In-Place Printing of Carbon Nanotube Transistors at Low Temperature

As the Internet of Things (IoT) continues to expand, there is increasing demand for custom low-cost sensors, displays, and communication devices that can grow and diversify the electronics ecosystem. The benefits to society of a vibrant, ubiquitous IoT include improved safety, health, and productivity as larger and more relevant datasets are able to be generated for fueling game-changing artificial intelligence and machine learning models. Printed carbon nanotube thin-film transistors (CNT-TFTs) have emerged as preeminent devices for enabling potentially transformative capabilities from, and widespread use of, IoT electronics. Still, despite intensive research over the past 15 years, there has yet to be the development of a streamlined, direct-write, in-place printing process, similar to today’s widely used inkjet or 3D printing technologies, where the substrate never leaves the printing stage and requires little to no post-processing. The development of such a process for producing CNT-TFTs could lead to the emergence of print-on-demand electronics, where direct-write printers are capable of printing distinct IoT sensing devices or even full IoT systems with little to no user intervention.

The work contained in this dissertation describes discoveries and innovations for streamlining and optimizing direct-write printed electronics using in-line or in-place methods, with primary focus on an in-place printing process for producing CNT-TFTs at relatively low temperature. The key enabling aspect of the in-place printing of CNT-TFTs was the development of aerosol jet-printable low-temperature conductive and dielectric inks that are functional immediately after printing. Additionally, the printed semiconducting CNT films required modified rinsing procedures for in-line processing, which proved to enhance performance. Notably, the resulting CNT-TFTs exhibited promising performance metrics with on/off-current ratios exceeding 103 and mobilities up to 11 cm2V-1s-1, while also operating under mechanical strain or after long-term bias stress, despite being printed with a maximum process temperature of only 80 °C. While optimizing these devices, various contact morphologies and configurations were investigated, where it was found that there was less variability in performance between sets of top-contacted devices, compared to bottom-contacts. Additionally, it was discovered that there are processing and performance trade-offs associated with various contact morphologies, with silver nanowires holding most value for in-place printing.

Although primary focus is given to aerosol jet-printed, CNT-based devices, this work also outlines another rapid, and potentially in-line, process for improving IoT-relevant electronics printed from a widely used direct-write method: fused filament fabrication. Here, using a high intensity flash lamp, the conductance of thermoplastic filaments are enhanced by up to two orders of magnitude. It was found that high-intensity light vaporizes the topmost layer of thermoplastic on metal-composite filaments, leaving behind a metallized surface layer in a technique referred to as flash ablation metallization (FAM). FAM was then used to enhance the performance of 3D printed circuit boards, demonstrating use in an immediately relevant application.

Overall, the development of in-place printed CNT-TFTs and the FAM process establish practical and scientific foundations for continued progress toward print-on-demand electronics. These foundations include: the development of low-temperature inks, rapid and in-line compatible process methods, and investigations of the impacts of various materials, device configurations, or process steps on electronic performance. Altogether, these developments have the potential to lower the time, costs, and overhead associated with printed electronics, moving the field closer to a point that is more accessible to industrialists, academics, and hobbyists alike.