Multiple <i>ZT</i> Peaks Caused by van Hove Singularities in Semiconducting Carbon Nanotube Films

The <i>ZT</i> figure-of-merit is an index for thermoelectric conversion efficiency that incorporates the Seebeck coefficient, electrical conductivity, and thermal conductivity. In conventional three-dimensional materials, <i>ZT</i> exhibits a local maximum at only one carrier density because these three thermoelectric parameters each have counteracting trends with carrier concentration. Therefore, optimizing the doping level to the single peak in the <i>ZT</i> value is a general strategy to improve the thermoelectric conversion efficiency. Here we show that such a conventional single-peak <i>ZT</i> result is not observed for materials with strong quantum confinement effects. Specifically, we present here a double-peak structure of <i>ZT</i> values observed in films of semiconducting single-walled carbon nanotubes (SWCNTs). The <i>ZT</i> value can be enhanced when the Fermi level is near the van Hove singularities (vHs) in the density of states of the SWCNTs. Using electrolyte gating to shift the Fermi level systematically, we observe two local maxima in <i>ZT</i> corresponding to the two vHs. Such multiple-peak characteristics of the <i>ZT</i> value in quantum materials will alter the concept for improving the thermoelectric conversion efficiency.