Two-Temperature EPR Measurements of Multi-Walled Carbon Nanotubes. Paweł Szroeder, Franciszek Rozpłoch and Waldemar Marciniak

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1 Solid State Phenomena Vol. 94 (2003) pp (2003) Trans Tech Publications, Switzerland doi: / Two-Temperature EPR Measurements of Multi-Walled Carbon Nanotubes Paweł Szroeder, Franciszek Rozpłoch and Waldemar Marciniak Instytut Fizyki UMK, ul. Grudzi dzka 5/7, PL Toru, Poland Keywords: electron paramagnetic resonance (EPR), multi-walled carbon nanotube (MWNT) Abstract. Two-temperature EPR measurements of macroscopic bundles of multi-walled carbon nanotubes were performed. The asymmetric resonance lines were observed. The contribution of localized -electron spin centers was higher than in pyrolytic graphite. The interpretation of these results is that the nanotube graphene sheet curvature causes an increase in the density of trapped electrons. Introduction The purpose of the experiments was to determine the contribution of the delocalized spins which give rise to electron paramagnetic resonance in multi-walled carbon nanotubes (MWNT). Like graphite, MWNTs are essentially sp 2 bonded systems that consist of coaxial graphene cylinders. The interlayer spacing between concentric nanotubes was slightly greater (1%-2%) than that found in graphite [1]. Depending on the chirality and diameter, the individual nanotubes are predicted to exhibit either metallic or semiconductiong properties. For metallic nanotubes, the bonding defects on cylindrical sheets as well as tube tips can serve as trapping sites for conduction electrons. The trapping sites are associated with interesting junction properties indicating possible applications. The localized spin centers near defect states show Curie-like temperature dependence; by contrast, the EPR temperature dependence of conduction electron spins is close to that predicted by the Pauli magnetism. The comparison of EPR signal intensity at two temperatures allows one to determine the relative contributions of both types of spin centers. The paper reports on the experiments performed on unoriented bulk material using two-temperature EPR measurements Experimental The purpose of the experiments was to determine the contribution of the delocalized spins which give rise to electron paramagnetic resonance in multi-walled carbon nanotubes (MWNT). Like graphite, MWNTs are essentially sp 2 bonded systems that consist of coaxial graphene cylinders. The interlayer spacing between concentric nanotubes was slightly greater (1%-2%) than that found in graphite [1]. Depending on the chirality and diameter, the individual nanotubes are predicted to exhibit either metallic or semiconductiong properties. For metallic nanotubes, the bonding defects on cylindrical sheets as well as tube tips can serve as trapping sites for conduction electrons. The trapping sites are associated with interesting junction properties indicating possible applications. The localized spin centers near defect states show Curie-like temperature dependence; by contrast, the EPR temperature dependence of conduction electron spins is close to that predicted by the Pauli magnetism. The comparision of EPR signal intensity at two temperatures allows one to determine the relative contributions of both types of spin centres. The paper reports on the experiments performed on unoriented bulk material using two-temperature EPR measurements. Results and discussion Line shape. The experimental EPR curves in bulk carbon nanotubes are shown in Fig 2. The shapes of the resonance line are strongly assymetric due to the skin effect. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-04/06/14,10:21:11)

2 276 Interfacial Effects and Novel Properties of Nanomaterials Fig. 1. SEM images of MWNTs bundles To discuss these asymmetric curves the modified Dyson's theory of CESR line shape in graphite was used [3]. The skin depth was estimated from resistivity. Electron transport phenomena in macroscopic bundles of closely packed MWNTs are dominated by intertube contacts or intratube barriers and the measured resistivity is 6.5 m cm at 300 K [4]. Assuming this value we find» 40 m at 10 GHz. The skin depth is comparable to the diameter of columnar MWNT's bundles seen in the SEM images. So the conclusion is that the resonance lines reflect CESR phenomena within MWNT's bundles as a whole The linewidth. The linewidth B in macroscopic bundles of close-packed MWNTs decreases with increasing temperature. It equals 31 Gs at 77 K and 12 Gs at room temperature. Knowledge of B allows determination of the spin relaxation time T2 (= T1 for metals). Using the formula T2 =1/ B [5] we find T2» s at room temperature and» s at 77 K. The amplitude asymmetry of resonance curve A/B (see inset in Fig. 2) is related to the square root of the ratio T D / T2. TD is the time required for a spin to diffuse across the skin depth. Using this relation we estimate TD» 10-8 s at 300 K and s at 77 K. Taking a Fermi velocity uf» 10-8 cm/s in nanotube bundles estimated by Song et al. using Hall measurements [4], and confirmed by Tian et al. on the basis of thermoelectric power experiments [6], we determine a mean free path L. Assuming that all the conduction electrons have the same Fermi velocity uf, then TD = (3/2)(d2/uFL). The room temperature value for TD, combined with d» 40 m, leads to a mean free path L» 6 m. While crude, this estimate suggests that conduction occurs mainly along the nanotube axis in the bulk material and that the hopping between the tubes within a bundle is not the dominant transport mechanism. The g value. The g-value of bulk MWNTs is equal to ± at room temperature and increases with decreasing temperature. At 77 K g becomes ± 0,003. The shift g of the gfactor with respect to the free electron value (gfree = ) and B reveals information about the intristic resistivity in graphite. B is determined by the Elliott mechanism which relates the spin relaxation time T1 ( B a 1/T1) to the resistivity scattering time tr = kt1 g2, k being a constant in the range Using this relation with care we find tr» s. These results allow us to give an

3 Solid State Phenomena Vol Fig. 2. The EPR lines in bundles of carbon nanotubes at 300 and 77 K. The absorptive and dispersive components of resonance curves are shown under recorded spectra. The inset shows the parameters of Dysanian lines. upper limit of the intristic resistivity m/ne 2 τ R 10-1 Ωcm for MWNTs bundles at room temperature, where n is a carrier density (10 18 cm -1 ) and m is a carrier mass, assuming free electron mass. This value is over one order higher than the dc resistivities of MWNT films (see Ref. 4) which is consistent with the interpretation that dc is dominated by the intertube contact resistances. Resonant intensity. The intensity of the absorptive part of the resonance line is contributed to by both the conduction electrons and localized spin centres at the tube tips and defects such as deformed azulene structures on the graphene cylinders. Two-temperature measurements allow the subdivision of the total intensity of the absorptive part of the line I into two components that display respectively Pauli- (I Pauli = A + BT) and Curie-like (I Curie = C/T) temperature behaviour, where I = I Pauli + I Curie. If X I Pauli /I denotes the contribution of delocalized electron states then I I TN TRT A + BTN TRT = X + ( 1 X ) (1) A + BT T RT N

4 278 Interfacial Effects and Novel Properties of Nanomaterials Neglecting that π-electrons contributes to the creation of holes in the valence band we estimate using Eq. 1 that ~ 80 % of spin states are delocalized and Pauli-like temperature behavior occurs. Two-temperature EPR spectra recorded in pyrolytic graphite shows that X increases with improvement of their crystallite structure by annealing [7]. In pyrolytic graphite annealed at 2873 K, X = Much lower X in bundles of MWNT can be interpreted as a consequence of the tubular geometry of graphene layers being conducive to the creation of trapping sites for conduction -electrons. Conclusions Assuming mixed Curie- and Pauli-like temperature dependence of EPR line intensity we use the two-temperature measurement method for estimating the contribution of delocalizad -electron states in resonance. The percentage of delocalized states is much lower than that in annealed pyrolytic graphite. This result suggests that a higher number of defects (f.ex. pentagon-heptagon pairs) in MWNTs are present than in graphite. Topological imperfections and open edges of graphitic domains on the tips, cause increase of the trapped electron density. That leads to a decrease in carier mobility and resistivity scattering time τ R. Acknowledgments This work was supported by N.Copernicus University Grant No. 380-F. References [1] Y. Saito, T. Yoshikawa, S. Bandow, M. Tomita, and T. Hayashi, Phys. Rev. B Vol. 48, (1993), p [2] S. Ijima, Nature Vol.354, (1991) p. 56 [3] A. M. Ziatdinov, N. M. Mishtshenko, Fiz. Tverd. Tel a Vol. 36, (1994), p. 2360, A. M. Ziatdinov, N. M. Mishtshenko, Fiz. Tverd. Tel a Vol. 29, (1987), p [4] S. N. Song, X. K. Wang, R. P. H. Chang, and J. B. Ketterson, Phys. Rev. Lett. Vol. 72, (1994), p. 697 [5] G. Feher and A. F. Kipp, Phys. Rev. Vol. 98, (1955) p. 337 [6] M. Tian, F. Li, L. Chen, and Z. Mao, Phys. Rev. B Vol. 58, (1998), p [7] F. Rozpłoch, Magnetooporno ujemna w w glu pirolitycznym, (UMK, Toru, Poland 1984), p.56

5 Interfacial Effects and Novel Properties of Nanomaterials / Two-Temperature EPR Measurements of Multi-Walled Carbon Nanotubes / DOI References [1] Y. Saito, T. Yoshikawa, S. Bandow, M. Tomita, and T. Hayashi, Phys. Rev. B Vol. 48, (1993), doi: /physrevb