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Inelastic electron scattering spectroscopy of graphite-acceptor compounds
Synthetic Metals, 3 (1981) 73 - 80
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© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
INELASTIC E L E C T R O N SCATTERING SPECTROSCOPY OF G R A P H I T E - A C C E P T O R COMPOUNDS
JOHN J. RITSKO and EUGENE J. MELE
Xerox Webster Research Center, Webster, N Y 14580 (U.S.A.) (Received May 19, 1980}
Summary Electron energy loss spectroscopy has been used to measure the valence and core electronic excitations up to 300 eV in low stage AsFs-graphite residual intercalation compounds. The results are compared with similar measurements on FeCla-graphite intercalation compounds and graphite which has been d o p e d substitutionally with a small amount of boron. The AsFs-graphite samples showed well defined intraband plasmons near 1.2 eV which exhibited dispersion similar to that observed in stage-2 FeC13graphite. The excitation spectra of carbon ls electrons in a stage-2 c o m p o u n d were used as a direct measurement of the Fermi level position which is shown to be 0.9 eV below that of pristine graphite. Using an empirical model for the density of states the charge transfer per carbon atom is estimated as 0.025 electrons in stage-2 AsFs-graphite.
Introduction The synthetic metals with the highest conductivity are the low stage AsFs-graphite intercalation c o m p o u n d s [ 1, 2]. In an effort to understand the transport properties of these materials on a microscopic level, spectroscopic techniques such as optical reflectivity [3 - 5], X-ray absorption [6, 7], X-ray photoemission [ 8 ] , magnetic susceptibility [9], and de Haas-van Alphen [10] measurements have been employed. Measurements of the absolute Pauli spin susceptibility of stage-1 and stage-2 AsF~ intercalation c o m p o u n d s determined the density of states at the Fermi energy in these materials [9]. Utilizing a theoretical band structure model, the Fermi level position and the a m o u n t of charge transfer were then calculated [9]. Using a three dimensional rigid band model for graphite the de Haas-van Alphen periods lead to a determination of the Fermi level from which the charge transfer was also calculated [10]. In an earlier series of electron energy loss measurements and calculations on low stage FeC13-graphite intercalation compounds [11 - 15], it was
74 shown that the spectra of intraband plasmons near 1 eV, and core electronic excitations near 285 eV, could provide an accurate measurement of the Fermi level position and thus, through a theoretical density of states model, give a measure of the amount of charge transfer. In the present paper the results of inelastic electron scattering measurements on an AsFs-graphite intercalation c o m p o u n d are presented and compared with previous measurements on FeCl~-graphite compounds and new studies of boron-doped graphite, which will be described in detail in a separate paper. The present results yield the Fermi level position directly and, coupled with the earlier measurement of the density of states at the Fermi level [9], determine an empirical model for the graphite density of states which is in reasonable agreement with theoretical models [ 16, 17 ]. Thus, a refined computation of the amount of charge transfer in FeC13 as well as AsF5 intercalated graphite is provided.
Experimental The measurements reported here are the energy loss spectra of fast (80 keV) electrons transmitted through thin, cleaved, self-supporting samples. The energy resolution was 0.11 eV and the m o m e n t u m transfer resolution was 0.055 A-1 . Samples were intercalated in a manner similar to that used in the photoemission studies [8]. The starting material, highly oriented pyrolytic graphite kindly provided by A. Moore of Union Carbide Corp., was cleaved with adhesive tape to be < 1 0 0 0 A thick as determined by visual inspection of transmitted light. Thicknesses < 1 0 0 0 A were subsequently verified by studying the probability of multiple inelastic scattering events. The thin samples were m o u n t e d on gold electron microscope specimen grids (3 mm dia.), placed in a stainless steel vacuum reaction chamber, and degassed at 200 °C for 1 1/2 h under a vacuum of < 1 0 -6 Tort. They were then exposed to >~1 atmosphere of AsF5 at room temperature for periods varying from 15 min to 2 1/2 d in an attempt to form the completely saturated stage-1 intercalation compound. Despite the thin nature of the samples, subsequent analysis indicated that a 15 min exposure was insufficient to completely intercalate AsF5, b u t that exposures of several hours to several days all yielded the same reproducible final product. Since, at present, the energy loss spectrometer can only study samples under vacuum at room temperature, the residual c o m p o u n d formed by pumping off the AsF 5 gas in the intercalation chamber was studied. This was done both with samples of the stage-1 intercalated material prepared as above at room temperature and by first cooling the sample to --- 196 °C then warming to -- 95 °C before pumping off excess AsF 5 [8]. The latter samples were slowly warmed to room temperature While pumping continued [8]. In contrast to large, thick, stage-1 AsF5-graphite samples which have been exposed to vacuum at room temperature [4, 8 ] , no exfoliation was observed in the small, thin samples. After the AsF5 had been pumped out, the samples were
75 transferred, inside an argon filled glove box (oxygen < 0 . 8 ppm, water 2 - 3 ppm), to a transfer chamber which was then attached to the spectrometer. The sample was thus exposed only to pure argon for a period of about 1/2 h before being admitted to the spectrometer vacuum.
Results and discussion Samples of stage-1 AsFs-graphite, when exposed to vacuum, revert to a stage-2 residual c o m p o u n d [ 7]. Detailed X-ray diffraction studies on large samples reveal that actually a mixture of residual compounds, stage-1' and stage-2', can occur which have gallery spacings a b o u t 0.5 A smaller than the corresponding non-residual c o m p o u n d s [8]. Removal of the AsF5 atmosphere causes the deintercalation of neutral AsF 5 molecules and the subsequent crystal structure change [ 7, 8]. But, most importantly, exposure to the vacuum does not remove the electrically active species and the charge transfer to the graphite is n o t greatly affected [8]. The oxidation reaction is n o t reversible [ 8]. If this is the case, then measurements of the Fermi level and charge transfer in the residual samples should be characteristic of the non-residual c o m p o u n d s as well. Although X-ray diffraction studies cannot easily be done on the thin specimens used in this study, the energy loss spectra themselves suggest that the samples are primarily a stage-2 residual compound. Typical energy loss spectra of the AsFs-graphite samples from 0 - 10 eV at q = 0.1 A-z (momentum polarized parallel to the graphite planes) are compared in Fig. 1 with those of pristine graphite (the dashed curve}. The most significant differences are an intraband plasmon at 1.26 eV and the down-shifted interband plasmon at 6.5 eV. These spectra are qualitatively similar to those for low stage FeCl3-graphite c o m p o u n d s [ 1 1 ] , and can be understood in a similar fashion [ 13 - 1 5 ] . The intraband plasmon at q ~ 0 is manifested in optical reflectivity studies as a sharp dip in the reflectivity [3 - 5]. At q = 0 the plasm o n peak nearly coincides with the midpoint of the reflectivity edge. The measured intraband plasmon energy is in reasonable agreement with expected values for stage-2 c o m p o u n d s based on reflectivity studies [3, 4, 8]. Moreover, the Peak height of the intraband plasmon relative to the interband n plasmon at 6.5 eV is quite similar to that observed in stage-2 FeC13graphite, whereas in the stage-1 FeCls-graphite c o m p o u n d the intraband and interband plasmons were nearly of equal peak height [11]. Incidentally, the intraband plasmon peak height actually increases somewhat with time after the AsF5 atmosphere is removed. The solid curve in Fig. 1 shows the final reproducible spectrum which appears after a b o u t one day under the spectrometer vacuum of 10 -s Torr. The d o t t e d curve indicates the plasmon shape after 5 h of pumping. Apparently the final residual c o m p o u n d requires a long time to form at room temperature, even in thin samples. The peak position of the interband ~ plasmon provides additional information a b o u t the stage of a c o m p o u n d since it depends primarily on the
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Fig. 1. Energy loss spectra for graphite (dashed curve) and the stage-2 AsFs-graphite residual compound (solid curve) at q = 0.1 A -1 parallel to the graphite planes. The dotted curve shows the spectrum measured 5 h after pumping off of excess .aLsF5. The solid curve is the final spectrum attained after 1 day of exposure to vacuum. Fig. 2. Intraband p|asmon dispersion in AsF 5 intercalated graphite: energ~ loss spectra at q = 0.1 A-1 , (b) 0.14 A- I , (c) 0.174 A-1 , (d) 0.21 A-1 , (e) 0.243 A-1 .
density of carbon planes [14, 15]. Pure graphite and graphite substitutionally doped with 0.5% boron both show interband plasmons at 7.0 eV (for q = 0.1 h -1) since the carbon plane density is the same in these materials. However, stage-1 and stage-2 FeCls-graphite exhibit interband plasmons at 5.89 and 6.27 eV, respectively [15]. Assuming a linear correlation between interband plasmon energy and carbon plane density, a stage-2 AsFs-graphite residual compound would show a plasmon at 6.37 eV, whereas a stage-1 compound plasmon would be at 6.05 eV. Hence, a stage-2 designation for the samples studied here seems appropriate since the interband plasmon appears at 6.5 eV. The momentum dependence of the intraband plasmon is shown in Fig. 2. The peak at 1.26 eV at q = 0.1 A-1 is 0.4 eV wide, which is noticeably wider than the stage-1 FeCls-graphite intraband plasmon (0.3 eV). This may be due to changes which occur in forming the residual compound and is observed optically as a broadening of the reflectivity edge [8]. As momentum is increased the plasmon broadens further and disperses to higher energy. The dispersion relation is quadratic, and the coefficient of the q2 term is 19 e V A 2 which is nearly identical with that for stage 2 FeClsgraphite. This is notably larger than the dispersion for stage-1 FeCls-graphite [11] where the coefficient of the q2 term is only 12.5 e V A 2. The dispersion is intimately related to the shape of the energy bands near the Fermi level and the spectrum of intra- and interband transitions [13]. Thus, the similarity in dispersion between stage-2 FeCl3-graphite and the AsFs-graphite studied here suggests that the latter are also primarily stage-2.
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Fig. 3. Carbon l s core electron excitation spectra: o, HOPG; e, AsF 5 intercalated graphite (stage-2 residual compound).
The present samples cannot contain significant regions of stage 3 or higher stages since these compounds show a definite (although yet unexplained) absorption at 0.55 eV [3 - 5]. Such a feature would easily be seen in the energy loss spectrum, since the signal to noise ratio in Fig. 1 is about the thickness of the solid line, and its absence rules out higher stages. Moreover, none of the earlier X-ray diffraction studies of samples intercalated to saturation and then subsequently exposed to vacuum reported evidence for higher stage compounds [7, 8]. A detailed analysis of intraband plasmon dispersion based on the graphite band structure can provide a measurement of the Fermi level position in graphite acceptor compounds [13, 15]. Such an analysis has recently been carried out for stage-2 FeCla-graphite where the interplane carboncarbon interactions are explicitly accounted for [18]. When applied to the data of Fig. 2 the Fermi level position is estimated to be 0.85 eV below that of pristine graphite for the assumed stage-2 residual compound. A much more direct measurement of the Fermi level appears in the spectra of excitations of the carbon is core electrons shown in Fig. 3 where the raw data for the AsFs-graphite samples are plotted along with measurements on pristine graphite. The carbon ls states are very sharp in energy (<0.1 eV) so that excitations to the e m p t y 2p states which make up the conduction band directly probe the distribution of e m p t y conduction band states. The spectrum is strongly influenced by the electron-hole interaction and an accurate theoretical model [ 12] is needed to describe quantitatively the data of Fig. 3. Nevertheless, it is quite apparent that the Fermi level has been lowered in the AsFs-graphite compound since the absorption threshold has been shifted from 284.1 eV to 283.2 eV in going from pristine to intercalated graphite. A detailed analysis of the lineshape shows that the Fermi
78 level in the present stage-2 AsFs-graphite residual compound is 0.9 + 0.1 eV below that of pristine graphite, as compared with 0.9 eV and 0.7 eV for stage-1 and stage-2 FeC13-graphite, respectively [ 15]. This direct measurement falls between earlier, less direct estimates of 1.1 eV from the susceptibility of a stage-2 non-residual compound [9], and 0.765 eV from de Haasvan Alphen measurements on a stage-2 residual compound [10]. Figure 3 also indicates that the main peak at 285.3 eV in pristine graphite is shifted up by 0.2 eV in the intercalated compound. This is due to the larger binding energy of the carbon Is electrons in the intercalated sample, since the net charge per carbon atom has been reduced by charge transfer to the acceptor intercalant. This 0.2 eV shift is the same as XPS chemical shifts reported for AsF5-graphite compounds [ 8], and is a more precise measurement of this quantity since the experimental resolution in the electron energy loss measurements is much better than in the X-ray photoemission studies [8]. However, this small chemical shift cannot be used in a simple way to accurately determine the charge transfer per carbon atom as is often done [8]. While it is true that Siegbahn's charge v s . binding energy shift curves are fitted by a straight line with slope 5.6 eV binding energy shift per unit electronic charge, there is a large scatter in the data for small charge transfer [19]. Hence, there must be a relatively large uncertainty associated with the charge transfer of 0.035 electrons/carbon atom determined from the 0.2 eV chemical shift and the slope of Siegbahn's plot [ 8]. Since the present work measures the Fermi level position to about 10% a more accurate determination of the charge transfer should be possible. Theoretical models of the graphite density of states within about 1.3 eV of the pristine graphite Fermi level indicate a nearly linear dependence of the density of states on energy [9, 12, 16, 17, 20, 21]. There are, however, significant differences in the slope of the straight line, which is determined largely by the strength of the assumed nearest neighbor interaction 70 [17, 20, 21]. Thus, measurements of the charge transfer, which rely on a theoretical model of the density of states {such as the Pauli susceptibility [9] and electron energy loss measurements [12, 15] ), are subject to rather large uncertainties [21]. For example, in stage-1 FeCla-graphite the Fermi level was measured as 0.9 +- 0.1 eV below that in pristine graphite, the same as in the present AsFs-graphite compound. Using a band structure model with 7o = 3.0 eV, a charge transfer of 0.015 electrons/carbon atom was computed [12, 15]. Had the model used in the analysis of the susceptibility data [9] been used, 70 = 2.9 eV [17], a charge transfer of 0.021 electrons/carbon atom would have been computed. This is at the upper limit of the original uncertainty [ 12, 15]. Given that the present experiments measure the Fermi level position in a stage-2 AsF5 residual compound, and that the oxidation of the graphite is not reversible when the AsF5 atmosphere is pumped off [8], the empirical density of states at the Fermi level for stage-2 AsF5graphite [ 9] can be used to fix the slope of the density of states line. Using 0.056 states/eV-carbon atom [9] and E~ = 0.9 eV, the charge transfer is estimated as 0.025 electrons/carbon atom in stage-2 AsFs-graphite. The
79 empirical density of states curve lies just above the earlier theoretical lines [9, 17] and corresponds to 70 - 2.8 eV, which is still quite reasonable. Values of 70 as low as 2.4 eV have been used [ 2 1 ] . Another way of seeing the difference caused by the empirical determination is to note that with the original theoretical density of states [9] the Fermi level position for stage-2 AsF5-graphite was predicted to lie 1.1 eV below that of pristine graphite, whereas the value measured here is 0.9 eV + 0.1 eV. The amount of charge transfer per intercalant molecule, f, in AsF5graphite has been the subject of much controversy [2 - 9]. Using the formula C20 AsF5 for the residual c o m p o u n d [8] we calculate f = 0.5 electrons for stage-2. This agrees well with the earlier estimates based on the susceptibility measurements [9]. f Is less than 2/3, implying that all the AsF5 is not reduced, and that even in the residual c o m p o u n d some neutral AsF5 is present [8]. The reason that f increases by a factor of 2 in going from stage1 to stage-2 is presumably because stage-1 contains relatively more neutral molecules. This is by contrast with the FeC13-graphite c o m p o u n d s where f is the same for stages 1 and 2 [15]. In the above analysis it was assumed that the oxidation of the graphite was completely irreversible on removal of the AsF5 atmosphere [ 8]. The slight down-shift (0.2 eV) of the reflectivity edge in forming the residual c o m p o u n d s may indicate that this is not entirely correct (although a change in stage complicates the analysis). Reduction in the partial pressure of neutral AsF5 will tend to push the oxidation reaction in the reverse direction and cause reverse charge transfer which would be indicated by a down-shift o f the intraband plasmon energy. Based on our theoretical analysis however, we estimate that a shift of 0.2 eV in plasmon energy would accompany a shift in Fermi level of a b o u t 0.1 eV, which is a b o u t the uncertainty in the Fermi level determination. Such a shift, however, would bring our empirical density of states curve into even better agreement with earlier theoretical models [9, 17].
Conclusions In this paper we have reported electron energy loss measurements of valence and core electronic excitations in AsF5-graphite residual compounds. Comparison has been made with similar data and analysis for stages 1 and 2 FeC13-graphite compounds. Analogous experiments were performed on pyrolytic graphite samples substitutionally doped with 0.5% boron kindly provided by A. Moore of Union Carbide Corp. In these samples, core excitation spectra show that the Fermi level is only 0.2 eV below that of pristine graphite, and that new absorption near 1 eV appears which is the precursor of the intraband plasmons measured in the low stage intercalation compounds. The details of these measurements and their analysis based on the three dimensional graphite band structure will be dealt with in a separate article. Here, we merely point o u t that the energy loss measurements are particularly suited to measurements of Fermi level position, electronic structure, and charge transfer in a variety of graphite-acceptor compounds.
80
The results of our measurements indicate that small, thin samples of highly oriented pyrolytic graphite, intercalated to saturation with gaseous AsF5 at room temperature and subsequently exposed to vacuum, form, primarily, a stage-2 residual compound. The dispersion of intraband plasmons near 1.2 eV and the carbon l s core electron excitation spectrum were measured. Analysis of the core electron spectrum showed the Fermi level to be 0.9 eV + 0.1 eV below that of pristine graphite, in good agreement with the Fermi level determined from an analysis of the intraband plasmon dispersion. Using an empirical determination of the density of states at the Fermi level [9], the charge transfer per carbon atom is estimated as 0.025 electrons. The charge transfer per intercalant molecule in stage-2 AsF 5graphite is estimated as 0.5. With our empirical density of states, refined estimates of the charge transfer in FeCla-graphite compounds can be made. In stages 1 and 2 FeC13-graphite the charge transfer per carbon atom is 0.025 and 0.012 electrons, respectively. The charge transfer per intercalant unit, f, is 0.16, independent of stage. These latter estimates are substantially larger than earlier results [12, 15].
References 1 G. M. T. Foley, C. Zeller, E. R. Falardeau and F. L. Vogel, Solid State Commun., 24 (1977) 371. 2 J. E. Fischer and T. E. Thompson, Phys. Today, 31 (7) (1978) 36. 3 L. R. Hanlon,E. R. Falardeau and J. E. Fischer, Solid State Commun., 24 (1977) 377. 4 L. R. Hanlon, E. R. Falardeau, D. Gu4rard and J. E. Fischer, Mater. Sci. Eng., 31 (1977) 161. 5 C. C. Shieh, R. L. Schmidt and J. E. Fischer, Phys. Rev. B, 20 (1979) 3351. 6 N. Bartlett, R. N. Biagioni, B. W. McQuillan, A. S. Robertson and A. C. Thompson, J. Chem. Soc., Chem. Commun., (1978) 200. 7 N. Bartlett, B. McQuillan and A. S. Robertson, Mater. Res. Bull., 13 (1978) 1259. 8 M. J. Moran, J. E. Fischer and W. R. Salaneck, J. Chem. Phys., 73 (1980) 629. 9 B. R. Weinberger, J. Kaufer, A. J. Heeger, J. E. Fischer, M. Moran and N. A. W. Holzwarth, Phys. Rev. Lett., 41 (1978) 1417. 10 Y. Iye, O. Takahashi and S. Tanuma, Solid State Commun., 33 (1980) 1071. 11 J . J . Ritsko and M. J. Rice, Phys. Rev. Lett., 42 (1979) 666. 12 E. J. Mele and J. J. Ritsko, Phys. Rev. Lett., 43 (1979) 68. 13 E. J. Mele and J. J. Ritsko, Solid State Commun., 33 (1980) 937. 14 J. J. Ritsko a~d E. J. Mele, Phys. Rev. B, 21 (1980) 730. 15 J. J. Ritsko and E. J. Mele, Physica, 99B (1980) 425. 16 G. S. Painter and D. E. Ellis, Phys. Rev. B, 1 (1970) 4747. 17 N. A. W. Holzwarth, Phys. Rev. B, in press. 18 E. J. Mele and J. J. Ritsko, to be published. 19 K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden, K. Hamrin, V. Gelius, T. Bergmark, L. O. Werme, R. Manne and Y. Baer, ESCA Applied to Free Molecules, North Holland, Amsterdam, 1969, p. 114. 20 J. W. McClure,Phys. Rev., 108 (1957)612. 21 J. Blinowski, N. H. Hau, C. Rigaux, J. P. Vieren, R. LeToullec, G. Furdin, A. H~rold and J. Melin, J. Phys. (Paris), 41 (1980) 47.
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© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
INELASTIC E L E C T R O N SCATTERING SPECTROSCOPY OF G R A P H I T E - A C C E P T O R COMPOUNDS
JOHN J. RITSKO and EUGENE J. MELE
Xerox Webster Research Center, Webster, N Y 14580 (U.S.A.) (Received May 19, 1980}
Summary Electron energy loss spectroscopy has been used to measure the valence and core electronic excitations up to 300 eV in low stage AsFs-graphite residual intercalation compounds. The results are compared with similar measurements on FeCla-graphite intercalation compounds and graphite which has been d o p e d substitutionally with a small amount of boron. The AsFs-graphite samples showed well defined intraband plasmons near 1.2 eV which exhibited dispersion similar to that observed in stage-2 FeC13graphite. The excitation spectra of carbon ls electrons in a stage-2 c o m p o u n d were used as a direct measurement of the Fermi level position which is shown to be 0.9 eV below that of pristine graphite. Using an empirical model for the density of states the charge transfer per carbon atom is estimated as 0.025 electrons in stage-2 AsFs-graphite.
Introduction The synthetic metals with the highest conductivity are the low stage AsFs-graphite intercalation c o m p o u n d s [ 1, 2]. In an effort to understand the transport properties of these materials on a microscopic level, spectroscopic techniques such as optical reflectivity [3 - 5], X-ray absorption [6, 7], X-ray photoemission [ 8 ] , magnetic susceptibility [9], and de Haas-van Alphen [10] measurements have been employed. Measurements of the absolute Pauli spin susceptibility of stage-1 and stage-2 AsF~ intercalation c o m p o u n d s determined the density of states at the Fermi energy in these materials [9]. Utilizing a theoretical band structure model, the Fermi level position and the a m o u n t of charge transfer were then calculated [9]. Using a three dimensional rigid band model for graphite the de Haas-van Alphen periods lead to a determination of the Fermi level from which the charge transfer was also calculated [10]. In an earlier series of electron energy loss measurements and calculations on low stage FeC13-graphite intercalation compounds [11 - 15], it was
74 shown that the spectra of intraband plasmons near 1 eV, and core electronic excitations near 285 eV, could provide an accurate measurement of the Fermi level position and thus, through a theoretical density of states model, give a measure of the amount of charge transfer. In the present paper the results of inelastic electron scattering measurements on an AsFs-graphite intercalation c o m p o u n d are presented and compared with previous measurements on FeCl~-graphite compounds and new studies of boron-doped graphite, which will be described in detail in a separate paper. The present results yield the Fermi level position directly and, coupled with the earlier measurement of the density of states at the Fermi level [9], determine an empirical model for the graphite density of states which is in reasonable agreement with theoretical models [ 16, 17 ]. Thus, a refined computation of the amount of charge transfer in FeC13 as well as AsF5 intercalated graphite is provided.
Experimental The measurements reported here are the energy loss spectra of fast (80 keV) electrons transmitted through thin, cleaved, self-supporting samples. The energy resolution was 0.11 eV and the m o m e n t u m transfer resolution was 0.055 A-1 . Samples were intercalated in a manner similar to that used in the photoemission studies [8]. The starting material, highly oriented pyrolytic graphite kindly provided by A. Moore of Union Carbide Corp., was cleaved with adhesive tape to be < 1 0 0 0 A thick as determined by visual inspection of transmitted light. Thicknesses < 1 0 0 0 A were subsequently verified by studying the probability of multiple inelastic scattering events. The thin samples were m o u n t e d on gold electron microscope specimen grids (3 mm dia.), placed in a stainless steel vacuum reaction chamber, and degassed at 200 °C for 1 1/2 h under a vacuum of < 1 0 -6 Tort. They were then exposed to >~1 atmosphere of AsF5 at room temperature for periods varying from 15 min to 2 1/2 d in an attempt to form the completely saturated stage-1 intercalation compound. Despite the thin nature of the samples, subsequent analysis indicated that a 15 min exposure was insufficient to completely intercalate AsF5, b u t that exposures of several hours to several days all yielded the same reproducible final product. Since, at present, the energy loss spectrometer can only study samples under vacuum at room temperature, the residual c o m p o u n d formed by pumping off the AsF 5 gas in the intercalation chamber was studied. This was done both with samples of the stage-1 intercalated material prepared as above at room temperature and by first cooling the sample to --- 196 °C then warming to -- 95 °C before pumping off excess AsF 5 [8]. The latter samples were slowly warmed to room temperature While pumping continued [8]. In contrast to large, thick, stage-1 AsF5-graphite samples which have been exposed to vacuum at room temperature [4, 8 ] , no exfoliation was observed in the small, thin samples. After the AsF5 had been pumped out, the samples were
75 transferred, inside an argon filled glove box (oxygen < 0 . 8 ppm, water 2 - 3 ppm), to a transfer chamber which was then attached to the spectrometer. The sample was thus exposed only to pure argon for a period of about 1/2 h before being admitted to the spectrometer vacuum.
Results and discussion Samples of stage-1 AsFs-graphite, when exposed to vacuum, revert to a stage-2 residual c o m p o u n d [ 7]. Detailed X-ray diffraction studies on large samples reveal that actually a mixture of residual compounds, stage-1' and stage-2', can occur which have gallery spacings a b o u t 0.5 A smaller than the corresponding non-residual c o m p o u n d s [8]. Removal of the AsF5 atmosphere causes the deintercalation of neutral AsF 5 molecules and the subsequent crystal structure change [ 7, 8]. But, most importantly, exposure to the vacuum does not remove the electrically active species and the charge transfer to the graphite is n o t greatly affected [8]. The oxidation reaction is n o t reversible [ 8]. If this is the case, then measurements of the Fermi level and charge transfer in the residual samples should be characteristic of the non-residual c o m p o u n d s as well. Although X-ray diffraction studies cannot easily be done on the thin specimens used in this study, the energy loss spectra themselves suggest that the samples are primarily a stage-2 residual compound. Typical energy loss spectra of the AsFs-graphite samples from 0 - 10 eV at q = 0.1 A-z (momentum polarized parallel to the graphite planes) are compared in Fig. 1 with those of pristine graphite (the dashed curve}. The most significant differences are an intraband plasmon at 1.26 eV and the down-shifted interband plasmon at 6.5 eV. These spectra are qualitatively similar to those for low stage FeCl3-graphite c o m p o u n d s [ 1 1 ] , and can be understood in a similar fashion [ 13 - 1 5 ] . The intraband plasmon at q ~ 0 is manifested in optical reflectivity studies as a sharp dip in the reflectivity [3 - 5]. At q = 0 the plasm o n peak nearly coincides with the midpoint of the reflectivity edge. The measured intraband plasmon energy is in reasonable agreement with expected values for stage-2 c o m p o u n d s based on reflectivity studies [3, 4, 8]. Moreover, the Peak height of the intraband plasmon relative to the interband n plasmon at 6.5 eV is quite similar to that observed in stage-2 FeC13graphite, whereas in the stage-1 FeCls-graphite c o m p o u n d the intraband and interband plasmons were nearly of equal peak height [11]. Incidentally, the intraband plasmon peak height actually increases somewhat with time after the AsF5 atmosphere is removed. The solid curve in Fig. 1 shows the final reproducible spectrum which appears after a b o u t one day under the spectrometer vacuum of 10 -s Torr. The d o t t e d curve indicates the plasmon shape after 5 h of pumping. Apparently the final residual c o m p o u n d requires a long time to form at room temperature, even in thin samples. The peak position of the interband ~ plasmon provides additional information a b o u t the stage of a c o m p o u n d since it depends primarily on the
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Fig. 1. Energy loss spectra for graphite (dashed curve) and the stage-2 AsFs-graphite residual compound (solid curve) at q = 0.1 A -1 parallel to the graphite planes. The dotted curve shows the spectrum measured 5 h after pumping off of excess .aLsF5. The solid curve is the final spectrum attained after 1 day of exposure to vacuum. Fig. 2. Intraband p|asmon dispersion in AsF 5 intercalated graphite: energ~ loss spectra at q = 0.1 A-1 , (b) 0.14 A- I , (c) 0.174 A-1 , (d) 0.21 A-1 , (e) 0.243 A-1 .
density of carbon planes [14, 15]. Pure graphite and graphite substitutionally doped with 0.5% boron both show interband plasmons at 7.0 eV (for q = 0.1 h -1) since the carbon plane density is the same in these materials. However, stage-1 and stage-2 FeCls-graphite exhibit interband plasmons at 5.89 and 6.27 eV, respectively [15]. Assuming a linear correlation between interband plasmon energy and carbon plane density, a stage-2 AsFs-graphite residual compound would show a plasmon at 6.37 eV, whereas a stage-1 compound plasmon would be at 6.05 eV. Hence, a stage-2 designation for the samples studied here seems appropriate since the interband plasmon appears at 6.5 eV. The momentum dependence of the intraband plasmon is shown in Fig. 2. The peak at 1.26 eV at q = 0.1 A-1 is 0.4 eV wide, which is noticeably wider than the stage-1 FeCls-graphite intraband plasmon (0.3 eV). This may be due to changes which occur in forming the residual compound and is observed optically as a broadening of the reflectivity edge [8]. As momentum is increased the plasmon broadens further and disperses to higher energy. The dispersion relation is quadratic, and the coefficient of the q2 term is 19 e V A 2 which is nearly identical with that for stage 2 FeClsgraphite. This is notably larger than the dispersion for stage-1 FeCls-graphite [11] where the coefficient of the q2 term is only 12.5 e V A 2. The dispersion is intimately related to the shape of the energy bands near the Fermi level and the spectrum of intra- and interband transitions [13]. Thus, the similarity in dispersion between stage-2 FeCl3-graphite and the AsFs-graphite studied here suggests that the latter are also primarily stage-2.
77 l
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o
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o
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o o Oo o ''°°oo~ "°o
o
.o.
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283
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,8 "o
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285 ENERGY (ev}
Fig. 3. Carbon l s core electron excitation spectra: o, HOPG; e, AsF 5 intercalated graphite (stage-2 residual compound).
The present samples cannot contain significant regions of stage 3 or higher stages since these compounds show a definite (although yet unexplained) absorption at 0.55 eV [3 - 5]. Such a feature would easily be seen in the energy loss spectrum, since the signal to noise ratio in Fig. 1 is about the thickness of the solid line, and its absence rules out higher stages. Moreover, none of the earlier X-ray diffraction studies of samples intercalated to saturation and then subsequently exposed to vacuum reported evidence for higher stage compounds [7, 8]. A detailed analysis of intraband plasmon dispersion based on the graphite band structure can provide a measurement of the Fermi level position in graphite acceptor compounds [13, 15]. Such an analysis has recently been carried out for stage-2 FeCla-graphite where the interplane carboncarbon interactions are explicitly accounted for [18]. When applied to the data of Fig. 2 the Fermi level position is estimated to be 0.85 eV below that of pristine graphite for the assumed stage-2 residual compound. A much more direct measurement of the Fermi level appears in the spectra of excitations of the carbon is core electrons shown in Fig. 3 where the raw data for the AsFs-graphite samples are plotted along with measurements on pristine graphite. The carbon ls states are very sharp in energy (<0.1 eV) so that excitations to the e m p t y 2p states which make up the conduction band directly probe the distribution of e m p t y conduction band states. The spectrum is strongly influenced by the electron-hole interaction and an accurate theoretical model [ 12] is needed to describe quantitatively the data of Fig. 3. Nevertheless, it is quite apparent that the Fermi level has been lowered in the AsFs-graphite compound since the absorption threshold has been shifted from 284.1 eV to 283.2 eV in going from pristine to intercalated graphite. A detailed analysis of the lineshape shows that the Fermi
78 level in the present stage-2 AsFs-graphite residual compound is 0.9 + 0.1 eV below that of pristine graphite, as compared with 0.9 eV and 0.7 eV for stage-1 and stage-2 FeC13-graphite, respectively [ 15]. This direct measurement falls between earlier, less direct estimates of 1.1 eV from the susceptibility of a stage-2 non-residual compound [9], and 0.765 eV from de Haasvan Alphen measurements on a stage-2 residual compound [10]. Figure 3 also indicates that the main peak at 285.3 eV in pristine graphite is shifted up by 0.2 eV in the intercalated compound. This is due to the larger binding energy of the carbon Is electrons in the intercalated sample, since the net charge per carbon atom has been reduced by charge transfer to the acceptor intercalant. This 0.2 eV shift is the same as XPS chemical shifts reported for AsF5-graphite compounds [ 8], and is a more precise measurement of this quantity since the experimental resolution in the electron energy loss measurements is much better than in the X-ray photoemission studies [8]. However, this small chemical shift cannot be used in a simple way to accurately determine the charge transfer per carbon atom as is often done [8]. While it is true that Siegbahn's charge v s . binding energy shift curves are fitted by a straight line with slope 5.6 eV binding energy shift per unit electronic charge, there is a large scatter in the data for small charge transfer [19]. Hence, there must be a relatively large uncertainty associated with the charge transfer of 0.035 electrons/carbon atom determined from the 0.2 eV chemical shift and the slope of Siegbahn's plot [ 8]. Since the present work measures the Fermi level position to about 10% a more accurate determination of the charge transfer should be possible. Theoretical models of the graphite density of states within about 1.3 eV of the pristine graphite Fermi level indicate a nearly linear dependence of the density of states on energy [9, 12, 16, 17, 20, 21]. There are, however, significant differences in the slope of the straight line, which is determined largely by the strength of the assumed nearest neighbor interaction 70 [17, 20, 21]. Thus, measurements of the charge transfer, which rely on a theoretical model of the density of states {such as the Pauli susceptibility [9] and electron energy loss measurements [12, 15] ), are subject to rather large uncertainties [21]. For example, in stage-1 FeCla-graphite the Fermi level was measured as 0.9 +- 0.1 eV below that in pristine graphite, the same as in the present AsFs-graphite compound. Using a band structure model with 7o = 3.0 eV, a charge transfer of 0.015 electrons/carbon atom was computed [12, 15]. Had the model used in the analysis of the susceptibility data [9] been used, 70 = 2.9 eV [17], a charge transfer of 0.021 electrons/carbon atom would have been computed. This is at the upper limit of the original uncertainty [ 12, 15]. Given that the present experiments measure the Fermi level position in a stage-2 AsF5 residual compound, and that the oxidation of the graphite is not reversible when the AsF5 atmosphere is pumped off [8], the empirical density of states at the Fermi level for stage-2 AsF5graphite [ 9] can be used to fix the slope of the density of states line. Using 0.056 states/eV-carbon atom [9] and E~ = 0.9 eV, the charge transfer is estimated as 0.025 electrons/carbon atom in stage-2 AsFs-graphite. The
79 empirical density of states curve lies just above the earlier theoretical lines [9, 17] and corresponds to 70 - 2.8 eV, which is still quite reasonable. Values of 70 as low as 2.4 eV have been used [ 2 1 ] . Another way of seeing the difference caused by the empirical determination is to note that with the original theoretical density of states [9] the Fermi level position for stage-2 AsF5-graphite was predicted to lie 1.1 eV below that of pristine graphite, whereas the value measured here is 0.9 eV + 0.1 eV. The amount of charge transfer per intercalant molecule, f, in AsF5graphite has been the subject of much controversy [2 - 9]. Using the formula C20 AsF5 for the residual c o m p o u n d [8] we calculate f = 0.5 electrons for stage-2. This agrees well with the earlier estimates based on the susceptibility measurements [9]. f Is less than 2/3, implying that all the AsF5 is not reduced, and that even in the residual c o m p o u n d some neutral AsF5 is present [8]. The reason that f increases by a factor of 2 in going from stage1 to stage-2 is presumably because stage-1 contains relatively more neutral molecules. This is by contrast with the FeC13-graphite c o m p o u n d s where f is the same for stages 1 and 2 [15]. In the above analysis it was assumed that the oxidation of the graphite was completely irreversible on removal of the AsF5 atmosphere [ 8]. The slight down-shift (0.2 eV) of the reflectivity edge in forming the residual c o m p o u n d s may indicate that this is not entirely correct (although a change in stage complicates the analysis). Reduction in the partial pressure of neutral AsF5 will tend to push the oxidation reaction in the reverse direction and cause reverse charge transfer which would be indicated by a down-shift o f the intraband plasmon energy. Based on our theoretical analysis however, we estimate that a shift of 0.2 eV in plasmon energy would accompany a shift in Fermi level of a b o u t 0.1 eV, which is a b o u t the uncertainty in the Fermi level determination. Such a shift, however, would bring our empirical density of states curve into even better agreement with earlier theoretical models [9, 17].
Conclusions In this paper we have reported electron energy loss measurements of valence and core electronic excitations in AsF5-graphite residual compounds. Comparison has been made with similar data and analysis for stages 1 and 2 FeC13-graphite compounds. Analogous experiments were performed on pyrolytic graphite samples substitutionally doped with 0.5% boron kindly provided by A. Moore of Union Carbide Corp. In these samples, core excitation spectra show that the Fermi level is only 0.2 eV below that of pristine graphite, and that new absorption near 1 eV appears which is the precursor of the intraband plasmons measured in the low stage intercalation compounds. The details of these measurements and their analysis based on the three dimensional graphite band structure will be dealt with in a separate article. Here, we merely point o u t that the energy loss measurements are particularly suited to measurements of Fermi level position, electronic structure, and charge transfer in a variety of graphite-acceptor compounds.
80
The results of our measurements indicate that small, thin samples of highly oriented pyrolytic graphite, intercalated to saturation with gaseous AsF5 at room temperature and subsequently exposed to vacuum, form, primarily, a stage-2 residual compound. The dispersion of intraband plasmons near 1.2 eV and the carbon l s core electron excitation spectrum were measured. Analysis of the core electron spectrum showed the Fermi level to be 0.9 eV + 0.1 eV below that of pristine graphite, in good agreement with the Fermi level determined from an analysis of the intraband plasmon dispersion. Using an empirical determination of the density of states at the Fermi level [9], the charge transfer per carbon atom is estimated as 0.025 electrons. The charge transfer per intercalant molecule in stage-2 AsF 5graphite is estimated as 0.5. With our empirical density of states, refined estimates of the charge transfer in FeCla-graphite compounds can be made. In stages 1 and 2 FeC13-graphite the charge transfer per carbon atom is 0.025 and 0.012 electrons, respectively. The charge transfer per intercalant unit, f, is 0.16, independent of stage. These latter estimates are substantially larger than earlier results [12, 15].
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