Aligning carbon nanotubes via aerosol jet printing for flexible electronics

• Main Author: Goh, Guo Liang
• Other Authors: Yeong Wai Yee
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Nanoelectronics; Engineering::Manufacturing::CAD/CAM systems
• Online Access: https://hdl.handle.net/10356/137163
• Institution: Nanyang Technological University

The alignment of carbon nanotubes (CNT) is paramount in determining the performance of the CNTs electronics devices due to the higher electron mobilities. Apart from the microstructures of the materials, high print resolution is also essential for the fabrication of miniature electronic component design, which is vital for achieving high performance printed electronics devices. As such, this work aims at achieving alignment of CNTs and high print resolution using aerosol jet printing (AJP) technique with the intention of producing high-performance electronics. AJP is a droplet deposition technique, where the aerosols of ink are formed and mixed with carrier gas flow and aerodynamically focused by sheath flow to form a tiny beam of aerosols for high-resolution printing. This work aims at utilizing evaporation-driven self-assembly (EDSA) phenomenon (also known as coffee ring effect) to form aligned CNTs in the printed trace. Sessile drop experiments, a fundamental representation of the AJP process, are conducted to understand the factors affecting the EDSA mechanism of CNTs during the evaporation process. Factors such as substrate temperatures, concentration, pH and drop size of CNTs suspensions have been studied and their effects on the evaporation dynamics and final deposition patterns are discussed. CNTs are found aligned along the liquid-air-substrate interface and the resulting microstructures are characterized by Raman spectroscopy and Herman orientation factor (f) to help understand the mechanism of EDSA process and optimize for achieving better CNTs alignment. Solvent evaporation time is found to be critical for good CNTs alignment with large drop evaporating at low substrate temperature favoring the formation of highly aligned CNTs (f >0.9). Subsequently, the process window of the AJP process for achieving good quality and high-resolution line traces is investigated. An analytical model for the AJP process, which can help explain and understand the process, is developed and presented. A tradeoff between the degree of alignment and the printed line width is observed. The lowest line width of preferentially aligned CNTs traces is approximately 16 µm with f ≈ 0.4. It is found that the print speed and the number of print passes are the main factors affecting the electrical conductivity of the printed CNTs traces. The preferentially aligned CNTs twin-lines exhibit almost 200-1000 times better electrical resistance as compared to the randomly and homogeneously distributed CNTs traces. The resistance per unit length of the printed CNTs traces was found to be tunable from 160 kΩ/cm to 1.1 GΩ/cm using suitable print parameters. Lastly, functional CNTs-based flexible sensors such as optically transparent motion sensors and pH sensor are fabricated as a proof-of-concept. These favorable findings ascertain the feasibility of fabricating preferentially aligned CNTs using aerosol jet printing and contributed to the scientific knowledge in the respect of having (i) determined the key factors and mechanisms of evaporation-driven self-assembly of CNTs, (ii) analyzed AJP process comprehensively from aerosol transportation to ink deposition, and (iii) determined the process windows for printing high-resolution preferentially aligned CNTs and correlating the microstructure to the process parameters.