Fully Printed Submicron Carbon Nanotube Thin-Film Transistors

Printed carbon nanotube thin-film transistors (CNT-TFTs) show promise for flexible, sustainable electronics due to their compatibility with a wide range of printing approaches and demonstrated recyclability. However, the limited resolution of printers has been a major barrier to realizing commercial applications of printed CNT-TFTs such as display backplane electronics, sensors, and back-end-of-line devices for chips. Previous reports have shown creative approaches to realizing submicron channel lengths using chemical processes, such as ink-to-ink repulsion and self-assembled monolayers, or mechanical manipulation in the form of post-print line-splitting. While these workarounds show possibility, there are challenges of repeatability due to process complexity, reliance on homogeneous chemical functionalization, and the difficulty of achieving fine control of transistor channel dimensions due to variability in line-edge roughness. In this work, we present fully printed submicron CNT-TFT channel lengths without chemical modification or physical manipulation post-printing using a new form of direct ink writing called capillary flow printing (CFP). The versatility of this printing technique is demonstrated by printing conducting (Ag), semiconducting (CNTs), and insulating (ion gel) inks on several types of substrates (SiO₂, Kapton, and paper) and through the fabrication of various TFT device structures. Notably, CFP of these CNT-TFTs yielded on-currents of 1.12 mA/mm when back-gated on Si/SiO₂, and 490 µA/mm when side-gated through ion gel on Kapton, demonstrating the strong transistor performance achievable with CFP. Mechanical bending and sweep rate resilience of devices printed on Kapton show the wide utility of CFP-fabricated devices for flexible applications. When benchmarked against state-of-the-art thin-film technology, the on-current of the fully printed CFP CNT-TFTs on Kapton rivals that of IGZO and LTPS devices at a channel length smaller than a tenth of the cleanroom-manufactured TFTs. In summary, the advancements of this work are three-fold: 1) the demonstration of submicron gaps between as-printed silver nanoparticle (AgNP) electrodes; 2) the development of a printing procedure to realize dense CNT films with a line width below 20 µm; and 3) the demonstration of fully printed CNT-TFTs with submicron channel lengths on a flexible substrate. Overall, this work highlights CFP as a viable fabrication method for submicron electronics through cleanroom-free printing techniques.