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Growth of copper nanowire arrays on NiTi shape memory alloy thin film
Surface & Coatings Technology 206 (2012) 4075–4078
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Growth of copper nanowire arrays on NiTi shape memory alloy thin film N. Bayat a, S. Sanjabi a,⁎, Z.H. Barber b a b
Nanomaterials Group, Department of Materials Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran Department of Materials Science & Metallurgy, Cambridge University, Pembroke Street, Cambridge CB2 3QZ, UK
a r t i c l e
i n f o
Article history: Received 18 November 2011 Accepted in revised form 30 March 2012 Available online 9 April 2012 Keywords: Copper nanowires NiTi thin film Shape memory alloys Al template
a b s t r a c t Copper nanowire arrays were synthesized on NiTi shape memory alloy thin films to improve thermal actuation. To grow the forest of Cu nanowires, an Al film was deposited on top of the NiTi by DC sputtering, anodized in oxalic acid to form anodic aluminum oxide (AAO), and then the copper was electrodeposited inside the nanochannels of the template. Growth of these Cu nanowires was studied by electrochemical methods: anodization and electrodeposition rates were characterized using current density versus time diagrams. The morphology and crystal structure of Cu nanowires were evaluated by field emission scanning electron microscopy and X-ray diffraction. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Shape memory alloy (SMA) thin films are used for rapid actuation in micro-electro-mechanical systems (MEMS) due to their high surface to volume ratio, in comparison with bulk SMAs. However, slow heat transport into/out of the SMA to switch between the austenitic/martensitic phases at the transformation temperature leads to reduced actuation speed [1]. Thus, the use of high thermal conductivity materials as heat transport paths into/out of the SMA has been suggested to increase cycling frequencies [2]. As previous studies have shown, carbon nanotubes (CNTs) have exceptionally high thermal conductivities (e.g. ~ 3000–6000 W/K m) [3,4], and could be used to increase cycling frequencies of SMA films by enhanced heat transport [5]. The basic requirement for this application is to obtain CNT forest growth on SMA films, while retaining a reversible martensitic transformation, as required for shape memory effect exploitation [5]. However, disadvantages stemming from the growth of CNTs by chemical vapor deposition (CVD) directly onto metal-alloy SMA films include: 1- The requirement for high temperatures (e.g. T > 700 °C), which may destroy the shape memory effect and superelasticity of the NiTi by precipitation of unwanted phases, and lead to oxidation of the SMA surface. 2- The requirement for deposition of a catalyst, typically by physical vapor deposition methods: this increases costs, and must subsequently be removed from the substrate or tips of the CNTs. 3- Overall increase in cost associated with CVD. ⁎ Corresponding author. Tel./fax: + 98 21 82883325. E-mail address: [email protected] (S. Sanjabi). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.03.093
Although thermal conductivities for single CNTs imply ten times faster heat conduction as compared to Cu (~400 W/K m), and thus a considerable increase in SMAs' cycling frequencies, synthesis of Cu nanowires at room temperature by electrodeposition inside a nanotemplate overcomes the disadvantages listed above. Also, there have been several studies dedicated to the fabrication of Cu nanowires, because of their potential application in a range of devices, such as wire-grid polarizers, electrostatically dissipative devices, and current collectors for Li-ion batteries. Copper has an important role in the electronics industry, based on its excellent electrical and thermal conductivity [6–16]. The template method, using anodized aluminum oxide (AAO) membrane, is a simple and effective approach to the synthesis of one-dimensional nanowires. This is a low cost method to achieve high density pores and uniform pore distribution for subsequent formation of electrodeposited nanowires. Electrodeposition in AAO templates is a very simple, efficient technique, in which the diameter and length of nanowires can be easily controlled by the anodizing parameters. High density nanowires fabricated by this technique are well ordered and parallel to each other in the AAO template. This characteristic has qualified nanowires for unique applications in mesoscopic physics and nanoscale devices [17–19]. In the present work, copper nanowires were synthesized at relatively low temperatures on NiTi SMA thin films by an electrochemical method. This process also offers low fabrication cost, and the deposition parameters are easily controlled. Fig. 1 shows a schematic of the fabrication steps. 2. Experimental procedure To obtain a template for subsequent growth of Cu nanowires, ~500 nm of Al was deposited (at room temperature) onto ~ 2 μm NiTi thin films on silicon substrates (10 mm ∗5 mm) [20]. At the
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N. Bayat et al. / Surface & Coatings Technology 206 (2012) 4075–4078
Fig. 1. Schematic of Cu nanowire synthesis on NiTi thin film.
initial steps of the experiments, it was found that electron beam deposition of the Al led to insufficient adhesion to the underlying NiTi film, and hence dc sputter deposition was used instead. A sputtering gas pressure of 0.5 Pa Ar, with a target–substrate distance of 40 mm, gave a deposition rate of ~ 20 nm/min. Al/NiTi films were subsequently annealed in a vacuum furnace (base pressure b 10 − 4 Pa) at 500 °C for 1 h, with heating and cooling rates of approximately 50 °C/min to anneal both Al and NiTi amorphous films. The annealed films were anodized in 0.3 M oxalic acid: the composite layer of Al/NiTi was set as the anode and stainless steel (316) was the cathode. The anodized aluminum was prepared at 15 °C using different anodizing voltages to obtain the optimum AAO structure. In order to monitor the anodization process, a multimeter was used to measure the current density, at constant voltage. A solution of 0.2 M CuSO4·5H2O and 0.1 M H3BO3 was used as electrolyte to deposit the copper. The electrolyte pH was adjusted to 3 by adding sulphuric acid. Here, the AAO template was the cathode and the pure copper sheet was set as the anode. Electrodeposition was performed at 0.4 V, the applied potential between cathode and anode, and bath temperature of 25 °C. A homemade electrochemical analyzer (SAMA500) was used to determine the time taken for nanochannel filling by measuring the current density versus time. After filling the AAO template with copper, the AAO/NiTi layer was etched in 1 M NaOH to remove the anodic alumina template. AAO template film and Cu nanowires were examined by observing the surface and fracture cross-sections in a Field Emission Scanning Electron Microscope (FE-SEM) using a S4160 Hitachi Japan. X-ray diffraction (XRD) used an Expert MPT with Cu-Kα (λ = 1.54056 Å) X-ray source to identify the film and Cu nanowire structure. 3. Results and discussion Fig. 2 shows a cross sectional FE-SEM image of the ~ 500 nm Al layer deposited onto ~ 2 μm amorphous NiTi film on a Si(100) substrate. Fig. 3 shows FE-SEM images of the anodized Al layer formed using anodization voltages of 40, 35, and 30 V. In these experiments all other anodization parameters were unchanged. As shown in Fig. 3a and b, at higher voltage the surface of the anodized layer was not uniform. The optimal result was obtained at 30 V (see Fig. 3c), showing a uniform surface with ordered nano-channels. Here the ~60 nm diameter nanopores are spaced at approximately 60 nm from each other. In order to ensure the best contact between the copper nanowires and the NiTi thin film surface, it was necessary to completely anodize the Al film. However, excessive anodization time may lead to damage on the SMA film surface. Hence, the anodization process was carefully monitored by measuring the current density as a function of time. As shown in Fig. 4, three regions can be identified during Al anodization: a) current density decrease on formation of oxide on the top surface of the Al film, b) constant current density during nanochannel
formation in the Al, and c) further current density change when the Al anodization process is complete (here, current density increase on reaching the NiTi film). From Fig. 4 we can observe that the growth of nanochannels begins after about 60 s, and in less than 380 s the anodization process is complete. The anodization rate is therefore estimated to be approximately 80 nm/min. The ~60–80 nm ordered nanochannels formed by anodization are next used to synthesize copper nanowires by reduction of Cu ions within them. Here, choice of a suitable electrodeposition voltage has an important role: if low voltages are used, nanowire growth is slow, but at high voltages uniformity becomes a problem. In this work, following optimization, the applied voltage used was −0.4 V/SCE and Fig. 5 shows a plot of current versus time. In zone (a) of Fig. 5, nanowires grow up from the NiTi film surface and the current density remains constant. After about 280 s the nanochannels are completely filled, and continued electrodeposition results in lateral growth across the spaces between them (zone b). On completely filling the lateral spaces between nanochannels, further electrodeposition leads to growth of a uniform surface layer (zone c). From this figure the growth rate of Cu nanowires within the AAO nanochannels is estimated to be 107 nm/min. Fig. 6 shows the copper nanowires before and after removal of the alumina template. As shown here, the copper has just begun to grow laterally, after filling the nanochannels (Fig. 6a), so that the diameter of the tops of the Cu nanowires appears larger than the diameter of the nanochannels themselves (see Fig. 6b). Fig. 7 shows the XRD scan from copper nanowires on a NiTi thin film. The (100) austenitic peak of NiTi film (after annealing of the amorphous film) is observed at 2θ = 42.7°. The Cu nanowires grow in a (100) crystallographic orientation, corresponding to the (100) plane of NiTi.
Fig. 2. FE-SEM image of deposited Al layer (~500 nm) on 2 μm NiTi film.
N. Bayat et al. / Surface & Coatings Technology 206 (2012) 4075–4078
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a
b
Fig. 5. Current–time diagram at − 0.4 V/SCE for electrodeposition of Cu in nanochannels of AAO layer.
4. Conclusions
c
Cu nanowire arrays, offering improved thermal actuation properties, have been fabricated on NiTi SMA thin films by filling AAO templates. By measuring the current density during anodization and during electrodeposition of copper nanowires, the rates of Al anodization and Cu electrodeposition were calculated as 79 and 107 nm/min, respectively. These data can be used to predict the time required for Al film anodization and AAO template filling on SMA films. This paper also can propose the possibility of growth of metallic elemental or alloy array nanowires instead of Cu on SMA thin films by using AAO film template for desired applications.
a
Fig. 3. Anodized Al layer on NiTi thin film following anodization in 0.3 M oxalic acid at 15 °C and voltages of a) 40 V, b) 35 V and c) 30 V.
b
Fig. 4. Current density as a function of time for anodization (at 30 V).
Fig. 6. Cu nanowires a) before, and b) after removal of alumina template in 1 M NaOH.
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N. Bayat et al. / Surface & Coatings Technology 206 (2012) 4075–4078
Fig. 7. XRD patterns of Cu nanowires on NiTi thin film.
References [1] K. Otsuka, T. Kakeshita, MRS Bull. 27 (2002) 91. [2] A. Ishida, V. Martynov, MRS Bull. 27 (2002) 111. [3] P. Kim, L. Shi, A. Majumdar, P.L. McEuen, Phys. Rev. Lett. 87 (2001) 215502.
[4] S. Berber, Y. Kwon, D. Tomanek, Phys. Rev. Lett. 84 (2000) 4613. [5] B.C. Bayer, S. Sanjabi, C. Baehtz, C.T. Wirth, S. Esconjauregui, R.S. Weatherup, Z.H. Barber, S. Hofmann, J. Robertson, Thin Solid Films 519 (2011) 6126. [6] S.Z. Chu, K. Wada, S. Inoue, S. Todoroki, Electrochim. Acta 48 (2003) 3147. [7] J.-L. Duan, J. Liu, H.-J. Yao, D. Mo, M.-D. Hou, Y.-M. Sun, Y.-F. Chen, L. Zhang, Mater. Sci. Eng. B 147 (2008) 57. [8] Z. Ying, F. Leung-Yuk Lam, H.U. Xijun, Y. Zifeng, Chin. Sci. Bull. 51 (2006) 2662. [9] G. Riveros, H. Gómez, A. Cortés, R.E. Marotti, E.A. Dalchiele, Appl. Phys. A 81 (2005) 17. [10] X. Wen, Y. Xie, C.L. Choi, K.C. Wan, X.-Y. Li, S. Yang, Langmuir 21 (2005) 4729. [11] G. Sharma, C.S. Chong, L. Ebin, V. Kripesh, C.L. Gan, C.H. Sow, Nanotechnology 18 (2007) 305306. [12] A. Ghaddar, J. Gieraltowski, F. Gloaguen, J. Appl. Electrochem. 39 (2009) 719. [13] S. Inoue, S.-Z. Chu, K. Wada, D. Li, H. Haneda, Sci. Technol. Adv. Mater. 4 (2003) 269. [14] T. Gao, G. Meng, Y. Wang, S. Sun, L. Zhang, J. Phys. Condens. Matter 14 (2002) 355. [15] Y.-H. Lee, I.-C. Leu, M.-T. Wu, J.-H. Yen, K.-Z. Fung, J. Alloys Compd. 427 (2007) 213. [16] R. Inguanta, S. Piazza, C. Sunseri, Appl. Surf. Sci. 255 (2009) 8816. [17] S.Z. Chu, K. Wada, S. Inoue, S. Todoroki, Surf. Coat. Technol. 169–170 (2003) 190. [18] Y.F. Mei, X.L. Wu, T. Qiu, X.F. Shao, G.G. Siu, P.K. Chu, Thin Solid Films 492 (2005) 66. [19] Y. Yang, H. Chen, Y. Mei, J. Chen, X. Wu, X. Bao, Solid State Commun. 123 (2002) 279. [20] S. Sanjabi, S.K. Sadrnezhaad, K.A. Yates, Z.H. Barber, Thin Solid Films 491 (2005) 190.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Growth of copper nanowire arrays on NiTi shape memory alloy thin film N. Bayat a, S. Sanjabi a,⁎, Z.H. Barber b a b
Nanomaterials Group, Department of Materials Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran Department of Materials Science & Metallurgy, Cambridge University, Pembroke Street, Cambridge CB2 3QZ, UK
a r t i c l e
i n f o
Article history: Received 18 November 2011 Accepted in revised form 30 March 2012 Available online 9 April 2012 Keywords: Copper nanowires NiTi thin film Shape memory alloys Al template
a b s t r a c t Copper nanowire arrays were synthesized on NiTi shape memory alloy thin films to improve thermal actuation. To grow the forest of Cu nanowires, an Al film was deposited on top of the NiTi by DC sputtering, anodized in oxalic acid to form anodic aluminum oxide (AAO), and then the copper was electrodeposited inside the nanochannels of the template. Growth of these Cu nanowires was studied by electrochemical methods: anodization and electrodeposition rates were characterized using current density versus time diagrams. The morphology and crystal structure of Cu nanowires were evaluated by field emission scanning electron microscopy and X-ray diffraction. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Shape memory alloy (SMA) thin films are used for rapid actuation in micro-electro-mechanical systems (MEMS) due to their high surface to volume ratio, in comparison with bulk SMAs. However, slow heat transport into/out of the SMA to switch between the austenitic/martensitic phases at the transformation temperature leads to reduced actuation speed [1]. Thus, the use of high thermal conductivity materials as heat transport paths into/out of the SMA has been suggested to increase cycling frequencies [2]. As previous studies have shown, carbon nanotubes (CNTs) have exceptionally high thermal conductivities (e.g. ~ 3000–6000 W/K m) [3,4], and could be used to increase cycling frequencies of SMA films by enhanced heat transport [5]. The basic requirement for this application is to obtain CNT forest growth on SMA films, while retaining a reversible martensitic transformation, as required for shape memory effect exploitation [5]. However, disadvantages stemming from the growth of CNTs by chemical vapor deposition (CVD) directly onto metal-alloy SMA films include: 1- The requirement for high temperatures (e.g. T > 700 °C), which may destroy the shape memory effect and superelasticity of the NiTi by precipitation of unwanted phases, and lead to oxidation of the SMA surface. 2- The requirement for deposition of a catalyst, typically by physical vapor deposition methods: this increases costs, and must subsequently be removed from the substrate or tips of the CNTs. 3- Overall increase in cost associated with CVD. ⁎ Corresponding author. Tel./fax: + 98 21 82883325. E-mail address: [email protected] (S. Sanjabi). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.03.093
Although thermal conductivities for single CNTs imply ten times faster heat conduction as compared to Cu (~400 W/K m), and thus a considerable increase in SMAs' cycling frequencies, synthesis of Cu nanowires at room temperature by electrodeposition inside a nanotemplate overcomes the disadvantages listed above. Also, there have been several studies dedicated to the fabrication of Cu nanowires, because of their potential application in a range of devices, such as wire-grid polarizers, electrostatically dissipative devices, and current collectors for Li-ion batteries. Copper has an important role in the electronics industry, based on its excellent electrical and thermal conductivity [6–16]. The template method, using anodized aluminum oxide (AAO) membrane, is a simple and effective approach to the synthesis of one-dimensional nanowires. This is a low cost method to achieve high density pores and uniform pore distribution for subsequent formation of electrodeposited nanowires. Electrodeposition in AAO templates is a very simple, efficient technique, in which the diameter and length of nanowires can be easily controlled by the anodizing parameters. High density nanowires fabricated by this technique are well ordered and parallel to each other in the AAO template. This characteristic has qualified nanowires for unique applications in mesoscopic physics and nanoscale devices [17–19]. In the present work, copper nanowires were synthesized at relatively low temperatures on NiTi SMA thin films by an electrochemical method. This process also offers low fabrication cost, and the deposition parameters are easily controlled. Fig. 1 shows a schematic of the fabrication steps. 2. Experimental procedure To obtain a template for subsequent growth of Cu nanowires, ~500 nm of Al was deposited (at room temperature) onto ~ 2 μm NiTi thin films on silicon substrates (10 mm ∗5 mm) [20]. At the
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N. Bayat et al. / Surface & Coatings Technology 206 (2012) 4075–4078
Fig. 1. Schematic of Cu nanowire synthesis on NiTi thin film.
initial steps of the experiments, it was found that electron beam deposition of the Al led to insufficient adhesion to the underlying NiTi film, and hence dc sputter deposition was used instead. A sputtering gas pressure of 0.5 Pa Ar, with a target–substrate distance of 40 mm, gave a deposition rate of ~ 20 nm/min. Al/NiTi films were subsequently annealed in a vacuum furnace (base pressure b 10 − 4 Pa) at 500 °C for 1 h, with heating and cooling rates of approximately 50 °C/min to anneal both Al and NiTi amorphous films. The annealed films were anodized in 0.3 M oxalic acid: the composite layer of Al/NiTi was set as the anode and stainless steel (316) was the cathode. The anodized aluminum was prepared at 15 °C using different anodizing voltages to obtain the optimum AAO structure. In order to monitor the anodization process, a multimeter was used to measure the current density, at constant voltage. A solution of 0.2 M CuSO4·5H2O and 0.1 M H3BO3 was used as electrolyte to deposit the copper. The electrolyte pH was adjusted to 3 by adding sulphuric acid. Here, the AAO template was the cathode and the pure copper sheet was set as the anode. Electrodeposition was performed at 0.4 V, the applied potential between cathode and anode, and bath temperature of 25 °C. A homemade electrochemical analyzer (SAMA500) was used to determine the time taken for nanochannel filling by measuring the current density versus time. After filling the AAO template with copper, the AAO/NiTi layer was etched in 1 M NaOH to remove the anodic alumina template. AAO template film and Cu nanowires were examined by observing the surface and fracture cross-sections in a Field Emission Scanning Electron Microscope (FE-SEM) using a S4160 Hitachi Japan. X-ray diffraction (XRD) used an Expert MPT with Cu-Kα (λ = 1.54056 Å) X-ray source to identify the film and Cu nanowire structure. 3. Results and discussion Fig. 2 shows a cross sectional FE-SEM image of the ~ 500 nm Al layer deposited onto ~ 2 μm amorphous NiTi film on a Si(100) substrate. Fig. 3 shows FE-SEM images of the anodized Al layer formed using anodization voltages of 40, 35, and 30 V. In these experiments all other anodization parameters were unchanged. As shown in Fig. 3a and b, at higher voltage the surface of the anodized layer was not uniform. The optimal result was obtained at 30 V (see Fig. 3c), showing a uniform surface with ordered nano-channels. Here the ~60 nm diameter nanopores are spaced at approximately 60 nm from each other. In order to ensure the best contact between the copper nanowires and the NiTi thin film surface, it was necessary to completely anodize the Al film. However, excessive anodization time may lead to damage on the SMA film surface. Hence, the anodization process was carefully monitored by measuring the current density as a function of time. As shown in Fig. 4, three regions can be identified during Al anodization: a) current density decrease on formation of oxide on the top surface of the Al film, b) constant current density during nanochannel
formation in the Al, and c) further current density change when the Al anodization process is complete (here, current density increase on reaching the NiTi film). From Fig. 4 we can observe that the growth of nanochannels begins after about 60 s, and in less than 380 s the anodization process is complete. The anodization rate is therefore estimated to be approximately 80 nm/min. The ~60–80 nm ordered nanochannels formed by anodization are next used to synthesize copper nanowires by reduction of Cu ions within them. Here, choice of a suitable electrodeposition voltage has an important role: if low voltages are used, nanowire growth is slow, but at high voltages uniformity becomes a problem. In this work, following optimization, the applied voltage used was −0.4 V/SCE and Fig. 5 shows a plot of current versus time. In zone (a) of Fig. 5, nanowires grow up from the NiTi film surface and the current density remains constant. After about 280 s the nanochannels are completely filled, and continued electrodeposition results in lateral growth across the spaces between them (zone b). On completely filling the lateral spaces between nanochannels, further electrodeposition leads to growth of a uniform surface layer (zone c). From this figure the growth rate of Cu nanowires within the AAO nanochannels is estimated to be 107 nm/min. Fig. 6 shows the copper nanowires before and after removal of the alumina template. As shown here, the copper has just begun to grow laterally, after filling the nanochannels (Fig. 6a), so that the diameter of the tops of the Cu nanowires appears larger than the diameter of the nanochannels themselves (see Fig. 6b). Fig. 7 shows the XRD scan from copper nanowires on a NiTi thin film. The (100) austenitic peak of NiTi film (after annealing of the amorphous film) is observed at 2θ = 42.7°. The Cu nanowires grow in a (100) crystallographic orientation, corresponding to the (100) plane of NiTi.
Fig. 2. FE-SEM image of deposited Al layer (~500 nm) on 2 μm NiTi film.
N. Bayat et al. / Surface & Coatings Technology 206 (2012) 4075–4078
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a
b
Fig. 5. Current–time diagram at − 0.4 V/SCE for electrodeposition of Cu in nanochannels of AAO layer.
4. Conclusions
c
Cu nanowire arrays, offering improved thermal actuation properties, have been fabricated on NiTi SMA thin films by filling AAO templates. By measuring the current density during anodization and during electrodeposition of copper nanowires, the rates of Al anodization and Cu electrodeposition were calculated as 79 and 107 nm/min, respectively. These data can be used to predict the time required for Al film anodization and AAO template filling on SMA films. This paper also can propose the possibility of growth of metallic elemental or alloy array nanowires instead of Cu on SMA thin films by using AAO film template for desired applications.
a
Fig. 3. Anodized Al layer on NiTi thin film following anodization in 0.3 M oxalic acid at 15 °C and voltages of a) 40 V, b) 35 V and c) 30 V.
b
Fig. 4. Current density as a function of time for anodization (at 30 V).
Fig. 6. Cu nanowires a) before, and b) after removal of alumina template in 1 M NaOH.
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N. Bayat et al. / Surface & Coatings Technology 206 (2012) 4075–4078
Fig. 7. XRD patterns of Cu nanowires on NiTi thin film.
References [1] K. Otsuka, T. Kakeshita, MRS Bull. 27 (2002) 91. [2] A. Ishida, V. Martynov, MRS Bull. 27 (2002) 111. [3] P. Kim, L. Shi, A. Majumdar, P.L. McEuen, Phys. Rev. Lett. 87 (2001) 215502.
[4] S. Berber, Y. Kwon, D. Tomanek, Phys. Rev. Lett. 84 (2000) 4613. [5] B.C. Bayer, S. Sanjabi, C. Baehtz, C.T. Wirth, S. Esconjauregui, R.S. Weatherup, Z.H. Barber, S. Hofmann, J. Robertson, Thin Solid Films 519 (2011) 6126. [6] S.Z. Chu, K. Wada, S. Inoue, S. Todoroki, Electrochim. Acta 48 (2003) 3147. [7] J.-L. Duan, J. Liu, H.-J. Yao, D. Mo, M.-D. Hou, Y.-M. Sun, Y.-F. Chen, L. Zhang, Mater. Sci. Eng. B 147 (2008) 57. [8] Z. Ying, F. Leung-Yuk Lam, H.U. Xijun, Y. Zifeng, Chin. Sci. Bull. 51 (2006) 2662. [9] G. Riveros, H. Gómez, A. Cortés, R.E. Marotti, E.A. Dalchiele, Appl. Phys. A 81 (2005) 17. [10] X. Wen, Y. Xie, C.L. Choi, K.C. Wan, X.-Y. Li, S. Yang, Langmuir 21 (2005) 4729. [11] G. Sharma, C.S. Chong, L. Ebin, V. Kripesh, C.L. Gan, C.H. Sow, Nanotechnology 18 (2007) 305306. [12] A. Ghaddar, J. Gieraltowski, F. Gloaguen, J. Appl. Electrochem. 39 (2009) 719. [13] S. Inoue, S.-Z. Chu, K. Wada, D. Li, H. Haneda, Sci. Technol. Adv. Mater. 4 (2003) 269. [14] T. Gao, G. Meng, Y. Wang, S. Sun, L. Zhang, J. Phys. Condens. Matter 14 (2002) 355. [15] Y.-H. Lee, I.-C. Leu, M.-T. Wu, J.-H. Yen, K.-Z. Fung, J. Alloys Compd. 427 (2007) 213. [16] R. Inguanta, S. Piazza, C. Sunseri, Appl. Surf. Sci. 255 (2009) 8816. [17] S.Z. Chu, K. Wada, S. Inoue, S. Todoroki, Surf. Coat. Technol. 169–170 (2003) 190. [18] Y.F. Mei, X.L. Wu, T. Qiu, X.F. Shao, G.G. Siu, P.K. Chu, Thin Solid Films 492 (2005) 66. [19] Y. Yang, H. Chen, Y. Mei, J. Chen, X. Wu, X. Bao, Solid State Commun. 123 (2002) 279. [20] S. Sanjabi, S.K. Sadrnezhaad, K.A. Yates, Z.H. Barber, Thin Solid Films 491 (2005) 190.