Elsevier

Applied Surface Science

Volume 254, Issue 2, 15 November 2007, Pages 621-626
Applied Surface Science

Positioning of cationic silver nanoparticle by using AFM lithography and electrostatic interaction

https://doi.org/10.1016/j.apsusc.2007.06.046Get rights and content

Abstract

One-dimensional metal lines of silver nanoparticles with a nano-sized width were generated onto silicon surface by using a nano-level lithography technique, field induced oxidation (FIO) by AFM, on self-assembled monolayer-modified Si wafers. This FIO technique provided SiO2 lines a width of less than 100 nm. Short-time immersion of partially anodized silicon surface which is covered by a cationic silanol surfactant ((CH3O)3SiCH2CH2CH2N(CH3)3+Cl)-monolayer into quaternary ammonium (HSCH2CH2N(CH3)3+Br)-covered silver nanoparticles readily and reproducibly gave nano-metal lines of silver onto silicon wafers. Hydrophilicity of the whole wafer surface was indispensable for homogeneously wetting the anodized SiO2 area with a nanodimensional width.

Introduction

Nanoparticles have been more and more intensively investigated as one of key materials of recent nanotechnology, catalysis, and biochemistry [1], [2], [3]. Especially, well-controllable nanostructures with low dimension on a substrate play a central role in the development of actual nanotechnology [4]. Self-organization or self-assembly techniques of nano-sized materials is intensively investigated for constructing such nanostructures. However, most of the self-organized structures of metal nanoparticles without templates are in two- or three-dimension. Metal one-dimensional arrays, which can be called as “nanowires”, have been reported by using various templates [5], special stabilizing reagents [6], or lithography [7]. Both soft templates, such as DNA, polymers and other organic molecules, and hard templates, such as micropore inorganic materials or crystals, are used. Such template systems for preparation of metal nanowires can be largely divided into two categories. One is the arrangement of metal nanoparticles in one dimension by using some temperates. The other one is direct preparation of metal nano-wires by reducing metal ions on templates. We and others used DNA molecules as soft templates for arrangement of cationic metal nanoparticles. Some solid templates, such as mesopore type metal oxides [8], can also be used for 1D arrangement of metal nanoparticles. Direct preparation method with DNA molecules as soft templates has been also proposed for Ag or Pd nanowires. Adsorption of metal cations (for example, Ag+ or Pd2+) on DNA molecules followed by reduction gave one-dimensional nanowires of the corresponding metals [9], [10]. In Table 1, various preparative processes of one-dimensional metal nanoparticle arrays and metal nanowires with nanodimensional widths by using organic or inorganic templates are listed [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24].

Nano-lithography on flat substrates has been also useful to construct one-dimensional metal nanoparticle assemblies. Scanning probe lithography or drawing with self-assembled monolayers has a high ability to organize materials on the nanometer scale [25]. Dip-pen technology is useful to draw one-dimensional nanoparticle arrays. On the other hand, anodic nano-oxidation by using an AFM probe is useful to provide SiO2 structures on Si substrates in nanometer order. With this technique, one can also remove SAM molecules selectively and generate SiO2 selectively, which was proposed by Sugimura and Nakagiri [26] Recently, Liu and coworkers followed Sugimura's approach and proposed the preparation of nanopatterned assembling of gold nanoparticles onto an amine-terminated SAM surface which was selectively oxidized by AFM [27]. However, such coordinative ligand adsorption of metal nanoparticles could not have densely packed nanoparticle structures and this process should have taken very long immersion time (12 h). Furthermore, smaller patterns showed lower efficiency for adsorption of nanoparticles [28]. Therefore, alternative approaches should be proposed.

On the other hand, densely-packed patterning of submicron-sized spherical particles by using patterning of SAM on silicon substrates was successfully carried out. Particle wires with a micron-width could be obtained on a silanol/alkyl patterned surface generated by UV-lithography [29]. But wetting behavior can be different on submicron (or smaller)-sized patterns. Therefore, more precisely designed molecular temperates should be proposed.

This paper reports the fabrication of 1D silver nanoparticle arrays by simple immersion of AFM-modified Si surface covered by cationic SAM into an aqueous dispersion of cationic Ag nanoparticles. AFM-modification of Si, that is, field induced oxidation (FIO) by AFM is a system of anodic oxidation of silicon by using conductive AFM cantilevers as negative electrodes. As for the monolayer on silicon substrate, we have used alkyl (hydrophobic)-SAM and quaternary ammonium (hydrophilic/cationic)-SAM. For nanosized patterns, hydrophilic monolayer may be useful to obtain better particulate structures. As SiO2 nanolines are negatively charged at ambient conditions [30], they can densely adsorb Ag nanoparticles with highly positively charged surface.

Section snippets

Experimental

Water was purified by a Mili-Q system (>18 MΩ). Thiocholine bromide (TCBr, HS(CH2)2N(CH3)3+Br), which was used as the stabilizer reagent in this study, was obtained by hydrolysis of acetylthiocholine bromide (Aldrich or ACROS) [5]. Its purity was checked by 1H NMR, FT-IR and elemental analysis. Other chemicals were used as received. Silver nanoparticles used here were produced by chemical reduction from AgCl according to the preparative method reported previously [31]. The nanoparticles were

Results and discussion

Cationic silver nanoparticles used here were prepared from insoluble metal source, AgCl by using NaBH4 as the reducing reagent [31]. Under vigorous stirring, after injection of aq. NaBH4 by using a syringe pump for dropwise injection, the colorless liquid containing AgCl powders gradually changed to a brown and homogeneous dispersion, which indicates the formation of the silver nanoparticles. No aggregate or precipitate was observed for several days. In order to remove excess stabilizing

Conclusion

One-dimensional nanoparticle arrays with the width of less than 100 nm could be generated by simple immersion of silicon substrate covered by quaternary ammonium groups. Wettability of the nanoparticle dispersions on the substrate surface is highly important for immobilization of nanoparticles. FIO-modified SiO2 area are is hydrophilic and anionic but when this area is very small and surrounded by hydrophobic surface, that is, the OTS-covered surface, the aqueous drops containing the silver

Acknowledgements

This work is partly supported by a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS). NS acknowledges the post-doctoral fellowship from JSPS. TY thanks the warm hospitality of Prof. H. Nishihara (Univ. Tokyo).

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