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.