Journal
NANO LETTERS
Volume 19, Issue 10, Pages 7370-7376Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.nanolett.9b03022
Keywords
thermoelectric; single-wall carbon nanotubes; nanomaterials; ionic liquid; electrical double-layer technique; one dimension
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Funding
- JSPS KAKENHI [JP17H06124, JP17H01069, JP18H01816, JP19J21142]
- JST CREST, Japan [JPMJCR17I5]
- U.S. Department of Energy Basic Energy Sciences [DEFG0206ER46308]
- U.S. National Science Foundation [ECCS-1708315]
- Robert A. Welch Foundation [C-1509]
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Semiconductors are generally considered far superior to metals as thermoelectric materials because of their much larger Seebeck coefficients (S). However, a maximum value of S in a semiconductor is normally accompanied by a minuscule electrical conductivity (sigma), and hence, the thermoelectric power factor (P = S-2 sigma) remains small. An attempt to increase a by increasing the Fermi energy (E-F), on the other hand, decreases S. This trade-off between S and sigma is a well-known dilemma in developing high-performance thermoelectric devices based on semiconductors. Here, we show that the use of metallic carbon nanotubes (CNTs) with tunable E-F solves this long-standing problem, demonstrating a higher thermoelectric performance than semiconducting CNTs. We studied the E-F dependence of S, sigma, and P in a series of CNT films with systematically varied metallic CNT contents. In purely metallic CNT films, both S and sigma monotonically increased with E-F, continuously boosting P while increasing E-F. Particularly, in an aligned metallic CNT film, the maximum of P was similar to 5 times larger than that in the highest-purity (>99%) single-chirality semiconducting CNT film. We attribute these superior thermoelectric properties of metallic CNTs to the simultaneously enhanced S and a of one-dimensional conduction electrons near the first van Hove singularity.
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