Our previous work demonstrated that Cr2Ge2Te6 based compounds with a layered structure and high symmetry are good candidates for thermoelectric application. However, the power factor of only ∼0.23 mW/mK2 in undoped material is much lower than that of conventional thermoelectrics. This indicates the importance of an electronic performance optimization for further improvements. In this work, either Mn- or Fe-substitution on the Cr site is investigated, with expectations of both carrier concentration control and band structure engineering. First-principle calculations indicate that an orbital hybridization between d orbitals of the doping atom and the p orbital of Te significantly increases the density of states (DOS) around the Fermi level. In addition, it is found that Mn doping is more favorable to improve the electrical properties than Fe doping. By tuning the carrier concentration via Mn doping, the peak power factor rises rapidly from 0.23 mW/mK2 to 0.57 mW/mK2 at 830 K with x = 0.05. Combined with the intrinsic low thermal conductivity, Cr1.9Mn0.1Ge2Te6 displays a decent zT of 0.63 at 833 K, a 2-fold value as compared to that of the undoped sample at the same direction and temperature.

Using the first-principles pseudopotential method and Boltzmann transport theory, we give a comprehensive understanding of the electronic and phonon transport properties of the thermoelectric material BiCuSeO. By choosing an appropriate hybrid functional for the exchange–correlation energy, we find that the system is a semiconductor with a direct band gap of ∼0.8 eV, which is quite different from those obtained previously using standard functionals. Detailed analysis of a three-dimensional energy band structure indicates that there is a valley degeneracy of eight around the valence band maximum, which leads to a sharp density of states and is responsible for a large p-type Seebeck coefficient. Moreover, we find that the density of states effective mass is much larger and results in a very low hole mobility for BiCuSeO. On the other hand, we discover two flat phonon branches contributed by the Cu and Se atoms, which can effectively block heat transfer. Combined with large atomic displacement parameters of the Cu atom, we believe that the intrinsically low lattice thermal conductivity in BiCuSeO is mainly caused by the Cu atoms, instead of the prevailingly believed Bi atoms. The thermoelectric figure-of-merit is also predicted and compared with available experimental results.

•The relaxation time is predicted by electron-phonon Wannier interpolation techniques.•The scattering term of the Seebeck coefficient can not be ignored.•The electronic thermal conductivity can be effectively reduced by nanostructuring.We combine first-principles calculations and Boltzmann transport theory to study the thermoelectric properties of intermetallic compound YbAl3. To accurately predict the electronic relaxation time, we use the density functional perturbation theory and Wannier interpolation techniques which can effectively treat the electron-phonon scattering. Our calculated transport coefficients of YbAl3 are in reasonable agreement with the experimentally measured results. Strikingly, we discover that in evaluating the Seebeck coefficient of YbAl3, the scattering term has a larger contribution than the band term and thus should be explicitly considered in the calculations, especially for the case with localized bands near the Fermi level. Moreover, we demonstrate that by reducing the sample size to less than ∼30 nm, the electronic thermal conductivity of YbAl3 can be sufficiently suppressed so that the thermoelectric figure of merit can be further enhanced.

•The Bi nanoribbon undergoes a topological transition from trivial to non-trivial edge states at a critical width of ∼6.5 nm.•The edge relaxation time exhibits unique energy dependence and is immune to nonmagnetic impurities or disorder.•Utilization of topological edge states might be a promising approach to cross the threshold of the industrial application of thermoelectricity.Using first-principles calculations combined with Boltzmann transport theory, we investigate the effects of topological edge states on the thermoelectric properties of Bi nanoribbons. It is found that there is a competition between the edge and bulk contributions to the Seebeck coefficients. However, the electronic transport of the system is dominated by the edge states because of its much larger electrical conductivity. As a consequence, a room temperature ZT value exceeding 3.0 might be achieved for both p- and n-type systems by fine tuning the relaxation time ratio between the edge and the bulk states.

Few-layer black phosphorus has recently emerged as a promising candidate for novel electronic and optoelectronic devices. Here we demonstrate by first-principles calculations and Boltzmann theory that black phosphorus could also have potential thermoelectric applications and a fair ZT value of about 1.1 can be achieved at elevated temperature. Moreover, such a value can be further increased to 5.4 by substituting the P atom with the Sb atom, giving a nominal formula of P0.75Sb0.25. Our theoretical work suggests that high thermoelectric performance can be achieved without using complicated crystal structures or seeking for low-dimensional systems.

The electronic and transport properties of the half-Heusler compound LaPtSb are investigated by performing first-principles calculations combined with semi-classical Boltzmann theory and deformation potential theory. Compared with many typical half-Heusler compounds, LaPtSb exhibits an obviously larger power factor at room temperature, especially for the n-type system. Together with the very low lattice thermal conductivity, the thermoelectric figure of merit (ZT) of LaPtSb can be optimized to a record high value of 2.2 by fine tuning the carrier concentration.

The thermoelectric properties of the distorted bismuth(110) layer are investigated using first-principles calculations combined with the Boltzmann transport equation for both electrons and phonons. To accurately predict the electronic and transport properties, the quasiparticle corrections with the GW approximation of many-body effects have been explicitly included. It is found that a maximum ZT value of 6.4 can be achieved for n-type systems, which essentially stemmed from the weak scattering of electrons. Moreover, we demonstrate that the distorted Bi layer retains high ZT values in relatively broad regions of both temperature and carrier concentration. Our theoretical work emphasizes that the deformation potential constant characterizing the electron–phonon scattering strength is an important paradigm for searching high thermoelectric performance materials.

The electronic and transport properties of the (10, 0) single-walled carbon nanotube are studied by performing first-principles calculations and semi-classical Boltzmann theory. It is found that the (10, 0) tube exhibits a considerably large Seebeck coefficient and electrical conductivity which are highly desirable for good thermoelectric materials. Together with the lattice thermal conductivity predicted by non-equilibrium molecular dynamics simulations, the room temperature ZT value of the (10, 0) tube is estimated to be 0.15 for p-type carriers. Moreover, the ZT value exhibits strong temperature dependence and can reach to 0.77 at 1000 K. Such a ZT value can be further enhanced to as high as 1.9 by isotopic substitution and chemisorptions of hydrogen on the tube surface.

As a new carbon allotrope, the recently fabricated graphdiyne has attracted much attention due to its interesting two-dimensional character. Here we demonstrate by multiscale computations that, unlike graphene, graphdiyne has a natural band gap, and simultaneously possess high electrical conductivity, large Seebeck coefficient, and low thermal conductivity. At a carrier concentration of 2.74 × 1011 cm−2 for holes and 1.62 × 1011 cm−2 for electrons, the room temperature ZT value of graphdiyne can be optimized to 3.0 and 4.8, respectively, which makes it an ideal system to realize the concept of “phonon-glass and electron-crystal” in the thermoelectric community.

The thermoelectric properties of two typical SiGe nanotubes are investigated using a combination of density functional theory, Boltzmann transport theory, and molecular dynamics simulations. Unlike carbon nanotubes, these SiGe nanotubes tend to have gear-like geometry, and both the (6, 6) and (10, 0) tubes are semiconducting with direct band gaps. The calculated Seebeck coefficients as well as the relaxation time of these SiGe nanotubes are significantly larger than those of bulk thermoelectric materials. Together with smaller lattice thermal conductivity caused by phonon boundary and alloy scattering, these SiGe nanotubes can exhibit very good thermoelectric performance. Moreover, there are strong chirality, temperature and diameter dependences of the ZT values, which can be optimized to 4.9 at room temperature and further enhanced to 5.4 at 400 K for the armchair (6, 6) tube.

The structural and electronic properties of a two-dimensional monolayer bismuth are studied using density functional calculations. It is found that the monolayer forms a stable low-buckled hexagonal structure, which is reminiscent of silicene. The electronic transport properties of the monolayer bismuth are then evaluated by using Boltzmann theory with the relaxation time approximation. By fitting first-principles total energy calculations, a modified Morse potential is constructed, which is used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. The room temperature ZT value of a monolayer bismuth is estimated to be 2.1 and 2.4 for the n- and p-type doping, respectively. Moreover, the temperature dependence of ZT is investigated and a maximum value of 4.1 can be achieved at 500 K.

The structural and electronic properties of a series of stoichiometric Bi2Te3 nanowires with two growth orientations [110] and [210] are studied by using density functional calculations. Our results indicate that the nanowires with [110] orientation are energetically more favorable than those with [210] orientation. All the investigated Bi2Te3 nanowires are found to be semiconducting and the band gaps of [110] nanowires monotonically increase with the decreasing cross-sectional width. For the [210] orientation, however, the band gaps exhibit an interesting width-dependent even–odd oscillation behavior. The electronic transport properties of these nanowires are then evaluated by using the semi-classical Boltzmann theory with the relaxation time approximation. For the phonon transport, the lattice thermal conductivity is predicted by using the non-equilibrium molecule dynamics simulations. Our theoretical calculations suggest that the thermoelectric performance of Bi2Te3 nanowires can be optimized at appropriate carrier concentration with particular orientation and cross-sectional size. The figure of merit (ZT value) can reach as high as 2.3 at 300 K and 2.5 at 350 K for the [210] nanowire with the width N = 5.

The room temperature thermoelectric properties of three kinds of small diameter carbon nanowires are investigated by using nonequilibrium Green’s function method and molecular dynamics simulations. Due to very low thermal conductance and a relatively high power factor, these nanowires are found to exhibit better thermoelectric performance than other low-dimensional carbon-based materials such as carbon nanotubes. Moreover, the ZT values of these systems can be further increased to about 10 by partial passivation of hydrogen, which greatly reduces both the electron and phonon contributions to the thermal conductance, but leaves the power factor less affected.

The structural, electronic and magnetic properties of BiSb nanoribbons (BSNRs) with different widths and edge configurations are investigated via the first-principles pseudopotential method. It is found that the pristine BSNRs with armchair edges (ABSNRs) are semiconductors and the band gaps exhibit a width dependent odd–even oscillation. In contrast, the pristine BSNRs with zigzag edges (ZBSNRs) are found to be metallic. When all the edge atoms are passivated by hydrogen, both the ABSNRs and ZBSNRs become semiconducting and the corresponding band gaps decrease monotonically with the increasing width. If, however, the edge atoms are partially passivated, the ABSNRs can be either semiconducting or metallic. Moreover, local magnetism appears when all the edge Sb atoms are passivated and there are one or more unsaturated Bi atoms. Using the nonequilibrium Green's function (NEGF) approach, we find that all the investigated odd-numbered ABSNRs have almost the same peak value of the power factor around the Fermi level. This is not the case for the even-numbered ABSNRs, where the peaks are twice that of when they are n-type doped. Our calculations indicate that BSNRs can have a very high room temperature figure of merit (ZT value), which makes them very promising candidates for thermoelectric applications.

Using the nonequilibrium Green's function method and nonequilibrium molecular dynamics simulations, we discuss the possibility of using silicene nanoribbons (SiNRs) as high performance thermoelectric materials. It is found that SiNRs are structurally stable if the edge atoms are passivated by hydrogen, and those with armchair edges usually exhibit much better thermoelectric performance than their zigzag counterparts. The room temperature ZT value of armchair SiNRs shows a width-dependent oscillating decay, while it decreases slowly with increasing ribbon width for the zigzag SiNRs. In addition, there is a strong temperature dependence of the thermoelectric performance of these SiNRs. Our theoretical calculations indicate that by optimizing the doping level and applied temperature, the ZT value of SiNRs could be enhanced to as high as 4.9 which suggests their very appealing thermoelectric applications.

The electronic transport of three kinds of ultrasmall single-wall carbon nanotubes are studied by using nonequilibrium Green’s function method. It is found that the transmission function displays a clear stepwise structure that gives the number of electron channels. The calculated power factor of these nanotubes can be optimized to much higher values in a wide temperature range. Using nonequilibrium molecule dynamics simulations, the lattice thermal conductivity of these nanotubes are predicated with quantum correction. Our calculations indicate that the (4,2) tube has relatively higher room temperature figure of merit (ZT value) compared with those of the (5,0) and (3,3) tubes. Moreover, the thermoelectric performance of these nanotubes can be greatly enhanced by surface design, formation of bundles, increasing the tube length, and so on, which significantly reduce the phonon and electron-derived thermal conductance.

Using density functional calculations, we study the structural, energetic, and electronic properties of a triplet form of (5,0) carbon nanotube. In contrast to the weak tube–tube interactions found in a bundle of large diameter nanotubes, the ultrasmall (5,0) tubes within the triplet are covalently connected and appear like a three-blade electric fan from a top view. The triplet is energetically most favorable and is the only semiconductor among all the small bundles of (5,0) tubes. Due to its unique atomic configuration, chemisorptions of hydrogen on the triplet show interesting site dependence. When the physisorptions are also included in the system, the hydrogen storage capacity can reach 10.4 wt %.

The structural, electronic, and thermoelectric properties of BiSb nanotubes are investigated using a multiscale approach which combines the first-principles pseudopotential method, semiclassic Boltzmann theory, and molecular dynamics (MD) simulations. Our calculations indicate that the gear-like nanotubes (g-NTs) are energetically more favorable than their hexagonal counterparts (h-NTs). All the g-NTs are found to be semiconducting and their electronic transport coefficients are calculated within the relaxation time approximation. At room temperature, the Seebeck coefficients exhibit obvious peaks near the band edge and their absolute values are very large and increase monotonically with increasing band gaps of the nanotubes. The MD simulations show that the investigated BiSb nanotubes have very low lattice thermal conductivity. The significantly enhanced ZT value suggests that by appropriate doping the BiSb nanotubes could be excellent candidates for the thermoelectric applications.

Using first-principles calculations, we study the structural, electronic and optical properties of a complex system where the 0.4 nm carbon nanotubes are confined inside the AlPO4-5 zeolite (AFI) channels. The interaction between the tube and AFI is described within the framework of local density approximation (LDA). Our results indicate that the AFI zeolite cannot be simply regarded as an inert template, and it has real effect on the physical properties of the carbon nanotubes grown inside.

The two-dimensional graphene-like carbon allotrope, graphyne, has been recently fabricated and exhibits many interesting electronic properties. In this work, we investigate the thermoelectric properties of γ-graphyne by performing first-principles calculations combined with Boltzmann transport theory for both electron and phonon. The carrier relaxation time is accurately evaluated from the ultra-dense electron-phonon coupling matrix elements calculated by adopting the density functional perturbation theory and Wannier interpolation, rather than the generally used deformation potential theory which only considers the electron-acoustic phonon scattering. It is found that the thermoelectric performance of γ-graphyne exhibits a strong dependence on the temperature and carrier type. At an intermediate temperature of 600 K, a maximum ZT value of 0.77 can be achieved for the n-type system and can be further enhanced to 1.4 by considering isotopic effect.The thermoelectric properties of γ-graphyne are accurately predicted by performing first-principles calculations combined with Boltzmann transport theory for both electron and phonon.

•A simple but effective Morse potential is constructed to accurately describe the interatomic interactions of CuInTe2.•The lattice thermal conductivity of CuInTe2 predicted by MD agrees well with those measured experimentally, as well as those calculated from phonon BTE.•Introducing Cd impurity or Cu vacancy can effectively reduce the lattice thermal conductivity of CuInTe2 and thus further enhance its thermoelectric performance.The lattice thermal conductivity of thermoelectric material CuInTe2 is predicted using classical molecular dynamics simulations, where a simple but effective Morse-type interatomic potential is constructed by fitting first-principles total energy calculations. In a broad temperature range from 300 to 900 K, our simulated results agree well with those measured experimentally, as well as those obtained from phonon Boltzmann transport equation. By introducing the Cd impurity or Cu vacancy, the thermal conductivity of CuInTe2 can be effectively reduced to further enhance the thermoelectric performance of this chalcopyrite compound.

The electronic and transport properties of the (10, 0) single-walled carbon nanotube are studied by performing first-principles calculations and semi-classical Boltzmann theory. It is found that the (10, 0) tube exhibits a considerably large Seebeck coefficient and electrical conductivity which are highly desirable for good thermoelectric materials. Together with the lattice thermal conductivity predicted by non-equilibrium molecular dynamics simulations, the room temperature ZT value of the (10, 0) tube is estimated to be 0.15 for p-type carriers. Moreover, the ZT value exhibits strong temperature dependence and can reach to 0.77 at 1000 K. Such a ZT value can be further enhanced to as high as 1.9 by isotopic substitution and chemisorptions of hydrogen on the tube surface.

Using the nonequilibrium Green's function method and nonequilibrium molecular dynamics simulations, we discuss the possibility of using silicene nanoribbons (SiNRs) as high performance thermoelectric materials. It is found that SiNRs are structurally stable if the edge atoms are passivated by hydrogen, and those with armchair edges usually exhibit much better thermoelectric performance than their zigzag counterparts. The room temperature ZT value of armchair SiNRs shows a width-dependent oscillating decay, while it decreases slowly with increasing ribbon width for the zigzag SiNRs. In addition, there is a strong temperature dependence of the thermoelectric performance of these SiNRs. Our theoretical calculations indicate that by optimizing the doping level and applied temperature, the ZT value of SiNRs could be enhanced to as high as 4.9 which suggests their very appealing thermoelectric applications.

The structural and electronic properties of a series of stoichiometric Bi2Te3 nanowires with two growth orientations [110] and [210] are studied by using density functional calculations. Our results indicate that the nanowires with [110] orientation are energetically more favorable than those with [210] orientation. All the investigated Bi2Te3 nanowires are found to be semiconducting and the band gaps of [110] nanowires monotonically increase with the decreasing cross-sectional width. For the [210] orientation, however, the band gaps exhibit an interesting width-dependent even–odd oscillation behavior. The electronic transport properties of these nanowires are then evaluated by using the semi-classical Boltzmann theory with the relaxation time approximation. For the phonon transport, the lattice thermal conductivity is predicted by using the non-equilibrium molecule dynamics simulations. Our theoretical calculations suggest that the thermoelectric performance of Bi2Te3 nanowires can be optimized at appropriate carrier concentration with particular orientation and cross-sectional size. The figure of merit (ZT value) can reach as high as 2.3 at 300 K and 2.5 at 350 K for the [210] nanowire with the width N = 5.

Using the first-principles pseudopotential method and Boltzmann transport theory, we give a comprehensive understanding of the electronic and phonon transport properties of the thermoelectric material BiCuSeO. By choosing an appropriate hybrid functional for the exchange–correlation energy, we find that the system is a semiconductor with a direct band gap of ∼0.8 eV, which is quite different from those obtained previously using standard functionals. Detailed analysis of a three-dimensional energy band structure indicates that there is a valley degeneracy of eight around the valence band maximum, which leads to a sharp density of states and is responsible for a large p-type Seebeck coefficient. Moreover, we find that the density of states effective mass is much larger and results in a very low hole mobility for BiCuSeO. On the other hand, we discover two flat phonon branches contributed by the Cu and Se atoms, which can effectively block heat transfer. Combined with large atomic displacement parameters of the Cu atom, we believe that the intrinsically low lattice thermal conductivity in BiCuSeO is mainly caused by the Cu atoms, instead of the prevailingly believed Bi atoms. The thermoelectric figure-of-merit is also predicted and compared with available experimental results.

Few-layer black phosphorus has recently emerged as a promising candidate for novel electronic and optoelectronic devices. Here we demonstrate by first-principles calculations and Boltzmann theory that black phosphorus could also have potential thermoelectric applications and a fair ZT value of about 1.1 can be achieved at elevated temperature. Moreover, such a value can be further increased to 5.4 by substituting the P atom with the Sb atom, giving a nominal formula of P0.75Sb0.25. Our theoretical work suggests that high thermoelectric performance can be achieved without using complicated crystal structures or seeking for low-dimensional systems.

The thermoelectric properties of the distorted bismuth(110) layer are investigated using first-principles calculations combined with the Boltzmann transport equation for both electrons and phonons. To accurately predict the electronic and transport properties, the quasiparticle corrections with the GW approximation of many-body effects have been explicitly included. It is found that a maximum ZT value of 6.4 can be achieved for n-type systems, which essentially stemmed from the weak scattering of electrons. Moreover, we demonstrate that the distorted Bi layer retains high ZT values in relatively broad regions of both temperature and carrier concentration. Our theoretical work emphasizes that the deformation potential constant characterizing the electron–phonon scattering strength is an important paradigm for searching high thermoelectric performance materials.

The electronic and transport properties of the half-Heusler compound LaPtSb are investigated by performing first-principles calculations combined with semi-classical Boltzmann theory and deformation potential theory. Compared with many typical half-Heusler compounds, LaPtSb exhibits an obviously larger power factor at room temperature, especially for the n-type system. Together with the very low lattice thermal conductivity, the thermoelectric figure of merit (ZT) of LaPtSb can be optimized to a record high value of 2.2 by fine tuning the carrier concentration.