Carbon-based Inks and Printing Processes for Environmentally Friendly Sensors and Transistors

Abstract

The semiconductor industry currently fabricates electronic devices using materials that are difficult to recycle and energy-intensive processes with significant waste products, resulting in considerable environmental impact. Yet, as the Internet-of-Things (IoT) continues to add sensors and circuitry to everyday objects, there is no end in sight to the proliferation of electronics throughout society, particularly devices that enable new flexible or wearable applications. Additive manufacturing, notably printing, could be a powerful tool for fabricating robust flexible electronics for the IoT through the use of energy-efficient processes and recyclable or biodegradable materials. The work outlined in this dissertation presents contributions to the advancement of printed materials that have environmentally sustainable attributes with the eventual goal of developing manufacturing-ready print processes for electronics that have no environmental impact.A versatile device for enabling many applications of sustainable electronics, from sensors to circuits, is the thin-film transistor (TFT). To realize a TFT requires the deposition of semiconducting, conducting, and insulating materials into a multilayered device structure. Carbon nanotubes (CNTs) are ideal semiconducting candidates for TFTs due to their high thin-film mobility, compatibility with a wide range of printing approaches, and low environmental impact. Further, CNT-TFTs may be used as a platform to benchmark the electrical properties of insulating and conducting printed materials. Among the layers comprising a CNT-TFT, a gate insulator (i.e., dielectric) has proven to be the most difficult to print due to challenges in film uniformity and post-processing requirements. This challenge was overcome by using nanocellulose as a printable ionic dielectric, showing better device performance than common ionic dielectrics used in the field. Further, the influence of form factor and surface groups on the ionic dielectric performance of nanocellulose were explored, revealing a potential dependence of SS on surface group. The impact of using aqueous high work function metals was also studied for their impact on CNT-TFT performance, demonstrating Au nanoparticle (NP) and PtNP contacts to the CNT channel. Impressively, the aqueous AuNP contacts had a lower contact resistance than previous solvent-based Au inks.While most of this dissertation work was completed using aerosol jet printing (AJP), significant material waste and limits on resolution (> 10 µm) reduced the environmental benefits of printing sustainable materials with AJP. Therefore, a direct-write printing process called capillary-flow printing (CFP) was used to realize as-printed submicron CNT-TFTs without the use of chemical modifications or physical manipulation. While reducing ink waste and energy consumption, CFP enabled demonstration of submicron fully printed devices in three easy print steps, yielding devices with performance that is competitive with that of state-of-the-art TFT technology for display backplanes, which rely on materials and processes with sizeable environmental footprints. Although the majority of the work discussed in this dissertation focuses on discoveries and advancements in printed CNT-TFTs, the final work presented highlights the versatility of an aqueous graphene ink aerosol jet printed into 3D microstructures with broad applicability beyond transistors. By developing this 3D print process on an aerosol jet printer, which typically prints in 2D, conductive 3D graphene microstructures were achieved without any post-processing. After extensive characterization, it was found that pillars can be printed below a 45° angle with respect to the substrate without altering the angle of the nozzle or substrate, which is comparable to extrusion-based 3D printers. Additionally, 3D microstructures were functionalized onto a graphene humidity sensor, showing that the use of 3D graphene trusses yields a nearly twofold improvement in device sensitivity. The work described herein marks a significant leap in reducing the environmental impact of printed electronics through use of aqueous inks for transistor and sensing applications as well as pushing the resolution limits of printing through the use of a capillary flow printer to create submicron transistors while minimizing material waste. Through this work, we showed that adopting environmentally conscious printing can continue to push electrical performance and resolution limits of printed electronics, ushering in a new era of printing. The world is eager for sustainable solutions weaved seamlessly into the manufacturing of electronics as opposed to dealing with the environmental impacts post-manufacturing. As the printed electronics industry works to integrate new practices such as those outlined in this work, there is still more work to be done exploring the vast range of materials, inks, and print processes that are suitable for not only well-established applications but also new uses for printed electronics that continue to emerge every day.