Browsing by Subject "Thin-film transistors"

The rapid maturation of the internet of things (IoT) has led to an ever-stronger drive for large-area, flexible, and/or customizable electronics for data collection, display, and communication. The use of printing technology for IoT and thin-film electronics has shown growing promise due to its potential in low-cost and high-throughput manufacturing, as well as the capability to handle a wide array of substrates, materials, and production techniques (e.g., mass-production or customizable). At the heart of many IoT devices are thin-film transistors (TFTs). Carbon nanotubes (CNTs) have been considered promising candidates for printed TFTs due to their extraordinary electronic properties and material attributes, such as mechanical flexibility and low-temperature processability. Despite the significant research progress in printing CNT-TFTs and demonstrating CNT-TFTs in applications, process variability has still remained a major obstacle to the translation of CNT-TFTs out of the lab and into products. In addition, most printing processes reported in the literature involve post-printing thermal treatments, limiting the throughput and efficiency of printing approaches. This dissertation contains scientific discoveries, technical advancements, and innovations that reduce the process variability of CNT-TFTs and lead to the development of print-in-place processes -- a series of rapid, versatile, and streamlined printing approaches for yielding CNT-TFTs without any postprocessing. The key enabling factor of variability reduction is to understand the impact of CNT ink temperature, a commonly overlooked factor, on the resultant ink deposition via aerosol jet printing (AJP). It was discovered that an appropriately lowered ink temperature benefits both long-term (~1 hour) and short-term (~1 minute) stability of AJP, resulting in fully printed TFTs with average mobility of 12.5 cm2/V·s and mobility variation as small as 4%. The streamlining of the print-in-place processes for CNT-TFTs involved identifying low-temperature processable materials and formulating corresponding printable inks from these materials. By printing silver nanowires (AgNW) as the electrical contacts and hexagonal boron nitride (h-BN) as the gate insulator, the maximum processing temperature of the print-in-place process was reduced to 80 °C with no additional thermal treatment required. Notably, the resultant devices (known as 1D-2D TFTs) showed relatively good performance, including an on/off-current ratio up to 3.5×105, channel mobility up to 10.7 cm2/V∙s, small gate hysteresis, and superb mechanical stability under bending. In addition, another print-in-place process was demonstrated by employing a side-gate configuration with an ion gel dielectric and graphene contacts. Compared to the fabrication process for 1D-2D TFTs, this 3-step process was even more streamlined, and the resulting devices exhibited more uniform performance at the cost of the on/off-current ratio. Besides variability reduction and streamlining, the versatility and environmental friendliness of printed CNT-TFTs were also enhanced by studying the use of all-aqueous inks for CNT-TFTs. The study unveiled the printing challenges imposed by aqueous CNT inks compared to the more commonly used inks that depend on harsh solvents, such as toluene. It was discovered that the ionic surfactant in the aqueous inks hinders the CNT adhesion with the substrate, and a multi-step process with interstitial rinsing was proposed to mitigate this issue. Through the combination of aqueous CNT, graphene, and crystalline nanocellulose (CNC) inks, water-only TFTs were printed without the use of any harsh chemical solvents, and most device layers are either recyclable or biodegradable. In addition, although this dissertation is primarily focused on the printing techniques used to make CNT-TFTs, the materials and methods developed in the works were also utilized to demonstrate a surface acoustic wave (SAW) tuning device, an uncommon application for CNT-TFTs. The phase-velocity tunability (∆v/v = 2.5% ) was exceptional and close to the theoretical limit, suggesting the promise of CNT-TFTs for SAW applications and that there exist numerous unexplored areas where CNT-TFTs could potentially be advantageous. Overall, the findings and advancements contributed by this dissertation advance the printing of CNT-TFT by addressing obstacles and demonstrating possibilities. These include the developed methodologies, such as the print-in-place approaches and the multi-step aqueous printing with interstitial rinsing, along with the discoveries that may trigger follow-up studies, such as the correlation between ink temperature and process variability. Combined, these contributions help to reduce the variability, lower the process duration, and enhance the versatility of printed CNT-TFTs, pushing the field toward ubiquitous use in real-world applications.

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.