Cage relaxation effects on the local density of states in a C60 derivative

19 July 1996

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical Physics Letters 257 (1996) 163-168

Cage relaxation effects on the local density of states in a C6o derivative L.-M. Yu a, V. Langlais h, j. Ghijsen a, G. Gensterblum a, Liangbing Gan c, R.L. Johnson d , R. Caudano a , I. Forbeaux b, J.-M. Themlin b, J.M. Debever h, Chinhui Huang c a L1SE, Facultds Universitaires Notre-Dame de la Paix, 61 rue de Bruxelles, B-5000 Bruxelles, Belgium b GPEC, Facultd des Sciences de Luminy-Case 901, F-13288 Marseille Cedex 09, France c Department o f Chemistry, Peking University, city, People's Republic of China Universiti~t Hamburg, H. lnstitut fftr Experimentalphysik, city, Germany Received 29 February 1996

Abstract

A (6-6) bonded fulleroid C6o(NH(CHCOOCH3) 2) has been deposited on a GeS(001) crystal by molecular beam expitaxy (MBE). XPS and LEED experiments suggest that the molecules are close packed in a (11 l) plane with the tail pointing to the vacuum. The local density of states of the film has been investigated by direct and inverse photoemission spectroscopies. Cage relaxation effects induced by (6-6) bond breaking has been localized in the ~r states around the HOMO-LUMO gap as evidenced by a 0.5 and 0.8 eV shift of the HOMO and LUMO feature towards higher and lower binding energies, respectively.

Following the discovery of fullerenes [1], extensive theoretical and experimental investigations have been dedicated to the determination of their structural and electronic properties. Recently, more and more interest has been directed to the study of fullerene derivatives. Among endohedral-, exohedral-, and hetero-fullerenes, the second field is the most developed one. It has been found that the most favorable exohedral chemical reaction is to break a double bond of C6o located at the junction of two hexagons ( 6 - 6 double bond) [2]. A number of these (6-6) exohedral C60 derivatives have already been synthesized and purified [2]. The interests of these new compounds lies not only in the added functional groups but also in the modified chemical bonds within the fullerene cage. It is well known that the

electronic properties of the fullerene cage are determined mainly by the pyramidalization angles of C sp 2 orbitals [3]. A 6 - 6 bond breaking changes the s - p hybridization to sp 3, which will locally release the strain presented in the fullerene cage without creating unfavorable 5 - 6 double bond. In this sense, the original fullerene cage will be distorted in a way depending on different chemical reactions, so that novel physical properties might be expected. In 1995, the first ab initio calculation was performed for a simple (6-6) fulleroid C60-CH 2. The structural relaxation and the charge inhomogeneity were found to spread no further than the second nearest neighbor of the added carbon atom [4]. Although the global electronic structures were predicted to be very similar to C60, we expect to

0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S 0 0 0 9 - 2 6 1 4 ( 9 6 ) 0 0 5 3 4 - 9

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L.-M. Yu et aL / Chemical Physics Letters 257 (1996) 163-168

~

I

N

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Fig. 1. Molecular structure of C6o-R ~.

observe a large modification in the local density of states (LDOS) close to the modified bonds in the cage. In this Letter, we present the first experimental evidence for the lattice relaxation effects on the LDOS of an epitaxial (6-6) fulleroid film collected by direct and inverse photoemission [5]. A C60 derivative, namely C60-R I (C60(NH(CHCOOCH3)2)) was selected as our model system. This compound was recently prepared by a photochemical reaction between C60 and glycine methyl ester in toluene/methanol, and purified by column chromatography on silica gel [6]. The formula and molecular structure depicted in Fig. 1 have been deduced from IR and NMR spectroscopic data [71. Filling C60-R 1 into a Kundsen cell, we have sublimated this compound in ultra-high vacuum on an in situ cleaved GeS(001) substrate at a cell temperature of around 450°C. In addition to carbon, oxygen and nitrogen fingerprints are clearly observed by X-ray photoemission spectra (XPS) recorded on the deposited film, indicating that the fulleroid does not decompose during sublimation. A sharp hexagonal LEED pattem is observed for films grown at a substrate temperature of around 150°C, regardless of the film thickness. The lattice parameter determined from the LEED p.attem yields a firstneighbor distance of about 10 A, which is surprisingly close nearest neighbor distance in C60 crystals. In a previous growth study of C60(lll)/GeS(001) [8], the cleaved GeS surface has been shown to be a perfect template for the growth of crystalline fullerite because of the close match between twice the lattice poarameter of GeS along its a axis (2 × 4.30 ,~ = 8.60 A) and the separation between rows of C60 clusters along the (101) or equivalent directions (8.68 ,~). The epitaxial relation has been evidenced by a detailed analysis of the LEED pattern, indicating that the C~9 film grows along the (111) orientation with their (101) direction parallel to the corrugation direc-

tion of the substrate. The identical LEED patterns observed for C6o-R ~and C6o imply that this fulleroid adopts the same epitaxial relation as C6o, which means that the fulleroid cages are closely packed in the (111) plane parallel to the surface, leaving the elongated adducts out of this plane. Moreover, since the van der Waals interaction of C60 with the substrate should be considerably larger than the one of the adduct with the substrate [9], one might expect, from an energetical point of view, that the tail points towards the vacuum. This will receive further support from the photoemission results presented later on.

The valence band of an in situ deposited 110 ,~ thick C60-R 1 film was investigated at room temperature by synchrotron radiation photoemission performed at Hasylab. The angular-integrated energydistribution curves (EDC) were collected with a cylindrical mirror analyzer, having an overall energy resolution better than 300 meV in the 25-110 eV photon energy range. EDCs have been recorded with 5 eV steps and a set of selected spectra is plotted in Fig. 2 (traces b-i). At the top of the figure, a HeI valence band spectrum of a thick C6o film [8] (trace

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Fig. 2. Energy dependent photoemission spectra of C ~ -R i (traces b-h) and the C6o valence spectrum (trace a) recorded with Hel. The assignment is based on Ref. [10].

L.-M. Yu et a l . / Chemical Physics Letters 257 (1996) 163-168

a) has been reproduced, upon which the assignment of the molecular orbital is sketched: (1) The valence features below 5 eV and above 10 eV are almost entirely derived from the pure 7r and cr states, respectively, while those between 5 - 1 0 eV binding energy contain states of both symmetries [10]. (2) The two v-derived bands possess odd (ungerade) and even (gerade) symmetry, respectively, and their photoionization cross sections oscillate with the final-state energies up to 120 eV above the highest occupied level. Nevertheless, the overall spectral shape remains almost the same for UPS spectra recorded with different photon energies [11-13]. The photoemission spectrum of C60-R 1 taken at the lowest photon energy ressembles very much the one of C60, indicating that the global nature of the electron states are similar in both molecular films. Considering the theoretically results obtained for C61H 2 [4], it can be safely assumed that the modification of the molecule is quite localized in a (6-6) monofulleroid. The amount of additional and modified bonds is small in comparison to the unaffected bonds in the C60 cage, so that the overall valence structures of C6o-R ~ will remain similar to that of the C6o when we sum all of the valence contributions from this fulleroid. This is actually the case when a very low photon energy is used for UPS since the probing depth is relatively high according to the universal curve of electron mean free path [14]. By increasing the photon energy, we can decrease the detection depth to a value smaller that one monolayer, say 5 ,~, so that we should expect to detect the topmost part of the molecules in the film. In other words, if the fulleroid are orientated with the adducts pointing out of the surface, we should observe the sum of LDOS near the adducts, which will certainly be different from the DOS of the whole molecule. In fact, the differences are directly visible in the spectra plotted in Fig. 2: With increasing photon energy, the overall valence feature changes, especially for the binding energy range between 5 - 1 0 eV where part of the valence feature of the adducts should appear. A shift of about 0.5 eV and 1.0 eV towards higher binding energy is observed for the pure w states and the pure tr states, respectively. This fact cannot be exclusively explained by the difference in solid state screening energy. If it was the case, one should only observe a rigid shift, and

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Fig. 3. The valence spectra of polyethylene (lower trace) and polyvinyl acetate (upper trace).

moreover, it should also be present in the lower photon energy spectra since the screening energy would affect all the molecules in the film. These observations indicated that the bond modifications of the C60 induced by the chemical group R~ are mainly appearing in the near surface region, which confirms that the adducts should point outward of the surface in the epitaxial C6o-R l film. In order to isolate the spectral contribution of the relaxed fullerene cage, let us first examine the valence band structure of R 1: Its lowest bonding energy state is determined by the lone pair orbital in C = O for the adducts, while the C - N , C - H , and C - N bonds give rise to ~ states, which should appear at higher binding energies• Since C = O is surrounded by pure tr bonds in R I, we assume that its ~r states should be quite localized so that the binding energy will not be affected much by the remaining part of the molecule. To estimate the lowest binding energy of C---O, we compare the valence spectrum of polyethylene (PE) and polyvinyl acetate (PVAc), as reproduced from Ref. [15] in Fig. 3. The valence spectrum of PE (trace a) gives the binding energies of the C - H chain, where C2p bands appear between 4•5-12.5 eV while C2s bands appear between 12.5-

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L.-M. Yu et a l . / Chemical Physics Letters 257 (1996) 163-168

22.5 eV. The leading peak at 5 eV in the PVAc valence spectrum (trace b) is definitely derived from the "rr states of the C - - O bond. The O2s structure appears mainly at a binding energy higher than 22 eV, while s - p hybridization between carbon and oxygen contributes to the spectral features between 6 . 0 - 21.0 eV. This estimate can further be confirmed by the HOMO location of the solid state CHaCOOCH 3. The gas phase ionization potential of this molecule has been determined to be 11.2 eV [16]. To obtain its solid state binding energy, one has to subtract the work function and the polarization energy. For most organic molecules, they are estimated around 5 and 0.5 eV, respectively, so that the HOMO should appear around 5.7 eV in this case. From the above consideration, we can conclude that the first two peaks in the valence spectra of Fig. 2 represent the pure -rr states of the fullerene cage. A quantitative analysis of these features should allow us to extract the spectral influence of cage relaxation in this (6-6) bonded fulleroid. The energy-dependent UPS spectra have been fitted with a set of asymmetric mixed Gaussian-Lorentzian peaks. The peak positions and the relative intensities (normalized to their sum) of the first two rr states are plotted in Fig. 4a and b (triangles and full lines), respectively. The same procedure has also been applied to the valence band spectra of pure C60 [13] and the corresponding data are presented with dots and dashed lines in the same figures. Clear modulations are observed for the relative intensities of the two 7r states in both cases with almost the same overall behaviors in the photon energy range between 40-70 eV, which confirms once again that both valence peaks are derived from the a'r bonds of the fullerene cage. Nevertheless, deviations in the relative intensities for both molecules are clearly visible for photon energies above 70 eV. This can be understood from the higher surface sensitivity in this photon energy range so that the spectra receive more influence from the LDOS from the relaxed fullerene cage, the conduction band of which has been modified up to 60 eV above the HOMO. Since the modulation of the photoemission cross section with photon energy is related to initialand final-state symmetry and parity selection rules [ l l - 1 3 ] , the modification of the binding energies yields information on the chemical bonding in their initial states. From Fig. 4b, it is evident that the

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binding energy of both "rr states stays constant at 2.15 and 3.45 eV respectively for C6o while they stay at these values only when the photon energy is lower than 40 eV for C6o-Rl. Above this value, a continuous shift towards higher binding energy is observed, which reaches about 0.4 eV at a photon energy of 90 eV. On the other hand, the separation between these two rr states remains constant for both molecules (see the two lowest curves in Fig. 4b). Since the minimum mean free path of the photon electron should appear between 40-100 eV in kinetic energy, we believe that the UPS spectra represent the summed valence states of the whole C60-R i molecule when the photon energy is smaller than 40 eV. By increasing the photon energy, the detection depth becomes smaller than the thickness of one monolayer so that the spectra reflects the sum of the partial LDOS of the molecules near the surface. Since the adducts stay on the topmost surface, the modified fullerene cage should give more and more

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L.-M. Yu et a l . / Chemical Physics Letters 257 (1996) 163-168

influence on the UPS recorded with higher photon energies. According to the calculation available for C61H 2 [4], the bond broken at a (6-6) bonding releases the strain of the fullerene cage by changing its curvature and decreasing the pyramidation angle [5]. The bond lengths of the four nearest 5 - 6 single bonds and the 6 - 6 double bonds are modified by 2%. Although the situation could be quantitatively different in our case, we can assume that a quarter of the 20 double bonds of C60, which contribute to the rr states, have been modified. Among the modified double bonds, one has been broken and four have been relaxed ressembling more to the (6-6) double bonds in higher fullerenes. From a comparison of the UPS spectra of C60 and C70 [5], it is indeed clear that the rr states with a lower pyramidation angle appear at a higher binding energy. In this sense, the shift of the binding energies in Fig. 4b is a direct experimental evidence of the relaxation of the fullerene cage caused by the (6-6) additional reaction. Since it has been predicted that the cage relaxation induces a stronger modification in the conduction band rather than in the valence band for C61H2, we carried out a kil-resolved inverse photoemission (KRIPES) measurement on the epitaxial C6o-R l layer. The normal emission KRIPES spectrum is plotted in Fig. 5 along with the spectrum of C6o taken under the same conditions. The three lowest C60 feature have been assigned to pure xr states [5] entirely derived from the double bonds in the fullerene cage. The conduction band structures of C60-Rl are very similar to those of C60, apart from a rigid shift of about 0.8 eV away from the Fermi level and a broadening of the peaks. This observation is coherent with the valence band broadening and shift in higher photon energy UPS spectra, since the detection depth of KRIPES is usually small [17]. In addition, the separation between the second and third ~r states increased about 0.3 eV, while the separation of the first two rr states stays the same as for C60. Nevertheless, no dispersion could be detected in these conduction bands, which indicates that the intermolecular interactions are not strongly modified with respect to those of C60. In conclusion, lattice relaxation of the fullerene cage in a 6 - 6 monofulleroid has been observed by direct and inverse photoemission. In our specific

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case, the decrease of the pyramidation angle pushes the HOMO and LUMO away by an order of 1 eV. These states are localized near to the adducts but the remaining fullerene cage keeps the characteristics of C60. This result emphasize that the electronic and optical properties of monofulleroid can be varied by a small distortion of the carbon cage as in the case of doped semiconductors. LMY, VL and JG acknowledge support from the large installation program of the EC. JG is a research associate of the NFSR (Belgium). This work is funded by the Belgian national program of Interuniversity Research Projects on 'Materials Characterization' initiated by the Belgian State Prime Minister's Office (Federal Services of Scientific, Technical and Cultural affairs) and by the Wallonia Region and by the Bundesminsterium for Bildung, Wissenschaft, Forschung und Technologie (BMBF).

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L.-M. Yu et al./ Chemical Physics Letters 257 (1996) 163-168

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