16814
J. Phys. Chem. 1995, 99, 16814-16816
Polymerization and Pressure-Induced Amorphization of C a and C,ot C. N. R. Rao,* A. Govindaraj, Hemantkumar N. Aiyer, and Ram Seshadri CSIR Centre of Excellence in Chemistry, Solid State & Structural Chemistry Unit and Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India Received: July 13, 1995; In Final Form: September 18, 1 9 9 9
Scanning tunneling microscopy of solid films of Cm and C70 clearly demonstrate the occurrence of photochemical polymerization of these fullerenes in the solid state. X-ray diffraction studies show that such a polymerization is accompanied by contraction of the unit-cell volume in the case of Cm and expansion in the case of c 7 0 . This is also evidenced from the STM images. These observations help to understand the differences in the amorphization behavior of Cm and c 7 0 under pressure. Amorphization of Cm under pressure is irreversible because it is accompanied by polymerization associated with a contraction of the unit cell volume. Monte Carlo simulations show how pressure-induced polymerization is favored in Cm because of proper orientation as well as the required proximity of the molecules. Amorphization of C70, on the other hand, is reversible because C70 is less compressible and polymerization is not favored under pressure.
It has been reported recently that Cm undergoes polymerization in the solid state involving 2+2 cycloaddition induced photochemically' or at high pressures and temperatures.* Cm is also known to undergo irreversible amorphization at high pressures, and the irreversibility has been attributed to the formation of cycloaddition product^.^ c 7 0 seems to undergo photopolymerization in the solid state involving 2+2 cycloaddition: but unlike Cm, it undergoes reversible amorphization at high pressure^.^ We have been interested in understanding the polymerization of Cm and c70 in the solid state6 and its relation to the nature of amorphization of these fullerenes. For this purpose, we have investigated photopolymerizationof solid films of the two fullerenes by scanning tunneling microscopy (STM) and X-ray diffraction and carried out certain simulation studies on the effects of pressure on these solids. The study has indeed revealed the subtle differences between Cm and C ~ O with regard to features of their polymerization and amorphization. Photopolymerization of solid films of Cm and c70 was studied by recording the STM images before and after UV irradiation. Typical STM images of Cm and c70 shown in Figure 1 bear clear evidence for the occurrence of photopolymerization. Photopolymerization of Cm (Figure la) does not change the symmetry of the top surface. The internal features of the molecules become visible after photopolymerizationbecause of the complete freezing of molecular m ~ t i o n .Internal ~ features become visible after photopolymerizationof C70 as well (Figure 1b), but the lattice symmetry of the top layer appears to change from fcc( 1 11)-like to fcc( lOO)-like. STM results indicate that in Cm, a lattice contraction of *13% occurs on polymerization while in C70, an expansion of =lo% is observed. X-ray diffraction measurements on Cm and C70 films confirm similar changes in the unit-cell volume. Accordingly, we see from Figure 2 that the 28 values decrease on photopolymerization of C70 while the opposite holds in the case of Cm. The changes in the unit cell volume can be understood by means of the model for the dimerization of C70 and Cm shown in Figure 2. The volume expansion in C70 arises due to the ellipsoidal shape of Supported by the Jawaharlal Nehru Centre for Advanced Scientific Research. * Correspondence to be addressed at the CSIR Centre of Excellence in Chemistry, Indian Institute of Science, Bangalore 560 012, India. Abstract published in Advance ACS Abstracts, October 15, 1995. @
lhd lh4 Figure 1. (a) STM images of Cm films on highly oriented pyrolytic graphite (HOPG) before and after UV irradiation. (b) STM images of films on HOPG before and after UV irradiation. The conditions Torr vacuum. were 254 nm UV, 12 h,
c70
the molecule as well as the high reactivity of the bonds radial to the capping pentagons toward 2+2 addition, i.e., the pentagons defining the 5-fold axis.* Since the molecules do not have to reorient for the polymerization of Cm to occur, the only effect is due to intermolecular bonding resulting in the observed contraction. The observation of unit-cell expansion in the case of C70 and contraction in the case of Cm is indeed noteworthy and has direct bearing on the amorphizationof these fullerenes.
0022-3654/95/2099-16814$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 46, 1995 16815
Letters
I1
p
I
PRISTINE
r””--”--”-
(a)
70
UV EXPOSED
60
UV EXPOSED
~
10
12
16
14
28
18
20
22
L i.w.060 i
Figure 2. XRD patterns of (a) c70 and (b) Cm on glass substrates before and after UV irradiation. Whereas the Cm lattice contracts on photopolymerization, for C70, the lattice is seen to expand. The 2+2 addition of Cm and C ~ (resulting O in photopolymerized products) is shown schematically. 1 .oo 0.95 d?
0.90 0.85
2 5 10 15 20 25 30
0
p(GPa)
of pressure in Figure 3a. These plots were made using the Murnaghan equation of state and experimentalcompressibilities. We see that solid Cm is more compressible than C70. Our NVT Monte Carlo studies’O also throw light on the nature of changes brought about on application of pressure. Among the various results obtained from such studies, the one most relevant to the context of polymerization under pressure is the following: On application of pressure, the Ca molecules approach one another much more closely than C70 molecules. The calculated closest approach between two Cm molecules at 300 K and 30 GPa is ~ 1 . A, 8 while it is e2.2 A in c70 (Figure 3b). Although the Ca molecules are randomly oriented in the solid up to moderate pressures, the molecules align themselves at higher pressures providing so as to be favorable for the 2+2 addition to occur. In the case of C70 on the other hand, the alignment of the molecules under pressure occurs with their long axes along the face and body diagonals of the unit cell so as to render the reactive end caps of the molecule to be relatively farther apart. Snapshots of the simulated Cm and C70 lattices under pressure are shown in Figure 3c. Clearly, polymerization under pressure is favored in the case of Cm but not in C70. Since the polymerization is also accompanied by volume contraction in the case of Carpressure would indeed favor such a process. This is not so in C70,where polymerization is accompanied by volume expansion. On the basis of these observations, we are able to understand why the amorphization of Cm under pressure is irreversible because of polymerization.
References and Notes
c60
c70
Figure 3. (a) Pressure dependence of the ratio of the pseudocubic lattice parameter a of Cm and c70 to the parameter uoat ambient pressure as given by the Mumaghan equation of state. Experimental compressibilities were employed to produce these plots. It is assumed that there are no intervening phase changes and that Cm remains cubic and C70, rhombohedral throughout. (b) Atom-atom radial distribution functions of Cm and C70 molecules at 0 and 30 GPa and 300 K from NVT Monte Carlo studies. The molecules of Cm approach closer at a given pressure than do C70. (c) Snapshots of the Cm lattice at 20 GPa and C70 lattice at 25 GPa obtained from NVT-MC simulations, looking down the (100) direction.
What is also interesting about Cm and c70 is that their compressibilities are quite different? The ratios of the pseudocubic lattice parameters of Cm and C70 with respect to their corresponding room-pressure values are shown as a function
(1) (a) Rao, A. M.; Zhou, P.; Wang, K. A.; Hager, G. T.; Holden, J. M.; Wang, Y.;Lee, W. T.; Bi, X. X.; Ecklund, P. C.; Comett, D. S.; Duncan, M. A.; Amster, I. J. Science 1993,259,955. (b) Weaver, J. H.; Pokier, D. M. Solid State Phys. 1994,423, 1. (c) Zhou, P.; Rao, A. M.; Wang, K. A.; Robertson, J. D.; Eloi, C.; Meier, M. S.; Rin, S. L.; Bi, X. X.; Ecklund, P. C.; Dresselhaus, M. S. Appl. Phys. Lett. 1992,60,2871. (d) Bacsa, W. S.; Lannin, J. S. Phys. Rev. 1994, B49 14750. (e) Menon, M.; Subbaswamy, K. R. Phys. Rev. 1994, B49 13966. (2) (a) Nunez-Reguero, M.; Marques, L.; Hodeau, J. L.; Bethoux, 0.; Perroux, M. Phys. Rev. Lett. 1995, 74,278. (b) Xu, C. H.; Scuseria, G. E. Phys. Rev. Lett 1995, 74, 274. (3) Rao, A. M.; Menon, M.; Wang, K. A.; Ecklund, P. C.; Subbaswamy, K. R.; Comett, D. S.; Duncan, M. A.; Amster, I. J. Chem. Phys. Lett. 1994, 224, 106. (4) (a) Yoo, C. S.; Nellis, W. J. Chem. Phys. Lett. 1992, 198,379. (b) Moshary, F.; Chen, N. H.; Silvera, I. F.; Brown, C. A.; Dom, H. C.; deVries, M. S.; Bethune, D. S. Phys. Rev. Lett. 1992, 69, 466. (c) Snoke, D. W.; Syassen, K.; Mittelbach, A. Phys. Rev. 1993, B47, 4146. ( 5 ) Chandrabhas, N.; Sood, A. K.; Muthu, D. V. S.; Sundar, C. S.; Bharathi, A.; Hariharan, Y.;Rao, C. N. R. Phvs. Rev. Lett. 1994, 73,341 1. (6) Since 2+2 cycloaddition is symmetry allowed under photochemical conditions but not under thermal conditions, under the application of pressure, it must take place by routes other than concerted, for example, via polar intermediates. (7) Both Cm and (270 are orientationally disordered in the solid state at ordinary temperatures. Moreover, the disorder is dynamic. This prevents the probing of intramolecular features by techniques such as STM. However, when the fullerenes are deposited as films on surfaces such as single crystal metals, ordered structures can be obtained because the molecules are attached to the surface relatively strongly because of charge transfer from the metal to the fullerenes. When the molecules polymerize, they cannot rotate freely and the internal structure becomes visible. For details see: (a) Wang, X. D.; Hashizume, T.; Sakurai. T. Mod. Phys. Lett. 1994.8, 1397. (b) Aiyer, H. N.; Govindaraj, A.; Rao, C. N. R. Bull. Mater. Sci. 1994, 17, 563. (c) Joachim, C.; Gimzewski, J.; Schittler, R.; Chavy, C. Phys. Rev. Lett. 1995, 74,2102. (d) Aiyer, H. N.; Govindaraj, A.; Rao, C. N. R. Philos. Mag. Lett., in print. (8) (a) Henderson, C. C.; Cahill, P. A. Science 1994, 263, 397. (b) Karfunkel, H. R.; Hrisch, A. Angew. Chem., Int. Ed. Engl. 1992.31. 1468. (c) Rathna, A.; Chandrasekhar, J. Curr. Sci. (India) 1993, 65, 768. (d) Govindaraj, A.; Rathna, A.; Chandrasekhar, J.; Rao, C. N. R. Proc. Indian Acad. Sci. (Chem. Sci.) 1993, 105, 303. (9) (a) Duclos, S. J.; Brister, K.; Haddon, R. C.; Kortan, A. R.; Thiel, F. A. Nature 1991,351,380. (b) Christides, C.; Thomas, I. M.; Dennis, T. J. S.; Prassides, K. Europhvs. Lett. 1993. 22. 545.
16816 J. Phys. Chem., Vol. 99, No. 46, 1995 (10) (a) NVT Monte Carlo simulations were performed on cells containing 32 rigid Ca or c70 molecules. The intermolecular potential was of the Lennard-Jones form. The unit cells for the simulation were taken from experiments referred to in the text. Visualization was done using the Insight11 software from BIOSYM Technologies, San Diego on Indigo11 workstations. For details of the simulation as well as the potential
Letters parameters used, see: (a) Cheng, A,; Klein, M. L. J . Phys. Chem. 1991, 95, 6750. (bj Girifalco, L. A. J . Phys. Chem. 1992, 98, 858. (c) Chakrabarti, A.: Yashonath, S.; Rao, C. N. R. Chem. Phys. Lett. 1993,215, 519.
JP951970P
