Selectively Tuning the Substrate Adhesion Strength of Aligned Carbon Nanotube Arrays via Thermal Postgrowth...

The excellent intrinsic properties of aligned nanofibers, such as carbon nanotubes (CNTs), and their ability to be easily formed into multifunctional 3D architectures motivate their use for a variety of commercial applications, such as batteries, chemical sensors for environmental monitoring, and energy harvesting devices. While controlling nanofiber adhesion to the growth substrate is essential for bulk-scale manufacturing and device performance, experimental approaches and models to date have not addressed tuning the CNT array-substrate adhesion strength with thermal processing conditions. In this work, facile "one-pot" thermal postgrowth processing (at temperatures T_p = 700-950 °C) is used to study CNT-substrate pull-off strength for millimeter-tall aligned CNT arrays. CNT array pull-off from the flat growth substrate (Fe/Al₂O₃/SiO₂/Si wafers) via tensile testing shows that the array fails progressively, similar to the response of brittle microfiber bundles in tension. The pull-off strength evolves nonmonotonically with T_p in three regimes, first increasing by 10 times through T_p = 800 °C due to graphitization of disordered carbon at the CNT-catalyst interface, and then decreasing back to a weak interface through T_p = 950 °C due to diffusion of the Fe catalyst into the substrate, Al₂O₃ crystallization, and substrate cracking. Failure is observed to occur at the CNT-catalyst interface below 750 °C, and the CNTs themselves break during pull-off after higher T_p processing, leaving residual CNTs on the substrate. Morphological and chemical analyses indicate that the Fe catalyst remains on the substrate after pull-off in all regimes. This work provides new insights into the interfacial interactions responsible for nanofiber-substrate adhesion and allows tuning to increase or decrease array strength for applications such as advanced sensors, energy devices, and nanoelectromechanical systems (NEMS).