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J. Phys. Chem. C 2007, 111, 16878-16882
Size-Induced Effects on the Superconducting Properties of Mo2C Nanoparticles Manoj K. Kolel-Veetil,† Syed B. Qadri,*,† Michael Osofsky,† Teddy M. Keller,† Ramasis Goswami,† and Stuart A. Wolf‡ NaVal Research Laboratory, Washington, DC 20375, and UniVersity of Virginia, CharlottesVille, Virginia 22904 ReceiVed: July 26, 2007; In Final Form: August 29, 2007
Nanoparticles of molybdenum carbide in two different phases were synthesized by the pyrolysis of a Moderived inorganic-organic hybrid system. The size and structure of the Mo2C nanoparticles were found to be uniquely influenced by the pyrolytic conditions: 2-4 nm particles were produced in the R-Mo2C phase at 850 °C, and larger particles with diameters ∼50-150 nm were formed in the β-Mo2C phase at temperatures above 850 °C, with a minor amount of the β-Mo2C phase being formed at this temperature. Resistivity measurements indicated that the samples containing the 50-150 nm particles were metallic with Tc’s above 4 K and the sample containing the 2-4 nm particles, including a minor contaminant fraction of the 50-150 nm particles, was insulating in behavior. During magnetic measurements, the samples with 50-150 nm particles appeared to be strongly diamagnetic and the sample with a majority of 2-4 nm particles was observed to be weakly diamagnetic.
Introduction The study of superconductivity in particles of very small dimensions has preoccupied researchers since the inception of the Bardeen, Cooper, and Schrieffer (BCS) theory,1 as a fundamental means for exploring the basic principles of physics at the transitional boundary between atomic and molecular species and condensed matter. The question as to what the lower size limit has to be of particles for the existence of superconductivity has been addressed by Anderson in the criterion named after him.2 According to the Anderson criterion, superconductivity would no longer be sustained when the mean (electronic) level spacing (d) at the Fermi level of a material becomes comparable or larger than its bulk superconducting energy gap (∆). As the ratio ∆/d yields the number of free-electron states that pair-correlate, which in essence is the number of Cooper pairs in a material, a value of ∆/d < 1 would clearly render a material not superconducting.3,4 The lower limit for the particle (grain) size that will permit superconducting ordering in a typical metal is generally calculated to be ∼3 nm.5 When this size limit is interpreted in terms of the number of electrons that are available in the particle to form a condensate, the number is of the order 103 for typical metals.2 In particular, when the particle size of a metallic superconductor is less than its superconducting coherence length, ξ0, then in completely isolated particles the lower size limit is predicted to be even larger than 3 nm.6 Recently, superconductivity has been reported in a crystalline Ga84-cluster compound that contains regularly arranged isolated Ga clusters, each containing 84 Ga atoms.7 These clusters, which are typically of 1.4 nm in size,8 were reported to exhibit a superconducting transition at Tc of ∼7.2 K and had an upper critical field of 13.8 T. The estimated mean level spacing and
the superconducting gap of this system are 45 and 1 meV, respectively, thereby giving a ∆/d value of ∼0.02, a value very low to be able to support superconductivity in this system. 69,71Ga NMR studies on this system have conclusively proven that the observed superconductivity is a result of a band-type conductivity arising from weak intercluster charge transfer among the Ga844- clusters, similar to doped C60 or other molecular superconductors.9-11 Further, in granular superconductor systems comprising nanoparticles of known superconductors and other insulating components, a loss in macroscopic (global) superconductivity is observed when the intergranular Josephson coupling length exceeds an optimum value. For example, in granular Al-Ge superconductors12,13 the transport properties of the granular mixtures changes from insulating to superconducting or vice versa depending on the change in the metal to insulator (Al to Ge) ratio. The transition to a global superconducting phase coherent state or to a superconducting insulator state (with Cooper pairs totally localized within individual grains) is observed to occur as a function of the intergranular Josephson coupling strength. Molybdenum carbide, in its various phases, is known to be a superconductor.14-21 The cubic R-Mo2C phase and the hexagonal β-Mo2C phase are known to be superconducting with Tc’s in the range of 6.7-7.319 and 8.1-12.2 K,18 respectively. The feasibility to produce Mo2C nanoparticles in either of these phases from a Mo-derived carboranylenesiloxane networked matrix system22 makes it an ideal system for exploring size effects on the associated superconducting properties of the two phases. In this Letter, we present the effects of the particle size on the formation of the different phases of Mo2C and on their superconducting properties. Experimental Section
* To whom correspondence should be addressed. E-mail: qadri@ anvil.nrl.navy.mil. † Naval Research Laboratory. ‡ University of Virginia.
The synthesis of the Mo-derived carboranylenesiloxane networked matrix system used in this study and its subsequent pyrolytic conversions to R- and β-molybdenum nanoparticles
10.1021/jp075920j CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007
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have been described elsewhere.22 X-ray analyses were performed with a Rigaku 18 kW X-ray generator and a high-resolution powder diffractometer. X-ray diffraction (XRD) scans of the samples were obtained by use of Cu KR-radiation from a rotating anode X-ray source. The temperature-dependent dc resistivity measurements were carried out by use of a standard four point probe configuration, with the sample being slowly lowered into a liquid helium Dewar. The resistivity values were estimated assuming that the samples were homogeneous and dense and thus should not be taken to be the true bulk values. Field-cooled and zero-field-cooled magnetization data were obtained in a 10 Oe field with a superconducting quantum interference device (SQUID) magnetometer for temperatures down to 1.75 K. A Philips CM-30 transmission electron microscope (TEM), operated at 300 kV, was used to obtain the microstructure and composition of the 850 and 1000 °C pyrolysis residues. Specimens for TEM analysis were made by crushing the material in ethanol into a fine powder and transferring a few drops of the liquid containing the fine powder onto a carbon-coated copper grid. Results and Discussion The pyrolytic conversions of cross-linked networked matrices formed from Mo-derived inorganic-organic hybrid carboranylenesiloxane oligomers produce ceramic mixtures containing Mo2C nanoparticles of controllable sizes. The size and the phase of the produced Mo2C nanoparticles were found to be uniquely influenced by the pyrolytic conditions. Smaller-sized particles in R-Mo2C phase, with diameter ∼2-4 nm, were formed at 850 °C, while larger-sized β-Mo2C nanoparticles, with diameter ∼50-150 nm, were formed at progressively higher temperatures such as 950, 1000, and 1250 °C. A small amount (∼10% by volume fraction of the crystalline products) of the larger β-Mo2C nanoparticles was also formed at 850 °C. In addition to the Mo2C nanoparticles, multiwalled carbon nanotubes (MWCNTs) were formed in the 1000 °C residue and MWCNTs and Mo nanoparticles were formed in the 1250 °C residue. Some amorphous materials were also observed in all of the samples. Figure 1 shows the resistivity, XRD scan, and magnetic susceptibility data for the sample prepared at 850 °C. The broad peaks in the XRD scan (Figure 1b) correspond to the cubic R-Mo2C phase (JCPDS 15-0457), and from their full width at half-maximum and by use of the well-known Scherrar’s equation, the particle sizes are estimated to be of the order of 2-4 nm. In addition, superimposed on these peaks are some (suggested) very weak sharp peaks, which can be attributed to a minute fraction of the hexagonal β-Mo2C phase (JCPDS 350787). The R-Mo2C nanoparticles are presumed to have evolved from the networked matrix with the highest possible concentrations of nascent molybdenum and carbon atoms. The volume fraction of the R-Mo2C nanoparticles was estimated to be about 90% from the TEM micrographs of the 850 °C pyrolysis product (panels a and b of Figure 2). The pyrolytic conditions at 850 °C appear to be more conducive to the substantial formation of the smaller-sized R-Mo2C nanoparticles than the larger-sized β-Mo2C nanoparticles. The broad baseline in the XRD spectrum also indicated the presence of some amorphous products in the pyrolytic residue. The resistivity data for the 850 °C sample plotted as a function temperature in Figure 1a suggested an insulating behavior despite the presence of the well-known superconducting species, R-Mo2C. This behavior is reminiscent of the semiconducting-like temperature dependence exhibited by the Ga84-cluster compound.7 Magnetization measurements (Figure 1c) showed a hint of superconductivity as evidenced
Figure 1. (a) The resistivity, (b) the XRD spectrum, and (c) the fieldcooled (FC) and zero-field-cooled (ZFC) magnetization in a 10 Oe field (nominal) of the 850 °C pyrolysis product.
by the weak diamagnetic response of the sample. There was no significant difference in the zero-field-cooled (ZFC) and the field-cooled (FC) signals. In comparison, in the case of the Ga84cluster superconductor, a clear diamagnetic signal was observed during magnetization measurements.7 To further understand and correlate the observed conductivity characteristics of the 850 °C sample with the Mo2C particle size, we analyzed samples treated at 950, 1000, and 1250 °C, respectively. The XRD analysis of the 950 °C pyrolysis product revealed the presence of both R-Mo2C and β-Mo2C nanoparticles. In this product, while the R-Mo2C nanoparticles were smaller in size (∼2-4 nm), as in the 850 °C product, the β-Mo2C nanoparticles were found to be larger (∼50-150 nm in size), as observed by the sharpening of the diffraction peaks for β-Mo2C in the XRD spectrum. The product exhibited a superconducting transition both by resistivity and by magnetic susceptibility measurements, producing strong diamagnetic responses. The onset of superconductivity was observed at 8 K. The onset Tc value was very
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Figure 2. TEM micrographs of the 850 °C pyrolysis product, containing predominantly (a) R-Mo2C nanoparticles and (b) R-Mo2C nanoparticles with a few β-Mo2C nanoparticles, and the 1000 °C pyrolysis product, containing essentially (c) larger β-Mo2C nanoparticles and (d) larger β-Mo2C nanoparticles with MWCNTs.
near the range of the reported Tc values (8.1-12.2 K) for β-Mo2C nanoparticles.18 The XRD spectrum (Figure 3b) of the pyrolysis product at 1000 °C predominantly contained larger β-Mo2C nanoparticles, which were also clearly evident in the TEM micrographs of the product (panels c and d of Figure 2). In addition, we observed XRD peaks associated with MWCNTs. The signature peak for the MWCNTs is evident at 2θ ) 25.9°. The parallel strands of MWCNTs (not shown) were also observable in the TEM micrographs of the pyrolytic residue. Resistivity measurements of the 1000 °C pyrolytic residue revealed a broad superconductive transition beginning at 8 K and with F ) 0, below 4 K (Figure 3a). The magnetization data shown in Figure 3c also exhibit a clear superconducting transition, as evidenced by the strong diamagnetic response of the sample. The diamagnetic response of the ZFC signal was of an order of magnitude greater than that of the FC signal. The product prepared at 1250 °C exhibited similar characteristics to those observed with the 1000 °C product in terms
of structure and superconducting properties. However, in the 1250 °C product, in addition to β-Mo2C nanoparticles and MWCNTs, the presence of Mo nanoparticles (JCPDS 42-1120) was also observed. The distinctly apparent superconducting property in the products prepared at 950, 1000, and 1250 °C as evident from their clear diamagnetic signals is reasonably accountable and seems to directly point to the commonly present large-sized β-Mo2C nanoparticles as its origin. The presence of other constituents such as MWCNTs, Mo nanoparticles, and amorphous residual carbon in these products can be discounted to produce a critical temperature onset at 8 K. While pure Mo has shown superconductivity at only 1 K,23 end-bonded MWCNTs have been reported recently to exhibit superconductivity with a transition temperature as high as 12 K.24 However, such an arrangement of the MWCNTs can only be achieved in a template-driven synthesis. In contrast, the origin of the weak diamagnetic signal as apparent for the 850 °C product that contains mainly the smaller 2-4 nm R-Mo2C particles and a
Superconducting Properties of Mo2C Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16881 order of 103) for particles of typical metals necessary to form a condensate capable of supporting superconductivity.2 However, recent calculations on small nanoclusters with the number of delocalized electrons in the range 102-103 have revealed that electronic pairing can be favorable in such clusters.3 These clusters possess shell structures, similar to those in atoms or nuclei, which contain highly degenerate energy levels, or group of energy levels, with very small energy spacing between electronic states and the Fermi level, producing a favorable situation for electronic pairing. Even though the calculations involved only simple metals (Ga, Al, Zn, Cd, etc.) and not transition metals, such a favorable electronic pairing situation could be expected to be present in clusters of transition-metal compounds such as in the 2-4 nm sized R-Mo2C particles. The similarities in the resistivity and magnetic susceptibility of the 850 °C product with 2-4 nm sized R-Mo2C particles and the Ga84-cluster compound raise the question as to whether a similar weak intercluster charge transfer is in operation in the “R-Mo2C particles” case. As these clusters are assumed to be neutral and not charged as in the case of Ga844- clusters, such a possibility is unlikely. However, the chance that the clusters have become charged due to interaction with the surrounding amorphous matrix material cannot be discounted. If the smallersized R-Mo2C particles were indeed the cause for the observed superconductivity, the greatly diminished signal suggests that only a small volume fraction of the R-Mo2C particles contributes to it, perhaps the fraction from the 2-4 nm sized particles with the proper size parameter supports the superconductivity. It is also likely that a large volume fraction of the R-Mo2C particles is not “perfectly” arranged in the crystals for supporting superconductivity at the annealing temperature of 850 °C. Perhaps, such a long-range order could be induced at higher annealing temperatures (950 °C and above). However, at such temperatures, the smaller-sized R-Mo2C particles are found to convert to the larger-sized β-Mo2C particles. The presence of a minor fraction of the larger-sized, 50150 nm, β-Mo2C particles in the 850 °C product also opens the possibility that the observed superconductivity is derived from such species. It is possible that such particles could exhibit localized superconductivity rather than global superconductivity due to reduced intergranular Josephson coupling.12,13 If in fact these particles are causing the observed superconductivity, then the 2-4 nm sized R-Mo2C particles could in fact act as the intergranular coupling agents. Summary
Figure 3. (a) The resistivity, (b) the XRD spectrum (including the lattice parameters of β-Mo2C), and (c) the field-cooled (FC) and zerofield-cooled (ZFC) magnetization in a 10 Oe field (nominal) of the 1000 °C pyrolysis product.
small fraction of the larger 50-150 nm β-Mo2C particles needs further elaboration. The size distribution of 2-4 nm of the smaller-sized R-Mo2C particles in the 850 °C product places them around the lower size limit (∼3 nm) that is required for permitting a superconducting ordering in a typical metal.5 The number of delocalized electrons available in the 2-4 nm sized Mo2C clusters containing roughly 5-10 Mo2C entities is in the order of 100-200. This is much below the predicted number of electrons (of the
Nanoparticles of Mo2C were prepared in the R-Mo2C and β-Mo2C phases, with R-Mo2C nanoparticles of dimensions ∼2-4 nm and β-Mo2C nanoparticles of dimensions ∼50-150 nm, from a Mo-derived inorganic-organic hybrid system. The pyrolytic conditions at 850 °C appear to be more conducive to the substantial formation of smaller-sized R-Mo2C nanoparticles, and the ones at higher temperatures of 950, 1000, and 1250 °C facilitated the formation of larger-sized β-Mo2C nanoparticles. Resistivity measurement of the sample formed at 850 °C containing a majority of the 2-4 nm sized R-Mo2C nanoparticles and a minor fraction of the 50-150 nm sized R-Mo2C nanoparticles exhibited a semiconducting-like temperature dependence. In contrast, resistivity measurements of the samples containing mainly the 50-150 nm sized R-Mo2C nanoparticles (the 950, 1000, and 1250 °C products) revealed superconducting transitions. During the magnetization measurements, a weak diamagnetic signal was observed for the 850 °C product and strong diamagnetic signals were observed for the 950, 1000,
16882 J. Phys. Chem. C, Vol. 111, No. 45, 2007 and 1250 °C products. The observed resistivity and magnetization data have been interpreted on the basis of the sizes of the R-Mo2C nanoparticles and the β-Mo2C nanoparticles. These results represent the first instance of an unusual behavior in the Mo2C system wherein the complex interplay among the precise arrangement of a particular Mo2C nanoscaled species, the topology within a single species, and its effect on the associated superconducting property is observed. Acknowledgment. The authors acknowledge the Office of Naval Research for its financial support of this work. References and Notes (1) Bardeen, J.; Cooper, L. N.; Schrieffer, J. R. Phys. ReV. 1957, 108, 1175. (2) Anderson, P. W. J. Phys. Chem. Solids 1959, 11, 26. (3) Kresin, V. Z.; Ovchinnikov, Y. N. Phys. ReV. B 2006, 74, 024514. (4) von Delft, J. Ann. Phys. Leipzig, Ger. 2001, 10 (3), 219. (5) Jaeger, H. M.; Haviland, D. B.; Orr, B. G.; Goldman, A. M. Phys. ReV. B 1989, 40 (1), 182. (6) Muhlschlegel, B.; Scalapino, D. J.; Denton, R. Phys. ReV. B 1972, 6, 1767. (7) (a) Hagel, J.; Kelemen, M. T.; Fisher, G.; Pilawa, B.; Wosnitza, J.; Dorman, E.; Lohneysen, H. v.; Schnepf, A.; Schno¨ckel, H.; Neisel, U.; Beck, J. J. Low Temp. Phys. 2002, 129 (314), 133. (b) Bono, D.; Schnepf,
Kolel-Veetil et al. A.; Hartig, J.; Schno¨ckel, H.; Nieuwenhuys, G. J.; Amato, A.; de Jongh, L. J. Phys. ReV. Lett. 2006, 97 (7), 077601. (8) Schnepf, A.; Schno¨ckel, H. Angew. Chem., Intl. Ed. 2001, 40, 711. (9) Bakharev, O. N.; Zelders, N.; Brom, H. B.; Schnepf, A.; Schno¨ckel, H.; Jos de Jongh, L. Eur. Phys. J. D 2003, 24, 101. (10) Bakharev, O. N.; Bono, D.; Brom, H. B.; Schnepf, A.; Schno¨ckel, H.; de Jongh, L. J. Phys. ReV. Lett. 2006, 96 (11), 117002. (11) Hartig, J.; Schnepf, A.; de Jongh, L. J.; Bono, D.; Schno¨ckel, H. Z. Anorg. Allg. Chem. 2007, 633, 63. (12) Shapira, Y.; Deutscher, G. Phys. ReV. B 1983, 27 (7), 4463. (13) Gerber, A.; Milner, A.; Deutscher, G.; Karpovsky, M.; Gladkikh, A. Phys. ReV. Lett. 1997, 78 (22), 4277. (14) Meissner, W. Z. Phys. 1930, 58, 570. (15) Meissner, W.; Frank, H. Z. Phys. 1930, 65, 30. (16) Meissner, W.; Frank, H.; Westerhoff, G. Z. Phys. 1932, 75, 521. (17) Toth, L. E.; Rudy, E.; Johnston, J.; Parker, E. R. J. Phys. Chem. Solids 1965, 26, 517. (18) Sadagopan, V.; Gatos, H. C. J. Phys. Chem. Solids 1966, 27, 235. (19) Morton, N.; James, B. W.; Wostenholm, G. H.; Pomfret, D. G.; Davies, M. R.; Dykins, J. L. J. Less-Common Met. 1971, 25, 97. (20) Toth, L. E.; Zbasnik, J. Acta Metall. 1968, 16, 1177. (21) Matthias, B. T.; Hulm, J. K. Phys. ReV. 1952, 87, 799. (22) Kolel-Veetil, M. K.; Qadri, S. B.; Osofsky, M.; Keller, T. M. Chem. Mater. 2005, 17 (24), 6101. (23) Geballe, T. H.; Matthias, B. T.; Corenzwit, E.; Hull, G. W., Jr. Phys. ReV. Lett. 1962, 8, 313. (24) Takesue, I.; Haruyama, J.; Kobayashi, N.; Murata, N.; Chiashi, S.; Maruyama, S.; Sugai, T.; Shinohara, H. Phys. Status Solidi B 2006, 243 (13), 3423-3429.
