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Two-dimensional (2D) materials may have potential applications in thermoelectric devices. In this work, the thermoelectric properties of orthorhombic group IV-VI monolayers AB (A=Ge and Sn; B=S and Se) are systematically investigated by the first-principles calculations and semiclassical Boltzmann transport theory. The spin-orbit coupling (SOC) is considered for their electron part, which produces observable effects on the power factor, especially for n-type doping. According to the calculated ZT, the four monolayers exhibit diverse anisotropic thermoelectric properties although they have a similar hinge-like crystal structure. The GeS along zigzag and armchair directions shows the strongest anisotropy, while SnS and SnSe show mostly isotropic efficiency of thermoelectric conversion. This can be explained by the strength of anisotropy of their respective power factor and electronic and lattice thermal conductivities. The calculated results show that the ZT between n- and p-type doping has little difference for GeS, SnS, and SnSe. It is found that GeSe, SnS, and SnSe show better thermoelectric performance compared to GeS in n-type doping and that SnS and SnSe exhibit higher efficiency of thermoelectric conversion in p-type doping. Compared to other many 2D materials, orthorhombic group IV-VI monolayers AB (A=Ge and Sn; B=S and Se) may possess better thermoelectric performance due to lower lattice thermal conductivities. Our work would be beneficial to stimulate further theoretical and experimental works. [ABSTRACT FROM AUTHOR]

Phonon transport of recently fabricated Na2He at high pressure is investigated from a combination of first-principles calculations and the linearized phonon Boltzmann equation within the single-mode relaxation time approximation. The calculated room-temperature lattice thermal conductivity s 149.19W m-1 K-1, which is very close to that of Si. It is found that low-frequency optical modes comprise 16% of the lattice thermal conductivity, while high-frequency optical modes have negli- gible contribution. The high lattice thermal conductivity is due to large group velocities, small Grüneisen parameters, and long phonon lifetimes. The size effects on lattice thermal conductivity are considered by cumulative thermal conductivity with respect to the phonon mean free path. To significantly reduce the lattice thermal conductivity, the characteristic length smaller than 100 nm is required and can reach a decrease of 36%. These results may be useful to understand thermal transport processes that occur inside giant planets. [ABSTRACT FROM AUTHOR]

We investigate the electronic structures and optical dielectric functions of the high temperature phase of Sr2 CrOsO6 with cubic structure by using Tran and Blaha's modified Becke and Johnson exchange potential. In the absence of spin-orbit coupling, the total spin moment is 0μB. When spin-orbit coupling is included, the small total spin moment and an unquenched Os orbital moment appear, and the spin non-conservation gap becomes smaller. The calculated net magnetic moment is smaller than the popular generalized gradient approximation result, and the spin non-conservation gap is larger. The optical dielectric functions with spin-orbit coupling are not very different from the ones without spin-orbit coupling. [ABSTRACT FROM AUTHOR]

Tuning the physical properties of transition metal dichalcogenide (TMD) monolayers by strain engineering is an area that has been widely studied, and recently a Janus TMD monolayer MoSSe has been synthesized. In this work, we systematically study the biaxial strain dependence of the electronic structures and transport properties of Janus TMD MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer by using generalized gradient approximation (GGA) and spin–orbit coupling (SOC). It is found that SOC has a noteworthy detrimental influence on power factor in p-type MoSSe, WSSe, n-type WSTe, p-type MoSeTe and WSeTe, and has a negligible influence on n-type MoSSe, MoSTe, p-type WSTe and n-type MoSeTe. This can be understood by considering SOC effects on their valence and conduction bands. For all six monolayers, the energy band gap firstly increases, and then decreases, when strain changes from a compressive one to a tensile one. It is found that strain can tune the strength of band convergence of both valence and conduction bands by changing the numbers and relative position of valence band extrema (VBE) or conduction band extrema (CBE), which can produce very important effects on their electronic transport properties. By applying appropriate compressive or tensile strain, both n- or p-type Seebeck coefficient can be enhanced by strain-induced band convergence, and then the power factor can be improved. Our works further enrich studies on the strain dependence of electronic structures and the transport properties of new-style TMD monolayers, and will motivate further experimental works. [ABSTRACT FROM AUTHOR]

Various two-dimensional (2D) materials with a graphene-like buckled structure have emerged, and the β-phase AsP monolayer has been recently proposed to be thermodynamically stable from first-principles calculations. The studies of thermal transport are very useful for these 2D materials-based nano-electronics devices. Motivated by this, a comparative study of strain-dependent phonon transport of AsP monolayers is performed by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). It is found that the lattice thermal conductivity () of the AsP monolayer is very close to the one of As monolayer with a similar buckled structure, which is due to neutralization between the reduction of phonon lifetimes and group velocity enhancement from As to AsP monolayer. The corresponding room-temperature sheet thermal conductance of AsP monolayer is 152.5 . It is noted that the increasing tensile strain can harden a long wavelength out-of-plane (ZA) acoustic mode, and soften the in-plane longitudinal acoustic (LA) and transversal acoustic (TA) modes. Calculated results show that of AsP monolayer presents a nonmonotonic up-and-down behavior with increased strain. The unusual strain dependence is due to the competition among the reduction of phonon group velocities, improved phonon lifetimes of ZA mode and nonmonotonic up-and-down phonon lifetimes of TA/LA mode. It is found that acoustic branches dominate the in the considered strain range, and the contribution from ZA branch increases with increased strain, while it is opposite for TA/LA branch. By analyzing cumulative with respect to phonon mean free path, tensile strain can modulate effectively the size effects on in the AsP monolayer. Our work enriches the studies of thermal transports of 2D materials with graphene-like buckled structures, and strengthens the idea to engineer thermal transport properties by simple mechanical strain, and stimulates further experimental works to synthesize AsP monolayers. [ABSTRACT FROM AUTHOR]

Lots of two-dimensional (2D) materials have been predicted theoretically and further confirmed in experiments, and have wide applications in nanoscale electronic, optoelectronic and thermoelectric devices. In this work, the thermoelectric properties of ATeI (A = Sb and Bi) monolayers are systematically investigated according to semiclassical Boltzmann transport theory. It is found that spin–orbit coupling (SOC) has an important effect on the electronic transport coefficients of p-type doping, but a negative influence on n-type doping. The room-temperature sheet thermal conductance is 14.2 for SbTeI and 12.6 for BiTeI, which is lower than that of most well-known 2D materials, such as the transition-metal dichalcogenide, group IV–VI, group VA and group IV monolayers. The very low sheet thermal conductance of ATeI (A = Sb and Bi) monolayers is mainly due to their small group velocities and short phonon lifetimes. The strongly polarized covalent bonds between A and Te or I atoms induce strong phonon anharmonicity, which gives rise to low lattice thermal conductivity. It is found that the high-frequency optical branches contribute significantly to the total thermal conductivity, which is obviously different from the usual picture, where there is little contribution from the optical branches. According to cumulative lattice thermal conductivity with respect to the phonon mean free path (MFP), it is difficult to further reduce the lattice thermal conductivity using nanostructures. Finally, the possible thermoelectric figure of merit ZT values of the ATeI (A = Sb and Bi) monolayers are calculated. It is found that p-type doping has much better thermoelectric properties than n-type doping. At room temperature, the peak ZT can reach 1.11 for SbTeI and 0.87 for BiTeI, respectively. These results make us believe that ATeI (A = Sb and Bi) monolayers may be potential 2D thermoelectric materials, which could stimulate further experimental work towards the synthesis of these monolayers. [ABSTRACT FROM AUTHOR]

Biaxial strain effects on the electronic structures and thermoelectric properties of monolayer are investigated by using generalized gradient approximation (GGA) plus spin–orbit coupling for the electron part and GGA for the phonon part. Calculated results show that a small compressive strain (about −3%) can induce semiconductor-to-metal transition, which can easily be achieved in experiment. Band convergence in the conduction bands is observed for unstrained , which can be removed by both compressive and tensile strain. Tensile strain can give rise to band convergence in the valence bands by changing the position of the valence band maximum, which can induce an enhanced Seebeck coefficient, and bring about high power factors. It is found that tensile strain can also reduce lattice thermal conductivity. More specifically, the lattice thermal conductivity at a strain of 4% can decrease by about 19% compared to the unstrained case at room temperature. According to the tensile strain effects on ZTe and lattice thermal conductivity, tensile strain can indeed improve the p-type efficiency of thermoelectric conversion. Our results demonstrate the potential of strain engineering in for applications in electronics and thermoelectricity. [ABSTRACT FROM AUTHOR]

Recently, has been predicted to be a strong topological insulator by the first-principle calculations. It is well known that topological insulators have a close connection to thermoelectric materials, such as the family. In this work, we investigate thermoelectric properties of by the first-principles calculations combined with the Boltzmann transport theory. The electronic part is carried out by a modified Becke and Johnson (mBJ) exchange potential, including spin–orbit coupling (SOC), while the phonon part is performed using a generalized gradient approximation (GGA). It was found that the electronic transport coefficients between the in-plane and cross-plane directions showed strong anisotropy, while lattice–lattice thermal conductivities demonstrated almost complete isotropy. Calculated results revealed a very low lattice thermal conductivity for , and the corresponding average lattice thermal conductivity at room temperature is 1.69 , which is comparable or lower than those of lead chalcogenides and bismuth-tellurium systems as classic thermoelectric materials. Due to the complicated scattering mechanism, calculating the scattering time τ is challenging. By using an empirical s, the n-type figure of merit ZT is greater than 0.40 in wide temperature ranges. Experimentally, it is possible to attain better thermoelectric performance by strain or tuning size parameters. This work indicates that may be a potential thermoelectric material, which can stimulate further theoretical and experimental work. [ABSTRACT FROM AUTHOR]

The electronic structures and thermoelectric properties of semiconducting transition-metal dichalcogenide monolayers (M = Zr, Hf, Mo, W and Pt; X = S, Se and Te) are investigated by combining first-principles and Boltzmann transport theory, including spin–orbital coupling (SOC). It is found that the gap decrease increases from S to Te in each cation group when the SOC is opened. The spin–orbital splitting has the same trend with the gap reducing. The calculated results show that SOC has a noteworthy detrimental effect on the p-type power factor, while it has a negligible influence in n-type doping except for the W cation group, which can be understood by considering the effects of SOC on the valence and conduction bands. For (X = S, Se and Te), SOC leads to an observable enhanced power factor in n-type doping, which can be explained by SOC-induced band degeneracy, namely the bands converge. Among all of the cation groups, the Pt cation group shows the highest Seebeck coefficient, which leads to the best power factor, if we assume that the scattering time is fixed. The calculated results show that (M = Zr, Hf, Mo, W and Pt) have the best p-type power factor of all the cation groups, and that (M = Zr and Hf), and (M = Mo and Pt) have a more excellent n-type power factor in their respective cation group. Therefore, these results may be useful for further theoretical prediction or experimental research of excellent thermoelectric materials from semiconducting transition-metal dichalcogenide monolayers. [ABSTRACT FROM AUTHOR]

We investigate pressure dependence of electronic structures and thermoelectric properties in BiTeI by using a modified Becke and Johnson exchange potential. Spin–orbit coupling (SOC) effects are also included due to giant Rashba splitting. Thermoelectric properties are illuminated through solving Boltzmann transport equations within the constant scattering time approximation. The calculated energy band gap of 0.36 eV agrees well with the experimental value of 0.38 eV. As the pressure increases, the energy band gap first decreases, and then increases. The Rashba energy has the opposite trend with the energy band gap. SOC has obvious detrimental influence on the power factor in both n-type and p-type doping. For low doping concentration, the power factor has the same trend with the energy band gap with increasing pressure, but shows a monotonic changing trend in high doping. It is found that the pressure can induce a significantly enhanced power factor in high n-type doping, which can be understood as pressure leading to two-dimensional-like density of states in the conduction bands. These results suggest that BiTeI may be a potential candidate for efficient thermoelectricity in n-type doping by pressure, turning an ordinary insulator into a topological insulator. [ABSTRACT FROM AUTHOR]

We investigate magnetic ordering and electronic structures of Cr2MoO6 under hydrostatic pressure. To overcome the band gap problem, the modified Becke and Johnson exchange potential is used to investigate the electronic structures of Cr2MoO6. The insulating nature at the experimental crystal structure is produced, with a band gap of 1.04 eV, and the magnetic moment of the Cr atom is 2.50 μB, compared to an experimental value of about 2.47 μB. The calculated results show that an antiferromagnetic inter-bilayer coupling–ferromagnetic intra-bilayer coupling to a ferromagnetic inter-bilayer coupling–antiferromagnetic intra-bilayer coupling phase transition is produced with the pressure increasing. The magnetic phase transition is simultaneously accompanied by a semiconductor–metal phase transition. The magnetic phase transition can be explained by the Mo–O hybridization strength, and ferromagnetic coupling between two Cr atoms can be understood by empty Mo-d bands perturbing the nearest O-p orbital. [ABSTRACT FROM AUTHOR]

The ferromagnetic insulator plays a major role in high-performance spintronic applications. So, it is very necessary to search for better ferromagnetic insulators that are compatible with current semiconductor technology. We investigate the electronic structures and magnetic properties of the superlattice by using Tran and Blaha’s modified Becke and Johnson exchange potential. The calculated results show that the superlattice is a ferromagnetic insulator, which combines ferromagnetic properties of with insulating properties of . The ferromagnetic stability is proven by magnetic energy differences for a series of lattice constants a. It is very interesting that both the majority-spin channel and spin non-conservation gaps firstly decrease with increasing lattice constants a, and then increase. However, the minority-spin channel gap firstly decreases, then remains nearly steady. These trends of gaps can be understood by changes of electronic band structures. If the superlattice was experimentally synthesized with ferromagnetic insulator properties, it should be used to design high-performance spintronic devices, achieving new functionality. [ABSTRACT FROM AUTHOR]

We investigate the energy band gap, dielectric functions and thermoelectric properties of Mg2Sn at hydrostatic pressure by using a modified Becke and Johnson exchange potential. It is very interesting that the energy band gap first increases with increasing pressure, and then decreases. The phonon calculations prove that no structural phase transition under the considered pressure is produced. When the pressure reaches 5.6 GPa, the energy band gap attains the biggest value, which is also a critical pressure with the Mg s-character near the high symmetry X-point transforming from the first conduction band to the second one. When the pressure increases to 50.7 GPa, the energy band gap closes, leading to a semiconductor-to-semimetal transition. As the pressure increases, the main peaks of the real and imaginary part of the dielectric functions of Mg2Sn move toward the high-energy region. The Seebeck coefficient and power factor for p-type doping change little with increasing pressure, but for n-type they vary greatly. The change trend of the Seebeck coefficient and power factor for n-type doping as a function of pressure is conic, whose critical pressure is just 5.6 GPa. [ABSTRACT FROM AUTHOR]