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Current Applied Physics 6S1 (2006) e48–e53

Selective attachment of functionalized nanospheres to carbonnanotubes-distributed template in a large area

S.K. Kim a, J. Zhang b, M. Lee a, H.Y. Choi a,*, H. Lee a

a Department of Chemistry, Hanyang University, Seoul 133-791, Republic of Koreab Department of Applied Physics, Harbin Institute of Technology, China

Received 21 July 2005Available online 27 March 2006

Abstract

An effective 3-D nanostructure for nano-sized devices was designed using single-walled carbon nanotubes (SWNTs) as a main frameof the structure. The –NH2 modified polystyrene (PS) nanosphere with an average diameter of 100 nm could form SWNT-PS nanospherecomposite and selective attachment to SWNT distributed on gold template by covalent combination, which were accomplished by usingtwo different processes. These results may enable carbon nanotubes to be applied to the construction of nanostructures.� 2006 Published by Elsevier B.V.

Keywords: Single-walled carbon nanotube; Nanostructure; Selective attachment

1. Introduction

A carbon nanotube has been widely investigated as anessential component for fabrication of nanoelectronicdevices and its numerous applications [1,2]. Especially,well-arrayed carbon nanotubes are highly desired to pre-pare chemical sensors [3], nanoprobes for scanning probemicroscopy [4], field emitter devices [5], etc. Besides theircapabilities as functional components, carbon nanotubesare good building blocks for organizing 3-D nanostructurewhich is another important factor in molecular electronics[6]. This study reports the selective attachment of nano-spheres to SWNTs on modified template using a covalentbinding in order to fabricate a basic structure for furtherapplication using carbon nanotubes. If the high selectivityof a specific chemical reaction can be controlled, we candesign an effective 3-D nanostructure for nano-sized devicesusing SWNTs as a main frame of the structure. To date,selective alignments of carbon nanotubes (CNTs) have beenbroadly investigated by growing CNTs through the place-

1567-1739/$ - see front matter � 2006 Published by Elsevier B.V.

doi:10.1016/j.cap.2006.01.010

* Corresponding author. Fax: +82 02 2296 0287.E-mail address: haiwon@hanyang.ac.kr (H.Y. Choi).

ment of catalyst at particular position [7]. Nevertheless,the integration of CNTs still suffers from the contaminationby impurities and the lack of uniformity in the structure ofCNTs when growing CNTs directly on templates. The puri-fication and functionalization of as-grown carbon nano-tubes enables CNTs to be processed easily in solution sothat the formation of nanostructures through the alignmentof CNTs would be possible in a large area.

In another hand, extraordinary mechanical properties ofCNTs have attracted passionate researches in CNT com-posites [8,9]. However, the strength enhancement of thecomposites does not have been accomplished comparedto other hybrid materials because of poor connectivity inthe composite and phase separation. This paper also pre-sents polymer nanosphere-CNT composite well networkedwith strong covalent bonding.

Selective attachment of polystyrene nanospheres to sin-gle-walled carbon nanotubes (SWNTs) was conducted byusing two different processes in order to accomplish thefabrication of CNT nanostructures. The size and func-tional groups of SWNTs were controlled using the shorten-ing and functionalizing process of SWNTs. The formationof amide bond through condensation reaction between

Scheme 1. Fabrication processes of selectively attached PS nanospheres to SWNTs-dispersed gold template. Two different approaches were conducted tofabricate 3-D nanostructures on template: (a) NH2 modified nanospheres dispersed in water were immersed in SWNTs-COCl aqueous solution and thencysteamine-covered gold substrate was additionally immersed in the solution and (b) the gold substrate was immersed in SWNTs-COCl/DMF suspension,and then the SWNTs-attached substrate after rinsing was immersed again in the PS aqueous solution.

Fig. 1. TEM images of (a) raw SWNTs and (b) acid chloride-functionalized SWNTs and (c) FT-IR spectra of raw SWNTs (a in (c)) and SWNTs-COCl(b in (c)) with baseline correction. The sharp peak at A: 1519 cm�1 corresponds to the stretch mode of the aromatic carbon–carbon bond in raw SWNTs.Bands B: 1700, C: 1210 and D: 790 cm�1 correspond to the formation of the group –COCl on the ends and side-wall of SWNTs.

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amine group and acid chloride group enabled PS nano-spheres to be attached strongly to the SWNTs.

2. Fabrication of SWNT-PS nanostructure

The purified single-walled carbon nanotubes werepurchased from Carbon Nanotechnologies Inc. (HiPCOSWNTs). The –NH2 modified polystyrene (PS) nano-spheres (�100 nm in diameter) dispersed in aqueous solu-tion (purchased from Bangs Laboratories, Inc.) were usedas received. The carboxylic acid groups-terminated SWNTs(SWNTs-COOH) were prepared by shortening and etchingprocesses using ultrasonification in mixed acids accordingto the reported procedure [10]. To consider further chemical

Fig. 2. SEM images of (a) composite matrix of SWNTs covalently combined wdispersed on template using the first process.

reaction with chemically modified surface, the carboxylicacid groups on SWNTs were converted into acid chloridegroups by treatment with thionylchloride. The acid chlo-ride-functionalized SWNTs (SWNTs-COCl) were dispersedin water and dimethyl formamide (DMF) by ultrasonic agi-tation and were immediately used in reaction with stirring.The formation of functional groups on SWNTs was con-firmed by FT-IR. The NH2 group-terminated gold sub-strate was prepared using self-assembly method with5 mM cysteamine solution in ethanol [11]. The NH2-termi-nated PS nanospheres were reacted with the SWNTs-COClunder stirring at room temperature for 24 h in all experi-ments. After completing the attachment, the reacted sub-strates were rinsed with deionized water under sonication

ith PS nanospheres and (b) selectively attached PS nanospheres to SWNTs

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and were dried under a stream of nitrogen. FT-IR (ABBBomem 100 Spectrometer, ABB Bomem Inc.), equippedwith a globar source and DTGS detector, was used to con-firm the functional groups of SWNTs. Transmission elec-tron microscope (TEM) and scanning electronMicroscope (SEM) were exploited to characterize the puri-fied SWNTs and the surface morphologies modified withSWNTs and PS nanospheres.

3. Results and discussion

Scheme 1 shows the fabrication processes of selectivelyattached PS nanospheres to SWNTs dispersed on gold tem-plate. Two different kinds of approaches were conducted tofabricate 3-D nanostructure on template; First, NH2 mod-ified nanospheres dispersed in water were immersed in

Fig. 3. SEM images of (a) covalently adsorbed SWNTs-COCl to NH2-termiSWNTs in a large area.

SWNTs-COCl aqueous solution and then cysteamine-cov-ered gold substrate was additionally immersed in the solu-tion (Scheme 1(a)). Second, the gold substrate wasimmersed in SWNTs-COCl/DMF suspension, and theSWNTs-attached substrate after rinsing was immersed inthe PS aqueous solution (Scheme 1(b)).

Fig. 1 shows TEM images of (a) raw SWNTs and (b)acid chloride-functionalized SWNTs and (c) their FT-IRspectra with baseline correction measured with KBr pellet.The shortened SWNTs had approximately the length from500 nm to 1 lm and the diameter from 30 nm to 50 nm.The sharp peak at A: 1519 cm�1 in the IR spectrum (a inFig. 1(c)) is from the stretch mode of the aromatic car-bon–carbon bond in raw SWNTs. After the reaction ofthe shortened SWNTs with SOCl2, the IR spectrum (b inFig. 1(c)) showed well defined bands at about B: 1700, C:

nated gold substrate and (b) selectively attached PS nanospheres on the

Fig. 4. SEM image of the nanostructures of PS nanospheres selectively combined with a bunch of SWNTs.

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1210, D: 790 cm�1 which are clearly related to the forma-tion of the group –COCl on the ends and side-wall ofSWNTs.

Fig. 2(a) shows the composite of polystyrene (PS) nano-spheres and single-walled carbon nanotubes prepared bycondensation reaction. When PS nanospheres with aminegroup termination were immersed in SWNTs-COCl aque-ous solution as shown in Scheme 1(a), PS-SWNTs aggre-gates were precipitated rapidly in the reactor just afterimmersing. Many defects on the side wall of SWNTs, pro-duced by cutting process using strong acids, as well as theboth ends of them act as active sites to chemically combinewith NH2 group-modified PS nanospheres. Thus, many PSnanospheres were adsorbed to SWNTs as shown inFig. 2(a). From the formation of chemical bonding, a num-ber of SWNTs were strongly combined with polystyrenenanospheres, resulted in a well-networked matrix. How-ever, the bulky composite was difficult to introduce to thesurface of template modified with a cysteamine self-assem-bled monolayer (SAM) even under severe stirring as shownin Fig. 2(b) because almost no reactive site on SWNTsremains after condensation reaction with NH2 group-mod-ified nanospheres.

As another approach to effectively fabricate 3-D nano-structure on a template, the chemical binding of SWNTs-COCl with NH2 group on gold plate was conducted at first.Then, the nanospheres-NH2 were reacted with SWNTs-COCl attached to the gold substrate. Fig. 3 shows (a) cova-lently adsorbed SWNTs-COCl to –NH2 modified goldsubstrate and (b) selectively attached PS nanospheres onthe SWNTs in a large area. The SWNTs were well distrib-uted with chemical bonding on gold substrate as shown in

Fig. 3(a). After a reaction of the nanospheres-NH2,selective attachment of the PS nanospheres only to thechemically attached SWNTs on gold substrate was accom-plished as shown in Fig. 3(b).

Fig. 4 shows wonderful 3-D nanostructures of PS nano-spheres selectively combined with a bunch of SWNTs. Theactive groups existing on the defects of the side-wall andthe ends of SWNTs enabled PS nanospheres to be attachedon SWNTs. By now, many of research groups havefocused on improving physical, chemical and electricalproperties of SWNTs through chemical attachments ofuseful substances on the side defects of SWNTs. The align-ment of carbon nanotubes also has been critical issue toapply them to the area of molecular electronics, field emis-sion display, etc. As shown in Fig. 4, the PS nanospheresattached on the SWNTs would not seem to change muchthe properties of SWNTs and also to improve the align-ment greatly. However, it is very important to selectivelyattach nanospheres onto SWNTs because these basic struc-tures can provide the potential possibility in application ofSWNTs.

4. Summary

In conclusion, we have demonstrated the fabrication of3-D nanostructure using SWNTs as a main frame of thestructure. The polystyrene nanospheres were selectivelyattached to SWNTs distributed on –NH2 modified goldtemplates. The strong networking of the SWNT and PScomposite could be formed by covalent bonding in thesolution, which can be used to prepare strong and light-weight nature of SWNT composites.

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Acknowledgements

This work was supported by the research fund of Hany-ang University (HY-2004-I) and Center for Nanostruc-tured Materials Technology under 21st Century FrontierR&D Programs.

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