Electronic structure of Sn-doped C60 film

~)

0038-1098/9255.00+ .00 Pergamon Press Ltd

Solid State Communications, Vol. 84, No. 8, pp. 793-798, 1992. Printed in Great Britain.

Electronic Structure of Sn-doped C60 Film Deng Junzhuo (1) , Lu Sihua
Xun Kun (2) ,

Wang Zuquan <3) ,

Yao Jun <1) ,

Lui Fengqin <1) ,

Wu Sicheng <~)
( 1 ) Synchrotron Radiation Laboratory, Institute of High Energy Physics, Academia Sinica, Beijing, 100039, China ( 2 ) Department of Physics, Yantai Uninersety, Yantai, 264005, China ( 3 ) Department of Physics, Peking University, Beijing, 100871, China

(Received September 10,1992 by Z. Z. G a n )

The Sn-doped C60 film was obtained by annealing a 1 0 0 ~ thick C60 film with 6 ~ thick Sn overlayer at 200°C. Photoelectron spectra show that the ratio of Sn atoms to C60 molecules in the film is about 3 : 1. The valence band and C ls core level spectra of the Sn-doped C60 film are similar to that of C60 film. In contrast with the expectation of the reported superconductive phase of Sndoped Ce0, no metallic condution band with a distinct Fermi level cutoff has been observed in the valence bands portion.

spectra clearly show a well defined Fermi edge.

The discovery of superconductivity in alkali-metal doped C60 films D3 stimulated a great

For the alkali-metal doped C60, the chemical in-

deal of research to correlate the superconductiv-

stability is a difficult problem for the film to be

ity of the fuUerides with their electronic struc-

used in device processing [63. Chert et al. [~3

ture. It is now known that the K-, Rb- and Cs-

have studied the electronic structures of the alka-

fuUerides form a superconducting fee-based A~

line-earth-metal doped Cs0 film by synchrotron

C60 phase F2,s'43. Photoemission studies Is3 have

radiation photoemission. They have found that

shown that the transition to the metallic state is

the metallic Srx C60 and Bax C60 are candidates

related to the occupation of energy levels de-

for superconductivity. Recently, Gu et al. E83

rived from the lowest unoccupied molecular or-

have reported the observation of the supercon-

bitals(LUMO) of C60 by electrons donated by

ductivity of Sn-doped C80 with good chemical

the alkali atoms. In the superconducting phase

stabitity. In this paper we present our experi-

the LUMO are half filled and the photoemlssion

mental results of the electronic properties of Sn793

Sn DOPED C60 FILM

794

Vol. 84, No. 8

doped C60 film studied by ultraviolet(UV) and

the contact-arc method. The soot then was ex-

X - r a y photoemission spectroscopy ( X P S ) . The

tracted by toluene in a Soxhlet extractor. The re-

peak positions in the valence band and C ls core

sultant powder was degassed in ultra-high vac-

level spectra of the Sn-doped C~0 film are the

eum so that molecules could be sublimed and

same as that of Ce0 film. Especially, in compari-

condensed onto clean A u E l l 0 ]

son with the alkali- metal and alkaline- earth-

pressure of 1. 5 )< 10 -9 mbar to produce films

metal fullerides Is'7],

of 100 _~ nominal thickness. Sn was evaporat-

no metallic conduction

substrate at a

band has been observed in the region between

ed from a high purity Sn source at about

the highest occupied molecular orbital ( H O M O )

ll00°C.

of C60 and the Fermi level. It indicated that no

film the Sn source was degassed m a n y times un-

electrons tramsfer f r o m Sn atoms to fill the LU-

til no pressure rise in the chamber during the Sn

MO of Ce0.

evaporation.

Before depositing the Sn onto the C~0

The deposition rate of Sn was

about 1/~ in 4 rain. The thickness d of Sn overThe experiments were carried out in a mul-

layer was determined by the standard formula I

ti-technique high v a c u u m system of VSW Scien-

= I 0 e x p ( - - d / ~ . g s e ) , here I and I0 are the inten-

tific Instruments Ltd.

sities of the C ls core level peak with kinetic en-

at the National Syn-

chrotron Radiation Laboratory in Beijing. The

ergy of 968 eV from the C60 film with and with-

combined UV and X - r a y photoelectron instru-

out the Sn overlayer respectively. The inelastic

ment used is equipped with H A 1 5 0 electron en-

mean free path ~.968= 16. 8 / k as ealcuated by

ergy analyzer and conventional UV and X - r a y

Penn D3. The amount of Sn deposition is noted

sources. The spectrometer was operated under

in /~ ngstroms, where, for reference, 1 /~ Sn

high resolution (10eV pass energy) and the full

corresponds to ~-- 3 . 7 X 1014 a t o m s / c m z and the

width at half m a x i m u m ( F W H M )

surface layer density of close-packed C60 is "~

of Au 4fr/2

was 0. 90eV. The overall instrumental resolu-

1 . 1 X 10 t4 molecules/cm 2.

tion for the valence band spectra with He I resonance radiation ( 2 1 . 2 e V ) was ---- 0. 2eV.

Fig. 1 ( a ) shows the C ls and Sn 3d core level spectra of the thick C60 film with 6 /~

C60 was synthesized in a stainless steel chamber using high-purity graphite electrode by

thick Sn overlayer on it. The C ls peak is located at 285. 0eV below F_~, which is the same as

I

795

Sn DOPED C60 FILM

Vol. 84, No. 8 I

I

I

I

,

i

t

i

I I

i

i I

I

I I

I I

L

/s

2b

285 283

B .E.(eV} Fig. 1. The XPS spectra of C ls and Sn 3d core levels for ( a ) 6_~ Sn on C60 film and

10

( b ) after annealing at 200"C for 20

5 B.E. (eV)

Ef:O

minutes. Fig. 2. The UPS spectra of Sn-C60 for (a) 100 that of pure Ce0 film (not shown here). The

C60 film on Au substrate, ( b ) 6 ] k

F W H M of C ls peak is 0. 95eV and it is a little

Sn on C~0 film and (c) after annealing

larger than the value 0. 89eV of pure C60 film.

at 200°C for 20 minutes. Insert shows

Fig. 1 (a) shows that the Sn 3d spin-orbit dou-

the normalized intensities of HOMO for

blet are located at 493. 5eV and 485.2eV lelow

( a ) , (b) and (c).

with F W H M of 1.10eV. toionization cross section of 5s and 5p electrons In Fig. 2 ( a ) , the distribution of occupied

in Sn are quite low D°] , and they are rather delo-

electronic states of Co0 film shows the highest oe-

~lized. So in Fig. 2 ( b ) , the contribution from

cupied molecular orbitals (HOMO) located at

the valence electrons of Sn overlayer is very

2. 5eV below F_~ with FWHM of 0. 78eV. Af-

weak,and it only increase the background a lit-

ter deposit 6 ~ thick Sn on it, Fig. 2 ( b ) shows

fie.

that all peak positions in the valence-band spectrum of C80 do not change. The intensities of all

After annealing the 6 ~

Sn/C60 film at

features from Ceo are decrease because of the ex-

200"C for 20 m i n . , Fig. 1 ( b ) shows that the

istenee of Sn overlayer. We note that the pho-

peak positions of C ls and Sn 3d core level and

796

Sn DOPED C60 FILM

Vol. 84, No. 8

the FWHM of Sn 3d doublet are the same as

transfer from Sn atoms to fill the LOMO of

that of Fig. 1 ( a ) , but the FWHM of C ls

C60, and the interaction between the valence

main peak increases from 0. 95eV to 1. 03eV.

band of Sn and C60 is very weak. No electron

Before annealing the intensity ratio of Sn 3d

transfer between Sn and C60 was confirmed by

doublet to C ls peak is about 3 . 0 , after anneal-

the core level studies. In the conducting phase

ing it decreases to 1. 6. It means that the Sn

of aclkali-metal fullerides K3C60 the C ls .r~ak

atoms diffused into C60 film and formed a Sn-

and the features in the valence band spectra

doped Co0 film. We note that the Sn-doped Coo

shifted toward the lower binding energy by

sample of Gu et al. [83 was prepared at 550"C

about 0. 3-0. 4eV. [5,,2] because of the increas-

for 30 days. In our experiments, the Sn-doped

ing final state hole-screening by the electrons in

Coo film will be evaporated ff the annealing tem-

the metallic conduction bands.

perature is higher than 250°C. The intensities

tioned above, after annealing the peak positions

of Sn 3d and C ls peak in Fig. 1 (b) have been

of C ls ad Sn 3d core level and the FWHMs of

used to estimate the ratio of Sn atoms to Coo

Sn 3d doulbet in the Sn doped C60 film remain

molecules in the Sn-doped C80 film. Following

constant as compared with the curve obtained

the method of Carley and Roberts [1 ~3and taking

before annaling (Fig. l ( a ) ) .

the photoionization cross section from Yeh and

As we men-

The insert in Fig. 2 shows the normalized

Lindau[l°], assuming the homogeneous distribu-

intensities of the HOMO of pure C60, 6_]k S n /

tion of Coo and Sn in the XPS detectable depth,

C60 and Sn-doped C60 films. The FWHM of the

the ratio of Sn atoms to C~0 molecules is about

HOMO peak increases from 0. 78 eV (C60 film)

3.1. The valence band spectrum of Sn-doped

to 0. 83 eV ( 6 ] t S n / C 6 0 ) through 0. 91 eV

Coo film is shown in Fig. 2 (c). All features in

(Sn-doped C60film) . Such broadening have ai-

the curve are similar to that of the C60 film

m been observed in the alkali-metal and aika-

(Fig.

1/ne-earth-metal fullerides[5,r], it could be ex-

2 (a)),

no changes of peak positions

have been observed. In comparison with the

plained by the lifting of the degeneracy of the

conducting alkali-metal and alkaline- earth- met-

HOMO. In the Sn-doped Coo film, Sn atoms

ai fulleridesIS'z] , no metallic conduction band

were intercalated into the interstitial tetrahedrai

appears in the region between the HOMO of Coo

or octahedral sites of the f. c.c. Co0. The inter-

and Fermi level. It means that no electrons

ealated Sn atoms destroyed the icosahedrai sym-

Sn DOPED C60 FILM

Vol. 84, No. 8

797

metry of C-so molecule. Electrons in the differ-

M. Palstra,A. P. Ramirez and A. R. Kortan,

ent orbitals of HOMO may have different inital

Nature 350(1991)600.

state energies and probably also different final

2. K. Holczer, O. Klein, S. M. Huang, R . B .

state hole-screening due to the Sn atoms. For

Kaner, K. T. Fu, R. L. Whetten and F.

the 6 ~ Sn/C.s0 film (Fig. 2 ( b ) ) , the broading

Diederieh, Scinee 252 (1991 ) 1154.

of the HOMO peak is smaller than that of Sndoped C60 film (Fig. 2 ( c ) ) ,

because only the

3. M. 3. Rosseinsky, A. P. Ramirez, S. H. Glarum, D. W. Murphy, R. C. Haddon.

Co0 molecules in the top layer were affected by

A. F. Hebard, T. T. M. Palstra, A. R. Kor-

the Sn overlayer. The broadenings of C ls

tan, S. M. Zahurak and A. V. Makhija.

main peak in Fig. 1 ( a ) and ( b ) could be ex-

Phys. Rev. Lett. 66(1991)2830. 4. S. P. Kelty, C. C. Chen and C. M. Lieber.

plained in the same way. In conclusion,

no matellic conduction

band in Sn-doped Coo film has been observed. The peak positions in the valence band and C ls core level spectra of Sn-doped Coo film are the same as in that of Coo film. It indicated that there is no electrons transfer between Sn and C60. The interaction of valence bands between

Nature 352

(1991)223.

5. P. 3. Benning, 3. L. Martins, 3. H. Weaver. L. P. F. Chibante and R. E. Smalley,Science 2 5 2 ( 1 9 9 1 ) 1417. G. K. Wertheim, 3. E. Rowe, D. N. E. Buchanan, E. E. Chaban, A. F. Habard, A. R. Kortan, A. V. Makhija and H. C. Haddon, Science 252 ( 1991 ) 1419. C. T. Chen, L. H. Tjeng, P. Rudolf,

Sn and Cs0 is very weak.

G. Meigs, J. E. Rowe, J. Chen, J. P. MeThe work was supported by the National Natural

Science

Fundation

of

china

No.

19174003. One of us, Xun Kun, is also indebted to Shandong Province for support through Grant No. 90A1111.

References

Cauley J r , A . B. Smith III ,A. R. McGhie, W. J. Romanow. and E. W. Plummer, Nature 352(1991)603. 6. J. Yao. F.Q. Liu, J. Z. Deng, K. Xun, Z. Q. Wang, S. H. Lu, S. C. W u , M. L. Jiang, Z. N. Gu and X. H. Zhou, Chinese Phys. Lett.

1. A. F. Hebard, M. J. Rosseinsky, R. C. Haddon, D. W. Murphy, S. H. Glarum, T . T .

9(1992)317.

7. Y. Chert, F. Stepriak, J. H. Weaver, L . P . F. Chibante and R. E. Smalley, Phys. Rev.

798

Sn DOPED C60 FILM B45(1992)8845.

Vol. 84, No. 8

Nulear Data Table 32(1085)1.

8. Z. N. Gu, J. X. Qian, Z. X. Jin, X. H.

Zhou, S. Q. Feng, W. S. Zhou and X.

11. A. F. Carley and M. W. Roberts, Pore. Roy. Sac. (London)A $63(1078)403.

Zhu, Solid state Commun. 82(1992)167.

12. D. M. Poirier, T. R. Ohno, G. H. Kroll,

9. D.R. Penn Phys. Rev. B35(1987)482~ J.

Y. Chert, P. J. Benning, J. H. Weaver,

Electron Spectrosc.

Relat.

Phenom.

9

(1976)29. 10. J.J. Yeh and I. Lindau, Atomic Data and

L. P. F. Chibante, R. E. SmaUey, Science 253(1991)100.