One of the interesting features of the single-wall c a r b o n n a n o t u b e ( S W N T ) i s i t s n a n o m e t e r s i z e i n n e r h o l l o w c a v i t y [ 1 , 2 ] . T h i s e n c o u r a g e s r e s e a r c h e r s t o s t u d y i t s u s e i n i n d u s t r i a l applications such as gas storage cylinders and one- d i m e n s i o n a l n a n o m e t e r m o l d s . R e c e n t l y , i t w a s s h o w n t h a t b u l k q u a n t i t i e s o f f u l l e r e n e m o l e c u l e s can be encapsulated in SWNTs [3, 4]. This class of material is called “ peapod” . The reason for this can be clearly seen in Fig. 1 . Novel Structures of C 60 and C 70 -Encapsulating Carbon Nanotubes r o t a t i o n i n s o l i d C 7 0 i s v e r y a n i s o t r o p i c , a n d t h e p h a s e t r a n s i t i o n r e l a t e d t o t h e m o l e c u l a r r o t a t i o n o c c u r s s u c c e s s i v e l y a t 2 8 0 K a n d 3 4 0 K . T h e difference between the two molecules in the phase t r a n s i t i o n i s a s c r i b e d t o t h e m o l e c u l a r s h a p e : t h e C 6 0 i s a p p r o x i m a t e d t o a s p h e r e w i t h a m e a n diameter of 0.71 nm while the C 70 can be likened to a r u g b y b a l l w i t h a s h o r t a x i s o f 0 . 7 1 2 n m a n d a long axis of 0.796 nm. In the present studies [6], we performed powder X- ra y di ff ra ct io n ( XR D ) me as ur em en ts of th e C 60 - and C 70 -peapods in a temperature range between 300 K and 1000 K to clarify the structural aspects. The XRD experiments were performed at BL02B2 o f S P r i n g - 8 a n d B L 1 B o f P F . F i g u r e 2 s h o w s e x a m p l e s o f th e o b s e r v e d X R D p a tt e r n s ta k e n a t r o o m t e m p e r a t u r e ( R T ) . A l t h o u g h t h e p e a k s a r e as si gn ed to a tw o- di me ns io na l tr ia ng ul ar la tt ic e in t h e b u n d l e s o f S W N T s , e a c h B r a g g p e a k i s si gn if ic an tl y br oa de ne d du e to th e sm al l co he re nt l e n g t h , ~ 2 0 n m . F o r t h i s r e a s o n , t h e X R D p e a k profiles are strongly modulated by the form factors of the tubes and fullerene molecules. W e notice a large depression of 1 0 peak intensity around Q ~ 6 ( 1 / n m ) o n t h e e n c a p s u l a t i o n . T h i s i s a c o m m o n f e a t u r e a s s o c i a t e d w i t h g a s a d s o r p t i o n i n s i d e SWNTs [7]. The other important feature is the appearance of n e w p e a k s i n d i c a t e d b y a r r o w s i n F i g . 2 i n t h e e n c a p s u l a t i o n . T h e s e p e a k s a r e a s s i g n e d t o t h e o n e - d i m e n s i o n a l c r y s t a l s o f f u l l e r e n e m o l e c u l e s inside the tubes. The C 60 intermolecular distance is e s t i m a t e d t o b e 0 . 9 7 n m . I n t h e c a s e o f C 7 0 , inter estin gly, there are two diffe rent inter molec ular distances, 1.0 nm and 1.1 nm, which correspond to two molecular configurations of standing and lying alignments inside the tubes, respectively ( Fig. 2 ). F i g u r e 3 s h o w s t h e t h e r m a l e x p a n s i o n o f i n t e r m o l e c u l a r d i s t a n c e s . A l t h o u g h t h e C 6 0 - pea pod s sho wed com pli cat ed beh avi or dep end ing o n t h e s a m p l e t r e a t m e n t , c a r e f u l m e a s u r e m e n t s s t r o n g l y s u g g e s t e d t h a t t h e i n t r i n s i c t h e r m a l expansion is substantially smaller than those for the Fig. 1. The TEM image of C 70 -peapods (left) and a sc he ma ti c vi ew of pe ap od s (r ig ht ). Th e sc al e is 10 nm . Th e in se t sh ow s a cr os s se ct io n of a b un dl e. A t h e o r e t i c a l c a l c u l a t i o n f o r C 6 0 - p e a p o d s predicts new features in the band structure around the Fermi level [5], suggesting a possibility of high- temperature superconductivity without any doping. H o w e v e r , i t m u s t c o m p e t e w i t h i n s t a b i l i t i e s s u c h a s t h e c h a r g e d e n s i t y w a v e ( C D W ) a n d t h e s p i n d e n s i t y w a v e ( S D W ) , b e c a u s e t h e f u l l e r e n e mo le cu le s fo rm an on e- di me ns io na l cr ys ta l in si de the tub e. The C 60 or C 70 mol ecu lar ori ent ati on is a n o t h e r i m p o r t a n t f r e e d o m . I n t h e c a s e o f s o l i d C 6 0 , i t i s k n o w n t h a t t h e C 6 0 e x h i b i t s q u a s i - f r e e rotation at each lattice site; thus the solid C 60 is a plastic crystal at room temperature ( T ). When T is lowered, an orientational ordering phase transition i s s h o w n a t 2 6 1 K . O n t h e o t h e r h a n d , t h e m o l e c u l a r 34 400 600 800 1000 1.03 1.02 1.01 1.00 0.99 T (K) d(T) / d(300K) Standing Lying Solid C 60 C 70 -peapods C 60 -peapods Intensity ( a.u. ) 4 6 8 10 12 14 16 Q (1/ nm ) 1 0 peak Empty (a) Observed (b) Simulated C 70 C 60 F i g . 3 . I n t e r m o l e c u l a r d i s t a n c e o f C 6 0 - a n d C 7 0 - p e a p o d s , n o r m a l i z e d a t 3 0 0 K , a s a f u n c t i o n o f t e m p e r a t u r e . Fig. 2. The observed and simulated XRD patterns for empty SWNTs, C 60 - and C 70 -peapods. solid C 60 and for the C 70 -peapods. Along with the rather short C 60 -intermolecular distance of 0.97 nm, the ins ide of the tub es is fou nd to be som e nov el en vi ro nm en t fo r th e C 60 mo le cu le s. On th e ot he r hand, the C 70 -peapods with the standing alignment sh ow a la rg e th er ma l ex pa ns io n co ef fi ci en t of th e i n t e r m o l e c u l a r d i s t a n c e , i n d i c a t i n g t h a t t h e r m a l l y a c t i v a t e d C 7 0 - s t u m b l i n g o c c u r s i n s i d e t h e t u b e s a b o v e 3 0 0 K . T h e a b s e n c e o f a c l e a r p h a s e t r a n s i t i o n i n s o l i d C 7 0 i s p r o b a b l y d u e t o t h e o n e - dimensionality of the C 70 -crystal. References [1] S. Iijima and T. Ichihashi, Nature (London) 363 (1993) 603. [2] D.S. Bethune et al. , Nature 363 (1993) 605. [3] B.W. Smith et al. , Nature 396 (1998) 323. [4] H. Kataura et al. , Synthetic Metals 121 (2001) 1195. [5] S. Okada et al. , Phys. Rev. Lett. 86 (2001) 3835. [6] H. Kataura, Y. Maniwa, M. Abe, A. Fujiwara, T. K o d a m a , K . K i k u c h i , H . I m a h o r i , Y . M i s a k i , S . Su zu ki an d Y. Ac hi ba , Ap pl . Ph ys . A 74 (2 00 2) 34 9; Y . M a n i w a , H . K a t a u r a , M . A b e , A . F u j i w a r a , R . Fujiwara, H. Kira, S. Suzuki, Y. Achiba, E. Nishibori, M. Takata, M. Sakata and H. Suematsu, submitted in Science. [7] Y. Maniwa et al. , Jpn. J. Appl. Phys. 38 (1999) L668 ; A. Fujiw ara et al . , Chem. Phys. Lett. 336 ( 2 0 0 1 ) 2 0 5 ; Y . M a n i w a e t a l . , P h y s . R e v . B 6 4 (2001) 241402(R). Y u t a k a M a n i w a a , H i r o m i c h i K a t a u r a a a n d Akihiko Fujiwara b (a) Tokyo Metropolitan University (b) Japan Advanced Institute of Science and Technology E-mail: maniwa @ phys.metro-u.ac.jp 35
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