Photoinduced electron doping of single-walled carbon nanotubes based on carboxamide photochemical reactions

Abstract

1,3-Dimethyl-2-arylbenzimidazole (DMBI) derivatives act as good electron dopants for semiconductors and form DMBI cations (DMBI⁺). However, in the presence of oxygen, inactive oxygen adducts (DMBI-Ox) are produced by the reaction of DMBI radicals with oxygen molecules, which decreases the doping efficiency. In this study, we found that UV irradiation of DMBI-Ox generates DMBI⁺ and hydroxyl ions that act as electron dopants. The underlying mechanism is likely the intramolecular cyclization of DMBI-Ox, followed by the elimination of hydroxyl ions. As an application of the new photoinduced reaction of DMBI-Ox, we carried out photoinduced electron doping of single-walled carbon nanotubes (SWCNTs) using DMBI-Ox to convert p-type SWCNTs into n-type SWCNTs. The photochemical reaction proceeded on the surface of the SWCNTs in the solid state, and the photodoped n-type SWCNTs showed air stability for 20 days owing to the generation of chemically stable DMBI⁺ on the negatively charged SWCNT surface. This doping method offers a facile preparation method for SWCNT sheets with a p–n pattern using photomasks. We fabricated planar-type thermoelectric generator (TEG) devices using p–n-patterned SWCNT sheets. The TEG device with four p–n sequences on a hot plate (ΔT = 35 °C) showed open-circuit voltage and maximum power density values of 0.68 mV and 0.504 nW, respectively. We believe that this new photochemical n-doping method will offer various applications based on the fine and facile patterning capabilities of these sheets.