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Recently, the experimentally synthesized Nb

Recently, the experimentally synthesized Nb2SiTe4 was found to be a stable layered narrow-gap semiconductor, and the fabricated field-effect transistors (FETs) based on few-layers Nb2SiTe4 are good candidates for ambipolar devices and mid-infrared detection (Zhao et al 2019 ACS Nano 13 10705–10). Here, we use first-principles combined with Boltzmann transport theory and non-equilibrium Green’s function method to investigate the thermoelectric transport coefficients of monolayer Nb2XTe4 (X = Si, Ge) and the gate voltage effect on the thermoelectric performance of the FET based on monolayer Nb2SiTe4. It is found that both monolayers have large p-type Seebeck coefficients due to the ‘pudding-mold-type’ valence band structure, and they both exhibit anisotropic thermoelectric behavior with optimal thermoelectric figure of merit of 1.4 (2.2) at 300 K and 2.8 (2.5) at 500 K for Nb2SiTe4 (Nb2GeTe4). The gate voltage can effectively increase the thermoelectric performance for the Nb2SiTe4-based FET. The high thermoelectric figure of merit can be maintained in a wide temperature range under a negative gate voltage. Furthermore, the FET exhibits a good gate-tunable Seebeck diode effect. The present work suggests that Nb2XTe4 monolayers are promising candidates for 2D thermoelectric materials and thermoelectric devices.

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI

Monolayer transition-metal dichalcogenides (TMDCs) MX2 (M = Mo, W, Zr, Hf, etc; X = S, Se, Te) have become well-known in recent times for their promising applications in thermoelectrics and field effect transistors. In this work, we perform a systematic study on the thermoelectric properties of monolayer ZrSe2 and HfSe2 using first-principles calculations combined with Boltzmann transport equations. Our results point to a competitive thermoelectric figure of merit (close to 1 at optimal doping) in both monolayer ZrSe2 and HfSe2, which is markedly higher than previous explored monolayer TMDCs such as MoS2 and MoSe2. We also reveal that the higher figure of merits arise mainly from their low lattice thermal conductivity, and this is partly due to the strong coupling of acoustic modes with low frequency optical modes. It is found that the figure of merits can be better optimized in n-type than in p-type. In particular, the performance of HfSe2 is superior to ZrSe2 at a higher temperature. Our results suggest that monolayer ZrSe2 and HfSe2 with lower lattice thermal conductivity than usual monolayer TMDCs are promising candidates for thermoelectric applications.

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI2-type) structural TiS2 monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS2 monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.

The half-metallic behavior of Au-V(Cr) quantum wires adsorbed on an armchair (5, 5) boron nitride nanotube is obtained by performing spin-polarized density functional calculations. The density of states shows a metallic property at the Fermi level for the majority spin channel and a semiconductor gap in the minority spin channel. The half-metallic behavior of the quantum wire/nanotube complex originates from the half-metallic behavior of the free-standing Au-V(Cr) quantum wires. The calculations of spin-polarized transport indicate that such a one-dimensional half-metallic magnet can be used as a spin filter.

The spin-polarized transport properties of a high-spin-state spin-crossover molecular junction with zigzag-edge graphene nanoribbon electrodes have been studied using density functional theory combined with the nonequilibrium Green's-function formalism. The molecular junction presents integrated spintronic functionalities such as negative differential resistance behavior, spin filter and the spin rectifying effect, associated with the giant magnetoresistance effect by tuning the external magnetic field. Furthermore, the transport properties are almost unaffected by the electrode temperature. The microscopic mechanism of these functionalities is discussed. These results represent a step toward multifunctional molecular spintronic devices on the level of the individual spin-crossover molecule.