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Structural, vibrational and electronic properties of C60 thin films investigated by high resolution electron energy loss spectroscopy
Journal of Ekctron Spectroscopy and Related Phenomena, 6465 (1993)835-342 03&3-2048/93/$06.00@ 1993 - Elsevier Science Publishers B.V. All rights reserved
835
Structural, vibrational and electronic properties of Cm thin films investigatedby high resolutionelectronenergyloss spectroscopy G. Gensterbluma, J.-J. Pireauxa, P.A. Thirya, R. Caudanoa, Ph. Lambinbs*, and A.G Lucasb aLaboratoire Interdisciplinaire de Spectroscopic Electronique, bLaboratoire de Physique des Solides, Institute for Studies in Interface Sciences Facultis Univetsitaires Notre-Dame de la P&x, 61, rue de Bruxelles, B-5000 Namur, Belgium
The vibrational structures of (260 films grown on different single crystal substrates reveal their degrees of order: the films are disordered on Si(lOO>,ordered on GaSe(0001) and crystalline on GeS(001). Because of an enhanced impact vs. dipole scattering contribution, the disordered films provide a rich vibrational information: Raman active modes are observed with intensities comparable to infrared active modes. The HREELS investigation of Cm films on Si(lO0) was extended to the visible and ultraviolet regions.
l.INTRODUCTION The recent “serendipitous” discovery of an almost spherical molecular cluster of carbon, (360, is a remarkable example of an unexpected fertilizing interaction between astrophysics and solid state physics, two disciplines that, at first sight, do not seem to share much in common. It is indeed the remok observation of interstellar carbon dust, that triggered solid state physics experiments aimed at producing carbon clusters that should present the unexplained strong ultraviolet absorption at 2175 k These experiments were successful in the sense that they led to the discovery of a new family of carbon molecules: the fullerenes. However, they did not provide a definitive answer to the interstellar dust composition whose ultraviolet signature was not reproduced.
* Research Associate of the Fund for Scientific Research
Belgian
National
Let us briefly recall the two important events that led to the discovery of C60. In 1985, traces of “soccerball” like molecules, named “buckminsterfullerenes” or briefly “fullerenes” were identified by Kroto and Smalley 111, in the carbon fragments produced by laser ablation of a graphite disk in a low background pressure of helium. Despite all their efforts, Smalley and Eroto did not succeed in producing more ‘than microscopic quantities of fullerenes. Significant amount of material could only be obtained five years later by Kratschmer and HufTinan [21, who sublimed graphite in an electric arc in a high residual pressure of helium (about lo4 Pa). Solid state physics and chemistry investigations could start at this point, and since then, an impressive wealth of information has been gathered on Cc0 and on the related fullerenes. In this paper, we report on a high resolution electron energy loss spectroscopy @IREELS) study of C60. This study was first undertaken on thin films deposited on an elemental semiconductor Si( 100). During the
836
preparation of the films, the ultimate goal was to achieve epitaxial growth. Besides some possible technological applications, epitaxial films provide an interesting alternative to single crystals for the study of solid state effects in fuller&e, the crystalline
face-centered-cubic (fee) room temperature phase of Gee. Indeed, the typical size of the state-of-the-art available single crystals is too small for most electron spectroscopies, especially if dispersion effects have to be investigated. We did not succeed in growing crystalline Gee layers on Si(lOO), because the interaction with the dangling bonds of Si, reduced drastically the mobility of the species adsorbed at the surface, thereby preventing the formation of an ordered layer which could induce the epitaxy. This is the reason why another set of experiments was conducted on lamellar semiconductor substrates: GaSe(OO01)and GeS(OO1). Within such layered materials, the atoms are covalently bonded inside lamellae, which are assembled together by van der Waals forces. C60 being itself a van der Waals solid, a weak interaction between the substrate and the impinging molecules should, at appropriate temperatures, enhance the adsorbate mobility during the nucleation and growth processes, favouring the formation of a wellordered first monolayer. Indeed, succesful epitaxial growth of Cm. could already be achieved on two layered materials: MO% 131 and mica [41. Among the two lamellar semiconductors used in the present study, GeS(001) substrates combine the advantages of an excellent unidimensional lattice match to fullerite and a characteristic surface corrugation favourable for van der Waals epitaxial growth of Ce(). Indeed, high quality crystalline films were obtained on GeS(001). The HREELS spectra recorded on crystalline C60 films are dominated by the dipole contribution of the infrared active
modes. By contrast, the spectra recorded on disordered films grown on SiUOO) provided a more COmplet.69vibrational fingerprint Of C&) as the disorder lifted the infrared selection rule, revealing both Raman and infrared active modes, with comparable intensities. The effect of the substrate selectivity illustrates nicely the interaction mechanisms in HREELS [51. Furthermore, the analysis
of the angular profiles of the elastic peak, which is analogous to a spot profile analysis in low energy electron diffraction (LEED) , provides some structural information on the coherent domain size of Cm. One important advantage of HBEELS in comparison to optical spectroscopies, is that the energy range of investigation can be extended at will in the same set of experiments. Medium resolution EELS spectra of Ce&3iClOO>were recorded in the visible and ultraviolet energy ranges. They also display many characteristic features related to individual electronic transitions or collective excitations that shed some light on the ek!ctrOnic strwtUre OfC&-J.
2. EXPERIMENTAL
DETAILS
The measurements reported here were performed in a two-chamber ultra high vacuum set up, equipped with conventional HBEELS and LEED systems. The purified carbon soot, containing up to 85 % ($0 and 15 % C70 was obtained from several other laboratories. The powder was loaded in the graphite or boron nitride crucible of a Knudsen cell and sublimed at 675 K at constant pressure below 2~10~~ mbar. The deposition rates ranged from 0.25 to 5 kmin. as measured by a quartz crystal microbalance. Because of a sizable difference between the vapour pressures of Gee and C7o, the low temperature of the cell enhances the sublimation of Ce() versus C7e so that the evaporated layers COnSiStOf almost pure C&J material.
337
Warn nu* 0
.
500
,
I
(1kn-j loo0 a
1500
zoo0
I
In Figure 1, we present a typical I-IKEELS spectrum recorded on a 60 A thick film of C,o Four peaks are deposited on Si(100). outstanding at 66,94, 156 and 194 meV, and six weaker bands are also visible. The interpretation of such a vibrational spectrum proceeds by considering the normal mode analysis of the free C6() molecule, which belongs to the Ih icosahedral point group symmetry. The resulting 174 eigenmodes produce 46 distinct modes which according to the usual notation, are classified as follows 171: + 4T1u + 4T2g + 5T2, + r=2Ag+Au+3Tlg 6Gg + 6Gu + 8Hg + 7Hu (1)
Figure 1. HEEELS spectrum of a 60 A thick film of Cm deposited on Si(lOO), recorded in the specular geometry at 45’ with 3.7 eV electrons. The ordinate shows the intensity in arbitrary units 161.
The substrates were either SiWO) wafers annealed in situ at 1000 K, or GaSe and GeS single crystals freshly cleaved in vacua before the sublimation of Cgo. The substrate temperature was varied between 300 K and 500 K during the deposition.
3. VIBRATIONAL
HBEELS
OF Cso
Among these, there are only four dipole active eigenmodes corresponding to the four threefold degenerate Tlu modes at 65, 71, 147 and 177 meV. The first two modes to the prominent contribute together HREELS peak at 66 meV, the instrumental resolution of 10.5 meV being not sufficient to resolve them. The other two infrared active modes are not clearly observed, except for the 177 meV one, which is present in the shoulder of the 194 meV peak. The remaining HEEELS features and especially the peaks at 94, 156 and 194 meV can be related to some of the 10 Eaman active modes of the molecule. Let us point out only the presence of the completely symmetrical radial breathing mode at 61 meV in the asymmetric left ban4 shoulder of the 66 meV peak. The detailed attribution of the energy loss peaks and their interpretation can be found elsewhere [61.
3.1. C~Si Thick films of C6o evaporated on SiUOO) at room temperature did not present any LEED pattern, implying that the films are fairly disordered. A coherent domain size of about 25 A, corresponding to three times the diameter of one C!~Ocage, could be roughly estimated from the angular profile of the HEEELS elastic peak.
3.2. C~Ga!3eWOOl> Upon deposition of C60 on GaSe(OO01) at room temperature, no LEED pattern was observed. On the other hand, sharp diffraction spots were visible after sublimation on a freshly cleaved substrate held at about 420 K. The LEED patterns could be interpreted as the superposition of
838
Wave number 0
500
R
1000
0
50
1500
2000
C,dGaSe
66
1
(l/cm)
I
100
150
200
250
Energy (mev)
Figure 2. HREELS spectrum of a 350 A thick ordered film of C& on GaSe@OOl) recorded in specular geometry at 45’, with 3.7 eV electrons, The ordinate shows the intensity in arbitrary units C8,91.
two hexagonal diffraction patterns rotated by 23’ with respect to one another, indicating the formation of C&11) domains with two different orientations. Figure 2 shows the HREELS spectrum recorded on a 350 A thick film with an electron impact energy of 3.7 eV [8,91. Note that the energy resolution is 7.5 meV, significantly better than for C6()deposited on Si(lO0) although both spectra were taken with the same spectrometer settings. The angular width of the elastic eak (4.7’) yields a rough estimation of 100 1 for the coherent domain size, thus sticient to These produce a diffraction pattern. arguments converge to confirm that the C60 films deposited on GaSe(0001) are polycrystalline. Before comparing the spectrum of Figure 2 to the one of Figure 1, we notice the pre-
sence of an intense energy loss at 29.5 meV. It is the fingerprint of an interface optical mode localized at the interface between C60 and Case. In the framework of the dielectric theory of energy loss, it can be easily shown that HBEELS is primarily an interface spectral since technique sensitive contributions are expected only from the matching of the electric fields and of the at the boundary electric displacement between two media characterized by different Such long dielectric functions [lOI. wavelength interface phonons were already observed in several epitaxial systems like CaFdSi(ll1) [ill and a GsAs-AlGaAs superlattice [121. It is interesting to note that interface phonons are here remotely observed under a 350 A thick layer of Cm even though the electrons do not penetrate the target. This is a consequence of the long range nature of the Coulomb interaction between the electron and the surface which governs the dipole vibration, scattering mechanism considered by the dielectric theory. No interface phonon was observed in the case of Si, because Si is infrared inactive. We now turn to a direct comparison of the CGn-related spectral features of Figures 1 and 2. More than on Si (Figure l), the strong peak at 66 meV dominates the spectrum of Figure 2 and now clearly shows an asymmetric tail at about 72 meV, confirming the interpretation of these two peaks as infrared active Tl, modes. On the other hand, compared to the 66, meV infrared active peak, the Raman active peak at 94 meV is now fairly reduced in intensity on GaSe, as are the Raman active peaks at 156 and 194 meV previously observed on Si. The other two infrared active T1, modes predicted by the theory are now visible on Figure 2, at 147 and 178 meV, respectively. We conclude that the HEEELS spectrum of C~GaSe(OOO1) closely reproduces an infrared absorption spectrum characteristic of dipole scattering.
839
Wavenumber (l/cm) 0
so0
loo0
1500
I
I
I
I
2ooo
I 66
C,/GeS(OOl)
I i .. ..
-50
0
50
loo
150
200
250
Energy (mev)
Figure 3. HREELS spectrum of a 600 A thick C60 film epitaxial on GeS(OOl), in the specular geometry (45’) with an electron energy of 3.5 eV. The ordinate shows the intensity in arbitrary units [91.
$3. C&GeS(ool) As demonstrated in the previous section, the ordered growth of C66 is favoured by van der Waals epitaxy on lamellar crystals. However, considering that the C&,-substrate interaction is certainly larger than the interaction between Cm molecules, it is clear that the substrate, acting as a template, imposes its periodicity to the first C6o layer. Under this respect, it seemed appropriate to find a lamellar crystal lattice presenting better commensurability to C60 fullerite than GaSe for which the lattice mismatch was 12%. The lattice mismatch reduces to 0.75% along the a-axis crystal direction of the GeS(OO1)plane. Furthermore, the GeS(OO1) surface exhibits the structure of a corrugated roof due to the alignment of well-separated Ge-S zig-zag chains, which provide a preferential adsorption direction in the
interchain grooves parallel to the b-axis. The completion of a COO monolayer could be accomplished, at the expenses of a very small lattice distortion, if the C60 rows skipped one interchain groove, systematically. This expectation was indeed realized and confirmed by the interpretation of sharp LEED patterns Cl31 clearly showing the orientation of the C,o layers deposited on GeS(OO1). High quality diffraction patterns were obtained for films thicknesses up to 1000 A and it is probable that large single crystals can be grown by this way. Figure 3 shows a HREELS spectrum recorded on a 690 A thick film with an electron energy of 3.5 eV and a resolution of 7 meV 191. The best resolution obtained on this system was 4.5 meV. However, in this case, due to the low oscillator strength of the infrared active modes of C6o, the intensity of the energy losses was so small, that it required prohibitive accumulation times, in order to get a reasonable signal. An angular scan of the elastic peak revealed an extremely sharp profile of 1.2’ FWHM, corresponding to the angular acceptance of the analyser. The long range crystalline order in the Cc;0 layers, already revealed by LEED is thus again confirmed by HREELS. Under these ideal conditions, the infrared dipole selection rule governs the HREELS spectrum which clearly emphasizes the four infrared active Tlu modes of C& (66, 72, 146 and 178 meVI allowing a quantitative interpretation of the data in the framework of the dielectric theory. For the resonant frequencies we observed a good agreement with the infrared spectroscopy (IRS) results. As for the oscillator strengths, the HREELS values were significantly higher than the IRS ones (by a factor of two>, probably because there is still an important impact contribution to the HREELS energy losses. Finally, it is worth mentioning that, at the left side of the 66 meV peak, we observe the interface optical phonon of CGo-GeS through a C,o layer thickness of 650 A.
040
We close this section on the vibrational properties of C60 fullerito by some concluding remarks. The HREELS spectra of c60 do not show the characteristic fingerprint expected from optical surface phonons of dielectric crystals like MgO 1141 or CaFg [ll], which in the specular geometry, are observed on an intensity scale comparable to the elastic peak intensity. The HREELS peaks should then be considered as arising from the excitation of molecular vibrations of the C60 clusters at the surface. Under this respect, they look more similar to the spectra recorded on polymer surfaces [151 which also present a significant impact contribution. Moreover, like on polymer surfaces, resonance effects participate in the impact scattering, when the incident electron energy matches some unoccupied orbitals of the molecule. Such effects have also been observed on C(3) and are presently under study DSI.
4. EELS OF C~&WOO) IN THE VISIBLE AND ULTRAVIOLET ENERGY RANGE In the same set of experiment, the range of energy losses was increased in order to access to the near infrared, visible and ultraviolet excitation spectrum of c60. It is the privilege of EELS, in comparison to optical spectroscopy to be capable of spanning such a broad range of energies with the same experiment set up. Individual electronic excitations in a solid request usually substantial momentum transfer to occur, so that the dipole scattering configuration is not very favourable, for their investigation. Therefore, because of the impossibility to increase our detector aperture in order to collect electrons deflected over a broader angle, we choose to study the most disordered films i.e. C6@i(loO), With the eXpeCk&iOn that the incoherent or diffuse elastic scattering would artificially have the same effect as increasing the collection angle.
Cso /Si( 100) 3,7 m
X5o
UJ
E&oei
II
I
I
I
I
I
0
2
4
6
8
Energy (eV)
Figure 4. EELS spectrum of 60 A C&WOO> recorded in the visible-ultraviolet energy range, in the specular geometry (45’) for two primary electron energies: 10 and 30 eV [61.
Figure 4 shows a spectrum taken on the same sample as in Figure 1, in the specular direction with electrons of 10 eV 161. For intensity purposes, the instrumental resolution was degraded down to 60 meV. The infrared vibrational fingerprint, squeezed by the expanded energy scale and by the lower energy resolution is hardly visible in the tail of the elastic peak. The first observation is a flat spectral region extending to the peak at 2.2 eV, which corresponds to the energy gap of ($0 fullerite. This value is consistent with a comparison between direct and inverse photoemission results which yield a value of 3.6 eV, when allowing for a local attractive electron-hole interaction of 1.6 eV. Excitonic effects are confirmed by the presence of a peak at 1.55 eV and of some structures in the onset of the 2.2 eV peak. The two outstanding peaks at 3.7 eV and 4.6 eV are observed in absorption spectroscopy (AS) with comparable lineshapes and intensities [161.
841
The steadiness
of the intensity
of the 2.2, electron energy (Figure 4 shows spectra taken at 10 and 30 eV), is at variance to the behaviour of the complex structure developping around 6 eV. This structure consists of at least two peaks : one around 5.5-5.8 eV, corresponding to one-electron transitions and a second at 6.3 eV whose intensity increases constantly with the excitation energy. This latter peak together with the 4.8 eV peak reproduces the typical “camel back’ structure described by “the” and Huffman as IWtschmer characteristic feature of the AS spectrum of C6@ However, instead of one doublet, the interstellar carbon dust shows only one broad absorption band in this spectral region. Recent calculations performed on complex fullerene structures 1181 predict that the assembly of concentric onion-like shells results in a filling of the gap between the two uv absorption bands, so that C66 might, after all, enter into the composition of the mysterious interstellar dust. The 6.3 eV peak is assigned to the of the x-electron excitation collective subsystem of the molecule, the so-called ICIndeed, it can be shown that plasmon. warping a graphite layer to model the C,o molecule as one spherical dielectric shell produces a dipole excitation around 6 eV which is the energy quantum for tangential I=1 density fluctuations of the x electrons of the molecule. The intensity variation of this peak is a consequence of the fact that the lobe associated to the dipole scattering cross section has a width equal to the plasmon energy divided by twice the electron energy. Consequently, the lobe shrinks towards the narrow analyser aperture, when the primary energy is increased. This effect enhances the By contrast, electron measured intensity. transitions observed at lower energies remain nearly independent of the primary energy, since they are excited by the quasi isotropic impact scattering mechanism. 3.7 and 4.8 eV peaks with primary
Figure 5. EELS spectrum of 60 A C&Si(lOO> in the uv and vuv energy ranges, for two electron energies: (a) 70 eV and (b) 150 eV 161. The intensity is displayed in arbitrary units.
5. EELS OF C&3iWO) WV RAJ’lGE
IN TECE W
TO
Extending the energy loss investigation to the vuv range, produces the EELS spectra shown on Figure 5, that have been taken with 70 eV and 150 eV primary energies, The “camel back’ feature respectively. by the although somewhat squeezed expanded energy scale, still fingerprints the spectra. The next peak at 7.6 eV corresponds to the lowest ionization energy of the Cm molecule measured by uv photolectron spectroscopy (UPS) 1191 Then, we find a series of peaks which can be correlated to WS lines. Note that HREELS bands in the vuv region reflect one-electron transitions of an N-electron system and need not to coincide with the UPS lines involving N-l electrons_ Finally we observe the emergence of a broad feature around 28 eV, when the primary electron energy increases. This structure is interpreted as arising from the
842
excitation of collective oscillations of the complete system of the 240 electrons of the valence shell of the molecule (o-plasmon).
3.
6. CONCLUSIONS
5.
This study demonstrates the capability and versatility of electron energy loss spectroscopy for the study of elementary excitations at a solid surface. Indeed, with the very same technique it was possible to cover a broad range of energies otherwise accessible only by the combined use of many different spectroscopies. As far as C66 is concerned, an important achievement is the realization of high quality crystalline films by van der Waals epitaxy on lamellar substrates. As a general result of this study it appears that even in the crystalline state, the vibrational and electronic structures of C6o retain most of their molecular character and that solid state effects should be weak. A precise quantification of these solid state effects is now feasible from band dispersion measurements in epitaxial layers.
6.
4.
7. 8.
9.
10. 11. 12.
7. ACKNOWLEDGMENTS We thank W. Kriitschmer, G. Sawatzky and J. Fischer for providing us with C6o material. This work was supported by the Belgian national program of Interuniversity Research Projects initiated by the State Prime Minister Cff%e (Science Policy Program-ming), by the Wallonia region and by the Belgian National Science Foundation.
13.
14. 15. 16. 17.
1. H.W. Kroto, J. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature 318 (1985)162. 2. W. Krlitschmer, L.D. Lamb, K. Fostiropoulos, D.R. Htiman, Nature 347 (1990) 354.
18. 19.
M. Sakurai, H. Tada, K. Saiki, A. Koma, Jpn. J. Appl. Phys. 30 (1991) L565R. D. &nicker, S. Schmidt, J.G. Skofronick, J.P. Toennies, R. Vollmer, Phys. Rev. B 44 (1991) 10995. P.A. Thiry, M. Liehr, J.-J, Pireaux, R. Caudano, Phys. Scripta 35 (1987) 368. G. Gensterblum, J.-J. Pireaux, P.A Thiry, R. Caudano, J.-P, Vigneron, Ph. Lambin, AA Lucas, W. Kriitschmer, Phys. Rev. L&t. 67 (1991) 2171; Phys. Rev. B 45 (1992) 13694. D.E. Weeks, W.G. Harter, J. Chem. Phys. 90 (1989) 4744. G. Gensterblum, L.-M. Yu, J.-J. Pireaux, P.A Thiry, R. Caudano, Ph. Lambin, AA. Lucas, W. KrFitschmer, J.E. Fischer, J. Phys. Chem. Solids 63 (1992) 1427. G. Gensterblum, L.-M. Yu, J.-J. Pireaux, P.A Thiry, R. Caudano, J.-M Themlin, S. Bouzidi, F. Coletti, J.-M. Debever, Appl. Phys. A 56 (1993) 175. Ph. Lambin, J.-P. Vigneron, and AA Lucas, Phys. Rev. B 32 (1985) 8203. M. Liehr, P.A Thiry, J.-J. Pireaux, R, Caudano, Phys. Rev. B 34 (1986) 7471. Ph. Lambin, J.-P. Vigneron, AA Lucas, P.A Thiry, M. Liehr, J.-J. Pireaux, R. Caudano, T. Kuech, Phys. Rev. Lett. 56 (1986) 1842. J.M. Tbemlin, S. Bouzidi, F. Coletti, J.M. Debever, G. Gensterblum, L.-M. Yu, J.-J. Pireaux, P.A. Thiry, Phys. Rev. B 46 (1992) 15602. P.A Thiry, M. Liehr, J.-J. Pireaux, R. Caudano, Phys. Rev. B 29 (1984) 4824. J.-J. Pireaux, P.A. Thiry, R. Caudano, P. Pfluger, J. Chem. Phys. 84 (1986) 6452. G. Gensterblum, private communication. J.P. Hare, H.W. Kroto, and R. Taylor, Chem. Phys. Lett. 177 (1991) 394. L. Henrard, A.A. Lucas, Ph. Lambin, Ap. J. 406 (1993) 92. D.L. Lichtenberger, K.W. Nebesny, Ch,D. Ray, D.R. Huffman, L.D. Lamb, Chem. Phys. Lett. 176 (1991) 203.
835
Structural, vibrational and electronic properties of Cm thin films investigatedby high resolutionelectronenergyloss spectroscopy G. Gensterbluma, J.-J. Pireauxa, P.A. Thirya, R. Caudanoa, Ph. Lambinbs*, and A.G Lucasb aLaboratoire Interdisciplinaire de Spectroscopic Electronique, bLaboratoire de Physique des Solides, Institute for Studies in Interface Sciences Facultis Univetsitaires Notre-Dame de la P&x, 61, rue de Bruxelles, B-5000 Namur, Belgium
The vibrational structures of (260 films grown on different single crystal substrates reveal their degrees of order: the films are disordered on Si(lOO>,ordered on GaSe(0001) and crystalline on GeS(001). Because of an enhanced impact vs. dipole scattering contribution, the disordered films provide a rich vibrational information: Raman active modes are observed with intensities comparable to infrared active modes. The HREELS investigation of Cm films on Si(lO0) was extended to the visible and ultraviolet regions.
l.INTRODUCTION The recent “serendipitous” discovery of an almost spherical molecular cluster of carbon, (360, is a remarkable example of an unexpected fertilizing interaction between astrophysics and solid state physics, two disciplines that, at first sight, do not seem to share much in common. It is indeed the remok observation of interstellar carbon dust, that triggered solid state physics experiments aimed at producing carbon clusters that should present the unexplained strong ultraviolet absorption at 2175 k These experiments were successful in the sense that they led to the discovery of a new family of carbon molecules: the fullerenes. However, they did not provide a definitive answer to the interstellar dust composition whose ultraviolet signature was not reproduced.
* Research Associate of the Fund for Scientific Research
Belgian
National
Let us briefly recall the two important events that led to the discovery of C60. In 1985, traces of “soccerball” like molecules, named “buckminsterfullerenes” or briefly “fullerenes” were identified by Kroto and Smalley 111, in the carbon fragments produced by laser ablation of a graphite disk in a low background pressure of helium. Despite all their efforts, Smalley and Eroto did not succeed in producing more ‘than microscopic quantities of fullerenes. Significant amount of material could only be obtained five years later by Kratschmer and HufTinan [21, who sublimed graphite in an electric arc in a high residual pressure of helium (about lo4 Pa). Solid state physics and chemistry investigations could start at this point, and since then, an impressive wealth of information has been gathered on Cc0 and on the related fullerenes. In this paper, we report on a high resolution electron energy loss spectroscopy @IREELS) study of C60. This study was first undertaken on thin films deposited on an elemental semiconductor Si( 100). During the
836
preparation of the films, the ultimate goal was to achieve epitaxial growth. Besides some possible technological applications, epitaxial films provide an interesting alternative to single crystals for the study of solid state effects in fuller&e, the crystalline
face-centered-cubic (fee) room temperature phase of Gee. Indeed, the typical size of the state-of-the-art available single crystals is too small for most electron spectroscopies, especially if dispersion effects have to be investigated. We did not succeed in growing crystalline Gee layers on Si(lOO), because the interaction with the dangling bonds of Si, reduced drastically the mobility of the species adsorbed at the surface, thereby preventing the formation of an ordered layer which could induce the epitaxy. This is the reason why another set of experiments was conducted on lamellar semiconductor substrates: GaSe(OO01)and GeS(OO1). Within such layered materials, the atoms are covalently bonded inside lamellae, which are assembled together by van der Waals forces. C60 being itself a van der Waals solid, a weak interaction between the substrate and the impinging molecules should, at appropriate temperatures, enhance the adsorbate mobility during the nucleation and growth processes, favouring the formation of a wellordered first monolayer. Indeed, succesful epitaxial growth of Cm. could already be achieved on two layered materials: MO% 131 and mica [41. Among the two lamellar semiconductors used in the present study, GeS(001) substrates combine the advantages of an excellent unidimensional lattice match to fullerite and a characteristic surface corrugation favourable for van der Waals epitaxial growth of Ce(). Indeed, high quality crystalline films were obtained on GeS(001). The HREELS spectra recorded on crystalline C60 films are dominated by the dipole contribution of the infrared active
modes. By contrast, the spectra recorded on disordered films grown on SiUOO) provided a more COmplet.69vibrational fingerprint Of C&) as the disorder lifted the infrared selection rule, revealing both Raman and infrared active modes, with comparable intensities. The effect of the substrate selectivity illustrates nicely the interaction mechanisms in HREELS [51. Furthermore, the analysis
of the angular profiles of the elastic peak, which is analogous to a spot profile analysis in low energy electron diffraction (LEED) , provides some structural information on the coherent domain size of Cm. One important advantage of HBEELS in comparison to optical spectroscopies, is that the energy range of investigation can be extended at will in the same set of experiments. Medium resolution EELS spectra of Ce&3iClOO>were recorded in the visible and ultraviolet energy ranges. They also display many characteristic features related to individual electronic transitions or collective excitations that shed some light on the ek!ctrOnic strwtUre OfC&-J.
2. EXPERIMENTAL
DETAILS
The measurements reported here were performed in a two-chamber ultra high vacuum set up, equipped with conventional HBEELS and LEED systems. The purified carbon soot, containing up to 85 % ($0 and 15 % C70 was obtained from several other laboratories. The powder was loaded in the graphite or boron nitride crucible of a Knudsen cell and sublimed at 675 K at constant pressure below 2~10~~ mbar. The deposition rates ranged from 0.25 to 5 kmin. as measured by a quartz crystal microbalance. Because of a sizable difference between the vapour pressures of Gee and C7o, the low temperature of the cell enhances the sublimation of Ce() versus C7e so that the evaporated layers COnSiStOf almost pure C&J material.
337
Warn nu* 0
.
500
,
I
(1kn-j loo0 a
1500
zoo0
I
In Figure 1, we present a typical I-IKEELS spectrum recorded on a 60 A thick film of C,o Four peaks are deposited on Si(100). outstanding at 66,94, 156 and 194 meV, and six weaker bands are also visible. The interpretation of such a vibrational spectrum proceeds by considering the normal mode analysis of the free C6() molecule, which belongs to the Ih icosahedral point group symmetry. The resulting 174 eigenmodes produce 46 distinct modes which according to the usual notation, are classified as follows 171: + 4T1u + 4T2g + 5T2, + r=2Ag+Au+3Tlg 6Gg + 6Gu + 8Hg + 7Hu (1)
Figure 1. HEEELS spectrum of a 60 A thick film of Cm deposited on Si(lOO), recorded in the specular geometry at 45’ with 3.7 eV electrons. The ordinate shows the intensity in arbitrary units 161.
The substrates were either SiWO) wafers annealed in situ at 1000 K, or GaSe and GeS single crystals freshly cleaved in vacua before the sublimation of Cgo. The substrate temperature was varied between 300 K and 500 K during the deposition.
3. VIBRATIONAL
HBEELS
OF Cso
Among these, there are only four dipole active eigenmodes corresponding to the four threefold degenerate Tlu modes at 65, 71, 147 and 177 meV. The first two modes to the prominent contribute together HREELS peak at 66 meV, the instrumental resolution of 10.5 meV being not sufficient to resolve them. The other two infrared active modes are not clearly observed, except for the 177 meV one, which is present in the shoulder of the 194 meV peak. The remaining HEEELS features and especially the peaks at 94, 156 and 194 meV can be related to some of the 10 Eaman active modes of the molecule. Let us point out only the presence of the completely symmetrical radial breathing mode at 61 meV in the asymmetric left ban4 shoulder of the 66 meV peak. The detailed attribution of the energy loss peaks and their interpretation can be found elsewhere [61.
3.1. C~Si
3.2. C~Ga!3eWOOl> Upon deposition of C60 on GaSe(OO01) at room temperature, no LEED pattern was observed. On the other hand, sharp diffraction spots were visible after sublimation on a freshly cleaved substrate held at about 420 K. The LEED patterns could be interpreted as the superposition of
838
Wave number 0
500
R
1000
0
50
1500
2000
C,dGaSe
66
1
(l/cm)
I
100
150
200
250
Energy (mev)
Figure 2. HREELS spectrum of a 350 A thick ordered film of C& on GaSe@OOl) recorded in specular geometry at 45’, with 3.7 eV electrons, The ordinate shows the intensity in arbitrary units C8,91.
two hexagonal diffraction patterns rotated by 23’ with respect to one another, indicating the formation of C&11) domains with two different orientations. Figure 2 shows the HREELS spectrum recorded on a 350 A thick film with an electron impact energy of 3.7 eV [8,91. Note that the energy resolution is 7.5 meV, significantly better than for C6()deposited on Si(lO0) although both spectra were taken with the same spectrometer settings. The angular width of the elastic eak (4.7’) yields a rough estimation of 100 1 for the coherent domain size, thus sticient to These produce a diffraction pattern. arguments converge to confirm that the C60 films deposited on GaSe(0001) are polycrystalline. Before comparing the spectrum of Figure 2 to the one of Figure 1, we notice the pre-
sence of an intense energy loss at 29.5 meV. It is the fingerprint of an interface optical mode localized at the interface between C60 and Case. In the framework of the dielectric theory of energy loss, it can be easily shown that HBEELS is primarily an interface spectral since technique sensitive contributions are expected only from the matching of the electric fields and of the at the boundary electric displacement between two media characterized by different Such long dielectric functions [lOI. wavelength interface phonons were already observed in several epitaxial systems like CaFdSi(ll1) [ill and a GsAs-AlGaAs superlattice [121. It is interesting to note that interface phonons are here remotely observed under a 350 A thick layer of Cm even though the electrons do not penetrate the target. This is a consequence of the long range nature of the Coulomb interaction between the electron and the surface which governs the dipole vibration, scattering mechanism considered by the dielectric theory. No interface phonon was observed in the case of Si, because Si is infrared inactive. We now turn to a direct comparison of the CGn-related spectral features of Figures 1 and 2. More than on Si (Figure l), the strong peak at 66 meV dominates the spectrum of Figure 2 and now clearly shows an asymmetric tail at about 72 meV, confirming the interpretation of these two peaks as infrared active Tl, modes. On the other hand, compared to the 66, meV infrared active peak, the Raman active peak at 94 meV is now fairly reduced in intensity on GaSe, as are the Raman active peaks at 156 and 194 meV previously observed on Si. The other two infrared active T1, modes predicted by the theory are now visible on Figure 2, at 147 and 178 meV, respectively. We conclude that the HEEELS spectrum of C~GaSe(OOO1) closely reproduces an infrared absorption spectrum characteristic of dipole scattering.
839
Wavenumber (l/cm) 0
so0
loo0
1500
I
I
I
I
2ooo
I 66
C,/GeS(OOl)
I i .. ..
-50
0
50
loo
150
200
250
Energy (mev)
Figure 3. HREELS spectrum of a 600 A thick C60 film epitaxial on GeS(OOl), in the specular geometry (45’) with an electron energy of 3.5 eV. The ordinate shows the intensity in arbitrary units [91.
$3. C&GeS(ool) As demonstrated in the previous section, the ordered growth of C66 is favoured by van der Waals epitaxy on lamellar crystals. However, considering that the C&,-substrate interaction is certainly larger than the interaction between Cm molecules, it is clear that the substrate, acting as a template, imposes its periodicity to the first C6o layer. Under this respect, it seemed appropriate to find a lamellar crystal lattice presenting better commensurability to C60 fullerite than GaSe for which the lattice mismatch was 12%. The lattice mismatch reduces to 0.75% along the a-axis crystal direction of the GeS(OO1)plane. Furthermore, the GeS(OO1) surface exhibits the structure of a corrugated roof due to the alignment of well-separated Ge-S zig-zag chains, which provide a preferential adsorption direction in the
interchain grooves parallel to the b-axis. The completion of a COO monolayer could be accomplished, at the expenses of a very small lattice distortion, if the C60 rows skipped one interchain groove, systematically. This expectation was indeed realized and confirmed by the interpretation of sharp LEED patterns Cl31 clearly showing the orientation of the C,o layers deposited on GeS(OO1). High quality diffraction patterns were obtained for films thicknesses up to 1000 A and it is probable that large single crystals can be grown by this way. Figure 3 shows a HREELS spectrum recorded on a 690 A thick film with an electron energy of 3.5 eV and a resolution of 7 meV 191. The best resolution obtained on this system was 4.5 meV. However, in this case, due to the low oscillator strength of the infrared active modes of C6o, the intensity of the energy losses was so small, that it required prohibitive accumulation times, in order to get a reasonable signal. An angular scan of the elastic peak revealed an extremely sharp profile of 1.2’ FWHM, corresponding to the angular acceptance of the analyser. The long range crystalline order in the Cc;0 layers, already revealed by LEED is thus again confirmed by HREELS. Under these ideal conditions, the infrared dipole selection rule governs the HREELS spectrum which clearly emphasizes the four infrared active Tlu modes of C& (66, 72, 146 and 178 meVI allowing a quantitative interpretation of the data in the framework of the dielectric theory. For the resonant frequencies we observed a good agreement with the infrared spectroscopy (IRS) results. As for the oscillator strengths, the HREELS values were significantly higher than the IRS ones (by a factor of two>, probably because there is still an important impact contribution to the HREELS energy losses. Finally, it is worth mentioning that, at the left side of the 66 meV peak, we observe the interface optical phonon of CGo-GeS through a C,o layer thickness of 650 A.
040
We close this section on the vibrational properties of C60 fullerito by some concluding remarks. The HREELS spectra of c60 do not show the characteristic fingerprint expected from optical surface phonons of dielectric crystals like MgO 1141 or CaFg [ll], which in the specular geometry, are observed on an intensity scale comparable to the elastic peak intensity. The HREELS peaks should then be considered as arising from the excitation of molecular vibrations of the C60 clusters at the surface. Under this respect, they look more similar to the spectra recorded on polymer surfaces [151 which also present a significant impact contribution. Moreover, like on polymer surfaces, resonance effects participate in the impact scattering, when the incident electron energy matches some unoccupied orbitals of the molecule. Such effects have also been observed on C(3) and are presently under study DSI.
4. EELS OF C~&WOO) IN THE VISIBLE AND ULTRAVIOLET ENERGY RANGE In the same set of experiment, the range of energy losses was increased in order to access to the near infrared, visible and ultraviolet excitation spectrum of c60. It is the privilege of EELS, in comparison to optical spectroscopy to be capable of spanning such a broad range of energies with the same experiment set up. Individual electronic excitations in a solid request usually substantial momentum transfer to occur, so that the dipole scattering configuration is not very favourable, for their investigation. Therefore, because of the impossibility to increase our detector aperture in order to collect electrons deflected over a broader angle, we choose to study the most disordered films i.e. C6@i(loO), With the eXpeCk&iOn that the incoherent or diffuse elastic scattering would artificially have the same effect as increasing the collection angle.
Cso /Si( 100) 3,7 m
X5o
UJ
E&oei
II
I
I
I
I
I
0
2
4
6
8
Energy (eV)
Figure 4. EELS spectrum of 60 A C&WOO> recorded in the visible-ultraviolet energy range, in the specular geometry (45’) for two primary electron energies: 10 and 30 eV [61.
Figure 4 shows a spectrum taken on the same sample as in Figure 1, in the specular direction with electrons of 10 eV 161. For intensity purposes, the instrumental resolution was degraded down to 60 meV. The infrared vibrational fingerprint, squeezed by the expanded energy scale and by the lower energy resolution is hardly visible in the tail of the elastic peak. The first observation is a flat spectral region extending to the peak at 2.2 eV, which corresponds to the energy gap of ($0 fullerite. This value is consistent with a comparison between direct and inverse photoemission results which yield a value of 3.6 eV, when allowing for a local attractive electron-hole interaction of 1.6 eV. Excitonic effects are confirmed by the presence of a peak at 1.55 eV and of some structures in the onset of the 2.2 eV peak. The two outstanding peaks at 3.7 eV and 4.6 eV are observed in absorption spectroscopy (AS) with comparable lineshapes and intensities [161.
841
The steadiness
of the intensity
of the 2.2, electron energy (Figure 4 shows spectra taken at 10 and 30 eV), is at variance to the behaviour of the complex structure developping around 6 eV. This structure consists of at least two peaks : one around 5.5-5.8 eV, corresponding to one-electron transitions and a second at 6.3 eV whose intensity increases constantly with the excitation energy. This latter peak together with the 4.8 eV peak reproduces the typical “camel back’ structure described by “the” and Huffman as IWtschmer characteristic feature of the AS spectrum of C6@ However, instead of one doublet, the interstellar carbon dust shows only one broad absorption band in this spectral region. Recent calculations performed on complex fullerene structures 1181 predict that the assembly of concentric onion-like shells results in a filling of the gap between the two uv absorption bands, so that C66 might, after all, enter into the composition of the mysterious interstellar dust. The 6.3 eV peak is assigned to the of the x-electron excitation collective subsystem of the molecule, the so-called ICIndeed, it can be shown that plasmon. warping a graphite layer to model the C,o molecule as one spherical dielectric shell produces a dipole excitation around 6 eV which is the energy quantum for tangential I=1 density fluctuations of the x electrons of the molecule. The intensity variation of this peak is a consequence of the fact that the lobe associated to the dipole scattering cross section has a width equal to the plasmon energy divided by twice the electron energy. Consequently, the lobe shrinks towards the narrow analyser aperture, when the primary energy is increased. This effect enhances the By contrast, electron measured intensity. transitions observed at lower energies remain nearly independent of the primary energy, since they are excited by the quasi isotropic impact scattering mechanism. 3.7 and 4.8 eV peaks with primary
Figure 5. EELS spectrum of 60 A C&Si(lOO> in the uv and vuv energy ranges, for two electron energies: (a) 70 eV and (b) 150 eV 161. The intensity is displayed in arbitrary units.
5. EELS OF C&3iWO) WV RAJ’lGE
IN TECE W
TO
Extending the energy loss investigation to the vuv range, produces the EELS spectra shown on Figure 5, that have been taken with 70 eV and 150 eV primary energies, The “camel back’ feature respectively. by the although somewhat squeezed expanded energy scale, still fingerprints the spectra. The next peak at 7.6 eV corresponds to the lowest ionization energy of the Cm molecule measured by uv photolectron spectroscopy (UPS) 1191 Then, we find a series of peaks which can be correlated to WS lines. Note that HREELS bands in the vuv region reflect one-electron transitions of an N-electron system and need not to coincide with the UPS lines involving N-l electrons_ Finally we observe the emergence of a broad feature around 28 eV, when the primary electron energy increases. This structure is interpreted as arising from the
842
excitation of collective oscillations of the complete system of the 240 electrons of the valence shell of the molecule (o-plasmon).
3.
6. CONCLUSIONS
5.
This study demonstrates the capability and versatility of electron energy loss spectroscopy for the study of elementary excitations at a solid surface. Indeed, with the very same technique it was possible to cover a broad range of energies otherwise accessible only by the combined use of many different spectroscopies. As far as C66 is concerned, an important achievement is the realization of high quality crystalline films by van der Waals epitaxy on lamellar substrates. As a general result of this study it appears that even in the crystalline state, the vibrational and electronic structures of C6o retain most of their molecular character and that solid state effects should be weak. A precise quantification of these solid state effects is now feasible from band dispersion measurements in epitaxial layers.
6.
4.
7. 8.
9.
10. 11. 12.
7. ACKNOWLEDGMENTS We thank W. Kriitschmer, G. Sawatzky and J. Fischer for providing us with C6o material. This work was supported by the Belgian national program of Interuniversity Research Projects initiated by the State Prime Minister Cff%e (Science Policy Program-ming), by the Wallonia region and by the Belgian National Science Foundation.
13.
14. 15. 16. 17.
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