Selective oxidation and removal of hydrocarbons from carbon nanotubes using reactive yttrium films

Abstract

Wrapping polymers are useful for sorting high purity suspensions of semiconducting carbon nanotubes (CNTs) in organic solvents, but for many microelectronic applications the wroapping polymer needs to be removed. Coating wrapped CNTs with yttrium metal, followed by oxidation and removal with dilute aqueous acid, has been used to etch wrapping polymer, but the mechanism, selectivity, extent of etching, and range of conditions over which etching occurs have not been reported. We use spectroscopic and physical measurements to characterize this process on thin films of an archetypical conjugated wrapping polymer (PFO-BPy), its amorphous char residue, CNTs (average diameter 1.5 nm), graphene, and other organic films. Exposure of a yttrium overcoated film of PFO-BPy to ambient air at 20 °C oxidizes a ∼0.5 nm layer of polymer, forming carbonate, carboxylate, and/or carbonyl groups that dissolve in dilute acid. Thicker layers of polymer are removed by repeated cycles. Similar results are observed for other organic films at 20 °C whereas CNTs and graphene are unaltered, providing the selectivity needed to remove carbon-based contaminants from sp² carbon based nanostructures. Increasing temperature to 250 °C increases polymer oxidation and removal to ∼2.5 nm per cycle; however, the CNTs and graphene are damaged.