In situ monitoring and control of carbon nanotube synthesis

Abstract

While carbon nanotubes (CNTs) have exceptional properties, synthesis --

especially of high-quality, ordered assemblies of CNTs such as vertically aligned arrays ("forests") - remains a barrier to their broader commercial adoption. In particular, high-throughput production of CNTs via chemical vapor deposition (CVD) requires improvements in both control (i.e. achieving uniformity in CNT size and alignment) and efficiency (i.e. maximizing the yield of CNTs relative to a population of catalyst nanoparticles). These developments are critical for applications such as thermal interface materials, electrical interconnects, and filtration membranes. This thesis utilizes in situ characterization techniques to explore the synthesis of CNT forests, and to tailor CNT morphology by dynamic control of process conditions.

First, the formation of CNT forests is studied with in situ environmental transmission electron microscopy (ETEM), revealing that carbon availability during particle formation enhances both the formation of Fe catalyst nanoparticles and CNT nucleation, resulting in a 10-fold increase in CNT density. Then, a machine learning model is presented to identify the phase of individual nanoparticles in ETEM videos, to correlate catalyst particle phase dynamics with CNT nucleation probability. Next, a benchtop CVD system with an in situ Raman probe is used to monitor CNT nucleation and density accumulation during forest growth, to identify the correlation between time-variant carbon exposure to the catalyst and resulting CNT crystallinity.

Finally, the morphological development and growth kinetics of the CNT forest are shown to be mechanochemically modulated, by applying controlled mechanical forces during growth and coupling real-time height measurements with ex situ small-angle X-ray scattering analysis of forests grown under various loads. Taken together, these findings may be applied to the design and operation of large-area and continuous-feed reactors for CNT manufacturing.