Selenium intercalated solid C60

5 April 1996

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 252 (1996) 101-106

Selenium intercalated solid C60 Kejian Fu a,1, Peng Zhang a, Tom L. Haslett a, Martin Moskovits a, Xijia Gu h a Department of Chemistry and Ontario Laser and Lightwaoe Research Centre, University of Toronto, Toronto M5S 1A1, Canada b Ontario Laser and Lightwaoe Research Centre, Toronto M5S 1A1, Canada

Received 4 December 1995

Abstract

Two new phases based on Se intercalated into C6o were produced in a high temperature reaction of the chalcogen with the fullerene. Based on laser-ablation time-of-flight mass spectroscopy the two phases are proposed to have stoichiometries (Se5)2C60 and (Se5)2(C60)2 in which the latter is proposed to contain dimeric C60. The two species are stable in air. Laser Raman spectroscopy indicated a lowering of the Ag pentagonal pinch mode of the C60 by 5 cm -~ and 11 cm -1 for (Se5)2C60 and (Se5)2(C6o)2, respectively, suggesting an electron transfer from the Se5 clusters to the C6o in the range 0.5 to 2 electrons per Se a.

1. Introduction

The exciting discovery that alkali-metal-intercalated fullerenes exhibit superconductivity suggested a larger paradigm for creating an entire class of new materials based on metal-intercalated fullerenes [1-4]. The large size of C60, whose hollow, truncated-icosahedral morphology (I h) closely approximates a sphere, means that when packed in its face-centered cubic crystalline structure, the 26% interstitial volume can accommodate fairly large guest atoms. The nearest ne!ghbour distance in solid C60 is approximately 10 A. However, the novelmaterials possibilities of intercalated fullerenes is not based on size alone. Among C60's novel properties

t Permanent address: Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. 0009-2614/96/$15.00 © 1996 Elsevier Science B.V. PH S 0 0 0 9 - 2 6 1 4 ( 9 6 ) 0 0 1 4 9 - 2

are its high electronegativity and the high degeneracy of its lowest-lying unfilled states, which allow it to accept up to six, or perhaps as many as twelve, electrons. The intercalation of alkali and alkali earth atoms, which are electron donors, has been investigated widely. Other types of donors with varying stoichiometries have also been reported [5-7]. More recently, it has been shown that large molecular units, such as 12 [8,9], S s [10] and P4 [11] can also be intercalated into solid C60. Large species of this sort change the intermolecular spacing and the packing of the fullerene significantly. Despite the large number of new fullerene intercalation materials reported a great deal remains to be explored. In this letter we report the intercalation of selenium clusters in solid C6o. Selenium is known to have both donor and acceptor properties. To date, only one report of C60/Se complexes has appeared in the form of a calculated molecular geometry for the C60Se species [12]. We

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K. Fu et al. / Chemical Physics Letters 252 (1996) 1 O1 - 106

have observed two solid phases based on the intercalation of selenium clusters: one seems to involve complexation of Se clusters with monomeric C60, the other seems to entail dimeric C60. The two phases were produced by reacting C60 and selenium at 673 K and 773 K respectively. Time-of-flight mass spectra and HPLC suggest that the two species are based on complexes in which 10 Se atoms (or ions) are associated with one and two C60 molecules, respectively. Laser micro-Raman spectra showed that the Ag(2) mode of solid C60 is shifted by 5 cm-1 and 11 cm-~ for the species with stoichiometries (Se5)2C60 and (Se5)2(C60) 2, respectively. We write the l0 Se atoms or ions as (Se5) 2, foreshadowing our proposal that the species involve Se 5 rings, which are known to be particularly stable. In addition to the above solid phases, other species including SesC x, Se6C x ( x = 4, 6, 12) were observed. These will be discussed elsewhere.

2. Experimental C6o was synthesized using the Huffman method, extracted and purified by column chromatography on alumina to a purity of 99.9%. The fullerene was then dried by outgassing in vacuum. Selenium powder (99.999%, Alfa) was used as received. Stoichiometric amounts of selenium and C60 powder were sealed in a Pyrex tube under vacuum and heating for several days in a gradient furnace. The temperature of solid C60 was maintained hotter than that of the selenium to prevent selenium from condensing directly onto the surface of the C60. More than twenty samples were prepared with highly reproducible results. The materials produced have remained stable for over a year. The samples prepared at 673 K and 773 K will be referred to as A and B, respectively. The materials prepared were examined by energy dispersive X-ray analysis (Hitachi S-570) and shown to consist almost exclusively of selenium and carbon with only a trace quantity of oxygen. Samples were analyzed by HPLC (Waters 600E with Novapak C18 reversed-phase column). Reasonable HPLC chromatograms were obtained despite the very low solubility of the C60/Se products in xylene. (Although CS 2 was found to be better solvent

for the new products it was not compatible with our HPLC columns.) Laser desorption time-of-flight mass spectrometry (TOF-MS) was used as one of the chief diagnostic tools to analyze the content of the fullerene-rich soots produced and the fullerene-based materials synthesized. Solid samples were pressed onto a single crystal silicon surface, which was circulated using an eccentric gear so that each laser pulse was intercepted by a fresh spot on the sample. Ions were generated by laser ablation using 532 nm light from the doubled output of a Q-switched mode-locked Nd3+:YAG laser. The ions were introduced directly into the acceleration region of a Wiley-McLaren type TOF-MS system (R.M. Jordan Co.), accelerated with 2 kV voltage, and detected with a multichannel plate, located at the end of a 1 m flight tube. The only unique feature of the apparatus is the fact that the laser beam counterpropagates with the ion beam. Two small holes were perforated in the acceleration grids in order to allow the laser to reach the sample. The signal was collected and stored in a digital oscilloscope (LeCroy 9400A). The pulse energy of the laser is adjusted to be just above the threshold for ion production so as to minimize fragmentation of the desorbing species. The negative ion TOF-MS spectra were found to be considerably simpler than those of the positive ions. Consequently most of the analysis was done using the negative ion TOF-MS spectra. High-performance liquid chromatography (Waters outfitted with Prep Novapak 25 mm × 100 mm Cartridges) separated and identified two species from the products of the reaction of selenium with C6o. Laser micro-Raman was performed on the two fractions. By recording the Raman spectra at various points within the sample, micro-Raman is capable of reporting its homogeneity in a general way. Raman spectra were excited with approximately 2 mW of 530.87 nm line of a Kr ion laser focused down to a 20 /zm diameter spot, corresponding to an intensity of 625 W / c m 2 at the sample. Raman spectra were recorded in a back-scattering geometry using a Triplemate spectrometer (SPEX Industries Inc., Model 1877D) equipped with a SPEX 1482D Micromate microscope and a liquid nitrogen cooled CCD detector (Princeton Instruments Inc. Model LN/CCD).

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3. Results and discussion The HPLC chromatogram of sample A contained two peaks with retention times of 5.67 and 9.20 min, representing one new species and C,,, respectively. For sample B, HPLC revealed a third peak with a retention time of 4.43 min (Fig. 1). The (negative ion) TOF-MS spectrum of sample A (Fig. 2) shows two species in addition to those that can be assigned to selenium clusters: a narrow peak corresponding to m/z of 720 (C,,) and a weaker, broader peak at m/z = 15 14 which corresponds to Se ,aC,, . The peak width results primarily from the wide natural isotope composition of selenium. The TOF-MS spectrum of sample B (Fig. 3) shows an additional peak at m/z = 2240 which corresponds to Se ,,,C ,20. For a control sample prepared by grinding selenium powder together with C, at room temperature (without heating) the TOF-MS spectrum showed no species with m/z above 720, thus eliminating the possibility that the Se,,C,, and Se,,C,20 species were formed during or subsequent to laser ablation. It also implies

Retention

Time

Fig. 1. Typical high-performance extracted solution of the material Se at 773 K (sample B).

0-l

10

1

15

20

!25

,

,

,

30

35

40

,

,

,

45

50

55

f

Flight time (mlerwecond)

Fig. 2. Laser-ablation the material produced

time-of-flight negative ion mass spectra of by reacting C, with Se at 673 K (sample

A).

that the species result from a reaction at elevated temperatures. It is natural to suggest that the two species detected by HPLC also produce the two new peaks in the TOF-MS spectra. Additionally, EDX analysis suggests that the elemental composition of sample A is 46.8% C, 51.9% Se and 1.3% 0. A material with stoichiometry C,Se,, would have an elemental composition of 47.6% C and 52.3% Se, in satisfactory agreement with the EDX result. (The small quantity of oxygen is likely due to atmospheric oxygen adsorbed on the powdered selenium/fullerene material.)

(mh)

liquid chromatogram of xylene produced by reacting C, with

Fig. 3. Laser-ablation the material produced B).

time-of-flight negative ion mass spectra of by reacting C, with Se at 773 K (sample

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The equilibrium cluster composition in selenium vapor has been studied extensively using Knudsen effusion mass spectroscopy. Although all selenium clusters Se x ( x = 2-7) are observed in the mass spectrum, Se 5, Se 6 and Se 7 predominate in the range 650-800 K. The positive ion TOF-MS spectrum of a type A sample is shown in Fig. 4. In the selenium cluster region Se~- is the most abundant ion. The electronic structure and the geometry of Se 5 have been studied both by theory and experiment. Hohl and Li et al. [13,14] applied both molecular dynamics and the density functional approach to Se 5. They suggest a cyclic ground state with C s symmetry. Reliable experimental structural information was deduced from the ultraviolet photoelectron spectrum of Se 5 [15]. The spectrum implies a low symmetry cyclical cluster in accord with the theory. We suggest that the Sel0C60 material is composed of positively charged Se 5 species intercalated in C60 in the ratio of 2 selenium clusters per fullerene molecule, hence (Se5)2C60. (We speculate, but cannot prove, that the cyclic pentamer enters the C6o lattice as a preformed unit.) This explains the unusual abundance of Se 5 clusters in the positive ion TOF-MS spectrum of sample A. The detection of only the two species, (5e5)2C60 and (Se5)2(C60)2 in the negative ion mass spectra of the two selenium/fullerene samples, that is, the absence of a range of species with varying proportions of Se 5 and C60, suggests that the binding strength of the selenium cluster to the fullerene is much stronger than the binding strength of the selenium/fullerene complexes to each other.

20-

ceo

15-

i

Ses

10"

5"

10

20

30 40 Time of flight ( microsecond)

50

60

Fig. 4. Time of flight positive ion mass spectra of an A-type sample.

/•1451 -~

1457

i-

A 146

1400

1450

1500

1550

Wavenuml~r (cm-1)

Fig. 5. LaserRaman spectraof the Ag(2) mode of C6o, (Se5)2C6o and (Se5)2(C60)2,respectively.

The laser micro-Raman spectra of the three solid samples with stoichiometries corresponding to C60, (Se5)2C60 and (Se5)2(C60) 2 are shown in Fig. 5. The Raman band of C6o at 1462 cm -1 is the pentagonal pinch mode belonging to the representation Ag. This mode is known to be particularly sensitive to the charge on the C60 molecule, its frequency decreasing with increased negative charge on the fullerene molecule. The mode is down-shifted to 1457 cmin (5e5)2C60 and to 1451 cm -l in (Se5)2(C60)2, implying that the positive charge on the Se 5 cluster is produced by donating electrons to the fullerene. Comparison with the known frequencies of these modes in K3C60 and K6C60 where the charge on the fullerene is known to be - 3 and - 6 respectively [16], suggests that the degree of red-shift observed for the species (Se5)2C60 and (Se5)2(C60)2 is caused by a charge of approximately - 1 on the fullerene moiety of A and - 2 on that of B, respectively. This, in turn, implies a charge of approximately +0.5 and + 1 on each of the Se 5 clusters in the two species (to the extent that the results obtained with the alkali fullerides can be assumed to carry over to other systems). The lower frequency Ag breathing mode of C6o is also downshifted from 497 cm-1 to 493 cm-1 in (Se5)2C60, as is the 270 cm-1 Hg mode to 268 cm -1. The observed red-shift for the seleniumcontaining fulleride is similar to that observed for the Ag modes of sulphur-doped C60 [17], consistent with the notion that the frequency is sensitive primarily to

K. Fu et a l . / Chemical Physics Letters 252 (1996) 101-106

the charge on the fullerene rather than to the identity of the intercalant. Increasing the temperature to 773 K apparently produces a species in which the Se 5 clusters are bonded to two C6o molecules. We suggest that they are, in fact, bonded to C60 dimers. The material so produced is found to be stable in air. The mechanism for the photoinduced solid state polymerization of C60 was suggested Eklund and coworkers [18] to be a photochemically allowed [2 + 2] type cycloaddition reaction between neutral C60 molecules that results in a linear polymer. The thermal reaction is forbidden according to the W o o d w a r d - H o f f m a n n rules. We suggest that the C60 dimers in the (Se5)2(C60) 2 species may be an ion-induced [2 + 2] cycloaddition reaction, as was suggested for the dimeric AiC60 (A = K, Rb) system [19]. (Of course we cannot tell if the dimerization occurs after charge transfer from only one or two Se 5 clusters.) The L U M O - H O M O gap in Se 5 clusters is 2.17 eV [14]. This suggests that the transfer of valence electrons would occur from the selenium to the C60, resulting in an anionic species with excess pseudo-p charge delocalized on the carbons. Under these circumstances the [2 + 2] cycloaddition becomes thermally allowed. During the dimerization, the h e x a g o n hexagon ( 6 - 6 ) bonds in the C6o cages are broken and bonds between two adjacent C6o'S are formed. Each C6o dimer would have two charged Se 5 clusters associated with it, i.e. (5e5)2(C60) 2. The dimerization also reduces the symmetry of the C60, specifically removing the center of inversion, so that both gerade and ungerade vibrations will be Raman active [20]. And, indeed, new Raman bands are observed in the spectrum of the dimeric species. The Hg (1) mode splits into three bands and the A s (2) mode is downshifted in frequency by l l cm -1, in good agreement with the calculations of Adams [21].

4. Conclusion The high temperature reaction of stoichiometric amounts of selenium with C60 produces two new solid phases with apparent stoichiometries (Se5)2C60 and (5e5)2(C60) 2. We propose that both species involve cyclic Se pentamers with positive charges

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ranging from + 0 . 5 to + 1 and that the latter involves a C6o dimer. In both cases the fullerene carries the conjugate negative charge. Laser-ablation time-of-flight mass spectra identified species with the above stoichiometries as the most abundant negative ions produced. Raman spectroscopy indicated a decrease in the frequencies of the pentagonal pinch mode of the fullerenes in agreement with the proposed charge-transfer between the Se 5 clusters and fullerene.

Acknowledgements The authors are grateful to NSERC and C E M A I D for financial support.

References [1] A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W.Murphy, S.H. Glarum, T.T.M.Palstra, A.P. Ramirez and A.R. Kortan, Nature 350 (1991) 600. [2] R.C.Haddon, Acc. Chem. Res. 25 (1992) 127. [3] L.Q. Jiang and B. E. Koel, Phys. Rev. Letters 72(1994) 140. [4] B.J. Benning, F. Stepniak and J.H.Weaver, Phys. Rev. B 48 (1993) 9086. [5] W. Andreoni, P. Giannozzi and M. Parrineloo, Phys. Rev. Letters 72 (1994) 848. [6] M. Fleming, M.J. Rosseinsky, A.P. Ramirez, D.W. Murphy, J.C. Tully, R.C. Haddon, T. Siegrist, R. Tycko, S.H.Glarum, P. Marsh, G. Dabbagb, S.M. Zahurak, A. V.Makhija and C. Hampton, Nature 352 (1991) 701. [7] Y. Yildirim, O. Zhou, J.E. Fischer, N. Bykovetz, R.A. Strongin, M.A. Cichy, A.B. Smith Ill, C.L. Lin and R. Jelinek, Nature 360 (1992) 568. [8] Q. Zhu, D.E. Cox, J.E. Fisher, K. Kniaz, A.R.McGhie and O. Zhou, Nature 355 (1992) 712. [9] P.V. Huorg, Solid State Commun. 88 (1993) 23. [10] G. Roth and P. Adelmann, Appl. Phys. A 56 (1993) 169. [11] I. W. Locke, A. D. Darwish, H. W. Kroto, K. Prassides, R.Taylor, D. R. M. Walton, ChEm. Phys. Letters 225 (1994) 186. [12] Z. Slanina, S.-L Lee, Bull. Inst. Chem., Acad. Sin. 42 (1995) 1. [13] D. Hohl, R.O.Jones, R. Car and M.Parrinello, Chem. Phys. Letters 139 (1987) 540. [14] Z. Q. Li, J.Z. Yu, K. Ohno, B. L. Gu, R.Czaijka, A.Kasuya, Y.Nishina and Y.Kawazo, Phys. Rev. B 52 (1995) 1524. [15] J. Becker, K. Rademann and F. Hensel, At. Mol. Clusters 19 (1991) 229.

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[16] X.J. Gu, K.J. Fu, P. Zhang, M. Moskovits, unpublished. [17] K. L. Akers, M. Moskovits, J. Electron Spectry. Relat. Phenom. 64/65 (1993) 871. [18] A. M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X.-X. Bi, P.C. Eklund, D,S. Comett, M.A. Duncan and l.J. Amster, Science 259 (1993) 955.

[19] S. Pekker, L. Forro, L. Mihaly and A. Janossy, Solid State Commun. 90 (1994) 349. [20] M.C. Martin, D. Koller, A.Rosenberg, C. Kendziom and L. Mihaly, Phys. Rev. B 51 (1995) 3210. [21] G. B. Adams, J.B. Page, O.F. Sankey and M. O'Keeffe, Phys Rev. B 50 (1994) 17471.