Indian Journal of ChemistryVol. 31 A&B, May 1992, pp, F92-F99

Studies on structural and electronic properties of undoped and dopedfullerenes C60 and C70

C S Sundar", Y Hariharan, A Bharathi, V S Sastry, G V N Rao, J Janaki, T Geethakumari, T S Radhakrishnan,A K Arora, T Shakuntala, M Yousuf, P Ch Sahu, N Subramanian, V S Raghunathant & M C Valsakumar

Materials Science Division, Metallurgy Divisiont, Indira Gandhi Centre for Atomic Research, Kalpakkam,India 603 102

Received 20 February 1992

This paper presents an overview of the experimental studies on fullerenes currently underway inthe Materials Science Division, Kalpakkam. These include the synthesis of fullerenes by arcing ofgraphite electrodes, followed by the chromatographic separation of C60 and C70 and their dopingwith alkali metals. Doping of C60 with K and Rb is observed to lead to superconductivity with trans-ition temperatures of 18 K and 28 K respectively. Preliminary results of several ongoing studies, inparticular the studies on structure of C70 as a function of pressure and temperature, are reported.

Kroto et al,' in 1985 discovered the existence ofstable carbon clusters of 60 atoms in the mass spec-trum of laser ablated graphite, for which they pro-posed a soccer ball structure and termed it as buck-minsterfullerene after the designer of geodesicdomes. The recent synthesis of macroscopic quan-tities of fullerenes by Kratschmer et aU by strikingan arc between graphite electrodes in a helium at-mosphere, has generated a flurry of interest in theclosed caged carbon molecules C60, C70 and in thestudy of their solid state properties. The study offullerenes received further impetus with the initialdiscovery of metallicity' in alkali doped C60 and thesubsequent announcement of superconductivity"with a T, = 18 Kin K doped C60. Currently, thestudy of fullerenes is an active area of research inseveral fronts: (i) studies on various methods of pro-duction with high yields"; (ii) studies on the structureof C60 and the structural transformations as a func-tion of temperature? and pressure"; (iii) experimen-tal and theoretical investigations of the electronic=?and vibrational properties 10, II , and studies on thedoping of fullerenes with alkali atoms and other ele-ments in order to obtain higher T,12-17.

In this paper, we present in detail, our experiencewith respect to synthesis and doping of fullerenes.With the availability of macroscopic quantities ofsamples, several experiments on the study of super-conductivity in the doped samples and studies onthe structural properties of pristine C60 and C70have been initiated. This paper presents a status re-port of these experiments.

Materials and Methods

Synthesis of fullerenesThe synthesis of fullerenes involves first the pro-

duction of carbon spot, which is mainly done byeither resistive heating of graphite electrodes or

SYNTHESIS AND DOPING OF FULLERENES

ARCING BETWEEN GRAPHITE ELECTRODESIN HELIUM ATMOSPHERE

• Pressure: 200 torr, Static• Arcing Condition: 20V, 200A

fCARBON SOOT 'MSHED IN DI-ETHYL ETHER,EXTRACT FULLERENES FROM THE CARBONSOOT USING A TOLUENE SOXHLET

tCHROMATOGRAPHIC SEPARATION OF C-eoC-70 USING NEUTRAL ALUMINA

• Eluent for C-eO: 5$ Toluene-n Hexane• Eluent for C-70: 20$ Toluene-n Hexane

tEVAPORATION OF MAGENTA COLOURED C-eoAND REDDISH BROWN C-70 SOLUTIONSTO OBTAIN POWDER SAMPLES

tDOPING OF C-eo WITH K & Rb BY VAPOURPHASE REACTION METHOD

• Sample handled within an Ar glove bag

Fig. 1- Flow chart for the synthesis and doping of fullerenes

SUNDAR et al.: STRUCTURAL & ElECTRONIC PROPERTIES OF UNDOPED & DOPED C60 & C70 F93

"contact arc" between graphite electrodes2,5,18,19.Other methods such as burning of benzene in flamehave also been reported 20. The soot containing var-ious carbon clusters is then separated in a chroma-tographic column to obtain C60 and C70• The flowchart for the synthesis of fullerenes and their dopingwith alkali metals is shown in Fig. 1.

Arcing of graphite electrodesThe schematic diagram of the apparatus used for

the production of carbon soot is shown in Fig.2. Itconsists of a pair of water-cooled graphite elec-trodes between which an arc is struck in a heliumatmosphere. The·electrodes are covered by a water-cooled copper hood in which the soot gets collectedand this whole assembly is enclosed within a bell jar.The bell jar which is connected to a rotary pumpwith liquid nitrogen trap can be evacuated and re-filled with helium to the desired pressure. An im-portant design feature of this set up is that one of theelectrodes can be moved inside the vacuum/heliumatmosphere by a spring loaded arrangement. Theelectrodes are connected to a low voltage (40 voltsrnax), high current (400 A max) AC power supplywith the live end connected to the stationary elec-trode. In a typical arcing run with this apparatus ap-proximately 3 em of 10 rom diameter graphite rodgets consumed in - 2 hr resulting in - 0.3 g ofsoot. In order to increase the output another versionhas been designed and fabricated in which the mov-able electrode can traverse 20 em through a Wilsonseal arrangement. The schematic diagram of this ap-paratus is shown in Fig.3. In this apparatus each op-eration lasting 4 hr yields 10 g of soot.

During the operation of this apparatus, the influ-ence of various parameters such as the helium .pres-

TO VAC I"~

Ic:RNTHFIl. \I I M:

TO VRC. I"UI'II"

Fig. 2-Schematic diagram of the graphite arcing apparatus

VI~W IiLR55

Fig. 3-Schematic diagram of the modified graphite arcing apparatus. In this version, the graphite electrodes can be moved by20 em through a Wilson seal arrangement.

F94 INDIAN J CHEM, SEe. A& B, MAY 1992

sure, the nature of graphite rods, the operating cur-rent and voltage, etc., on the quality and yield ofsoot have been tried. Helium pressure has been vari-ed between 100 and 300 torr with the best resultsobtained for 200 torr. Both static and dynamic (con-trolled leak rate) helium conditions have been triedand best performance was seen under the former.The experiments on the variation of operating vol-tage indicated that for 10 mm graphite rods bestperformance was seen under the operating condi-tion of 20 volts, 200 A. Smaller diameter rods (3mm) could be operated with lower voltages (10volts). The performance of this system improvedsignificantly after the incorporation of a perforatedtantalum sheet above the electrodes (see Fig.2). Theexact reason for this as to whether it is due to theavoidance of photo-disintegration" or due to thelonger retention of the carbon vapours in the regionof the arc, thus enabling the formation of fullerenemolecules-? is not clear at the moment.

Separation of C60 and C70The soot collected from the arcing apparatus is

washed in diethyl ether to get rid of possible hydro-carbons and then introduced in the Soxhlet appara-tus containing toluene. In - 24 hr of operation, areddish brown solution containing fullerenes in tol-uene was obtained. This was concentrated by eva-poration in a distillation apparatus at 120°C.Chromatographic separation of C('o and C70 wasachieved using a column, I meter long and 3.5 emdiameter, in which - 500 g of neutral grade alumi-na was loaded. The alumina was heat treated at20(tC for 24 hi"to get rid of all the moisture beforepacking into the chromatographic column. The co-lumn was conditioned with n-hexane and the Sox-hlet extract (5% solution in n-hexane) was pouredinto the column. A clear separation of purple andbrown bands corresponding to C60 and C70 wasseen. Cf>(Iwas eluted using a 5% toluene-n-hexanemixture resulting in the collection of a clear magentacoloured solution. This was followed by the elutionof a clear solution, after which the reddish brownC70 solution was eluted out using a 20% toluene-a-hexane mixture. The magenta solution was evapo-rated to remove n-hexane and toluene and blackcrystallites of C60 were obtained. Under an opticalmicroscope, several transparent reddish crystalscould be seen. Scanning electron microscopic stud-ies, carried out using Philips model PSEM-501 mic-roscope, clearly indicated the cubic morphology ofthese crystals with an average size of 40 urn. Theevaporation of the reddish brown solution C70 intoluene yielded needle shaped black crystallites.The typical yield of Coo and C70 powder together

was observed to be - 5%, with C60 and C70 in theratio of 80:20. For various arcing conditions that wehave tried, we do not observe a 100% yield of C60ashas been reported in ref. 19. The .chromatographi-cally separated Coo and C70 powders have beencharacterised and used in further studies on alkalidoping of fullerenes. In addition to the preparationof powder samples, thin films of C60have been dep-osited on glass and NaCl substrates by subliming theC60 powder by heating in vacuum at 500°.

Results and Discussion

CharacterisationSeveral experimental techniques have been used

to characterise and study the fullerenes. These in-clude mass spectrornetry+", optical absorption inthe infra-red, visible and ultraviolet+", X-ray dif-fraction?", Raman spectroscopy+", and NMR26. Inour experiments, the chromatographically separat-ed C60and ClO have been characterised by ultravio-let-visible absorption spectroscopy and X-ray dif-fraction.

The optical absorption spectrum was recorded inthe U'V-visible range for the separated C60 and C70dissolved in n-hexane using a Shimadzu, model UV-240, spectrometer. The absorption spectrum re-corded at room temperature in the range of 300nm-700 nm is shown in Fig.4. The spectrum of C60shows a dominant absorption band at 320 nm and adouble-hump structure centred at 404 and 408 nmrespectively. In the case of C70, broad humps areseen at 550, 470 nm and in the near UV region at378, 359 and 331 nm. The dominant spectral fea-tures in C60and C70are in agreement with those re-ported by Ajie et a/.24. As a test of the efficacy ofchromatographic separation, in particular the possi-ble contamination of Coo in C70' the absorptionspectra were recorded with higher sensitivity in therange of 380-430 nm (see Fig.5). The characteristicdouble hump structure seen in the absorptionspectrum of C60 can not be discerned in the spec-trum of C7O'On the basis of this, the content of Coo1.1r.------,-----·-r-·--·----,----

wuZ..m0:o'"'"..

500 -----600----·-70U

WMELEI'!GTH (nml

- C60

- - - C70

.- --.---'---LOO

Fig, 4-Absorption spectra of C611 and C70 in n-hexane in therange of 300-700 nm

SUNDAR etaL: SlRUCTURAL & ElECfRONIC PROPERTIES OF UNDOPED & DOPED c., & C711 F95

0.4,.-----,.--------,

\\

~ \\ ~~~ 1-I-

ViZWI-

Z

10 20 25 30152 e (Degree)

Fig.6-X-ray diffraction pattern of C~{).The diffraction lineshave been indexed to fee structure

in the chromatographically separated C70 is estimat-ed to be < 2%.

The icosahedral C60 molecules have been shown"to crystallise in the fcc structure with completeorientational disorder at room temperature; theorientational order develops at temperature lessthan 249 K. The room temperature X-ray diffrac-tion pattern of C60 powder measured using CuKaradiation is shown in Fig.6. The diffraction patterncould be indexed to fec structure with a lattice con-stant of 14.2 ± 0.1 A. Further, as noted in ref. 7, it isseen that the diffraction peak corresponding to(200) index is not observed. This arises 7 due to theaccidental vanishing of molecular form factor whenhollow spherical C60 clusters of radius r (3.55 A) are

>->-Vizw>-z

35

o

"o

.'::.1 __ -:.L ..LI •• ._~_

15 20 25 305 10

2 e (Degree)Fig. 7-X-ray diffraction pattern for C70. The lines have been in-

dexed to a distorted hep structure

packed in a fcc lattice with a lattice constant of 4* r(14.2 A).

There is as yet no definitive published crystalstructure of C70. Theoretical predictions indicatethat these prolate spheroidal molecules crystallise inthe hcp structure+', The experimental X-ray powderdiffraction pattern of C70 is shown in Fig.7. Thesehave been at present indexed to a distorted hexa-gonal packed structure with the lattice parameters a= 9.547 A, b = 21.277 A, c = 17.205 A, and y =119.610. For an unequivocal identification of struc-ture, further experiments on single crystals of Cm,and calculations of diffraction intensities are inprogress.

A lkali doping of C60In the first report" of superconductivity (T, = 18

K) in K doped C60 films, the doping was carried outby the vapour phase reaction method. Holczer eta/.12 have described in detail the protocol for thedoping of K(and Rb) in bulk C60. From the studieson the diamagnetic shielding as a function of initialK composition, they have identified the supercon-ducting composition to be K3C60. Stephens et al.2xhave found the structure of K3C60 to consist of K at-oms located in the tetrahedral and octahedral holesof the fec C60 lattice. In addition to K doped ChO'su-perconductivity has been observed in other alkalidoped Co,; Rb doped Co.r'F, = 28K)12.13, RbxK3_.C60(Tc = 19K t030K)14,Csdoped C(,()(Tc= 30K)16,and CSxRb3_x Coo(T, = 33K)IS. The highest transitiontemperature observed17so far is in RbfTI doped Cfi) witha T, of 45 K. Since the initial report of the preparationof K doped Cr,oby the vapour phase reaction meth-od, other synthesis procedures have been tried outwith a view to obtaining better control over the stoi-chiometry, and increasing the superconducting vo-lume fraction. The difficulties in the handling of

35

F96 INDIANJCHEM,SEC.A&B,MAY 1992

small quantities of air sensitive alkali metals inside adry box have been overcome to some extent bytreating C60 with alkali/heavy metal amalgams In.Another method to obtain better control over thestoichiometry involves first the preparation of satu-ration product (KnC60), which is subsequently react-ed with known amount of Cno to obtain K,Cno29• Asolution route for the synthesis of K doped C60 hasbeen described by Wang et al.30 in which Cno dis-solved in toluene is reacted with K chips in an air-free environment.

In the present experiments, stoichiometric quan-tities of CnlJ and K (corresponding to the composi-tion K,Coo) were loaded into a 6 mrn dia, 20 cm longpyrex tube fitted with a stop cock. The mixture washeat treated in vacuum (10.5 torr) at 250°C for 24 hr.The mixture was reground and heat treated again invacuum at 250°C for 24 hr. The reaction product( - 20 mg) was reground, pelletiscd and sintered at250°C for 24 hr. A pair of sliced pellets was loadedinto the tefton capsule for AC susceptibility mea-surements. Finely ground powder was loaded into aLindemann glass capillary and sealed for X-raymeasurements. All operations involving handling ofthe sample were carried out inside an argon filledglove bag.

The X-ray diffraction pattern of K doped C60 isshown in Fig.8. The broad background is due to theglass capillary. The diffraction lines could be in-dexed to a fee structure with a lattice constant of14.3 ± 0.1 A. The diffraction lines correspondingto K are also indexed and it is seen that very smallamount of unreacted K, if any, is present. Compar-ing with the diffraction pattern of Coo(see Fig.6), it isseen that the (200) line is still absent and the domi-nant change due to K doping is in the relative inten-sities of the (220) and (311) diffraction lines. As in-dicated by Stephens et al/", this arises due to dopingof Kat (1/4,1/4,1/4) and (1/2,0,0) positions in theunit cell.

1'-_.

I ~>-:'=1Lf),Z'w'I-Z

oo'"

-,,

10 20 30 40 502 8 (Degree)

Fig. 8-X-ray diffraction pattern of K doped C60. The lines havebeen indexed to fee structure. The diffraction line corresponding

to K are also indicated

AC susceptibility measurements were carried outat 941 Hz using a mutual inductance coil and lock-inamplifier. The variation of norma lised susceptibilityas a function of temperature, as measured using acalibrated carbon resistor, shows a sharp supercon-ducting transition (cf, Fig.9) with a onset at 18 K anda transition width (25 to 75%) of 3 K. The transitioncurve compares favourably with the best results re-ported so far". The superconducting volume frac-tion with respect to Nb standard is estimated to be10%. Superconductivity in K doped C60 is observedto degrade and finally disappear on exposure of thesample to atmosphere. We have also observed thedegradation of superconductivity in a sample sub-jected to prolonged (1 week) heat treatment in vac-cum at 250°. The implications of this with respect tothe initial stoichiometry for sample preparation, dif-fusion of K inside the Coo matrix during heat-treat-ment and phase decomposition of KxC60 are beingexamined.

Doping of ChO with Rb was carried out along simi-lar lines to that of K, except that the heat-treatmentin this case was carried out at 440°C. Figure 10 showsthe X-ray ·diffraction pattern of the Rb doped C60·

100-0 ..- .... ..- .. -

>- 80-0...::::;iiiti'w 60-0u

SUNDAR et al: STRUC11JRAL & ElECfRONIC PROPERTIES OF UNDOPED & DOPED C60 & C70 F97

iooo ..,,.., .>- 80-0•...::;g;

~w 60-0usIF>o~ 400

~II:

!f 200

Rb3 e60

II

/."20-0~' ••••• ·r ..:.....-:~""''''.I • I I I ' It" I

10 15 20 25 30TEMPERATURE ( K )

35

Fig. ll-Normalised susceptibility versus temperature in Rbdoped C6U' indicating superconducting transition at 28 K

The lines could be indexed to fcc structure with alattice constant of 14.4 ± 0.1 A. A comparison withthe diffraction patterns of C60 (Fig.o), and K3C60(Fig.8) shows that the (200) line is present. Thisarises because of the expansion of the lattice so thatthe fortuitous condition a = 4* r is no longer satisfi-ed", The results of AC susceptibility measurementson the Rb doped sample are shown in Fig.Il , Asharp superconducting transition with aT, (onset) at28 K is clearly seen. The superconducting volumefraction is estimated to be 7%.

On-going studiesWith the availability of macroscopic quantities

(100 mg) of well characterised fullerenes, furtherwork on the study of structural and electronic pro-perties as a function of temperature, pressure anddoping, using a variety of experimental techniquessuch as X-ray diffraction, resistivity, AC susceptibil-ity, photoluminescence, Raman spectroscopy andpositron annihilation spectroscopy have been in-itiated. A brief account of these experiments is pro-vided in the following.

Structural studies on C70In contrast to detailed structural studies on C60 as

a function of temperature and pressure, there is asyet no published results on C70, Energy dispersiveX-ray diffraction measurements have been carriedout in C70 as a function of pressure, under truly hy-drostatic conditions, in the range of 0-20 GPa, usinga diamond anvil cell. Dramatic changes in the dif-fraction pattern have been observed beyond 13 GPaand work is in progress to identify the high-pressurephase!'. It may be remarked that in the case of C60,having an fcc structure under ambient conditions,no structural transition has been observed underhydrostatic conditions upto 20 GPa, whereas under

>-....ViZI&J•...~

C'0+ 2!i'Co 4!iOt

(Q)

n115 •• IS •7.5 • IS40 29 (DEGREE)

>-•...Viffi~~

C10

+ 2!it

o 4!iOt

(b)

iUII!! II!! Z.5B!I liS 115 17.514!1

29 (DEGREE)

Fig. 12-X-ray diffraction pattern of C70 at room temperatureand 4500 in the range of (a) 7.5 to 11.5 degrees and (b) )3.5 to21.5 degrees. Notice the disappearance of certain peaks at hightemperature, in addition to the shifting of peaks towards smaller

angles

non-hydrostatic conditions, a transition to crystal-lographic structure of lower symmetry is seen at 16GPa7.

C60, which crystallises into an fcc structure at roomtemperature undergoes a structural transformationto sc structure below 249 K and this is associatedwith the development of orientational order amongthe icosahedral molecules. With a view to lookingfor similar structural transition in C70, X-ray diffrac-tion experiments have been carried out as a functionof .emperarure (under vacuum) upto 450°C. It isseen from Figs 12a and 12L that as the temperatureis increased, apart from the shifting of peaks tow-ards smaller angles due to thermal expansion, someof the peaks present in the room temperature spec-trum disappear at 450°C. These changes are ob-served to be irreversible. Work is in progress toidentify t~e structural changes with temperature.

PhotoluminescenceRecent photoluminescence measurements= on

single crystals of C60 show a band centred at 13200

F98 INDIAN J CHEM, SEe. A& B, MAY 1992

cm', and this shifts to lower wave numbers with theapplication of pressure upto 3_2GPa_This indicatesa large reduction in band gap with pressure and byan extrapolation of this trend the band gap is pre-dicted to collapse at - 13 GPa32_ In pursuance ofthis interesting suggestion on possible metallisationof C('ounder pressure, we have carried out resistiv-ity measurements as a function of pressures, Ourmeasurements so far indicate that C60 remains in theinsulating phase up to 6 GPa. Further work on in-creasing the pressure range of resistivity measure-ments is envisaged. Our photoluminiscence mea-surements indicate that in addition to the overlapp-ing four band structure seen in the range of 14000 to12000 ern:', structures are also seen in the higherwave number region of 20000 to 16000 cm'. Workis in progress to understand these features in the lu-minescence spectra.

Positron annihilation spectroscopyPositron lifetime measurements in sintered pellets

of C(,o,indicate a single component with a lifetime of360 ps (Fig.l Ja). There is a considerable interest inunderstanding the origin of this lifetime componentas to whether it arises from annihilations associatedwith the localisation of the positron within the C60cluster, or from the delocalised positron in the in-terstitial regions of the crystal. Theoretical calcul-ations of the positron density distribution in C60have been carried out and these indicate (Fig.13b)that the positrons probe predominantly the intersti-tial region along the C60-C(,o bonds rather than theregion inside the C60 cluster. In the light of this dis-position of positron density within the C60 crystal, itcan be anticipated that the positron annihilationparameters will be sensitive to probe the effects ofpressure and doping of C60 crystal.

Doping studiesIn addition to the studies on K and Rb doped C60,

attempts to intercalate with other dopants, and stud-ies on the structure and superconducting-propertiesof the resultant products are also being carried out.In the course of these studies we have observed thatPb can be incorporated along with Kinta C60 andthe resultant samples are superconducting with T, of18 K. These samples exhibit much sharper transitionand are also observed to be more resistant to degrada-tion in comparison to K doped C60• Systematic studieson the PbIK doped samples of various stoichiome-tries are in progress.Summary

The details of the arcing of graphite electrodesand chromatographic separation for the synthesis ofpure C60 and C70 have been described. Doping of

12~-----~--~--~--~"-----.

11(0)

109

~a Bug..J 6

I4 .#~30~-~5~0--~OO~-~1~50~-~2~OO~--2~570-~3dOO

CHANNEL NO.

Fig. 13-(a) Positron lifetime spectrum in CM' (b) Positron dens-ity distribution in (001) plane of fcc C6U' The dotted circles in-dicate the C60 cluster. The maxima of the positron density isalong the Ch()-C60 bonds of the crystal rather than in the region

inside the cluster

C60 with K and Rb results in superconducting sam-ples with T, of 18 K and 28 K respectively. The prel-iminary results of several on-going experiments onboth pristine and doped fullerenes have been pre-sented.

Acknowledgement .The authors would like to thank Shri NChinnasa-

my, Smt M Premila, Smt Padma Gopalan and KumM Shailaja for assistance in various aspects of theexperiments reported in this paper. The enthusiasticsupport and encouragement from Dr KanwarKrishan and various members of the Materials Sci-ence Division is gratefully acknowledged.References

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SUNDAR et aL: STRUCTURAL & ELECTRONIC PROPERTIES OF UNDOPED & DOPED C60 & C70 F99

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