Toward Synthesis and Characterization of Unconventional C66 and

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Towards Synthesis and Characterization of Unconventional C and C Fullerenes Inside Carbon Nanotubes 66

68

Viktor Zólyomi, Herwig Peterlik, Johannes Bernardi, M#onika Bokor, Istvan Laszlo, Janos Koltai, Jeno Kurti, Martin Knupfer, Hans Kuzmany, Thomas Pichler, and Ferenc Simon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509755x • Publication Date (Web): 25 Nov 2014 Downloaded from http://pubs.acs.org on November 27, 2014

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The Journal of Physical Chemistry

Towards synthesis and characterization of unconventional C66 and C68 fullerenes inside carbon nanotubes Viktor Zólyomi,∗ † ‡ Herwig Peterlik,¶ Johannes Bernardi,§ Mónika Bokor,‡ István László, János Koltai,⊥ Jen® Kürti,⊥ Martin Knupfer,# Hans Kuzmany,¶ Thomas Pichler,¶ and Ferenc Simon¶ @ , ,

,

Physics Department, Lancaster University, LA1 4YB, Lancaster, United Kingdom, Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences, P. O. B. 49, H-1525, Budapest, Hungary, Faculty of Physics, University of Vienna, Strudlhofgasse 4, A-1090 Wien, Austria, University Service Centre for Transmission Electron Microscopy (USTEM), Technische Universität Wien, Wiedner Hauptstrasse 8 - 10 / 052, A-1040 Wien, Austria, Department of Theoretical Physics, Budapest University of Technology and Economics, Budafoki út 8., H-1111 Budapest, Hungary, Department of Biological Physics, Eötvös University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary, Leibniz Institute for Solid State and Materials Research, Helmholtzstrasse 20, 01069, Dresden, Germany, and Budapest University of Technology and Economics, Institute of Physics and Condensed Matter Research Group of the Hungarian Academy of Sciences, H-1521 Budapest, PO BOX 91, Hungary E-mail: [email protected]

Abstract

To whom correspondence should be addressed † Physics Department, Lancaster University, LA1 4YB, Lancaster, United Kingdom ‡ Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences, P. O. B. 49, H-1525, Budapest, Hungary ¶ Faculty of Physics, University of Vienna, Strudlhofgasse 4, A-1090 Wien, Austria § University Service Centre for Transmission Electron Microscopy (USTEM), Technische Universität Wien, Wiedner Hauptstrasse 8 - 10 / 052, A-1040 Wien, Austria Department of Theoretical Physics, Budapest University of Technology and Economics, Budafoki út 8., H-1111 Budapest, Hungary ⊥ Department of Biological Physics, Eötvös University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary # Leibniz Institute for Solid State and Materials Research, Helmholtzstrasse 20, 01069, Dresden, Germany @ Budapest University of Technology and Economics, Institute of Physics and Condensed Matter Research Group of the Hungarian Academy of Sciences, H-1521 ∗

We present compelling evidence pointing to the possible synthesis of unconventional C and C fullerenes in the interior of single-walled carbon nanotubes. The production proceeds from C -toluene/benzene clathrates encapsulated inside the nanotubes using heat-driven nano-testtube chemistry. All isomers violate the so-called isolated pentagon rule and are stabilized solely by the proximity of the wall of the host nanotube. We present detailed characterization of the unconventional fullerenes using Raman spectroscopy, C isotope labeling of the benzene molecules, transmission electron microscopy, X-ray diractometry, and rst principles calculations. Multiple isomers of both C and C are identied in the sample. We argue that our method opens the way to high-yield 66

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Budapest, PO BOX 91, Hungary

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synthesis of unconventional fullerenes.

e.g.

Thus SWCNTs act like nanoscale

reaction chambers, allowing for chemical reactions to take place within their conned interior. These reactions have been in the center

Carbon nanostructures are at the forefront of

of attention as of late since the dimerization re-

present day materials science due to their rich

actions of fullerenes and metallofullerenes have

physics and chemistry as well as their high ap-

been imaged

plication potential. In particular, single-walled

using transmission electron

NTs are not limited to the fusion of fullerenes

nanoscale chemical reaction chambers: they can

however, as recent experiments have shown that

be lled with various encapsulated molecules

e.g. hydrogenation of fullerenes can be achieved

which can undergo chemical reactions yielding

inside a SWCNT 16 . In this work we show that

A more uncommon example

the nanoscale chemical reaction chamber envi-

of carbon nanostructures are unconventional

ronment in the interior of SWCNTs is suitable

fullerenes, exotic fullerene molecules which violate the isolated pentagon rule:

in situ

microscopy 15 . Chemical reactions inside SWC-

carbon nanotubes (SWCNTs) 1,2 can function as

for the synthesis of unconventional fullerene

they con-

molecules which would normally be unstable

tain adjacent pentagons which destabilize the

but are stabilized by the SWCNT environment.

molecule 3 . Such unconventional fullerenes are

Usually,

typically stabilized by heteroatoms and do not exist in pure carbon form 47 .

Raman spectroscopy 13 and isotope engi-

neering 14 .

Introduction

a new material.

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DWCNTs

are

grown

from

C 60 -

peapods produced by either subliming C 60 in a sealed glass ampule containing SWCNTs 17 or

In the follow-

ing we present a combined experimental and

by mixing the tubes into a solution of fullerenes

theoretical study reporting the stabilization of

with a solvent 18 . However, it was recently re-

unconventional C 66 and C68 fullerenes inside

ported that inner tubes can also be grown from

carbon nanotubes via nano-testtube chemistry.

non-fullerene molecules such as benzene and

This chemistry is well established and is a way

toluene when these are retained inside the tubes

to stabilize reactive species and to lower the

using fullerenes 19 . Using 13 C isotope labeling of

formation energy of novel structures: here we

the solvents, the solvent related carbon fraction

prove for the rst time that C 66 and C68 isomers encapsulated inside nanotubes are stabi-

was found to be about 10 %. Therein it was not recognized that this value corresponds to

lized without any additional heteroatoms.

roughly 6 solvent related carbon atoms per C 60

One of the simplest examples of in-the-

in the inner tube, suggesting that inner tubes

nanotube chemistry is the synthesis of an in-

are formed from the fusion of one solvent per

ner SWCNT from fullerene molecules. This is

one C60 molecule.

achieved from the so-called peapods, SWCNTs

To better understand the growth mechanism

lled with fullerenes 8 . Apart from fundamental

of the inner tubes from the organic molecules

interest, peapods are thought to lead to appli-

and the fullerenes, we studied the reaction of

cations in nanodevices such as e.g. memory de-

the two components at intermediate tempera-

vices or nanomechanical oscillators 9 , and they

tures of 800 ◦ C. To our surprise, we found that

have been in the center of attention for numer-

an intermediate phase consisting of unconven-

ous reasons such as their possible use in spin-

tional fullerene molecules is formed as a pre-

tronics when using N@C 60 as the lling material 10 or their use as precursors to double-

cursor of the inner tubes. Here, we present our results from high-resolution transmission elec-

walled carbon nanotubes (DWCNTs). Anneal-

tron microscopy (HRTEM), diractometry, Ra-

ing of peapods to temperatures above 1200 ◦ C

man spectroscopy, and nuclear magnetic reso-

results in the formation of an inner tube from

nance (NMR) spectroscopy, along with a vibra-

the encapsulated fullerenes 11,12 , and the result-

tional analysis to show that the intermediate

ing DWCNTs have made possible the exten-

phase consists of C 66 (see Figure 1) and C 68 fullerenes. Such fullerenes necessarily contain

sive study of small diameter nanotubes using

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adjacent pentagons which are energetically unfavorable 3 and thus make these molecules quite unstable (as opposed to conventional fullerenes which obey the so-called isolated pentagon rule; this rule states that no two pentagons can touch each other on the surface of the fullerene molecule). However, heteroatoms either at endohedral locations 4,6 or covalently attached to the fullerenes can stabilize such unconventional fullerenes. Examples for the latter include the C64 H4 7 and C50 Cl10 5 fullerenes, where the heteroatoms are bound to the vertices of the adjacent pentagons. In our experiments there are no heteroatoms present, yet we conrm the synthesis of not one but three distinct isomers of C66 and an isomer of C68 , all of which are stable inside the host SWCNT. This suggests that the conned environment inside the SWCNT stabilizes the unconventional fullerenes. We argue that our method enables the high-yield synthesis of heteroatom-free unconventional fullerenes, allowing for extensive experimental studies of such exotic molecules. The high-yield synthesis of unconventional fullerenes inside SWCNTs is also important for practical reasons. Due to their asymmetric structure they possess nite dipole moments which makes it possible to utilize them as nanoscale oscillators if controlled by an external electric eld. C 59 N+ molecules have already been suggested as molecules that could function as high frequency nano-oscillators inside SWCNTs 20 . Since recent experimental studies have found that even C 60 is electromechanically coupled to the nanotube in a peapod 9 , the C66 molecules studied here should be of great use in nanoelectromechanical systems due to their inherent dipole moment and economic growth process.

Figure 1: Structure of the most stable isomer of the C66 molecule (left) and its Schlegel diagram (right). The carbon atoms and bonds in the trimer of adjacent pentagons are highlighted to emphasize the unconventional nature of this fullerene. This molecule possesses C s symmetry, signicantly lower than the I h symmetry of C60 . identied by the observation of their characteristic Raman modes, the use of isotope labeled solvents and the theoretical analysis of the vibrational modes. C 60 (Hoechst, Super Gold Grade C60 , purity 99.9%) was co-encapsulated with the organic molecules benzene (C 6 H6 ), chlorobenzene (C6 H5 Cl) and toluene (C7 H8 ) similarly to a previous work (all solvents 1 from Sigma-Aldrich) 19 . SWCNTs from a commercial source (Nanocarblab, Moscow, Russia) were opened by heating in air at 450 ◦ C for 0.5 h. The SWCNT material was suspended in the solvents which contained dissolved C 60 with a concentration of 1 mg/ml. This mixture was sonicated for 2 hours which results in the encapsulation of both the fullerenes and the organic molecules. The resulting material was ltered to separate the SWCNT from the solvent and dissolved fullerenes and it was sonicated again in the corresponding pure solvent with excessive amounts to remove any non-encapsulated C 60 , which was followed by a nal ltering. The resulting pieces of free-standing "Bucky-papers" were heat treated in dynamic vacuum at varying temperatures and durations. Highest yield of encapsulated unconventional fullerenes was achieved by annealing at 800 ◦ C for 1 hour. We

Experimental and theoretical methods The unconventional fullerenes, three dierent isomers of C66 and an isomer of C68 encapsulated inside SWCNTs are synthesized by heat treating an encapsulated mixture of C 60 and organic solvent molecules. The fullerenes are

Although toluene and benzene do not function as conventional solvents in our experiments, rather, they act as reagent materials, we refer to them as "solvents" throughout the paper for sake of simplicity. 1


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in 21 and references therein. Free standing lms of about 100 nm eective thickness, which is thin enough to avoid multiple scattering, were prepared by dropping from a suspension of the SWCNTs in acetone on a KBr crystal until a visible layer is formed. The KBr is dissolved in water and the oating SWCNT lms are transferred to a 200 mesh platinum TEM grid. Density functional theory (DFT) calculations were performed using the Gaussian 03 software package 22 with the B3LYP hybrid exchange-correlation functional in a 631G* basis set. A scaling factor of 0.975 was applied to the calculated spectra to match the PPM frequencies using C 60 as reference; the C60 reference was taken from measurements of C60 peapods to account for a frequency correction due to interaction between the nanotube and the fullerenes. DFT studies were only performed on the most stable isomers; these were chosen from all possible topologies based on the energetics of their optimized geometries according to the density functional tight binding (DFTB) method 23 . The required topologies were constructed using the topological coordinate method 24,25 . The Schlegel diagrams of the three most stable isomers of C 66 and C68 are shown in Figure 2.

also used 13 C enriched benzene ( 13 C6 H6 , Eurisotop, France) to track the benzene related carbons in the unconventional fullerenes through the isotope shifts in the Raman spectrum.

Summary of methods

X-ray diractometry:

X-ray diraction was measured with Cu K α radiation from a rotating anode X-ray generator and a pinhole camera, equipped with a two-dimensional, positionsensitive detector. Radial averages of the two-dimensional spectra yielded the scattering curves as a function of q = 4π/λ sin θ, with 2θ being the scattering angle and λ = 0.1542 nm being the X-ray wavelength. The strong increase in scattering intensity, that is always observed for SWCNT towards small q , was subtracted by a power-law. Raman spectroscopy was performed with a Dilor xy triple monochromator spectrometer excited with the lines of an Ar-Kr gas discharge laser.

Theory:

Raman spectroscopy:

Transmission electron microscopy:

High-resolution transmission electron microscopic (HRTEM) studies were performed on a TECNAI F20 eld emission microscope equipped with a Gatan Image Filter (GIF 2001) operated at 120 kV. The relatively low electron energy is a compromise between resolution and the inevitable radiation damage to the fullerenes. Electron transparent samples were prepared by dropping a suspension of the peapod materials in N,N-dimethylformamide on a holey carbon TEM grid. Nuclear magnetic resonance (NMR) spectroscopy was performed with the spin-echo technique in a 2 T magnetic eld at ambient conditions. The 1 H amount in the samples can be determined by integrating the NMR signal for long repetition times, when even the slowly relaxing nuclei contribute to the signal. Electron energy loss spectroscopy (EELS) was performed in a transmission geometry at a 170 keV in a purpose built spectrometer which combines both high energy (180 meV) and momentum resolution (0.6 nm−1 ). Details can be found

a.

b.

c.

Nuclear magnetic resonance:

d.

e.

f.

Electron energy loss spectroscopy:

Figure 2: Schlegel diagrams of the rst (a), second (b), and third (c) most stable isomer of C 66 , and the rst (d), second (e), and third (f) most stable isomer of C68 .


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X-ray diraction results

a)

In Figure 3 we show the X-ray diraction proles for a usual C60 peapod and for the C60 +toluene peapod material after heat treatment. It is well known that the Bragg peaks of the hexagonal SWCNT bundle structure (arrows in Figure 3) are modulated in the presence of the one-dimensional C 60 chain due to the form factor of the latter 26 . Namely, the rst Bragg peak around 4 nm −1 is suppressed and the second around 6.5 nm −1 is relatively enhanced. The rst Bragg peak is equally suppressed in the heat treated C 60 +toluene material, which indicates the presence of a onedimensional linear chain inside the SWCNTs. However the second Bragg peak is markedly dierent in this latter material: its intensity is reduced and a shoulder on the high q side appears as we show in panel b) of Figure 3. Both observations are compatible with the structure observed with HRTEM, i.e. with the presence of dimer-like structures which may form from C66 .

Empty-SWCNT

* Scattering Intensity (arb.u.)

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C60 peapod

C60+toluene peapod

1D-C60 chain

4

8

12 -1 q (nm )

16

b)

EELS results

6

7 -1 q (nm )

In panel a) of Figure 4 we show the electron energy loss spectra of the C 60 +toluene peapod sample before and after the 800 ◦ C annealing. EELS on C60 peapods 27,28 allows one to prove the presence of the encapsulated C 60 molecules by means of detecting a sharp onset of the C1s core level excitation spectrum (starting at around 284 eV), which is absent in the SWCNT reference material. This sharp feature is unique to the C60 fullerene and is related to its well dened electronic structure due to the high molecular symmetry 27 . The ngerprint of the encapsulated C 60 molecules is better observed by subtracting the EELS spectrum of the SWCNT reference from that of the peapod sample after an appropriate scaling. In panel b) of Figure 4 we show the result of this subtraction. The subtracted data for the C60 +toluene peapod sample shows a clear structure which resembles the EELS data of pure C60 , however for the annealed peapod sample, this well dened structure is lost. This

8

Figure 3: a) X-ray diraction prole for an empty SWCNT, the C60 peapod, and the heat treated C60 +toluene material. Arrows show the Bragg peaks of the hexagonal SWCNT bundle structure. The data are normalized by the graphite [002] peak around 18.5 nm −1 . The form factor of the 1D-C60 chain is also shown for comparison. b) Close up of the linear C 60 chain related peak for the two peapod materials. Note the appearance of a shoulder at higher q 's at around 7 nm−1 which suggests the presence of fullerene dimers.


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indicates that the highly symmetric structure of the C60 molecules is lost which supports the proposed reaction scheme between the fullerene and the toluene. The scaled subtraction also yields the number of carbon atoms which reside on the fullerenes and on the SWCNTs since the C1s EELS spectrum provides an absolute measure of the number of carbon atoms. We obtained that 37 ± 5% of the inner space of the SWCNTs is occupied by C 60 in the starting C60 +toluene sample. This value is to be compared to 65 ± 5%, which was found for encapsulating C60 's by the sublimation reaction 27,28 . It is known that the full inner SWCNT space is not available for the fullerenes. Thus the C60 amount in the present sample can be interpreted such that about half of the inner volume is occupied by the fullerenes and the rest by the toluene molecules.

a)

C60+toluene peapod C60+toluene peapod, annealed SWCNT reference

b)

C60+toluene peapod

Results

C60+toluene peapod, annealed SWCNT reference

284

286

288

290

292

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294

Electron energy (eV)

Figure 5: HRTEM micrograph of C 60 fullerenes encapsulated from toluene solution before (left) and after (right) 800 ◦ C vacuum annealing. Note the loose packing and the additional sidegroup like features for the sample with toluene and the close packing and spherical structures for the annealed sample as highlighted in the blow-up insets. The scale bar in the bottom right corner of the left hand image corresponds to a length of 5 nm.

Figure 4: a) Electron energy loss spectra of the C60 +toluene peapod before and after annealing and the SWCNT reference sample. Arrow indicates a narrow shoulder due to the encapsulated fullerenes around 284.5 eV, which feature is absent in the latter two samples. b) The EELS spectra of the two peapod samples after subtracting the background signal following appropriate scaling.

In Figure 5 we show the HRTEM micrograph for the peapod sample where the C 60 was encapsulated from toluene solution. Qualitatively identical result was observed when benzene was used. A loose packing of the encapsulated C 60 is observed with an average C 60 -C60 center to ACS Paragon Plus Environment

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center distance of 1.7(1) nm. In addition, some additional features are observed which resemble small planar molecules whose plane is parallel to the HRTEM axis of view. In peapods, which are prepared with the standard C 60 vapor phase method, a lattice constant of 0.97(2) nm 29 is observed with no additional features to those of spherical C 60 30,31 . This suggests that the solvent molecules separate the fullerenes in a one-by-one fashion, leading us to conclude that we observe an in-the-nanotube fullerenetoluene/benzene clathrate structure. This is supported by a quantitative Raman analysis of the amount of encapsulated fullerenes, which shows that in the presence of the solvent molecules about 50 % of the available volume is lled with fullerenes while the rest is taken up by the toluene/benzene molecules (see Ref. 19 ). The 1:1 ratio is further supported by 13 C isotope labeling of the solvents as discussed in Ref. 19 and mentioned above. In contrast to the unheated material we observe a close packing and the vanishing of the additional features after the 800 ◦ C vacuum annealing. This can be explained by a chemical reaction between the fullerenes and the toluene molecules which results in larger fullerenes. Several other facts support this identication which we discuss below. While the Raman spectra of these unconventional fullerenes dier signicantly from that of C60 , they contain sharp, well-dened modes which prove the presence of well-dened molecules. Furthermore, resonance behavior is found in the Raman spectra at specic energies which would be unlikely in an amorphous material. In order to gain insight into the composition of these unconventional fullerenes, we used selective isotope enrichment. We repeated the synthesis procedure with natural carbon C 60 mixed with 13 C enriched benzene molecules. (The isotope enrichment also served to conrm that the solvent does not escape from the tubes and occupies about half the inner volume; this was tested by annealing the sample at 1250 ◦ C where the lling material transforms into an inner tube inside the host SWCNT, in agreement with our previous work 19 .)

C60+toluene unheated 1466.7 *

C60+benzene

Raman signal (arb.u.)

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heated

1273.7

1466.0

1244.1

C60+toluene heated

1273.6 1289.4

1466.2

1235.4 1183.9

13

C60+ C benzene 1465.9

heated

1221.7 1262.1

1200

1300

1400

1500

-1

Raman shift (cm )

Figure 6: Measured and tted Raman spectra of the C60 encapsulated into SWCNTs with toluene or benzene. Spectra were taken with a 488 nm laser excitation and were normalized to the Raman G mode of the SWCNTs. Topmost spectrum shows the toluene-lled sample before the 800 ◦ C heat treatment; the spectra of the other two unheated samples (with natural carbon benzene and with 13 C-enriched benzene) are very similar (not shown). A weak, SWCNT related mode is marked with an asterisk at 1427 cm−1 . We also performed electron energy loss spectroscopy (EELS) on the annealed sample. The EELS spectrum of the C 60 molecule is very characteristic due to the high icosahedral symmetry of C60 , so much that even C 70 has a much less structured EELS spectrum. We found that the EELS spectrum looks nothing like that of a C60 molecule, which proves that whatever molecules are produced inside the SWCNT at 800 ◦ C, they have a lowered symmetry. The X-ray study also suggests that the C60 merged with the benzene/toluene molecules into a larger molecule, since we observe a dierent lattice constant than what is expected for C60 -peapods. In Figure 6 we show the Raman spectra of



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the starting and the annealed peapod materi-

the Raman spectra.

We must rst note

als in the range where the strongest fullerene

here that one cannot observe the modes of

modes are observed.

The spectra are domi-

toluene/benzene in the starting material due to

nated by the Raman D mode of the SWCNTs

the low density of the encapsulated molecules

at 1350 cm

and more importantly the lack of resonant Ra-

−1

(another SWCNT related mode −1

) and

man enhancement with the applied visible laser

the pentagonal pinch mode (PPM) of the C 60 at 1466 cm−1 , while a number of additional

excitations, hence the new modes cannot orig-

modes appear which we assign to the uncon-

served modes are relatively strong when the

ventional fullerenes. These new Raman modes

2.54 eV (488 nm) laser energy is used and dis-

emerge in the 1100-1300 cm

spectral range

appear rapidly for other laser excitations. For

upon heat treatment of the co-encapsulated

C60 , it is known that the HOMO-LUMO gap

is marked with an asterisk at 1427 cm

−1

organic solvent molecules and C 60 .

inate from residual solvent molecules. The ob-

is about 2.5 eV 34,35 thus the maximum Raman

At the

same time, the intensity of the 1466 cm

−1

C60

intensity corresponds to the resonant enhance-

PPM decreases relative to the SWCNT modes.

ment of the Raman signal. For other fullerenes,

When C70 fullerenes are co-encapsulated with

such as C70 , similar Raman enhancement is ob-

toluene/benzene, no new modes appear upon

served 34 , so the observed behavior is consistent

heat treatment, therefore the reaction appears

with the presence of fullerene molecules of some

to be specic for the C 60 fullerene. The new modes appear for heat treatments in a relatively

kind in the sample.

small temperature window:

for temperatures lower than 750 ◦ C and higher than 850 ◦ C the

eral observations. First, the absolute intensity

new modes do not appear. The optimum for the

the encapsulated C 60 , which indicates that they

occurrence of the new modes takes place at 800

are related. The fact that the PPM intensity

C heat treatment. If a sample with the new

decreases upon the heat treatment hints that

modes present is heated to temperatures above

some vibrational spectral weight is shifted to-

◦

The above conclusion is supported by sevof the new Raman modes is similar to that of

900 ◦ C, the new modes vanish. It is known that ◦

wards the other vibrational modes, although

C results in a grad-

no strict spectral weight conservation applies.

ual decay of the fullerenes and in a slow fusion

Second, the relatively sharp appearance of the

heat treatment above 900 into inner nanotubes 32 .

new Raman modes suggests that these originate from larger fullerenes. It shows that the reac-

These observations suggest that the new modes

originate

from

a

molecule

that

tion product is a well-dened molecule and not

is

a cluster of amorphous carbon.

an intermediate phase between the starting We also

Third, we performed a theoretical study of

found that toluene or benzene needs to enter

the unconventional fullerenes that are the prime

the tubes together with the fullerenes: a pea-

candidates for the molecules produced in our

pod material which was prepared by the vapor

experiments.

method and was sonicated in the solvent did not show the extra modes after the heat treatment.

vent based sample, each C 60 molecule gains an additional six carbon atoms on average. Since

This proves that a reaction takes place locally

the measurements show that the solvent and

between the two kinds of molecules inside the

C60 molecules alternate in the peapod, two sce-

SWCNTs. On the contrary, the new modes ap-

narios are expected: either each C 60 molecule

pear after annealing samples which were kept

incorporates exactly one of its two neighbor-

molecules and the inner nanotubes.

In the case of the benzene sol-

in air for 1.5 years after the co-encapsulation

ing solvent molecules and thereby forms a C 66

of toluene/benzene and C 60 , which shows that the otherwise volatile toluene/benzene can be

molecule, or a C 60 is merged with both neighboring solvent molecules which would result in

conned to the interior of the tubes with C 60 .

the formation of C 72 and a large amount of

We identify the new modes as vibrational

residual C60 . The latter scenario can be ruled out based on the HRTEM images which show

modes of larger fullerenes, as supported by


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Table 1: Summary of the theoretical results for the Raman active modes of C 66 and C68. Etot (meV) is the calculated total energy per carbon atom relative to that of C 60 according to the B3LYP functional with a 6-31G* basis set (with the zero point energy correction taken into account), d (D) is the dipole moment, while ω1 − ω8 (cm−1) are the calculated frequencies of the eight most Raman intense modes in the 11501300 cm−1 range, in order of descending intensity. Note that the relative energy for the most stable isomer of C68 is negative; this is perfectly ne and goes with the trend that the energy per carbon atom in fullerene molecules decreases as the size of the molecule increases, since this means the average curvature decreases and the structure approaches graphene (see e.g. Fig. 4 in Ref. 33). C66 Cs Etot 9.088 d 2.345 ω1 1184.7 ω2 1301.7 ω3 1303.4 ω4 1233.7 ω5 1234.9 ω6 1261.4 ω7 1176.6 ω8 1177.9

C66 C2v 11.808 0.678 1178.6 1268.7 1146.8 1257.7 1269.9 1241.9 1245.4 1175.3

C66 C2 C68 C2(1) C68 C2(2) 14.847 -1.107 2.176 2.854 2.776 2.519 1238.2 1202.2 1319.5 1244.8 1302.7 1315.9 1168.1 1189.3 1292.9 1189.9 1288.9 1261.5 1199.9 1244.8 1201.1 1274.3 1254.4 1245.7 1286.4 1289.5 1251.5 1294.3 1297.7 1316.0

that the size of the fullerenes does not vary signicantly. Therefore we expect that this sample contains C66 molecules. We expect a similar situation when toluene is used as a solvent, however, since toluene adds seven carbon atoms per C60 on average, the expectation is that C 66 and C68 molecules are formed in a one to one ratio. Based on these expectations, we focused on the theoretical study of the most stable isomers of C66 and C68. Since C66 and C68 have a total of 4478 and 6332 possible isomers (excluding enantiomers), respectively, we relaxed the geometry for each of them using the density functional tight binding (DFTB) method 23 . The most stable isomer of C66 has a Cs point group symmetry in which one of the pentagons in the two adjacent pentagon pairs is shared such that three pentagons lay next to each other in a trimer arrangement (see Figure 1); this result agrees with previous calculations in Ref. 33 . The second (C2v ) and third (C2) most stable isomers contain two separate pairs of adjacent pentagons, while all other isomers have more than two pairs. In the case of C68 we found that both the rst and second most stable isomers have a C 2 symme-

C68 Cs 9.536 3.731 1216.9 1304.4 1226.4 1190.6 1217.4 1204.2 1294.7 1202.9

try (these will be denoted as C 2(1) and C2(2), respectively) while the third most stable isomer has a Cs symmetry. We found that the most stable isomer is the one reported in Ref. 33 . The symmetry groups and the total energies of the aforementioned six molecules are listed in Table 1. For these we performed a full geometry optimization and calculated the Raman spectrum using rst principles density functional theory. The calculated frequencies are given in Table 1. Comparing the experimental data with the calculations it seems that several dierent isomers appear in the measured samples, one of which can be identied as the most stable, Cs symmetry isomer of C66. The detailed analysis leads to the conclusion that both samples contain more than one isomer, with the benzene solvent based one containing two isomers (both of them C66) and the toluene based one containing three (two isomers of C 66 and one of C68). The 1183.9 cm−1 and 1235.4 cm−1 lines in the toluene based sample originate from the Cs isomer of C66. The 1244.1 cm−1 line in the benzene based sample and the 1289.4 cm −1 line in the toluene based sample stem from the C 2 isomer of C66 and the C2(2) isomer of C68, re-


9


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Page 10 of 17

spectively. The last line in each samplewhich

out. Strictly speaking the intermediate prod-

are almost at the same frequency, hence we expect that they originate from the same isomer of

uct which we identify as C 66 and C68 molecules could be anything between C 60 and short nan-

C66 cannot be identied suggesting that these

otubes. Mass spectroscopy would in principle

originate from another isomer of C 66 not stud-

allow us to draw a more denitive conclusion

ied here.

Likewise, we can conclude that the

but this unfortunately cannot be performed

C2v isomer of C66 and the C2 (1) and Cs isomers of C68 are not present in the sample.

since removing the encapsulated molecules from

To summarize, we identify four out of the six

ertheless the amount of indirect evidence avail-

measured Raman lines, two of which appear to

able to us makes for a compelling case for the

the nanotube is not possible at this time. Nev-

originate from the C s isomer of C66 , while the

synthesis of C66 and C68 , based on arguments

third very likely stems from the C 2 isomer of C66 and the fourth from the C 2 (2) isomer of

as follows.

C68 ; the other two measured lines come from

tional fullerenes are normally stabilized by het-

another, unidentied isomer of C 66 . The assign-

eroatoms attached to the energetically unfa-

ment of isomers is summarized in Table 2. We

vorable neighboring pentagon vertices. This is

emphasize that the Raman spectra combined

needed because the vertices in the joining of

As mentioned in the Introduction, unconven-

with the other experimental data together form

neighboring pentagons have a strong sp 2 bond

compelling evidence of the synthesis of C 66 and C68 .

strain due to the unfavorable bond angles, and

Table 2: Assignment of the measured Raman lines of the unconventional fullerenes to the isomers of C 66 and C68. Column ωexp (cm−1) lists the measured frequencies. In the last column, u.i. stands for unidentied isomer. The 1273.7 cm−1 line of the benzene solvent based sample and the 1273.6 cm −1 line of the toluene solvent based sample is expected to originate from the same isomer.

is given to these carbon atoms by covalently at-

solvent

ωexp

isomer

benzene

1244.1

C66 C2

benzene

1273.7

C66 u.i.

toluene

1183.9

toluene

1235.4

C66 Cs C66 Cs

toluene

1273.6

C66 u.i.

toluene

1289.4

C68 C2 (2)

this strain is removed by the sp 3 character that tached heteroatoms 5,7 . We found no evidence that heteroatoms are attached to the fullerenes, which leads us to conclude that the nanoreactor interior of the SWCNTs already stabilizes the unconventional fullerenes. We considered the presence of hydrogen on the vertices joining the pentagons as it is a candidate

for such a functional group 7 . However, we can exclude the presence of such hydrogen for the following reasons:

i) the molecular structure

shows that there is not much space for such functional groups inside the SWCNT, ii) we

observed no Raman modes around 3000 cm −1 where the C-H modes are expected to appear,

iii) 1 H NMR measurements did not nd evidence of an excess amount of protons in the material investigated, suggesting that the amount of hydrogen in the annealed sample is negligible.

In the case of the chlorobenzene solvent

Discussion

it may be conceivable that Cl atoms stabilize

Above we presented experimental and theo-

therefore we can safely rule this possibility out

the molecule but that would require even more space inside the nanotube than hydrogenation, as well.

retical evidence of the formation of unconventional fullerenes inside SWCNTs.

Beyond the apparent lack of heteroatoms,

It must be

two more interesting observations deserve fur-

noted that much of the evidence is indirect

ther discussion.

and therefore other possibilities cannot be ruled

First, the HRTEM images

clearly show that the C 66 /C68 molecules form


Page 11 of 17

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dimers inside the SWCNT. While it cannot be completely excluded that neighboring C 66 /C68 molecules may form dimers through covalent bonding similar to C60 dimers, it is more likely that dimerization takes place through dipoledipole interaction. The dipole moment is between 0.68 and 3.73 Debye at the B3LYP level for the three most stable isomers of C 66 /C68 (see Table 1) which is more than sucient to keep a pair of C66 /C68 together. Second, the selectively isotope enriched samples exhibit some interesting features. When 13 C benzene and natural fullerenes are encapsulated, the PPM does not shift at all, while the 1244.1 cm−1 and 1273.7 cm−1 modes do shift, but not equally: the 1244.1 cm −1 mode shifts to 1221.7 cm−1 which corresponds to 1.8 % isotope shift or 44 % enrichment and the 1273.7 cm−1 mode shifts to 1262.1 cm −1 , corresponding to 0.91 % isotope shift or 22 % enrichment. Since we cannot completely rule out that some residual C60 is present in the sample, the most straightforward explanation why the PPM does not shift is that the peak dominantly originates from residual C60 , which of course does not shift since only the solvent was isotope enriched. This is plausible if we assume that there is more than just the fraction of a percent of residual C60 in the sample because, while the PPM-like modes of the unconventional fullerenes are at similar frequencies as the PPM of C 60 , their absolute intensity should be overwhelmed by that of the PPM of C60 due to the high symmetry of the latter molecule. However, this still does not explain why the two modes of C 66 shift by dierent amounts, neither of which agree with the level of enrichment (9 %). This unexpected isotope shift suggests that the way the benzene molecules incorporate into the C60 molecules may not be random, as if it was random then the isotope shift would be the same for every mode due to averaging over the random distribution of isotopes. Rather, this mode-dependent isotope shift suggests that the benzene molecule incorporates into the C 60 molecules the exact same way all the time, and these atoms move with greater amplitude in the normal mode than the other atoms of the C 66 molecule. While this explanation is supported

by the calculations in that the atoms do indeed tend to move with largely uneven amplitudes according to the normal modes of the most intense Raman lines in the relevant frequency region, it is dicult to prove quantitatively since we do not know the exact isotope distribution in the enriched C66 . (Below we study one particular arrangement to illustrate that the isotopic frequency shifts are not the same for every mode if the isotope distribution is not random, but one would need to know the exact arrangement of isotopes to arrive at a quantitatively conclusive result based on this argument.) Alternatively, it might be possible that, due to the heavier solvent molecules involved in the growth reaction, the isomers formed in the sample using the isotope enriched solvent are slightly dierent from the ones formed in the original sample. If so, the base frequencies are already dierent, which would explain why the isotope shift of the frequencies does not correlate with the enrichment level of the sample. What is even more interesting is that the intensity ratio of the two C 66 modes also changes with isotope substitution, which naively suggests that the normal modes change enough such that the Raman intensities are inuenced as well, which is surprising. It may be possible however that the ratio of the two isomers in the SWCNT changes when isotope enriched solvent is used, which would provide a straightforward explanation to the change of the intensity ratio. Identifying the isomers

We identify four out of the six measured new Raman lines, two of which appear to originate from the Cs isomer of C66 , while the third very likely stems from the C 2 isomer of C66 and the fourth from the C2 (2) isomer of C68 ; the other two measured lines come from another, unidentied isomer of C66 . This conclusion is straightforward to reach by comparing the measured and calculated Raman lines as follows. We compare all the peaks in the relevant frequency range in the calculated Raman spectrum with the measurements. Whenever there is a close match for a Raman line, we consider the fullerene isomer in question as a can-


11


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didate to explain the measured lines. If one of the calculated lines is missing from the experiments, then we assume that the line is too weak to be visible hence all other calculated lines with a lower intensity will be neglected as well. If, however, the missing line falls into the frequency range of the D band of the nanotube, then we can safely assume that the line is not visible in the measurements due to it being overwhelmed by the D band, hence Raman lines with weaker intensity can still be considered. To illustrate this identication method let us consider the example of the C s isomer of C66 . Its most intense line (at 1184.7 cm −1 ) is extremely close to the 1183.9 cm −1 measured line. The second and third most intense modes can be ignored as they fall into the D band regime. The fourth and fth most intense modes have nearly the same frequency, falling very close to the measured line at 1235.4 cm −1 . The remaining modes have much weaker intensities and are neglected. Thus we assign two of the measured Raman lines to the Cs isomer of C66 . The rest of the peaks can be assigned to one of the isomers following similar arguments. In one case we nd no match with the calculations leaving us to assume that it comes from an isomer we did not take into account in the DFT calculations. This line is present in both samples, hence assuming that they come from the same fullerene it has to be an isomer of C 66 . The take home message of this analysis is that for each of the three most stable isomers of C66 and C68 only a handful of the Raman active modes have a high enough intensity to be visible in the experimentally relevant frequency range and most of those are covered by the D band, implying that it is not possible to explain the measurements with a single isomer of either molecule. Therefore the samples must contain multiple isomers, most of which we identify in Table 2.

Page 12 of 17

Figure 7: Schlegel diagram of the most stable isomer of C66 with a 13 C content of 9.1 % (i.e. 6 out of 66 atoms are 13 C ). The 13 C isotopes are highlighted. random, we calculated the vibrational modes of the most stable isomer of C 66 with a 13 C content of 9.1 % (i.e. 6 out of 66 atoms are 13 C ) in a specic arrangement. We chose six carbon atoms in the C 66 molecule such that by removing them the dangling bonds of the remaining 60-atom molecule could be trivially connected into the C60 molecule without having to rearrange the existing bonds. (This is possible in more than one way, we chose one particular arrangement just to illustrate how the modes shift when the isotope distribution is xed.) These 6 atoms were replaced with 13 C while the rest of the atoms were 12 C. We compared the frequencies of the vibrational modes of the enriched molecule and the original C 66 . The results are summed up in Table 3. The last column of Table 3 is the virtual 13 C content for each mode: this number is the percentage 13 C content that the molecule should have in the case of a fully random isotope distribution (in which case all atoms could be considered to have the same average mass) in order for the given mode to exhibit the particular iso-

Isotope shift analysis

In order to illustrate the inhomogeneity of the isotopic downshift of the vibrational frequencies of C66 when the isotope distribution is not


12

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tope shift found in this calculation. As it can be

seen, this virtual 13 C content is far from being

Table 3: Comparison of the calculated vibrational frequencies of natural C 66 and 13 C enriched C66 for the most stable isomer of C66 in the frequency range 11501300 cm−1. ωnat (cm−1) is the calculated vibrational frequency of the natural carbon C66 molecule, while ωenriched (cm−1) is the calculated vibrational frequency of the 13C enriched molecule, and ωshif t (cm−1) is the isotopic frequency shift. V13C (%) is the virtual 13C content as calculated for each mode; this would be constant if the 13C isotopes were randomly distributed.

a constant value. Due to the xed positions of

the 13 C isotopes, the normal modes in which the heavier isotopes move less will exhibit a smaller shift, while the rest will exhibit a larger shift. In extreme cases this leads to either very large or very small (in one case even negative) virtual 13 13

C content. (Note: the average of the virtual C content of the modes roughly corresponds

to 9.1 %, as expected.) These results illustrate

that if the spatial distribution of 13 C isotopes in a 13 C enriched molecule is not random, then

the dierent vibrational modes will not exhibit the same relative isotope shift that one would expect in the case of a random isotope distribution. Since we see in our measurements that

ωnat

ωenriched

ωshif t

V13C

1154.7

1141.6

13.1

28.1

1155.5

1153.1

2.4

5.1

1158.2

1154.5

3.7

7.8

1168.7

1164.4

4.2

8.8

1177.9

1170.6

7.3

15.2

Conclusions

1176.6

1175.0

1.6

3.3

1176.6

1177.6

-0.9

-1.9

1184.7

1177.6

7.2

14.8

We have demonstrated that an intermedi-

1200.3

1196.7

3.7

7.4

ate growth product is formed inside single-

1203.7

1196.6

7.1

14.4

walled carbon nanotubes by co-encapsulating

1212.1

1205.9

6.2

12.5

C60 fullerenes together with organic solvent

1228.7

1217.8

10.9

21.8

two separate Raman active modes of C 66 exhibit dierent relative isotope shifts, we argue

that the spatial distribution of 13 C isotopes in the C66 molecules in our experiments is not random.

molecules and annealing the sample at 800 ◦ C

1228.7

1228.5

0.2

0.4

for 1 hour. Through a combination of dierent

1233.7

1225.3

8.4

16.7

experimental methods we have presented evi-

1234.9

1232.7

2.2

4.4

dence that the resulting cage-like molecules are

1236.5

1217.8

18.6

37.4

1236.5

1228.5

7.9

15.7

1244.6

1239.5

5.1

10.0

1246.7

1243.5

3.2

6.3

1248.1

1245.4

2.7

5.2

1255.4

1254.9

0.5

1.0

1261.4

1258.9

2.5

4.7

1273.2

1270.7

2.5

4.8

1281.2

1278.8

2.4

4.5

1282.4

1273.5

8.9

17.1

1282.4

1281.8

0.6

1.2

1288.2

1273.5

14.7

28.2

1288.2

1281.8

6.4

12.1

1298.9

1297.3

1.7

3.1

well-dened structures. We have identied candidates for the observed molecules as isomers of C66 and C68 unconventional fullerenes based on the ratio of carbon atoms between the starting fullerenes and solvent molecules. This identication is conrmed by rst principles calculations on the Raman spectrum of the three most stable isotopes; we also expect an isotope of C 68 to be present in the sample when toluene is used as a solvent.

We found that these molecules

have a nite dipole moment which makes them useful in nanoelectromechanical applications of peapods such as nanoscale oscillators. We have also shown that no heteroatoms are attached to the unconventional fullerenes inside the nan-



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otube, indicating that the conned environment of the interior of the nanotube is able to stabilize these molecules on its own. While the evidence found for the formation of C 66 and C68 is indirect and therefore ambiguous, the combination of spectroscopical ndings presented here form a compelling case in favor of our interpretation of the experiments. This novel stabilization of unconventional fullerenes enables highyield growth of heteroatom-free unconventional fullerenes in a controlled environment, thereby opening up a new possibility to study in-thenanotube chemistry and to ne tune the electronic properties of the carbon nanotubes with the help of encapsulated materials.

fullerene encaging a scandium dimer. ture 2000, 408, 426427.

Page 14 of 17

Na-

(5) Xie, S.-Y.; Gao, F.; Lu, X.; Huang, R.G.; Wang, C.-R.; Zhang, X.; Liu, M.-L.; Deng, S.-L.; Zheng, L.-S. Capturing the Labile Fullerene[50] as C 50 Cl10 . Science 2004, 304, 699670. (6) Shi, Z. Q.; Wu, X.; Wang, C. R.; Lu, X.; Shinohara, H. Isolation and characterization of Sc2 C2 @C68 : a metal-carbide endofullerene with a non-IPR carbon cage. Angew Chem Int Ed Engl. 2006, 45, 2107 2111. (7) Wang, C. R.; Shi, Z. Q.; Wan, L. J.; Lu, X.; Dunsch, L.; Shu, C. Y.; Tang, Y. L.; Shinohara, H. C 64 H4 : Production, Isolation, and Structural Characterizations of a Stable Unconventional Fullerene. J. Am. Chem. Soc. 2006, 126, 66056610.

Acknowledgement Work supported by the Hungarian State Grants (OTKA) CNK80991, K81492, and K84078, the Austrian Science Funds (FWF) project Nr. P21333-N20, by the European Research Council Grant Nr. ERC-259374-Sylo, the Marie Curie ERG project Carbotron (PERG08-GA2010-276805), and by the New Széchenyi Plans Nr. TÁMOP-4.2.1/B-09/1/KMR-2010-0002 and TÁMOP-4.2.1/B-09/1/KMR-2010-0003.

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Graphical TOC Entry

C66 molecule.


17

Toward Synthesis and Characterization of Unconventional C66 and

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