NANO LETTERS
Single Crystalline Nanowires of Lead Can Be Synthesized through Thermal Decomposition of Lead Acetate in Ethylene Glycol
2003 Vol. 3, No. 8 1163-1166
Yuliang Wang,† Thurston Herricks,‡ and Younan Xia*,† Department of Chemistry, and Department of Materials Science and Engineering, UniVersity of Washington, Seattle, Washington 98195 Received June 12, 2003
ABSTRACT This paper describes a solution-phase route to the facile synthesis of lead nanowires with lateral dimensions as thin as 35 nm and lengths up to 250 µm. These lead nanowires were single crystalline in structure, and electron transport measurements on individual nanowires (using the four-probe method) indicate a phase transition to the superconducting state around 7.13 K, a temperature similar to the value (7.20 K) reported for bulk lead. Different from polycrystalline samples, no significant variation in the transition temperature was observed for nanowires with diameters down to 50 nm.
Nanowires have been a subject of intensive research in recent years as researchers are intrigued by the interesting properties and applications of nanowires.1 Most of the reported work in this area has focused on metals, semiconductors, and dielectric materials.2-4 There are only a limited number of studies dealing with the synthesis and characterization of nanowires from materials that might exhibit superconducting properties. Among the published results, Yang et al. have demonstrated the synthesis of superconducting nanowires of MgB2 by reacting single crystalline nanowires of B with the vapor of Mg.5 Several other groups have prepared nanowires of Pb by templating against channels in porous membranes or steps on Si substrates.6 Although it was possible (as demonstrated by Schwarzacher and Jalochowski et al.) to obtain single crystalline nanowires of Pb by carefully controlling the deposition conditions, most of the samples prepared using these methods were polycrystalline. It also remains a considerable challenge to increase the lengths of these nanowires to scales beyond 20 µm. Another disadvantage of the membrane-assisted methods is that the number of wires that can be produced in each run of synthesis is restricted by the surface density of pores. For a typical anodic alumina (or polycarbonate) membrane, the surface density of pores is usually below 1012/cm2. In addition, defects on the surfaces of the channels may lead to the formation of poorly defined nanowires that often exhibit kinks and uneven * Corresponding author. † Department of Chemistry. ‡ Department of Materials Science and Engineering. 10.1021/nl034398j CCC: $25.00 Published on Web 06/26/2003
© 2003 American Chemical Society
thickness. Here we report a solution-phase method for the synthesis of single crystalline nanowires of Pb with diameters as thin as ∼35 nm and lengths up to ∼250 µm. Superior to the template-based method, this approach allowed for the production of Pb nanowires in relatively large quantities (on a scale close to the Avogadro’s number). The nanowires could also be easily collected from the reaction solution through centrifugation and then be deposited onto patterned electrodes for transport measurements. Different from previous measurements that were performed on arrays of multiple Pb nanowires using the two-probe technique,6 here the superconducting transition was measured on individual Pb nanowires of different sizes using the four-probe method (in an effort to avoid contact resistances). Lead nanowires were generated by refluxing ethylene glycol solutions that contained both lead acetate (the precursor) and poly(vinyl pyrrolidone) (PVP). In a typical synthesis, 0.05 g Pb(CH3COO)2‚3H2O and 0.05 g PVP (Mw ≈ 55,000), both dissolved in 5 mL ethylene glycol, were simultaneously added drop by drop (using syringes) into 10 mL boiling ethylene glycol hosted in a three-neck flask. The reaction was protected with a continuous flow of N2 gas and kept under constant magnetic stirring. A gray color appeared in the solution after about ∼30 min, indicating the formation of metallic lead. After another ∼30 min, the final product (i.e., Pb nanowires) was quickly separated from the hot reaction solution through centrifugation. If necessary, the collected Pb nanowires could also be washed with acetone and ethanol to remove PVP deposited on the surfaces of these
nanowires. In this approach, the production of Pb was believed to originate from the thermal decomposition of lead acetate, a process that has been investigated in the solid phase using techniques such as thermogravimetry (TG) and differential thermal analysis (DTA).7 In these studies, both Pb and PbO had been identified as the final products depending on the reaction temperature (up to 450 °C) and environment (under N2 or in air). The minimum temperature required for the formation of pure Pb was around 325 °C. When the decomposition reaction was performed in a solution phase such as ethylene glycol (bp ) 198 °C) and under the protection of N2 gas, we found that Pb could be obtained as the predominant product (together with acetic acid as a byproduct) at the boiling temperature of ethylene glycol. In this case, the decomposition temperature of lead acetate was greatly reduced by dissolving the precursor in a liquid phase to become molecular species. It was also found that PVP had to be introduced in appropriate quantities to provide a good control over the morphological evolution of resultant Pb nanostructures. The formation of Pb nanowires was believed to follow the Ostwald ripening process8 that was recently demonstrated for use by our group in the synthesis of Ag nanowires.9 During the reaction, the majority of Pb atoms (coming from thermal decomposition of lead acetate) were precipitated out as Pb nanoparticles (5-10 nm in size). Some of these nanoparticles were able to grow into large crystals with their sizes on the scale of submicrometers. In the following steps, the large Pb crystals could serve as the seeds to initiate the growth of nanowires as the small Pb nanoparticles were continuously dissolved. In this mechanism, the formation of nanowires was realized through the spontaneous transfer of Pb atoms from the surfaces of Pb nanoparticles to the surfaces of microscale seeds. Figure 1 shows the SEM and TEM images of a sample that was collected at the early stage of Pb nanowire synthesis. Figure 1A shows a scanning electron microscopy (SEM) image that displays three different forms of lead: small Pb nanoparticles (the source of Pb atoms for the growth of wires); large Pb crystals (the seeds); and the wires growing out from the corners of Pb seeds. The inset of this figure gives a magnified view of the roots of several nanowires. This image clearly indicates that the growth of all Pb wires were initiated from the surfaces (in particular, the corners) of seeding crystals. Figure 1B gives a transmission electron microscopy (TEM) image of the same sample, detailing the small Pb nanoparticles at a much higher magnification. The sizes of these nanoparticles are less than 10 nm. The selected-area electron diffraction (SAED) pattern shown in the inset of this image confirmed that these small particles were nanocrystallites of facecentered cubic lead. The exact role of PVP in this synthesis is yet to be completely understood. It is believed that its partial role was to prevent the aggregation of Pb nanoparticles in the nucleation stage. In addition, PVP molecules might be able to modulate the growth kinetics of the Pb seeds, as controlled probably by their different adsorption energies on various crystallographic facets of face-centered cubic lead. Depend1164
Figure 1. (A) SEM image of a sample collected after lead acetate had been introduced for 45 min. The inset provides a closer look at the roots of several nanowires. (B) TEM image of the same sample in an effort to better resolve the small Pb nanoparticles seen in (A). The inset shows a typical SAED pattern recorded from the random assembly of these nanoparticles.
ing on the ratio between PVP to Pb(CH3COO)2, the final product could display a range of different morphologies such as wires, hexagonal flakes, and triangular plates. When the ratio between PVP and Pb(CH3COO)2 was around 3.5, the content of Pb nanowires contained in a typical, as-synthesized sample was usually greater than 80% (by weight). The nanowires could be easily separated from the particles by centrifuging the product solution at 2000 rpm, with ethanol as the dispersion medium. For a typical synthesis, although 80% of the nanowires had their diameters confined to the range of 80-100 nm, wires as thin as 35 nm were also observed in the final product. By varying the concentration of lead acetate and the ratio between lead acetate and PVP, the yields of nanowires with various diameters could also be varied. A systematic study on this synthesis is under way, and the results will be published in an upcoming full paper. We have characterized the Pb nanowires using a number of techniques. Figure 2A shows the SEM image of a typical example of as-synthesized Pb wires with diameters of 85 ( 5 nm. This image demonstrates the copius quantity that could be easily achieved using this approach. Figure 2B shows a TEM image of the same sample in which Pb wires as small as ∼35 nm in diameter could also be observed. The inset displays a typical SAED pattern that was recorded from an individual nanowire, and all diffraction spots could be indexed to face-centered cubic lead. The diffraction pattern was essentially unchanged when the electron beam was Nano Lett., Vol. 3, No. 8, 2003
Figure 2. (A) SEM image of as-synthesized Pb nanowires that were 85 ( 5 nm in diameter and up to 250 µm in length. (B) TEM image of another sample showing Pb nanowires with two different sizes. The inset gives a SAED pattern taken from an individual wire of 90 nm in diameter. (C) HRTEM image taken from one side of an individual Pb nanowire. The fringe spacing along the [111] axis is indicated. (D) The XRD pattern recorded from a large number of Pb nanowires separated from the reaction solution by centrifugation.
scanned across each individual nanowire, suggesting that the Pb nanowires prepared using this procedure were single crystals. Electron diffraction studies also indicated a growth direction along the [110] axis. Figure 2C is a high-resolution TEM image showing the edge of a Pb nanowire. With the use of fast Fourier-transfer technique, we could resolve the lattice spacing of this single crystalline nanowire and index it to that of face-centered cubic Pb. Because the melting point of Pb is relatively low (328 °C), the wires were highly susceptible to the beam damage (especially for a relatively thinner region like the edge) as they were exposed to a flux of high-energy electrons. In this figure, we noted that the right edge of this wire had stacking defects, which could have resulted from electron-induced damage. Figure 2D shows the X-ray diffraction (XRD) pattern taken from a sample containing many Pb nanowires. Together with the SAED pattern shown in Figure 2B, it could be concluded that the Pb nanowires were crystallized in the pure facecentered cubic phase. We also investigated the temperature-dependent, electron transport properties associated with individual Pb nanowires of different diameters. In a typical procedure, the freshly prepared Pb nanowire was deposited from a dilute dispersion in ethanol (via slow evaporation of solvent) onto four gold electrodes that had been patterned on a glass slide. Due to the low melting point of lead, good contacts between the lead nanowire and the gold electrodes could be ensured by focusing the high-energy electron beam of a scanning electron microscope on the contacted region to briefly anneal the contacts by heating. For the same reason, the currents used in all measurements were controlled below 1 µA to avoid the possible burning of individual thin nanowires of Nano Lett., Vol. 3, No. 8, 2003
Figure 3. Plots showing the dependence of resistance on temperature for Pb nanowires of two different sizes. The resistance was normalized against the value recorded at room temperature. The inset shows the SEM image of a typical sample used for the fourprobe measurement, where a single Pb nanowire was deposited on four gold electrodes equally separated by 25 µm. The electrodes denoted with I+ and I- were used as sources for current feeding; V+ and V- were used for measuring the potential drop. The resistances of both nanowires exhibited a clear transition to the superconducting state at a critical temperature similar to that of bulk lead.
Pb. Silver paste and copper wires were used to connect the leads of gold electrodes to the outside. A Keithley 236 sourcemeter was used to provide currents of various values and the voltages were measured using a Keithley 155 microvoltimeter. At room temperature, the resistance measured for individual Pb nanowires could vary from hundreds of ohms to several kiloohms, depending on the lateral dimensions of the nanowires. We note that several groups have already observed the phase transition from metal to superconductor for Pb nanowires prepared by templating against porous membranes.6a-6d All these measurements were, however, performed on parallel arrays of multiple nanowires by directly attaching contacts to the top and bottom surfaces of a membrane (used as the template). The measured resistance was, therefore, an averaged response from multiple nanowires that were inevitably different in size, structure, and crystallinity. Here we focused on individual nanowires of Pb by monitoring their changes in resistance with temperature using the four-probe method (as shown in the inset of Figure 3). A continuous flow cryostat (SuperTran-VP, Janis Research) was employed to conduct the measurements of resistance at temperatures down to 4.2 K. Figure 3 shows the dependence of normalized resistance on temperature for two individual Pb nanowires that were 50 and 85 nm in diameter, respectively. Both nanowires displayed a sharp decrease in resistance when the temperature was reduced down to 7.13 K, a value similar to the critical temperature Tc (7.20 K) reported for bulk lead. This observation provides another evidence to support our claim about the single crystallinity of the Pb nanowires synthesized using the present method. According to previous studies on electrodeposited Pb nano1165
wires, polycrystallinity is expected to introduce disorders into the wires, and thus cause a reduction in the transition temperature.6a,6d Note that the resistance in this figure was not completely reduced to zero; this may be due to the fact that during our measurement, when temperature was still high compared to Tc, the resistance heat produced in the system could slowly transform the surface of the wire to a thin layer of semiconductive lead oxides. Such an oxidation reaction on the surface of lead nanowires has also been observed by other groups when the wires were exposed to air and/or an aqueous environment for a certain period of time.6d As limited by the capability of our current instrument, we have not been able to perform transport measurements on Pb nanowires thinner than 50 nm in diameter. In summary, we have demonstrated a practical approach to the large-scale synthesis of single crystalline nanowires of Pb with lateral dimensions as thin as 35 nm and lengths up to ∼250 µm. The four-probe measurements on individual nanowires suggested a phase transition to the superconducting state around 7.13 K. No significant variation in the transition temperature was observed for Pb nanowires of two different diameters (85 and 50 nm). The present study suggests that single crystalline nanowires of Pb with diameters down to 50 nm could still be used as superconductors at temperatures below 7.13 K. Acknowledgment. This work has been supported in part by an AFOSR-DURINT subcontract from SUNY Buffalo, a Career Award from the National Science Foundation (DMR-9983893), and a Fellowship from the David and Lucile Packard Foundation. Y.X. is an Alfred P. Sloan Research Fellow (2000) and a Camille Dreyfus Teacher Scholar (2002). T.H. thanks the Center for Nanotechnology at the UW for an IGERT Nanotechnology Fellowship Award supported by the NSF (DGE-9987620). We thank Professor
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David Cobden for suggesting the synthetic target described in this paper. References (1) See, for example, (a) a special issue on one-dimensional nanostructures, AdV. Mater. 2003, 15, 341. (b) a special issue on nanowires, MRS Bull. 1999, 24, 20. (c) Hu, J.; Odom, T.; Lieber, C. Acc. Chem. Res. 1999, 32, 435. (2) Nanowires of metals: (a) Kovtyukhova, N.; Mallouk, T. Chem. Eur. J. 2002, 8, 4355. (b) Murphy, C.; Jana, N. AdV. Mater. 2002, 14, 80. (c) El-Sayed, M. Acc. Chem. Res. 2001, 34, 257. (d) Song, J.; Wu, Y.; Messer, B.; Kind, H.; Yang, P. J. Am. Chem. Soc. 2001, 123, 10398. (e) Martin, C.; Mitchell, D. Electroanal, Chem. 1999, 21, 1. (3) Nanowires of semiconductors: (a) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165. (b) Chung, S.; Yu, J.; Heath, J. Appl. Phy. Lett. 2000, 76, 2068. (c) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. Nature 2001, 409, 66. (d) Dai, Z.; Gole, J.; Stout, J.; Wang, Z. J. Phys. Chem. B 2001, 106, 1274. (e) Holmes, J.; Johnston, K.; Doty, R.; Korgel, B. Science 2000, 287, 1471. (4) Nanowires of dielectrics: (a) Gu, G.; Zheng, B.; Han, W.; Roth, S.; Liu, J. Nano Lett. 2002, 2, 849. (b) Hu, J.; Jiang, Y.; Meng, X.; Lee, C.; Lee, S. Chem. Phys. Lett. 2003, 367, 339. (c) Yang, P.; Lieber, C. Science 1996, 273, 1836. (5) Wu, Y.; Messer, B.; Yang, P. AdV. Mater. 2001, 13, 1487. (6) (a) Sharifi, F.; Herzog, A.; Dynes, R. Phy. ReV. Lett. 1993, 71, 428. (b) Dubois, S.; Michel, A.; Eymery, J.; Duvail, J.; Piraux, L. J. Mater. Res. 1999, 14, 665. (c) Michotte, S.; Piraux, L.; Dubois, S.; Pailloux, F.; Stenuit, G.; Govaerts, J. Physica C 2002, 377, 267. (d) Yi, G.; Schwarzacher, W. Appl. Phys. Lett. 1999, 74, 1746. (e) Jalochowski, M.; Bauer, E. Surf. Sci. 2001, 480, 109. (f) Jalochowski, M.; Bauer, E. Prog. Surf. Sci. 2001, 67, 79. (g) Pang, Y.; Meng, G.; Zhang, L.; Shan, W.; Gao, X.; Zhao, A.; Mao, Y. J. Phys.: Condens. Matter 2002, 14, 11729. (h) Pang, Y.; Meng, G.; Zhang, L.; Qin, Y.; Gao, X.; Zhao, A.; Fang, Q. AdV. Funct. Mater. 2002, 12, 719. (7) (a) Leibold, R.; Huber, F. J. Therm. Anal. 1980, 18, 493. (b) Mu, J.; Perlmutter, D. Thermochim. Acta 1981, 49, 207. (c) Mohamed, M.; Halawy, S.; Ebrahim, M. Thermochim. Acta 1994, 236, 249. (8) Roosen, A.; Carter, W. Physica A 1998, 261, 232. (9) (a) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (b) Sun, Y.; Yin, Y.; Mayers, B.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736.
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