Oxide-Free Titanium–Carbon Nanotube Hybrid Conductors from Inert Atmosphere Hot Filament Chemical Vapor Deposition

Bulk metal–carbon nanotube (CNT) conductors are an attractive composite that can lead to a high electrical conductivity at a lower physical density for mass-sensitive applications. Titanium is a standard adhesion metal to enhance the electrical contact between metals and CNT materials due to its favorable nanoscale wettability and lower contact resistance, but suffers from a high reactivity with oxygen. Hot filament chemical vapor deposition (CVD) is a technique to deposit nanoscale metals within the bulk CNT network and has been applied in the present work to fabricate an oxide-free titanium adhesion layer. Purified CNT roving is Joule-heated in the presence of cyclopentadienyl(cycloheptatrienyl) titanium(II) [CHT-Ti-Cp] vapor under a continuous flow of 95% Ar/5% H2 gas to maintain inert conditions during deposition. Control over deposited mass is shown as a function of the CHT-Ti-Cp mass with the corresponding nanoscale morphologies imaged by scanning electron microscopy (SEM). Raman spectroscopy measurements of Ti-CNT samples produced in an inert atmosphere confirmed their oxide-free nature compared to Ti-CNT samples deposited in the presence of trace O2 that exhibited standard rutile TiO2 peaks between 200 and 800 cm–1. Electrical conductivity measured at 300 K for an oxide-free Ti-CNT hybrid with a conformal adhesion coating is 185 kS/m, an increase of ∼2.5× over purified CNTs and an ∼4× improvement over as-received CNTs. Temperature-dependent conductivity modeling from 300 to 500 K shows a significant reduction in the tunneling energy barrier from 43 meV for the purified CNTs to 11 meV in the oxide-free Ti-CNT conductor. Thus, the enhanced nanoscale transport from lower contact resistances at CNT junctions with oxide-free Ti can directly improve metal matrix–CNT conductors.