Superconductivity of C60 Langmuir−Blodgett Films Doped with

3932

Langmuir 1996, 12, 3932-3937

Superconductivity of C60 Langmuir-Blodgett Films Doped with Potassium: Low-Field Signal and Electron Spin Resonance Study† Ping Wang,‡ Yusei Maruyama,§ and Robert M. Metzger*,‡ Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487-0336, and Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Received November 21, 1995. In Final Form: March 28, 1996X Langmuir-Blodgett films of C60 were doped with potassium, and the doping was followed in situ by both microwave low-field signal (LFS) and electron spin resonance. The superconducting critical temperature was Tc (onset) ) 12.9 K for a nominal 14-layer film at optimum doping. The four-probe conductivity of an underdoped film as a function of temperature showed semiconducting behavior.

Introduction The discovery of superconductivity in potassium-doped C601 has aroused great interest in fullerenes. The superconducting transition temperature Tc of alkali-metaldoped C60 ranges from 18 K to K3C601 to 33 K for Rb1Cs2C602 and to 40 K at about 15 kbar for Cs3C60.3 Thin films (96 nm) of K-doped C60 have a superconducting onset around 16 K;1 in a single crystal of K3C60, the superconducting transition was observed at 19 K (resistivity measurement).4 We previously reported superconductivity (Tc around 8 K) in a K-doped C60 Langmuir-Blodgett (LB) multilayer film.5 Due to air sensitivity of the alkali-metal-doped samples and the potential for inhomogeneous doping,1 it is difficult to study the doping process in situ and to find optimal doping conditions by routine resistivity or SQUID (superconducting quantum interference device) measurements. The low magnetic field microwave absorption, or low-field signal (LFS), due to the Meissner-Ochsenfeld effect, has been detected for high Tc superconductors,6-8 organic superconductors,9-11 alkali-metal-doped C60 superconductors,12 and K-doped C60 LB films;5 the LFS signal has proved to be a sensitive and simple method for the * To whom correspondence should be addressed. † Supported in part by AFOSR F49620-92-J-0529DEF. ‡ University of Alabama. § Institute for Molecular Science. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600-601. (2) Tanigaki, K.; Ebbesen, T. W.; Saito, S.; Mizuki, J.; Tsai, J. S.; Kubo, Y.; Kuroshima, S. Nature 1991, 352, 222-223. (3) Palstra, T. T. M.; Zhou, O.; Iwasa, Y.; Sulewski, P. E.; Fleming, R. M.; Zegarski, B. R. Mater. Res. Soc. Symp. Proc. 1995, 359, 285-288. (4) Maruyama, Y.; Inabe, T.; Ogata, H.; Achiba, Y.; Suzuki, S.; Kikuchi, K.; Ikemoto, I. Chem. Lett. 1991, 1849-1852. (5) Wang, P.; Metzger, R. M.; Bandow, S.; Maruyama, Y. J. Phys. Chem. 1993, 97, 2926-2927. (6) Blazey, K. W.; Mu¨ller, K. A.; Bednorz, J. G.; Berlinger, W.; Amoretti, G.; Buluggiu, E.; Vera, A.; Matocotta, F. C. Phys. Rev. 1987, B36, 7241-7243. (7) Sastry, M. D.; Kadam, R. M.; Dalvi, A. G. I.; Phatak, G. M.; Iyer, R. M. Physica 1989, C163, 1589-1590. (8) Janes, R.; Singh, K. K.; Blunt, J.; Edwards, P. P. J. Chem. Soc., Faraday Trans. 1990, 86, 3829-3830. (9) Haddon, R. C.; Glarum, S. H.; Chichester, S. V.; Ramirez, A. P.; Zimmerman, N. M. Phys. Rev. 1991, B43, 2643-2647. (10) Bohandy, J.; Kim, B. F.; Adrian, F. J.; Moorjani, K.; Arcangelis, S. D.; Cowan, D. O. Phys. Rev. 1991, B43, 3724-3727. (11) Zakhidov, A. A.; Ugawa, A.; Yakushi, K.; Imaeda, K.; Inokuchi, H.; Khairullin, I. I.; Khabibullaev, P. K. Physica 1991, C185-C189, 2669-2670. (12) Zakhidov, A. A.; Ugawa, A.; Imaeda, K.; Yakushi, K.; Inokuchi, H.; Kikuchi, K.; Ikemoto, I.; Suzuki, S.; Achiba, Y. Solid State Commun. 1991, 79, 939-946.

S0743-7463(95)01084-5 CCC: $12.00

study of negligibly small amounts of superconducting phases in non-superconducting or superconducting matrices13 and air-sensitive samples.5 In particular, the LFS method does not require direct contacts to the sample, so it is the best method to detect the doping time dependence and air-sensitive superconducting phases. We present here the dependence of superconductivity upon potassium doping of C60 LB films studied by LFS, combined with conventional electron spin resonance (ESR) and conductivity measurements. Experimental Section C60 (stated purity 99.9%) was obtained from Texas Fullerenes Corp. and used without further purification. C60 was first dissolved in CS2, and then the solution was diluted with methylene chloride (1:10 volume ratio CS2:CH2Cl2) to a final concentration range between 10-5 and 10-4 M. This C60 solution was carefully spread on a subphase of either pure water (Millipore Milli-Q, resistivity 16 MΩ cm) or an aqueous solution of phenol (10-2 M) at room temperature (air temperature 25 °C, water temperature 20 °C) in a Lauda film balance (Langmuir trough) set on a vibration-free table in a room provided with filtered air. The insoluble Langmuir film of C60 was formed by slowly compressing the barrier after waiting for more than 20 min for the solvent to evaporate. The pressure-area isotherm was measured for several dropping solution concentrations (in the range 4.72 × 10-5 to 2.67 × 10-4 M), to establish at which concentration and dropping solution volume a true Langmuir monolayer is obtained.14 For vertical transfer of the film onto solid substrates (Langmuir-Blodgett method), dilute dropping solutions were used (10-5 M), but to provide enough material for transfer after film compression, three to four times more solution was used than is needed for a true Langmuir monolayer. The films were transferred at a film pressure of 25 mN/m, either onto glass slides coated with gold electrodes (for conductivity measurements) or onto strips of poly(ethylene terephthalate)15 (for LFS measurements). The transfer ratios were not measured. The C60 LB films on a sheet of poly(ethylene terephthalate) were cut into about 3 mm × 30 mm strips using titanium scissors to prevent possible contamination. Several such strips were placed in one side of a 5 mm diameter quartz ESR tube. An excess amount of potassium metal was rapidly introduced into the other side of the tube, then the tube was evacuated to a pressure of 1 × 10-4 Torr for 2 h, and the potassium was distilled by slow heating at the same pressure; finally, the tube was sealed (13) Khairullin, I. I.; Zakhidov, A. A.; Khabibullaev, P. K.; Iqbal, Z.; Baughman, R. H. Synth. Metals 1989, 33, 243-256. (14) Bulho˜es, L. O. S.; Obeng, Y. S.; Bard, A. J. Chem. Mater. 1993, 5, 110-114. (15) Ikegami, K.; Kuroda, S.; Sugi, M.; Nakamura, T.; Tachibana, H.; Matsumoto, M.; Kawabata, Y. J. Phys. Soc. Jpn. 1992, 61, 37523766.

© 1996 American Chemical Society

Superconductivity of C60 LB Films

Langmuir, Vol. 12, No. 16, 1996 3933

Figure 2. Surface area-pressure isotherm for pure C60 on an aqueous subphase at 20 °C: (a) 150 µL of 4.72 × 10-5 M solution (solvent, 1:10 volume ratio CS2:CH2Cl2); (b) 300 µL of 2.67 × 10-4 M solution (solvent, 1:10 volume ratio CS2:CH2Cl2). Table 1. C60 LB Film Transfer and Doping Conditions for LFS and ESRa sample

Nl

Ns

Ts (°C)

TK (°C)

L1 L2 L3

14 14 14

10 10 4

150 150 170

200 200 200

a N is the number of up-down strokes of the dipper for the LB l film transfer; Ns is the number of strips used for the LSF and ESR measurements, Ts is the temperature of the sample side and TK is the temperature of potassium side, during the doping process.

Figure 1. Potassium doping system for conductivity measurement. during pumping. The K doping of C60 LB films was performed by means of a double furnace method, which had been constructed for preparing alkali-metal graphite intercalation compounds.16 The ESR tube, with K in one end and the C60 LB films in the other, was placed in this furnace, where the temperatures were maintained at TK ) 120-200 °C (K side) and Ts ) 190-200 °C (C60 LB films side). The doping process was carried out in steps, interspersed with LFS measurements. ESR spectra and LFS were measured with a Bruker ESP300E spectrometer operated at X-band, equipped with an Oxford Instruments ESR 900 liquid He flow cryostat (temperature range 2.7-300 K). The temperature was monitored with a Au-0.007% Fe versus chromel thermocouple. For most multiply overlapped lines, a computer fit to the spectra allowed calculation of g-factors of the contributing ESR lines. The C60 LB film on a 10 × 10 mm2 glass slide coated with Au electrodes was placed in a glass tube, with K chips placed in the bottom and a side-arm connected to a vacuum system (Figure 1). For the electrical conductivity measurement, four Au wires were connected to the Au electrodes by Au paste and attached with Ag paste to four Pt feed-through wires sealed through a vacuumtight glass flange. Then the system was closed by a cylindrical layer of epoxy resin (Varian Torrseal), and pumped for more than 24 h at the pressure of 8 × 10-7 Torr. Potassium was slowly and carefully distilled with a heating gun. Then the C60 LB film side was wrapped with heating tape (100 V, 65 W, 0.3 W/cm2); its temperature was controlled by a Nakamura Temperature Control Unit to vary the temperature at the C60 LB film side during doping. The K-side was placed into an oil bath, whose temperature was slowly increased during the doping process. The resistance change on doping was continuously recorded in situ by a two-probe method using an Advantest R8340A Ultra High Resistance Meter or a Keithley 2001 Multimeter, depending on the resistance range. After doping was completed, as indicated by minimum resistance, the sample was annealed at 100 °C for several hours, the glass vessel was flushed with He gas (2 cmHg) three times, and the LB film side was sealed off at the point indicated in Figure 1. (16) Nixon, D.; Parry, G. J. Phys. 1992, D1, 291.

The sealed glass vessel was placed on a Cu sample holder of a cryostat, wrapped to it by Teflon tape, and covered by a cylindrical shroud sealed by an In seal. The cryostat was inserted into a liquid helium Dewar. The temperature dependence of the conductivity for the K-doped C60 LB film was measured by a standard four-probe method.

Results LB Films. Figure 2 shows the pressure-area (Π-A) isotherm for pure C60 solution at the air/water interface at two spreading solution concentrations but summarizes a thorough study of the concentration dependence. Over a pure aqueous subphase, at low spreading solution concentration (10-4 M), the occupied area per molecule was about 30 Å2, indicating that the Langmuir film was a multilayer, presumably because of clustering before film compression.19 The quality of multilayer films on a mixed water/phenol subphase was observed to be better than that on a pure water subphase.20 We found that a 50-layer film, trans(17) Wragg, J. L.; Chamberlain, J. E.; White, H. W.; Kra¨tschmer, W.; Huffman, D. R. Nature 1990, 348, 623-624. (18) Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; de Vries, M. S.; Hunziker, H. E.; Wendt, H. R. Nature 1990, 348, 621-622. (19) Wang, P.; Shamsuzzoha, M.; Wu, X.; Lee, W. J.; Metzger, R. M. J. Phys. Chem. 1992, 96, 9025-9028. (20) Tomioka, Y.; Ishibashi, M.; Kajiyama, H.; Taniguchi, Y. Langmuir 1993, 9, 32-35.

3934 Langmuir, Vol. 12, No. 16, 1996

Wang et al.

Figure 3. LFS and EPR spectra for C60 LB film sample L1 after 480 min of K-doping: (a) LFS (microwave power P ) 10 mW, modulation amplitude Hmod ) 5 G, receiver gain G ) 1 × 104); (b) ESR spectrum (P ) 0.5 mW, Hmod ) 0.5 G).

ferred onto a glass slide from the mixed phenol/water subphase, presented, under an optical microscope, smoother images than a similar multilayer film transferred from a pure water subphase. However we could not transfer a Langmuir monolayer to a substrate due to technical problems (e.g., ratio of film area before compression to area after compression; clustering of molecules during transfer). Therefore, the LB films used for K doping were obtained from multilayer Langmuir films. As previously, the thickness of a transferred C60 LB film was monitored by ellipsometry:19 film thicknesses between 45 and 100 nm were measured on several spots on the film, for a nominal number of layers of C60 LB multilayer film Nl ) 14. Potassium Doping. Twelve samples of pure C60 LB films were doped with potassium under various conditions, of which three are listed in Table 1: temperature of the potassium side Tk, temperature of the C60 LB films side Ts, the nominal number of layers of C60 LB multilayer film Nl, and the number of C60 LB film strips used Ns. Besides the above variables, the doping results of the C60 LB films were also dependent on the amount of potassium introduced into the ESR tube and the distance between the C60 LB films and the potassium in the ESR tube.

LFS and ESR. The results of LFS and ESR spectral studies can be divided into three regions, depending on the doping time td and the sample properties after doping: (a) undoped region, (b) optimally doped region, and (c) overdoped region. (a) Undoped Region. For td ) 0, there is no LFS absorption to the lowest available temperature T ) 2.7 K. There is a single, intense, very narrow ESR line (g ) 1.9933, line width ∆Hpp ) 1.8 G) at 2.7 K, and no other signal in the range of 1500-4000 G, in contrast with the spectral features seen by Zakhidov et al.12 The origin of this narrow line is not known. (b) Optimally Doped Region. In sample L1, after 480 min of doping, the color of the C60 LB film strips in the ESR tube changed: two of the C60 LB film strips turned to a very beautiful shiny black color with metallic luster, six strips turned brown, and the other two strips turned slightly darker, compared with the initial yellowish color of the undoped film. The LFS signal, with hysteresis, can be detected; the ESR spectrum also shows some changes from the undoped sample. Figure 3 presents the results. The intensity of LFS (Figure 3a) decreased slightly with increasing temperature, until the LFS disappeared at 12.9 K; this indicates Tc (onset) ≈ 12.9 K. Compared with the

Superconductivity of C60 LB Films

Langmuir, Vol. 12, No. 16, 1996 3935

Figure 4. LFS and ESR spectra for C60 LB film sample L2 after 690 min K-doping: (a) LFS (P ) 10 mW, Hmod ) 5 G, G ) 1 × 104); (b) ESR spectrum (P ) 0.5 mW, Hmod ) 0.5 G).

LFS for a bulk K-doped C60 sample (where LFS is 102 larger than background),11 the intensity of the LFS for the K-doped C60 LB films is very weak (at best five times larger than the noise level). Even for well-doped state, the LFS is relatively “noisy”. The ESR spectrum (Figure 3B) also showed changes after doping: a broader ESR line (∆Hpp ) 11.7 G), overlapped with a narrow line (∆Hpp ) 2.6 G), was detected. The average g-factor was 2.002 24 (because of spectral overlap, it is difficult to determine the g-factor for each of them separately). The broad signal showed a Curielike temperature dependence until it disappeared above 25 K; the narrow one showed a Pauli-like temperature dependence above 180 K and a Curie-like behavior in the lower temperature range. Similar behavior was observed in bulk K-doped C60.11 In sample L2, after 690 min of doping, the color of most C60 LB film strips became black or dark brown. The LFS still exists at and below 12 K (Figure 4a) and is very similar to that of sample L1. The ESR spectrum of sample L2 (Figure 4b) shows two lines, with ga ) 2.00294 and gb ) 2.0004, in agreement with the result of Kosaka et al.,21 who assign ga to the superconducting phase and gb to the K4C60 overdoped

phase. Therefore, after 690 min of doping, sample L2 has partially converted into the non-superconducting K4C60 state. (c) Overdoped Region. After 3300 min of doping, sample L3 was found to be “overdoped”: the LFS and ESR spectra are shown in Figure 5. The LFS intensity has decreased, and no clear signal could be detected (Figure 5a); the hysteresis has disappeared, i.e., the sample has become non-superconducting. The ESR spectrum (Figure 5b) changed from what was seen in the optimally doped regime. Three ESR signals could be detected, with ga ) 2.00294, gb ) 2.0004, and gc ) 1.9926; these three peaks were attributed21 to K3C60, K4C60, and K6C60, that is, the three phases coexist. Since K-doping of C60 LB films can be inhomogeneous even at very long doping times, some small region of the C60 LB films may still be in an “optimally doped” superconducting K3C60 state (but so small that a clear LFS is not seen), and other regions are in the K4C60 and K6C60 states. With increasing temperature, the three peaks gradually became indistinct. (21) Kosaka, M.; Tanigaki, K.; Hirosawa, I.; Shimikawa, Y.; Kuroshima, S.; Ebbesen, T. W.; Mizuki, J.; Kubo, Y. Chem. Phys. Lett. 1993, 203, 429-432.

3936 Langmuir, Vol. 12, No. 16, 1996

Wang et al.

Figure 5. LFS and ESR spectra for C60 LB film sample L3 after 3300 min of K-doping: (a) LFS (P ) 10 mW, Hmod ) 5 G, G ) 1 × 104); (b) ESR spectrum (P ) 0.5 mW, Hmod ) 0.5 G). Table 2. C60 LB Film Doping Conditions for Conductivity Measurementsa sample

Nl

Ts (°C)

Tk (°C)

td (h)

FRT,min (Ω cm)

E1 E2 E3

20 40 110

140 100 100

70 90 94

4 5.5b 2.5c

30 0.79 0.34

a N is the number of LB up-and-down strokes of the dipper for l the LB film transfer, Ts is the temperature of the sample side, and TK is the temperature of the potassium side, during the doping process, td is the doping time, and FRT,min is the minimum resistivity measured at room temperature. b After 4.5 h of doping, the twoprobe in situ resistance of the sample decreased to 214 kΩ; the sample was then annealed for 12 h at 100 °C. Then the sample was doped for another 1 h, until the resistance dropped to 65.7 kΩ and then annealed for 3 h at 100 °C, with no further change in resistance. c After 2 h of doping, the two-probe in situ resistance of the sample decreased to 87 kΩ; the sample was then annealed for 3 days at 100 °C, after which the resistance increased to 3.18 MΩ; doping for a further 0.5 h yielded a final resistance of 5.1 kΩ, which did not change appreciably despite 24 h of annealing at 100 °C.

Electrical Conducitivity. Three C60 LB films on glass slides covered by Au strips were prepared for potassium doping. The resistance of a neat LB film of C60 was greater than 1010 Ω at room temperature. Doping conditions and results are listed in Table 2. After doping, the resistance decreased by 7 orders of magnitude. Due to experimental difficulties, samples E1 and E2 were destroyed before they could be introduced into the low temperature conductivity system; the temperature dependence of the electrical conductivity could only be measured for sample E3. Figure 6 shows the temperature dependence of conductivity (120 K < T < 300 K), which presented a typical semiconducting curve, with activation energy 9.53 × 10-2 eV. The

Figure 6. Temperature dependence of resistivity for C60 LB film sample E3 doped with potassium.

semiconducting behavior for this K-doped C60 LB film is due to insufficient doping time (the sample did not reach either the metallic or the superconducting state) or to domain structures and inhomogeneities in the sample. Discussion The film thickness implied in Tables 1 and 2 should be discussed briefly. The LB film transfer is, surprisingly, reported to be Z-type, i.e., occurs only on the upstroke.22,23 Given our procedures, the Langmuir films before transfer may be several C60 molecules thick. If N1 ) 14 in Table (22) Nakamura, T.; Tachibana, H.; Yumura, M.; Matsumoto, M.; Azumi, R.; Tanaka, M.; Kawabata, Y. Langmuir 1992, 8, 4-6. (23) Williams, G.; Pearson, C.; Bryce, M. R.; Petty, M. C. Thin Solid Films 1992, 209, 150-152.

Superconductivity of C60 LB Films

Langmuir, Vol. 12, No. 16, 1996 3937

1 corresponded to 14 Z-type transfer of true monolayers (each 10 Å thick), then the LB film would be a minimum of 14 nm thick, this is an idealized lower limit. Since the Langmuir films before transfer were several molecules thick, the thickness for a 14-layer film could be several times 14 nm; this agrees with the film thickness (45-100 nm) measured by ellipsometry. It should be remembered that the sample thickness is by no means uniform across the sample.19 The LFS signal and the clear hysteresis in the LFS observed (Figures 3a and 4a) in K-doped C60 LB films confirms, in accord with earlier findings,5 that there is a superconducting state with Meissner-Ochsenfeld flux exclusion in C60 LB films doped with potassium. This LFS signal with hysteresis should not be confused with another type of nonhysteretic LFS signal observed in conducting polymers at relatively high temperatures.24 Compared with bulk or signal crystal materials,7-12 the LFS intensity is much weaker, and the spectrum of LFS is noisier, presumably because the only LB films are too thin. The detection of superconductivity in C60 LB films is difficult, due to the small film thickness and its sensitivity to local K doping, compared to the bulk or single crystal case. Although we have confirmed the earlier report14 that monolayer packing at the air-water interface is possible for C60, we could not develop a reliable transfer of such a Langmuir monolayer of C60 from the air-water interface to any substrate: (i) starting with very dilute dropping solutions, the LB barrier had to be moved too close to the sample pickup arm to develop a significant film pressure, so several compressions would have to be performed to transfer an LB multilayer; (ii) we could not measure the transfer ratios; (iii) it is not clear whether a C60 Langmuir monolayer, during or after transfer to a substrate, would not recognize into small clusters. This situation frustrated the original purpose of our study, which was to investigate how the dimensionality could influence superconductivity. Bulk K3C60 is a three-dimensional superconductor; organic superconductors and inorganic cuprate superconductors are thought to be two-dimensional; a thin LB multilayer is a two-dimensional structure, so it was hoped that a study of LB films of C60, maybe interspersed with nonsuperconducting C70 monolayers, would provide a reliable system for study 2D ot 3D crossover phenomena in superconductivity. Since C60 is not amphiphilic, it does not behave in the classical LB sense; so far, nobody has studied derivatized C60 molecules with hydrophilic or hydrophilic groups for their possible superconductivity.

The ESR spectrum of C60 LB film doped with potassium shows three coexistent stages of doping, K3C60, K4C60, and K6C60. The conductivity measurement on our K-doped C60 LB film indicated a semiconducting state, because the sample was either not optimally doped or inhomogeneous. Superconductivity was detected in a conventional (nonLB) 1000 Å thick K-doped C60 thin film by a four-probe conductivity measurement, with Tc (midpoint) ) 8 K, ∆T (10%-90%) ) 10 K,25 while for a 960 Å film Tc (onset) ) 16 K, Tc (min. Ω) ) 5 K.1 Our earlier result for LB films (N1 ) 50) was Tc (onset) ) 8.1 K.5 The variation in onset temperatures is probably linked to film thickness. It is clear that thin films have lower Tc than a bulk sample or a single crystal, but the exact thickness dependence of Tc must await future work. After the present work was completed and submitted for publication, we were informed of a parallel effort26 which found superconductivity (onset as 23 K) in LB films of C60 exposed to heated rubidium azide, detected by a very weak LSF signal; no superconductivity was detected in LB films exposed to K metal, Rb metal, or KN3; the EPR signal (g ) 2.0010) was interpreted as due to a metallic Rb1C60 phase.26 The first report on superconductivity in LB films of C60 exposed to K (Tc (onset) ) 8 K),5 the present report (Tc (onset) ) 12.9 K for LB films of C60 exposed to K), and the independent study (Tc (onset) ) 23 K for LB films of C60 films exposed to RbN3)26 all agree that the superconductivity occurs in a small inhomogeneous region of a very small sample. The LFS signals shown in Figure 3a are somewhat stronger than those seen in Figure 3 of ref 26, but it is difficult to compare intensities, since the total amount of sample is not specified in ref 26, and inhomogenous doping is a constant menace. It is clear, however, that a study of the thickness dependence of superconductivity in LB films of A3C60 (A ) K, Rb, Cs) may be soon amenable to systematic study.

(24) Zakhidov, A. A.; Khairullin, I. I.; Sokolov, V. Y.; Baughman, R. H.; Iqbal, Z.; Maxfield, M.; Ramakrishna, B. L. Synth. Metals 1991, 41-43, 3717-3727.

(25) Maruyama, Y. Unpublished results. (26) Ikegami, K.; Kuroda, S.-I.; Matsumoto, M.; Nakamura, T. Jpn. J. Appl. Phys. Lett. 1995, 34, L1227.

Acknowledgment. We are grateful to the United States Air Force Office of Scientific Research (Contract AFOSR F49620-92-J-0529DEF) for their support. We thank Dr. S. Bandow and Dr. A. A. Zakhidov (Institute for Molecular Science, IMS) for valuable discussions. We are also indebted to Dr. K. Imaeda (IMS) for loaning us the double furnace system. P.W. appreciates the hospitality and financial support of IMS (Visiting Scientist Fellowship). LA9510846