Perspective pubs.acs.org/JPCL
Emerging Carbon and Post-Carbon Nanomaterial Inks for Printed Electronics Ethan B. Secor† and Mark C. Hersam*,†,‡,§ †
Department of Materials Science and Engineering, ‡Department of Chemistry, and §Department of Medicine, Northwestern University, Evanston, Illinois 60208, United States ABSTRACT: Carbon and post-carbon nanomaterials present desirable electrical, optical, chemical, and mechanical attributes for printed electronics, offering low-cost, large-area functionality on flexible substrates. In this Perspective, recent developments in carbon nanomaterial inks are highlighted. Monodisperse semiconducting single-walled carbon nanotubes compatible with inkjet and aerosol jet printing are ideal channels for thin-film transistors, while inkjet, gravure, and screen-printable graphene-based inks are better-suited for electrodes and interconnects. Despite the high performance achieved in prototype devices, additional effort is required to address materials integration issues encountered in more complex systems. In this regard, post-carbon nanomaterial inks (e.g., electrically insulating boron nitride and optically active transition-metal dichalcogenides) present promising opportunities. Finally, emerging work to extend these nanomaterial inks to three-dimensional printing provides a path toward nonplanar devices. Overall, the superlative properties of these materials, coupled with versatile assembly by printing techniques, offer a powerful platform for next-generation printed electronics.
he emerging field of printed electronics channels multidisciplinary research into a technological framework with transformative possibilities. By integrating solutionprocessed electronic materials with high-throughput and additive manufacturing technologies, printed electronics offers a range of attributes and applications complementary to conventional circuits.1 In particular, the large-area, mechanically flexible, and low-cost nature of printed devices enables the practical and economical realization of a range of functionalities including displays, photovoltaics, smart packaging, and distributed sensing. Liquid-phase processing of printed and flexible electronics motivates the development of novel materials and inks, ranging from organic semiconductors and sol−gel metal oxides to metal and ceramic nanoparticles.2,3 Nanomaterials are particularly promising in this regard because suitable dispersion engineering can yield liquid-phase inks compatible with printing technologies while maintaining unique and desirable materials properties. In particular, carbon nanomaterials and emerging two-dimensional (2D) nanomaterials offer a complementary suite of properties that are wellsuited for this field.4,5 This Perspective will highlight recent progress and future opportunities in the development of these nanomaterials for printed electronics, focusing on successful strategies for the preparation and printing of inks based on semiconducting single-walled carbon nanotubes (s-SWCNTs) and pristine graphene, as well as emerging research in materials integration, post-carbon nanomaterial inks, and three-dimensional (3D) printing. Attributes of Carbon Nanomaterials. Carbon nanomaterials constitute a unique class of materials with a wide range of applications. The sp2 chemical bonding of carbon atoms in SWCNTs and graphene provides exceptional electronic,
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© 2015 American Chemical Society
Carbon nanomaterials combine a number of desirable features amenable to a wide range of printed electronics applications. thermal, and mechanical properties.6 In addition, the nanoscale physical structure and resulting electronic band structure of these materials lead to desirable optical properties and exotic physics.7 As a result, academic and industrial interest in these materials has expanded rapidly in recent years for a broad spectrum of applications including electronics, optoelectronics, sensors, energy storage, and composites.8,9 As illustrated in Figure 1, carbon nanomaterials combine a number of desirable features amenable to a wide range of printed electronics applications. With proper solvent, surfactant, or stabilizer chemistry, SWCNTs and graphene can be dispersed to produce inks with widely tunable rheology that is compatible with diverse printing methods. Their high charge mobility offers a promising alternative to conventional printed semiconductors with s-SWCNT devices already exceeding the mobility of stateof-the-art organic transistors.4 The robust stability of carbon nanomaterials to mechanical, chemical, and thermal stressors further enables their integration in flexible devices, improves device reliability in harsh environments, and offers broad process compatibility for integration with dissimilar materials. In addition, with appropriate functionalization and dispersion, Received: November 17, 2014 Accepted: January 28, 2015 Published: January 28, 2015 620
DOI: 10.1021/jz502431r J. Phys. Chem. Lett. 2015, 6, 620−626
The Journal of Physical Chemistry Letters
Perspective
Developments in s-SWCNT Inks. The development of purified s-SWCNT inks and their incorporation in printing processes closely follow strategies to selectively disperse or sort semiconducting and metallic SWCNTs.15,16 For example, density gradient ultracentrifugation has been used to isolate dispersions of s-SWCNT with >99% purity using aqueous surfactant mixtures.17,18 Frisbie and co-workers demonstrated the application of these aqueous s-SWCNT inks for aerosol-jetprinted ambipolar thin-film transistors (TFTs), inverters, and ring oscillators.19,20 In addition, s-SWCNT dispersions can be transferred to the organic solvent N-cyclohexyl-2-pyrrolidone for inkjet printing, a strategy developed by Dodabalapur and coworkers.21 The selective dispersion of s-SWCNTs using aromatic polymers also presents a promising strategy for the development of inks.22,23 Cui and co-workers have used this strategy to isolate large-diameter s-SWCNTs for inkjet-printed transistors and driving circuits.24,25 These three basic ink chemistries, namely, aqueous surfactant solutions, semiconducting polymer wrapping, and neat organic solvent dispersion, are shown schematically in Figure 2a−c. All three strategies have demonstrated promising transistor metrics including field-effect mobilities as high as 10−50 cm2/V·s and current on/off ratios of 105−107. Despite these initial successes, a more comprehensive understanding of the nuanced effects of defects and doping is needed to facilitate deterministic control of device polarity and threshold voltage. Atmospheric exposure has been widely exploited for adventitious p-type doping of s-SWCNTs, which is attributed to oxygen and water adsorbates.26 However, reliable and stable device operation further requires minimization of the trap state density. A broader understanding and control of transport polarity and threshold voltage, related to doping and trap state densities, will enable more versatile and reliable application of s-SWCNT electronics.27 Furthermore, improvements in the processing of monodisperse s-SWCNT populations will benefit electronic and optoelectronic applications.15 Uniformity in the electronic energy levels will reduce potential charge traps in s-SWCNT network devices, and
Figure 1. Applications of printable carbon nanomaterials. From center to edge, the structure, key attributes, and resulting application areas are delineated for s-SWCNTs and graphene.
these materials are biocompatible, facilitating a range of applications in biointegrated devices.10−13 The incorporation of s-SWCNTs and graphene in printed electronics therefore offers numerous opportunities, both as an alternative to existing materials and in enabling novel applications, as will be outlined below. It should be noted that metallic carbon nanotube inks are omitted from this discussion due to their advanced stage of technological maturity and the fact that detailed discussion of this topic can be found elsewhere.14
Figure 2. Semiconducting SWCNT inks for printed TFTs. Illustration of three different s-SWCNT ink chemistries including (a) dispersion with aqueous surfactants, (b) polymer wrapping, and (c) direct dispersion in select organic solvents. (d) Representative atomic force microscopy image of a dense s-SWCNT random network constituting the channel of the devices. Panel (b) adapted by permission from Macmillan Publishers Ltd.: Nature Communications, ref 23, Copyright 2011. Panel (d) reprinted with permission from ref 21, Copyright 2013, AIP Publishing LLC. 621
DOI: 10.1021/jz502431r J. Phys. Chem. Lett. 2015, 6, 620−626
The Journal of Physical Chemistry Letters
Perspective
Figure 3. Pristine graphene inks for printed electronics. (a,b) Illustration of two different chemistries for pristine graphene inks, specifically direct dispersion in select organic solvents and polymer-stabilized dispersion using EC for (a) and (b), respectively. (c) Target plot illustrating the tradeoffs between the ink chemistries for a range of desirable attributes, highlighting the room for improvement through alternative ink chemistries or further development of existing methods.
applications, such as chemical sensors and TFTs, the properties of pristine graphene are beneficial, particularly the ability to modulate the carrier concentration. In other applications, such as electrodes and interconnects, electrical conductivity is a primary metric. In addition, the ink chemistry has an effect on the printing characteristics, including the resolution and stability of printed lines on various substrates. A high graphene concentration is favorable as thicker lines are more conductive, and a high loading mitigates the need for time-consuming multiple-pass printing. Finally, the versatility of the ink chemistry, in terms of tuning the viscosity and drying kinetics, is a key parameter for adapting the inks to additional, and particularly higher-throughput, printing methods.
control of the chiral vector and handedness will facilitate broad tuning of optoelectronic properties. In addition to understanding and tuning the electronic structure, the ability to tailor the physical structure of sSWCNT devices is desirable. In particular, the random network structure of s-SWCNTs, shown in Figure 2d, has many junctions between nanotubes, leading to reduced mobility relative to single SWCNT devices. The electrical resistance associated with these junctions is approximately 100 kΩ, compared to the intrinsic resistance of SWCNTs of 1−10 kΩ/ μm, indicating that junction resistance will dominate transport for the short SWCNTs (
