Synthesis of C60 intercalated graphite

Vinay Guptaa,*, Peter Scharffa, Katrin Rischa, Henri Romanusb, Rene Mullera

aDepartment of Physics, Ilmenau Technical University, Waimerstrasse, Ilmenau 98684, GermanybDepartment of Material Science, Ilmenau Technical University, Ilmenau 98684, Germany

Received 20 March 2004; received in revised form 10 May 2004; accepted 20 May 2004 by C.E.T. Goncalves da Silva

Available online 7 June 2004

Abstract

We report for the first time direct intercalation of C60 into graphite. In this new carbon solid, the two dimensional layers of

C60, stacked between every graphite layers (stage 1), showed long range ordering with average separation of 1.27 nm between

C60 layers as evident from transmission electron microscopy. An up-shift of Graphite Raman E2g2 mode and down-shift of

fullerenes Raman Ag2 pentagonal pinch mode was observed due to charge transfer from graphite to C60.

q 2004 Elsevier Ltd. All rights reserved.

PACS: 61.46. þ W; 68.08.Bc; 68.37.Hk; 68.37.Lp; 81.05.Zx; 81.07.De

Keywords: A. Fullerenes; B. Chemical synthesis; C. Transmission electron microscopy

The discovery of fullerenes [1] and subsequent mass

production [2] spurred intensive research to evaluate

physical, chemical and electronic properties of this material.

This resulted in the startling discovery that insulator

fullerenes become conducting upon intercalation with alkali

metals [3] and even superconducting at transition tempera-

tures up to 33 K [4]. A new class of superconductors can

also be obtained using Fullerenes intercalated graphite as

host. Such compounds have been predicted theoretically

[5–7]. Saito et al. [5] designed a model of C60-intercalated

graphite and suggested that it should be stable. They calculated

potassium co-intercalated C60-graphite compound to be a

promising candidate to exhibit superconductivity. Fuhrer et al.

[8] used an alternative approach by co-intercalation of C60 into

K-graphite and showed a resistive transition at 19.5 K but only

in a small fraction of the samples. This might be due to poor

arrangement of C60 into graphite. Since it may not be easy for

large C60 molecules to diffuse through small, closely spaced

and carbon bonded K atoms so large scale structural ordering is

not possible and therefore annealing was required to improve

ordering and to obtain superconducting transition. Moreover,

it was difficult to differentiate in their experiment whether the

intercalation is K or graphite induced and since K–C60

intercalation compounds are well known, so C60

intercalation in graphite could not be proved conclus-

ively. Here we have synthesised for the first time an all

carbon solid by directly combining two carbon allo-

tropes, i.e. C60 and graphite.

The type of graphite used in the present study was

detailed in Ref. [9]. C60 (99.95% pure) and graphite, in 2:1

weight ratio, were taken in a quartz tube (diameter: 1.1 cm

and length 5.9 cm) and vacuum sealed. The tube was put

into a furnace at 600 8C for two weeks. After the reaction,

the C60 intercalation graphite was cleaned first in toluene to

remove any absorbed C60 and then dispersed in ethanol in an

ultrasonic bath to make a homogenous suspension. A drop

of this solution was put on Quantifoil R2/2 grid and dried.

Transmission electron microscopy measurements were done

on Tecnia 20S-TWIN TEM operating at electron accelerat-

ing voltage of 200 kV.

High-resolution transmission electron microscopy

(HRTEM) image of the original graphite is shown in Fig.

1(a). The interlayer spacing was 0.337 nm. A HRTEM

image of intercalated graphite is shown in Fig. 1(b). The

average distance between two adjacent parallel layers was

1.27 nm. This material showed hexagonal symmetry as

ascertained by selected area diffraction image as shown in

0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2004.05.018

Solid State Communications 131 (2004) 153–155

www.elsevier.com/locate/ssc

* Corresponding author. Tel.: þ49-367-720-0774; fax: þ49-367-

769-3171.

E-mail address: drvinaygupta@netscape.net (V. Gupta).

Fig. 2. The radius of C60 can be taken as 0.355 nm [2]. The

Vander Walls diameter is 0.29 nm in the case of C60 as

detailed in Ref. [2]. By adding these values for stage 1

compound, the calculated C60 center-to-center distance is

shown in Fig. 3, in good agreement with the experimentally

obtained value.

Raman spectroscopy is a powerful tool for studying the

phonon characteristics of graphite intercalation compounds

(GICs) of both donor and acceptor type. In particular, the

stage dependence of Raman active E2g2 mode reveals that

charge density in the graphitic layers in contact with an

intercalated species is different from that of the graphitic

layers that are not in contact, hence graphite E2g2 mode split

for stages $ 2 and the original graphite E2g2 mode

disappears completely for pure stage 1 GICs [10]. In the

case of acceptor guest species, Raman active E2g2 mode of

graphite up-shifts from its 1582 cm21 position and exceeds

the 1600 cm21 value [10] but in the case of C60 intercalated

graphite, a low charge transfer and hence a low up-shift was

predicted by Saito et al. [5]. A Raman spectrum (blue laser,

488 nm) of the C60 intercalated graphite is shown in Fig. 4.

The graphite E2g2 mode up-shifted from its usual 1582 cm21

position for original graphite to 1589.1 cm21 due to electron

transfer from graphite to C60. C60 can behave as an electron

acceptor similar to alkali–metal intercalated fullerenes [3].

The pentagonal pinch mode (Ag2) of pristine C60 at

1469 cm21 [3] was down-shifted to 1446.1 cm21 (Fig. 4).

Such shifts have been observed previously and attributed to

the softening of the bond stretching modes due to electron

addition to the antibonding molecular orbitals [11]. Electron

transfer from graphite to C60, can partially fill the t1u level

[3] of C60 and contribute to the conductivity of C60 making it

highly conducting. Intercalation process here should be

similar to the alkali–metal intercalated fullerenes [3,12,13].

A disorder peak of graphite is also visible at 1370 cm21.

Finally novel C60 intercalated graphite, synthesized here,

can be potential host for new co-intercalation of alkali–

metals. Since alkali–metals can be easily accommodated

into large space between C60 molecules then C60 between K

atoms, due to their smaller size, a long range ordering of the

system corresponding to higher Tc than Fuhrer et al. [8] can

Fig. 1. (a) High resolution transmission electron microscopy image of the original graphite. (b) High resolution transmission electron

microscopy images of the C60 intercalated graphite, a C60 is shown by arrow.

Fig. 2. Selected area diffraction pattern of C60 intercalated graphite.

Fig. 3. Calculation of the C60 center-to-center distance in the

intercalated graphite.

V. Gupta et al. / Solid State Communications 131 (2004) 153–155154

be easily achieved in the bulk samples in reproducible

manner and can lead to better superconductors.

Acknowledgements

V. Gupta is thankful to Humboldt foundation for

financial support. V.G. role was decisive in this work.

P.S., K.R., H.R. and R.M.’s contribution was limited to

technical assistance.

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Fig. 4. Room temperature Raman spectra of C60 intercalated graphite.

V. Gupta et al. / Solid State Communications 131 (2004) 153–155 155