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Intercalation of potassium from the surface of graphite
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Surface Science 287/288 (1993) 178-182 North-Holland
Intercalation J.C. Barnard,
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of potassium from the surface of graphite
K.M. Hock and R.E. Palmer
Cauendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB.7 OHE, UK
Received 28 August 1992; accepted for publication 24 November 1992
We have investigated the intercalation of potassium adlayers from the surface of graphite into the bulk. Time-dependent HREELS and LEED data is presented from the submonolayer dispersed phase of potassium on graphite at 50 K, together with HREELS data from a multilayer of potassium on graphite in a range of temperatures from 80 to 220 K. The dispersed phase shows intercalation at 50 K while the multilayer was found to be stable against intercalation up to a temperature of approximately 150 K. This difference is explained in terms of the differing mobilities of the potassium atoms in the two phases; the dispersed phase is fluid whereas the multilayer phase is a solid. The data supports the generally held view that potassium intercalates via the step edges of the graphite layers.
1. Introduction
Graphite intercalation compounds (GICs) have attracted considerable interest in the past because of their interesting electronic and structural properties [l-3]. The alkali-metal GICs are extensively studied [l], while the kinetics of the intercalation process are less well known [l]. One topic of particular interest is the process of diffusion of the alkali-metal intercalants [4]. It is generally accepted that in the intercalation process most of the alkali-metal atoms diffuse from the graphite plane step edges into the interlayer spaces, as opposed to diffusing through the graphite planes. For example, Wu and Ignatiev [5-71 have shown that the intercalation of potassium into graphite is strongly favoured by a high step density on the (0001) graphite surface. In this paper a high-resolution electron energy-loss spectroscopy (HREELS) study of the intercalation of potassium adlayers on graphite is presented. The variation of the intercalation rate with both sample temperature and potassium coverage is investigated. The low-coverage phase diagram of potassium on graphite has been the subject of recent 0039.6028/93/$06.00
HREELS and low-energy electron diffraction (LEED) studies [8,9] which have also prompted a theoretical study [lo]. The results of the experimental investigations may be summarised as follows: at very low coverages ( < 0.1 monolayers) a very dilute potassium overlayer structoure is seen, with a K-K spacing as large as 60 A (Ak = 0.2 A- ’ in LEED measurements), which compresses continuously as the coverage is increased, until a structure is seen with a LEED peak at Ak = 0.45 A-‘, which corresponds to a coverage equivalent to that of a “7 x 7” potassium overlayer. This continuously compressing phase is a fluid phase [ll] and is referred to as the dispersed phase of potassium on graphite. The doping of the graphite surface with the potassium atoms also shifts the energy of the low-lying graphite plasmon, first reported by Jensen et al. [12]. The plasmon energy increases with increasing coverage until the critical coverage, beyond which the dispersed phase no longer compresses, after which it remains at 0.32 eV. As the coverage is increased further, 2 x 2 close-packed potassium structures appear, co-existing with the dispersed phase in the coverage regime of 0.1 to 1 monolayer. The 2 x 2 structure is a solid [ll] and is signified by
0 1993 - Elsevier Science Publishers B.V. All rights reserved
J.C. Barnard et al. / Intercalation of potassium from the surface of graphite
the appearance of a 1.5 A- ’ LEED peak and new features in the EEL spectra at 1.2, 1.5 and 2.2 eV. It is thought that these peaks may be characteristic of the edges of the 2 X 2 islands on the surface [13]. As the coverage is increased to one monolayer, the intensity of these features decreases. As the coverage is increased beyond one monolayer, a new mode at 2.7 eV appears in the EEL spectrum which is associated with the potassium surface plasmon [141 together with a reduction in the plasmon intensity at 0.32 eV and the appearance of a broad feature at 1.5 eV. Thereafter as more layers are deposited, the 2.7 eV mode increases in intensity and the 1.5 eV mode disappears. In this paper we report a time-dependent HREELS and LEED study of the dispersed phase
179
and the multilayer phase of potassium on graphite. We find that the multilayer is stable against intercalation at sample temperatures up to approximately 150 K, above which intercalation is observed to take place over a period of approximately 1 h. This result appears to be consistent with the results of Law et al. [15]. However, the dispersed phase is seen to intercalate at temperatures as low as 50 K. We attribute this to the fluid nature of this phase as opposed to the solid multilayer phase. This implies that the potassium atoms are more mobile in the dispersed phase, which enables them to intercalate via sites such as step edges. The results also indicate that when moving through the phase diagram from a high to low coverage during intercalation, the local structure of the potassium
(a) 1
I
I
I
I
1
2
3
4
h
-1
0
-0.2
0
Energy Loss (eV)
0
1
2
Energy Loss (eV)
0.4
0.6
0.8
0.6
0.8
Wavevector (A-‘)
(b)
-1
0.2
3
4
-0.2
0
0.2
0.4
Wavevector (A-‘)
Fig. 1. EEL spectra (on specular) and LEED profiles for a low coverage of potassium on HOPG (- 0.1 monolayer) taken at 50 K. The electron beam energy was 19 eV and the angle of incidence was 60” with respect to the surface normal. (a) Taken immediately after dosing. (b) Taken at a time 30 min later.
180
J.C. Barnard et al. / Intercalation of potassium from the surface of graphite
atoms on the surface is always maintained in dynamic equilibrium at temperatures as low as 50 K.
2. Experimental The experiments were performed with a highresolution electron energy loss spectrometer which is housed in a stainless-steel ultra-high vacuum chamber with a base pressure of < 3 x lo-” mbar. The electron energy-loss spectrome-
AbO.0 Hrs T=80K
ter was operated with an energy resolution of 25 meV FWHM and an angular resolution of - 1.5” FWHM. The monochromator is fixed whilst the analyser can rotate in the scattering plane from 55” to 180” with respect to the monochromator. In this manner the spectrometer can also be used to perform a one-dimensional, low-current LEED measurement by recording the elastically scattered electron intensity as the electron analyser is rotated [ 161. The sample used was a crystal of highly oriented pyrolitic graphite (HOPG) [l], a form of
(a)
i
At=kOCl Hk T = 84K
At=3.15 Hrs T = 160K
(d)
At=4.00Hrs
W
T = 200K
-.;, i. l;. x50
‘..:v;r \ >
IIIl[IIIiiI4?~ Ak4.15 Hrs
At=3.00 Hrs T = 143K
1
Energy
2 3 Loss (eV)
if)
T = 220K
4
-1
il
0 1 2 3 Energy Loss (eV)
4
Fig. 2. EEL spectra (on specular) for a multilayer coverage of potassium on HOPG. The electron beam energy was 19 eV and the angle of incidence was 60”. (a) Taken immediately after dosing, with a sample temperature of 80 K. (b) Taken 2 h later. also at 80 K. (c-f) Taken at later times (as labelled) as the sample temperature was allowed to increase to 220 K.
J. C. Barnard et al. / Intercalation of potassium from the surface of graphite
graphite composed of many microcrystallites (typically of size N 1 pm) that are oriented along the c-axis to within 1” but are azimuthally disordered. Hence, with this form of graphite, no information about the surface structure is lost by using a one-dimensional LEED scan as opposed to a conventional LEED system. The sample was cleaved in air with tape prior to insertion into the vacuum chamber and was cleaned in situ by resistive heating to N 1100 K. The sample temperature was monitored with a four-wire resistance thermometer. To obtain a sample temperature of 50 K for experiments on the dispersed phase, the sample was cooled by a continuous liquid helium flow [17]. For the experiments on the multilayer coverages the sample was initially cooled and maintained at 80 K with liquid nitrogen, and then allowed to warm up slowly. Potassium was dosed onto the graphite surface from a SAES getter source.
3. Results Fig. 1 shows data taken from the dispersed phase at 50 K. The first scan (fig. la) is taken immediately after dosing. The EELS features evident at 1.2, 1.5 and 2.1 eV indicate that the coverage is within the co-existence region of the phase diagram, where the dispersed phase co-exists as “7 X 7” areas alongside 2 X 2 close-packed islands. The modified graphite plasmon is seen in the EEL spectrum at its saturation value of 0.32 eV, as is expected for this coverage; The LEED scan shows a strong peak at 0.45 A-’ which is indicative of the “7 x 7” structure. The second scan (fig. lb) is taken at a time 30 min later. The EELS features at 1.2, 1.5 and 2.1 eV are no longer seen which indicates that there are no close-packed regions on the surface. Also there is a small shift in the plasmon frequency, now at = 0.17 eV enerq loss, and a shift of the LEED peak to _ 0.38 A- ‘. It should be noted here that the simultaneous shifts of both these peaks implies that as the coverage of potassium on the surface reduces, the local surface structure is modified such that the equilibrium structure at the lower coverage results. It is clear therefore
181
that the total amount of potassium on the surface has reduced. Desorption can be discounted [18] because the temperature is too low and therefore this observation must be the result of intercalation from the potassium layer into the bulk. Figs. 2a to 2f show data obtained from a higher coverage of potassium on graphite. The sample was dosed at 80 K. Fig. 2a shows the EEL spectrum shortly after dosing. As can be seen, the layer supports a mode at 2.7 eV energy loss, with no other features apparent. This indicates, by comparison with previous results [8,19], that the coverage is in the multilayer regime. From the evaporation time, we can estimate that the coverage is of the order of 10 monolayers. Fig. 2b was taken 2 h later with the sample still kept at 80 K. There is minimal change in the EELS spectrum which indicates that there has been little change in the adlayer. Figs. 2c to 2f were obtained as the temperature of the multilayer potassium film was allowed to increase. No change was observed until the sample had reached a temperature of 160 K (fig. 2d). At this point broad features appear in the EELS spectrum at around 1 eV, indicating some definite change in the adlayer. These features are probably attributable to the loss peaks seen in the submonolayer potassium EEL spectra at lower temperature, as shown in fig. la, indicating a reduction in the coverage of potassium on the surface. At 200 K (fig. 2e) we see the complete disappearance of the 2.7 eV peak, while the features at 1.2-1.5 eV are still evident. Once again it is apparent that as the coverage reduces the surface structure modifies to that given by the phase diagram [8]. By 220 K (fig. 2f) the EEL spectrum shows no potassium features, indicating that there is no potassium left on the surface. Again desorption can be discounted on temperature grounds [18] and therefore the potassium must have intercalated into the bulk.
4. Discussion As has been shown, relatively rapid intercalation of the dispersed phase occurs at temperatures as low as 50 K, whereas the multilayer is
182
J.C. Barnard et al. / Intercalation of potassium from the surface of graphite
stable at temperatures up to approximately 150 K. The dispersed phase is a fluid, whereas the close-packed phase is a solid. Therefore, the potassium atoms within the dispersed phase are expected to be much more laterally mobile than those in the close-packed phase, and it is this factor which we believe explains the differing stabilities against intercalation. The potassium atoms in the dispersed phase have sufficient lateral mobility at temperatures as low as 50 K to find sites such as the edges of steps through which it is generally thought that intercalation takes place [6,7]. Conversely, the atoms in the close-packed structures are locked in position at low temperature and cannot intercalate unless the temperature is increased. Note that therefore the adsorption of more potassium on the surface actually inhibits the rate of intercalation into the bulk.
5. Conclusions In conclusion, we have observed the intercalation of potassium from adsorbed layers on the surface of graphite. The dispersed phase is found to be unstable with respect to intercalation over a period of 30 min at 50 K, whereas the closepacked phase is stable against intercalation at temperatures up to 150 K. In the light of these results we conclude that, because it is a fluid phase, the potassium atoms in the dispersed phase have sufficient mobility to reach sites such as step edges and intercalate at 50 K. Conversely, the potassium atoms in the solid close-packed phase, having a much reduced mobility, cannot intercalate until the sample temperature has reached approximately 150 K. At all coverages we find that a local equilibrium is maintained on the surface.
Acknowledgements
J.C.B. acknowledges financial support from the Science and Engineering Research Council. K.M.H. thanks Caius College, Cambridge, for the award of a studentship. R.E.P. acknowledges financial support by the Royal Society. HOPG samples were supplied by Dr. A.W. Moore of Union Carbide, Ltd.
References [l] MS. Dresselhaus and G. Dresselhaus, Adv. Phys. 30 (1980) 139. [2] S.A. Solin, Adv. Chem. Phys. 49 (1982) 455. [3] S.A. Safran, Solid State Phys. 40 (1987) 143. [4] H. Zabel, A. Magerl, A.J. Dianoux and J.J. Rush, Phys. Rev. Lett. 50 (1983) 2094. [5] N.J. Wu and A. Ignatiev, J. Vat. Sci. Technol. 20 (1982) 896. (61 N.J. Wu and A. Ignatiev, Solid State Commun. 4h (1983) 59. [7] N.J. Wu and A. Ignatiev, Phys. Rev. B 28 (1983) 7288. [8] Z. Li, K.M. Hock and R.E. Palmer, Phys. Rev. Lett. 67 (1991) 1562. [9] Z. Li, K.M. Hock, R.E. Palmer and J.F. Annett, J. Phys. Condensed Matter 3 (1991) SlO3. [lo] H. Ishida and R.E. Palmer, Phys. Rev. B 46 (1992) 15484. [ll] J. Cui, J.D. White, R.D. Diehl, J.F. Annett and M.W. Cole, Surf. Sci. 279 (19921 149. [12] E.T. Jensen, R.E. Palmer, W.A. Allison and J.F. Annett, Phys. Rev. Lett. 66 (19911492. [13] K.M. Hock and R.E. Palmer, Surf. Sci. 284 (19931 349. [14] J.B. Swan, Phys. Rev. 135 (1964) A1467. [15] A.R. Law, M.T. Johnson and H.P. Hughes. Surf. Sci. 152/153 (1985) 284. 1161 E.T. Jensen and R.E. Palmer, Rev. Sci. Instrum. 60 (1989) 3065. [17] R.E. Palmer, P.V. Head and R.F. Willis, Rev. Sci. Instrum. 58 (1987) 1118. [18] P. Sjiivall and B. Kasemo, submitted. [19] K.M. Hock, PhD Thesis, University of Cambridge, 1992.
Surface Science 287/288 (1993) 178-182 North-Holland
Intercalation J.C. Barnard,
.. .
kurface ...
. . .
>. ..>.
.:. ‘. .. j:..
:>:
‘:’
science
‘: .. .... i:.:. ‘.‘.... :..:....::::,.,...:_:, .. .. ,,.. ... .: 3.::” . . :: .’ ->.:‘y<::::.:.: .~:,,.: .: .::: ::.::‘::::.:.::,:::._:: . . ‘)
..::._
of potassium from the surface of graphite
K.M. Hock and R.E. Palmer
Cauendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB.7 OHE, UK
Received 28 August 1992; accepted for publication 24 November 1992
We have investigated the intercalation of potassium adlayers from the surface of graphite into the bulk. Time-dependent HREELS and LEED data is presented from the submonolayer dispersed phase of potassium on graphite at 50 K, together with HREELS data from a multilayer of potassium on graphite in a range of temperatures from 80 to 220 K. The dispersed phase shows intercalation at 50 K while the multilayer was found to be stable against intercalation up to a temperature of approximately 150 K. This difference is explained in terms of the differing mobilities of the potassium atoms in the two phases; the dispersed phase is fluid whereas the multilayer phase is a solid. The data supports the generally held view that potassium intercalates via the step edges of the graphite layers.
1. Introduction
Graphite intercalation compounds (GICs) have attracted considerable interest in the past because of their interesting electronic and structural properties [l-3]. The alkali-metal GICs are extensively studied [l], while the kinetics of the intercalation process are less well known [l]. One topic of particular interest is the process of diffusion of the alkali-metal intercalants [4]. It is generally accepted that in the intercalation process most of the alkali-metal atoms diffuse from the graphite plane step edges into the interlayer spaces, as opposed to diffusing through the graphite planes. For example, Wu and Ignatiev [5-71 have shown that the intercalation of potassium into graphite is strongly favoured by a high step density on the (0001) graphite surface. In this paper a high-resolution electron energy-loss spectroscopy (HREELS) study of the intercalation of potassium adlayers on graphite is presented. The variation of the intercalation rate with both sample temperature and potassium coverage is investigated. The low-coverage phase diagram of potassium on graphite has been the subject of recent 0039.6028/93/$06.00
HREELS and low-energy electron diffraction (LEED) studies [8,9] which have also prompted a theoretical study [lo]. The results of the experimental investigations may be summarised as follows: at very low coverages ( < 0.1 monolayers) a very dilute potassium overlayer structoure is seen, with a K-K spacing as large as 60 A (Ak = 0.2 A- ’ in LEED measurements), which compresses continuously as the coverage is increased, until a structure is seen with a LEED peak at Ak = 0.45 A-‘, which corresponds to a coverage equivalent to that of a “7 x 7” potassium overlayer. This continuously compressing phase is a fluid phase [ll] and is referred to as the dispersed phase of potassium on graphite. The doping of the graphite surface with the potassium atoms also shifts the energy of the low-lying graphite plasmon, first reported by Jensen et al. [12]. The plasmon energy increases with increasing coverage until the critical coverage, beyond which the dispersed phase no longer compresses, after which it remains at 0.32 eV. As the coverage is increased further, 2 x 2 close-packed potassium structures appear, co-existing with the dispersed phase in the coverage regime of 0.1 to 1 monolayer. The 2 x 2 structure is a solid [ll] and is signified by
0 1993 - Elsevier Science Publishers B.V. All rights reserved
J.C. Barnard et al. / Intercalation of potassium from the surface of graphite
the appearance of a 1.5 A- ’ LEED peak and new features in the EEL spectra at 1.2, 1.5 and 2.2 eV. It is thought that these peaks may be characteristic of the edges of the 2 X 2 islands on the surface [13]. As the coverage is increased to one monolayer, the intensity of these features decreases. As the coverage is increased beyond one monolayer, a new mode at 2.7 eV appears in the EEL spectrum which is associated with the potassium surface plasmon [141 together with a reduction in the plasmon intensity at 0.32 eV and the appearance of a broad feature at 1.5 eV. Thereafter as more layers are deposited, the 2.7 eV mode increases in intensity and the 1.5 eV mode disappears. In this paper we report a time-dependent HREELS and LEED study of the dispersed phase
179
and the multilayer phase of potassium on graphite. We find that the multilayer is stable against intercalation at sample temperatures up to approximately 150 K, above which intercalation is observed to take place over a period of approximately 1 h. This result appears to be consistent with the results of Law et al. [15]. However, the dispersed phase is seen to intercalate at temperatures as low as 50 K. We attribute this to the fluid nature of this phase as opposed to the solid multilayer phase. This implies that the potassium atoms are more mobile in the dispersed phase, which enables them to intercalate via sites such as step edges. The results also indicate that when moving through the phase diagram from a high to low coverage during intercalation, the local structure of the potassium
(a) 1
I
I
I
I
1
2
3
4
h
-1
0
-0.2
0
Energy Loss (eV)
0
1
2
Energy Loss (eV)
0.4
0.6
0.8
0.6
0.8
Wavevector (A-‘)
(b)
-1
0.2
3
4
-0.2
0
0.2
0.4
Wavevector (A-‘)
Fig. 1. EEL spectra (on specular) and LEED profiles for a low coverage of potassium on HOPG (- 0.1 monolayer) taken at 50 K. The electron beam energy was 19 eV and the angle of incidence was 60” with respect to the surface normal. (a) Taken immediately after dosing. (b) Taken at a time 30 min later.
180
J.C. Barnard et al. / Intercalation of potassium from the surface of graphite
atoms on the surface is always maintained in dynamic equilibrium at temperatures as low as 50 K.
2. Experimental The experiments were performed with a highresolution electron energy loss spectrometer which is housed in a stainless-steel ultra-high vacuum chamber with a base pressure of < 3 x lo-” mbar. The electron energy-loss spectrome-
AbO.0 Hrs T=80K
ter was operated with an energy resolution of 25 meV FWHM and an angular resolution of - 1.5” FWHM. The monochromator is fixed whilst the analyser can rotate in the scattering plane from 55” to 180” with respect to the monochromator. In this manner the spectrometer can also be used to perform a one-dimensional, low-current LEED measurement by recording the elastically scattered electron intensity as the electron analyser is rotated [ 161. The sample used was a crystal of highly oriented pyrolitic graphite (HOPG) [l], a form of
(a)
i
At=kOCl Hk T = 84K
At=3.15 Hrs T = 160K
(d)
At=4.00Hrs
W
T = 200K
-.;, i. l;. x50
‘..:v;r \ >
IIIl[IIIiiI4?~ Ak4.15 Hrs
At=3.00 Hrs T = 143K
1
Energy
2 3 Loss (eV)
if)
T = 220K
4
-1
il
0 1 2 3 Energy Loss (eV)
4
Fig. 2. EEL spectra (on specular) for a multilayer coverage of potassium on HOPG. The electron beam energy was 19 eV and the angle of incidence was 60”. (a) Taken immediately after dosing, with a sample temperature of 80 K. (b) Taken 2 h later. also at 80 K. (c-f) Taken at later times (as labelled) as the sample temperature was allowed to increase to 220 K.
J. C. Barnard et al. / Intercalation of potassium from the surface of graphite
graphite composed of many microcrystallites (typically of size N 1 pm) that are oriented along the c-axis to within 1” but are azimuthally disordered. Hence, with this form of graphite, no information about the surface structure is lost by using a one-dimensional LEED scan as opposed to a conventional LEED system. The sample was cleaved in air with tape prior to insertion into the vacuum chamber and was cleaned in situ by resistive heating to N 1100 K. The sample temperature was monitored with a four-wire resistance thermometer. To obtain a sample temperature of 50 K for experiments on the dispersed phase, the sample was cooled by a continuous liquid helium flow [17]. For the experiments on the multilayer coverages the sample was initially cooled and maintained at 80 K with liquid nitrogen, and then allowed to warm up slowly. Potassium was dosed onto the graphite surface from a SAES getter source.
3. Results Fig. 1 shows data taken from the dispersed phase at 50 K. The first scan (fig. la) is taken immediately after dosing. The EELS features evident at 1.2, 1.5 and 2.1 eV indicate that the coverage is within the co-existence region of the phase diagram, where the dispersed phase co-exists as “7 X 7” areas alongside 2 X 2 close-packed islands. The modified graphite plasmon is seen in the EEL spectrum at its saturation value of 0.32 eV, as is expected for this coverage; The LEED scan shows a strong peak at 0.45 A-’ which is indicative of the “7 x 7” structure. The second scan (fig. lb) is taken at a time 30 min later. The EELS features at 1.2, 1.5 and 2.1 eV are no longer seen which indicates that there are no close-packed regions on the surface. Also there is a small shift in the plasmon frequency, now at = 0.17 eV enerq loss, and a shift of the LEED peak to _ 0.38 A- ‘. It should be noted here that the simultaneous shifts of both these peaks implies that as the coverage of potassium on the surface reduces, the local surface structure is modified such that the equilibrium structure at the lower coverage results. It is clear therefore
181
that the total amount of potassium on the surface has reduced. Desorption can be discounted [18] because the temperature is too low and therefore this observation must be the result of intercalation from the potassium layer into the bulk. Figs. 2a to 2f show data obtained from a higher coverage of potassium on graphite. The sample was dosed at 80 K. Fig. 2a shows the EEL spectrum shortly after dosing. As can be seen, the layer supports a mode at 2.7 eV energy loss, with no other features apparent. This indicates, by comparison with previous results [8,19], that the coverage is in the multilayer regime. From the evaporation time, we can estimate that the coverage is of the order of 10 monolayers. Fig. 2b was taken 2 h later with the sample still kept at 80 K. There is minimal change in the EELS spectrum which indicates that there has been little change in the adlayer. Figs. 2c to 2f were obtained as the temperature of the multilayer potassium film was allowed to increase. No change was observed until the sample had reached a temperature of 160 K (fig. 2d). At this point broad features appear in the EELS spectrum at around 1 eV, indicating some definite change in the adlayer. These features are probably attributable to the loss peaks seen in the submonolayer potassium EEL spectra at lower temperature, as shown in fig. la, indicating a reduction in the coverage of potassium on the surface. At 200 K (fig. 2e) we see the complete disappearance of the 2.7 eV peak, while the features at 1.2-1.5 eV are still evident. Once again it is apparent that as the coverage reduces the surface structure modifies to that given by the phase diagram [8]. By 220 K (fig. 2f) the EEL spectrum shows no potassium features, indicating that there is no potassium left on the surface. Again desorption can be discounted on temperature grounds [18] and therefore the potassium must have intercalated into the bulk.
4. Discussion As has been shown, relatively rapid intercalation of the dispersed phase occurs at temperatures as low as 50 K, whereas the multilayer is
182
J.C. Barnard et al. / Intercalation of potassium from the surface of graphite
stable at temperatures up to approximately 150 K. The dispersed phase is a fluid, whereas the close-packed phase is a solid. Therefore, the potassium atoms within the dispersed phase are expected to be much more laterally mobile than those in the close-packed phase, and it is this factor which we believe explains the differing stabilities against intercalation. The potassium atoms in the dispersed phase have sufficient lateral mobility at temperatures as low as 50 K to find sites such as the edges of steps through which it is generally thought that intercalation takes place [6,7]. Conversely, the atoms in the close-packed structures are locked in position at low temperature and cannot intercalate unless the temperature is increased. Note that therefore the adsorption of more potassium on the surface actually inhibits the rate of intercalation into the bulk.
5. Conclusions In conclusion, we have observed the intercalation of potassium from adsorbed layers on the surface of graphite. The dispersed phase is found to be unstable with respect to intercalation over a period of 30 min at 50 K, whereas the closepacked phase is stable against intercalation at temperatures up to 150 K. In the light of these results we conclude that, because it is a fluid phase, the potassium atoms in the dispersed phase have sufficient mobility to reach sites such as step edges and intercalate at 50 K. Conversely, the potassium atoms in the solid close-packed phase, having a much reduced mobility, cannot intercalate until the sample temperature has reached approximately 150 K. At all coverages we find that a local equilibrium is maintained on the surface.
Acknowledgements
J.C.B. acknowledges financial support from the Science and Engineering Research Council. K.M.H. thanks Caius College, Cambridge, for the award of a studentship. R.E.P. acknowledges financial support by the Royal Society. HOPG samples were supplied by Dr. A.W. Moore of Union Carbide, Ltd.
References [l] MS. Dresselhaus and G. Dresselhaus, Adv. Phys. 30 (1980) 139. [2] S.A. Solin, Adv. Chem. Phys. 49 (1982) 455. [3] S.A. Safran, Solid State Phys. 40 (1987) 143. [4] H. Zabel, A. Magerl, A.J. Dianoux and J.J. Rush, Phys. Rev. Lett. 50 (1983) 2094. [5] N.J. Wu and A. Ignatiev, J. Vat. Sci. Technol. 20 (1982) 896. (61 N.J. Wu and A. Ignatiev, Solid State Commun. 4h (1983) 59. [7] N.J. Wu and A. Ignatiev, Phys. Rev. B 28 (1983) 7288. [8] Z. Li, K.M. Hock and R.E. Palmer, Phys. Rev. Lett. 67 (1991) 1562. [9] Z. Li, K.M. Hock, R.E. Palmer and J.F. Annett, J. Phys. Condensed Matter 3 (1991) SlO3. [lo] H. Ishida and R.E. Palmer, Phys. Rev. B 46 (1992) 15484. [ll] J. Cui, J.D. White, R.D. Diehl, J.F. Annett and M.W. Cole, Surf. Sci. 279 (19921 149. [12] E.T. Jensen, R.E. Palmer, W.A. Allison and J.F. Annett, Phys. Rev. Lett. 66 (19911492. [13] K.M. Hock and R.E. Palmer, Surf. Sci. 284 (19931 349. [14] J.B. Swan, Phys. Rev. 135 (1964) A1467. [15] A.R. Law, M.T. Johnson and H.P. Hughes. Surf. Sci. 152/153 (1985) 284. 1161 E.T. Jensen and R.E. Palmer, Rev. Sci. Instrum. 60 (1989) 3065. [17] R.E. Palmer, P.V. Head and R.F. Willis, Rev. Sci. Instrum. 58 (1987) 1118. [18] P. Sjiivall and B. Kasemo, submitted. [19] K.M. Hock, PhD Thesis, University of Cambridge, 1992.