A* is 2.0021 and that of peak D is 1.988. Both of the observed A* peaks, for the annealed purified nanotubes and the annealed crude tubes, have almost the same g-values. Moreover, the temperature dependence of their ESR intensities reflect Pauli-like behavior, as described in the following paragraph. The relative spin density of a)

peak A* for annealed purified nanotubes is about twice that for annealed crude tubes. These results indicate that peak A* corresponds to conduction electrons of annealed nanotubes and the initial ratio of the nanotubes to the crude tubes is about 1 : 1. Peak D is not observed for purified nanotubes, and its intensity shows Curie-like dependence on temperature. Therefore, peak D is thought to be related to defects in the nanoparticles or amorphous carbon. The g-values of peak A* for the annealed purified tubes and the annealed crude tubes (both are = 2.0022) shift greatly from that of the non-annealed tubes. The g-values for the non-annealed purified and crude tubes were measured to be 2.012 and 2.019, respectively, for the peak (A) corresponding to the conduction electrons [ 1 ]. The g-value shift implies that the electronic properties of nanotubes are changed by annealing. The temperature dependences of the g-values of peak A (non-annealed purified tubes) and of peak A* (annealed purified tubes) are shown in Fig. 2. Notice that the g-value of peak A decreases with increasing temperature over most of the temperature range. The inverse temperature dependence in low-temperature regions can be explained by the presence of localized spins (for instance, dangling bonds at the tips of the nanotubes) interacting with the conduction electrons [ 15]. The average g-value of polycrystalline graphite can be expressed as g = -~(gll + 2 g ± ) [16]. Here, g~_ does not depend on temperature and is equal to 2.0026+0.0002. While the g-value anisotropy is b) 296K

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M. Kosaka et al. / Chemical Physics Letters 233 (1995) 47-51 2.03

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gll - g ± = 0.047 at room temperature in pure graphite, it increases with decreasing temperature. The average g-value of graphite powder is expected to be close to 2.018 at room temperature. The observed g-value region for non-annealed nanotubes and the overall temperature dependence are quite similar to graphite powder. This temperature dependence of peak A cannot easily be explained if the nanotubes were perfect as discussed further down. In contrast to non-annealed nanotubes, the g-value of the annealed ones is nearly constant irrespective of temperature and is less than 2.0022 below room temperature. This value, which is close to that of free electrons, and its temperature dependence are different from graphite. In graphite, only glb (c axis) depends on temperature [ 16]. This is explained by changes in the interlayer interaction in graphite which is strongly dependent on temperature [ 17]. The observed temperature independence of the g-value for the annealed nanotubes is easily explained if the nanotubes are perfect closed cylindrical structures since then, unlike graphite, the interlayer spacing should remain relatively constant. The difference in temperature dependences of the gvalues was also observed for crude nanotubes. The gvalue of peak A (non-annealed crude tubes) strongly depends on temperature while the g-value of peak A* (annealed crude tubes) remains nearly constant. The tips of purified nanotubes are open [ 2,11 ] and remain so after annealing. On the other hand, the tips of crude tubes are closed. Therefore the same g-value

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behavior for annealed purified tubes and annealed crude tubes implies that the tip structure is not a critical factor in the overall electronic properties of the nanotubes. This is perhaps not surprising considering the large aspect ratio of the nanotubes. The difference in g-value of the nanotubes before and after annealing must be due to structural changes in the cylindrical part of the nanotubes, such as loss of defects by the annealing process. As mentioned in Section l, high-pressure experiments and intercalation studies indicate that the structure of nanotubes and nanoparticles in crude nanotubes samples are far from being perfectly closed structures [ 9 ]. It was proposed that nanotubes are better described as being composed of discontinuous graphitic sheets in a paper-mache model [9]. Transmission electron studies have shown that nanoparticles are indeed sometimes made of rolled-up graphitic sheets [ 18]. On the other hand, oxidation experiments show that nanotubes are preferentially oxidized from the tip inwards, layer by layer [ 10,11 ]. If the paper-mache model was the dominant structure, one would expect oxidation to start at all the edges of the discontinuous structure, not just at the tips. Furthermore, Raman studies indicate that the graphitic domain is large [ 19]. In summary, we have a set of experiments, including these ESR results, which indicate the presence of defects, and on the other hand, the paper-mache model appears to be in contradiction with other results. All this can perhaps be understood by considering two important points: (i) quality of the samples and (ii) the variety of defects that can be present in nanotubes. (i) The comparison of results is complicated by the fact that the quality of crude nanotube samples vary from group to group depending on the design of the machine used [12]. Some samples contain mostly nanoparticles and fused material specially if the carbon arc is not properly cooled [ 12]. Furthermore, it is not yet known how the quality of individual nanotubes is affected by the conditions of their production. (ii) Nanotubes may contain a variety of defects just like graphite sheets. One possibility is the open edge of a graphitic domain as suggested in the paper-mache model [9]. Another set of defects are topological imperfections such as pentagon-heptagon pairs. These are clearly present in greater number than one might have expected [20]. There are others including heptagon-triangle pairs that have been observed inside the

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M. Kosaka et al. / Chemical Physics Letters 233 (1995) 47-51

hexagonal lattice of graphite [21,22]. Scanning tunneling microscopy experiments have revealed that atoms are sometimes missing in the graphitic sheet [23]. Thus the difficulty in reconciling the various observations lies in the fact that each crude sample contains not only a particular distribution of nanotubes and nanoparticles but also a particular distribution in the type of defects present. Only when we have a pure welldefined sample of nanotubes, will we perhaps be able to say something on the nature of the defects. Most importantly, the observation here that the nanotubes are strongly affected by the annealing shows that we will not be able to compare measured electronic properties with calculations using crude samples of nanotubes. The temperature dependence of the intensity of peak A for the annealed purified nanotubes is shown in Fig. 3, and clearly the intensity does not depend on temperature between 4 and 296 K. This Pauli-like temperature dependence shows that peak A corresponds to the conduction electrons, and that the annealed nanotubes include metallic and/or narrow gap semiconducting nanotubes, as do the non-annealed tubes [ 1 ]. The Pauli paramagnetic susceptibility is estimated to be 4.5 × 10 -8 emu/g. The susceptibility of metallic and/ or narrow gap semiconducting nanotubes is estimated to be more than 4.5 X 10 - s emu/g, assuming the existence of wide band gap semiconducting nanotubes, which are undetectable by ESR. This is about three

times larger than that for graphite. Since the susceptibility of these semiconductors must be less than that for semi-metals, metallic nanotubes must actually be present in the samples, even though the ratio of metallic tubes to semiconducting tubes cannot be determined from this experiment. The Peierls transition expected for quasi one-dimensional metals [5] is not observed above 4 K probably because nanotubes have some twodimensional character [ 24].

4. Conclusions

We have studied the effect of annealing on the electronic properties of nanotubes. We observed the CESR for annealed nanotubes whose intensity is independent of temperature, as well as non-annealed nanotubes. These observations show that the annealed tubes also include metallic and/or narrow gap semiconducting nanotubes. However, the g-value of the annealed tubes and its temperature dependence changes significantly after annealing. These results suggest that some kinds of defects are initially present in nanotubes which are then removed by annealing. The ESR studies for annealed nanotubes indicate that the electronic properties of intrinsic nanotubes are different from those of graphite. Further studies on the electronic properties of nanotubes will not be meaningful unless the tubes are purified and carefully annealed. Only then is there a chance that the measured properties are intrinsic and not that of defects.

Acknowledgement

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The authors would like to thank M. Endo for annealing the samples and Y. Okamoto, A. Oshiyama and Y. Mochizuki for their useful discussions.

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