Theoretical Study On The Thermoelectric Properties Of Black Phosphorus And Related Low-dimensional Structures

The emerging global need for energy production, conservation, and management has promoted in the more effective means of power generation. A potential energy is from waste heat by using thermoelectric materials. The heat can come from the combustion of fossil fuel, automobile exhaust, and the process of chemical reaction. For this point, the thermoelectric materials play an important role in the power generation and energy conservation. Thermoelectric devices are scalable, reliable, quiet operation, no moving parts and maintenance-free. The efficiency of a thermoelectric material is determined by the dimensionless figure of merit (ZT= S2?T/?). However, these transport parameters (S?? and ?) are closely interrelated and related to the crystal and electronic structure, as well as the carrier concentration. As a result, the ZT value of some traditional bulk thermoelectric materials such as Bi2Te3, PbTe and Si1-xGex has remained at about 1.0 for several decades, whereas a ZT?3 is necessary to compete with conventional refrigerators or power generators. In 1993, the theoretical work of Hicks et al. reported that low-dimensional structures could exhibit significantly higher ZT values because of improved power factors on account of quantum confinement and decreased thermal conductivity caused by phonon boundary scattering. Since then, many efforts have been made to synthesize low-dimensional or nanoscale materials, such as Bi2Te3/Sb2Te3 superlattice structure, PbSeTe/PbTe quantum-dot superlattice, Si nanowires and nanostructured BiSbTe bulk alloys. However, the above mentioned materials synthesized by complicated fabrication technologies usually contain the elements are either expensive or toxic, which are not suitable for large-scale applications. Recently, the possibility of using black phosphorus (BP) and its low-dimensional structures as thermoelectric materials has attracted growing interest. Moreover, the phosphorus element is earth-abundant, low-cost and environmentally friendly. In this dissertation, we use a multiscale approach which includes density functional theory (DFT), semiclassical Boltzmann theory for electrons and phonons, and classical molecular dynamics (MD) simulations to investigate the structural, electronic, phonon, and transport properties of black phosphorus and related low-dimensional structures, and explore their possibility as high-performance thermoelectric materials.We first estimated the thermoelectric properties of bulk black phosphorus. It is found that the electronic and thermal transport in BP exhibit strong orientation dependence. Although the lattice thermal conductivity is large along the armchair direction, the thermoelectric efficiency can reach 1.1 at 800 K since we can get a large power factor. We also show the effect of substitution of P atom with Sb atom on the thermoelectric properties of BP. It is found that Sb substitution can not only enhance the electronic transport of BP by increasing the DOS around the Fermi level, but also suppress the lattice thermal conductivity because of the mass difference between Sb and P atom. As a result, the optimal ZT value of the substituted system (P0.75Sb025) is 4 times higher than that of pristine BP at 800 K.As for the low-dimensional system, we first studied the thermal transport properties of five phosphorene allotropes (?-, ?-, ?-,?- and ?-phase). It is found that the a-phosphorene exhibits considerable anisotropic thermal transport, while it is less obvious in the other four phosphorene allotropes. The highest thermal conductivity is found in the ?-phosphorene, followed by ?-, ?- and ?-phase. The much low thermal conductivity of ?-phosphorene is consistent with its relatively complex atomic configuration and can be attributed to a low phonon relaxation time and high scattering phase space. Combined with good electronic transport properties, it is very promising to use ?-phosphorene as high-performance thermoelectric material.We also investigated the transport properties of Bi2Te3 consisting of one quintuple layer (QL). It is found that the band gap of an isolated QL is considerably larger than that of bulk Bi2Te3. The electronic transport of the QL is then evaluated using the semiclassical Boltzmann theory within the relaxation time approximation. By fitting the energy surface from first-principles calculations, a suitable Morse potential is constructed and used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. By optimizing the carrier concentration of the system, the ZT of Bi2Te3 QL can be enhanced to a relatively high value. Moreover, the ZT value exhibits strong temperature dependence and can reach as high as 2.0 at 800 K. This value can be further increased to 2.2 by the substitution of Bi atoms with Sb atoms, giving nominal formula of (Bi0.25Sb0.75)2Te3.Finally, we studied the electronic and thermal transport properties of phosphorene nanoribbons with different width and edge configurations. It is found that the armchair phosphorene nanoribbons are semiconducting while the zigzag nanoribbons are metallic. By passivating the edge phosphorus atoms with hydrogen, the zigzag series also become semiconducting, while the armchair series exhibit a larger band gap than their pristine counterpart. The band gaps of semiconducting PNRs decrease monotonically with increasing ribbon width. Due to a relatively large band gap, the Seebeck coefficient of semiconducting PNRs exhibits a large value around the Fermi level. With the help of phonon Boltzmann transport equation, we found the armchair nanoribbons exhibit a lower lattice thermal conductivity than that of zigzag nanoribbons. Combined with good electronic transport properties, it is very promising to use APNR as high-performance thermoelectric material.