J. Lee, M. Abdulhafez, G. Tomaraei, M. Bedewy
University of Pittsburgh,
Keywords: carbon nanotubes, chemical vapor deposition, nucleation density, aligned nanotubes, catalysis
Summary:Synthesis of vertically aligned carbon nanotubes (CNTs), commonly referred to as CNT forests, is needed for many applications that leverage their unique anisotropic mass/energy transport properties. Hence, understanding the population dynamics governing the simultaneous growth and self-organization of 10s-100s of billions of CNTs during chemical vapor deposition (CVD) are important for controlling forest density, diameter distributions, atomic defects, and tube lengths.1–3 In particular, the following important synthesis questions represent major challenges toward the manufacturing of CNT-based devices: (1) How can catalyst formation be controlled to tune CNT diameters? (2) What determines CNT nucleation density? (3) How can catalytic lifetime be extended to produce longer CNTs? And (4) how atomic defect density in CNTs can be optimized? Importantly, independent control on each of these aspects has been challenging, because of the coupled nature of the CVD process. In growing CNT forests via catalytic CVD, thin metal films deposited on a substrate are first converted into nanoparticles by dewetting at high temperature in a reducing atmosphere. Upon introducing a hydrocarbon gas, some of these nanoparticles nucleate CNTs that continue to grow until catalytic deactivation of individual nanoparticles leads to collective growth self-termination. Usually, the conditions for the dewetting and nucleation steps are coupled in typical hot-walled reactors. In our work, we developed a dynamic growth recipe based on a custom-designed multizone reactor,4,5 in which we decoupled the catalyst preparation step from CNT nucleation step by rapid thermal processing. We uniquely show that nucleation density of CNTs in a forest has a negative correlation with the nucleation and growth temperature regardless of the dewetting temperature.6 For the same nucleation temperature of 720 °C, density of CNT forests remained nearly constant with varying dewetting temperature from 500 to 900 °C. Moreover, we demonstrate a threefold increase of catalytic lifetime for the same growth temperature by increasing catalyst formation temperature. These insights enable decoupling density, diameters, and lengths of low-defect CNTs in forests, for tailoring their collective properties for applications in thermal interfaces and electrical interconnects.