Paper TF-MoA3
Al₂O₃ and W/Al₂O₃ Atomic Layer Deposition on Quantities of Multiwalled Carbon Nanotubes

Monday, October 15, 2007, 2:40 pm, Room 613/614

Atomic layer deposition (ALD) can be employed to coat individual single-walled (SW) and multi-walled (MW) carbon nanotubes (CNTs). We have performed Al₂O₃ and W/Al₂O₃ ALD on quantities of multiwalled carbon nanotubes (MWCNTs) in a rotary reactor designed for ALD on high surface area nanoparticles. Al₂O₃ ALD was performed using trimethylaluminum (TMA) and H₂O. W ALD was performed using WF₆ and Si₂H₆. Al₂O₃ ALD on MWCNTs yielded nanospheres that grew with the number of trimethylaluminum and H₂O reaction cycles. Al₂O₃ ALD is believed to nucleate only at defect sites on the surface because the graphene surface of MWCNTs is unreactive. The NO₂/TMA nucleation procedure developed by Farmer and Gordon [Nano Letters 6, 699 (2006)] for ALD on SWCNTs was used to obtain very conformal Al₂O₃ ALD films on gram quantities of MWCNTs. The Al₂O₃ ALD films grew linearly with the number of TMA/H₂O reaction cycles. This Al₂O₃ ALD film is not covalently attached to the MWCNTs. Evidence for only a physisorption interaction was provided by ALD-coated MWCNTs where the Al₂O₃ ALD coating had broken to yield "ALD macaroni on a CNT string". W ALD also grew on the conformal Al₂O₃ ALD coating to create W/Al₂O₃ bilayers. X-ray photoelectron spectroscopy indicated that the surface of the metallic W was oxidized to form WO₃ upon exposure to atmosphere. The W ALD oxidization should be avoided by passivation with Al₂O₃ ALD. Calculations show that a metallic W ALD coating significantly enhances the CNT conductivity.