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Immobilization of Gold Nanoparticles onto Silicon Surfaces by Si-C Covalent Bonds Yoshinori Yamanoi,† Tetsu Yonezawa,*,†,‡ Naoto Shirahata,§ and Hiroshi Nishihara*,† Department of Chemistry, School of Science, The University of Tokyo, PRESTO, Japan Science and Technology Corporation (JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimo-shidami, Moriyama, Nagoya 463-8560, Japan Received December 23, 2003 A series of ω-alkene-1-thiol-stabilized gold nanoparticles were prepared by a wet process and then covalently linked to a hydrogen-terminated silicon(111) surface with Si-C bonds via a thermal hydrosilylation reaction. The modified silicon surfaces were observed mainly by high-resolution scanning electron microscopy (HR-SEM). The HR-SEM images revealed that the gold nanoparticles protected with 2-propene-1-thiol (C3) were covalently bound to the hydrogen-terminated silicon surface, after which the nanoparticles self-fused; on the other hand, gold nanoparticles protected with 5-hexene-1-thiol (C6) or 10-undecene-1thiol (C11) were linked to the hydrogen-terminated silicon surface independently. These surfaces were stable in air and can be stored for several months without detectable decomposition.
In the past decade, considerable attention has been devoted to the preparation of metal nanoparticles because they have unusual properties different from those of the bulk state and potential applications in optical, electronic, catalytic, and magnetic materials.1 To build nanostructures using colloidal nanoparticles, much effort has been made to assemble and immobilize such particles on solid surfaces. Silicon is clearly the most important material in modern technology, and there has been strong interest in the chemical modification of its surface.2 Several techniques have been published on the self-assembly of ordered monolayer arrays on silicon surfaces. One approach to modifying silicon surfaces is the use of silicasupported alkylsiloxane layers.3 Although these layers are easily prepared by reacting alkyltrichlorosilane with an oxidized silicon surface, an insulating oxide layer is not desirable for many applications, such as electronic devices or sensor materials. An alternative strategy is to prepare monolayers that are covalently bonded to a hydrogen-terminated silicon surface via a thermal hydrosilylation reaction.4-6 This reaction directly forms * To whom correspondence should be addressed. E-mail:
[email protected] (T.Y.) or
[email protected] (H.N.). Fax and telephone: +81-3-5841-4348. † The University of Tokyo. ‡ Japan Science and Technology Corporation (JST). § National Institute of Advanced Industrial Science and Technology (AIST). (1) For recent reviews on nanoparticles, see: (a) Kotov, N. A. Layerby-layer Assembly of Nanoparticles and Nanocolloids: Intermolecular Interactions, Structure, and Material Perspectives. In Multilayer Thin Films; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003; pp 207-269. (b) Yonezawa, T.; Toshima, N. Polymer-stabilized Metal Nanoparticles. In Advanced Functional Molecules and Polymers; Nalwa, H. S., Ed.; Gordon and Breach: London, U.K., 2001; pp 65-86. (2) For a recent review, see: Buriak, J. M. Chem. Rev. 2002, 102, 1272. (3) For a recent review, see: Ulman, A. Chem. Rev. 1996, 96, 1533. (4) For a recent review, see: Buriak, J. M. J. Chem. Soc., Chem. Commun. 1999, 1051. (5) Recently, Buriak and co-workers reported hydrosilylation on a porous silicon surface, see: (a) Buriak, J. M. Adv. Mater. 1999, 11, 265. (b) Choi, H. C.; Buriak, J. M. Chem. Mater. 2000, 12, 2151. (c) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491. (d) Schmeltzer, J. M.; Porter, L. A.; Stewart, M. P.; Buriak, J. M. Langmuir 2002, 18, 2971.
Scheme 1. Immobilization of ω-Alkene-1-thiol-Functionalized Gold Nanoparticles onto a Hydrogen-Terminated Silicon(111) Surface
stable silicon-carbon covalent bonds on the silicon surface. A variety of organic monolayers have been demonstrated to attach covalently to silicon surfaces through siliconcarbon bonds. In this paper, we describe the ω-alkene1-thiol-functionalized gold nanoparticle covalently linked to a hydrogen-terminated silicon(111) surface through a thermal hydrosilylation reaction (Scheme 1). To our best knowledge, this is the first approach to attach nanosized materials by using hydrosilylation. This strategy can be applied to molecular architectures on semiconductor surfaces, leading to a promising technique for electronic applications such as the single-electron transistor. An excellent example for this research is ω-alkene-1thiols, which undergo typical reactions for both aliphatic thiols and olefins. The aliphatic thiols can act as surfacestabilizing reagents in the formation of nanoparticles, and the CdC bond can participate in the thermal hydrosilylation reaction on the hydrogen-terminated silicon(111) surface. The preparation of gold nanoparticles 2a-c is summarized in Scheme 2. These compounds were prepared from HAuCl4 and the corresponding alkenethiol (1a-c)7 (6) For recent reports on the fabrication of a hydrogen-terminated silicon surface via a thermal hydrosilylation reaction, see: (a) Yamada, T.; Inoue, T.; Yamada, K.; Takano, N.; Osaka, T.; Harada, H.; Nishiyama, K.; Taniguchi, I. J. Am. Chem. Soc. 2003, 125, 8039. (b) Ara, M.; Tada, H. Appl. Phys. Lett. 2003, 83, 578. (7) Propene-1-thiol (1a) was purchased from TCI and used without further purification. 5-Hexene-1-thiol (1b) and 10-undecene-1-thiol (1c) were prepared from corresponding alkenyl bromide substrates and thiourea according to well-known procedures. Speziale, A. J. Org. Synth. 1963, IV, 401.
10.1021/la036437c CCC: $27.50 © 2004 American Chemical Society Published on Web 01/24/2004
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Scheme 2. Preparation of the ω-Alkene-1-thiol-Functionalized Gold Nanoparticles
according to the modification method based on the literature.8 Nanoparticles 2a-c were isolated as black, air-stable powders in good yields by reprecipitation with acetonitrile at room temperature. With the use of sodium borohydride, not all the CdC bonds were kept, but the several alkenyl thiolates on a particle were hydrogenated during the formation of functionalized gold nanoparticles, as judging from elemental analyses and the integration of 1H NMR. The ratios of alkyl thiolates to alkenyl thiolates on particles were 58/42, 50/50, and 17/83 for 2a, 2b, and 2c, respectively. The particle sizes were characterized by transmission electron microscopic (TEM; Hitachi HF-2000) observation at an accelerating voltage of 200 kV. TEM photographs and the diameter distributions of the gold nanoparticles are shown in Figure 1. Gold nanoparticle 2a varied remarkably in size (mean particle size, 3.1 nm; standard deviation, 1.6 nm). On the other hand, nearly monodispersed gold nanoparticles were obtained in the preparation of 2b (mean particle size, 3.4 nm; standard deviation, 1.2 nm). When the stabilizer was changed from 5-hexene-1thiol (1b) to 10-undecene-1-thiol (1c), the particles obtained were smaller with a narrower size distribution (mean particle size, 1.9 nm; standard deviation, 0.6 nm). The average size and size distribution of these gold nanoparticles probably depended on the interaction between the stabilizers and the particle surfaces. This indicates that dense and highly hydrophilic interaction between long aliphatic chains effectively stabilized AuCl4and the resulting gold nanoparticles. On the basis of the average core diameters obtained by the TEM images and elemental analysis results, the average nanoparticle compositions were estimated to be Au954(SC3H5)215, Au1224(SC6H11)347, and Au202(SC11H21)77 for 2a, 2b, and 2c, respectively. The relationships between the core size and the number of alkenyl thiolates on a particle surface agreed well with the data previously reported by Murray et al.9 We then covalently immobilized these gold nanoparticles onto a silicon surface. A hydrogen-terminated silicon(111) surface was prepared by etching cleaned pieces of silicon in a 1% HF solution followed by treatment with 40% ammonium fluoride as described in our previous report.10 The dehydrated toluene dispersion of gold nanoparticles (2a-c; 30 mg/10 mL)11 was added to the hydrogen-terminated silicon(111) surface under a nitrogen atmosphere at 50 °C. After 24 h of immersion, the substrate surface was ultrasonically washed with toluene to remove the excess unbounded nanoparticles and then was dried (8) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (9) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (10) Shirahata, N.; Yonezawa, T.; Miura, Y.; Kobayashi, K.; Koumoto, K. Langmuir 2003, 19, 9107. (11) To complete the reactions promptly, the gold nanoparticles 2a-c are added in great excess (more than 1000 times the required quantity).
Figure 1. TEM images and histograms showing the size distributions of gold nanoparticles 2a (a), 2b (b), and 2c (c).
with a stream of nitrogen. We used high-resolution scanning electron microscopy (HR-SEM; Hitachi S-5200, 30 kV, resolution =0.5 nm) analysis to study the silicon surfaces modified by these procedures. Figure 2 shows silicon(111) surfaces treated with different alkenethiolstabilized gold nanoparticles. As the figure shows, the chain length of alkenethiol has a major effect on the immobilized surface structure. We first explain the silicon surface modified with C3 alkenethiol-stabilized gold nanoparticle 2a. In this case, the agglomerated gold nanoparticles are clearly discerned on the silicon surface (Figure 2a). This phenomenon can be explained by the fluctuation of gold atoms on the nanoparticle surface in several-angstrom scales, after which the atoms readily aggregate through metallic core fusion between particles. According to the literature, this aggregation allows them to reduce the surface energy of particles when small surface-stabilizing reagents are used.12 The authors also previously reported the immediate fusion of immobilized gold nanoparticles when DNA was reacted with gold nanoparticles that were protected by thiol having a short (ca. 0.5 nm) alkyl chain and by the quaternary ammonium bromide moiety, even at room temperature.13 In addition, we were able to make one particle layer of 2a on the silicon surface by heating for a long time. On the other hand, C6 and C11 alkenethiol-stabilized gold nanoparticles did not aggregate (the thiols used were 0.8- and 1.4-nm long, (12) (a) Buffat, P.; Borel, J.-P. Phys. Rev. A 1976, 13, 2287. (b) Lewis, L. J.; Jensen, P.; Barrat, J.-L. Phys. Rev. B 1997, 56, 2248. (c) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, M.; Schiffrin, D. J. Nature 1998, 396, 444. (13) Yonezawa, T.; Onoue, S.; Kimizuka, N. Chem. Lett. 2002, 1172.
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Figure 2. HR-SEM images of the silicon surface covalently linked to nanoparticles 2a (a), 2b (b), and 2c (c).
respectively). A number of protrusions with 4-5-nm length are observed as a result of independently immobilized gold nanoparticles. These surfaces retained their original topographic appearance with little or no apparent contamination on a scale of several nanometers (Figure 2b,c). These are in good agreement with the gold nanoparticle size observed by TEM. In addition, Figure 2b demonstrates the creation of an ordered particle arrangement in some parts. The average core-core spacing found in the ordered area is about 2 nm, close to twice the ligand length (0.8 nm). Subsequent TEM observations of the particle dispersion after reaction showed little improvement in the size distribution. These modified surfaces were stable in air and can be stored for several months without detectable decomposition. This strategy should be a promising technique to
covalently fix nanometer-sized materials onto a silicon surface. Our group is now undertaking a more detailed investigation. Acknowledgment. The authors thank Hitachi Science Systems and Hitachi High Technologies for experimental assistance with the HR-SEM observation. Discussions on experimental conditions with Professor K. Koumoto and Mr. Y. Matsushita (both of Nagoya University) are also acknowledged. Supporting Information Available: Synthetic procedures and characterization data for new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. LA036437C
