Enhancing thermal stability of n-type conduction in carbon nanotubes via cation replacement mediated by bicyclic guanidinium salts†

The development of thermally stable n-type carbon nanotubes (CNTs) is crucial for their implementation in pn junction devices. In previous work, we introduced an ion replacement technique to stabilize chemically p-doped CNTs, demonstrating the control of hole density and the stabilization of doped states through separate doping and anion replacement processes. This study extends the methodologies to n-type doping by substituting the cation with a specific dopant or stabilizer. The exceptional reduction capability of the cobalt-based complex was evident from the negative Seebeck coefficient, the markedly high electrical conductivity, and the reduction in work function of the doped CNTs. Additionally, the selection of the anion is critical for successful cation replacement, as explored through complex chemistry perspectives. The n-type CNTs, coordinated with bicyclic guanidinium cations, showed improved thermal stability compared to their as-doped counterparts. Lastly, we discuss the thermoelectric properties (with the power factor up to 100 μW m⁻¹ K⁻²) as prospective applications for n-type CNTs in energy harvesting. This foundational work proposes a strategy for engineering n-type CNTs with optimized doping levels and enhanced stability.