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The electronic structure of ferric chloride intercalated graphite
425
T H E E L E C T R O N I C S T R U C T U R E OF FERRIC C H L O R I D E I N T E R C A L A T E D G R A P H I T E John J. R I T S K O and Eugene J. M E L E Xerox Webster Research Center, Webster, New York 14580, USA
The electronic excitation spectrum in energy and momentum space of stage 1 and stage 2 FeEl3 intercalated graphite has been measured from 0.2 to 287 eV by high resolution (0.1 eV) electron energy loss spectroscopy. The dispersion of intraand interband plasmons was measured and the dielectric function E(q, E) computed by a Kramers-Kronig analysis of the energy loss data. Critical points in the dielectric function and the interband plasmon dispersion indicate that the carbon 7r bands are only weakly perturbed. A tight binding model of the carbon ~" bands is used to compute E(q,E) and the intraband plasmon dispersion thus providing an accurate measure of the amount of charge transfer. In addition, the spectrum of excitations of carbon IS core states to empty carbon 2P states directly probes the new states which appear after intercalation and provides a direct measurement of Fermi level lowering in intercalated graphite. Charge transfer per intercalant layer is constant in the various compounds and with respect to pure graphite the Fermi level has been lowered by 0.9 and 0.7 eV in stage 1 and stage 2 FeC13-graphite, respectively.
1. Introduction The intercalation compounds of graphite are known to achieve electrical conductivities which rival those of the best metals [1, 2]. This work is aimed at obtaining a microscopic understanding of the nature of the high conductivity in these materials. In particular, we need to know to what extent the electronic structure of the graphite layers is modified by the presence of intercalant layers in low stage compounds. A quantitative measurement of the amount of charge transfer between carbon and intercalant is essential. Moreover, in order to fully understand the donor-acceptor interaction in these materials one must know which species, carbon or intercalant, determines the amount of charge transfer. Finally, an accurate measurement of the Fermi level position is needed so that coupled with an accurate model for the electronic energy band structure the density of states at the Fermi level and the electrical conductivity can be computed. One method for probing the electronic structure of solids is by measuring the electronic excitation spectrum using optical absorption or rcflectivity spectroscopy. Radical changes in the reflectivity spectra of intercalated graphites have been measured in a number of compounds [3-5]. However, these measurements have been confined to the region below about 3 eV. A method for measuring the electronic excitation spectrum over a very large energy region is by inelastic electron scattering spectroscopy which at small scattering angles measures electric dipole excitations as in optical spectroscopy and which extends the excitation spectrum into
Physica 99B (1980) 425--429(~) North-Holland
momentum
space at larger scattering angles
[6]. 2. Experiment In the experiment the energy loss spectra of 80 keV electrons transmitted through thin (~1000 ~,) cleaved samples were measured from 0.2 to 287 eV as a function of scattering angle, 0, or m o m e n t u m transfer q. The energy resolution was 0.11 eV over the entire spectral region and the m o m e n t u m resolution was 0.055 ~-1. Samples were prepared from distilled anhydrous FeCI3 and thin slabs of highly oriented pyrolytic graphite in an evacuated pyrex tube using a two zone heating system similar to most previous studies [7]. For stage 1 the graphite was held at 320°C and FeCI3 at 300°C while for stage 2 the graphite was held at 350°C and FeCI3 at 295°C. X-ray diffraction studies using Cu K~ radiation showed narrow lines ~<0,2° wide for all materials. In the stage 1 c o m p o u n d small regions of unintercalated material produced a diffraction peak characteristic of pure graphite whose intensity was ~ 1 0 % that of the stage 1 compound, while higher stages were present at the < 1 % level. O u r stage 2 c o m p o u n d showed a considerable amount of unintercalated graphite and so in addition we studied a very fine sample of this material prepared by C. Underhill. Measurements of the different stage 2 samples produced in the different laboratories showed minor differences but were essentially consistent, and we are confident that the results reported here for all of the materials we studied are characteristic of well staged intercalation compounds.
426 3. Valence excitations
The energy loss spectra measured in pristine and stage 1 FeCI3 intercalated graphite (C6.6FEC13) in the region from 0--10 eV are shown in fig. 1. These data were obtained at a small scattering angle (q = 0.l ,~ l) where the direction of momentum transfer is parallel to the graphite planes. In pristine graphite there is a low steplike knee near 1 eV corresponding to the onset of 7r--*vr* interband transitions and a strong plasmon peak at 7.0 eV due to a collective oscillation of all the ~ electrons [8, 9]. In F e C l 3 graphite the Fermi level is lowered due to charge transfer to the intercalant. The new intraband excitations now possible give rise to a sharp intraband plasmon which for q = 0.1 A ~ occurs at 1.14 eV in C66FEC13 and at 1.10 eV in the stage 2 compound (ClzsFeC13). The 7.0 eV 7r plasmon in graphite is shifted to 5.9 eV in C6.6FEC13 and 6.25 eV in C1zsFeC13. The 7r plasmons in the intercalated compounds are shifted to lower energy because charge transfer reduces the total oscillator strength of the 7r--* 7r* excitations and since the density of carbon planes is lowered, the oscillator strength is further reduced. Reduction in the oscillator strength reduces the plasmon energy but the momentum dependence of 7r
0
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ENERGY (eV)
Fig. 1, Valence excitation spectrum: relative energy loss probability normalized to the highest peak in pristine graphite (dashed curve) and C6.6FeC13 (solid curve) at q = 0.1 ~-1.
plasmons in pristine and intercalated materials is quite similar indicating that the 7r band structure is not significantly altered [6]. The momentum dependence of the intraband plasmon was shown to exhibit positive quadratic dispersion characteristic of a free-electron gas but with a very rapid falloff in intensity indicating strong band structure effects [6]. Since freeelectron gas models could not explain this behavior [6] we incorporated the band structure into an accurate calculation of the momentum dependent dielectric response function and computed the plasmon dispersion and momentum dependent intensity. In this calculation a tight binding model is fitted to the two dimensional graphite 7r bands which were originally computed by an ab initio variational method using a linear-combination of atomic-orbitals basis of Bloch states [10]. The tight binding model reproduces the density of states near the Fermi level very accurately and allows a straightforward calculation of the polarizability of a single graphite plane to be made [11]. The macroscopic dielectric response function is then computed by calculating proper spatial averages of the volume averaged polarization and electric fields. The contribution of the FeCl3 is taken into account by assuming a constant background dielectric constant, E~ = 5. Absorption in FeCI3 begins at 2.0eV and the only effect on the dielectric response function near the plasmon energy should be a nearly energy independent polarizability. Intra- and interband excitations are explicitly included and the plasmon energy is given by the zeros of el(q, E). The results are given in fig. 2 for a family of curves which differ by the amount of charge transfer or the Fermi momentum in the carbon planes. The experimental data, see ref. 6, are shown as discrete points. Clearly the plasmon dispersion is very sensitive to the amount of charge transfer and the data are well fitted by curve c so that the charge transfer per carbon atom in C6.6FEC13 is 0.015+_0.005electrons/carbon atom. Since the coupling between planes is important and nontrivial, calculations for ClzsFeCI3 will be the object of a subsequent publication. In addition to providing a good fit to the measured plasmon dispersion the model also predicts a very rapid falloff of the plasmon strength with increasing q. The rapid falloff is due to the large wavevector of the measured
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Fig. 2. Intraband plasmon dispersion: solid curves are theoretical dispersion relations for indicated a m o u n t s of charge transfer in electrons per carbon atom. Solid dots experimental data ref. 6. O p e n circle is extrapolated from fit to experimental data.
plasmons relative to the Fermi wavevector of the hole states created by the intercalation reaction, k~ (holes) = 0.15 A-~. For the integrated plasmon strength our model predicts a q-2.6 dependence which agrees very well with the observed value q-2S [6]. Further insight into the excitations of the valence electrons is given by an experimental determination of the dielectric function by a Kramers-Kronig analysis of the energy loss data. Spectra were recorded up to 100eV, corrected for multiple inelastic scattering and properly normalized by using the oscillator strength sum rule for the energy loss function, Im - l / E , and a value for E~(q) at zero energy given by our theoretical model. The results were sensitive to the theoretical input only in the low energy region below ~0.5 eV. The results shown in fig. 3 include a computation of E2 for pristine graphite obtained by a similar method, el crosses zero near 1 and 6 eV resulting in the strong plasmon peaks shown in fig. 1. Below 2 eV ~2 in C6.6FeCl 3 is very much weaker than in pristine graphite because the original 7r ~ ~'* interband transitions
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Fig. 3. Dielectric function of C6.6FEC13 from K r a m e r s - K r o n i g analysis of energy loss spectra at q = 0.1 A.-L Real part, ~ , dashed curve; imaginary part, ~2, solid curve, e2 of pristine graphite is the dotted curve.
which began at very low energies (~-0 eV) now show a gap due to the lowering of the Fermi level. They are replaced by intraband excitations and a weak interband threshold at about 1.2_+ 0.2 eV. Peaks at 2.75, 3.9, 8 and 9 eV are due to FeCI3 [6]. The Q-point interband peak at 4.3 eV in pristine graphite is still seen in the intercalated material as a shoulder at the same energy, and the or band transitions near 13 and 15 eV are also observed in C6.6FeCi3. It is expected that the strength of the pure graphite critical points in the joint density of states will be reduced in the intercalated material since the density of carbon planes is very much reduced, but the fact that these critical points are apparent in the compound is an indication that the graphite band structure is not significantly perturbed. As a check on the accuracy of the calculated dielectric function, the normal incidence reflectivity was computed and compared with recent measurements [12]. Excellent agreement was obtained with the absolute values and shape of the reflectivity measured. In addition, the lowering of the plasma edge by ~ 0 . 0 4 e V in going from stage 1 to stage 2 was confirmed [12[. 4. Core excitations The electronic excitations of carbon IS core electrons are important since transitions from
428 these localized atomic states occur only to empty p orbitals localized on the carbon atoms. At forward scattering angle (0 = 0) the transition dipole matrix element selects only the Pz orbital along the c-axis which makes up the graphite 7r band. The experimental results are given in fig, 4. In pure graphite, the absorption edge is characterized by a broad, nearly symmetric peak centered at 285.35 - 0.05 eV approximately 1 eV above the threshold which corresponds to the Fermi level. In the intercalated compounds charge is removed from the carbon valence band, lowering the Fermi energy and creating new empty states which were occupied before the intercalation reaction. The carbon 1S excitation spectrum now includes transitions into these empty states and a new peak appears below the original pristine graphite threshold. Interaction of the final states with the core hole left behind causes the shape of this spectrum to deviate significantly from the original density of empty 7r states. Theoretical calculations which include the final state electron hole interaction in a contact approximation [11] yield an excellent description of the shape of the absorption spectrum and indicate the degree of Fermi energy lowering (which is approximately given by the width of the small peak at threshold). We find the lowering of
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the Fermi level to be 0 . 9 - 0 . 1 eV for C6.6FeCl3 and 0.7---0.1 eV for C12.5FEC13. Using our model theoretical density of states this corresponds to a charge transfer of 0.015 +- 0.003 electrons/carbon atom in stage 1 and 0.0075 --- 0.0025 electrons/carbon atom in stage 2. These results confirm the determination of charge transfer deduced from the plasmon dispersion in fig. 2. Moreover, the charge transfer per carbon atom in stage 2 is exactly one half that in stage 1 leading to the conclusion that the charge transfer per intercalant layer is constant independent of stage. Thus, in the case of FeCl3-graphite the intercalant determines the amount of charge transfer. This agrees with earlier Mossbauer measurements on this system [13]. In contrast, in the case of AsF5 intercalation compounds the charge transfer appears to be determined by the carbon [14].
5. Summary The electronic structure of the carbon in FeC13 intercalated graphite (stages 1 and 2) is accurately represented by that of two dimensional graphite with the Fermi level lowered by 0.9 eV and 0.7 eV in stage 1 and stage 2 compounds, respectively. Critical points in the dielectric
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284 285 ENERGY LOSS (eV)
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Fig. 4. Raw data of carbon IS excitation spectra: open circles, pristine graphite; crosses, C6.6FEC13; triangles C~25FeC13. The point with error bars indicates uncertainty due to counting statistics and energy resolution.
429
function as obtained by a Kramers-Kronig analysis and the dispersion of interband zr plasmons imply that the band structure is not significantly perturbed on intercalation. The amount of charge transfer per carbon atom is 0.015 electrons in C6.6FeCl 3 a s determined by the dispersion of the intraband plasmon using a theoretical calculation of the momentum dependent dielectric function. This is in agreement with charge transfer as determined from the lowering of the Fermi level as evident in the interband ~ r ~ 7r* gap in E2 at 1.2--_0.2 eV (at q = 0.1 k -t) and as directly measured in the carbon 1S electron core excitation spectrum. In C125FEC13 the charge transfer is 0.0075 electrons/carbon atom. Charge transfer per intercalant unit ( F e C l 3 ) is 0.10 and is independent of stage.
We are grateful to C. Underhill for his excellent samples of stage 2 FeCl3-graphite and to A. Moore for the highly ordered pyrolytic graphite we used in preparing our own samples.
References [1] J.E. Fischer and T.E. Thompson, Phys. Today 31 (1978) 36. [2] F.L. Vogel, G.M.T. Foley, C. Zeller, E.R. Falardeau and J. Gan, Mat. Sci. and Eng. 31 (1977) 261. [3] J.E. Fischer, T.E. Thompson, G.M.T. Foley, D. Guerard, M. Hoke and F.L. Lederman, Phys. Rev. Lett. 37 (1976) 769. [4] L.R. Hanion, E.R. Falardeau, D. Guerard and J.E. Fischer, Mat. Sci. and Eng. 31 (1977) 161. [5] M. Zanini and J.E. Fischer, Mat. Sci. and Eng. 31 (1977) 169. [6] J.J. Ritsko and M.J. Rice, Phys. Rev. Lett. 42 (1979) 666 and refs. cited therein. [7] C. Underhill, S.Y. Leung, G. Dresselhaus and M.S. Dresselhaus, Sol. St. Comm. 29 (1979) 769 and refs. cited therein. [8] K. Zeppenfeld, Z. Phys. 243 (1971) 229. [9] E.A. Taft and H.R. Philipp, Phys. Rev. A 138 (1965) 197. [10] G.S. Painter and D.E. Ellis, Phys. Rev. B 1 (1970) 4747. [11] E.J. Mele and J.J. Ritsko, Phys. Rev. Lett. 43 (1979) 68 and to be published. [12] J. Perrachon, Thesis, University of Pennsylvania (1978). [13] J.G. Hooley, M.W. Bartlett, B.V. Liengme and J.R. Sams, Carbon 6 (1968) 681. [14] B.R. Weinberger, J. Kaufer, A.J. Heeger, J.E. Fischer and M. Moran, N.A.W. Hoizwarth 41 (1978) 1417.
T H E E L E C T R O N I C S T R U C T U R E OF FERRIC C H L O R I D E I N T E R C A L A T E D G R A P H I T E John J. R I T S K O and Eugene J. M E L E Xerox Webster Research Center, Webster, New York 14580, USA
The electronic excitation spectrum in energy and momentum space of stage 1 and stage 2 FeEl3 intercalated graphite has been measured from 0.2 to 287 eV by high resolution (0.1 eV) electron energy loss spectroscopy. The dispersion of intraand interband plasmons was measured and the dielectric function E(q, E) computed by a Kramers-Kronig analysis of the energy loss data. Critical points in the dielectric function and the interband plasmon dispersion indicate that the carbon 7r bands are only weakly perturbed. A tight binding model of the carbon ~" bands is used to compute E(q,E) and the intraband plasmon dispersion thus providing an accurate measure of the amount of charge transfer. In addition, the spectrum of excitations of carbon IS core states to empty carbon 2P states directly probes the new states which appear after intercalation and provides a direct measurement of Fermi level lowering in intercalated graphite. Charge transfer per intercalant layer is constant in the various compounds and with respect to pure graphite the Fermi level has been lowered by 0.9 and 0.7 eV in stage 1 and stage 2 FeC13-graphite, respectively.
1. Introduction The intercalation compounds of graphite are known to achieve electrical conductivities which rival those of the best metals [1, 2]. This work is aimed at obtaining a microscopic understanding of the nature of the high conductivity in these materials. In particular, we need to know to what extent the electronic structure of the graphite layers is modified by the presence of intercalant layers in low stage compounds. A quantitative measurement of the amount of charge transfer between carbon and intercalant is essential. Moreover, in order to fully understand the donor-acceptor interaction in these materials one must know which species, carbon or intercalant, determines the amount of charge transfer. Finally, an accurate measurement of the Fermi level position is needed so that coupled with an accurate model for the electronic energy band structure the density of states at the Fermi level and the electrical conductivity can be computed. One method for probing the electronic structure of solids is by measuring the electronic excitation spectrum using optical absorption or rcflectivity spectroscopy. Radical changes in the reflectivity spectra of intercalated graphites have been measured in a number of compounds [3-5]. However, these measurements have been confined to the region below about 3 eV. A method for measuring the electronic excitation spectrum over a very large energy region is by inelastic electron scattering spectroscopy which at small scattering angles measures electric dipole excitations as in optical spectroscopy and which extends the excitation spectrum into
Physica 99B (1980) 425--429(~) North-Holland
momentum
space at larger scattering angles
[6]. 2. Experiment In the experiment the energy loss spectra of 80 keV electrons transmitted through thin (~1000 ~,) cleaved samples were measured from 0.2 to 287 eV as a function of scattering angle, 0, or m o m e n t u m transfer q. The energy resolution was 0.11 eV over the entire spectral region and the m o m e n t u m resolution was 0.055 ~-1. Samples were prepared from distilled anhydrous FeCI3 and thin slabs of highly oriented pyrolytic graphite in an evacuated pyrex tube using a two zone heating system similar to most previous studies [7]. For stage 1 the graphite was held at 320°C and FeCI3 at 300°C while for stage 2 the graphite was held at 350°C and FeCI3 at 295°C. X-ray diffraction studies using Cu K~ radiation showed narrow lines ~<0,2° wide for all materials. In the stage 1 c o m p o u n d small regions of unintercalated material produced a diffraction peak characteristic of pure graphite whose intensity was ~ 1 0 % that of the stage 1 compound, while higher stages were present at the < 1 % level. O u r stage 2 c o m p o u n d showed a considerable amount of unintercalated graphite and so in addition we studied a very fine sample of this material prepared by C. Underhill. Measurements of the different stage 2 samples produced in the different laboratories showed minor differences but were essentially consistent, and we are confident that the results reported here for all of the materials we studied are characteristic of well staged intercalation compounds.
426 3. Valence excitations
The energy loss spectra measured in pristine and stage 1 FeCI3 intercalated graphite (C6.6FEC13) in the region from 0--10 eV are shown in fig. 1. These data were obtained at a small scattering angle (q = 0.l ,~ l) where the direction of momentum transfer is parallel to the graphite planes. In pristine graphite there is a low steplike knee near 1 eV corresponding to the onset of 7r--*vr* interband transitions and a strong plasmon peak at 7.0 eV due to a collective oscillation of all the ~ electrons [8, 9]. In F e C l 3 graphite the Fermi level is lowered due to charge transfer to the intercalant. The new intraband excitations now possible give rise to a sharp intraband plasmon which for q = 0.1 A ~ occurs at 1.14 eV in C66FEC13 and at 1.10 eV in the stage 2 compound (ClzsFeC13). The 7.0 eV 7r plasmon in graphite is shifted to 5.9 eV in C6.6FEC13 and 6.25 eV in C1zsFeC13. The 7r plasmons in the intercalated compounds are shifted to lower energy because charge transfer reduces the total oscillator strength of the 7r--* 7r* excitations and since the density of carbon planes is lowered, the oscillator strength is further reduced. Reduction in the oscillator strength reduces the plasmon energy but the momentum dependence of 7r
0
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5
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7
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ENERGY (eV)
Fig. 1, Valence excitation spectrum: relative energy loss probability normalized to the highest peak in pristine graphite (dashed curve) and C6.6FeC13 (solid curve) at q = 0.1 ~-1.
plasmons in pristine and intercalated materials is quite similar indicating that the 7r band structure is not significantly altered [6]. The momentum dependence of the intraband plasmon was shown to exhibit positive quadratic dispersion characteristic of a free-electron gas but with a very rapid falloff in intensity indicating strong band structure effects [6]. Since freeelectron gas models could not explain this behavior [6] we incorporated the band structure into an accurate calculation of the momentum dependent dielectric response function and computed the plasmon dispersion and momentum dependent intensity. In this calculation a tight binding model is fitted to the two dimensional graphite 7r bands which were originally computed by an ab initio variational method using a linear-combination of atomic-orbitals basis of Bloch states [10]. The tight binding model reproduces the density of states near the Fermi level very accurately and allows a straightforward calculation of the polarizability of a single graphite plane to be made [11]. The macroscopic dielectric response function is then computed by calculating proper spatial averages of the volume averaged polarization and electric fields. The contribution of the FeCl3 is taken into account by assuming a constant background dielectric constant, E~ = 5. Absorption in FeCI3 begins at 2.0eV and the only effect on the dielectric response function near the plasmon energy should be a nearly energy independent polarizability. Intra- and interband excitations are explicitly included and the plasmon energy is given by the zeros of el(q, E). The results are given in fig. 2 for a family of curves which differ by the amount of charge transfer or the Fermi momentum in the carbon planes. The experimental data, see ref. 6, are shown as discrete points. Clearly the plasmon dispersion is very sensitive to the amount of charge transfer and the data are well fitted by curve c so that the charge transfer per carbon atom in C6.6FEC13 is 0.015+_0.005electrons/carbon atom. Since the coupling between planes is important and nontrivial, calculations for ClzsFeCI3 will be the object of a subsequent publication. In addition to providing a good fit to the measured plasmon dispersion the model also predicts a very rapid falloff of the plasmon strength with increasing q. The rapid falloff is due to the large wavevector of the measured
427 20
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Fig. 2. Intraband plasmon dispersion: solid curves are theoretical dispersion relations for indicated a m o u n t s of charge transfer in electrons per carbon atom. Solid dots experimental data ref. 6. O p e n circle is extrapolated from fit to experimental data.
plasmons relative to the Fermi wavevector of the hole states created by the intercalation reaction, k~ (holes) = 0.15 A-~. For the integrated plasmon strength our model predicts a q-2.6 dependence which agrees very well with the observed value q-2S [6]. Further insight into the excitations of the valence electrons is given by an experimental determination of the dielectric function by a Kramers-Kronig analysis of the energy loss data. Spectra were recorded up to 100eV, corrected for multiple inelastic scattering and properly normalized by using the oscillator strength sum rule for the energy loss function, Im - l / E , and a value for E~(q) at zero energy given by our theoretical model. The results were sensitive to the theoretical input only in the low energy region below ~0.5 eV. The results shown in fig. 3 include a computation of E2 for pristine graphite obtained by a similar method, el crosses zero near 1 and 6 eV resulting in the strong plasmon peaks shown in fig. 1. Below 2 eV ~2 in C6.6FeCl 3 is very much weaker than in pristine graphite because the original 7r ~ ~'* interband transitions
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which began at very low energies (~-0 eV) now show a gap due to the lowering of the Fermi level. They are replaced by intraband excitations and a weak interband threshold at about 1.2_+ 0.2 eV. Peaks at 2.75, 3.9, 8 and 9 eV are due to FeCI3 [6]. The Q-point interband peak at 4.3 eV in pristine graphite is still seen in the intercalated material as a shoulder at the same energy, and the or band transitions near 13 and 15 eV are also observed in C6.6FeCi3. It is expected that the strength of the pure graphite critical points in the joint density of states will be reduced in the intercalated material since the density of carbon planes is very much reduced, but the fact that these critical points are apparent in the compound is an indication that the graphite band structure is not significantly perturbed. As a check on the accuracy of the calculated dielectric function, the normal incidence reflectivity was computed and compared with recent measurements [12]. Excellent agreement was obtained with the absolute values and shape of the reflectivity measured. In addition, the lowering of the plasma edge by ~ 0 . 0 4 e V in going from stage 1 to stage 2 was confirmed [12[. 4. Core excitations The electronic excitations of carbon IS core electrons are important since transitions from
428 these localized atomic states occur only to empty p orbitals localized on the carbon atoms. At forward scattering angle (0 = 0) the transition dipole matrix element selects only the Pz orbital along the c-axis which makes up the graphite 7r band. The experimental results are given in fig, 4. In pure graphite, the absorption edge is characterized by a broad, nearly symmetric peak centered at 285.35 - 0.05 eV approximately 1 eV above the threshold which corresponds to the Fermi level. In the intercalated compounds charge is removed from the carbon valence band, lowering the Fermi energy and creating new empty states which were occupied before the intercalation reaction. The carbon 1S excitation spectrum now includes transitions into these empty states and a new peak appears below the original pristine graphite threshold. Interaction of the final states with the core hole left behind causes the shape of this spectrum to deviate significantly from the original density of empty 7r states. Theoretical calculations which include the final state electron hole interaction in a contact approximation [11] yield an excellent description of the shape of the absorption spectrum and indicate the degree of Fermi energy lowering (which is approximately given by the width of the small peak at threshold). We find the lowering of
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I
I
the Fermi level to be 0 . 9 - 0 . 1 eV for C6.6FeCl3 and 0.7---0.1 eV for C12.5FEC13. Using our model theoretical density of states this corresponds to a charge transfer of 0.015 +- 0.003 electrons/carbon atom in stage 1 and 0.0075 --- 0.0025 electrons/carbon atom in stage 2. These results confirm the determination of charge transfer deduced from the plasmon dispersion in fig. 2. Moreover, the charge transfer per carbon atom in stage 2 is exactly one half that in stage 1 leading to the conclusion that the charge transfer per intercalant layer is constant independent of stage. Thus, in the case of FeCl3-graphite the intercalant determines the amount of charge transfer. This agrees with earlier Mossbauer measurements on this system [13]. In contrast, in the case of AsF5 intercalation compounds the charge transfer appears to be determined by the carbon [14].
5. Summary The electronic structure of the carbon in FeC13 intercalated graphite (stages 1 and 2) is accurately represented by that of two dimensional graphite with the Fermi level lowered by 0.9 eV and 0.7 eV in stage 1 and stage 2 compounds, respectively. Critical points in the dielectric
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Fig. 4. Raw data of carbon IS excitation spectra: open circles, pristine graphite; crosses, C6.6FEC13; triangles C~25FeC13. The point with error bars indicates uncertainty due to counting statistics and energy resolution.
429
function as obtained by a Kramers-Kronig analysis and the dispersion of interband zr plasmons imply that the band structure is not significantly perturbed on intercalation. The amount of charge transfer per carbon atom is 0.015 electrons in C6.6FeCl 3 a s determined by the dispersion of the intraband plasmon using a theoretical calculation of the momentum dependent dielectric function. This is in agreement with charge transfer as determined from the lowering of the Fermi level as evident in the interband ~ r ~ 7r* gap in E2 at 1.2--_0.2 eV (at q = 0.1 k -t) and as directly measured in the carbon 1S electron core excitation spectrum. In C125FEC13 the charge transfer is 0.0075 electrons/carbon atom. Charge transfer per intercalant unit ( F e C l 3 ) is 0.10 and is independent of stage.
We are grateful to C. Underhill for his excellent samples of stage 2 FeCl3-graphite and to A. Moore for the highly ordered pyrolytic graphite we used in preparing our own samples.
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