Co&m Vol. 7.2, No. 6. pp. 497-506, 1984 Printed in the U.S.A

owa23/84 $3.00 + .fxl Pergamon Press Ltd.

SURFACE STRUCTURE AND ELECTRICAL CONDUCTIVITY OF NATURAL AND

ARTIFICIAL GRAPHITES

D. MARCHAND, C. FRETIGNY, M. LAGUFB and A. P. LEGRAND Laboratoire de Physique Quantique. E.R.A. 676. E.S.P.C.I., 10 rue Vauquelin, 75231 Paris Cedex 05,

France

E. MCRAE, 1. F. MARECHE and M. LELAURAIN Laboratoire de Chimie du Solide Mineral, L.A. 1.58, Service de Chimie Minerale Appliquee,

Universite de Nancy I, BP 239, 54506 Vandoeuvre Ies Nancy Cedex, France

(Received 23 September; in revised form 3 January 1984)

Ahstraet-The importance of both the crystallographic and chemical characterization of the graphite substrate is discussed as regards an understanding of intercalation. The influence of defects on the transport properties of synthetic metals is recalled. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) are suitable tools for estimating the mean size of crystallites and for determining possible surface contamination.

After a description of the cleaning procedure for graphite surfaces, we show that deposition of cesium, followed by thermal diffusion, leads to a clear revelation of crystallite boundaries. Graphite substrates of various origin and quality are submitted to AES and LEED. Purity and crystallographic structure are compared. The electrical conductivity, both parallel and perpendicular to the layer planes, has been measured. The results are discussed in relation to crystallite size. No correlation is found between this parameter and the dispersion of c-axes as determined by X-ray diffraction.

1. INTRODUCXIO~

The size of the crystallites in pyrographite increases with the temperature at which the carbon is deposited and with the duration of the subsequent annealing. In the case of small crystallite graphites, the low- temperature conductivity may be limited by bound- ary scattering due to the finite size of the domains[l, 21. This has been experimentally honeyed in materials in which the diameters of crystallites were limited to a few hundred angstroms. Measurements of the low-temperature mobility and the thermal conductivity as well as X-ray analyses have shown good agreement as regards the variation of crystallite size with deposition temperatures[3,4]. Moore[S] indicated in-plane dimensions, &, of the order of 25-40 pm for HOPG. The boundary scatter- ing would be a limiting factor only at very low temperatures.

As regards the synthetic metals based on graphite, a correlation has been found between the resistivity of the initial graphite and that of its intercalation compounds, for a given intercalant and stage[6]. Ubbelohde has pointed several times to the cor- relation between the quality of the initial graphite and the intercalation threshold[7-91.

Structural faults or the presence of heteroatoms (Cl, H, 0, S, Si) present in the graphite in quantities inversely related to the annealing temperature (HTT) influence the intercalation by lowering the Fermi level. They thus favor the intercalation of electron donors and hinder that of acceptors.

Hooley[lO] has shown that the basal surface trig- gers the intercalation, which starts between planes

close to the surface before occurring at the centre of the sample, after superficial adsorption of the reac- tant on the external (0~~) plane. There are two principal factors which influence the surface reac- tivity of the graphite substrate when preparing syn- thetic metals: the dimensions of the crystallites and the presence of faults[ll].

The above considerations show the important role played by the crystallite dimensions as regards chem- ical reactivity and electrical transport, both in graph- ite and in its intercalation compounds. We therefore propose the use of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) to characterize the average size of the crystallites and the chemical purity of the first several atomic layers of the graphite host. The high anisotropy of this mate- rial, rendering it quasi-two-dimensional, suggests that these surface characteristics are representative of the bulk properties. Using standard X-ray diffraction techniques, the 0002 reflexions of the same samples were analyzed to determine the mosaic spread A,.

Finally, the variation of both the basal plane and the c-axis electrical resistivities, p. and pC, were deter- mined from room temperature to liquid helium temperature.

2. PRLNCIPAL TYPES OF GRAPHFfE UTILIZED IN SYNTHESES OF ~ERCALA~ON COMETS

2.1 Natural graphites

From a structural viewpoint, the simplest host material is a flake of natural, single crystal graphite. Such flakes are encountered essentially in limestone: this is indeed the case for the crystals from Ceylon or

497

498 D. MARCHAND et al.

for the Ticonderoga crystals from New York State known since 1883 and in which the natural graphite constitutes interstratified layers throughout the gneiss. Another type of natural graphite results from metamorphized combustible materials (Norway). The degree of purity of both types lies between 90 and 97% and typical dimensions of the single crystals are of the order of a millimeter, the thickness being a few hundreths of a millimeter.

2.2 ~rtt~~ia~ gr~hites These are generally highly oriented pyro~aphites

(HOPG) formed by high-tem~rature cracking of a hydrocarbon followed by annealing at high tem- perature, generally under pressure. The resulting ma- terial is highly oriented along the c-axis (the deviation is of the order of 1”) but the layer planes are made up of crystallites which are randomly oriented with respect to each other, and on the average, several tens of microns in diameter.

(a)

3. CHARACTERIZATION OF A GRAPHITE SURFACE BY

LOW ENERGY ELECTRON DIFFRACTION AND

AUGER ELECTRON SPECTROSCOPY

Although the surface of the carbon matrix plays a primordial role in the synthesis of intercalation com- pounds, it is almost never examined by surface analysis techniques (AES, XPS, SIMS, ISS, etc.). Furthermore, because of the strongly two- dimensional character of the substrate, its crys- tallography is representative of that of the bulk so that LEED is a tool well adapted for studying this structure.

Various authors[l2-141 have shown that graphite (b)

is very stable and that the (~0~) plane reacts very little with most elements. Chemiso~~on on a graph- ite substrate occurs on the edges of the outermost planes. The physisorption of impurities can occur at low temperature, but at 1O-‘o Torr this does not take place within the duration of an experiment.

3.1 Procedure for cleaning the crystallographic faces of graphite

Conventional surface cleaning procedures fail in the case of graphite. Ion-bombardment yields a com- pletely amorphous surface and heating gives surface contamination from segregation of buIk impurities. However, an air-cleaved {~~) surface. inserted quickly into a vacuum system and annealed (-770 K; -5 hr) produces a clean surface as judged Cc) by AES[lS]. In contrast, the (1070) and (1120) sur- Fig. 1. LEED patterns (incident energy of 72 ev). (a) Si

faces fractured in air and analysed within minutes by crystal graphite (Norway). (b) Union Carbide (HAPG) 1. (c)

X-ray photoelectron spectroscopy (XPS) show con- Union. Carbide HOPG, type ZYD.

siderable oxygen contamination [ 161.

is noted. The reciprocal lattice of the surface is the 3.2 LEED study on a (0001) face of single or poly- same as that of the bulk. When the surface is SP fept crystalline graphite by the incident electron beam, no modification of ’ the

A clean, natural single crystal of graphite gives a diffraction pattern occurs, showing that the surfac x is diffraction pattern, made up of single spots on a light highly uniform (long distance orientational ord ler). continuous background (Fig. la). No su~rstructure With a crystal which is not clean, the diffraction spots

Surface structure and electrical conductivity of natural and artificial graphites 499

are more smeared out on a more significant back- ground.

Figure l(b) shows the LEED pattern of a variety of HOPG known as HAPG (Highly Annealed Pyro- graphite). This material is obtained by extensive thermal annealing without any applied stress. The pattern is intermediate between that of the hexagon representing a single crystal diffracting under the incident electron beam and a circle which would be given by a large number of diffracting crystals ran- domiy oriented with respect to each other in a plane, as observed on Fig. l(c) where the diffraction pattern is obtained from the ZYD quality of HOPG.

The escape depth of the electrons goes through a minimum of the order of 5 A for an energy of about one hundred eV. This is thus a first indication concerning the number of graphite layers par- ticipating in the diffraction, knowing that the inter- planar distance is 3.35 A. The symmetry criterion of the diffraction pattern, as defined by Albinet et al.[17], can also be used. These authors have shown that, for an incident beam with energy less than 7OeV, the hexagonal symmetry of the diffraction pattern signifies that only the first atomic layer diffracts. Between 70 and 2OOeV, the trigonal sym- metry corresponds to diffraction by two atomic lay- ers. Finally, beyond 200eV, the symmetry becomes hexagonal once again and corresponds to diffraction by a number of layers greater than or equal to three.

4. DIM~SIONS AND REVEALING OF THE CRYSTALLITRS OF AN HOPG

4.1 LEED evaluation Knowing the dimension of the focal point of the

incident electron beam ( N 150 pm), the average char- acteristic surface area of the crystallites of an HOPG

I- \ \

sample can be deduced by counting the number of spots observed on a diffractogram within an angular sector of 60”, since this will also be the number of single crystal domains. The average size of the do- mains in the HAPG was thus determined from the LEED pattern given in Fig. l(b): this area lies between 5 x 10m3 and IO-*mm2. For the ZYD HOPG (Fig. Ic) for which the diffraction pattern is a circle, the average area of the crystallites is certainly less than 5 x 10m3mm2.

4.2 R~ea~~ng the ~~~~ari~~ between ~r~~ta~l~te~ Using a technique already described[lI?], a few

layers of a first-stage cesium intercalation compound CsC, are synthesized in situ and yield the LEED pattern and AES spectrum presented in Fig. 2. The sample is afterwards taken out from the ultra-high- vacuum chamber and ~~anentiy exposed to air. Segregation and oxidation of cesium along the grain boundaries occur, causing a local exfoliation, which permits a direct observation of microcrystals using optical microscopy (Fig. 3). The average dimensions of the microcrystals thus determined are in agreement with those evaluated by LEED.

5. SURFACE AND MOSAIC SPREAD CHARAaERIZATION OF THE PRINCIPAL

TYPES OF ARTIFICIAL GRAPHITE

After cleavage under a stream of dry nitrogen, the samples were imme~ately transferred to ultra-high vacuum ( N 1O-‘o Torr). Their surface contamination was evaluated by AES, the crystallographic quality and crystallite size estimated by LEED following the technique indicated above. The incident energy of the electrons varied from 7 1 to 102 eV.

t----c------r--- ------ r

240 250 260 Energy

__~-_-.+.___ _-_#_.-_ 270 280 290 (@VI

Fig. 2. Auger spectrum and LEED pattern of first stage GIC CsC,, at 150 K, synthesized under ultra-high vacuum[ll]. The outer circle of the LEED pattern corresponds to the graphite lattice, the two inner circles to the 2 x 2 intercalated cesium structure. This suggests a CsCs surface stoichiometry. The thin 285 eV peak in the Auger spectrum (solid curve), is characteristic of CsC,. The dashed line shows the Auger

spectrum of pure graphite.

500 D. MARCHAND et al.

i. M~~lo~ap~c microscope picture of a HAF’G substrate. (a) Initial state. @) After absorption and diffusion of cesium.

bulk

The samples analyzed were either PGCCL from “Le Carbone Lorraine” or HAPG and HOPG (ZYB and ZYD) supplied by “Union Carbide”. The c-axis mosaic spread was measured in each case using 0002 X-ray diffraction (MO&). The experimental results are grouped in Table 1. Figure 4 shows representative Auger spectra and diffraction patterns of the different samples analyzed.

As regards the AES spectra, silica is the only contaminant noted in some samples. From the peak- to-peak intensity of the Auger transition, its concen- tration is estimated at a few percent of a monolayer. It may have migrated from the graphite blocks surrounding the pyrographite during the compression-annealing since silicon is sometimes used as a catalyst during the production of these blocks. (Silicon has been shown to improve the mechanical properties along the c-axis through the formation of silicon carbide [ 191.)

The experimental results do not show any cor- relation between the average crystallite dimensions and the mosaic spread. Samples 1 and 4, which have about the same spread, are made up of crystallites of vastly different size. The ZYB HOPG gives average dimensions superior to those observed for the ZYD. The width of the circles observed in LEED patterns in conjunction with the mosaic spread shows that the sensitivity of the method is of the order of 1”.

6. ELECI-RICXL TRANSPORT ~~~ER~A~ON

6.1 Resistivity measurement techniques It was deemed desirable to carry out the transport

measurements on the same samples as those used for the LEED, AES and mosaic-spread studies. The four materials examined (samples 1,2,4 and 7 of Table I) were rectangular, a few millimeters long and wide and from 50 to 200 pm in thickness.

The c-axis resistivity measurements were made by

Surface structure and electrical ~nductivity of natural and artificial graphites 501

--.

1oocv ---

E (eV) --.__. ____+ I

au

Fig. 4. Auger spectra and LEED patterns for graphite samples: 1 (4a). 4 (4b) and 7 (4~) described in Tables I and 2.

502 D. MARCHAND ef al.

Table 1. Data relevant to LEED and Auger studies. The crystallite size, L,, is the ratio of the incident electron beam diameter, D, at the focal point to the square root of the number of spots, n, observed in

a 60” angular sector (L, = D/h). XXXX, no detectable contamination

Sarple Ref Sarple Source Mosaic Bear Surface spread

Crystallite energy cantarinant size

XL=Carbone Lorraine UC=Union Carbide 1 (degree) (eV) (microns)

1 CL-PGCCL 1 72 center of a thick disc

Si02 ~60

2 CL-PGCCL 2 64 xxxx CSO center of a thin disc

3 UC-HOP6 1 71 Si02 490 type ZYB

4 UC-HOP6 55' 102 Si02 150 type ZYB

5 UC-HAP6 48' 72 Si02 470

6 UC-HOP6 1'36' 72 xxxx (40 type ZYD

7 UC-HOP6 36' 94 xxxx +40 type ZYB

applying two contacts to each face, one for current

injection, the second as a potential probe, thus avoid- ing contact resistance problems through the use of

this four-point technique. The in-plane measurements were made using the

van der Pauw (vdP) method[20]. The theoretical foundation of this method is based on the conformal mapping of the specimen (of arbitrary shape) onto a semi-infinite half plane. The sample must be. homo- genous in thickness, be singly connected, e.g. it must contain no isolated voids, and the steady state current density J must satisfy the relationships:

V.J=O and VxJ=O.

Under these conditions, the specific resistivity can be determined without knowing the actual current pat- tern. Experimentally, four ohmic, line electrodes de- noted 1, 2, 3 and 4, are placed at arbitrary positions on the periphery, their projections on the surface are point contacts. A current is passed through any two adjacent contacts (say 1 and 2) and the voltage measured across the other two (3 and 4). A cyclic permutation is then made, so that the current now passes through 2 and 3 and the potential measured across 1 and 4. This leads to two values of resistance, noted R,&R,J and &,&). For an isotropic sam-

ple, or for one which is isotropic in the plane of measurement, the in-plane resistivity can be calcu- lated:

P = & (R” + &) *f&I&) (1)

in which t denotes the sample thickness and f a tabulated correction factor determined from the fol- lowing equation:

(R, - RB)/(RA + RB) = Cflln 2) arc cash

((exp tin 2/!))/2). (2)

It was later shown that for the more general case of a material having a diagonal resistivity tensor with components p,,, pz2 and pr3 and a normal to the sample with direction cosines I,, f, and lJ, the mea- sured resistance R, is expressed as:

R,2 = f @sP,,&~ + PUP,&? + ~11~22~~). (3)

From a practical point of view, this technique has been applied to the study of a wide variety of materials but it is beyond the scope of this paper to

Surface structure and electrical ~nductivity of natural and artificial graphites 503

review the field. Certain comments concerning its applicability to anisotropic, multi-domain materials, such as those of this study, are however to the point. As in any technique, the measured resistivity is a function of the different charge carrier scattering mechanisms called into play: interactions between the carriers and the phonons, inclusions, impurities, grain boundaries. The importance of the per- turbations and the errors that may be introduced into electrical measurements of the vdP type by spatially ordered or randomly dispersed conductive or non- conductive inclusions or by inhomogeneities arising from impurity diffusion have been reviewed by Weider[21] with emphasis on their relevance to the analysis of thin film data. However, in high quality HOPG, the level of impurities or inclusions is min- imal and their interaction with the charge carriers can be considered negligible with respect to grain bound- ary and phonon scattering. Let us therefore first examine the vdP technique as applied to multi- domain materials.

The effect of grain boundary scattering becomes significant when the grain size is of the same order of magnitude as the mean free path (m.f.p.) of the charge carriers. Bismuth has proved to be a good candidate for determining if this technique gives valid results when applied to granular materials since the electronic m.f.p. at room-temperature is of the order of one micron, significantly greater than the grain size. Chaudhuri and Pa1[22], for instance, have stud- ied bismuth films in which the film thickness was varied from 800 to 2500 A. The experimental plot of the ratio of film to bulk resistivity as a function of film thickness was successfully accounted for by the Mayadas-Shatzkes theory[23] which treats grain boundary ~atte~ng together with the normal back- ground scattering caused by phonons and defects. This illustrates that not only can the vdP technique be applied to granular materials but that it can be used explicitly to investigate the interaction between these boundaries and the charge carriers.

Going one step further, Nanba[24] applied a slightly modified vdP technique to the measurement of the conductivity of powder compacts. To ensure maximum contact between the compact and the probes, these were placed on a circle of radius b, somewhat less than the radius of the compact, CI. Theoretical analysis showed that eqns (1) and (2) above were modified only to the extent that the In 2 term becomes In C (C = 4(a* + b2)4/(a4 + b’)). When the contacts are on the periphery (a = b) the Nanba and vdP formulae are identical. The predictions of this author were experimentally verified using com- pacts of CuFe,O,. Thus the vdP formulation remains valid even in this limiting case.

The arguments presented above show that the vdP technique is not limited to single-crystal materials. One further point should be brought up concerning its applicability to graphite, a highly anisotropic material. Equation (3) uses the resistivity components pi,, pzz and pj3. In HOPG, the resistivity in the plane,

p,, is isotropic and significantly less than that perpen- dicular to the layer planes, pc. We associate p,, = p22 with p0 and pj3 with pc. Suppose now that the true crystallographic c-axis is at a small angle 0 with the normal of the sample. For small 0, (3) becomes:

Pa = PI f (PP - PJ@/2. (4)

The graphite samples of this study are poly- crystalline and contain a large number of individually anisotropic domains, each with its c-axis making a small angle with the normal to the macroscopic sample. This angle is of the order of 1” and corre- sponds to the mosaic spread. When this is very small, the contribution of the c-axis component to the total measured sheet resistivity will be small; for materials of high anisotropy or greater mosaic spread, the measured sheet resistivity now includes, to an in- creasing extent, both pa and pt. We have accounted for this in our work through the use of eqn (4) to extract pa, using the measured value of p, at each temperature, taking Q equal to one half the mosaic spread A, and using the uncorrected vdP value for the sheet resistivity p,. This appro~mation is, of course, somewhat crude since by convention the single value implied in the term “mosaic spread” is merely the FWHM of the 0002 reflection and the true crystallite distribution should in fact be used.

The vdP method has been applied to HOPG by Dillon et al. [2.5]. However, in their study, the mosaic spread was about 0.2-0.3”, so that the difference between pa and p,? in (4) was negligible. The deter- minations using this method and the conventional bridge method both gave the same values to within less than lv/,.

In summa~, it should be pointed out that the polycrystalline nature of HOPG imposes no intrinsic limit on the use of the vdP technique and that, as with other methods, the basal plane resistivity thus deter- mined is close to the single crystal value, inasmuch as the m.f.p., except at the lowest temperatures, is much smaller than the average crystallite dimensions. Spain has recently pointed out that even at low tem- perature, the m.f.p., A,,< 0.4&[26]. Thus the only necessary correction to be applied when using the vdP method is that relative to the c-axis contribution.

6.2 c-axis and basal plane ~~u~e~ents Figure 5 shows the variation of c-axis resistivity.

Above a characteristic temperature, denoted T,., the variation is of the form In pc = a - bT, i.e. p, = A exp (- BT), in which a, b, A and B are constants, and T is the temperature. The parameters A and E have been calculated in Table 2. Similar thermal variations have been noted before for several types of graphite. In their study on HOPG, Spain et al.[27] noted that pe generally increases with crys- talline perfection. The linear dependence of In pe vs temperature has been experimentally noted by Kawamura et af. [28] in their work on similar types of graphite and by KIein[I] in his study of pyrographites

D. MARCHAND et al. 504

1

10"

lo-:

PC tn.4 1

T(K)

100 200 300

Fig. 5. Variation of the c-axis resistivity (CJ x em) vs tem- perature for samples 1 ( .), 4 (O), 2 (A) and 7 (A).

deposited at temperatures 1900-2500°C. Saturation was sometimes noted as the temperature was lowered, but never a decrease. This is discussed further below. The results of the basal plane measurements are shown in Fig. 6. Here again, the variations are similar to those noted in other studies[3, 27, 291.

6.3 Conductivity anisotropy From the results of the measurements presented

above, it is possible to calculate the variation of the anisotropy, p,/p, as a function of temperature (Fig. 7). Recent review papers[3,4,30] show that our room temperature values are comparable to those of the literature. Over the entire temperature range studied, the shape of the curves of Fig. 7 closely resembles that of the results given by Kawamura et al. [28].

lo/ijf . . T(K)

0 100 200 300

Fig. 6. Basal plane resistivity (@I x em) for samples 1 ( ), 4 (0) and 7 (A).

6.4 Relationship between crystallite size and electrical resistivity

Let us examine the electrical resistivity results in the light of the data concerning crystallite size and mosaic spread of Table 1. On the one hand, there does not seem to be any correlation between A, and the conductivity in either direction but, on the other hand, it is clear that sample 4, with a value of L, much greater than the others, is the only sample in which pc decreases as the temperature is lowered below T, (Fig. 5). It has the most rapid initial decrease in p, as the temperature decreases (Fig. 6), the greatest value of residual resistivity ratio (Table 2) and the largest value of anisotropy at low tem- peratures (Fig. 7). While these remarks might have been expected in earlier work[l-3,271 in which the

Table 2. Data relevant to resistivity measurements. In-plane resistivity p, in (JQ x em). At 290 K, p, in units of lo-* 62 x em. The anisotropy has to be multiplied by 103. At 4.2 K, pC in units of 10-l D x cm. The anisotropy has to be multiplied by 104. Z’, is the temperature below which the relationship p, = A exp ( - LIT) is no longer valid. RRR is the residual resistivity ratio: p. (290 K)/p&4.2 K). X: not

measured

SAHPLE T = 290 K T = 4.2 K a axis c axis

Ref. Symbol 0, Oc P&Pa Pa Qc Pc’@a RRR T, 102xA 103x8

1 . 36.0 8.9 2.21 7.1 2.4 3.38 5.1 160 2.70 3.81

2 A x 9.7 x X 2.9 X X 340 3.45 4.34

4 0 40.5 9.0 2.22 4.0 1.9 4.81 10.1 170 2.95 4.09

7 A 44.5 5.5 1.33 6.0 1.6 2.39 5.6 440 2.05 4.54

Surface structure and electrical conductivity of natural and artificial graphites 505

I

‘OS: PC/&

combined effect of the coherence length and the diffracting thickness renders more quantitative evalu- ation difficult. The structural characterization of the

J principal types of graphite used in the research on l . , :

graphite intercalation compounds has not led to any

l . correlation between the size of the graphite crys- . & tallites and the mosaic spread around the c-axis. l .

104- ‘. %> Furthermore, no well defined relationship has been

“9..

:

found between these parameters and the c-axis or

‘*_ l *. basal plane resistivity. However, the exceptionally

‘a l r C”

large domains lead to specific behaviour in both ‘1,., ” *?

“< :.. directions: a drop in c-axis resistivity below a certain

‘. ‘0.

critical temperature and a high value of basal plane i c f ,.

“I residual resistivity ratio.

I I It is hoped that multi-technique characterizations

lo3 T(K) of the graphite substrate such as these will allow

0 100 200 300 better reproductibility and greater control of the physical properties of the synthetic metals based on

Fig. 7. Conductivity anisotropy p,/p, vs temperature for intercalation into such carbon matrices. samples I ( ), 4 (0) and 7 (A).

crystallites were comparable in size to the m.f.p. at low temperature, a similar conclusion here is not as obvious, the crystallite dimensions in our study all being quite large.

The fall of the c-axis resistivity below about 50 K appears to be a characteristic only of the highest quality materials (in the sense that their basal plane properties approach those of single crystals). This has been noted elsewhere[27, 311 and discussed by Spain[3], though no explanation has been put for- ward.

It is interesting to note as well that a large number of graphite intercalation compounds have been syn- thesized using graphite sample 4[6]: these have yielded low room-temperature values of resistivity upon intercalation with gallium trichloride and higher values of residual resistivity ratio than often observed with acceptor compounds[32].

7. CONCLUSIONS

The comprehension and control of the segregation process which undoubtedly plays an important role in the synthesis of graphite intercalation compounds implies a knowledge of the structural quality and chemical composition of the sample surface. The significant damage of a (OOOf) graphite surface by a very low intensity ion beam shows the impossibility of using secondary ion mass spectroscopy (SIMS) to analyze the surface contamination of this material. Furthermore, the surface sensitivity of X-ray micro- analysis techniques is insufficient. Auger spec- troscopy is thus best adapted for an elementary analysis of the surface of graphite samples.

As concerns the crystallites, the LEED experiments and the revelation of the grain boundaries by the sorption of a reagent have allowed the estimation of their average size. X-Ray analysis confirms that the domain size is greater than one micron, but the

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A. W. Moore, Chemistry and Physics of Carbon (Edited by P. L. Walker and P. A. Thrower), Vol. 11, pp. 69-187. Marcel Dekker, New York (1973). A. W. Moore, A. R. Ubbelohde and D. A. Young, Proc. Roy. Sot. A280, 153 (1964). E. McRae, These de Doctorat d’ttat. Universiti de Nancy. France (1982). A. R. Ubbelohde, Proc. Roy. Sot. A304, 25 (1968). A. R. Ubbelohde, Carbon 7, 523 (1969). A. R. Ubbelohde, J. Chim. Phys. No. sphc. 64-70 (1969). J. G. Hooley, Mat. Sci. Engng 31, 17 (1977). A. Herold, Muf. Sci. Engng 31, 1 (1977). J. J. Lander and J. Morrison, J. Appl. Phys. 35, 3593 (1964); Surface Sci. 6, 1 (1967). G. F. Amelio and E. J. Scheibner, Surface Sci. 11, 242 (1968). G. R. Hennig, Chemistry and Physics of Carbon, Vol. 2, pp. 149. Marcel Dekker, New York (1966). J. Suzanne, J. P. Coulomb and M. Bienfait, Surface Sci. 44, 141 (1974). R. G. Musket, W. McLean, C. A. Colmenares, D. M. Makowiecki and W. J. Siekhaus, Appl. of Surface Sci. 10, 143 (1982). G. Albinet, J. P. Biberian and M. Bienfait, Phys. Rev. 33, 2015 (1971). D. Marchand, M. Lag&% and A. P. Legrand, 3rd Inc. Carbon Conf., p. 85. Baden-Baden (1980). H. S. Shim and G. W. Hollenberg, Carbon 13, 539 (1975). L. J. Van der Pauw, Philips Res. Rep. 13, 1 (1958); 16, 187 (1961). H. H. Wieder, Thin Solid Films 31, 123 (1976). S. Chaudhuri and A. K. Pal, J. Appl. Phys. 48, 3455 (1977). A. F. Mayadas and M. Shatzkes, Phys. Rev. Bl, 1382 (1970). N. Nanba, Jap. J. Appl. Phys. 17, 445 (1978). R. 0. Dillon, I. L. Spain, J. A. Woollam and W. H. Lowrey, J. Phys. Chem. Solids 39, 907 (1978). 1. L. Spain, Chemistry and Physics of Carbon (Edited by P. L. Walker and P. A. Thrower), Vol. 16, pp. 119-304. Marcel Dekker, New York (1981).

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29. L. A. Pendrys, C. Zeller and F. L. Vogel, J. Mat. Sci. Synth. Met. 8, 163 (1984). 15, 2103 (1980).