High-Purity Semiconducting Single-Walled Carbon Nanotubes: A Key Enabling Material in Emerging Electronics (2017)

Semiconducting single-walled carbon nanotubes (sc-SWCNTs) are emerging as a promising material for high-performance, high-density devices as well as low-cost, large-area macroelectronics produced via additive manufacturing methods such as roll-to-roll printing. Proof-of-concept demonstrations have indicated the potential of sc-SWCNTs for digital electronics, radiofrequency circuits, radiation hard memory, improved sensors, and flexible, stretchable, conformable electronics. Advances toward commercial applications bring numerous opportunities in SWCNT materials development and characterization as well as fabrication processes and printing technologies. Commercialization in electronics will require large quantities of sc-SWCNTs, and the challenge for materials science is the development of scalable synthesis, purification, and enrichment methods. While a few synthesis routes have shown promising results in making near-monochiral SWCNTs, gram quantities are available only for small-diameter sc-SWCNTs, which underperform in transistors. Most synthesis routes yield mixtures of SWCNTs, typically 30% metallic and 70% semiconducting, necessitating the extraction of sc-SWCNTs from their metallic counterparts in high purity using scalable postsynthetic methods. Numerous routes to obtain high-purity sc-SWCNTs from raw soot have been developed, including density-gradient ultracentrifugation, chromatography, aqueous two-phase extraction, and selective DNA or polymer wrapping. By these methods (termed sorting or enrichment), >99% sc-SWCNT content can be achieved. Currently, all of these approaches have drawbacks and limitations with respect to electronics applications, such as excessive dilution, expensive consumables, and high ionic impurity content. Excess amount of dispersant is a common challenge that hinders direct inclusion of sc-SWCNTs into electronic devices. At present, conjugated polymer extraction may represent the most practical route to sc-SWCNTs. By the use of polymers with a π-conjugated backbone, sc-SWCNTs with >99.9% purity can be dispersed in organic solvents via a simple sonication and centrifugation process. With 1000 times less excipient and the flexibility to accommodate a broad range of solvents via diverse polymer constructs, inks are readily deployable in solution-based fabrication processes such as aerosol spray, inkjet, and gravure. Further gains in sc-SWCNT purity, among other attributes, are possible with a better understanding of the structure-property relationships that govern conjugated polymer extraction. This Account covers three interlinked topics in SWCNT electronics: metrology, enrichment, and SWCNT transistors fabricated via solution processes. First, we describe how spectroscopic techniques such as optical absorption, fluorescence, and Raman spectroscopy are applied for sc-SWCNT purity assessment. Stringent requirements for sc-SWCNTs in electronics are pushing the techniques to new levels while serving as an important driver toward the development of quantitative metrology. Next, we highlight recent progress in understanding the sc-SWCNT enrichment process using conjugated polymers, with special consideration given to the effect of doping on the mechanism. Finally, developments in sc-SWCNT-based electronics are described, with emphasis on the performance of transistors utilizing random networks of sc-SWCNTs as the semiconducting channel material. Challenges and advances associated with using polymer-based dielectrics in the unique context of sc-SWCNT transistors are presented. Such transistor packages have enabled the realization of fully printed transistors as well as transparent and even stretchable transistors as a result of the unique and excellent electrical and mechanical properties of sc-SWCNTs.