Fine tuning of the Fermi level of single-walled carbon nanotubes with onium salts and application for thermoelectric materials

Abstract

Fermi level is one of the most important parameters for control of semiconductor polarity and improvement of thermoelectric performances. Although single-walled carbon nanotubes (SWCNTs) are promising candidates for thermoelectric materials, control of the Fermi level of SWCNTs with chemical dopants is still challenging. Reported here is the fine-tuning of the Fermi level of SWCNTs with onium salts. The Fermi level of SWCNTs was very sensitive to substituent size on the cation and nucleophilicity of the anion, and the Seebeck coefficients markedly changed between −50 μV K−1 and 68 μV K−1. The optimum Fermi level of p-type SWCNTs that exhibits the maximum power factor was realized by ammonium chloride or pyridine hydrochloride, and the power factor (PF) was improved by about 7 times as compared with non-doped SWCNTs. SWCNTs doped with onium salts were stable in both p-type and n-type states, and thermoelectric performances did not change more than 100 days in the air.