AsF5 intercalated graphite: Conductivity and the anisotropy of the skin depth via NMRspectroscopyH. A. Resing, M. J. Moran, and Gerald Ray Miller Citation: The Journal of Chemical Physics 76, 1706 (1982); doi: 10.1063/1.443208 View online: http://dx.doi.org/10.1063/1.443208 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/76/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Reduced conductivity in the terahertz skin-depth layer of metals Appl. Phys. Lett. 90, 122115 (2007); 10.1063/1.2716066 Graphite Intercalation Compounds Phys. Today 40, 64 (1987); 10.1063/1.881095 Magnetic and electronic properties of HOPG/MoF6 graphite intercalation compounds: An ESR study J. Chem. Phys. 83, 3859 (1985); 10.1063/1.449096 Fractional ionization and the identification of intercalated species in AsF5–graphite J. Chem. Phys. 73, 629 (1980); 10.1063/1.440164 Graphite intercalation compounds Phys. Today 31, 36 (1978); 10.1063/1.2995104

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AsFs intercalated graphite: Conductivity and the anisotropy of the skin depth via NMR spectroscopy

H. A. Resing and M. J. Moran

Naval Research Laboratory, Washington. D. C. 20375

Gerald Ray Miller

Naval Research Laboratory. Washington, D. C. 20375 and Department o/Chemistry) University 0/ Maryland, College Park, Maryland 20742 (Received 17 August 1981; accepted 3 November 1981)

Intercalation of AsF, into highly oriented pyrolytic graphite (HOPG) produces a compound which is a highly anisotropic, two-dimensional electrical conductor. Despite the high conductivity in the graphite planes, an 19F NMR experiment can detect all of the fluorine in the sample when the c axis of the HOPG is oriented perpendicular to the rf magnetic field B I' It is not necessary to pulverize the sample. The signal intensity is reduced when the c axis is oriented parallel to B I' thereby enabling estimates to be made of the skin depth ( - 121l m) and the in-plane conductivity ( - 2.7 X 10' [) - 1 cm -I).

Doped polyacetylene and intercalated graphite are anisotropic electrical conductors, showing one-dimen­sional1,2 and two-dimensionae conductivity, respective­ly. For graphite, let (Jc be the conductivity normal to the sheets and (Ja that in the plane. The anisotropy «(J/ (Jc) in acceptor-intercalated graphite can be as high as 106• We show experimentally that an oscillating mag­netic field B1 can penetrate two-dimensional conductors uniformly when the axis of poor conductivity is perpen­dicular to the axis of polarization of ii1 • In this orienta­tion, eddy currents are negligible, the interior of the body is not shielded, and nuclear magnetic resonance (NMR) experiment detects all of the nuclear spins in the body. If the axis of poor conductivity is parallel to B1, eddy currents are induced in the plane of easy con­ductivity and NMR detects only those nuclei within the skin depth. From these two experiments, it is possi­ble to estimate the conductivity in the plane of easy con­ductivity. A more important result is that it is not necessary to pulverize the graphite to reduce the di­mensions of a crystal to less than the skin depth calcu­lated from the conductivity in the easy plane. Indeed, the Signal ~trength is maximized by orienting c perpen­dicular to B 1 •

Eddy currents are induced in isotropic conductors by Ii1 regardless of sample orientation. Since only those nuclei within the skin depth are detected by NMR,4,5 pulverizing such samples can increase the number of nuclei within the skin depth and thereby increase the Signal. This gain may be offset by the spreading out of the spectrum when anisotropic chemical (or Knight) sfiifts or quadrupolar interactions are present. Calcu­lations show that the shape of a bulk solid also influences the Signal strength, with a foil more favorable than wires or spheres. 6,7,8 In principle, these calculations offer a starting point for analysis of the two-dimension­al conductivity problem. We could then flatten a sphere into an oblate ellipsoid approximating a graphite flake. In practice, even the spherical isotropic conductor prob­lem has not yielded closed form solutions,7 so we rely

a) Permanent address.

1706 J. Chern. Phys. 76(4), 15 Feb. 1982

on an approximate theory to accompany our experimen­tal demonstration that NMR can probe the whole of a specimen of a graphite acceptor compound.

A specimen of highly oriented pyrolytic graphite (HOPG) of dimensions 5 x 5 mm in the graphite plane was intercalated to first stage with AsF 5 (3 atm). 9 The stage was determined by uniform color and chemical shift. 10 After intercalation, the specimen dimension parallel to the graphite c axis was O. 5± 0.05 mm. The method of intercalation was that of Falardeau et al. 11

The intercalation chamber was a 10 mm o. d. tube with a flat bottom, which was inserted into the 2 cm long NMR coil so that the sample was centered. The speci­men was examined with its c axis perpendicular to the constant field Bo (Fig. 1) in the special orientations C1B1 [Fig. l(a)] and c /I 81 [Fig. l(b)j. For each orien­tation, 1000 scans were recorded at 56.4 MHz. The signal of the AsFs gas (3 atm) external to the graphite served as a check on the Q of the system; the depth of the sample tube in the coil was the same for both orien­tations studied.

b)

0)

-40 -50 -60 -70 -80 0, ppm

FIG. 1. 19F NMR spectra for AsFs interoalated to first stage in highly oriented pyrolytio graphite, (a) no eddy currents, (b) eddy currents; line at - 70, gas; line at - 49, interoalated AsF5•

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Resing. Moran. and Miller: AsF 5 intercalated graphite 1707

FIG. 2. Representation of an anisotropic conductor in an os­cillating magnetic field; 6 is the skin depth.

Two spectra are compared in Fig. 1. For Fig. 1(b), with c 1\ .81, the gain is 16 times that of Fig. 1(a). The integrated signal intensity of the gaseous AsF 5 is the same within 15%, indicating that there is no gross dif­ference in Q for the two orientations. For c 1\.81 [Fig. l(b)J, the NMR signal intensity for the intercalated AsFs is only 1/20 that in the orientation c1..81 [Fig. 1(al). We ascribe this difference to a difference in shielding by eddy currents. Given this orientation, dependent difference in the coupling of the specimen with the os­cillating field, and given the dependence of probe tuning on such coupling, 12 the 15% difference in the signal in­tensity of the gaseous AsF 5 is not significant.

In Fig. 2(b), for c 1\ B1, the eddy currents produced by .8 1 are sketched. The current loops have an orientation such that their resultant magnetic fields instantaneously oppose B1, as required by Lenz's law. The eddy cur­rents serve to shield the interior of the body. (They lie within individual graphite planes of high conductivity.) Only nuclei within the skin depth are seen. In contrast, for c 1. Bl [Fig. 21a)J, current loops of the proper orien­tation to oppose Bl lie in a plane of extremely low con­ductivity. Therefore, no shielding of the interior of the sample is expected and all of the nculei are observed.

For the c 1\ Bl orientation, we make the simplifying assumption that we observe all of the nuclei within the skin depth 6 and none of the remaining nuclei in the sample. Then the fraction of the nuclei seen is just 26/t and this is equal to the ratio of the intensities in the two experiments I,,/l~ "" 1/20. Thus,

6 = (rrflJ.(J)"1/2 =tl,,/(2Ij .l "" 12 IJ.m , (1)

where f is the frequency in Hz, IJ. is the permeability of free space, and (J is the conductivity. We find the con­ductivity to be 2. 7X 105 n-1 cm-1, in the range observed by others. 13.14 The agreement is gratifying, but, more importantly, it verifies the model of the NMR experi­ment described. The conductivity along the c axis13 is only 10-6 that in_the plane, 13.14 which leads to a skin depth in the c 1 B 1 orientation many times the dimen­sions of the specimen.

For two-dimensional conductors, the orientation of the sample with the normal to the plane of easy conduc­tivity perpendicular to 'iit maximizes the Signal, mini-

mizes or eliminates skin depth problems, and enables one to measure directly the angular dependence of anisotropic parameters such as the chemical shift.

The same considerations applied to highly anisotropic one-dimensional conductors lead us to conclude that eddy currents and shielding should be unimportant for any orientation in the Bl field, a conclusion previously reached by others. 15

ACKNOWLEDGMENT

We thank Dr. Arthur Moore of Union Carbide for donating samples of highly oriented pyrolytic graphite.

ly. W. Park, M. A. Druy, C. K. Chiang, A. G. MacDiarmid, A. J. Heeger, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Lett. Ed. 17, 195 (1979).

2C. R. Fincher, Jr., D. L. Peebles, A. J. Heeger, M. A. Druy, Y. Matsumura, A. G. MacDiarmid, H. Shirakawa, and S. Ikeda, Solid State Commun. 27, 489 (1978).

3The anisotropy in graphite and in graphite intercalculation compounds formed with donors and with acceptors has been studied by many groups. See, for example, C. Zeller, G. M. T. Foley, E. R. Falardeau, and F. L. Vogel, Mater • Sci. Eng. 31, 255 (1977), and references therein.

4F. J. Dyson, Phys. Rev. 98, 349 (1955). 'Reviewed by 1. D. Weisman, L. J. Swartzendruber, and

L. H. Bennett, in Measurement of Physical Properties, Techniques of Metals Research, edited by R. F. Bunshah (Interscience, New York, 1972), Vol. VI, pt. 2, Sec. 9.

SM. Mehring, D. Kotzur, and O. Kanert, Phys. Status Solidi B 53, K25 (1972).

7D. E. Demeo, M. Bogdan, and Gh. Mihailescu, Rev. Roum. Phys. 24, 487 (1979).

3D. E. Demeo, M. Bogdan, and L. Jakab, Rev. Roum. Phys. 24, 699 (1979).

9L . Chun-Hsu, H. Selig, M. Rabinovitz, 1. Agranat, and S. Sarig, Inorg. NucL Chern. Lett. 11, 601 (1975). We followed the procedure given.

10Unpublished data gathered by the authors show a clear distinc­tion between the 19F chemical shifts of first and second stage graphite intercalation compounds of AsF5•

lIE. R. Falardeau, L. R. Hanlon, and T. E. Thompson, Inorg. Chern. 17, 301 (1978).

12p. Daugaard, H. J. Jakobsen, A. R. Garber, and P. D. Ellis, J. Mag. Reson. 44, 224 (1981).

13G. M. T. Foley, C. Zeller, E. R. Falardeau, andF. L. Vogel, Solid State Commun. 24, 371 (1977).

14(a) L. V. Interrante, R. S. Markiewicz, and D. W. McKee, Synth. Met. 1, 287 (1979/80), (b) S. C. Singhal and A. Ker­nick, ibid. 3, 247 (1981); (c) T. E. Thompson, E. M. McCarren and N. Bartlett, ibid. 3, 255 (1981); (d) M. J. Moran, J. W. Milliken, C. Zeller, R. A. Grayeski, and J. E. Fischer, ibid. 3, 269 (1981).

l"Michael Cohen, S. K. Khanna, W. J. Gunning, A. F. Garito, andA. J. Heeger, SolidStateCommun. 17,367 (1975). See also Y. Tomkiewicz, D. Garrod, and A. Taranko, Bull. Am. Phys. Soc. 20, 497 (1975); Arnold H. Kahn, J. Appl. Phys. 46, 4965 (1975).

J. Chern. Phys., Vol. 76, No.4, 15 February 1982

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