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Surface intercalation of graphite by lanthanum
427
Journal of Electron Spectroscopy and Related Phenomena, 68 (1994) 427430 0368.2048/94/$07,00 @I 1994 Elsevier Science B.V. All rights reserved
SURFACE
INTERCALATION
OF GRAPHITE
BY LANTHANUM
G. V. Prudnikova, A. G. Vjatkin, A. V. Ermakov, A. M. Shikin and V. K. Adamchuk Institute of Physics, St.-Petersburg
State University, St.-Petersburg,
198904, Russia
Investigation of La - adsorption onto the @OOl>graphite surface with following annealing was performed by AES and STM . It was shown that the low temperatre annealing leads to formation of La carbide phase, and the high temperature annealing is followed by La - intercalated graphite formation. The C(KVV) Auger line of carbide shows the characteristic triplet structure. La - intercalated graphite is characterized by reconstruction of the surface crystalline structure and appearance in C(KVV) Auger spectra of the high energy feature connected with formation of the high density of states near the Fermi-level.
1. INTRODUCTION This work is devoted to the study of the electron spectra and surface crystalline structure changes under adsorption of lanthanum onto graphite surface and thermal annealing of the adsorbed La-layers with the aim of the lanthanum-intercalated graphite formation, Interest to the investigation of such systems is motived by the similarity to alkali-metal graphite intercalation compounds (AGIC), which display a novel electronic, chemical and structural properties (including superconductivity) [ 1 ].On the other hand the interaction of light rare-earth metals with the sp-valence band elements (carbon, nitrogen1 can lead to formation of systems characterized by the sharp-structured density of states in the Fermi-level region that allows to suppose the formation of a new interesting properties for such systems [2,3]. We have not found any mentions concerning the existence of Iaintercalated graphite in literature. Therefore it was interesting to study the processes of Laadsorption and thermal annealing which result in the La-intercalated graphite formation. This work is devoted to the investigation of (La-C) chemical interaction and surface reconstruction taking place under the adsorption of La onto (0001) graphite surface and thermal SSDI0368-2048(94)02143-N
annealing.The investigation was performed by the use of Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM) methods.
2. EXPERIMENTAL DETAILS AES measurements were performed by the use of the 4-grid LEED optic analyzer, It operated with the energy resolution of 0.25% with 0.5 V amplitude modulation and the primary electron energy of 1 Kev . Experiments were carried out under vacuum conditions of 1O”l’ Torr. Pieaces of highly oriented pyrolytic graphite (HOPG) with the preliminary cleaved (0001) surface were used as the substrates. Lanthanum was evaporated by heating of a Labead melted on a thin (W-Re) wire. The thickness of deposited layers was monitored by a quartz microbalance method. The annealing of the Lagraphite systems was performed by heating of back-placed Ta-foil. STM measurement was performed at the bias voltage of 100 mV and tunneling current of 1 nA in the constant current mode. The lateral resolution was calibrated by the comparison with the atomic resolution STM image of (0001) graphite surface obtained with the same STM tip. To minimize the thermal drift
428
all STM images were measured one hour latter after the mounting of the sample into the STM.
3.RESULTS
AND DISCUSSION
The typical C(KVV) Auger electron spectra observed for different stages of La-adsorption onto the @OOl)graphite surface and subsequent thermal annealing are shown in fig.1, (curves 1,2,3,5). To reveal the characteristic features and compare with the results of other works [4-6 ] the dN/dE modes of the experimental Auger spectra are presented in fig.1.b. The spectra of d-metal carbide and alkali-metal intercalation compound from works 16] and [7 ] are also represented in fig. 1, (curves 4 and 6)) respectively, The N(E) Auger spectra presented in fig. 1.a are shown after the correction on the secondary electron background by Shirley method using the procedure similar to [9 I. From analysis of the Auger spectra (curves 1, 2, fig.0 one can see that the structure of the C(KVV) Auger line is significantly changed under la-adsorption onto the @OOl)surface of graphite. New features at energies of 238 eV and 254 eV appear in the spectra (marked in fig.1 by A and A’). In dN/dE mode it corresponds to the positions of the minima equal to 241 eV and 257 eV. For La-coverages more than 3-4 monolayers the C(KVV1 Auger line is characterized by triplet structure (see curves 2). Similar Auger spectra with characteristic triplet structure are observed for d-metal carbides [4,5,7]. For comparison the Auger spectrum for vanadium carbide from [7 ] is represented in fig.1, curves 4. Formation of the characteristic triplet structure is usually connected with the formation of Me(d)-C(p) hybrid bonding and antibonding states in the valence band. Under lowtemperature annealing of thick layer of lanthanum on graphite at 600’ C (curves 3 in fig.11 the triplet structure becomes more pronounced. Thus, from the analysis of the C(KW) Auger line changes we can conclude that the formation of the La-carbide chemical bonds takes place under room-temperature adsorption of lanthanum onto @OOl) graphite surface and low-temperature annealing of La-graphite
A
‘
\
-i-T
/,
240
,
260
a ,~ L6 280
240
260
280
KINETIC ENERGY, eV Figure 1. The C(KVV) Auger spectra for different stages of La-adsorption onto the @OOl)graphite surface and thermal annealing of the adsorbed La-layer in mode- (a) and dN/dE N(E) mode - (b). (1) - for pure graphite; (2) - for La-coverage of 4 monolayers; (3) - after low-temperature annealing; (4) - for vanadium carbide ( from [6 3); (5) - after high-temperature annealing; (6) - for AGIC (from [7 1). system. The high - temperature annealing of the adsorbed la-layer at the temperature of 12001400’ C leads to the additional changes of the
429
C(KVV) Auger spectra (curves 5 in fig. 1). The new feature appears at the energy of 280 eV. It is marked in fig.1 by B. Such feature is also observed in Auger spectra for AGIC [7,8]. For comparison the Auger spectrum of AGIC from [7] is represented. The appearance of this feature in Auger spectra can testify to the formation of intercalated graphite phase, that is characterized by high density of states near the Fermi-level. The formation of such intercalated phase is connected with reconstruction of the crystalline structure, incorporating of the lanthanum atoms into the space between graphite layers and electron charge redistribution between metal atoms and sp2 -planes of graphite [lO,ll 1.
To observe the surface crystalline structure changes under the intercalation process the STM ‘images of the La-intercalated graphite and pure (0001) graphite surfaces were obtained. They are displayed in fig.2 a,b. On the right sides of
rorlmted
fig3a and 2b the profiles of the electron density distribution along atomic rows are also shown. It is seen from fig.2 the (0001) graphite surface is characterized by the centered hexagonal symmetry. The distance between the nearestneighbor maxima in the ST,M density profile equals approximatelly to 2,SA that corresponds to the distance between the sites with coincidental carbon atoms in the first and second atomic layers of graphite {lo]. From STM image of the high temperature annealing phase (fig.2.b) we can see that the reconstruction of surface crystalline structure takes place. The STM image and the electron density profile show an increase of surface structure period relative to that for the pure graphite surface. Similar changes of the surface structure are also observed for the first stage AGIC [11,12]. There are several surface structures for different types of AGIC - ( @X fl) StIWtUre for c&i-cm [ 12 ] Of (2x2) structure for other alkali metals (K, Cs) -
glrrphlte
CbJ,
Figure 2 . STM images and the electron density profiles for the surfaces of graphite NOOl) - (a) and La - intercalated graphite - (b)
430
GIC with composition CaMe [ 111. In the case of La-intercalated graphite the electron density profile along the scanning line shows the period of about 4,2&. The directions of the electron density profiles are shown in fig.2.a and 2.b by solid lines. It corresponds to the formation of o-like structure. Possible thermal drift can distort the symmetry of STM image and the distances, especially along the direction perpendicular to the scanning line. According to this fact, now we can not identify the symmetry more exactly and type of the surface structure of La-intercalated graphite. The existence of ( 1(5x @, flx2> structure and others is possible. For careful diagnostics of the surface crystalline and electronic structure of La-intercalated graphite phase the additional investigation are needed.
4. CONCLUSION The results of this work can testify that the formation of the La-carbide phase takes place under La-adsorption onto the @OOl)graphite surface and low temperature annealing of this system (600’ C) . The C (KVV) Auger spectra of this La-C phase is characterized by the carbidelike triplet structure. The high temperature annealing (12001400’C) of the La-graphite system leads to the formation of La-intercalated graphite. This La-C phase is characterized by the feature on the high energy side of C(KVV) Auger peak. It is connected with the formation of high density of states near the Fermi-level. STM image of the the La-intercalated graphite shows reconstruction of the surface with the increase of
the surface crystalline structure period and the formation of 13-like symmetry.
REFERENCES
1. M.S.Dresselhaus, G.Dresselhaues, Advances in Phys., 3O.tl981) 139. 2. M.R. Norman, D.D. Koelling, A.J. Freeman, Phys.Rev.B, 31 (1985) 6251. 3, D.D. Koelling, B.D. Dunlap, G.W. Crabtee, Phys.Rev.B, 31 (1985) 4966. 4. D.E.Ramaker,Appl. Surf. Sci.,21 (1985) 243. 5. D.E. Ramaker, Critical Reviews in Solid St. and Mater.Sci., 17(3) (1991) 211. 6. J.M. Shulga, G.L. Gutsev, J. Electr. Spectr. (1984) 39. Relat.Phenom. 34 7. M.Laguqs, D.Marchand, CFretign, J. Vat. Sci. Technol., A 5 (1987) 1292. 8. J.S. Murday, B.I. Dunlap, F.L. Hutson II, P.Oelhafen, Phys.Rev.3, 24 (1981) 4764. 9. V.K. Adamchuk, A.M. Shikin, J. Electron Spectr. Relat. Phenom., 52 (1990) 103. lO.W.Eberhardt,L.T.McGovern, E,W.Plummer, J.E.Fisher, Phys.Rev. Lett., 44 (1980) 200. 1l.N. Gunasekara, T. Takahashi, F. Maeda, T. Sagawa, H. Suematsu. 2. Phys. B.Condensed Matter., 70 (1988) 349. 12.D. Tomanek, S.G. Lonie, Phys. Rev. B., 37 (1988) 8327. 13.S.P. Keltly, C.M. Lieber, Phys. Rev, B , 40 (1989) 5856. 14.D. Anselmetti, R. Wiesendanger, H.J.Guntherodt, Phys. Rev. B, 39 (1989) 11135.
Journal of Electron Spectroscopy and Related Phenomena, 68 (1994) 427430 0368.2048/94/$07,00 @I 1994 Elsevier Science B.V. All rights reserved
SURFACE
INTERCALATION
OF GRAPHITE
BY LANTHANUM
G. V. Prudnikova, A. G. Vjatkin, A. V. Ermakov, A. M. Shikin and V. K. Adamchuk Institute of Physics, St.-Petersburg
State University, St.-Petersburg,
198904, Russia
Investigation of La - adsorption onto the @OOl>graphite surface with following annealing was performed by AES and STM . It was shown that the low temperatre annealing leads to formation of La carbide phase, and the high temperature annealing is followed by La - intercalated graphite formation. The C(KVV) Auger line of carbide shows the characteristic triplet structure. La - intercalated graphite is characterized by reconstruction of the surface crystalline structure and appearance in C(KVV) Auger spectra of the high energy feature connected with formation of the high density of states near the Fermi-level.
1. INTRODUCTION This work is devoted to the study of the electron spectra and surface crystalline structure changes under adsorption of lanthanum onto graphite surface and thermal annealing of the adsorbed La-layers with the aim of the lanthanum-intercalated graphite formation, Interest to the investigation of such systems is motived by the similarity to alkali-metal graphite intercalation compounds (AGIC), which display a novel electronic, chemical and structural properties (including superconductivity) [ 1 ].On the other hand the interaction of light rare-earth metals with the sp-valence band elements (carbon, nitrogen1 can lead to formation of systems characterized by the sharp-structured density of states in the Fermi-level region that allows to suppose the formation of a new interesting properties for such systems [2,3]. We have not found any mentions concerning the existence of Iaintercalated graphite in literature. Therefore it was interesting to study the processes of Laadsorption and thermal annealing which result in the La-intercalated graphite formation. This work is devoted to the investigation of (La-C) chemical interaction and surface reconstruction taking place under the adsorption of La onto (0001) graphite surface and thermal SSDI0368-2048(94)02143-N
annealing.The investigation was performed by the use of Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM) methods.
2. EXPERIMENTAL DETAILS AES measurements were performed by the use of the 4-grid LEED optic analyzer, It operated with the energy resolution of 0.25% with 0.5 V amplitude modulation and the primary electron energy of 1 Kev . Experiments were carried out under vacuum conditions of 1O”l’ Torr. Pieaces of highly oriented pyrolytic graphite (HOPG) with the preliminary cleaved (0001) surface were used as the substrates. Lanthanum was evaporated by heating of a Labead melted on a thin (W-Re) wire. The thickness of deposited layers was monitored by a quartz microbalance method. The annealing of the Lagraphite systems was performed by heating of back-placed Ta-foil. STM measurement was performed at the bias voltage of 100 mV and tunneling current of 1 nA in the constant current mode. The lateral resolution was calibrated by the comparison with the atomic resolution STM image of (0001) graphite surface obtained with the same STM tip. To minimize the thermal drift
428
all STM images were measured one hour latter after the mounting of the sample into the STM.
3.RESULTS
AND DISCUSSION
The typical C(KVV) Auger electron spectra observed for different stages of La-adsorption onto the @OOl)graphite surface and subsequent thermal annealing are shown in fig.1, (curves 1,2,3,5). To reveal the characteristic features and compare with the results of other works [4-6 ] the dN/dE modes of the experimental Auger spectra are presented in fig.1.b. The spectra of d-metal carbide and alkali-metal intercalation compound from works 16] and [7 ] are also represented in fig. 1, (curves 4 and 6)) respectively, The N(E) Auger spectra presented in fig. 1.a are shown after the correction on the secondary electron background by Shirley method using the procedure similar to [9 I. From analysis of the Auger spectra (curves 1, 2, fig.0 one can see that the structure of the C(KVV) Auger line is significantly changed under la-adsorption onto the @OOl)surface of graphite. New features at energies of 238 eV and 254 eV appear in the spectra (marked in fig.1 by A and A’). In dN/dE mode it corresponds to the positions of the minima equal to 241 eV and 257 eV. For La-coverages more than 3-4 monolayers the C(KVV1 Auger line is characterized by triplet structure (see curves 2). Similar Auger spectra with characteristic triplet structure are observed for d-metal carbides [4,5,7]. For comparison the Auger spectrum for vanadium carbide from [7 ] is represented in fig.1, curves 4. Formation of the characteristic triplet structure is usually connected with the formation of Me(d)-C(p) hybrid bonding and antibonding states in the valence band. Under lowtemperature annealing of thick layer of lanthanum on graphite at 600’ C (curves 3 in fig.11 the triplet structure becomes more pronounced. Thus, from the analysis of the C(KW) Auger line changes we can conclude that the formation of the La-carbide chemical bonds takes place under room-temperature adsorption of lanthanum onto @OOl) graphite surface and low-temperature annealing of La-graphite
A
‘
\
-i-T
/,
240
,
260
a ,~ L6 280
240
260
280
KINETIC ENERGY, eV Figure 1. The C(KVV) Auger spectra for different stages of La-adsorption onto the @OOl)graphite surface and thermal annealing of the adsorbed La-layer in mode- (a) and dN/dE N(E) mode - (b). (1) - for pure graphite; (2) - for La-coverage of 4 monolayers; (3) - after low-temperature annealing; (4) - for vanadium carbide ( from [6 3); (5) - after high-temperature annealing; (6) - for AGIC (from [7 1). system. The high - temperature annealing of the adsorbed la-layer at the temperature of 12001400’ C leads to the additional changes of the
429
C(KVV) Auger spectra (curves 5 in fig. 1). The new feature appears at the energy of 280 eV. It is marked in fig.1 by B. Such feature is also observed in Auger spectra for AGIC [7,8]. For comparison the Auger spectrum of AGIC from [7] is represented. The appearance of this feature in Auger spectra can testify to the formation of intercalated graphite phase, that is characterized by high density of states near the Fermi-level. The formation of such intercalated phase is connected with reconstruction of the crystalline structure, incorporating of the lanthanum atoms into the space between graphite layers and electron charge redistribution between metal atoms and sp2 -planes of graphite [lO,ll 1.
To observe the surface crystalline structure changes under the intercalation process the STM ‘images of the La-intercalated graphite and pure (0001) graphite surfaces were obtained. They are displayed in fig.2 a,b. On the right sides of
rorlmted
fig3a and 2b the profiles of the electron density distribution along atomic rows are also shown. It is seen from fig.2 the (0001) graphite surface is characterized by the centered hexagonal symmetry. The distance between the nearestneighbor maxima in the ST,M density profile equals approximatelly to 2,SA that corresponds to the distance between the sites with coincidental carbon atoms in the first and second atomic layers of graphite {lo]. From STM image of the high temperature annealing phase (fig.2.b) we can see that the reconstruction of surface crystalline structure takes place. The STM image and the electron density profile show an increase of surface structure period relative to that for the pure graphite surface. Similar changes of the surface structure are also observed for the first stage AGIC [11,12]. There are several surface structures for different types of AGIC - ( @X fl) StIWtUre for c&i-cm [ 12 ] Of (2x2) structure for other alkali metals (K, Cs) -
glrrphlte
CbJ,
Figure 2 . STM images and the electron density profiles for the surfaces of graphite NOOl) - (a) and La - intercalated graphite - (b)
430
GIC with composition CaMe [ 111. In the case of La-intercalated graphite the electron density profile along the scanning line shows the period of about 4,2&. The directions of the electron density profiles are shown in fig.2.a and 2.b by solid lines. It corresponds to the formation of o-like structure. Possible thermal drift can distort the symmetry of STM image and the distances, especially along the direction perpendicular to the scanning line. According to this fact, now we can not identify the symmetry more exactly and type of the surface structure of La-intercalated graphite. The existence of ( 1(5x @, flx2> structure and others is possible. For careful diagnostics of the surface crystalline and electronic structure of La-intercalated graphite phase the additional investigation are needed.
4. CONCLUSION The results of this work can testify that the formation of the La-carbide phase takes place under La-adsorption onto the @OOl)graphite surface and low temperature annealing of this system (600’ C) . The C (KVV) Auger spectra of this La-C phase is characterized by the carbidelike triplet structure. The high temperature annealing (12001400’C) of the La-graphite system leads to the formation of La-intercalated graphite. This La-C phase is characterized by the feature on the high energy side of C(KVV) Auger peak. It is connected with the formation of high density of states near the Fermi-level. STM image of the the La-intercalated graphite shows reconstruction of the surface with the increase of
the surface crystalline structure period and the formation of 13-like symmetry.
REFERENCES
1. M.S.Dresselhaus, G.Dresselhaues, Advances in Phys., 3O.tl981) 139. 2. M.R. Norman, D.D. Koelling, A.J. Freeman, Phys.Rev.B, 31 (1985) 6251. 3, D.D. Koelling, B.D. Dunlap, G.W. Crabtee, Phys.Rev.B, 31 (1985) 4966. 4. D.E.Ramaker,Appl. Surf. Sci.,21 (1985) 243. 5. D.E. Ramaker, Critical Reviews in Solid St. and Mater.Sci., 17(3) (1991) 211. 6. J.M. Shulga, G.L. Gutsev, J. Electr. Spectr. (1984) 39. Relat.Phenom. 34 7. M.Laguqs, D.Marchand, CFretign, J. Vat. Sci. Technol., A 5 (1987) 1292. 8. J.S. Murday, B.I. Dunlap, F.L. Hutson II, P.Oelhafen, Phys.Rev.3, 24 (1981) 4764. 9. V.K. Adamchuk, A.M. Shikin, J. Electron Spectr. Relat. Phenom., 52 (1990) 103. lO.W.Eberhardt,L.T.McGovern, E,W.Plummer, J.E.Fisher, Phys.Rev. Lett., 44 (1980) 200. 1l.N. Gunasekara, T. Takahashi, F. Maeda, T. Sagawa, H. Suematsu. 2. Phys. B.Condensed Matter., 70 (1988) 349. 12.D. Tomanek, S.G. Lonie, Phys. Rev. B., 37 (1988) 8327. 13.S.P. Keltly, C.M. Lieber, Phys. Rev, B , 40 (1989) 5856. 14.D. Anselmetti, R. Wiesendanger, H.J.Guntherodt, Phys. Rev. B, 39 (1989) 11135.