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Cs induced work function changes on the graphite (0001) surface
L956
SURFACE
Surface Science 177 (1986) L956-L962 North-Holland, Amsterdam
SCIENCE
LETTERS
Cs INDUCED WORK FUNCTION CHANGES ON THE GRAPHITE (0001) SURFACE Z.P. HU *, N.J. WU ** and A. IGNATIEV Depmment
of Ph_wcs, Unrw-SIQJ
of HouFton Unrversrty Park, Houston, TX 77004, USA
Received 20 November 1985; accepted for publication 10 June 1986
The retarding potential method has been used to measure the work function changes due to Cs adsorption on the graphite (0001) surface. The work function decreases sharply and linearly with increasing Cs coverage up to a coverage B = 1 X 1014 atoms/cm2 after which there is no significant change. The work function minimum is reached at the onset of the (6 x &)R30° LEED pattern with a resultant A@ of -2.44 eV. The characteristic retarding field current-voltage curves also yield information about localized electron states at the Cs/graphite surface
Work function changes induced by alkali atom adsorption have been the subject of investigation for many years. A number of experiments dealing with work function change determined by absolute or relative methods [l-6] and numerous theoretical calculations have been devoted to it [7-121. The main reason for this comes from the interest in alkali adsorption enhancing the electron emission of metals and in understanding the mechanism of chemisorption using simple monovalent adsorbates. The work function change induced by alkali metal adsorbates has been described previously within a classical model [7] and a quantum-mechanical model [S-12]. The classical model explained the work function change as due to complete ionization of the alkali atoms upon adsorption. The adsorbate ions and their images form dipole moments P, with the work function change expressed as: A@ = 4mePN,,
(1)
where N, is the number of adatoms per unit area. Alkali atoms, however, are not completely ionized upon adsorption and many details cannot be explained by this model. The quantum-mechanical model considers the work function change as due to the change of the charge distribution accompanying the adsorption of an * Permanent address: University of Science and Technology, Hefei, People’s Rep. of China. ** Permanent address: Institute of Physics, Academy of Science, Beijing, People’s Rep. of China.
0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Z. P. Hu et al. / Cs rnduced work functron
changes
I.951
alkali species. The valence electrons of the alkali are polarized towards the interface and a polarized overlayer is formed thus lowering the work function. The predictions of theory have been well born out by experiment, however, even though a fair amount of theoretical and experimental work has been done on work function changes under alkali adsorption, there are no studies of work function changes induced by alkali adsorption on graphite. Our current interests in alkali metal adsorption on graphite have resulted in this study of work function charges upon adsorption of cesium. A standard four-grid LEED-AES apparatus mounted in an ultra-high Torr) was used for this investigation. vacuum chamber (pressure - 5 x lo-” The retarding potential or diode method was used [13] for the work function measurement. A variable retarding potential was placed on the sample and scanned through the energy of the incident electron beam (- 50 eV). The electron current through the sample was measured as a function of the retarding potential. The single crystal graphite sample was cleaved in air by the nitrogen gas method [14] and mounted on an UHV sample manipulator [15] with the capabilities of direct heating of the sample to 1400 K and cooling to 30 K. The graphite surface was cleaned in UHV at 600°C for 4 h with subsequent alkali adsorption monitored on a region of the sample that had a low step density (< 0.01) as defined by a 3-fold LEED pattern [14]. Cesium was evaporated onto the graphite from a thermal getter source mounted about 11 cm from the sample. The amount evaporated was regulated by controlling both the current through the source and the time of evaporation. Typical evaporation rates were - 2 x 1012 atoms/mm cm2. The graphite sample was held at 80 K during Cs evaporation. Fig. 1 shows the ring LEED pattern observed after the evaporation of 6 x 1013 Cs atoms/cm* onto the graphite (0001) surface. As Cs exposure is increased the diffraction rings expand continuously until a new surface phase is nucleated at a coverage of about 0.07 with additional phases being observed at higher coverages [16]. The diffraction ring observation is consistent with a low density Cs surface structure where Cs atoms are randomly but uniformly distributed on the graphite surface. The diffraction ring radius is therefore directly related to the Cs interatomic spacing in the adlayer and it can also be used to define surface coverage in the low coverage (8 < 1 X 1014 atoms/cm2) regime (table 1). It should be noted that such behavior (i.e. ring structures) was not observed on (0001) graphite surfaces with 6-fold symmetry, i.e. surfaces with high step density [14]. Such steps affect the homogeneous distribution of the Cs on the graphite. The change in work function upon Cs adsorption is obtained from the retarding potential current-voltage plots shown in fig. 2. These plots are for Cs coverages of 1 X 1013 < B < 1 X 1014 as determined by the LEED ring
L958
Z. P. Hu et al. / Cs Induced work functron changes
‘._ /
0 ‘I A’
I
‘L
(‘6) \_/
-‘\
:
/:
/--\
Fig. 1. A ring diffraction
Table 1 Cs induced
work function
Curve
y
0.13 0.15 0.16 0.18 0.20
pattern taken at 72 eV and a Cs coverage (corresponding to curve 4 in fig. 3).
changes
on the graphite
d
(Ae)
(‘Q
(eW
18.9 16.4 15.4 13.7 12.3
-0.75 - 1.37 - 1.70 - 1.92 - 1.97 - 2.07 -2.12
of
- 6~10”~
atoms/cm2
(0001) surface
N (10’4 atoms/ cm2)
AK, (v)
0.32 0.43 0.49 0.61 0.76 0.86 0.96
0.53 0.58 0.52 0.50 0.45 0.47 0.47
A& (v)
0.58 1.25 1.85 2.10 2.25 2.40 2.38
1,s (PA)
I, (PA)
2.0 1.8 1.8 2.1 2.4 2.6 3.0
4.2 1.5 1.5 1.5 1.3 0.8 0.7 0.5
The curve numbers in the table correspond to the curves in fig. 2. y is the ratio of the diameter of the small diffraction ring to the distance between two opposrte graphite diffraction spots. d is the average distance between two nearest neighbor cesium atoms (d = 2.46 A/y). AV,, is the width of the high retarding potential sigmoid feature in the curve. AVY 1s the width of the lower retarding potential sigmoid feature. I,, and Is are the maximum currents corresponding to the two sigmoid features.
2. P. Hu et al. / Cs rndured work funrtton changes
44
45
46
Retarding
47
46
’ 49
L959
Oo 50
Voltage (V)
Fig. 2. The retarding voltage-cu~ent
curves for increasing coverages of Cs on graphite (curves 0 to 7). The curve numbers correspond to the curve numbers in table 1. Note the double sigmoid structure in curves 1-5.
patterns. The curves are expected to be single sigmoid shaped with the voltage value at the inflection point related to the work function of the surface. Curves 0 and 7 of fig. 2 shows this expected behavior with, however, the remaining curves showing 2 sigmoid steps in each curve. By definition, the work function is the minimum amount of work required to remove an electron to infinity, specifically from the highest lying electron state in the system. This state is at the Fermi level which in the Cs/graphite system has both alkali and carbon interlayer bonding character [17-191. In the experiment, the work function of the surface is given by the sigmoid step at highest retarding potential (i.e. smallest difference between retarding potential and electron beam energy, E). It can be seen in fig. 2 that this work function value changes (retarding potential increases, i.e. work function decreases) with increasing Cs exposure. This is more clearly seen in fig. 3 where measured change in work function is plotted as a function of Cs coverage.
L960
Z. P. Hu et al. / Cs induced work functronchanges
12345678 Cs Density
(x 1014 atoms/cm*)
Fig. 3. The change in work function versus Cs coverage on graphite.
The shift to higher retarding potential and the increase in intensity with coverage in the high retarding potential sigmoid feature in the current-voltage curves (the feature defining the work function) is due to the shift in Fermi energy (EF) and the increase in DOS at E, with increasing alkali metal concentration. Note that the Cs coverage range of the data in fig. 2 is localized to extremely low coverage (table 1) where uniform coverage of the Cs is observed (LEED ring structures). In addition, there are no additional Cs structural phases observed in this coverage region [16]. It can also be seen that the decrease in work function is quite linear at coverages less than 1 X 1014 atoms/cm’ showing the applicability of eq. (1) to the low coverage regime. Using eq. (l), the initial dipole moment PO, can be extracted from the initial slope of the curve of fig. 3. The initial dipole moment is found to be P,, = 9.0 k 0.4 debye. This is significantly larger than found for cesium on Ta [20], W [20] and Ni [4], and indicates the possibility of greater charge transfer from Cs to graphite than in the metallic substrate systems. As coverage increases, depolarization occurs between the Cs adatoms resulting in a deviation from linearity in the work function versus coverage curve [l]. At Cs coverages greater than 1 x 1014 atoms/cm2 the change in work function remains relatively constant with a slight minimum corresponding to the attainment of the most intense (6 x J?;)R30° LEED pattern for the surface. The (6 X &)R30” structure is currently being studied by LEED intensity-energy analysis, however, it is expected that the structural model will be that of close packed Cs atoms with intra-layer atomic spacing of - 4.26 A,
Z. P. Hu et al. / Cs induced work function chunges
L961
i.e. 19% smaller than the spacing found in solid Cs but not as small as that expected for Cs in the ionic state. This structure corresponds to full monolayer Cs coverage on graphite. The large compression of the in-plane lattice parameter for monolayer Cs on graphite is also observed to a lesser degree for monolayer K on graphite (7% compression) [21,22]. In addition, the highly compressed (0 x fi)R30° phase is also observed for bulk stage-l K intercalated graphite subject to > 15 kbar hydrostatic pressure [23,24]. At the (6 X fi)R30” point, the work function change is A+ = - 2.44 eV. This is to be compared with the published work function values of graphite (4.7 eV) [25] and bulk cesium (2.00-2.14 eV) [1,20]. It should be noted that although no drastic minimum can be observed in the A@ versus 8 curve of fig. 3, the final work function of the Cs/graphite system at coverage of 7 X 1014 atoms/cm’ is close to that of bulk Cs. The second (lower retarding potential) sigmoid feature in curves l-5 of fig. 2 is not strictly a work function feature since it does not represent the minimum work required to remove an electron. The second feature does not significantly shift with Cs coverage, but does decrease in intensity and broaden. It is well to note again that within the region of exposures for which the second sigmoid feature is observed (fig. 2) no other phases of the Cs are observed on the graphite surface and no other phases are expected from the measured surface phase diagram [16]. Therefore, it is expected that the second feature is not a result of a phase separation at the surface, but is due to surface electronic structure. The feature is believed to be due to remnants of the graphite r bands [26] which become overshadowed by the increase in DOS at E, and the shift of E, upon alkali metal adsorption. In summary, the change in work function upon Cs adsorption has been measured by the retarding potential method for the graphite (0001) surface. The work function decreases linearly at low coverage with the change in work function reaching a shallow minimum of A@ = - 2.44 eV at a coverage of 5 X 1014 atoms/cm*. This corresponds to the existence of a (fi X fi)R30” surface structure on the surface. The retarding field measurements also show the relative shift and increase in intensity of alkali induced states at E, as compared to the remnants of the 7~ band graphite states located at E, of pristine graphite. Helpful discussions with C.S. Ting and W. Cai are gratefully acknowledged. The work was partially supported by the R.A. Welch Foundation.
References [l] J. Holzl and F.K. Schulte, Solid Surface Physics, Vol. 85 (Springer, New York, 1979). [2] A.G. Fedorus and A.G. Naumovets, Surface Sci. 21 (1970) 426.
L962
Z. P. Hu et al. / Cs Induced work function changes
[3] A.U. MacRae, K. Muller, J.J. Lander and J. Morrison, Surface Sci. 15 (1%9) 483. [4] R. Gerlach and T. Rhodin, Surface Sci. 19 (1970) 403. [5] G. Ertl and J. Kuppers, Low Energy Electrons and Surface Chemistry (Verlag Chemie, Weinheim, 1974) ch. 8. [6] S. Anderson and U. Jostell, Surface Sci. 46 (1978) 625. [7] I. Langmuir, J. Am. Chem. Sot. 54 (1932) 2798. [S] R.W. Gurney, Phys. Rev. 47 (1935) 479. [9] D. Newns, Phys. Rev. 178 (1969) 1123. [lo] P.W. Anderson, Phys. Rev. 129 (1961) 41. [ll] N.D. Lang and W. Kohn, Phys. Rev. B7 (1973) 3541. [12] E. Wimmer, A.J. Freeman, J.R. Hiskes and A.M. Karo, Phys. Rev. B28 (1983) 3074. [13] A.G. Knapp, Surface Sa. 34 (1973) 289. [14] N.J. Wu and A. Ignatiev, Phys. Rev. B25 (1982) 2983. [15] N.J. Wu and A. Ignatiev, Rev. Sci. Instr. 56 (1985) 752. [16] Z.P. Hu, N.J. Wu and A. Ignatiev, Phys. Rev. B, in press. [17] M. Postemak, A. Baldereschi, A.J. Freeman, E. Wimmer and M. Weinert, Phys. Rev. Letters 50 (1983) 761. [18] M. Lagues, D. Marchand, C. Fretigny and A. Legrand, Solid State Commun. 49 (1984) 739. [19] G. Volpilhac, F. Achard, L. Ducasse and J. Hoarau, J. Phys. Cl8 (1985) 1613. [20] D.L. Fehrs and R.E. Stickney, Surface Sci. 24 (1971) 309. [21] N.J. Wu and A. Ignatiev, J. Vacuum Sci. Technol. 20 (1982) 896. [22] A. Ignatiev and W.C. Fan, J. Vacuum Sci. Technol.. in press. [23] C. Fverst, J. Fischer, J. Axe, J. Hastings and D. McWhan, Phys. Rev. Letters 50 (1983) 357. [24] J. Bloch, H. Katz, D. Moses, V. Cajipe and J. Fisher, Phys. Rev. B31 (1985) 6785. [25] R.F. Willis, B. Fitton and G. Painter, Phys. Rev. B9 (1974) 1926. [26] I. McGovern, W. Eberhardt, E. Plummer and J. Fischer, Physica B99 (1980) 415.
SURFACE
Surface Science 177 (1986) L956-L962 North-Holland, Amsterdam
SCIENCE
LETTERS
Cs INDUCED WORK FUNCTION CHANGES ON THE GRAPHITE (0001) SURFACE Z.P. HU *, N.J. WU ** and A. IGNATIEV Depmment
of Ph_wcs, Unrw-SIQJ
of HouFton Unrversrty Park, Houston, TX 77004, USA
Received 20 November 1985; accepted for publication 10 June 1986
The retarding potential method has been used to measure the work function changes due to Cs adsorption on the graphite (0001) surface. The work function decreases sharply and linearly with increasing Cs coverage up to a coverage B = 1 X 1014 atoms/cm2 after which there is no significant change. The work function minimum is reached at the onset of the (6 x &)R30° LEED pattern with a resultant A@ of -2.44 eV. The characteristic retarding field current-voltage curves also yield information about localized electron states at the Cs/graphite surface
Work function changes induced by alkali atom adsorption have been the subject of investigation for many years. A number of experiments dealing with work function change determined by absolute or relative methods [l-6] and numerous theoretical calculations have been devoted to it [7-121. The main reason for this comes from the interest in alkali adsorption enhancing the electron emission of metals and in understanding the mechanism of chemisorption using simple monovalent adsorbates. The work function change induced by alkali metal adsorbates has been described previously within a classical model [7] and a quantum-mechanical model [S-12]. The classical model explained the work function change as due to complete ionization of the alkali atoms upon adsorption. The adsorbate ions and their images form dipole moments P, with the work function change expressed as: A@ = 4mePN,,
(1)
where N, is the number of adatoms per unit area. Alkali atoms, however, are not completely ionized upon adsorption and many details cannot be explained by this model. The quantum-mechanical model considers the work function change as due to the change of the charge distribution accompanying the adsorption of an * Permanent address: University of Science and Technology, Hefei, People’s Rep. of China. ** Permanent address: Institute of Physics, Academy of Science, Beijing, People’s Rep. of China.
0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Z. P. Hu et al. / Cs rnduced work functron
changes
I.951
alkali species. The valence electrons of the alkali are polarized towards the interface and a polarized overlayer is formed thus lowering the work function. The predictions of theory have been well born out by experiment, however, even though a fair amount of theoretical and experimental work has been done on work function changes under alkali adsorption, there are no studies of work function changes induced by alkali adsorption on graphite. Our current interests in alkali metal adsorption on graphite have resulted in this study of work function charges upon adsorption of cesium. A standard four-grid LEED-AES apparatus mounted in an ultra-high Torr) was used for this investigation. vacuum chamber (pressure - 5 x lo-” The retarding potential or diode method was used [13] for the work function measurement. A variable retarding potential was placed on the sample and scanned through the energy of the incident electron beam (- 50 eV). The electron current through the sample was measured as a function of the retarding potential. The single crystal graphite sample was cleaved in air by the nitrogen gas method [14] and mounted on an UHV sample manipulator [15] with the capabilities of direct heating of the sample to 1400 K and cooling to 30 K. The graphite surface was cleaned in UHV at 600°C for 4 h with subsequent alkali adsorption monitored on a region of the sample that had a low step density (< 0.01) as defined by a 3-fold LEED pattern [14]. Cesium was evaporated onto the graphite from a thermal getter source mounted about 11 cm from the sample. The amount evaporated was regulated by controlling both the current through the source and the time of evaporation. Typical evaporation rates were - 2 x 1012 atoms/mm cm2. The graphite sample was held at 80 K during Cs evaporation. Fig. 1 shows the ring LEED pattern observed after the evaporation of 6 x 1013 Cs atoms/cm* onto the graphite (0001) surface. As Cs exposure is increased the diffraction rings expand continuously until a new surface phase is nucleated at a coverage of about 0.07 with additional phases being observed at higher coverages [16]. The diffraction ring observation is consistent with a low density Cs surface structure where Cs atoms are randomly but uniformly distributed on the graphite surface. The diffraction ring radius is therefore directly related to the Cs interatomic spacing in the adlayer and it can also be used to define surface coverage in the low coverage (8 < 1 X 1014 atoms/cm2) regime (table 1). It should be noted that such behavior (i.e. ring structures) was not observed on (0001) graphite surfaces with 6-fold symmetry, i.e. surfaces with high step density [14]. Such steps affect the homogeneous distribution of the Cs on the graphite. The change in work function upon Cs adsorption is obtained from the retarding potential current-voltage plots shown in fig. 2. These plots are for Cs coverages of 1 X 1013 < B < 1 X 1014 as determined by the LEED ring
L958
Z. P. Hu et al. / Cs Induced work functron changes
‘._ /
0 ‘I A’
I
‘L
(‘6) \_/
-‘\
:
/:
/--\
Fig. 1. A ring diffraction
Table 1 Cs induced
work function
Curve
y
0.13 0.15 0.16 0.18 0.20
pattern taken at 72 eV and a Cs coverage (corresponding to curve 4 in fig. 3).
changes
on the graphite
d
(Ae)
(‘Q
(eW
18.9 16.4 15.4 13.7 12.3
-0.75 - 1.37 - 1.70 - 1.92 - 1.97 - 2.07 -2.12
of
- 6~10”~
atoms/cm2
(0001) surface
N (10’4 atoms/ cm2)
AK, (v)
0.32 0.43 0.49 0.61 0.76 0.86 0.96
0.53 0.58 0.52 0.50 0.45 0.47 0.47
A& (v)
0.58 1.25 1.85 2.10 2.25 2.40 2.38
1,s (PA)
I, (PA)
2.0 1.8 1.8 2.1 2.4 2.6 3.0
4.2 1.5 1.5 1.5 1.3 0.8 0.7 0.5
The curve numbers in the table correspond to the curves in fig. 2. y is the ratio of the diameter of the small diffraction ring to the distance between two opposrte graphite diffraction spots. d is the average distance between two nearest neighbor cesium atoms (d = 2.46 A/y). AV,, is the width of the high retarding potential sigmoid feature in the curve. AVY 1s the width of the lower retarding potential sigmoid feature. I,, and Is are the maximum currents corresponding to the two sigmoid features.
2. P. Hu et al. / Cs rndured work funrtton changes
44
45
46
Retarding
47
46
’ 49
L959
Oo 50
Voltage (V)
Fig. 2. The retarding voltage-cu~ent
curves for increasing coverages of Cs on graphite (curves 0 to 7). The curve numbers correspond to the curve numbers in table 1. Note the double sigmoid structure in curves 1-5.
patterns. The curves are expected to be single sigmoid shaped with the voltage value at the inflection point related to the work function of the surface. Curves 0 and 7 of fig. 2 shows this expected behavior with, however, the remaining curves showing 2 sigmoid steps in each curve. By definition, the work function is the minimum amount of work required to remove an electron to infinity, specifically from the highest lying electron state in the system. This state is at the Fermi level which in the Cs/graphite system has both alkali and carbon interlayer bonding character [17-191. In the experiment, the work function of the surface is given by the sigmoid step at highest retarding potential (i.e. smallest difference between retarding potential and electron beam energy, E). It can be seen in fig. 2 that this work function value changes (retarding potential increases, i.e. work function decreases) with increasing Cs exposure. This is more clearly seen in fig. 3 where measured change in work function is plotted as a function of Cs coverage.
L960
Z. P. Hu et al. / Cs induced work functronchanges
12345678 Cs Density
(x 1014 atoms/cm*)
Fig. 3. The change in work function versus Cs coverage on graphite.
The shift to higher retarding potential and the increase in intensity with coverage in the high retarding potential sigmoid feature in the current-voltage curves (the feature defining the work function) is due to the shift in Fermi energy (EF) and the increase in DOS at E, with increasing alkali metal concentration. Note that the Cs coverage range of the data in fig. 2 is localized to extremely low coverage (table 1) where uniform coverage of the Cs is observed (LEED ring structures). In addition, there are no additional Cs structural phases observed in this coverage region [16]. It can also be seen that the decrease in work function is quite linear at coverages less than 1 X 1014 atoms/cm’ showing the applicability of eq. (1) to the low coverage regime. Using eq. (l), the initial dipole moment PO, can be extracted from the initial slope of the curve of fig. 3. The initial dipole moment is found to be P,, = 9.0 k 0.4 debye. This is significantly larger than found for cesium on Ta [20], W [20] and Ni [4], and indicates the possibility of greater charge transfer from Cs to graphite than in the metallic substrate systems. As coverage increases, depolarization occurs between the Cs adatoms resulting in a deviation from linearity in the work function versus coverage curve [l]. At Cs coverages greater than 1 x 1014 atoms/cm2 the change in work function remains relatively constant with a slight minimum corresponding to the attainment of the most intense (6 x J?;)R30° LEED pattern for the surface. The (6 X &)R30” structure is currently being studied by LEED intensity-energy analysis, however, it is expected that the structural model will be that of close packed Cs atoms with intra-layer atomic spacing of - 4.26 A,
Z. P. Hu et al. / Cs induced work function chunges
L961
i.e. 19% smaller than the spacing found in solid Cs but not as small as that expected for Cs in the ionic state. This structure corresponds to full monolayer Cs coverage on graphite. The large compression of the in-plane lattice parameter for monolayer Cs on graphite is also observed to a lesser degree for monolayer K on graphite (7% compression) [21,22]. In addition, the highly compressed (0 x fi)R30° phase is also observed for bulk stage-l K intercalated graphite subject to > 15 kbar hydrostatic pressure [23,24]. At the (6 X fi)R30” point, the work function change is A+ = - 2.44 eV. This is to be compared with the published work function values of graphite (4.7 eV) [25] and bulk cesium (2.00-2.14 eV) [1,20]. It should be noted that although no drastic minimum can be observed in the A@ versus 8 curve of fig. 3, the final work function of the Cs/graphite system at coverage of 7 X 1014 atoms/cm’ is close to that of bulk Cs. The second (lower retarding potential) sigmoid feature in curves l-5 of fig. 2 is not strictly a work function feature since it does not represent the minimum work required to remove an electron. The second feature does not significantly shift with Cs coverage, but does decrease in intensity and broaden. It is well to note again that within the region of exposures for which the second sigmoid feature is observed (fig. 2) no other phases of the Cs are observed on the graphite surface and no other phases are expected from the measured surface phase diagram [16]. Therefore, it is expected that the second feature is not a result of a phase separation at the surface, but is due to surface electronic structure. The feature is believed to be due to remnants of the graphite r bands [26] which become overshadowed by the increase in DOS at E, and the shift of E, upon alkali metal adsorption. In summary, the change in work function upon Cs adsorption has been measured by the retarding potential method for the graphite (0001) surface. The work function decreases linearly at low coverage with the change in work function reaching a shallow minimum of A@ = - 2.44 eV at a coverage of 5 X 1014 atoms/cm*. This corresponds to the existence of a (fi X fi)R30” surface structure on the surface. The retarding field measurements also show the relative shift and increase in intensity of alkali induced states at E, as compared to the remnants of the 7~ band graphite states located at E, of pristine graphite. Helpful discussions with C.S. Ting and W. Cai are gratefully acknowledged. The work was partially supported by the R.A. Welch Foundation.
References [l] J. Holzl and F.K. Schulte, Solid Surface Physics, Vol. 85 (Springer, New York, 1979). [2] A.G. Fedorus and A.G. Naumovets, Surface Sci. 21 (1970) 426.
L962
Z. P. Hu et al. / Cs Induced work function changes
[3] A.U. MacRae, K. Muller, J.J. Lander and J. Morrison, Surface Sci. 15 (1%9) 483. [4] R. Gerlach and T. Rhodin, Surface Sci. 19 (1970) 403. [5] G. Ertl and J. Kuppers, Low Energy Electrons and Surface Chemistry (Verlag Chemie, Weinheim, 1974) ch. 8. [6] S. Anderson and U. Jostell, Surface Sci. 46 (1978) 625. [7] I. Langmuir, J. Am. Chem. Sot. 54 (1932) 2798. [S] R.W. Gurney, Phys. Rev. 47 (1935) 479. [9] D. Newns, Phys. Rev. 178 (1969) 1123. [lo] P.W. Anderson, Phys. Rev. 129 (1961) 41. [ll] N.D. Lang and W. Kohn, Phys. Rev. B7 (1973) 3541. [12] E. Wimmer, A.J. Freeman, J.R. Hiskes and A.M. Karo, Phys. Rev. B28 (1983) 3074. [13] A.G. Knapp, Surface Sa. 34 (1973) 289. [14] N.J. Wu and A. Ignatiev, Phys. Rev. B25 (1982) 2983. [15] N.J. Wu and A. Ignatiev, Rev. Sci. Instr. 56 (1985) 752. [16] Z.P. Hu, N.J. Wu and A. Ignatiev, Phys. Rev. B, in press. [17] M. Postemak, A. Baldereschi, A.J. Freeman, E. Wimmer and M. Weinert, Phys. Rev. Letters 50 (1983) 761. [18] M. Lagues, D. Marchand, C. Fretigny and A. Legrand, Solid State Commun. 49 (1984) 739. [19] G. Volpilhac, F. Achard, L. Ducasse and J. Hoarau, J. Phys. Cl8 (1985) 1613. [20] D.L. Fehrs and R.E. Stickney, Surface Sci. 24 (1971) 309. [21] N.J. Wu and A. Ignatiev, J. Vacuum Sci. Technol. 20 (1982) 896. [22] A. Ignatiev and W.C. Fan, J. Vacuum Sci. Technol.. in press. [23] C. Fverst, J. Fischer, J. Axe, J. Hastings and D. McWhan, Phys. Rev. Letters 50 (1983) 357. [24] J. Bloch, H. Katz, D. Moses, V. Cajipe and J. Fisher, Phys. Rev. B31 (1985) 6785. [25] R.F. Willis, B. Fitton and G. Painter, Phys. Rev. B9 (1974) 1926. [26] I. McGovern, W. Eberhardt, E. Plummer and J. Fischer, Physica B99 (1980) 415.