Ab initio study of uniaxial strain-induced thermoelectric property tuning of individual single-wall carbon nanotubes

Abstract

Carbon nanotubes (CNT) have garnered significant importance as thermoelectric (TE) material due to their high TE performance owing to their inherent fascinating one-dimensional quantum confinement of charge carriers. Among various methods to tune and enhance TE properties, the impact of strain on the properties of CNT-based TE devices is rarely explored, though it is immanently present, specifically on multi-functional CNT fibers. In this study, we present the tunability and enhancement of electronic and TE properties of single-wall carbon nanotubes (SWCNTs) by strain engineering within the framework of density functional theory and Boltzmann transport theory. Semiconducting to metallic transition with a zigzag pattern of band gap (E_g) and Seebeck coefficient (S) variation was observed for uniaxial strain due to shifting of the Dirac points. Within the smaller range of strains, the E_g and S for the (10,0) SWCNT exhibited a positive correlation with tensile strain and demonstrated a negative correlation with compressive strain, while it showed a reverse trend for (11,0) SWCNT. In contrast, for (9,0) SWCNT, the E_g and S increased under both tensile and compressive strains. The SWCNTs exhibited a significant enhancement of thermoelectric transport properties while they underwent tensile and compressive strains. Applying uniaxial stress enhanced the Seebeck coefficient of SWCNTs to 1580 μV K⁻¹. We achieved the enhancement of thermoelectric power factor as follows: 2.2 mW m⁻¹ K⁻² (−9%), 1.3 mW m⁻¹ K⁻² (−3%), 1.15 mW m⁻¹ K⁻² (−3%), and 1.24 mW m⁻¹ K⁻² (−9%) for (6,6), (9,0), (10,0), and (11,0) SWCNTs, respectively. The overall performance parameter thermoelectric figure of merit, zT was tuned up to 1.56 when the strain varied from −9% to +9%, a fewfold increment compared to its relaxed state. The optimum range of doping for SWCNTs was found to be within 5 × 10¹⁹ cm⁻³– 1 × 10²¹ cm⁻³ to achieve the best thermoelectric power factor and zT. The insights gained from this study will be indispensable for diverse applications in the solid-state and especially in fiber-based wearable nanoelectronic devices such as strain sensors, thermoelectric energy harvesters, and thermoelectric coolers.