Half-metallic band structure and spin transport characteristics of a 1T-FeCl2 monolayer.

Recent experiments indicated that both layered Bi 2 O 2 Se and Bi 2 O 2 Te are promising thermoelectric materials with low thermal conductivities. However, theoretical study on the thermoelectric properties, especially the phonon transport properties, is rare. In order to understand the thermoelectric transport mechanism, we here investigate the electron and phonon transport properties by using the first-principles calculations combined with the Boltzmann transport theory.Our results indicate that both Bi 2 O 2 Se and Bi 2 O 2 Te are semiconductors with indirect energy gaps of 0.87 eV and 0.21 eV within spin-orbit coupling, respectively. Large Seebeck coefficient and power factor are found in the p-type than the n-type for both compounds. Low lattice thermal conductivities at room temperature are obtained, 1.14 W m −1 K −1 for Bi 2 O 2 Se and 0.58 W m −1 K −1 for Bi 2 O 2 Te, which are close to the experimental values. It is found that the low-frequency optical phonon branches with higher group velocity and longer lifetime also make a main contribution to the lattice thermal conductivity. Interestingly, the lattice thermal conductivity exhibits obvious anisotropy especially for Bi 2 O 2 Te. These results are helpful for the understanding and optimization of thermoelectric performance of layered Bi 2 O 2 Se and Bi 2 O 2 Te.

Atomically thin 2D magnetic materials have attracted increasing interest due to their promising applications in spintronic devices. Although more and more 2D intrinsic ferromagnetic materials were theoretically predicted, few of...

Two-dimensional intrinsic ferromagnets with high spin polarization and high Curie temperature are rare and very important for developing novel nanoscale spintronic materials and devices. In this work, by using the first-principles calculations, we confirm that Mn 2 C 6 Se 12 and Mn 2 C 6 S 6 Se 6 monolayers are perfect and nearly Dirac spin gapless semiconductors with 100% spin polarization, high Fermi velocities, high Curie temperatures, and large magnetic anisotropic energies. Within the spin−orbit coupling, the Mn 2 C 6 Se 12 monolayer has nontrivial topological properties with a nonzero Chern number, which is confirmed by the calculated Berry curvature, anomalous Hall conductance, and chiral edge states. We also reveal from the first-principles combined with the nonequilibrium Green's function method that both monolayers exhibit perfect spin transport properties such as the spin-filtering effect, the negative differential resistance effect, and high magnetoresistance. These results suggest that Mn 2 C 6 Se 12 and Mn 2 C 6 S 6 Se 6 monolayers are promising candidates for the realization of the quantum anomalous Hall effect and the 2D spintronic devices.

Leveraging the unique physical properties, two-dimensional (2D) materials have circumvented the disadvantages of conventional epitaxial semiconductors and held great promise for potential optoelectronic applications. So far, two main detector architectures including photodiode based on a van der Waals P-N junction or Schottky junction and phototransistor based on individual 2D materials or hybrids have been well developed. However, a trade-off between responsivity and speed always exists in those technologies thus hindering the overall performance improvement. Here, we propose a new device concept by sandwiching the 2D anisotropic semimetal between p-type and n-type semiconductors in the out-of-plane direction, called PSN architecture, realizing the improvement of each parameter including broad spectral coverage, fast speed, high sensitivity, power-free and polarization-sensitive. We stack the p-type 2H-MoTe 2 , Weyl semimetal 1T-MoTe 2 and n-type SnSe 2 layer-by-layer constructing vertical sandwich structure where the top and bottom layers contribute to the internal built-in electric field, the intermediate layer can facilitate the exciton dissociation and act as infrared polarized light sensitizers. As a result, this PSN device exhibits broadband photo-response from 405 to 1,550 nm without external bias supply. At optical communication band (1,310 nm), operating at self-driven mode and room temperature, the responsivity and detectivity can reach up to 64.2 mA•W -1 and 2.2×10 11 Jones, respectively, along with fast speed on the order of millisecond. Moreover, the device simultaneously exhibits exceptional detection capability for infrared polarized light, demonstrating the anisotropic photocurrent ratio of 1.55 at 1,310 nm and 2.02 at 1,550 nm, which is attributed to the strong in-plane optical anisotropy of middle 1T-MoTe 2 layer. This work develops a new photodetector scheme with novel PSN architecture toward broadband, self-power, polarized light sensing and imaging modules.

Although 2D phosphorene has exhibited promising applications in electronic and optoelectronic devices with high carrier mobility and high on/off ratio, the nonmagnetism in pristine phosphorene hinders its potential applications in spintronic devices. In this work, using first-principles, Monte Carlo simulations, and the nonequilibrium Green’s function method, we investigate the structural, magnetic, and electronic properties as well as the spin transport and spin thermoelectric transport properties for Mn-doped monolayer blue phosphoren (Blue-P). We find that Mn-doped Blue-P is structurally stable and thermally stable, and it exhibits ferromagnetism above room temperature. Interestingly, the doped system is a half-metal with complete spin polarization of electrons around the Fermi level, which is robust with respect to the doping concentration. The spin transport properties indicate an excellent spin-filtering effect and a high magnetoresistance ratio (up to 105%), which are explained from the calculated bias-voltage- and spin-dependent band structures and the spin-dependent transmission eigenstates and pathways. In addition, the spin thermoelectric transport properties show a thermal spin filtering effect, a large spin Seebeck coefficient, and a low thermal conductivity. A high spin thermoelectric figure of merit accompanied by a small charge thermoelectric figure of merit can be obtained by adjusting the chemical potential and the temperature. These results suggest that 2D Mn-doped Blue-P is a promising candidate for versatile spintronic and spin caloritronic applications.

Future spintronic devices on the nanoscale require low-dimensional materials with high spin polarization. Transition-metal trichlorides have received much attention because 2D ferromagnetism is observed in them such as the ferromagnetic semiconductor of CrI3 monolayer and the ferromagnetic Dirac spin gapless semiconductor of VCl3 monolayer with 100% spin polarization. What about their spin transport properties? Here, we design the magnetic tunnel junction of VCl3/CoBr3/VCl3 with the electrode of the spin gapless semiconductor of VCl3 monolayer and the tunneling barrier of the nonmagnetic semiconductor of CoBr3 monolayer and explore the spin-polarized bias-voltage-dependent tunneling current. Our first-principles calculations combined with nonequilibrium Green's function indicate that VCl3/CoBr3/VCl3 exhibits a high tunnel magnetoresistance ratio (up to 4.5 × 1012%) and a perfect spin filtering effect, which make the VCl3 monolayer useful in 2D spintronic devices. The physical origins of these versatile spin transport properties are discussed in terms of the spin gapless semiconductor property of the VCl3 monolayer and the spin-dependent transmission spectrum.

Recently, a new two-dimensional (2D) semiconductor SnSe2 monolayer has been grown by molecular beam epitaxy, and weak ferromagnetic behavior above room temperature in Mn-doped SnSe2 thin films was also observed experimentally.