Controlling Electronic States of Few-walled Carbon Nanotube Yarn via Joule-annealing and p-type Doping Towards Large Thermoelectric Power Factor

Abstract

Flexible, lightweight and robust thermoelectric (TE) materials have attracted much attention to convert waste heat from low-grade heat sources, such as human body, to electricity. Carbon nanotube (CNT) yarn is one of the potential TE materials owing to its narrow band-gap energy, high charge carrier mobility, and excellent mechanical property, which is conducive for flexible and wearable devices. Herein, we propose a way to improve the power factor of CNT yarns fabricated from few-walled carbon nanotubes (FWCNTs) by two-step method; Joule-annealing in the vacuum followed by doping with ptype dopants, 2,3,5,6-tetrafluo-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Numerical calculations and experimental results explain that Joule-annealing and doping modulate the electronic states (Fermi energy level) of FWCNTs, resulting in extremely large thermoelectric power factor of 2250 µW m −1 K −2 at a measurement temperature of 423 K. Joule-annealing removes amorphous carbon on the surface of the CNT yarn, which facilitates doping in the subsequent step, and leads to higher Seebeck coefficient due to the transformation from (semi) metallic to semiconductor behavior. Doping also significantly increases the electrical conductivity due to the effective charge transfers between CNT yarn and F4TCNQ upon the removal of amorphous carbon after Joule-annealing. Nowadays, personnel electronic devices are ubiquitous in our daily lives. However, these devices heavily rely on batteries for operation while there are various forms of heat (energy) dissipated into our environment including our body heat as a waste 1,2. Therefore, flexible thermoelectric (TE) device becomes a promising technology to power the personal electronic devices from surrounding waste heat. The performance of the TE materials is usually determined by the dimensionless figure of merit, ZT = S 2 σ T/κ, where S is the Seebeck coefficient or thermopower (V K −1), σ is the electrical conductivity (S cm −1), T is the absolute temperature (K) and κ is the thermal conductivity (W m −1 K −1) 3. Power factor, P = S 2 σ can also be used alternatively to explain the thermoelectric performance of materials 4-7. Recently, TE performance has been optimized through formation of nanocomposite with conducting polymers, point defect engineering, energy filtering, controlling the density of state, and so on 8-10. Consequently, many semiconductors that provide the best TE performance are costly to fabricate since they are based on inorganic counterparts. Due to the scarcity of raw materials, and toxicity, inorganic semiconductors have a lot of drawbacks for fabrication of ubiquitous TE generators 11,12. Carbon nanotube (CNT) yarn is one of the potential ubiquitous TE materials because flexible, wearable, lightweight and robust TE devices can be easily fabricated from CNT yarn 9. Moreover, the carrier type of the CNT yarn can be easily altered by simple doping, paving it for using in various wearable or flexible TE applications 1,13,14. Several methods have been attempted to improve the T...

Enhancement of the mechanical and thermal transport properties of carbon nanotube yarns by boundary structure modulation

“…According to transmission electron microscopy images (JEM-2100F, JEOL), the original CNTs were characterized as multiwalled CNTs with two to five layers (supplementary figures 1 and 2 (available online at stacks.iop.org/NANO/ 33/235707/mmedia)) and a diameter distribution of 2-7 nm (supplementary figures 3(a) and (b)). Post-Joule annealing was performed on the CNT yarns in vacuum (10 -4 Pa) at 2100-2600 K for 1 min [21,26]. The CNT yarns were broken at a Joule annealing temperature of 2716 K. Scanning electron microscopy (SEM, JSM-6060LA, JEOL) images of the CNT yarns with and without Joule annealing showed a twisting angle of 4°-5°(figure 1).…”

“…These processes are irrelevant to graphene formation at the boundary of CNTs; therefore, the critical temperature (T , c, Energy 1860 K) of energy gap broadening does not match the critical temperature of the amorphous-to-graphene transition (T , c, 2Dg 2150 K). Owing to the energy gap modulation, the electrical conductivities (s) of CNT yarns with and without Joule annealing cannot be compared at a specific temperature (figure 3(a)) [26]. Here, we focus on the s 0 values and electrical conductivity without considering the energy gap of the CNT yarns, as shown in figure 3(c).…”

A hierarchically modified fibre-reinforced polymer composite laminate with graphene nanotube coatings operating as an efficient thermoelectric generator

“…Moreover, CNTs possess tunable semiconducting characteristics as their n- or p-type behaviour may be tailored via specific doping strategies resulting in exceptional versatility in terms of TE properties. 13,27–32…”