Resistivity of single crystal C60 and effect of oxygen

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Solid State Communications,Vol. 84, No. 8, pp. 827-829, 1992. Printed in GreatBritain.

RESISTIVITY

OF S I N G L E

CRYSTAL

C60

0038-1098/9255.00+.00 PergamonPress Ltd

AND EFFECT

OF O X Y G E N

Takeshi Arai, Youichi Murakami, Hiroyoshi Suematsu, Koichi Kikuchi*, Yohji Achiba*, Isao Ikemoto* Department of Physics, University of Tokyo, Hongo, Bunkyoku, Tokyo 113, *Department of Chemistry, Tokyo Metropolitan University, Minami-Ohsawa, Hachioji, Tokyo 192-03. (Received 12 September 1992 by H. Kamimura) A crucial effect of oxygen on resistivity !3 of a C60 single crystal and the temperature (160-570K) dependence of 13 have been studied. The resistivity increases by a factor of 104 on absorption of oxygen; the estimated concentration is 4% 02 per C60. The processes of oxygen absorption and desorption are reversible. The activation energy in p of the oxygen-free crystal is 0.26 and 0.15 eV above and below Tc=250K, respectively, which is ascribed to an impurity level.

1.

Introduction

resistivity from the slope at a high voltage region in the forward bias. We estimate the experimental error of a factor 2 for the absolute value. The resistance of a sample becomes very large at low temperatures to be comparable to that of the sample holder, that is, R~2.0xl010 f~: this is the measurable limit, 6x109 f~cm in P-

According to the recent band calculation the C60 crystal is a typical semiconductor with the direct band gap of 1.5 eV. 1) Experimental studies of the electronic structure has been investigated through the photoemission, 2) photoconduction, 3) and transport phenomena. 4) The photoemission measurement of C60 films yields the gap energy 1.9 eV, 2) while recent combined experiments of photoemission and inverse photoemission have given the energies of 2.l-2.2 eV 5) and 2.3 e v i l ) The photoconduction study for a mixture film of C60 and C70 gives the optical band edge of 1.6 eV, although the mobility edge is observed at 2.0 eV. 3) The electric resistivity, which was measured by Mort et al., 4) is found to be 1014 Ilcm for C60/C70 films at room temperature, and the band gap is obtained as 1.9 eV in the high temperature phase. However, we have so far no reliable data of the semiconductive transport parameters in the single crystal of C60, such as the gap energy and impurity levels. In this report we present the electric resistivity of a high-purity single crystal in a wide temperature region, and particularly emphasize the crucial effect of oxygen absorption and desorption on resistivity. 2.

3.

Experimental Results and Discussion

First we present a remarkable effect of oxygen on resistivity in a C60 single crystal. Fig.l shows the temperature dependence of resistivity of an as-grown crystal stored in atmosphere. The measurement was made in vacuum of less than 10-4 Pa. With increasing tempera~re in the first run (the upper trace, RUN#l) the resistivity p, which is beyond the measurable limit R>2xl010 f~ at room temperature, decreases steeply above 500K and seems to saturate around 580K. In this

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Experimental Procedures

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As the starting material of C60 we used powder samples with the purity of 99.8% (the content of C60 in total fullerenes), and baked them in vacuum ( < 10-4 Pa ) at 300 °C for the removal of solvent. The single crystals were prepared by the sublimation method. 7) A typical size of the crystal was 0.5x0.5x0.3 mm 3. The mosaic spread of the crystal was typically 0.05 °. The resistivity measurement was made by the two contact method with use of silver paste. In this case we measure the total resistivity of a sample and contacts, and also suffer the rectifying junction effect. While the contact resistivity was negligibly small in our case compared with that of the sample itself, the junction effect brings serious errors for the bulk resistivity measurement. In order to overcome the junction effect we measured the I-V characteristic at each measuring point and determined the

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Vol. 84, No. 8

RESISTIVITY OF SINGLE CRYSTAL C60

run temperature was held at 415 K for 1 hour, 466 K for 2 hours, 521 K for 4 hours and 580 K for 10 hours to ensure the thermal and chemical equilibrium in the sample; the corresponding processes are shown by kinks on the trace. Then, when decreasing temperature (the lower trace, RUN#2), p takes a different way from RUN#l, and has a smaller value by a factor of 10-4 to 10-5 than the initial value. The resistivity, which is drastically reduced by the heat treatment in vacuum, however, recovers to the initial value by the exposure to oxygen at r(~om temperature (RUN#3). The kinetics of absorption process will be described later. Prior to RUN#3 we have exposed the sample to nitrogen gas of 0.8 atm. (80 kPa), but observed only a few percent increase of resistivity, a negligibly small effect compared with the case of oxygen. These facts mean that a considerable amount of oxygen can be absorbed in C60 crystals by only a few days exposure to oxygen gas, and that absorbed oxygen can be easily rem(wed by the heat treatment in vacuum. When increasing temperature in vacuum (RUN#4) after oxygen absorption, the resistivity deceases in a way similar to RUN#1 in essence but a little deviated to smaller values. The difference may be ascribed to incomplete absorpsion of oxygen. When heating again at 570 K and decreasing temperature (RUN#5), the resistivity reproduces approximately same value of RUN#2 in essence. After RUN#5 the sample is again exposed to 0.21 atm. (21 kPa) of oxygen (the partial pressure of oxygen in air) at room temperature (RUN#6). Fig.2 shows the time development of resistivity. The resistivity recovers rapidly in initial 2 hours, but the kinetics of the oxygen absorption continues for 200 hours. The measurement of the further time development is limited by our measurable limit. Thus, the present experiments can be understood by the oxygen desorption and absorption effects, and it is found that the resistivity drastically increases by oxygen absorption. The effect of oxygen absorption is also observed in the magnetic susceptibility measurement. Fig.3 shows the susceptibility of the heat-treated powder sample before and after the exposure to oxygen gas ( 1.0 atm., 12 hours ) at room temperature. The susceptibility increases paramagnetically after the exposure, and the

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increment of susceptibility A× obeys the Curie-Weiss law. This means that oxygen is included in the sample with a paramagnetic moment. Assuming that oxygen exists in the form of 0 2 and has the spin S=I as in the gas state, we can estimate the 0 2 content of 4.0 % molecule per C60. The content is significantly large as the bulk absorption. From the geometric consideration an oxygen molecule should be accommodated only at the octahedral site in the face-centered-cubic structure of C60: the tetrahedral site is too small for an 0 2 molecule. This phenomenon is on the line of the recent high pressure experiment by Schirber et al., 8) who have reported that oxygen is reversibly incorporated into C60, and occupies 15 % of octahedral sites at 1 kbar (0.1 GPa), that is, the content of 15 % mol per C60. On the other hand, a different type of oxygen effect has been reported,9,10,11) in which oxygen is chemically bonded to a C60 molecule, and oxidation process is irreversible. As to the mechanism of the resistivity change of resistivity by oxygen we can consider some possibilities. One is the structural change due to the oxygen absorption, but the X-ray diffraction study reveals no evident difference between the C60 single crystal stored in atmosphere and the heat-treated crystal at 623K: the lattice constants are respectively, 14.166-+0.006/~ and 14.171 _+0.006/~; there is no significant difference within the experimental errors. The second is the surface doping effect of oxygen. The semiconductor surface is generally very sensitive to atmosphere, and oxygen may react with the surface of C60. The resistivity in such a case will decrease due to the heavy doping effect, because a large amount of oxygen is absorbed in our case. However, our result is opposite to this expectation. A most plausible is the carrier compensation effect: oxygen, possibly existing in the form of 02 but ionized a little negatively, acts as an acceptor and compensates electrons on donor levels, which leads the reduction of carrier density and the high resistivity. Next we present the temperature dependence of resistivity of the oxygen-tree single crystal, namely the crystal being heat-treated in vacuum at 570K; the resistivity from 160 to 570 K is shown in Fig.4. The

RESISTIVITY OF SINGLE CRYSTAL C60

Vol. 84, No. 8

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Fig.4. The temperature dependence of the electrical resistivity of an oxygen-free C60 single crystal. Inset shows the temperature dependence of the resistivity near the structual phase transition.

829

configuration of neighboring C60 molecules below and above Tc. Gelfand et al. 15) discussed the change of band structure associated with by the orientational order and disorder, but they gave no quantitative discussion. We need a detailed band calculation taking account of the precise crystal structures above and below Tc, especially in the tree rotating state. In conclusion a few percent of oxygen molecules is absorbed in C60 crystals at ambient pressure and temperature, and desorbed above 300 °C in vacuum. The processes are reversible. On absorption of oxygen, resistivity of a C60 single crystal increases by a factor of more than 104 , which is plausibly ascribed to the donor compensation by oxygen acceptors. The impurity conduction is observed for the oxygen-free C60 single crystal, and the impurity level energy is determined as 0.26 and 0.15 eV for the face-centered-cubic and simplecubic phases, respectively. Acknowledgements, We thank to Mr. T. Higuchi and K. Minakuchi for preparing C60 single crystals. This work is supported by Grant-in-Aid for Scientific Research (B) from The Ministry of Education, Science and Culture of Japan. References

resistivity shows a remarkable discontinuous change at Tc, the phase transition temperature of rotational disorder of C60 molecules, and it obeys a function exp( -EA/kT ) below and above To, EA being the activation energy. The observed EA is 0.15 eV for TTc. The gap energy deduced from EA ( Eg=2EA ) is much smaller than Eg obtained from calculation 1) and optical experiments.2,3,5, 6) Therefore, the observed temperature dependence of p is due to the impurity conduction, and E A corresponds to the impurity level energy in our case. A hump observed at 450 K, which is negligibly small in the first run, increases in intensity with thermal cycles. It seemed to be related to defects induced by the thermal cycles, although the origin is open at present. The observed impurity level may be related to the trapping level observed by Takahashi et al. 12,13) in the photoemission measurement, by a slight doping of Rb into C60 they observed an energy shift of the valence band toward the high-binding-energy side and suggested that in pure C60 there is a trapping level at 0.3 eV below the conduction band bottom. The level energy in photoemission, which was measured at room temperature, is consistent with our result of E A in resistivity. In this model we can identify the impurity as a donor. We note a most remarkable feature of EA, namely the value tbr TTc. If we assume the same impurity for the conductions below and above To, this fact suggests some significant changes in the band structure upon the phase transition. In fact the lattice constant is found to contract by 0.344 % at To, 14) so that we could expect the increase of the transfer integral and sequently the broadening of conduction and valence bands, which may lead to the reduction of band mass and the enhancement of dielectric constant. However, beside the abrupt change at Tc the lattice constant has a similar magnitude of temperature variation for T>Tc and for T
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