Rotational dynamics of confined C60 fromnear-infrared Raman studies under high pressureYonggang Zoua, Bingbing Liua,1, Liancheng Wanga, Dedi Liua, Shidan Yua, Peng Wanga, Tianyi Wanga, Mingguang Yaoa,Quanjun Lia, Bo Zoua, Tian Cuia, Guangtian Zoua, Thomas Wågbergb, Bertil Sundqvistb, and Ho-Kwang Maoa,c,1

aState Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China; bDepartment of Physics, Umeå University, S-90187 Umeå, Sweden;and cGeophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015

Contributed by Ho-Kwang Mao, October 30, 2009 (sent for review September 15, 2009)

Peapods present a model system for studying the properties ofdimensionally constrained crystal structures, whose dynamicalproperties are very important. We have recently studied therotational dynamics of C60 molecules confined inside single walledcarbon nanotube (SWNT) by analyzing the intermediate frequencymode lattice vibrations using near-infrared Raman spectroscopy.The rotation of C60 was tuned to a known state by applying highpressure, at which condition C60 first forms dimers at low pressureand then forms a single-chain, nonrotating, polymer structure athigh pressure. In the latter state the molecules form chains with a2-fold symmetry. We propose that the C60 molecules in SWNTexhibit an unusual type of ratcheted rotation due to the interactionbetween C60 and SWNT in the ‘‘hexagon orientation,’’ and thecharacteristic vibrations of ratcheted rotation becomes more ob-vious with decreasing temperature.

molecular rotation � peapods

Because of its high symmetry and relatively weak intermo-lecular interactions, the fullerene C60 presents a nearly ideal

system for studying different crystal structures possible in one-dimensionally constrained nanostructure systems, such as C60peapods [C60 molecules inserted into single walled carbon nano-tubes (SWNTs)] (1). Such nanostructures are emerging as a classof nanoscale materials with tunable mechanical, electronic,thermal, and optical properties (2, 3). In contrast to bulk C60crystals, C60 inserted in SWNTs form monomeric linear chainsin which each C60 has only two nearest neighbors, as comparedto 12 neighbors in crystalline bulk C60 (4). As a result theintermolecular interactions in these self-assembled carbon nano-structures and the dynamic behavior of the encapsulated mol-ecules are expected to be different from those in solid fullerenes.It is well known that the solid C60 crystallizes into a face centeredcubic structure at room temperature, and the molecules performnearly free rotations (5). For the peapods, recent theoreticalsimulations suggest that the C60s have two different orientationswith low potential energy when they are trapped into SWNTs,the pentagonal and hexagonal orientations (6–8) with a penta-gon and a hexagon perpendicular to the tube axis, respectively.Very recent experimental studies on peapods by inelastic neu-tron scattering (INS) and nuclear magnetic resonance (NMR)suggest that the linear arrays of C60 molecules still rotate insideSWNTs at room temperature, either showing quasi-free rotationor being dynamically disordered with high orientational mobility(9, 10). However, how the C60 molecules rotate inside SWNTs,especially at room temperature, is still an interesting openquestion. It is not only necessary to search another effectiveexperimental method to study the possible rotation of confinedC60 in detail, but also very important to study the rotation froma different perspective. Considering that C60 rotates down tovery low temperature, it would be helpful to employ othertechniques to change the rotational state in a way similar to thetemperature effect, which can effectively change the rotationalstate and stop the C60 molecules in a known state from which theinitial rotational state can be deduced. Motivated by this idea, we

have used near-infrared (NIR) Raman spectroscopy and highpressure technique to study the rotational dynamics of peapods.

Raman spectroscopy is a valuable technique for investigatingthe vibrational properties of C60 inside SWNT (11, 12). Inparticular, intermediate frequency modes (IFM) are very closelyrelated with the orientational state of C60 (13), providing anapproach to study the dynamics of rotation of C60. To the bestof our knowledge, these modes have previously only beendetected for pristine peapods at very low temperature and byusing high energy excitation lasers with obvious irradiationeffects (14). Because NIR Raman spectroscopy is also able todetect such modes it provides us with an approach to study therotational state of C60, detect the presence of covalent polymerbonds, and determine the type of polymeric C60 phase formedinside the SWNT.

In this report, we have carried out NIR excitation Ramanspectroscopy to reveal the intrinsic IFM vibrations of peapods,both at room temperature and down to 100 K. The rotationalstate of C60 was tuned to a known state by an increase of theintermolecular interaction between C60 molecules induced byapplying external pressure, under which condition C60 first formscovalently bonded dimers, and thus slows down its free rotation,at low pressure and then forms a single-chain polymer structurewith a known molecular orientation at very high pressure. Bycarefully comparing the changes in the IFM modes for thesepeapods in the various states, we propose that the C60 moleculesinside SWNTs exhibit a ratcheted rotation with a preferred‘‘hexagonal orientation’’ due to the interaction between C60 andSWNT walls.

Results and DiscussionThe typical Raman spectrum of pristine C60-peapods obtainedusing NIR excitation (830 nm) at room temperature is comparedwith the spectra of a bulk C60 sample and of pure SWNTs in Fig.1. The spectrum of C60@SWNT can approximately be consid-ered as a weighted sum of the spectra of C60 and pure SWNT,with weak lines from the nested fullerenes and strong lines fromthe host SWNTs. It consists mainly of four characteristic regions:(i) the low frequency modes �200 cm�1, mainly radial breathingmodes (RBM) for SWNTs (15); (ii) the high-frequency rangebetween 1,500 and 1,600 cm�1, containing tangential modes (theG band) for SWNTs (15, 16); (iii) fullerene modes near 1,470cm�1, i.e., the most prominent Ag (2) mode for nested C60, whichis upshifted from its position at 1,469 cm�1 in pristine C60 to1,474 cm�1 due to the interaction between C60 and tubes (15);(iv) and the intermediate frequency modes (IFMs) from 200 to1,000 cm�1. Here, we are particularly interested in the IFMs. Itshould be noted that these Raman peaks are not simply a

Author contributions: B.L. designed research; Y.Z., B.L., L.W., D.L., S.Y., P.W., P.W., M.Y.,Q.L., B.Z., T.C., G.Z., T.W., B.S., and H.-K.M. performed research; Y.Z., B.L., T.C., T.W., and B.S.analyzed data; and Y.Z., B.L., B.S., and H.-K.M. wrote the paper.

The authors declare no conflict of interest.

1To whom correspondence may be addressed. E-mail: liubb@jlu.edu.cn or h.mao@gl.ciw.edu.

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superposition of the individual modes for fullerene and SWNTs,but instead there are significant splits and shifts. We first assignthese modes to either C60 or SWNTs or both by carefullycomparing with original Raman spectra of C60 and emptySWNTs from the literature (17). It is clear that the Raman modesat 582, 806, and 860 cm�1 belong to the host SWNT, while the710 cm�1 peak is a combination between a broad mode comingfrom the SWNTs and a sharp Hg(3) mode of the C60. Except forthese peaks, all other modes originate from nested C60. We notethat the most obvious changes relative to the spectra of the purecomponents are the split modes of Hg(1) and Hg(2), which havebeen changed from 270 cm�1 to 292, 306, and 326 cm�1 for theHg(1) mode and from 433 cm�1 into 434 and 452 cm�1 for theHg(2) mode, while the Hg(3) (710 cm�1) and Hg(4) (771 cm�1)modes keep the same frequencies as in bulk C60. These featuresare remarkably different from the results obtained using a blueexcitation laser (488 nm), for which these modes were not foundeven at low temperatures down to 5 K (14, 18). The Ramanspectra are; however, in good agreement with a recent theoret-ical prediction for peapods (12), indicating that we have indeedobserved intrinsic IFM vibrations for C60 inside SWNT by usingNIR excitation at room temperature.

The clear line splits indicate that the symmetry of C60 haschanged and that the molecules are not in a completely freerotational state inside SWNTs. This is consistent with recent INSand NMR measurements. The splitting is caused by the inter-action between C60 and its carbon environment, either theSWNT wall, the two C60 neighbors, or both. We know that theupward shift of the Ag (2) mode from its intrinsic position canbe related to the interaction between C60s and tubes, andcorresponds to an apparent compression by 1–2 GPa appliedpressure, or a cooling by several tens of Kelvin from the NMRresults, which implies that the interaction between C60 and tubesindeed exists (9, 15). Regarding the C60–C60 interaction betweenadjacent molecules, its strength depends on the filling factor ofC60 inside SWNT and it is thus expected that the interactionincreases with increasing filling ratio. However, theoreticalsimulations show that the Raman spectrum of C60 is only weaklydependent on the filling ratio, while the RBM modes of theSWNTs change remarkably. This study indicates that relative tothe intermolecular interaction the interaction between C60 andtubes is stronger and thus plays the main role in influencing thevibrational properties of trapped C60 (12). Similarly, a recentNMR study also suggests that the C60 intermolecular interactionis negligibly small compared with the interaction between the C60and the SWNT (10), supporting our discussion.

Let us analyze the IFM in more detail. According to previousRaman spectroscopy results for bulk C60, IFM modes �1,000cm�1 are mainly from radial modes of the C60 cage and can bebroadly divided into two groups, pentagonal radial modes �500

cm�1 [Hg(1) and Hg(2)] and hexagonal radial vibrations abovethis wavenumber [Hg(3) and Hg(4)] (13). It is worth noting thatin our experiment, splitting mainly occurs in pentagonal radialmodes while there is no obvious change in hexagonal radialvibrations, implying that anisotropic interactions act on therotating C60. Thus, the interaction between tube and pentagonson C60 is slightly larger than that between tube and hexagons atroom temperature. To clearly observe this phenomenon, wefurther carried out Raman measurements at low temperaturewith the results shown in Fig. 2. It is seen that the Raman spectraare quite similar at all temperatures. A striking feature is that therelative intensity ratio between the Hg(2)/Hg(4) modes increasesdramatically with decreasing temperature, clearly indicating arelative increase of the tube-pentagon interaction comparedwith the tube-hexagon interaction.

As C60 molecules rotate inside SWNTs, such anisotropicinteractions further indicate that C60s are not rotating freely buthave a preferred relative orientation, in which C60s are subjectedto the strongest interaction with the tubes (i.e., the lowestpotential energy) and thus slow down and settle into thisparticular state. Considering that the theory predicts that fixedfullerenes inside SWNT have either pentagon-to-pentagon ori-entations or hexagon-to-hexagon orientations, we believe thateither the pentagon or the hexagon orientational state is theenergetically favored state where C60s slow down. In the hexagonorientation, the equator consists of a ‘‘zig-zag’’ ring ofCOCACOCACOC bonds, and parts of six pentagons are closeto the wall. We illustrate both orientation states in Fig. 3.

From our Raman results that pentagons interact much morestrongly with the tube than hexagons, we conclude that C60prefers to ratchet in the hexagonal orientation, where pentagonshave a stronger interaction with the tube than hexagons, and thatthe hexagon-to-hexagon orientation is the most likely favorablestate.

We calculate the energy difference with all possible orienta-tions of C60 in SWNTs with different diameters. SWNT (9, 9),(10, 10), and (11, 11) have been chosen because the correspond-ing diameters are similar to the peapod in our sample (19). Itturns out that for C60 in SWNTs the energy varies very little withthe orientation. However, the hexagonal orientation has thelowest energy, as shown in Fig. 3, indicating that C60s rotate inthe SWNTs and prefer to ratchet in the hexagonal state, whichis consistent with our experiment.

To further confirm the hexagon-to-hexagon orientation, weforced the rotational state to change by introducing covalentbonds between two adjacent C60s in SWNTs. The Ag(2) mode ofnested C60 is down-shifted from 1,474 cm�1 to 1,469 cm�1 for

Fig. 1. Raman spectra of SWNT, bulk C60 and peapod samples excited by an830-nm laser at room temperature.

Fig. 2. Temperature dependence of the vibrational properties of peapods.(Left): Raman spectra of peapods between 200 and 800 cm�1 at the temper-atures 100 K (solid red curve), 180 K (dashed blue curve), and 260 K (dottedblack curve), respectively. Right: temperature dependence of the relativeintensity ratio Hg(2)/Hg(4) for the peapods. The straight line has been fitted tothe data.

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dimers, and further to 1,465 cm�1 for the single-chain polymeras discussed in detail in our previous work (15). The dimerizationprocess was induced by applying a gentle pressure of 5 GPa in ourstudy. The IFM modes of the Raman spectrum for dimericpeapods are shown in Fig. 4B, and for comparison we show thecorresponding data for pristine peapods in Fig. 4A and for thechain polymer peapods in Fig. 4C. It is important to notice thatthe hexagon radial modes Hg(3) and Hg(4) show specific finger-print changes after dimerization, such that the original lines splitinto lines at 710 � 725 cm�1 and 754 � 771 � 779 cm�1,respectively, due to formation of covalent bonds between C60molecules (arrows). This feature has not been observed inmonomeric peapods but is in good agreement with the split inbulk C60 dimers except for the line at 725 cm�1, which might bederived from the Hg(3) mode, and changed due to interactionbetween C60 and SWNT. At the same time the pentagon radialmodes Hg(1) and Hg(2) are almost the same as in monomericpeapods except that their intensities are significantly enhancedand that the Hg(1) mode splits even further with a peak at 267

cm�1, which is also a characteristic feature in dimeric bulk C60.This indicates that the interaction between C60 and SWNT wasalso enhanced during the dimerization process, in agreementwith the fact that the molecular orientation implies that bothhexagons and pentagons now directly face the wall (Fig. 3C).We also observe a significant enhancement of the formerlyweak modes at 362 and 389 cm�1, which are not visible in bulkC60 dimer, nor for pristine SWNT. These two modes arerelated to the filling factor of C60 in SWNT from theoreticalcalculations, such that the higher the filling factor, the strongerare these modes (12). Another peak appears at 642 cm�1, andmight be the infrared active vibration mode Hu(3) of C60.These features show that C60 dimers inside peapods feel adifferent interaction with their environment than those in bulkC60. This tendency can be understood from the fact thatfullerenes with pentagon-to-pentagon orientations cannotdimerize (polymerize) and that only hexagon-to-hexagon ori-entation can bring double bonds into sufficient close proximityto give rather easy dimerization (20).

Based on the above studies, we thus tentatively propose threepossible types of rotational dynamics for the confined C60: (i)quasi-free rotation with a preferred hexagon-to-hexagon orien-tation state, i.e., ratcheted rotation; (ii) uniaxial rotation withhexagonal symmetry; that is, a hexagon oriented perpendicularto the tube axis; (iii) vibrational rotation, similar to the situationin the simple cubic (SC) phase of bulk C60, in which one of thepreferred orientations is the hexagonal one. Because recent INSresults rule out the third case (10), we consider below the firstand second possibilities.

To distinguish between the two possible types of rotationaldynamics, we then linked further C60s together, forming singlechain polymeric structures inside the peapods by applying ahigher pressure, 23 GPa. The Raman spectrum obtained afterthis treatment is shown in Fig. 4C. It is clearly seen that thespectrum has many similarities to that for the dimer peapod.There is no significant change in the IFMs of C60, except for asharp peak at 443 cm�1, which might be attributed to the shiftedHg(2) mode found to be characteristic for the single-chainpolymeric phase. This is normally observed at 454 cm�1 butcould be shifted due to encapsulation inside SWNT (21). Anoticeable change was also found in the relative intensity at400–500 cm�1 near the Hg(2) mode, which might be due topressure induced shortening of nanotubes (22) or to defectscreated in the SWNT wall during the pressure-induced polymer-ization process (23). A similar effect has been found in C70peapods (24). We know that monomeric C60s may possibly rotateuniaxially around the tube axis, but such uniaxial rotation shouldbe difficult, if not impossible, for a long chain of polymeric C60.For dimers uniaxial rotation might still be possible, but because

Fig. 3. Schematic images of different molecular orientations. (A) pentagonorientation, (B) hexagon orientation, (C) polymer orientation. i, ii, iii: Thecalculated total energy of rotating C60 in SWNTs with different diameters, (9,9), (10, 10), and (11, 11). iv: A rotating path that covers all of the threeorientations mentioned in the text; rotation from c to a to b corresponding tothe horizontal axis in the energy spectra (i, ii , iii).

Fig. 4. Raman spectra and schematic images. Pristine peapod sample (A), of peapods containing C60 dimers (B), and of peapods containing single-chain polymer(C). Squares, characteristic peaks of pristine peapods; star, a peak only detected for peapods containing dimers or polymers; arrows point to vibration peaks,which are also observed in bulk C60 polymer crystal.

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of the similarities between the spectra for peapods containingdimers and longer chains we conclude that the second case, theuniaxial rotation model, probably is not correct.

After ruling out the uniaxial rotation, we thus think that themost probable rotation mode of the C60 molecules confinedinside SWNT is a quasi-free rotation with preferred ‘‘hexagonalorientation’’; that is, ratcheted rotation. Our result does notcontradict previous experimental data, including INS and NMRstudies, and can be used to explain these data very well. Anotherimportant point is that our data can also help us to interpret thepolymerization process of C60 in SWNTs. The polymer has beeneffectively generated inside the peapod under high pressure atroom temperature or at elevated high temperature (20). Thesynthesis of such a one-dimensional chain polymer is differentfrom that of the bulk chain polymer (orthorhombic phase),where nearest neighbor C60 molecules link up along the (110)direction in an fcc lattice, a process that can be induced only athigh temperature and high pressure. It is not surprising thatlow-temperature polymer synthesis is possible in peapods, be-cause the C60 molecules in peapods prefer to stop at the hexagonorientation, which is advantageous for the formation of covalentbonds between C60 molecules (20). Recently, the authors becameaware of an NMR study on the dynamic properties of C60 insideSWNT (29). We recommend reading this article as a comple-ment to our work.

ConclusionsIn summary, we have experimentally studied the rotationaldynamics of C60 molecules in C60@SWNTs peapods by usingNIR Raman spectroscopy of the IFMs. The rotational state ofC60 was tuned to a known state by applying high pressure, atwhich C60 molecules first form covalently bonded dimers andslow down their free rotation at low pressure, then form single-chain polymer structures with 2-fold symmetry at high pressure.The C60 molecules in nanotubes exhibit an unusual type ofratcheted rotation hindered by hexagon orientation. The char-

acteristic vibration of ratcheted rotation becomes more obviouswith decreasing temperature. This study has important implica-tions for the future utilization of peapods in nano-materialsbased on this type of carbon structures, and increased our basicunderstanding of the dynamics of a unique dimensionally con-strained crystal structure. In addition, it has provided an effec-tive method to understand confinement effects, which are im-portant for many applications in various fields, such as superhardand electronic materials, geology, and geophysics.

MethodsThe peapod sample was prepared using a vapor phase reaction synthesismethod, as reported previously elsewhere. The filling factor of C60 insideSWNT is �80% (15, 25). The dimer and single-chain polymer structures wereinduced by applying high pressures, 5 GPa and 23.5 GPa, respectively, at roomtemperature using a diamond anvil cell with Ar as the pressure medium asshown in our previous work (15, 26). These pristine and polymeric peapodsamples were characterized by using Raman spectroscopy (Renishaw inVia)with a NIR (830-nm) excitation laser at room temperature. The measurementson pristine peapods were performed in a range of temperatures varying from300 to 100 K. We used the Discover module of the Materials Studio software(Accelrys Software, Inc.), to calculate the energy difference with all possibleorientations of C60 in SWNTs with different diameters, we built our model onthe experimentally suggested parameters (27). The COMPASS (28) force fieldis used.

ACKNOWLEDGMENTS. This work was supported by the National ScienceFoundation of China (10979001, 10674053, 20773043, and 10574053), theCultivation Fund of the Key Scientific and Technical Innovation Project (2004–295) of the Ministry of Excellence of China, the National Basic ResearchProgram of China (2005CB724400 and 2001CB711201), the Program forChangjiang Scholar and Innovative Research Team in University (IRT0625), the2007 Cheung Kong Scholars Program of China, the National Found for Fos-tering Talents of Basic Science (J0730311), and the Project for Scientific andTechnical Development of Jilin province. The Swedish Research Council hasalso provided an exchange grant through the Swedish Research Links pro-gram. This material is based upon work supported as part of the EFree, anEnergy Frontier Research Center funded by the U.S. Department of Energy,Office of Science, Office of Basic Energy Sciences under Award NumberDE-SC0001057.

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