Solution-Processed Clean SWCNTs and Their Use as Templates for One-Dimensional van der Waals Heterostructures

Abstract

Single-walled carbon nanotubes (SWCNTs) have shown exceptional electrical, optical, mechanical, and thermal properties. Solution processing is a critical first step to harness these nanomaterials for applications in electronics, biomedicine, and energy technologies. However, dispersion of SWCNTs in solutions requires assistance by surfactants or polymers, which cannot be cleanly removed easily and become unwanted contaminants, resulting in degraded performance of SWCNTs.In this dissertation, I developed strategies to attain clean, solution-processed SWCNTs and further demonstrated their applications as templates for the synthesis of van der Waals heterostructures. We investigated the role of surfactants in dispersing SWCNTs and found that the highest efficiency in dispersing SWCNTs occurs at the critical micelle concentration of surfactants, which is well below the typically required surfactant concentrations. Furthermore, we synthesized a thermally removable surfactant, ammonium deoxycholate (ADC) which can be removed cleanly at a relatively low temperature without damaging the SWCNT structure. Compared to a commonly used surfactant, sodium deoxycholate (DOC), ADC features the same anion, but contains an ammonium (NH4+) cation in place of the metal ion (Na+). ADC exhibits the same high dispersion efficiency for SWCNTs as DOC, but the peak thermal decomposition temperature of ADC is nearly 70 oC lower than that of DOC. A two-step annealing process can remove this new nanotube surfactant while keeping the SWCNTs intact, even with a small diameter of just 0.76 nm. This work also reveals the chemical origin of residues from thermal annealing of surfactant-processed carbon nanomaterials. The clean SWCNTs enable the synthesis of van der Waals heterostructure consisting of pure chiral single-wall carbon nanotubes nested in boron nitride (SWCNT@BN). Transmission electron microscopy and electron energy-loss spectroscopic mapping confirm the successful synthesis of SWCNT@BN from the solution-purified nanotubes. The photoluminescence peak of (7,5)-SWCNT@BN heterostructure is found to redshift by 10 nm relative to that of (7,5)-SWCNT and the Raman G peak of (7,5)-SWCNTs downshift by 10 cm-1 after BN coating.