Multicolored Carbon Nanotubes: Decorating Patterned Carbon Nanotube Microstructures with Quantum Dots Xiaodai Lim, † Yanwu Zhu, † Fook Chiong Cheong, † Nurmawati Muhammad Hanafiah, § Suresh Valiyaveettil, §,‡ and Chorng-Haur Sow †,‡, * † Department of Physics, Blk S12, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore, ‡ Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore, and § National University of Singapore Nanoscience and Nanotechnology Initiative, 2 Science Drive 3, Singapore 117542, Singapore C arbon nanotubes (CNTs) are fasci- nating nanomaterials with unique mechanical, electrical, and optical properties. 1–6 Suitable for a wide variety of applications, their versatility is truly remark- able. Since its discovery by Iijima, 1 CNTs have continued to captivate researchers. In recent years, great progress has been made in the synthesis and application studies of hybrid nanomaterial systems. One example of a hybrid system includes a controlled as- sembly of various nanomaterials onto CNTs template. There have been continuous ef- forts on novel studies involving alteration of physical properties of CNTs via the use of organic, 7 inorganic, 8 and biological species 9,10 to produce functionalized CNTs for further applications. 11 For example, Zhu et al. 12 fabricated CNTs beaded with ZnO nanoparticles by the simple heating of Zn- coated CNT. The hybrid nanomaterial exhib- its tunable nonlinear optical absorption. Among these modifications, interactions between CNTs and semiconducting nano- particles such as quantum dots (QDs) ap- pear to be a well-matched and exciting combination in changing both optical 7 and electronic properties 13 of CNTs. QDs possess size tunable optical and electronic properties resulting from quan- tum confinement. 14 Such properties allow them to be suitable candidates for applica- tions in solar cells 15 and light emitting de- vices. 16 Having a high resistance to photo- bleaching, QDs are attractive materials for optoelectronics 17 and in vivo biosensing. 18,19 The technique of assem- bling QDs onto CNTs has drawn great atten- tion. Ravindran et al. 20 were able to as- semble CdSe/ZnS nanoparticles onto both ends of CNTs via a 1-ethyl-3(3-dimethyl- aminopropyl) carbodiimide HCl (EDC) reac- tion. The process coupled the carboxyl group in the CNTs to the amine group on the QDs produced through reacting metha- nol cleaned QDs suspended in toluene with 2-aminoethane thiol (AET) to form an amide linkage. These QDs-CNT-QDs hetero- structures show great potential in electronic device applications. Robel et al. 15 made use of tetraoctylammonium bromide (TOAB) as a linker to secure CdS nanocrys- tallites on CNTs which exhibited the ability to respond to visible light and could be uti- lized in light-harvesting and optoelectron- ics applications. Having a possible potential of using such QDs/CNTs junctions on nano- chips for future devices, it is thus highly de- sirable to achieve a simple and cost- effective technique to assemble QDs onto CNT microstructures on a large-scale basis. An extension of these studies involves further developing of techniques to achieve assembly of these nanoparticles onto pat- *Address correspondence to physowch@nus.edu.sg. Received for review February 17, 2008 and accepted May 27, 2008. Published online June 11, 2008. 10.1021/nn800101f CCC: $40.75 © 2008 American Chemical Society ABSTRACT In this work, techniques to create patterned array of multiwalled nanotube (MWNT) microstructures decorated with quantum dots (QDs) were presented. Using aligned array of intertwined MWNTs as the supporting template, a droplet of solution comprising QDs was deposited onto the MWNTs. When the solution evaporated away, QDs were left behind on the MWNT template. Coupled with the technique of laser pruning, a wide variety of QDs-decorated MWNT microstructures were created. In addition, the aligned array of MWNTs was found to be an effective nanosieve that could effectively sort out QDs with a size difference of 0.5 nm. In this case, a droplet of solution comprising QDs of different sizes was placed on aligned array of MWNTs. As the solution spread across as well as trickled down the MWNTs, the smaller QDs were found to venture further and deeper into the MWNTs. Again coupled with laser pruning, fluorescence microscopy revealed multicolored MWNT microstructures due to preferential decoration of these QDs with difference sizes. As a result, multicolored/ multicomponents hybrid functional materials were achieved. KEYWORDS: carbon nanotubes · quantum dots · self-assembly · micropattern · nanosieve ARTICLE www.acsnano.org VOL. 2 ▪ NO. 7 ▪ 1389–1395 ▪ 2008 1389