| Thermoelectric (TE) materials are functional materials that can realize the direct conversion between electricity and thermal energies depending on the movement and interaction of carriers in solids. The TE devices have no moving parts and are therefore both reliable and durable. Also, they are scalable and lightweight, and no pollutants are released to the environment. Thus TE materials are environmentally friendly and have immense values and application prospects. As the world energy crisis and environmental pollution become increasingly serious, people pay close attention to the exploration of new TE materials which have become a fashion in the research field of materials.The efficiency of a TE material is determined by the nondimensional figure of merit, ZT = S2σT/κ, where T is the absolute temperature, S is the Seebeck coefficient,σis the electrical conductivity, andκis the thermal conductivity. S2σis known as the power factor, representing the electrical transport performance. A good TE material has a high ZT value, which requests a large Seebeck coefficient (S), a high electrical conductivity (σ), and a low thermal conductivity (κ). But it is difficult to simultaneously optimize these variables in conventional bulk materials because the three TE parameters as a function of carriers (electrons or holes) concentrations are coupled with each other. Recently, with the development of the nanomaterial synthesis technology, both theoretical and experimental studies have proved that the TE performance in nanostructured materials is better than that in bulk. For example, Bi2Te3/Sb2Te3 superlattices and PbTe/PbSeTe quantum dots show a large ZT enhancement up to 2.4 and 1.6, respectively at room temperature. But it is still difficult and expensive to fabricate materials such as Bi, Te, Pb, Sb, and Se into synthetic nanostructures for large-scale energy conversion purposes. In 2008, Boukai and Hochbaum et al. successfully synthesized Si nanowires (SiNWs) and reported a 100-fold improved ZT value over bulk Si near room temperature, demonstrating the TE potential in SiNWs. Si is the most abundant and widely used material in the semiconductor industry for low-cost and high-yield processing. This experimental work thus opens up a new strategy on the search of Si-based nanostructured TE materials. But the exact physical mechanism of this 100-fold TE enhancement remains unknown. Therefore, the first part of our work is to establish the physical origin of the high TE capability in SiNWs by extensively investigating their electronic, transport, and lattice vibrational behaviors.We calculated the power factors of SiNWs and bulk Si using first-principles electronic structure calculations in the framework of Boltzmann transport theory. Our calculations show that the power factors of SiNWs are not significantly affected by the low dimensional structures, i.e., possessing the similar transport coefficients (S andσ) to the bulk values. We have calculated the Si-Si pair distribution function and charge densities for bulk Si and SiNWs. One observes that the the bonding situation in SiNWs remains largely unchanged over bulk Si, with the similar sp3 bonding characters. We thus clarified that the preserved tetrahedral bonding characters in SiNWs are responsible for the negligible changes of electronic transport properties compared with those in bulk Si. In contrast, lattice heat transport of SiNWs has been largely revised by the spatial confinement. The reduced size and dimension of the SiNWs increases the influences of the surfaces, leading to a significant modification of the vibrational properties and reduces the phonon group velocities. Meanwhile, optic modes of nanomaterials are low in frequency, which strongly scatter the heat-carrying acoustic modes to reduce phonon scattering time. The reduced phonon group velocity and phonon scattering relaxation time, in SiNWs jointly result in a decrease of the lattice thermal conductivity. This accounts for the experimental results and signifies the TE potential in SiNWs. We suggest that our work is greatly helpful for the deep understanding of SiNWs and sheds strong light on the search or design of new low dimensional TE materials through phonon engineering.It is well known that doping and alloying are effective methods to decrease the thermal conductivities and enhance ZT values of materials. We here take SiGe alloys as an example. SiGe alloys are efficient TE materials at high temperature, with a high ZT value ~ 1 at 1000 K. The improved performance is attributed to their much lower thermal conductivies than Si and Ge. Then, is the performance of nanostructured SiGe materials better than that of the conventional SiNWs? To answer this question, we carried out the second part of our work, which is to theoretically predict the TE performance of heterostructured Si/Ge nanowires. In this section, we examined the TE properties of Si/Ge nanowires and presented the effective routes to improve their performance by electron and phonon engineering. It is devided into two aspects. One is to predict the TE potential of Ge/Si core-shell nanowires. The other is to explore the TE performance of Si/Ge superlattice nanowires, and to enhance their TE capabilities by tailoring the structural properties.Firstly, we have extensively explored the TE properties of the Ge/Si core-shell structured nanowires. Our results show that ZT value of Ge/Si core-shell nanowire with p-type doping can reach 0.85 at 300 K, significantly larger than the observed ZT value of 0.36 in pure SiNWs. In addition, we provide an optimal carrier concentration of 2.64×1025 m-3 to achieve the maximal ZT value. By the deeply examination of the electronic and lattice vibrational properties, we find that the underlying mechanism for this enhanced ZT is mainly attributed to the reduced lattice thermal conductivity in Ge/Si core-shell structure. Moreover, we suggest that appropriate Ge content in Ge/Si core-shell nanowire may further optimize its ZT value. The current research proposed a way to design high-performance TE materials through a proper construct of heterostructured materials and will stimulate future research on other core-shell nanowires. Secondly, we predicted that ZT value higher than 1 at 300 K is achievable forSi/Ge superlattice nanowires. This suggests that Si/Ge superlattice nanowire is a promising candidate for TE applications, much better than the known SiNWs. Since superlattice nanowires, consisting of a series of interlaced nanodots of two different materials, benefit from both the superlattice and nanowires, they have strong quantum confinement effects and distinct transport properties from conventional nanowires, and are of great interest for thermoelectricity. The electronic band structure calculations of Si/Ge superlattice nanowires indicate that the electronic states near the band gap are more localized and the discrete, thus giving rise to the"minigap"between the neighboring conduction bands. Such"minigap"is mainly responsible for their enhanced Seebeck coefficient. Meanwhile, the heterogeneous interfaces between Si and Ge zones can reduce the lattice thermal conductivity by increasing the phonon scattering at the segment interfaces. Besides, our calculations show that the electrical transport (S2σ) in Si/Ge superlattice nanowires strongly depends on their unique electronic structures. It is possible to further enhance the TE performance by decreasing wire diameter and superlattice period, or adopting different constituent lengths and optimal doping concentrations for a given superlattice period. The dependence of TE capabilities on parameters, such as diameter, segment length, doping level etc. can also be extended to other superlattice nanowires. We believe that our work will inevitably stimulate future experimental exploration on the high TE performance of superlattice nanowires.
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