Functionally Graded Carbon Nanotubes by Dynamic Control of Morphology during Chemical Vapor Deposition

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This grant will support basic research aimed at revealing the fundamental mechanisms underlying the process of manufacturing aligned carbon nanotubes for emerging applications of national interest. Carbon nanotubes are tubes that are smaller than one ten-thousandth of a human hair and can be thought of as atom-thick sheets that are wrapped seamlessly into tubes. The superior chemical and physical properties of individual nanotubes, as well as the collective unique directionality of energy and mass transport through large populations of aligned carbon nanotubes underpin their potential in many critical technological areas. The commercial production of carbon nanotubes exceeds several thousand tons per year and are used in products that rely on the random dispersion nanotubes of various lengths and diameters into a matrix to enhance the composite properties. However, emerging applications such as high power-density devices, 3D nanoelectronics, nanoporous membranes, and structural materials require more precise control of the spatial variation of sizes, order and hierarchical morphology of aligned carbon nanotube ensembles. This research looks to producing, through new understanding of their initial nucleation and subsequent growth, large arrays of vertically aligned carbon nanotubes which are all identical in width and length. These 'forests' of identical nanotubes have excellent mechanical properties and can be used in new application areas where the random length of the nanotubes would lead to poor performance. Research in this project will contribute knowledge towards building the process-structure-property relationship necessary for pushing emerging applications of nanotubes closer to market, leading to enhancing the American economic competitiveness, while advancing university-level education of molecular-scale manufacturing. Public outreach will be facilitated by the continued development of YouTube channel explaining nanotechnology-related topics to a general audience.The collective properties of vertically aligned carbon nanotubes (VACNTs) are dependent on the spatially varying morphology across their macroscopic forest-like structure. However, there is currently a lack of understanding of the mechanochemical factors governing the nucleation kinetics of large CNT populations, leading to an inability to control these variations in a repeatable fashion during the simultaneous growth of billions of CNTs. This project aims at filling this knowledge gap to enable precise control of spatially-engineered VACNTs grown by rapid thermal chemical vapor deposition (RT-CVD) and allow predicting their properties. This will be achieved by experimental growth of VACNTs under periodic growth conditions with in situ monitoring of height kinetics, combined with ex situ X-ray-based morphological characterization and electron microscopy, as well as stochastic modeling of the rates of catalytic activation and deactivation of CNTs nucleating from substrate-bound catalyst nanoparticles. Hence, the results generated from this project will enable predictably tuning CVD-grown CNTs in a process that is relevant to industrial CNT production.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.