In situ deposition and patterning of single-walled carbon nanotubes by laminar flow and controlled flocculation in microfluidic channels.

The remarkable electrical, mechanical, and chemical properties of single-walled carbon nanotubes (SWNTs) create interest in their potential use as semiconducting or conducting elements in sensors, nanoscale transistors, and in large-area electronic systems. As a result, considerable research focuses on developing techniques for depositing and patterning SWNTs from solution, as individuals or aggregates, with well-controlled coverage and alignment. Langmuir–Blodgett techniques and various schemes of deposition from solution have been studied extensively. Although these methods exhibit attractive features, they each have some combination of disadvantages, such as low deposition rate, inability to control tube alignment and/or to deposit over large areas, required chemical modification of the SWNTs or substrates, or the need for organic solvents that are incompatible with plastic device components (organic electrodes, semiconductors, or substrates). A recently described alternative technique uses a controlled flocculation process to drive individual tubes out of an aqueous suspension and onto a substrate. The deposition speed of this approach can be high, the tube coverage (i.e. the number of tubes per unit area) can be controlled over a wide range, and the form of the tubes (isolated individual tubes or bundles) can be defined by the processing conditions. The approach is also compatible with a wide range of substrates and it does not require chemical modification of the tubes or the substrates. However, this method does not allow the local orientation of the deposited nanotubes to be defined in a spatially dependent manner. Depositing and patterning SWNTs in a single step are also difficult, and secondary procedures must be used to transfer the nanotubes onto substrates incompatible with spin coating (e.g., curved surfaces). Herein we introduce a technique for patterning and depositing SWNTs through the use of controlled flocculation in laminar microfluidic networks. This method generates welldefined patterns of aligned SWNT films with controlled density and alignment on a variety of flat and curved surfaces. We present the essential chemical and physical aspects of this approach, demonstrate the important capabilities, and present examples for use in the fabrication of organic electronic devices. In the controlled flocculation approach, an aqueous suspension of surfactant-stabilized SWNTs is combined with a solvent, such as methanol, that is miscible with water and exhibits a strong affinity for the surfactant. When the methanol lowers the concentration of surfactant available to interact with the SWNTs to values below the critical micellar concentration, the tubes are no longer well suspended. In this situation, interactions between tubes can lead to the formation of bundles, and interactions with adjacent solid surfaces can cause the tubes to coat these surfaces. The introduction of streams of methanol and aqueous suspensions of SWNTs into a microfluidic channel in which fluid flow is laminar leads to diffusive mixing only near the liquid–liquid interface between the two streams flowing in parallel. The interaction between the methanol and the SWNT suspension at this interface leads to the deposition of SWNTs on the solid surfaces near this interface. The velocity profiles of the local flow fields determine the orientation of the tubes. The duration of the flows and their velocities as well as the concentration of the SWNT suspension determine the coverage. Figure 1a shows this process as implemented with microfluidic streams joined by a Y-junction. For the work described herein, we form the fluidic channels by placing a stamp of the elastomer poly(dimethylsiloxane) (PDMS) against the surface of a substrate to be patterned (Figure 1b). Syringe pumps introduce the SWNT solution and methanol [*] J.-U. Park, M. A. Meitl, S.-H. Hur, Prof. Dr. J. A. Rogers Department of Materials Science and Engineering Department of Electrical and Computer Engineering Department of Chemistry Beckman Institute, and Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana, IL (USA) Fax: (+1)271-244-1190 E-mail: jrogers@uiuc.edu

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