Superconducting Fullerene Nanowhiskers

We synthesized superconducting fullerene nanowhiskers (C60NWs) by potassium (K) intercalation. They showed large superconducting volume fractions, as high as 80%. The superconducting transition temperature at 17 K was independent of the K content (x) in the range between 1.6 and 6.0 in K-doped C60 nanowhiskers (KxC60NWs), while the superconducting volume fractions changed with x. The highest shielding fraction of a full shielding volume was observed in the material of K3.3C60NW by heating at 200 °C. On the other hand, that of a K-doped fullerene (K-C60) crystal was less than 1%. We report the superconducting behaviors of our newly synthesized KxC60NWs in comparison to those of KxC60 crystals, which show superconductivity at 19 K in K3C60. The lattice structures are also discussed, based on the x-ray diffraction (XRD) analyses.


Introduction
Fullerenes were discovered in 1985 [1], and the superconductivity of a potassium metal-doped fullerene was reported in 1991 [2]. Potassium metal (K)-doped fullerides K x C 60 [0 ≤ x ≤ 6] are particularly interesting because their structures and electronic properties are strongly related to the doping concentration. They exhibit an fcc structure at x = 0 and 3, a bct structure at x = 4, and a bcc structure at x = 6 [3]. The compound, K 3 C 60 , shows superconductivity below 19 K, while the others OPEN ACCESS exhibit insulating, semiconducting or metallic properties. In general, superconducting K 3 C 60 bulk samples have been synthesized mainly by three methods, i.e., a solid-solid reaction, vapor evaporation, and a reaction using liquids. Much effort has been expended to produce the K 3 C 60 superconductor, but a large volume fraction was difficult to obtain by a simple heating method. In addition, the obtained bulk superconductors of K 3 C 60 by the above methods were usually in powder form. The above two were the problems inherent in the bulk application of K 3 C 60 superconductors.
A recent study of fullerene-based supramolecular nanoarchitectures [4][5][6][7][8][9] is opening new possibilities for the application of fullerene materials; such applications include sensors, transistors, catalysts, and fuel cell electrodes. Fullerene nanowhiskers were obtained from the interface between the fullerene-saturated solution and fullerene-insoluble solvent. They vary in length from microns to centimeters. Most of the cuprate superconductors form powders or bulk polycrystals, which were encapsulated in metal tubes for making superconducting wires. This makes the process complicated and the wire heavy. In this study, we propose fullerene nanowhiskers for use as flexible and lightweight superconducting wire because of the advantages in the nanowhisker form.

Observation of Morphologies
The morphology of C 60 nanowihskers (C 60 NWs) used in this study are shown in Figure 1(a) observed using a scanning electron microscope (SEM). The shapes of C 60 NWs are mostly hexagonal prisms with the growth axis along the [001] direction [10]. C 60 NWs grow in a hexagonal crystal structure by the liquid-liquid interfacial method (LLIP method) [4,5], and the hexagonal structure then turns to the fcc structure while the solvent is drying. The C 60 NWs are composed of C 60 units bound by van der Waals force. Figure 1(c) shows the micrograph of C 60 NWs observed using a transmission microscope (TEM), indicating disordered nanopores formed at the stage of the drying process in the LLIP method. The 10 mg C 60 NWs were taken in a quartz tube with an appropriate amount of K (molar ratio K/C 60 = 0-6.0). The quartz tube was then sealed under an evacuated condition at about 3 × 10 −3 Pa and heated at 200 °C for 24-36 h for K intercalation into C 60 NWs. The SEM micrograph and TEM image of the K-intercalated samples of K 3.3 C 60 NWs are shown in Figure 1(b) and 1(d), respectively. Basically, the morphology of the K-doped samples looks the same as the pristine samples showing the nanopore structure. The energy dispersive x-ray analysis (EDAX) revealed that K was taken in the C 60 NWs. In this report, all of the compositions denote nominal ones. We also prepared K-doped C 60 crystals to compare the K-doped C 60 NWs. Several micro-cracks were observed in K 3.3 C 60 crystals, while no crack was observed in K 3.3 C 60 NWs. K-doping into the C 60 crystal expands the lattice of the C 60 crystals; therefore, cracks seemed to be induced because of the boundary strain from the lattice mismatch between K-doped and undoped lattices. On the other hand, in the case of K-doped C 60 NWs, the disordered nanopores play a possible role in reducing the lattice strain. Figure 2 shows the superconducting transitions of K 3.3 C 60 NWs and K 3.3 C 60 crystals heated at 200 °C for 24 h, which were measured upon warming under 20 Oe after cooling in a zero-field. The nominal K composition indicates the ratio vs. the C 60 unit. The left and right ordinates of the graph set separately for K 3.3 C 60 NWs and K 3.3 C 60 crystals are the magnetic moment normalized by the applied magnetic field and the weight of the samples. In K-doped C 60 materials, the only fcc phase of stoichiometric K 3 C 60 is a superconductor. We added a 10% excess amount of K, more than the crystallographically limiting composition K 3 C 60 NW and K 3 C 60 , because the K adsorption on the surface of crystallites or in the nanopores was considered to ensure complete reaction. As shown in the figure, there is a large difference of the superconducting signals between the two samples. The signal of the nanowhisker sample was 200 times larger than that of the crystal sample. The T c at 17 K in K 3.3 C 60 NWs is lower than the reported T c at 19 K in K 3.3 C 60 crystals, as shown in Figure 2. We believe the T c reduction in K 3.3 C 60 NWs can be caused from the disordered nanopores as seen in Figure 1(d).

Superconducting Properties and X-ray Diffraction Patterns
The powder x-ray diffraction (XRD) patterns of C 60 , K 3.3 C 60 crystals, C 60 NWs, and K 3.3 C 60 NWs are shown in Figure 3. The XRD patterns of non-doped C 60 crystals and C 60 NWs showed an fcc structure, as previously reported [3]. In addition, those of K 3.3 C 60 crystals and K 3.3 C 60 NWs were also identified to be the fcc structure phase as the peaks were indexed in the figure. The lattice parameters of four samples, C 60 , K 3.3 C 60 crystals, C 60 NWs, and K 3.3 C 60 NWs, were calculated to be 1.4180(5), 1.4180(2), 14.188(4), and 1.4200(5) nm, respectively. Therefore, K was hardly intercalated into the sites in the K 3.3 C 60 crystals because its lattice parameter was almost identical to that of C 60 crystals. This result is consistent with the difference of the superconducting shielding volume fractions between K 3.3 C 60 NWs and K 3.3 C 60 crystals by the nominal compositions. According to reported processes for K-doping to C 60 , so far, a period longer than several days or several weeks is required to form the superconducting K 3 C 60 phase [11,12]. We investigated the formation rate of the superconducting phase by measuring the superconducting volume fractions at 5 K as shown in Figure 4. In the case of K 3.3 C 60 NWs, it was saturated at around 24 h with heating at 200 °C. On the other hand, the shielding fraction of K 3.3 C 60 crystals hardly increased up to 36 h. Such a small volume fraction of the superconducting K x C 60 has already been reported, for example, by Hebard et al. [2] and Murphy et al. [3] Hebard et al. reported approximately 1% by heating at 200 °C for 36 h. We believe that the nanopores of K 3.3 C 60 NWs, shown in Figure 1(d), assist in the reaction of K-doping and the migration to form the K 3 C 60 NW superconducting phase, as explained in Figure 2. The nanopores in K 3.3 C 60 NWs play an important role for the formation of superconducting phase.    The K composition indicates the nominal ratio vs. C 60 unit in the C 60 NWs. The ordinate of the graph is the magnetic moment normalized by the applied magnetic field and the volume of the samples. None of the samples before heating showed any anomalies within a temperature range between 2 K and 30 K. The onset T c 's of the materials after heat treatment at 200 °C were almost the same (17 K) independently of the K composition, while the superconducting shielding fractions depended on the K-ratio. Figure 6 shows the compositional dependence of the superconducting volume fractions in K x C 60 NWs (nominal composition) heated at 200 °C for 24 h in comparison with those in K x C 60 reported by Holczer et al. [11]. Figure 6. K content dependence of shielding volume fractions in K x C 60 NWs (at 5 K) compared with the result by Hoczer et al. [11] for K x C 60 (at 4.2 K).

K-Compositional Dependence of Shielding Volume Fraction in K x C 60 NWs
Their heating procedure consists of three stages which are: (1) the first mixing stage heated at 200 °C for 20-24 h, (2) the second diffusion stage heated to 200 °C for 22 h and (3) the final relaxation stage heated at 250 °C for 6 h or more hours. Basically, their result coincides with our result in K x C 60 NWs. The shielding volume fractions are normalized by the volume of the perfect diamagnetism (−1/4). The maximum fraction was observed at around 3.0-3.3 by the nominal K compositions. This value coincides with the carrier concentration at the T c maximum in alkali-doped C 60 superconductors [12]. In the K x C 60 system, the superconducting phase is a line compound of K 3 C 60 with the full occupancy of two tetrahedral and one octahedral sites by K in the fcc structure. The three electrons transferred from K to C 60 occupy the triply degenerated t 1u orbital, which becomes half-filled, and the high density of states at the Fermi level. We believe that this logic of K 3 C 60 is analogical for K 3 C 60 NW. On the other hand, there is a difference in the case of x = 6.0 in K x C 60 NW, showing some superconducting volume fraction. Murphy et al. explained that the non-superconducting bcc phase (K 6 C 60 ) was formed as a kinetically facile phase at the first step of the reaction [13]. Regarding our XRD measurement for the nominal K 6 C 60 NW in the heating condition at 200 °C by 36 h, no bcc phase was detected in contrast to the Murphy's explanation. Since the lattice parameter of K 6.0 C 60 NW is 1.4210(9) nm, which is almost the same as 1.4200(5) nm of K 3.3 C 60 NW, an excessive amount of K over 3.0 might stay at the surface or in the nanopores of C 60 NW.

Experimental
The typical dimensions of the fullerene nanowhiskers (C 60 NWs) used in this experiment were 0.54 ± 0.16 μm in average diameter and 4.43 ± 2.63 μm in average length. The C 60 NWs were prepared by using the liquid-liquid interfacial method (LLIP method) [4,5]. The schematic diagram is illustrated in Figure 7. A C 60 -saturated toluene solution was taken in a glass bottle, and isopropyl alcohol was slowly added. The C 60 NWs form at the interface of the two solutions, then the nanowhiskers were filtered and dried in vacuum at 100 °C for 2 h [14]. According to the previous report [15], the residual toluene solvent in C 60 NWs is estimated to be about 0.2 mass %. Ten mg C 60 NWs and an appropriate amount of potassium (K) were placed together into a thin quartz tube. The nominal K compositions were set at 0.0, 1.6, 2.3, 3.0, 3.3, 4.0, 4.6 and 6.0 mole ratio vs. C 60 in K x C 60 NWs. We also prepared pristine and K-doped C 60 crystals using the same procedures for a comparison with K x C 60 NWs. This process was conducted in a glove box to prevent potassium from oxidizing. The quartz tube was sealed under a vacuum condition at 3 × 10 −3 Pa, followed by heating at 200 °C for 1-36 h in an electric oven. After the heat treatment, superconducting properties and structure analyses were performed as follows. Superconducting transitions were measured using a superconducting quantum interference device (SQUID) magnetometer (MPMS-5S, Quantum Design, San Diego, CA, USA) as the samples were kept in the quartz tube. The shapes and microstructures of those samples were observed with a scanning electron microscope (SEM 25kV, Hitachi SU-70, Tokyo, Japan) and a transmission electron microscope (TEM 400kV, JEOL JEM-4010, Tokyo, Japan). Their qualitative micro-analysis was achieved with an energy dispersive X-ray analyzer (EDAX, AMETEK, Mahwah, USA). The information of the crystal structure was analyzed by powder X-ray diffraction (XRD RINT-TTR3, RIGAKU, Akishima, Japan). To prevent the XRD samples from being oxidized, we used a tiny amount of paraffin oil to cover them on the holder plate. In the XRD patterns, the diffraction of oil was subtracted from the raw data.

Conclusions
We proved that K intercalation to C 60 NWs, rather than the process for K 3 C 60 , forms superconducting K 3 C 60 NWs with a short heating process. No bcc phase of K 6 C 60 NW was observed. The superconducting shielding volume fraction by heating at 200 °C for 24 h gave high values up to 80%, in contrast to the value of K-doped C 60 , K 3.3 C 60 , which was lower than 1%. This contrasting difference of the superconducting shielding fraction might be associated with the nanopores in K x C 60 NWs. These nanopores play an important role in the properties of K x C 60 NWs.