Printed Carbon Nanotube Thin Films for Electronic Sensing
With the advent of the internet-of-things (IoT) and a more connected digital ecosystem, new electronic sensors and systems are needed. Printing has been identified as a means of fabricating low-cost electronics on non-rigid, large-area substrates. Printed electronics have been demonstrated to have the required electrical and mechanical properties to facilitate new and unique flexible electronic sensors for the IoT. One printable material that has demonstrated significant promise, specifically when compared to more traditional printed semiconductors, is solution-processed carbon nanotubes (CNTs). While some work has been done to facilitate the fabrication of CNT thin-film transistors (TFTs), little work has been done to assess the viability and potential of CNT-TFTs and other CNT thin films for real-world sensing applications.
The work contained in this dissertation describes the use of aerosol jet printing to fabricate CNT-TFTs, and the resulting study of their capability for various sensing applications. Aerosol jet printing allows for printing all the materials necessary for a fully-functional CNT-TFT, including the semiconducting thin film, conducting contacts and gate, and insulating gate dielectric. Using this system, flexible and fully printed CNT-TFTs were developed and characterized. Fully printed transistors were fabricated with field-effect mobilities as a high as 16 cm2/(Vs). The transistors were also resilient to substantial bending/strain, showing no measurable performance degradation after 1000 bending cycles at a radius of curvature of 1 mm.
The printed CNT-TFTs were evaluated for several sensing applications, including environmental pressure sensing and point-of-care biological sensing. The biological sensors, which were electronically transduced immunoassays, consisted of an antifouling polymer brush layer to enhance the CNT-TFT sensitivity and printed antibodies for detection of target analytes. Unparalleled sensitivity in unfiltered biological milieus was realized with these printed biosensors, detecting protein concentrations as low as 10 pg/ml in whole blood. In addition to demonstrating an electronically transduced TFT-based biosensor, work was done to develop a stable platform with high yield that will provide the means for a deeper understanding of the biosensing mechanisms of transistor-based sensors. As part of this biosensor platform development, novel solution-gated CNT-TFTs were demonstrated, with stable operation in ionic solutions for periods as long as 5 hours.
Another important electronic sensing technique is capacitive-based sensing. Using aerosol jet printed carbon nanotubes, a capacitive sensor has been developed and demonstrated for measuring insulating material thickness. The sensors rely on the fringing field between two adjacent electrodes interacting with the material out-of-plane, and that interaction being perturbed differently based on the thickness of the overlaid material. This sensor was also demonstrated in a one-dimensional array, which can be used to map tire tread thickness from the outside of the tire.
Overall, this dissertation explores the use of printed carbon nanotubes for diverse sensing applications. While this work provides real-world demonstrations that have potential impact for the IoT, there are also substantial scientific advancements made. Namely, insight into biosensing mechanisms, operation of solution-gated nanomaterial-based transistors, and demonstration of porosity and thickness effects on printed capacitive sensor electrodes.
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