Development of Air-Stable n-Type Thermoelectric Single-Walled Carbon Nanotubes Doped By Benzimidazole Drivative
Authors: Nakashima Yuki , Fujigaya Tsuyohiko , Nakashima Naotoshi 
Published: February 2020

Despite the remarkable growth of microelectronics such as wearable and portable devices, remote sensors, etc. on the market in recent years, most of the microelectronics are still powered by batteries that are required to recharge and replace at stated periods. These electronic devices require energy autonomy for an extended working time without the need for the intervention of users. One possible solution for powering these wireless microelectronics without a battery is to harvest energy from the human body by using thermoelectric (TE) generation which generates electricity especially from temperature difference. To generate electricity via Seebeck effect is promising. Seebeck effect is the phenomenon that a temperature difference between hot and cold ends of the TE generators that produces a voltage. The conversion efficiency of TE generators can be evaluated by the dimensionless figure of merit, ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. TE generator with a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity are expected to get good TE properties. To fabricate the TE devices, p- and n-type materials are necessary and the study of TE materials using inorganic semiconductors such as Bi2Te3 have been done for a long time as they illustrate high performance more than ZT = 1. However, these materials contain rare elements which show toxicity and poor processability. Therefore among the several candidates including conducting polymers, transition metal dichalcogenide and carbon nanotubes (CNTs) single-walled CNTs (SWNTs) are emerged as the promising candidate due to their non-toxicity, processability, abundant resources together with the remarkable electrical conductivity, potentially large Seebeck coefficient and light weight. However, due to the air oxidation, SWNTs only show p-type semiconducting property in air. In the development of the CNT-based TE devices, instability of the n-type CNT has been a central issue, n-type SWNT are easily oxidized due to an electrophilic chemisorption of oxygen on the charge polarization.1 Previously, we reported n-type SWNT by encapsulating cobaltcence to SWNT, however, n-type stability lasts only 30 days in air.2 Therefore, in this study we explored an air-stable n-type thermoelectric material by covering the anion part of the SWNT surface by using cationic molecules as a dopant (n-type dopant). In this strategy, 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzo [d] imidazole (DMBI) was chosen, since it is stable in the atmosphere, forms a stable cationic state by hydride transfer3 and is known as n-type dopant for other carbon materials, such as graphene and fullerene.4 We evaluated the air stability of SWNT doped by DMBI and found that the n-type nature of the DMBI-doped SWNT was stable at least for 90 days at room temperature.5

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5) Nakashima, Y.; Fujigaya, T.; Nakashima, N. in preparation.