Control and Reproducibility in Aerosol Jet Printed Carbon Nanotube Thin-Film Transistors: From Print-in-Place to Water-Based Processes
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.
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