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Metal nanorod production in silicon matrix by electroless process
Applied Surface Science 255 (2009) 4670–4672
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Metal nanorod production in silicon matrix by electroless process Shinji Yae *, Tatsuya Hirano, Takashi Matsuda, Naoki Fukumuro, Hitoshi Matsuda Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 June 2008 Received in revised form 7 December 2008 Accepted 8 December 2008 Available online 11 December 2008
Metal filled Si nanopores, that is, metal nanorods in an Si matrix, are produced by an electroless process that consists of three steps: (1) electroless displacement deposition of metal nanoparticles from a metal salt solution containing HF; (2) Si nanopore formation by metal-particle-enhanced HF etching; and (3) metal filling in nanopores by autocatalytic electroless deposition. Ag nanoparticles produce Si nanopores whose sizes are a few tens of nm in diameter and ca. 50 nm deep. Au nanoparticles produce finer and straighter nanopores on Si than the Ag case. These nanopores are filled with a Co or a Co–Ni alloy by autocatalytic deposition using dimethylamine-borane as a reducing agent. Phosphinate can be used as a reducing agent for the Au-deposited-and-pore-formed Si. The important feature of this process is that the metal nanoparticles, that is, the initiation points of the autocatalytic metal deposition, are present on the bottoms of the Si nanopores. ß 2008 Elsevier B.V. All rights reserved.
PACS: 62.23.Pq, 81.05.Rm, 81.15.Pq, 81.65.Cf, 82.45.Jn, 82.45.Vp Keywords: Metal nanoparticle Porous silicon Metal-particle-enhanced HF etching Electroless deposition Magnetic recoding Cobalt
Porous silicon (Si) consisting of nanometer- to micrometer-sized pores is usually prepared by electrochemical etching under anodic bias in a fluoride-containing solution [1,2]. Metal-particle-enhanced hydrofluoric acid (HF) etching is an electroless method that can produce porous Si by immersing metal-particle-modified Si in an HF solution without a bias [3–14]. The structure of porous Si changes with the etching conditions [8,10,12–14]. An electroless deposition method can deposit metals on insulating substrates without a bias [15]. We have been studying the electroless displacement deposition of metal nanoparticles on Si [16] and autocatalytic electroless deposition of magnetic cobalt (Co) alloy thin films [17,18]. In this study, we combine these methods to produce metal filled Si nanopores, that is, metal nanorods in an Si matrix by an electroless process. This simple and low cost process is not only interesting for making such a structure [19,20], but is also favorable to apply ultrahigh-density perpendicular magnetic recording media [21].
hydrofluoric acid (HF), nitric acid, acetic acid, and water) and a 7.3 M (M = mol dm 3) HF solution. Metal nanoparticles were deposited on p-Si wafers by electroless displacement deposition using a 1 mM metal salt solution containing 0.15 M HF. The deposition of silver (Ag) and gold (Au) nanoparticles uses silver nitrate and tetrachloroauric (III) acid as metal salts. Si nanopores were prepared by metal-particle-enhanced HF etching using a 7.3 M HF aqueous solution at 298 K under dark conditions. For metal filling in Si nanopores, the autocatalytic deposition of Co was adopted. The deposition solution of Co using dimethylamineborane (DMAB) as a reducing agent included 0.05 M of cobalt sulfate (CoSO4), 0.025 M of DMAB, 0.2 M of sodium succinate, and 0.5 M of ammonium sulfate. The deposition solution of cobalt– nickel (Co–Ni) alloy using DMAB as its reducing agent included 0.05 M of CoSO4, 0.05 M of nickel sulfate, 0.05 M of DMAB, 0.2 M of sodium citrate, and 0.5 M of boric acid. Surface and cross-sectional inspections of the specimens were performed with a scanning electron microscope (SEM, Hitachi S-900).
2. Experimental
3. Results
1. Introduction
Single-crystal p-type Si wafers (CZ (1 0 0), ca. 1 Vcm) were washed with acetone and etched with CP-4A (a mixture of
* Corresponding author. Tel.: +81 79 267 4911; fax: +81 79 267 4911. E-mail address: [email protected] (S. Yae). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.020
Ag nanoparticles of 4 1010 cm 2 in particle density were deposited on p-Si substrates by immersion in displacement deposition solution at 313 K for 120 s. The deposited particles were connected to each other and formed non-spherical shapes. The particle density and morphology of the Ag nanoparticles resembled our previously reported results for Ag nanoparticle
S. Yae et al. / Applied Surface Science 255 (2009) 4670–4672
Fig. 1. SEM image of Ag-particle-deposited p-Si wafer prepared by immersion of pSi in 1 mM silver nitrate aqueous solution containing 0.15 M HF at 278 K for 30 s.
Fig. 2. Cross-sectional SEM image of Ag-particle-deposited p-Si wafer after metalparticle-enhanced HF etching in 7.3 M HF solution under dark conditions for 10 min.
deposition on n-Si substrates under the same conditions [16]. The decrease in immersion time and solution temperature reduced the size of the nanoparticles. Fig. 1 shows a SEM image of the p-Si surface after immersion in the Ag deposition solution at 278 K for 30 s. Spherical nanoparticles, 7–30 nm in diameter and 1.8 1011 cm 2 in particle density, are scattered with a few tens of nm distance between each particle.
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Fig. 3. Cross-sectional SEM image of Co filled Si nanopores prepared by immersing Ag-deposited-and-pore-formed p-Si wafer in the autocatalytic deposition solution using DMAB as a reducing agent. White ellipse indicates one of typical nanorods.
For producing nanopores by metal-particle-enhanced HF etching, the Ag-particle-deposited p-Si was immersed in the HF solution for 10 min. Fig. 2 shows a SEM image of p-Si after etching. The nanopores, a few tens of nm in diameter and ca. 50 nm deep, were formed. The diameter of the pores resembled the Ag nanoparticles. An Ag nanoparticle is present on the bottom of each pore. On visual inspection, the etched p-Si had a bright surface and showed no photoluminescence. These results indicate that the etching was led by Ag particles and no microporous layer was formed. The Ag-deposited-and-pore-formed p-Si was immersed in the solution for the autocatalytic electroless deposition of Co. No deposition reaction proceeded when using phosphinate as a reducing agent. By using dimethylamine-borane as a reducing agent, the pore-formed p-Si was covered with a conducting Co metal thin film by immersion in the deposition solution. Fig. 3 shows a cross-sectional SEM image of the pore-formed p-Si sample after immersion in the solution for 120 s. The pores of the p-Si substrate were filled with deposited Co metal, that is, Co nanorods were produced in the Si matrix. The Ag nanoparticles remain on the bottom of the pores. Neither flat Si nor porous Si, which has no Ag particles on the surface, can initiate the deposition of Co. These results clearly show that the autocatalytic electroless deposition of Co was initiated by the Ag nanoparticles on the bottom of each pore, and thus the nanopores were completely filled with Co.
Fig. 4. SEM images of electroless process for producing metal filled Si nanopores by using Au nanoparticles. (A) p-Si surface after displacement deposition of Au nanoparticles; (B) after metal-particle-enhanced HF etching; (C, D) after autocatalytic deposition of Co–Ni alloy using DMAB as a reducing agent; (B), (C) and (D): cross-section. White ellipses in (B) and (D) indicate one of typical nanopores and nanorods, respectively.
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S. Yae et al. / Applied Surface Science 255 (2009) 4670–4672
Fig. 4 shows the results for Au nanoparticles. Displacement deposition of Au on p-Si obtained a higher particle density of 5.5 1011 cm 2 and a smaller particle size of 4–15 nm than those of Ag particles by immersing the p-Si in the Au deposition solution at 278 K for 10 s (Fig. 4A). Finer and straighter nanopores than the Ag case were formed by immersing the Au-nanoparticle-deposited p-Si in the HF solution for 15 min (Fig. 4B). The diameter of the pores resembled that of the Au nanoparticles, which remain on the bottom of each nanopore. The autocatalytic electroless deposition of Co, Ni, and Co–Ni alloys was initiated on the Au-deposited-andpore-formed p-Si in both cases of reducing agents, phosphinate and DMAB. Fig. 4C shows a cross-sectional SEM image of Audeposited-and-pore-formed p-Si after immersion in an electroless deposition solution of Co–Ni–B for 120 s. An 85-nm-thick metal film continuously covered the surface of the p-Si. The interface between the metal film and the p-Si is not clear in the image. Fig. 4D shows another part of the sample of Fig. 4C. There is a cleft between the metal film and the p-Si surface that indicates that the metal film was detached from the p-Si substrate by cleaving the sample for cross-sectional inspection. Nanorods are confirmed on the Si side surface of the metal film. No nanorod is seen on the opposite side surface of the metal film. The diameter of the nanorods is consistent with the size of the nanopores of p-Si and thus the Au nanoparticles. These nanorods indicate that the p-Si nanopores were filled with electrolessly deposited metal, Co–Ni alloy in Fig. 4 case. The above results clearly show that the metal filled Si nanopores, the metal nanorods, were produced in an Si matrix by a three-step electroless wet process: displacement deposition of the metal nanoparticles, nanopore formation by metal-particleenhanced HF etching, and autocatalytic electroless metal filling in the silicon nanopores. 4. Discussion The temperature lowering of the electroless displacement deposition solution of Ag reduced the size of the deposited particles and increased their density. This result is explained as follows. The size and particle density for the present case of a solution temperature of 278 K and an immersion time of 30 s are similar to previously reported results for 313 K and 1 s [16]. Since nucleation activity of Ag on Si is very high [16], the temperature lowering from 313 to 278 K only reduced the growth rate of the Ag particles. The low growth rate, that is, the low anodic dissolution rate of Si, caused no detachment of the Ag particles from the Si surfaces. Thus, the temperature lowering of the deposition solution reduced the size of the deposited Ag particles and increased their density. The nanopore formation on Si, which was led by the metal particles, was caused by localizing the Si dissolution under the metal particles. The mechanism of the metal-particle-enhanced HF etching is described using a local galvanic cell consisting of the local cathode reduction of oxygen on the metal particles and the local anode oxidation of Si [8,10,12]. The absence of a photogenerated charge under dark conditions localized the anode reaction in the vicinity of the metal particles. The Ag nanoparticles did not initiate autocatalytic electroless deposition using phosphinate as a reducing agent; the Au
nanoparticles did. They were caused by the difference of the catalytic activity for the anodic oxidation of the phosphinate between the two metals. Ohno et al. reported that the anodic current attributable to the dissolution of Ag was obtained in a phosphinate solution [22]. Thus, Ag has no catalytic activity for the anodic oxidation of phosphinate, that is, the anode reaction of autocatalytic electroless deposition. They also reported that Au has even higher catalytic activity for the anodic oxidation of phosphinate than palladium [22]. 5. Conclusion In this study, we produced metal filled Si nanopores, that is, metal nanorods in an Si matrix by an electroless process that consists of three steps: (1) displacement deposition of metal nanoparticles; (2) Si nanopore formation by metal-particleenhanced HF etching; and (3) metal filling in nanopores by autocatalytic deposition. Metal nanoparticles that remain on the bottoms of the Si nanopores exhibit catalytic activity for the initiation of autocatalytic metal deposition. Thus, Si nanopores are completely filled with the metal from their bottoms. Acknowledgments The present work was partly supported by CREST and Research for Promoting Technological Seeds from JST, and Grants-in-Aid for Education and Research from Hyogo Prefecture and for Scientific Research (C) (20560676) from JSPS. References [1] V. Lehmann, Electrochemistry of Silicon, Wiley-VCH, Weinheim, 2002. [2] C. Le´vy-Cle´ment, in: S. Licht (vol. Ed.), A.J. Bard, M. Stratmann (series Eds.), Encyclopedia of Electrochemistry, vol. 6, Wiley-VCH, Weinheim, 2002 (Chapter 3.2). [3] K.W. Kolasinski, Curr. Opin. Solid State Mater. Sci. 9 (2005) 73. [4] X. Li, P.W. Bohn, Appl. Phys. Lett. 77 (2000) 2572. [5] S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, H. Matsuda, Electrochem. Commun. 5 (2003) 632. [6] K. Peng, Y. Yan, S. Gao, J. Zhu, Adv. Funct. Mater. 13 (2003) 127. [7] T. Hadjersi, N. Gabouze, E.S. Kooij, A. Zinine, A. Ababou, W. Chergui, H. Cheraga, S. Belhousse, A. Djeghri, Thin Solid Films 459 (2004) 271. [8] S. Yae, H. Tanaka, T. Kobayashi, N. Fukumuro, H. Matsuda, Phys. Stat. Sol. (c) 2 (2005) 3476. [9] K. Tsujino, M. Matsumura, Electrochem. Solid-State Lett. 8 (2005) C193. [10] S. Yae, T. Kobayashi, T. Kawagishi, N. Fukumuro, H. Matsuda, ECS Proc. 2004–19 (2006) 141. [11] H. Asoh, F. Arai, S. Ono, Electrochem. Commun. 9 (2007) 535. [12] S. Yae, M. Abe, T. Kawagishi, K. Suzuki, N. Fukumuro, H. Matsuda, Trans. Mater. Res. Soc. Jpn. 32 (2007) 445. [13] K. Tsujino, M. Matsumura, Electrochim. Acta 53 (2007) 28. [14] C. Chartier, S. Bastide, C. Le´vy-Cle´ment, Electrochim. Acta 53 (2008) 5509. [15] M. Paunovic, M. Schlesinger, Fundamentals of Electrochemical Deposition, 2nd. ed., John Wiley & Sons, New York, NY, 2006. [16] S. Yae, N. Nasu, K. Matsumoto, T. Hagihara, N. Fukumuro, H. Matsuda, Electrochim. Acta 53 (2007) 35. [17] H. Matsuda, O. Takano, H. Gohuku, P.J. Grundy, J. Magn. Magn. Mater. 110 (1992) 227. [18] N. Fukumuro, J. Nishiyama, S. Yae, H. Matsuda, Trans. Inst. Met. Finish 83 (2005) 281. [19] H. Sato, T. Homma, K. Mori, T. Osaka, S. Shoji, Electrochemistry 73 (2005) 275. [20] K. Kobayashi, F.A. Harraz, S. Izuo, T. Sakka, Y.H. Ogata, J. Electrochem. Soc. 153 (2006) C218. [21] K. Yasui, T. Morikawa, K. Nishio, H. Masuda, Jpn. J. Appl. Phys. 44 (2005) L469. [22] I. Ohno, O. Wakabayashi, S. Haruyama, J. Electrochem. Soc. 132 (1985) 2323.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Metal nanorod production in silicon matrix by electroless process Shinji Yae *, Tatsuya Hirano, Takashi Matsuda, Naoki Fukumuro, Hitoshi Matsuda Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 June 2008 Received in revised form 7 December 2008 Accepted 8 December 2008 Available online 11 December 2008
Metal filled Si nanopores, that is, metal nanorods in an Si matrix, are produced by an electroless process that consists of three steps: (1) electroless displacement deposition of metal nanoparticles from a metal salt solution containing HF; (2) Si nanopore formation by metal-particle-enhanced HF etching; and (3) metal filling in nanopores by autocatalytic electroless deposition. Ag nanoparticles produce Si nanopores whose sizes are a few tens of nm in diameter and ca. 50 nm deep. Au nanoparticles produce finer and straighter nanopores on Si than the Ag case. These nanopores are filled with a Co or a Co–Ni alloy by autocatalytic deposition using dimethylamine-borane as a reducing agent. Phosphinate can be used as a reducing agent for the Au-deposited-and-pore-formed Si. The important feature of this process is that the metal nanoparticles, that is, the initiation points of the autocatalytic metal deposition, are present on the bottoms of the Si nanopores. ß 2008 Elsevier B.V. All rights reserved.
PACS: 62.23.Pq, 81.05.Rm, 81.15.Pq, 81.65.Cf, 82.45.Jn, 82.45.Vp Keywords: Metal nanoparticle Porous silicon Metal-particle-enhanced HF etching Electroless deposition Magnetic recoding Cobalt
Porous silicon (Si) consisting of nanometer- to micrometer-sized pores is usually prepared by electrochemical etching under anodic bias in a fluoride-containing solution [1,2]. Metal-particle-enhanced hydrofluoric acid (HF) etching is an electroless method that can produce porous Si by immersing metal-particle-modified Si in an HF solution without a bias [3–14]. The structure of porous Si changes with the etching conditions [8,10,12–14]. An electroless deposition method can deposit metals on insulating substrates without a bias [15]. We have been studying the electroless displacement deposition of metal nanoparticles on Si [16] and autocatalytic electroless deposition of magnetic cobalt (Co) alloy thin films [17,18]. In this study, we combine these methods to produce metal filled Si nanopores, that is, metal nanorods in an Si matrix by an electroless process. This simple and low cost process is not only interesting for making such a structure [19,20], but is also favorable to apply ultrahigh-density perpendicular magnetic recording media [21].
hydrofluoric acid (HF), nitric acid, acetic acid, and water) and a 7.3 M (M = mol dm 3) HF solution. Metal nanoparticles were deposited on p-Si wafers by electroless displacement deposition using a 1 mM metal salt solution containing 0.15 M HF. The deposition of silver (Ag) and gold (Au) nanoparticles uses silver nitrate and tetrachloroauric (III) acid as metal salts. Si nanopores were prepared by metal-particle-enhanced HF etching using a 7.3 M HF aqueous solution at 298 K under dark conditions. For metal filling in Si nanopores, the autocatalytic deposition of Co was adopted. The deposition solution of Co using dimethylamineborane (DMAB) as a reducing agent included 0.05 M of cobalt sulfate (CoSO4), 0.025 M of DMAB, 0.2 M of sodium succinate, and 0.5 M of ammonium sulfate. The deposition solution of cobalt– nickel (Co–Ni) alloy using DMAB as its reducing agent included 0.05 M of CoSO4, 0.05 M of nickel sulfate, 0.05 M of DMAB, 0.2 M of sodium citrate, and 0.5 M of boric acid. Surface and cross-sectional inspections of the specimens were performed with a scanning electron microscope (SEM, Hitachi S-900).
2. Experimental
3. Results
1. Introduction
Single-crystal p-type Si wafers (CZ (1 0 0), ca. 1 Vcm) were washed with acetone and etched with CP-4A (a mixture of
* Corresponding author. Tel.: +81 79 267 4911; fax: +81 79 267 4911. E-mail address: [email protected] (S. Yae). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.020
Ag nanoparticles of 4 1010 cm 2 in particle density were deposited on p-Si substrates by immersion in displacement deposition solution at 313 K for 120 s. The deposited particles were connected to each other and formed non-spherical shapes. The particle density and morphology of the Ag nanoparticles resembled our previously reported results for Ag nanoparticle
S. Yae et al. / Applied Surface Science 255 (2009) 4670–4672
Fig. 1. SEM image of Ag-particle-deposited p-Si wafer prepared by immersion of pSi in 1 mM silver nitrate aqueous solution containing 0.15 M HF at 278 K for 30 s.
Fig. 2. Cross-sectional SEM image of Ag-particle-deposited p-Si wafer after metalparticle-enhanced HF etching in 7.3 M HF solution under dark conditions for 10 min.
deposition on n-Si substrates under the same conditions [16]. The decrease in immersion time and solution temperature reduced the size of the nanoparticles. Fig. 1 shows a SEM image of the p-Si surface after immersion in the Ag deposition solution at 278 K for 30 s. Spherical nanoparticles, 7–30 nm in diameter and 1.8 1011 cm 2 in particle density, are scattered with a few tens of nm distance between each particle.
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Fig. 3. Cross-sectional SEM image of Co filled Si nanopores prepared by immersing Ag-deposited-and-pore-formed p-Si wafer in the autocatalytic deposition solution using DMAB as a reducing agent. White ellipse indicates one of typical nanorods.
For producing nanopores by metal-particle-enhanced HF etching, the Ag-particle-deposited p-Si was immersed in the HF solution for 10 min. Fig. 2 shows a SEM image of p-Si after etching. The nanopores, a few tens of nm in diameter and ca. 50 nm deep, were formed. The diameter of the pores resembled the Ag nanoparticles. An Ag nanoparticle is present on the bottom of each pore. On visual inspection, the etched p-Si had a bright surface and showed no photoluminescence. These results indicate that the etching was led by Ag particles and no microporous layer was formed. The Ag-deposited-and-pore-formed p-Si was immersed in the solution for the autocatalytic electroless deposition of Co. No deposition reaction proceeded when using phosphinate as a reducing agent. By using dimethylamine-borane as a reducing agent, the pore-formed p-Si was covered with a conducting Co metal thin film by immersion in the deposition solution. Fig. 3 shows a cross-sectional SEM image of the pore-formed p-Si sample after immersion in the solution for 120 s. The pores of the p-Si substrate were filled with deposited Co metal, that is, Co nanorods were produced in the Si matrix. The Ag nanoparticles remain on the bottom of the pores. Neither flat Si nor porous Si, which has no Ag particles on the surface, can initiate the deposition of Co. These results clearly show that the autocatalytic electroless deposition of Co was initiated by the Ag nanoparticles on the bottom of each pore, and thus the nanopores were completely filled with Co.
Fig. 4. SEM images of electroless process for producing metal filled Si nanopores by using Au nanoparticles. (A) p-Si surface after displacement deposition of Au nanoparticles; (B) after metal-particle-enhanced HF etching; (C, D) after autocatalytic deposition of Co–Ni alloy using DMAB as a reducing agent; (B), (C) and (D): cross-section. White ellipses in (B) and (D) indicate one of typical nanopores and nanorods, respectively.
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S. Yae et al. / Applied Surface Science 255 (2009) 4670–4672
Fig. 4 shows the results for Au nanoparticles. Displacement deposition of Au on p-Si obtained a higher particle density of 5.5 1011 cm 2 and a smaller particle size of 4–15 nm than those of Ag particles by immersing the p-Si in the Au deposition solution at 278 K for 10 s (Fig. 4A). Finer and straighter nanopores than the Ag case were formed by immersing the Au-nanoparticle-deposited p-Si in the HF solution for 15 min (Fig. 4B). The diameter of the pores resembled that of the Au nanoparticles, which remain on the bottom of each nanopore. The autocatalytic electroless deposition of Co, Ni, and Co–Ni alloys was initiated on the Au-deposited-andpore-formed p-Si in both cases of reducing agents, phosphinate and DMAB. Fig. 4C shows a cross-sectional SEM image of Audeposited-and-pore-formed p-Si after immersion in an electroless deposition solution of Co–Ni–B for 120 s. An 85-nm-thick metal film continuously covered the surface of the p-Si. The interface between the metal film and the p-Si is not clear in the image. Fig. 4D shows another part of the sample of Fig. 4C. There is a cleft between the metal film and the p-Si surface that indicates that the metal film was detached from the p-Si substrate by cleaving the sample for cross-sectional inspection. Nanorods are confirmed on the Si side surface of the metal film. No nanorod is seen on the opposite side surface of the metal film. The diameter of the nanorods is consistent with the size of the nanopores of p-Si and thus the Au nanoparticles. These nanorods indicate that the p-Si nanopores were filled with electrolessly deposited metal, Co–Ni alloy in Fig. 4 case. The above results clearly show that the metal filled Si nanopores, the metal nanorods, were produced in an Si matrix by a three-step electroless wet process: displacement deposition of the metal nanoparticles, nanopore formation by metal-particleenhanced HF etching, and autocatalytic electroless metal filling in the silicon nanopores. 4. Discussion The temperature lowering of the electroless displacement deposition solution of Ag reduced the size of the deposited particles and increased their density. This result is explained as follows. The size and particle density for the present case of a solution temperature of 278 K and an immersion time of 30 s are similar to previously reported results for 313 K and 1 s [16]. Since nucleation activity of Ag on Si is very high [16], the temperature lowering from 313 to 278 K only reduced the growth rate of the Ag particles. The low growth rate, that is, the low anodic dissolution rate of Si, caused no detachment of the Ag particles from the Si surfaces. Thus, the temperature lowering of the deposition solution reduced the size of the deposited Ag particles and increased their density. The nanopore formation on Si, which was led by the metal particles, was caused by localizing the Si dissolution under the metal particles. The mechanism of the metal-particle-enhanced HF etching is described using a local galvanic cell consisting of the local cathode reduction of oxygen on the metal particles and the local anode oxidation of Si [8,10,12]. The absence of a photogenerated charge under dark conditions localized the anode reaction in the vicinity of the metal particles. The Ag nanoparticles did not initiate autocatalytic electroless deposition using phosphinate as a reducing agent; the Au
nanoparticles did. They were caused by the difference of the catalytic activity for the anodic oxidation of the phosphinate between the two metals. Ohno et al. reported that the anodic current attributable to the dissolution of Ag was obtained in a phosphinate solution [22]. Thus, Ag has no catalytic activity for the anodic oxidation of phosphinate, that is, the anode reaction of autocatalytic electroless deposition. They also reported that Au has even higher catalytic activity for the anodic oxidation of phosphinate than palladium [22]. 5. Conclusion In this study, we produced metal filled Si nanopores, that is, metal nanorods in an Si matrix by an electroless process that consists of three steps: (1) displacement deposition of metal nanoparticles; (2) Si nanopore formation by metal-particleenhanced HF etching; and (3) metal filling in nanopores by autocatalytic deposition. Metal nanoparticles that remain on the bottoms of the Si nanopores exhibit catalytic activity for the initiation of autocatalytic metal deposition. Thus, Si nanopores are completely filled with the metal from their bottoms. Acknowledgments The present work was partly supported by CREST and Research for Promoting Technological Seeds from JST, and Grants-in-Aid for Education and Research from Hyogo Prefecture and for Scientific Research (C) (20560676) from JSPS. References [1] V. Lehmann, Electrochemistry of Silicon, Wiley-VCH, Weinheim, 2002. [2] C. Le´vy-Cle´ment, in: S. Licht (vol. Ed.), A.J. Bard, M. Stratmann (series Eds.), Encyclopedia of Electrochemistry, vol. 6, Wiley-VCH, Weinheim, 2002 (Chapter 3.2). [3] K.W. Kolasinski, Curr. Opin. Solid State Mater. Sci. 9 (2005) 73. [4] X. Li, P.W. Bohn, Appl. Phys. Lett. 77 (2000) 2572. [5] S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, H. Matsuda, Electrochem. Commun. 5 (2003) 632. [6] K. Peng, Y. Yan, S. Gao, J. Zhu, Adv. Funct. Mater. 13 (2003) 127. [7] T. Hadjersi, N. Gabouze, E.S. Kooij, A. Zinine, A. Ababou, W. Chergui, H. Cheraga, S. Belhousse, A. Djeghri, Thin Solid Films 459 (2004) 271. [8] S. Yae, H. Tanaka, T. Kobayashi, N. Fukumuro, H. Matsuda, Phys. Stat. Sol. (c) 2 (2005) 3476. [9] K. Tsujino, M. Matsumura, Electrochem. Solid-State Lett. 8 (2005) C193. [10] S. Yae, T. Kobayashi, T. Kawagishi, N. Fukumuro, H. Matsuda, ECS Proc. 2004–19 (2006) 141. [11] H. Asoh, F. Arai, S. Ono, Electrochem. Commun. 9 (2007) 535. [12] S. Yae, M. Abe, T. Kawagishi, K. Suzuki, N. Fukumuro, H. Matsuda, Trans. Mater. Res. Soc. Jpn. 32 (2007) 445. [13] K. Tsujino, M. Matsumura, Electrochim. Acta 53 (2007) 28. [14] C. Chartier, S. Bastide, C. Le´vy-Cle´ment, Electrochim. Acta 53 (2008) 5509. [15] M. Paunovic, M. Schlesinger, Fundamentals of Electrochemical Deposition, 2nd. ed., John Wiley & Sons, New York, NY, 2006. [16] S. Yae, N. Nasu, K. Matsumoto, T. Hagihara, N. Fukumuro, H. Matsuda, Electrochim. Acta 53 (2007) 35. [17] H. Matsuda, O. Takano, H. Gohuku, P.J. Grundy, J. Magn. Magn. Mater. 110 (1992) 227. [18] N. Fukumuro, J. Nishiyama, S. Yae, H. Matsuda, Trans. Inst. Met. Finish 83 (2005) 281. [19] H. Sato, T. Homma, K. Mori, T. Osaka, S. Shoji, Electrochemistry 73 (2005) 275. [20] K. Kobayashi, F.A. Harraz, S. Izuo, T. Sakka, Y.H. Ogata, J. Electrochem. Soc. 153 (2006) C218. [21] K. Yasui, T. Morikawa, K. Nishio, H. Masuda, Jpn. J. Appl. Phys. 44 (2005) L469. [22] I. Ohno, O. Wakabayashi, S. Haruyama, J. Electrochem. Soc. 132 (1985) 2323.