J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 2, 2 0 0 3, 1205 – 1207
Ordered Ag nano-particle arrays derived from Ag nano-stripes
A. GOTOH, K. UCHIDA, A. KAWAI, K. KAMIYA, F. IKAZAKI, S. SANO, A. TSUZUKINational Institute of Advanced Industrial Science and Technology, Shimoshidami, Moriyama-ku,Nagoya, 463-8560 JapanE-mail: [email protected]
The preparation of the Ag nano-stripes made from a Agaerosol of less than 5 nm diameter in the presence of asurfactant (dodecanethiol) at ca. 150 ◦C was previouslydescribed [1]. The slight heating (from 150 to 175 ◦C)of the prepared Ag nano-stripes produced an orderedAg nano-particle array, which consisted of uniform 3–4 nm Ag nano-particles with the two-dimensional ar-ray of 8 × 6 particles per 50 nm square. This methodof preparing ordered uniform nano-particle arrays istotally different from the conventional ones, most ofwhich have applied the technique of self-assembly ofparticles or lithography.
Nano-structures such as two- or three-dimensionalordered arrays consisting of uniform nano-particleshave a potential for the exhibition of quantum effectsleading to the possibility of light-emitting diodes andquantum dot lasers [2, 3], because these devices requirea very large number of very faint quantum effect sig-nals emitted from ordered uniform nano-particles. Inaddition, such nano-structures can also be used for thefuture development of high-density storage targets ofthe order of 1 terabits per square inch, which wouldrequire fabrications on a nanometer scale [4, 5].
A variety of different methods such as the self-assembly of particles or photo-lithography have beenused in attempts to prepare such nano-structures; how-ever, the self-assembly has not produced well-definedenough particle arrangements to exhibit quantum ef-fects in terms of the orientation and regularity of nano-particles [6]. This is probably an intrinsic tendency be-cause the method of self-assembly is a mere assembly ofparticles based on an interfacial phenomenon betweennano-particles and solvent liquid during the evapora-tion of solvent [7–10]. This property of self-assemblymight give limitations to the applications.
On the other hand, the method of photo-lithographygives well-defined particle patterns [11]; however, thedegree of precision obtained using this technique is nowof the order of sub-micron (0.1–1 µm), not nano-meteras desired [12].
A novel method to overcome these drawbacks is de-scribed here, which is totally different from the con-ventional ones such as self-assembly. The procedure isillustrated in Fig. 1a, which is based on the breakdownof Ag nano-stripes into equal-sized pieces probably dueto the thermal force of the dodecanethiol molecule. Thepattern of the Ag nano-stripes made from a very smallaerosol of less than ca. 5 nm probably consists of alter-nate nano-sized stripes of Ag and dodecanethiol surfac-tant as shown in Fig. 1b, details of which are described
elsewhere [1]. The thiol functional group (-SH) in thedodecanethiol stripes is presumed to link with the sil-ver in the Ag stripes. In addition, dodecanethiol is anoily liquid having a boiling point of ca. 200 ◦C, indi-cating that dodecanethiol is subject to thermal effectswith increasing temperatures close to the boiling point.Consequently, in this case, a slight increase from 150 ◦Cto 175 ◦C for the nano-Ag stripes caused a significantchange in the breakdown of Ag nano-stripes into ap-proximately homogeneous sized particles. Based onthis consideration, this process could be called a kindof self-organization.
Fig. 2 shows a TEM micrograph of the preparedordered Ag nano-particles array, which exhibits someunique properties that are different from those obtainedby conventional methods. First, a closer examination ofFig. 2 shows that the prepared nano-particles are 3–4 nmin size, uniform, and not round but wedge-shaped, sug-gesting that the Ag nano-stripes could have been bro-ken into particles by certain external forces resultingfrom the thermal effect of the dodecanethiol moleculeslinked to the Ag stripes. Second, they form a certaindirectional array, that is to say, an isolated linear (one-dimensional) array of nano-particles as shown in Fig. 2.Third, they show an accurate two-dimensional particlearray of 8 × 6 particles per 50 nm square, which is notisotropic probably due to anisotropy of the Ag nano-stripes. In contrast, the major conventional method ofself-assembly does not produce any specific directionalarray. The electron diffraction analysis of the preparednano-particles array shows a halo, Fig. 2, indicatingthat the nano-particles are amorphous. The elementalanalysis (EDX) was described in the previous paper [1].In addition, this result turns our attention to the tran-sition process from the Ag nano-stripes to the orderedAg nano-particles array.
Fig. 3 shows a TEM micrograph of the transition pro-cess from the nano-stripes to the ordered nano-particlearray, in the top-left part of which the early stage ofthe production process can be seen. This micrographdemonstrates that the present method is totally differ-ent from conventional ones.
In conclusion, a novel method to fabricate an or-dered Ag nano-particle array is described, which isdifferent from the conventional ones such as self-assembly. This method features a slight additional heat-ing from 150 ◦C to 175 ◦C of Ag nano-stripes derivedfrom a nano-sized Ag aerosol in the presence of dode-canethiol. The prepared ordered particle array consistsof uniform Ag nano-particles with a 3–4 nm size and
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(a)
(b)
Figure 1 (a) Experimental procedure and (b) a typical TEM image of Ag nano-stripes prepared in the previous paper [1].
Figure 2 Ordered Ag nano-paricle arrays derived from nano-stripes. Inset: electron diffraction pattern.
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Figure 3 Transition process from nano-stripes to ordered nano-particle arrays.
wedge-like angular shape, and forms an accurate two-dimensional anisotropic structure of 8 × 6 particles per50 nm square.
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Received 25 November 2002and accepted 6 May 2003
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