Theoretical Study On The Thermoelectric Properties Of Carbon Nanotubes And Related One-dimensional Structures

As fossil fuels are being exhausted and the increasing challenges of global warming, it is urgently needed to develop clean and renewable sources of energy. Thermoelectric materials have attracted much attention from the science community because they can directly convert waste heat into electric power and vice versa. For conventional materials, the transport coefficients are coupled with each other, and it is usually difficult to greatly improve their ZT values. Both theoretical prediction and experimental studies indicate that low-dimensional materials or nanostructures could exhibit much higher thermoelectric performance on account of improved power factor caused by quantum confinement as well as decreased thermal conductivity caused by enhanced phonon boundary scattering. However, the experimental realization of such system remains a big challenging for the best thermoelectric materials which usually contain one ore more elements of Bi, Sb, Te, Co, Pb and Ag. Moreover, they are not suitable for large-scale fabrications and applications because of the complicated and expensive synthesis technique. Recently, it was reported that carbon nanotubes, carbon nanowires, and other one-dimensional carbon nanomaterials were successfully synthesized. Compared with the conventional materials, such carbon nanomaterials are environmentally friendly and easily to be mass produced. In this dissertation, we use a combination of density functional theory (DFT), nonequilibrium Green’s function (NEGF) method, and nonequilibrium molecular dynamics (NEMD) simulations to investigate the electronic, phonon, and transport properties of carbon nanotubes and related one-dimensional structures, and try to explore their possible applications as eco-friendly and high-performance thermoelectric materials.We first study the thermoelectric properties of three kinds of ultrasmall (4A) carbon nanotubes. It is found that the quantized transmission function displays a clear stepwise structure, which indicates ballistic transport of electrons in the carbon nanotubes. The power factor of these carbon nanotubes can be optimized to much higher values in a wide temperature range. Our calculations indicate that the semiconducting (4,2) tube exhibits higher room temperature ZT value (1.6) than (3,3) and (5,0) tubes because of the higher power factor and lower electronic thermal conductance. Although the ZT values of the pristine tube is not very high, the thermoelectric performance of these nanotubes can be greatly enhanced by hydrogen adsorption, formation of bundles, and increasing the tube length, which significantly reduce the electron and phonon induced thermal conductance. Our calculations suggest that ultra-small carbon nanotubes may be promising candidates for high-performance thermoelectric applications.We then investigate the thermoelectric performance of a series of carbon nanotubes with larger diameters (including the zigzag (7,0),(8,0),(10,0),(11,0),(13,0),(14,0) and the chiral (4,2),(5,1),(6,2),(6,4),(8,4),(10,5)), and discuss how their ZT values change with temperature, tube diameter, and chirality. It is found that these tubes could have higher ZT values at appropriate doping level and operating temperature. The tubes with an intermediate diameter (7～8A)(e.g.,(10.0) and (6,4)) are found to exhibit higher ZT values than others. Moreover, the thermoelectric performance can be significantly enhanced by isotope impurities, isoelectronic substitution, and surface design, thus the ZT values can be optimized to about4.0. Compared with the ultra-small (4A) nanotubes, tube (10,0) can be more easily fabricated in and selected from the experiments, which make it is more favorable to be applied as high-performance thermoelectric materials in the near future.We have also investigated the room temperature thermoelectric properties of three kinds of5A carbon nanowires with typical orientations:[100],[110], and [111]. Although these nanowires are metallic in the pristine form, those with zero transmission windows near the Fermi level can be optimized to exhibit higher power factor. Due to relatively high power factor and very low thermal conductance,[100] and [111] CNWs are found to exhibit better thermoelectric performance than other carbon nanomaterials such as carbon nanotubes. Moreover, the ZT of these systems can be further improved to value in excess of10by partial hydrogen passivation. which can reduce the thermal conductance by about50%but leave the power factor less affected. Our calculated results indicate that small diameter carbon nanowires could be very promising high-performance thermoelectric materials.Carbon nanotubes are usually self-arranged into a two-dimensional hexagonal array structures during the synthesis process. To make a better comparison with related experimental works, we discuss the thermoelectric properties of an array of (10,0) carbon nanotubes. Our theoretical calculations indicate that the electronic transport coefficients of (10,0) array are no longer quantized. At intermediate temperature, the power factor of array (10,0) can be optimized to higher values via appropriate doping, but it is still much lower than that of free standing tube (10,0). On the other hand, it is found that the lattice thermal conductance of array (10,0) is about20%lower than that of the tube (10,0), which can be attributed to the enhanced phonon scattering by the tube-tube interactions. Overall, the (10,0) array can be doped to exhibit the highest ZT value of1.2at800K.