- Email: [email protected]
Yttrium–barium–copper–oxygen nanorods synthesized by laser ablation
9 June 2000
Chemical Physics Letters 323 Ž2000. 180–184 www.elsevier.nlrlocatercplett
Yttrium–barium–copper–oxygen nanorods synthesized by laser ablation Y.F. Zhang, Y.H. Tang, X.F. Duan, Y. Zhang, C.S. Lee, N. Wang, I. Bello, S.T. Lee ) Center of Super-Diamond and AdÕanced Films (COSDAF) and Department of Physics and Materials Science, City UniÕersity of Hong Kong, Hong Kong, China Received 16 March 2000
Abstract Nanorods of yttrium–barium–copper–oxygen ŽYBCO. compound have been synthesized by laser ablation of a high Tc superconductor YBa 2 Cu 3 O 7 in an oxygen atmosphere. The YBCO nanorods were straight and with a uniform diameter. The diameters of the YBCO nanorods distributed between 18 and 96 nm with lengths up to several micrometers. Transmission electron microscopy showed that individual YBCO nanorods were single crystals with an orthorhombic lattice Ž a s 6.23, ˚ . and their axis was along the w001x direction. The growth mechanism of YBCO nanorods is b s 9.62, c s 7.02 A suggested to be similar to the oxide-assisted growth of Si nanowires. q 2000 Elsevier Science B.V. All rights reserved.
Quasi-one-dimensional ŽQ1D. crystals are an important topic for fundamental and technological research. Quantum confinement of electrons in one-dimensional systems provides an unique approach for manipulating their optical, electrical and magnetic properties. In recent years, significant progress has been made in Q1D nanorods or nanowires w1–6x. The Q1D organic molecular-chain superconductor ŽTMTSF. 2 PF6 has been reported to possess unusual properties, such as higher critical field and novel anisotropy inversion behavior w7,8x. Although its critical temperature Tc for superconducting is very low ŽTc - 1.2 K., the Q1D high Tc superconductor would possess unusual behaviors at temperatures as high as that of liquid nitrogen. Due to synthetic difficulties,
) Corresponding author. Fax: q852-2784-4696; e-mail: [email protected]
to our knowledge there has been no report on the synthesis of high Tc Q1D crystalline nanorods. Recently, we introduced the oxide-assisted growth mechanism of nanowires. Via this mechanism we succeeded in preparing uniform-sized nanowires of Si and SiO 2 in bulk-quantities by laser ablation and thermal evaporation techniques w9–12x by using a source of Si and SiO 2 . In this letter, we show that YBCO nanorods can be prepared by apparently a similar laser ablation technique. The YBCO nanorods were mixed with particles of YBCO. Transmission electron microscopy ŽTEM., energy dispersive spectroscopy ŽEDS. and electron energy loss spectroscopy ŽEELS. were used to characterize the YBCO nanorods. The laser ablation synthesis was carried out in a deposition system made of a quartz tube placed inside a tube furnace w3x. The target was placed inside the quartz tube with O 2 as the carrying gas.
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 5 0 4 - 2
Y.F. Zhang et al.r Chemical Physics Letters 323 (2000) 180–184
Production of YBCO nanorods involved laser ablating YBCO species from a YBa 2 Cu 3 O 7 target by a pulsed laser beam. The target was a YBa 2 Cu 3 O 7 disk, which can become superconducting at liquid nitrogen temperatures. The laser used was an excimer laser with a power of 400 mJ at a wavelength of 248 nm and a frequency of 10 Hz. The duration of every laser pulse was 34 ns. O 2 flowed through the pre-evacuated quartz tube at 1 atm and 100 sccm during the experiment. In the center of the quartz tube where the target was located, the temperature was 6958C. At ; 15 cm downstream, YBCO nanorods were formed on the inner wall of the quartz tube where the temperature was between 600 and 6908C. The nanorods with length up to several micrometers grew protruding from some large YBCO grains deposited on the wall of the quartz tube. No YBCO nanorods were observed at places where the temperature was lower than ; 6008C, while nanoclusters and amorphous mixtures of YBCO were deposited. The TEM observations were carried out in either a Philips CM20 TEM equipped with EDS or a Philips CM200 TEM equipped with EELS at room temperature. TEM samples were prepared by transferring a small piece of the deposit from the inside surface of the quartz tube to TEM micro-grids. Fig. 1 shows the representative morphologies of the nanorods. The nanorods are normally straight with various diameters and grew on large particles. Most
Fig. 1. Representative morphologies of the nanorods mixed with YBCO particles synthesized by laser ablation of YBa 2 Cu 3 O 7 in an oxygen atmosphere.
181
Fig. 2. TEM micrograph of the nanorods grown on YBCO particles at the regions of Ža. ;6308C and Žb. ;6808C.
of the nanorods are shorter than a micrometer, but some are up to several micrometers in length. The diameters of the nanorods distributed from 18 to 96 nm. The nanorods were easily broken during the preparation of the TEM samples. The nanorods grown at the location where the temperature was ; 6308C ŽFig. 2a. were relatively short and thin compared to those grown at higher temperatures. Fig. 2 shows that the nanorods grew nearly perpendicularly to the surface of the large particle. The surface of the rods in Fig. 2a was not smooth and the core of the rods appeared also nonuniform according to the mass density contrast within the rods. Fig. 2b shows several nanorods that grew with different lengths on a large particle at ; 6808C. These nanorods are straight, smooth in surface, and uniform in diameter throughout the entire length of the rod. Fig. 3a shows a high-resolution TEM ŽHRTEM. image of a nanorod having a diameter of about 86 nm. The nanorod was essentially single-crystal-
182
Y.F. Zhang et al.r Chemical Physics Letters 323 (2000) 180–184
Fig. 3. High-resolution TEM images of a nanorod Ža. before and Žb. after being radiated for half an hour by the electron beam at 200 kV in HRTEM.
line as evidenced from the lattice fringes. No other phases were observed either on the tip or in the core of the rod. The rod surface was relatively smooth. After being irradiated by the electron beam for half an hour at 200 kV in HRTEM, the surface of the nanorod was changed to a saw-tooth shape ŽFig. 3b., although the lattice fringes of the single-crystalline structure of the nanorod remained the same. The mechanism for this phenomenon is not understood. The crystalline structure of the nanorod shown in Fig. 3a was investigated by selected-area electron diffraction ŽSAED., which is shown in Fig. 4A. By rotating the nanorod around its axis for 18.568, 24.368, 30.648, and 40.248, we obtained the SAED patterns in Figs. 4B, 4C, 4D, and 4E, respectively. From these patterns the crystalline structure of the nanorod was determined to be an orthorhombic system and a p-type lattice. The lattice constants were deduced from the diffraction data to be a s 0.962, b s 0.623, c s 0.702 nm. The determined lattice parameters of the nanorods cannot be matched to the structural data of any of the reported crystalline materials including all the reported YBCO compounds. This suggests that the structure of the nanorods could be a new phase. The morphology and structure of the nanorods were stable and remained intact upon storage under ambient conditions for several months. This may be understood as the nanorods were synthesized in an oxygen ambient of one atmosphere pressure. The nominal atomic ratio YrBarCurO in the mixture of the nanorods and the large YBCO particle was determined by EDS to be similar to that in the YBa 2 Cu 3 O 7 target. Attempts to measure the chemical composition of individual nanorods by using either EDS or EELS with a nanometer electron beam were less successful. Although the characteristic peaks from elements Y, Ba, Cu and O could be identified in the measured EDS spectra, the EDS signal from a single nanorod was too weak to give a reliable estimate of the atomic ratio. Only the energy of EELS edges of O-K and Ba-M 4,5 are within the measuring range of our EELS spectrometer. Fig. 5 shows the EELS spectrum of a single nanorod. Both O-K and Ba-M 4,5 edges could be well identified, which yielded an atomic ratio BarO of about 1:5. This BarO value is a bit smaller than 2r7 in the target material YBa 2 Cu 3 O 7 , but the
Y.F. Zhang et al.r Chemical Physics Letters 323 (2000) 180–184
183
Fig. 4. ŽA. SAED patterns of the nanorod shown in Fig. 3a. SAED patterns ŽB., ŽC., ŽD., and ŽE. were taken by rotating the nanorod around its axis for 18.568, 24.368, 30.648, and 40.248, respectively.
difference is within the experimental error expected from EELS using a nanometer electron beam.
The growth mechanism of YBCO nanorods is not clear. We speculate that YBCO nanorods grew via
184
Y.F. Zhang et al.r Chemical Physics Letters 323 (2000) 180–184
element composition is complicated in that the simple picture presented above is far from satisfactory and more work is needed for clarification. In summary, freestanding and straight YBCO nanorods with diameters ranging from 18 to 96 nm and lengths up to several micrometers have been synthesized by laser ablating a target made of high Tc superconductor YBa 2 Cu 3 O 7 . The YBCO nanorods were single crystalline with an orthorhombic lattice Ž a s 6.23, b s 9.62, c s 7.02.. Their axes lay along the w001x direction. EELS and EDS identified elements of Y, Ba, Cu, and O in the nanorods. The growth mechanism of YBCO nanorods was proposed to be similar to the oxide-assisted VLS growth of Si nanowires w11x. The growth process was associated with a liquid growth tip and its subsequent cool-down to form a crystal under the growth tip. Fig. 5. The EELS spectrum of a single nanorod showing the O-K and Ba-M 4,5 edges.
Acknowledgements the oxide-assisted vapor-liquid-solid ŽVLS. growth mechanism w11x similar to that of Si and SiO 2 nanowires. Initially, hot nanoclusters of YBCO species generated by laser ablation deposited on the quartz tube surface to form particles. A liquid would be formed at the protruding sites of nanometer size or at the tip of nanorods of high curvature due to melting point lowering associated with nano-size and high curvature Žtypically hemispherical in shape.. The nanoclusters of YBCO ablated from the target could be absorbed readily at the liquid sites where the solubility of YBCO was high. As the liquid cooled down and crystallized the site would confine and direct the growth of the crystalline nanowires. Once a crystalline core under a hemisphere-shaped island was formed on a particle surface, the crystalline lattice of the core would continue to develop along the rod. The diameters of the nanorods were controlled by the diameters of the liquid growth tips. The nanorod diameter decreased with decreasing substrate temperature due to the increased cooling effect. The smallest diameter of the grown rod was found to be 18 nm in our experiments. However, if the temperature was too low, the crystallinity of the nanorods would become poor or the liquid state at the tip could not last for the nanorod to grow. The growth mechanism of the YBCO nanorod of multi-
The authors wish to thank Professor F.H. Li for useful discussions. The financial supports from the Strategic Research Grants of the City University of Hong Kong and the Research Grants Council of Hong Kong are gratefully acknowledged.
References w1x A.M. Morales, C.M. Lieber, Science 279 Ž1998. 208. w2x T. Ono, H. Saitoh, M. Esashi, Appl. Phys. Lett. 70 Ž1997. 1852. w3x Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 Ž1998. 1835. w4x N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 283 Ž1998. 368. w5x C.R. Martin, Chem. Mater. 261 Ž1996. 1316. w6x S.K. Chkarvarti, J. Vetter, J. Radiat. Meas. 29 Ž1998. 149. w7x A.G. Lebed, Phys. Rev. B 59 Ž1999. R721. w8x I.J. Lee, M.J. Naughton, G.M. Danner, P.M. Chaikin, Phys. Rev. Lett. 78 Ž1997. 3555. w9x Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T. Lee, J. Crystal Growth 197 Ž1999. 136. w10x N. Wang, Y.F. Zhang, Y.H. Tang, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 73 Ž1998. 3902. w11x S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bulletin 24 Ž1999. 36. w12x N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev. B 58 Ž1998. R16024.
Chemical Physics Letters 323 Ž2000. 180–184 www.elsevier.nlrlocatercplett
Yttrium–barium–copper–oxygen nanorods synthesized by laser ablation Y.F. Zhang, Y.H. Tang, X.F. Duan, Y. Zhang, C.S. Lee, N. Wang, I. Bello, S.T. Lee ) Center of Super-Diamond and AdÕanced Films (COSDAF) and Department of Physics and Materials Science, City UniÕersity of Hong Kong, Hong Kong, China Received 16 March 2000
Abstract Nanorods of yttrium–barium–copper–oxygen ŽYBCO. compound have been synthesized by laser ablation of a high Tc superconductor YBa 2 Cu 3 O 7 in an oxygen atmosphere. The YBCO nanorods were straight and with a uniform diameter. The diameters of the YBCO nanorods distributed between 18 and 96 nm with lengths up to several micrometers. Transmission electron microscopy showed that individual YBCO nanorods were single crystals with an orthorhombic lattice Ž a s 6.23, ˚ . and their axis was along the w001x direction. The growth mechanism of YBCO nanorods is b s 9.62, c s 7.02 A suggested to be similar to the oxide-assisted growth of Si nanowires. q 2000 Elsevier Science B.V. All rights reserved.
Quasi-one-dimensional ŽQ1D. crystals are an important topic for fundamental and technological research. Quantum confinement of electrons in one-dimensional systems provides an unique approach for manipulating their optical, electrical and magnetic properties. In recent years, significant progress has been made in Q1D nanorods or nanowires w1–6x. The Q1D organic molecular-chain superconductor ŽTMTSF. 2 PF6 has been reported to possess unusual properties, such as higher critical field and novel anisotropy inversion behavior w7,8x. Although its critical temperature Tc for superconducting is very low ŽTc - 1.2 K., the Q1D high Tc superconductor would possess unusual behaviors at temperatures as high as that of liquid nitrogen. Due to synthetic difficulties,
) Corresponding author. Fax: q852-2784-4696; e-mail: [email protected]
to our knowledge there has been no report on the synthesis of high Tc Q1D crystalline nanorods. Recently, we introduced the oxide-assisted growth mechanism of nanowires. Via this mechanism we succeeded in preparing uniform-sized nanowires of Si and SiO 2 in bulk-quantities by laser ablation and thermal evaporation techniques w9–12x by using a source of Si and SiO 2 . In this letter, we show that YBCO nanorods can be prepared by apparently a similar laser ablation technique. The YBCO nanorods were mixed with particles of YBCO. Transmission electron microscopy ŽTEM., energy dispersive spectroscopy ŽEDS. and electron energy loss spectroscopy ŽEELS. were used to characterize the YBCO nanorods. The laser ablation synthesis was carried out in a deposition system made of a quartz tube placed inside a tube furnace w3x. The target was placed inside the quartz tube with O 2 as the carrying gas.
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 5 0 4 - 2
Y.F. Zhang et al.r Chemical Physics Letters 323 (2000) 180–184
Production of YBCO nanorods involved laser ablating YBCO species from a YBa 2 Cu 3 O 7 target by a pulsed laser beam. The target was a YBa 2 Cu 3 O 7 disk, which can become superconducting at liquid nitrogen temperatures. The laser used was an excimer laser with a power of 400 mJ at a wavelength of 248 nm and a frequency of 10 Hz. The duration of every laser pulse was 34 ns. O 2 flowed through the pre-evacuated quartz tube at 1 atm and 100 sccm during the experiment. In the center of the quartz tube where the target was located, the temperature was 6958C. At ; 15 cm downstream, YBCO nanorods were formed on the inner wall of the quartz tube where the temperature was between 600 and 6908C. The nanorods with length up to several micrometers grew protruding from some large YBCO grains deposited on the wall of the quartz tube. No YBCO nanorods were observed at places where the temperature was lower than ; 6008C, while nanoclusters and amorphous mixtures of YBCO were deposited. The TEM observations were carried out in either a Philips CM20 TEM equipped with EDS or a Philips CM200 TEM equipped with EELS at room temperature. TEM samples were prepared by transferring a small piece of the deposit from the inside surface of the quartz tube to TEM micro-grids. Fig. 1 shows the representative morphologies of the nanorods. The nanorods are normally straight with various diameters and grew on large particles. Most
Fig. 1. Representative morphologies of the nanorods mixed with YBCO particles synthesized by laser ablation of YBa 2 Cu 3 O 7 in an oxygen atmosphere.
181
Fig. 2. TEM micrograph of the nanorods grown on YBCO particles at the regions of Ža. ;6308C and Žb. ;6808C.
of the nanorods are shorter than a micrometer, but some are up to several micrometers in length. The diameters of the nanorods distributed from 18 to 96 nm. The nanorods were easily broken during the preparation of the TEM samples. The nanorods grown at the location where the temperature was ; 6308C ŽFig. 2a. were relatively short and thin compared to those grown at higher temperatures. Fig. 2 shows that the nanorods grew nearly perpendicularly to the surface of the large particle. The surface of the rods in Fig. 2a was not smooth and the core of the rods appeared also nonuniform according to the mass density contrast within the rods. Fig. 2b shows several nanorods that grew with different lengths on a large particle at ; 6808C. These nanorods are straight, smooth in surface, and uniform in diameter throughout the entire length of the rod. Fig. 3a shows a high-resolution TEM ŽHRTEM. image of a nanorod having a diameter of about 86 nm. The nanorod was essentially single-crystal-
182
Y.F. Zhang et al.r Chemical Physics Letters 323 (2000) 180–184
Fig. 3. High-resolution TEM images of a nanorod Ža. before and Žb. after being radiated for half an hour by the electron beam at 200 kV in HRTEM.
line as evidenced from the lattice fringes. No other phases were observed either on the tip or in the core of the rod. The rod surface was relatively smooth. After being irradiated by the electron beam for half an hour at 200 kV in HRTEM, the surface of the nanorod was changed to a saw-tooth shape ŽFig. 3b., although the lattice fringes of the single-crystalline structure of the nanorod remained the same. The mechanism for this phenomenon is not understood. The crystalline structure of the nanorod shown in Fig. 3a was investigated by selected-area electron diffraction ŽSAED., which is shown in Fig. 4A. By rotating the nanorod around its axis for 18.568, 24.368, 30.648, and 40.248, we obtained the SAED patterns in Figs. 4B, 4C, 4D, and 4E, respectively. From these patterns the crystalline structure of the nanorod was determined to be an orthorhombic system and a p-type lattice. The lattice constants were deduced from the diffraction data to be a s 0.962, b s 0.623, c s 0.702 nm. The determined lattice parameters of the nanorods cannot be matched to the structural data of any of the reported crystalline materials including all the reported YBCO compounds. This suggests that the structure of the nanorods could be a new phase. The morphology and structure of the nanorods were stable and remained intact upon storage under ambient conditions for several months. This may be understood as the nanorods were synthesized in an oxygen ambient of one atmosphere pressure. The nominal atomic ratio YrBarCurO in the mixture of the nanorods and the large YBCO particle was determined by EDS to be similar to that in the YBa 2 Cu 3 O 7 target. Attempts to measure the chemical composition of individual nanorods by using either EDS or EELS with a nanometer electron beam were less successful. Although the characteristic peaks from elements Y, Ba, Cu and O could be identified in the measured EDS spectra, the EDS signal from a single nanorod was too weak to give a reliable estimate of the atomic ratio. Only the energy of EELS edges of O-K and Ba-M 4,5 are within the measuring range of our EELS spectrometer. Fig. 5 shows the EELS spectrum of a single nanorod. Both O-K and Ba-M 4,5 edges could be well identified, which yielded an atomic ratio BarO of about 1:5. This BarO value is a bit smaller than 2r7 in the target material YBa 2 Cu 3 O 7 , but the
Y.F. Zhang et al.r Chemical Physics Letters 323 (2000) 180–184
183
Fig. 4. ŽA. SAED patterns of the nanorod shown in Fig. 3a. SAED patterns ŽB., ŽC., ŽD., and ŽE. were taken by rotating the nanorod around its axis for 18.568, 24.368, 30.648, and 40.248, respectively.
difference is within the experimental error expected from EELS using a nanometer electron beam.
The growth mechanism of YBCO nanorods is not clear. We speculate that YBCO nanorods grew via
184
Y.F. Zhang et al.r Chemical Physics Letters 323 (2000) 180–184
element composition is complicated in that the simple picture presented above is far from satisfactory and more work is needed for clarification. In summary, freestanding and straight YBCO nanorods with diameters ranging from 18 to 96 nm and lengths up to several micrometers have been synthesized by laser ablating a target made of high Tc superconductor YBa 2 Cu 3 O 7 . The YBCO nanorods were single crystalline with an orthorhombic lattice Ž a s 6.23, b s 9.62, c s 7.02.. Their axes lay along the w001x direction. EELS and EDS identified elements of Y, Ba, Cu, and O in the nanorods. The growth mechanism of YBCO nanorods was proposed to be similar to the oxide-assisted VLS growth of Si nanowires w11x. The growth process was associated with a liquid growth tip and its subsequent cool-down to form a crystal under the growth tip. Fig. 5. The EELS spectrum of a single nanorod showing the O-K and Ba-M 4,5 edges.
Acknowledgements the oxide-assisted vapor-liquid-solid ŽVLS. growth mechanism w11x similar to that of Si and SiO 2 nanowires. Initially, hot nanoclusters of YBCO species generated by laser ablation deposited on the quartz tube surface to form particles. A liquid would be formed at the protruding sites of nanometer size or at the tip of nanorods of high curvature due to melting point lowering associated with nano-size and high curvature Žtypically hemispherical in shape.. The nanoclusters of YBCO ablated from the target could be absorbed readily at the liquid sites where the solubility of YBCO was high. As the liquid cooled down and crystallized the site would confine and direct the growth of the crystalline nanowires. Once a crystalline core under a hemisphere-shaped island was formed on a particle surface, the crystalline lattice of the core would continue to develop along the rod. The diameters of the nanorods were controlled by the diameters of the liquid growth tips. The nanorod diameter decreased with decreasing substrate temperature due to the increased cooling effect. The smallest diameter of the grown rod was found to be 18 nm in our experiments. However, if the temperature was too low, the crystallinity of the nanorods would become poor or the liquid state at the tip could not last for the nanorod to grow. The growth mechanism of the YBCO nanorod of multi-
The authors wish to thank Professor F.H. Li for useful discussions. The financial supports from the Strategic Research Grants of the City University of Hong Kong and the Research Grants Council of Hong Kong are gratefully acknowledged.
References w1x A.M. Morales, C.M. Lieber, Science 279 Ž1998. 208. w2x T. Ono, H. Saitoh, M. Esashi, Appl. Phys. Lett. 70 Ž1997. 1852. w3x Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 Ž1998. 1835. w4x N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 283 Ž1998. 368. w5x C.R. Martin, Chem. Mater. 261 Ž1996. 1316. w6x S.K. Chkarvarti, J. Vetter, J. Radiat. Meas. 29 Ž1998. 149. w7x A.G. Lebed, Phys. Rev. B 59 Ž1999. R721. w8x I.J. Lee, M.J. Naughton, G.M. Danner, P.M. Chaikin, Phys. Rev. Lett. 78 Ž1997. 3555. w9x Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T. Lee, J. Crystal Growth 197 Ž1999. 136. w10x N. Wang, Y.F. Zhang, Y.H. Tang, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 73 Ž1998. 3902. w11x S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bulletin 24 Ž1999. 36. w12x N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev. B 58 Ž1998. R16024.