Single-crystalline superconducting MgB2 nanowires

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Dec 6, 2009 - the precursor solution was prepared. 3 mmol of MgBr2·6H2O and 6 mmol of NaBH4 were dissolved separately in 10 ml and. 20 ml of ethanol, ...
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Single-crystalline superconducting MgB2 nanowires

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IOP PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGY

Supercond. Sci. Technol. 22 (2009) 075017 (3pp)

doi:10.1088/0953-2048/22/7/075017

Single-crystalline superconducting MgB2 nanowires Alok K Jha and Neeraj Khare1 Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India E-mail: [email protected]

Received 6 February 2009, in final form 19 March 2009 Published 10 June 2009 Online at stacks.iop.org/SUST/22/075017 Abstract We report the synthesis of superconducting MgB2 nanowires using a two-step process involving the sol–gel technique and heat treatment in pure argon atmosphere at 820 ◦ C. High resolution transmission electron microscopy revealed that MgB2 nanowires are single crystalline. The nanowires are 0.5–1.0 µm long with diameters ∼50–100 nm. Superconductivity in these nanowires has been confirmed by DC magnetization measurements indicating the superconducting transition temperature ∼38 K.

atmosphere. Chemical vapour transport has also been used to synthesize MgB2 nanowires [16]. In this work, we report the synthesis of MgB2 nanowires by using a two-step process involving the sol–gel technique and annealing in an argon atmosphere. The use of argon gas in the synthesis has an advantage over diborane gas as argon is low cost and its handling is much easier. The handling of diborane is difficult because it is toxic and explosive as well. Superconductivity in the as-prepared MgB2 nanowires is confirmed by DC magnetization measurements. Our method enables us to get free MgB2 nanowires which can be manipulated for superconducting circuit applications.

1. Introduction The discovery of superconductivity in MgB2 at an elevated transition temperature (Tc ) of ∼39 K [1] has stimulated a lot of interest because of its possible practical applications in superconducting devices. MgB2 has a simple AlB2 -type hexagonal crystal structure, comprising of interleaved twodimensional boron and magnesium layers [2, 3]. Many groups have reported the synthesis of superconducting MgB2 in various forms: bulk [4], thin film [5], wires [6] and tapes [7]. One-dimensional (1D) nanostructures, including nanotubes [8], nanowires [9] and nanobelts [10], are ideal candidates for a fundamental understanding of the behaviour of the given matter in downsized geometries. It is, therefore, interesting to synthesize MgB2 nanowires and study the effect of dimensionality and size on superconductivity. As compared to the synthesis of bulk and thin films of MgB2 , there have been only a few reports on the synthesis of MgB2 nanostructures. MgB2 nanoparticles of size 40–100 nm have been synthesized through mechanical alloying [11]. Polycrystalline MgB2 nanowires (50–400 nm in diameter) have been synthesized by reacting boron nanowires with Mg vapour [12]. Ma et al [13] reported the synthesis of MgB2 nanowires by direct pyrolysis of MgB2 nanoparticles in which MgB2 nanowires (10–20 nm in diameter) were produced on the edges of the precursor nanoparticles. The synthesis of aligned single-crystal superconducting MgB2 nanowires on nanochannels on Al2 O3 substrates has also been reported [14]. Nath and Parkinson [15] have reported the synthesis of bulk MgB2 nanowires using a heat treatment in a diborane–N2

2. Experimental details MgB2 nanowires have been synthesized using a two-step process involving the sol–gel technique. In the first step a gel of the precursor solution was prepared. 3 mmol of MgBr2 ·6H2 O and 6 mmol of NaBH4 were dissolved separately in 10 ml and 20 ml of ethanol, respectively. The solutions were sonicated for 20 min in order to dissolve the solutes completely. 0.1 mmol of CTAB (cetyltrimethylammonium bromide) surfactant was added to each solution and the solutions were further sonicated. The Mg bromide solution was then added dropwise to the sodium borohydride solution. The reaction mixture was left in an open environment for several hours (12–14 h) which led to the formation of the gel. In the second step, the gel was loaded in a quartz boat and placed inside a furnace for heat treatment at 820 ◦ C for 10 min. The tube of the furnace was flushed with highly pure argon gas in order to keep the reaction atmosphere as inert as possible. As the presence of even a minute amount

1 Author to whom any correspondence should be addressed.

0953-2048/09/075017+03$30.00

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© 2009 IOP Publishing Ltd Printed in the UK

Supercond. Sci. Technol. 22 (2009) 075017

A K Jha and N Khare

Figure 1. (a) Transmission electron micrograph (TEM) image of the MgB2 nanowires. The scale is 100 nm and the magnification is 53 000×. (b) A higher resolution TEM image of one of the MgB2 nanowires. The scale is 100 nm and the magnification is 175 000×.

in order to remove the by-products which could be formed during the chemical reaction: MgBr2 + 2NaBH4 −→ MgB2 + 2NaBr + 4H2 . Initial structural characterization was done using an x-ray diffractometer (XRD: X’Pert pro XRD MPD Panalytical). The resultant black powder was predominantly MgB2 with small traces of MgB4 and MgO. We have used a transmission electron microscope (TEM: Philips CM12) as well as a high resolution transmission electron microscope (HRTEM: Technai G20-stwin (200 kV)) to finally characterize the MgB2 nanowires. DC magnetization measurement of the as-prepared MgB2 nanowires was carried out at low temperature (15–60 K) using an MPMS SQUID VSM (Quantum Design). For this measurement, 20 mg of the MgB2 sample was used.

3. Results and discussion Figure 1(a) shows the transmission electron micrograph (TEM) of the as-prepared MgB2 nanowires. The nanowires are found to be 0.3–1.0 µm in length and 50–100 nm in diameter. The aspect ratio of MgB2 nanowires is ∼8–10. The hexagonal cross section of one of the nanowires can be seen in the TEM image at a higher resolution (figure 1(b)). The TEM image shows that the nanowires are solid, straight and quite uniform in diameter along their lengths. The high resolution transmission electron micrograph (HRTEM) image (figure 2) shows that these nanowires are crystalline in nature. The diffraction fringes can easily be seen from this micrograph. The interplanar spacing in this HRTEM

Figure 2. High resolution transmission electron micrograph (HRTEM) image and the selected-area electron diffraction (SAED) pattern (inset) of the MgB2 nanowire. The interplanar spacing is ˚ 2.73 A.

of oxygen could lead to the formation of MgO, the high purity argon gas was passed through two oxytraps connected in series before entering into the tube of the furnace. After 10 min of heat treatment at 820 ◦ C the sample was slowly cooled to room temperature over a period of 12–14 h. The resultant black powder product was washed in deionized water and in ethanol 2

Supercond. Sci. Technol. 22 (2009) 075017

A K Jha and N Khare

synthesized by a two-step process using the sol–gel technique in an argon atmosphere. The SAED pattern of MgB2 nanowires reveals that it has a single-crystal hexagonal structure. The superconducting transition temperature of the nanowires was 38 K as observed by DC magnetization measurements. These superconducting nanowires can be used as building blocks in superconducting devices and in nanoscale electronics.

Acknowledgments The authors are grateful to Professor A K Grover (TIFR, India) for the use of MPMS SQUID VSM. One of us (AKJ) is grateful to CSIR, New Delhi, India for the award of a Junior Research Fellowship.

Figure 3. Variation of magnetization of MgB2 nanowires with temperature showing the superconducting transition temperature at 38 K.

References [1] Nagamatsu J, Nakagawa N, Muranaka T, Zenitani Y and Akimitsu J 2001 Nature 410 63 [2] Jones M E and Marsh R E 1952 J. Am. Chem. Soc. 76 1434 [3] Choi H J, Roundy D, Sun H, Cohen M L and Loule S G 2002 Nature 418 758 [4] Badr M H and Ng K W 2003 Supercond. Sci. Technol. 16 668 [5] Eom C B et al 2001 Nature 411 558 [6] Jin S, Mavoori H, Bower C and Dover R B 2001 Nature 411 563 [7] Kumakura H, Matsumoto A, Fujii H and Togano K 2001 Appl. Phys. Lett. 79 2435 [8] Ijima S 1991 Nature 354 56 [9] Morales A M and Liber C M 1996 Science 273 1836 [10] Pan Z W, Dai Z R and Wang Z L 2001 Science 291 1947 [11] Gumbel A, Eckert J, Fuchs G, Nenkov K, Muller K H and Schultz L 2002 Appl. Phys. Lett. 80 2725 [12] Wu Y, Messer B and Yang P 2001 Adv. Mater. 13 1487 [13] Ma R, Bando Y, Mori T and Golberg D 2003 Chem. Mater. 15 3194 [14] Yang Q, Sha J, Ma X, Ji Y and Yang D 2004 Supercond. Sci. Technol. 17 L31 [15] Nath M and Parkinson B A 2006 Adv. Mater. 18 1865 [16] Lai S H, Liu S C and Lan M D 2007 J. Cryst. Growth 304 460

˚ which corresponds to the image has been calculated as 2.73 A, (100) plane of the MgB2 . The selected-area electron diffraction (SAED) image (inset) clearly shows that the nanowires are single crystalline. Figure 3 shows the variation of magnetization of MgB2 nanowires with temperature at an applied field of 20 Oe. The onset temperature of superconducting transition is at 38 K, which is very close to the superconducting transition temperature of bulk MgB2 . Earlier reports on MgB2 nanowires showed Tc ∼ 33 K [12, 14], 38.6 K [15] and 35 K [16]. In the MgB2 system, Tc is generally sensitive to the presence of impurities and even a small amount of impurities can lead to the reduction of Tc . However, in our case Tc of the nanowires is close to 39 K, indicating that the nanowires are chemically pure which is also supported by our HRTEM studies.

4. Conclusions In summary, single-crystal superconducting MgB2 nanowires with diameters ∼50–100 nm and 0.3–1.0 µm long have been

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