- Email: [email protected]
Thermal and adsorbate induced plasmon energy shifts in graphite
Surface Science 524 (2003) L77–L83 www.elsevier.com/locate/susc
Surface Science Letters
Thermal and adsorbate induced plasmon energy shifts in graphite Michael Gleeson, Bengt Kasemo, Dinko Chakarov
*
Department of Applied Physics, Chalmers University of Technology and University of G€oteborg, S-412 96 G€oteborg, Sweden Received 2 May 2002; accepted for publication 14 October 2002
Abstract Electron-energy-loss spectroscopy has been used to study plasmon excitations in pristine and alkali metal (Na, K, Cs) dosed highly oriented pyrolitic graphite in the temperature range 85–400 K. The energy and line width of the pband Ekc plasmon in pristine graphite was found to be extremely sensitive to substrate temperature and to the presence of adsorbates in qualitative accord with previously reported observations and theoretical calculations. After increasing monotonously with coverage (following square root dependence) the plasmon energy levels off above specific alkali coverages (hNa 1:45, hK 0:25, and hCs 0:2) at a common plasmon energy hx 250 meV. The results are interpreted as evidence for formation of alkali-like bands in the topmost graphite layers. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Graphite; Plasmons; Electron energy loss spectroscopy (EELS); Alkali metals
The electronic excitation at surfaces and interfaces and the charge transfer upon adsorption are topics of fundamental interest in surface physics. In this Letter we address these two issues by using electron energy loss spectroscopy (EELS) to study plasmon properties of graphite. Graphite is a semimetal with very low density of states (DOS) near the Fermi level EF due to the small overlap of the p-bands. Adsorption of alkalis offers the possibility to modify electron density in the conduction band by several orders of magnitude, making the
*
Corresponding author. Tel.: +46-31-772-3375; fax: +46-31772-3134. E-mail address: [email protected] (D. Chakarov).
system of particular interest. Jensen et al. [1] presented a band structure calculation describing the temperature dependence of the graphite plasmon frequency supported by EEL spectra. In Fig. 1 we show analogous measurements from highly oriented pyrolitic graphite (HOPG) (Union Carbide, grade XYA) at sample temperatures between 87 and 270 K obtained with an ELS22 spectrometer [2]. Our spectra reproduce qualitatively well those reported in Ref. [1]: A broad peak is observed superimposed on the strong loss continuum typical for graphite. The peaks are well resolved at temperatures above 115 K and their maxima shift towards higher energies with increasing sample temperature. The spectra in Fig. 1 are taken in single energy scans while simultaneously, stepwise decreasing
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 2 4 8 3 - 4
CE RF A LE C E TT S E R C IE S N
SU
L78
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
reproducible; the temperature dependent energy, and the relative intensity of the energy losses changed beyond the statistical error limits. We were further puzzled by the disagreement in the absolute energy values and peak widths, compared to the spectra from Ref. [1], in spite of our better energy resolution and good counting rate. We then changed the cleaning procedure increasing the annealing temperature to 1400 K at which we kept the sample for at least 5 min. Typical spectra taken after applying this treatment are shown in Fig. 2. The difference between the spectra in Figs. 1 and 2 (for the respective temperatures) is obvious: With the new cleaning procedure the plasmon energy loss peak lies at lower energy and appears only as a shoulder on the intense elastic peak. The shape of the spectrum at 307 K (Fig. 2) resembles closely those presented by Oshima et al. [4], which were taken with similar energy resolution and (presumably) at room temperature. The measured values for the plasmon energy are in excellent agreement with theoretical calculations [1,9] and with experimentally obtained
Fig. 1. EEL spectra obtained from HOPG prepared by standard cleaning procedure (see text). The spectra are taken during cooling of the sample to the indicated temperatures (indicated temperature is correct within DT ¼ 3 K). Initial electron energy E0 ¼ 25 eV; specular scattering at 60°.
the sample temperature, as indicated. Our routine experimental procedure (for the sample mounting and initial cleaning, see Ref. [2]) includes annealing at 1100–1200 K in UHV (P < 2:1010 Torr) for several minutes and fast (15 min) cooling down to 83–90 K before exposing the sample to the electron beam in the scattering chamber of the spectrometer. This procedure has been proven to ensure that no traces of contamination are detectable as judged by TDS and AES [2]. In the course of experiments we found that the spectral features in Fig. 1 are not satisfactorily
Fig. 2. EEL spectra obtained from the same sample after annealing at 1450 K (see text). The dashed curves represent ðhxÞ1:3 empirical scaling with the energy loss hx The loss energies at 35 meV (86 K) and 55 meV (307 K) are in good agreement with theoretical plasmon energy of the free carriers at the Fermi surface.
SU N IE SC RS CE E A TT R F LE CE
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
values [10]. Adopting the above described preparation procedure we were no longer able to reproduce the spectra in Fig. 1. Several characteristic features of the electronic excitations seen in Fig. 2 may be noted. First, the elastic peak indicates a broadened energy distribution of 8 meV as compared with the primary electron beam width that is less than 6 meV (as measured in the straight-through geometry). Second, the background intensities in the spectra are more than three orders of magnitude higher than those reflected from a typical metal surface [3] and are distributed over a wide energy range (up to 3 1:3 eV) following an ð hxÞ empirical scaling with the energy loss. The high background intensities are sharply peaked around the specular direction, a typical feature of the dipole excitation mechanism. Third, there is a shoulder at 35 meV (T ¼ 86 K) and at 55 meV (T ¼ 307 K) suggesting a weak peak with a width of more than 50 meV. Such a big width (compared for example to phonon losses in graphite [4] or plasmon losses in metals [5]) is indicative of a stronger damping of electronic excitations in graphite compared to metals. The loss energy is in good agreement with the theoretical plasmon energy of the free carriers in graphite at the Fermi surface predicted by Boyle and Nozieres [6], and with those measured in other experiments by different techniques [7]. All these characteristics motivate these spectral features to be assigned to the graphite p-band plasmon excitation polarized parallel to the c-axis. The low plasmon energy reflects the semimetallic band structure of graphite with a very low electron density in the conduction band. There are (at least) two questions arising from our observations that needs to be answered: (i) can we verify the suspected contamination and, if so, what is the origin of the contamination and (ii) why does the contamination induce plasmons that follow more or less perfectly the theoretically predicted temperature dependence for clean graphite? The most probable contamination in our case are traces of alkali metals, Na, K, and Cs accumulated in the course of previous experiments performed with the same graphite sample [2], that eventually segregate to the surface. When depos-
L79
ited as submonolayers on the graphite surface, these alkali metals desorb thermally at T 6 500 K. The desorption is complicated by the phenomenon of inter-diffusion, or intercalation, that also is thermally activated. The heats of formation of graphite intercalation compounds (GIC) are in the range 1.15–1.5 eV for stages I to V GICÕs [8], i.e. in the same range as the desorption energies, thus placing the intercalation process in competition with the thermal desorption. With the charge transfer to the substrate as a driving force, alkali metal intercalation is strongly dependent on the morphology of graphite surface. It is believed that the main intercalation channel is through the graphite basal plane defects and sample edges. Intercalated, as well as ‘‘trapped’’ alkali atoms (on defect sites or edges) may well serve as a source of surface contamination. Evidently, the experiments demonstrate that the short annealing to 1100 K employed by several laboratories, including ours, is not sufficient to completely remove intercalated or defect bonded alkalis. On the other hand, the sensitivity limits of TDS and AES are about 1 atomic percent that in turn explains why these methods fail to detect them. As it will be shown below, plasmon energies below 70 meV correspond to alkali atoms concentrations below these detection limits. In order to check more carefully for the effect of contamination we performed experiments with controlled depositions of Na, K, and Cs on HOPG applying higher temperature cleaning procedure. Fig. 3 illustrates such measurements at 89 K. The intensity of graphite plasmon loss increases and shifts to higher energy with increasing alkali coverage. Additional plasmon losses, including the multiple losses [5] and other electronic excitations, with higher energy, that appear with increasing alkali coverages were also systematically studied. These results will be presented in a forthcoming paper. Fig. 4 summarizes our observations by showing the frequency of the Na, K and Cs induced plasmon modes as a function of the square root of the alkali coverage. It is commonly accepted through many theoretical and experimental studies that in GICÕs electrons are donated to the previously (almost) empty carbon p anti bonding band, raising the
CE RF A LE C E TT S E R C IE S N
SU
L80
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
Fig. 3. EEL spectra for different Na, K, and Cs coverages on graphite at 89 K. One monolayer, 1 ML is defined to correspond to 6:3 1014 Na atoms cm2 ; 4:8 1014 K atoms cm2 and 3:8 1014 Cs atoms cm2 ; for coverage calibration see Ref. [2]. Experimental conditions as in Fig. 2. In each case the spectrum is taken in single energy scan (20 min) immediately after high temperature (1400 K, 5 min) annealing and new alkali deposition. Note the elastic intensity change with increasing coverage.
SU N IE SC RS CE E A TT R F LE CE
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
Fig. 4. Energy of the Na, K, and Cs induced plasmon losses as a function of the square root of the alkali coverage. The data points are extracted from experimental EEL spectra as those presented in Fig. 3 by subtracting the background obtained from clean graphite at 86 K (see Fig. 2) where the spectral features are minimal.
Fermi energy and increasing the density of occupied states relative to those of pristine graphite [2,8]. In a photo emission study Oelhafen et al. [11] reported an anomalous increase in the DOS at EF for AC8 compounds (A ¼ K, Rb, and Cs). In contrast, no such strong spectral features at EF were observed for GICÕs with lower alkali concentration. The authors considered the observation as an evidence for formation of alkali metal-like band. The charge transfer leads to filling of the graphite conduction band and the compound becomes more ‘‘metallic’’ [11]. However, existing electron structure calculations conflict on the position of the metal band relative to the EF , and consequently also with respect to the amount of charge transfer of the metal valence electrons to the graphite p band and about the character of the alkali metal induced bands near EF . Within the free electron model the plasma oscillations can be expected to occur near a frequency [12]: x ¼ ð4pge2 =mm eÞ1=2 ; where g is the density of the charge carriers that participate in the oscillations, m is the effective mass, m and e are the electronic mass and charge; e is the dielectric constant. On the basis of the work function measurements one may speculate that in the low coverage
L81
regime (hNa < 1:45, hK < 0:25; hCs < 0:2) the total charge transfer, which is assumed to result in a corresponding increased density of the charge carriers g, is proportional to the alkali coverage, hA (number of alkali atoms). The plasmon energy pffiffiffiffiffiffi hx should then depend linearly on hA . This is in fact what we observe in Fig. 4: The different initial slopes for Na, K, and Cs reflect different amounts of charge transfer per atom. Based on these data we calculate the charge transfer to be 0.1–0.2, 0.4–0.5 and 0.6–0.8e per atom for Na, K, and Cs, respectively. The result confirms the concept of incomplete alkali ionization when adsorbed on graphite [13] and is in agreement with the values obtained using work function measurements data [2]. The ‘‘saturation’’ energy (250 meV, see Fig. 4) of the alkali induced graphite plasmons intriguingly coincide (within the error limits). This observation may have far reaching consequence for the understanding of the fundamental process of alkali metals––graphite charge transfer. Graphite work function (4.7 eV) decreases linearly with DUNa 1:95 eV (hNa ¼ 1:25); DUK 2:15 eV (hK ¼ 0:25) and DUCs 2:4 (hCs ¼ 0:2); our data are in full agreement with [14]. At this point the curve levels off without a minimum, which otherwise is a distinctive feature upon alkali adsorption on metal or semiconductor surfaces [15]. (The lack of a minimum is currently explained by the dominance of the ‘‘band energy’’ term [16], the energy involved in raising the Fermi energy of the substrate due to an increased electron density in the conduction band.) The work functions of the alkalicovered graphite surface differ with more than 0.2 eV at saturation coverage. Assuming a rigid band shift (thus taking only the ‘‘bulk’’ contribution to the work function into account) we are facing a contradiction: As we demonstrate, to a first approximation the value of g determines the plasmon energy hx (Fig. 4). On the other hand a different position of EF suggests different gÕs due to the steepness of the graphite p band (at the K point of the BZ). Obviously, detailed analysis of the different contributions to the work function (bulk and surface double layer), as well as the nature of the charge transfer (as measured in this study) must be considered to resolve the issue.
CE RF A LE C E TT S E R C IE S N
SU
L82
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
It is interesting and should be pointed out that the leveling off of the surface plasmon energy occurs at coverages that exactly correspond to the overlayer transition from disperse to condensed phase. (For K and Cs; Na is exception in this respect forming clusters even at low coverage.) Referring to the calculations by Gadzuk [17], the coupled surface plasmon modes of a thin alkali film on top of a graphite surface will be with low but even now much higher energy than the observed (250 meV). On the other hand, (as it is recognized in [17]), charge transfer (or charge redistribution) from the film to the substrate adjacent layers might additionally decrease the plasmon energy. Furthermore, the boundary conditions of the alkali film upon condensation are not well defined and can influence the result as well. Whether the observed loss at 250 meV remains for the coupled surface plasmon or not, we cannot at present explain why all the alkali related plasmons have the same energy. Another interesting observation is the temperature dependence of the energy of the alkali adsorbate induced graphite plasmons. Our experiments with controlled depositions demonstrate a small decrease of hx with increased temperature. The energy shift is accompanied by substantial intensity drop and broadening of the plasmon peak. Qualitatively the observations resemble those in our previous LEED experiments for K and Cs on graphite [2]: the diffraction patterns (characteristically an alkali induced ring) disappear at elevated temperatures (up to 400 K but still below the thermal desorption temperature) and eventually come into view back after cooling, but with lower intensity (sharpness). These observations were interpreted in terms of a thermally activated intercalation process of the alkali metal atoms into the subsurface regions. We therefore conclude that intercalated K and Cs induce graphite plasmons with approximately 25% energy down shift compared to the energy that corresponds to the plasmons induced by the same number of atoms on the surface. The measured values are hxK 190 15 meV at hK ¼ 0:25 and hxCs 170 20 meV at hCs ¼ 0:2. We measure smaller relative shifts at lower initial coverages. At a first glimpse the obtained values contradict the
assumption of a complete ionization of intercalated alkalis, as this is accepted for K in KC8 compound [8]. However, additionally to the change in the boundary conditions [17] the plasmon energy difference reflects the different charge densities donated per graphite unit cell in the cases of alkali layer before condensation (dispersed phase) and the same number of adatoms distributed as stage I GIC. In the first case the donated charge is distributed, as a minimum, over one graphite layer and in the second case over a minimum of two layers. On this ground we speculate that the observed temperature dependence of the plasmon energy presented in Fig. 1 reflects the dynamics of the temperature stimulated processes of alkali intercalation and segregation. The observed energy shift is probably affected by one more phenomenon that needs to be mentioned: The clean graphite surface is inert and UHV residual gases have low sticking coefficients at temperatures 85 K. However, the presence of small amounts of alkali metals on the graphite surface increases dramatically the reactive properties and sticking [18]. We have proved that exposures of O2 or H2 O on alkaliprecovered graphite induce substantial changes in hx. One may expect the charge transfer ‘‘neutralization’’ to escalate with decreasing sample temperature. In conclusion, we have demonstrated the extreme sensitivity of graphite surface plasmons towards contamination, doping and temperature. Our experimental observations reveal peculiar leveling off of the plasmon energy levels above specific alkali coverages (hNa 1:45, hK 0:25, and hCs 0:2) at a common plasmon energy hx 250 meV. We also demonstrate that EELS of surface plasmons is a proper experimental tool to study the charge transfer in intercalated compounds and the kinetics of graphite intercalation.
Acknowledgements We acknowledge the enlightening discussions € sterlund, Lars Wallden, and Bill Gadwith Lars O zuk. The work was partially supported by TFR contract 98-293.
SU N IE SC RS CE E A TT R F LE CE
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
References [1] E.T. Jensen, R.E. Palmer, W. Allison, Phys. Rev. Lett. 66 (1991) 492. € sterlund, D.V. Chakarov, B. Kasemo, Surf. Sci. 420 [2] L. O (1999) 174. [3] R.E. Palmer, J.F. Annett, R.F. Willis, Phys. Rev. Lett. 58 (1987) 2490. [4] C. Oshima, T. Aizawa, R. Souda, Y. Ishizawa, Y. Sumiyoshi, Solid State Commun. 65 (1988) 1601. [5] K.D. Tsuei, E.W. Plummer, A. Liebsch, K. Kempa, P. Bakshi, Phys. Rev. Lett. 64 (1990) 44. [6] W.S. Boyle, P. Nozieres, Phys. Rev. 111 (1958) 782. [7] J. Geiger, H. Katterwe, B. Schr€ oder, Zeitschrift f€ ur Physik 241 (1971) 45. [8] M.S. Dresselhaus, G. Dresselhaus, Adv. Phys. 30 (1981) 139.
L83
[9] H. Raether, Excitations of Plasmons and Inter band Transitions by Electrons Springer Tracts in Modern Physics, Springer-Verlag, Berlin, 1980. [10] G.R. Hennig, J. Chem. Phys. 43 (1965) 1201. [11] P. Oelhafen, P. Pfluger, E. Hauser, H. G€ untherodt, Phys. Rev. Lett. 44 (1980) 197. [12] R.H. Ritchie, Phys. Rev. 106 (1957) 874. [13] A. Sandell, P.A. Br€ uhwiler, A. Nilsson, P. Bennich, P. Rudolf, N. Mortensson, Surf. Sci. 429 (1999) 309. [14] Z.P. Hu, N.J. Wu, A. Ignatiev, Surf. Sci. 177 (1986) L956. [15] T. Aruga, Y. Murata, Prog. Surf. Sci. 31 (1990) 61. [16] H. Ishida, R.E. Palmer, Phys. Rev. B 56 (1992) 15484. [17] J.W. Gadzuk, Phys. Rev. B 1 (1970) 1267. [18] C. Janiak, R. Hoffmann, P. Sj€ ovall, B. Kasemo, Langmuir 9 (1993) 3427.
Surface Science Letters
Thermal and adsorbate induced plasmon energy shifts in graphite Michael Gleeson, Bengt Kasemo, Dinko Chakarov
*
Department of Applied Physics, Chalmers University of Technology and University of G€oteborg, S-412 96 G€oteborg, Sweden Received 2 May 2002; accepted for publication 14 October 2002
Abstract Electron-energy-loss spectroscopy has been used to study plasmon excitations in pristine and alkali metal (Na, K, Cs) dosed highly oriented pyrolitic graphite in the temperature range 85–400 K. The energy and line width of the pband Ekc plasmon in pristine graphite was found to be extremely sensitive to substrate temperature and to the presence of adsorbates in qualitative accord with previously reported observations and theoretical calculations. After increasing monotonously with coverage (following square root dependence) the plasmon energy levels off above specific alkali coverages (hNa 1:45, hK 0:25, and hCs 0:2) at a common plasmon energy hx 250 meV. The results are interpreted as evidence for formation of alkali-like bands in the topmost graphite layers. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Graphite; Plasmons; Electron energy loss spectroscopy (EELS); Alkali metals
The electronic excitation at surfaces and interfaces and the charge transfer upon adsorption are topics of fundamental interest in surface physics. In this Letter we address these two issues by using electron energy loss spectroscopy (EELS) to study plasmon properties of graphite. Graphite is a semimetal with very low density of states (DOS) near the Fermi level EF due to the small overlap of the p-bands. Adsorption of alkalis offers the possibility to modify electron density in the conduction band by several orders of magnitude, making the
*
Corresponding author. Tel.: +46-31-772-3375; fax: +46-31772-3134. E-mail address: [email protected] (D. Chakarov).
system of particular interest. Jensen et al. [1] presented a band structure calculation describing the temperature dependence of the graphite plasmon frequency supported by EEL spectra. In Fig. 1 we show analogous measurements from highly oriented pyrolitic graphite (HOPG) (Union Carbide, grade XYA) at sample temperatures between 87 and 270 K obtained with an ELS22 spectrometer [2]. Our spectra reproduce qualitatively well those reported in Ref. [1]: A broad peak is observed superimposed on the strong loss continuum typical for graphite. The peaks are well resolved at temperatures above 115 K and their maxima shift towards higher energies with increasing sample temperature. The spectra in Fig. 1 are taken in single energy scans while simultaneously, stepwise decreasing
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 2 4 8 3 - 4
CE RF A LE C E TT S E R C IE S N
SU
L78
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
reproducible; the temperature dependent energy, and the relative intensity of the energy losses changed beyond the statistical error limits. We were further puzzled by the disagreement in the absolute energy values and peak widths, compared to the spectra from Ref. [1], in spite of our better energy resolution and good counting rate. We then changed the cleaning procedure increasing the annealing temperature to 1400 K at which we kept the sample for at least 5 min. Typical spectra taken after applying this treatment are shown in Fig. 2. The difference between the spectra in Figs. 1 and 2 (for the respective temperatures) is obvious: With the new cleaning procedure the plasmon energy loss peak lies at lower energy and appears only as a shoulder on the intense elastic peak. The shape of the spectrum at 307 K (Fig. 2) resembles closely those presented by Oshima et al. [4], which were taken with similar energy resolution and (presumably) at room temperature. The measured values for the plasmon energy are in excellent agreement with theoretical calculations [1,9] and with experimentally obtained
Fig. 1. EEL spectra obtained from HOPG prepared by standard cleaning procedure (see text). The spectra are taken during cooling of the sample to the indicated temperatures (indicated temperature is correct within DT ¼ 3 K). Initial electron energy E0 ¼ 25 eV; specular scattering at 60°.
the sample temperature, as indicated. Our routine experimental procedure (for the sample mounting and initial cleaning, see Ref. [2]) includes annealing at 1100–1200 K in UHV (P < 2:1010 Torr) for several minutes and fast (15 min) cooling down to 83–90 K before exposing the sample to the electron beam in the scattering chamber of the spectrometer. This procedure has been proven to ensure that no traces of contamination are detectable as judged by TDS and AES [2]. In the course of experiments we found that the spectral features in Fig. 1 are not satisfactorily
Fig. 2. EEL spectra obtained from the same sample after annealing at 1450 K (see text). The dashed curves represent ðhxÞ1:3 empirical scaling with the energy loss hx The loss energies at 35 meV (86 K) and 55 meV (307 K) are in good agreement with theoretical plasmon energy of the free carriers at the Fermi surface.
SU N IE SC RS CE E A TT R F LE CE
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
values [10]. Adopting the above described preparation procedure we were no longer able to reproduce the spectra in Fig. 1. Several characteristic features of the electronic excitations seen in Fig. 2 may be noted. First, the elastic peak indicates a broadened energy distribution of 8 meV as compared with the primary electron beam width that is less than 6 meV (as measured in the straight-through geometry). Second, the background intensities in the spectra are more than three orders of magnitude higher than those reflected from a typical metal surface [3] and are distributed over a wide energy range (up to 3 1:3 eV) following an ð hxÞ empirical scaling with the energy loss. The high background intensities are sharply peaked around the specular direction, a typical feature of the dipole excitation mechanism. Third, there is a shoulder at 35 meV (T ¼ 86 K) and at 55 meV (T ¼ 307 K) suggesting a weak peak with a width of more than 50 meV. Such a big width (compared for example to phonon losses in graphite [4] or plasmon losses in metals [5]) is indicative of a stronger damping of electronic excitations in graphite compared to metals. The loss energy is in good agreement with the theoretical plasmon energy of the free carriers in graphite at the Fermi surface predicted by Boyle and Nozieres [6], and with those measured in other experiments by different techniques [7]. All these characteristics motivate these spectral features to be assigned to the graphite p-band plasmon excitation polarized parallel to the c-axis. The low plasmon energy reflects the semimetallic band structure of graphite with a very low electron density in the conduction band. There are (at least) two questions arising from our observations that needs to be answered: (i) can we verify the suspected contamination and, if so, what is the origin of the contamination and (ii) why does the contamination induce plasmons that follow more or less perfectly the theoretically predicted temperature dependence for clean graphite? The most probable contamination in our case are traces of alkali metals, Na, K, and Cs accumulated in the course of previous experiments performed with the same graphite sample [2], that eventually segregate to the surface. When depos-
L79
ited as submonolayers on the graphite surface, these alkali metals desorb thermally at T 6 500 K. The desorption is complicated by the phenomenon of inter-diffusion, or intercalation, that also is thermally activated. The heats of formation of graphite intercalation compounds (GIC) are in the range 1.15–1.5 eV for stages I to V GICÕs [8], i.e. in the same range as the desorption energies, thus placing the intercalation process in competition with the thermal desorption. With the charge transfer to the substrate as a driving force, alkali metal intercalation is strongly dependent on the morphology of graphite surface. It is believed that the main intercalation channel is through the graphite basal plane defects and sample edges. Intercalated, as well as ‘‘trapped’’ alkali atoms (on defect sites or edges) may well serve as a source of surface contamination. Evidently, the experiments demonstrate that the short annealing to 1100 K employed by several laboratories, including ours, is not sufficient to completely remove intercalated or defect bonded alkalis. On the other hand, the sensitivity limits of TDS and AES are about 1 atomic percent that in turn explains why these methods fail to detect them. As it will be shown below, plasmon energies below 70 meV correspond to alkali atoms concentrations below these detection limits. In order to check more carefully for the effect of contamination we performed experiments with controlled depositions of Na, K, and Cs on HOPG applying higher temperature cleaning procedure. Fig. 3 illustrates such measurements at 89 K. The intensity of graphite plasmon loss increases and shifts to higher energy with increasing alkali coverage. Additional plasmon losses, including the multiple losses [5] and other electronic excitations, with higher energy, that appear with increasing alkali coverages were also systematically studied. These results will be presented in a forthcoming paper. Fig. 4 summarizes our observations by showing the frequency of the Na, K and Cs induced plasmon modes as a function of the square root of the alkali coverage. It is commonly accepted through many theoretical and experimental studies that in GICÕs electrons are donated to the previously (almost) empty carbon p anti bonding band, raising the
CE RF A LE C E TT S E R C IE S N
SU
L80
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
Fig. 3. EEL spectra for different Na, K, and Cs coverages on graphite at 89 K. One monolayer, 1 ML is defined to correspond to 6:3 1014 Na atoms cm2 ; 4:8 1014 K atoms cm2 and 3:8 1014 Cs atoms cm2 ; for coverage calibration see Ref. [2]. Experimental conditions as in Fig. 2. In each case the spectrum is taken in single energy scan (20 min) immediately after high temperature (1400 K, 5 min) annealing and new alkali deposition. Note the elastic intensity change with increasing coverage.
SU N IE SC RS CE E A TT R F LE CE
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
Fig. 4. Energy of the Na, K, and Cs induced plasmon losses as a function of the square root of the alkali coverage. The data points are extracted from experimental EEL spectra as those presented in Fig. 3 by subtracting the background obtained from clean graphite at 86 K (see Fig. 2) where the spectral features are minimal.
Fermi energy and increasing the density of occupied states relative to those of pristine graphite [2,8]. In a photo emission study Oelhafen et al. [11] reported an anomalous increase in the DOS at EF for AC8 compounds (A ¼ K, Rb, and Cs). In contrast, no such strong spectral features at EF were observed for GICÕs with lower alkali concentration. The authors considered the observation as an evidence for formation of alkali metal-like band. The charge transfer leads to filling of the graphite conduction band and the compound becomes more ‘‘metallic’’ [11]. However, existing electron structure calculations conflict on the position of the metal band relative to the EF , and consequently also with respect to the amount of charge transfer of the metal valence electrons to the graphite p band and about the character of the alkali metal induced bands near EF . Within the free electron model the plasma oscillations can be expected to occur near a frequency [12]: x ¼ ð4pge2 =mm eÞ1=2 ; where g is the density of the charge carriers that participate in the oscillations, m is the effective mass, m and e are the electronic mass and charge; e is the dielectric constant. On the basis of the work function measurements one may speculate that in the low coverage
L81
regime (hNa < 1:45, hK < 0:25; hCs < 0:2) the total charge transfer, which is assumed to result in a corresponding increased density of the charge carriers g, is proportional to the alkali coverage, hA (number of alkali atoms). The plasmon energy pffiffiffiffiffiffi hx should then depend linearly on hA . This is in fact what we observe in Fig. 4: The different initial slopes for Na, K, and Cs reflect different amounts of charge transfer per atom. Based on these data we calculate the charge transfer to be 0.1–0.2, 0.4–0.5 and 0.6–0.8e per atom for Na, K, and Cs, respectively. The result confirms the concept of incomplete alkali ionization when adsorbed on graphite [13] and is in agreement with the values obtained using work function measurements data [2]. The ‘‘saturation’’ energy (250 meV, see Fig. 4) of the alkali induced graphite plasmons intriguingly coincide (within the error limits). This observation may have far reaching consequence for the understanding of the fundamental process of alkali metals––graphite charge transfer. Graphite work function (4.7 eV) decreases linearly with DUNa 1:95 eV (hNa ¼ 1:25); DUK 2:15 eV (hK ¼ 0:25) and DUCs 2:4 (hCs ¼ 0:2); our data are in full agreement with [14]. At this point the curve levels off without a minimum, which otherwise is a distinctive feature upon alkali adsorption on metal or semiconductor surfaces [15]. (The lack of a minimum is currently explained by the dominance of the ‘‘band energy’’ term [16], the energy involved in raising the Fermi energy of the substrate due to an increased electron density in the conduction band.) The work functions of the alkalicovered graphite surface differ with more than 0.2 eV at saturation coverage. Assuming a rigid band shift (thus taking only the ‘‘bulk’’ contribution to the work function into account) we are facing a contradiction: As we demonstrate, to a first approximation the value of g determines the plasmon energy hx (Fig. 4). On the other hand a different position of EF suggests different gÕs due to the steepness of the graphite p band (at the K point of the BZ). Obviously, detailed analysis of the different contributions to the work function (bulk and surface double layer), as well as the nature of the charge transfer (as measured in this study) must be considered to resolve the issue.
CE RF A LE C E TT S E R C IE S N
SU
L82
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
It is interesting and should be pointed out that the leveling off of the surface plasmon energy occurs at coverages that exactly correspond to the overlayer transition from disperse to condensed phase. (For K and Cs; Na is exception in this respect forming clusters even at low coverage.) Referring to the calculations by Gadzuk [17], the coupled surface plasmon modes of a thin alkali film on top of a graphite surface will be with low but even now much higher energy than the observed (250 meV). On the other hand, (as it is recognized in [17]), charge transfer (or charge redistribution) from the film to the substrate adjacent layers might additionally decrease the plasmon energy. Furthermore, the boundary conditions of the alkali film upon condensation are not well defined and can influence the result as well. Whether the observed loss at 250 meV remains for the coupled surface plasmon or not, we cannot at present explain why all the alkali related plasmons have the same energy. Another interesting observation is the temperature dependence of the energy of the alkali adsorbate induced graphite plasmons. Our experiments with controlled depositions demonstrate a small decrease of hx with increased temperature. The energy shift is accompanied by substantial intensity drop and broadening of the plasmon peak. Qualitatively the observations resemble those in our previous LEED experiments for K and Cs on graphite [2]: the diffraction patterns (characteristically an alkali induced ring) disappear at elevated temperatures (up to 400 K but still below the thermal desorption temperature) and eventually come into view back after cooling, but with lower intensity (sharpness). These observations were interpreted in terms of a thermally activated intercalation process of the alkali metal atoms into the subsurface regions. We therefore conclude that intercalated K and Cs induce graphite plasmons with approximately 25% energy down shift compared to the energy that corresponds to the plasmons induced by the same number of atoms on the surface. The measured values are hxK 190 15 meV at hK ¼ 0:25 and hxCs 170 20 meV at hCs ¼ 0:2. We measure smaller relative shifts at lower initial coverages. At a first glimpse the obtained values contradict the
assumption of a complete ionization of intercalated alkalis, as this is accepted for K in KC8 compound [8]. However, additionally to the change in the boundary conditions [17] the plasmon energy difference reflects the different charge densities donated per graphite unit cell in the cases of alkali layer before condensation (dispersed phase) and the same number of adatoms distributed as stage I GIC. In the first case the donated charge is distributed, as a minimum, over one graphite layer and in the second case over a minimum of two layers. On this ground we speculate that the observed temperature dependence of the plasmon energy presented in Fig. 1 reflects the dynamics of the temperature stimulated processes of alkali intercalation and segregation. The observed energy shift is probably affected by one more phenomenon that needs to be mentioned: The clean graphite surface is inert and UHV residual gases have low sticking coefficients at temperatures 85 K. However, the presence of small amounts of alkali metals on the graphite surface increases dramatically the reactive properties and sticking [18]. We have proved that exposures of O2 or H2 O on alkaliprecovered graphite induce substantial changes in hx. One may expect the charge transfer ‘‘neutralization’’ to escalate with decreasing sample temperature. In conclusion, we have demonstrated the extreme sensitivity of graphite surface plasmons towards contamination, doping and temperature. Our experimental observations reveal peculiar leveling off of the plasmon energy levels above specific alkali coverages (hNa 1:45, hK 0:25, and hCs 0:2) at a common plasmon energy hx 250 meV. We also demonstrate that EELS of surface plasmons is a proper experimental tool to study the charge transfer in intercalated compounds and the kinetics of graphite intercalation.
Acknowledgements We acknowledge the enlightening discussions € sterlund, Lars Wallden, and Bill Gadwith Lars O zuk. The work was partially supported by TFR contract 98-293.
SU N IE SC RS CE E A TT R F LE CE
M. Gleeson et al. / Surface Science 524 (2003) L77–L83
References [1] E.T. Jensen, R.E. Palmer, W. Allison, Phys. Rev. Lett. 66 (1991) 492. € sterlund, D.V. Chakarov, B. Kasemo, Surf. Sci. 420 [2] L. O (1999) 174. [3] R.E. Palmer, J.F. Annett, R.F. Willis, Phys. Rev. Lett. 58 (1987) 2490. [4] C. Oshima, T. Aizawa, R. Souda, Y. Ishizawa, Y. Sumiyoshi, Solid State Commun. 65 (1988) 1601. [5] K.D. Tsuei, E.W. Plummer, A. Liebsch, K. Kempa, P. Bakshi, Phys. Rev. Lett. 64 (1990) 44. [6] W.S. Boyle, P. Nozieres, Phys. Rev. 111 (1958) 782. [7] J. Geiger, H. Katterwe, B. Schr€ oder, Zeitschrift f€ ur Physik 241 (1971) 45. [8] M.S. Dresselhaus, G. Dresselhaus, Adv. Phys. 30 (1981) 139.
L83
[9] H. Raether, Excitations of Plasmons and Inter band Transitions by Electrons Springer Tracts in Modern Physics, Springer-Verlag, Berlin, 1980. [10] G.R. Hennig, J. Chem. Phys. 43 (1965) 1201. [11] P. Oelhafen, P. Pfluger, E. Hauser, H. G€ untherodt, Phys. Rev. Lett. 44 (1980) 197. [12] R.H. Ritchie, Phys. Rev. 106 (1957) 874. [13] A. Sandell, P.A. Br€ uhwiler, A. Nilsson, P. Bennich, P. Rudolf, N. Mortensson, Surf. Sci. 429 (1999) 309. [14] Z.P. Hu, N.J. Wu, A. Ignatiev, Surf. Sci. 177 (1986) L956. [15] T. Aruga, Y. Murata, Prog. Surf. Sci. 31 (1990) 61. [16] H. Ishida, R.E. Palmer, Phys. Rev. B 56 (1992) 15484. [17] J.W. Gadzuk, Phys. Rev. B 1 (1970) 1267. [18] C. Janiak, R. Hoffmann, P. Sj€ ovall, B. Kasemo, Langmuir 9 (1993) 3427.