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Carbon nanotubes (CNTs) have garnered significant importance as thermoelectric (TE) ma- terial due to their high TE performance owing to their inherent fascinating property of 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 de- vices is rarely explored, though it is immanently present, specifically on multi-functional CNT fibers. We presented the tunability with enhanced electronic and TE properties of single-wall CNTs (SWCNTs) by strain engineering and doping technique within the frame- work of density functional theory and Boltzmann transport theory. Semiconductor-metallic- semiconductor transition with a zigzag pattern of band gap (Eg) and Seebeck coefficient (S) variation was observed for uniaxial strain due to shifting of the Dirac points. The SWC- NTs exhibited a significant enhancement of their thermoelectric transport properties while underwent tensile and compressive strains. Applying uniaxial stress enhanced the Seebeck coefficient of SWCNTs from 1172 to 1580 µV/K for (11,0) SWCNT. We achieved the en- hancement of thermoelectric power factor (PF) 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 few-fold incre- ment compared to its relaxed state. The optimum range of doping for SWCNTs was found to be within 5 × 1019cm−3 – 1 × 1021cm−3 to attain the best thermoelectric PF and zT .
The unique advantages associated with CNT’s one-dimensionality can be extracted by tun- ing the Fermi level near singularity points of the density of states at the band edges by appropriately doping the CNT bundle. Though halogen compounds act as excellent dopants among the various doping agents used in the CNT system, the impact of their orientation and placement in the CNT system is rarely explored. In this thesis, we explored the impact of halogen doping on the electronic and thermoelectric properties of of SWCNT bundle. We investigated the impact of halogen (ICl, IBr, and Br2) molecules filling the tube, inter- calation between adjacent SWCNTs, and their orientation on the thermoelectric properties of semiconducting SWCNT network through density functional theory-based ab initio cal- culations and Boltzmann transport theory. Our calculated halogen-doped SWCNT system showed improved thermoelectric performance. Notably, the system’s PF can be increased from 0.28 to 2.4 mW m−1 K−2, more than an eight-fold increment of its value obtained at the Fermi level. On the other hand, ICl intercalation of the SWCNT bundle shifted the peak thermoelectric PF below its Fermi level, and it can be attained by doping the CNT bundle with additional acceptor atoms. We achieved the best enhancement of thermoelectric PF from zero for pristine CNT to 0.3 mW m−1 K−2 when the ICl molecules were horizontally aligned outside the CNT in close spacing with adjacent dopants. It can further be pulled up
to 1.82 mW m−1 K−2 by doping the CNT with an acceptor concentration of 7.1×1021 cm−3. The IBr and Br2 doped SWCNT bundle increased the PF to 0.286 mW m−1 K−2 and 0.122 mW m−1 K−2, respectively. We discussed the underlying physics behind the alteration of thermoelectric properties of uniaxially strained and halogen-doped SWCNTs. The insights gained from this investigation would help designing and fabricating substrate-supported and flexible thermoelectric nanogenerators and coolers for applications to power up wearable electronics and IoT-based remote devices where thermal energy is abundant. |
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