Optical transparency in 2D ferromagnetic WSe2/1T-VSe2/WSe2multilayer with strain induced large anomalous Nernst conductivity.

Transparent two-dimensional (2D) magnetic materials may bring intriguing features and are indispensable for transparent electronics. However, it is rare to find both optical transparency and room-temperature ferromagnetism simultaneously in a single 2D material. Herein, we explore the possibility of both these features in 2D WSe2/1T-VSe2(1ML)/WSe2and WSe2/1T-VSe2(2ML)/WSe2heterostructures by taking one monolayer (1ML) and two monolayers (2ML) of 1T-VSe2using first-principles calculations. Further, we investigate anomalous Hall conductivity (AHC) and anomalous Nernst conductivity (ANC) using a maximally localized Wannier function. The WSe2/1T-VSe2(1ML)/WSe2and WSe2/1T-VSe2(2ML)/WSe2systems show Curie temperatures of 328 and 405 K. Under biaxial compressive strain, the magnetic anisotropy of both systems is switched from in-plane to out-of-plane. We find a large AHC of 1.51 e2/h and 3.10 e2/h in the electron-doped region for strained WSe2/1T-VSe2(1ML)/WSe2and WSe2/1T-VSe2(2ML)/WSe2systems. Furthermore, we obtain a giant ANC of 3.94 AK-1m-1in a hole-doped strained WSe2/1T-VSe2(2ML)/WSe2system at 100 K. Both WSe2/1T-VSe2(1ML)/WSe2and WSe2/1T-VSe2(2ML)/WSe2are optically transparent in the visible ranges with large refractive indices of 3.2-3.4. Our results may suggest that the WSe2/1T-VSe2/WSe2structure possesses multifunctional physical properties and these features can be utilized for spintronics and optoelectronics device applications such as magnetic sensors, memory devices, and transparent magneto-optic devices at room temperature.

Electric field-induced switching of anomalous Nernst conductivity in the 2D MoTe2/VSe2 heterostructure.

Generation of a transverse electric current by a longitudinal charge or heat current is receiving extensive research efforts because of its potential applications in information-processing devices. Therefore, we investigated the electric field-dependent Curie temperature, anomalous Hall conductivity (AHC), and anomalous Nernst conductivity (ANC) of the 2H-MoTe2/1T-VSe2 heterostructure. The MoTe2/VSe2 heterostructure had a Curie temperature of 270 K and the Curie temperature was substantially increased to 355 K under an electric field. We obtained the electric field-induced switching of the AHC in the electron-doped system, whereas no switching was found in the hole-doped system. Also, the electric field-dependent ANC of the MoTe2/VSe2 heterostructure was investigated. The electric field-dependence of the ANC was more prominent in the electron-doped system. We obtained a large ANC of 2.3 A K-1 m-1 when the electric field was applied from VSe2 to MoTe2 layers and this was switched to -0.6 A K-1 m-1 with an opposite electric field. This finding may indicate that the 2D MoTe2/VSe2 heterostructure can be used for potential applications in energy conversion and spintronic devices.

High thermoelectric performance in two dimensional chalcogenides systems: GaSe and GaTe.

Among the group-III chalcogenides, the two-dimensional (2D) GaSe and GaTe materials have been synthesized, but recent theoretical studies have raised controversial results regarding their thermoelectric (TE) properties. Hereby, systematically investigated the temperature and carrier concentration dependent TE properties of 2D GaSe and GaTe. We found that the GaSe had an indirect band gap of 2.94 eV while the GaTe had an indirect band gap of 1.88 eV. Both materials had almost the same Seebeck coefficients, but the p-type GaTe had the longest carrier relaxation time. We obtained the largest electrical conductivity over the thermal conductivity ratio in p-type GaTe compared with all other systems. This results in a very high p-type ZT of 0.91. Moreover, this high ZT performance is only changed by approximately 7% in a wide range of temperatures (300-700 K) and carrier concentration (1011-1013 hole cm-2). Compared with previously reported results, we find that it is necessary to consider the carrier relaxation time and spin-orbit coupling effect for determining reliable TE property. Overall, we propose that the p-type GaTe have outstanding TE property, and it can be utilized for potential TE device applications.

Electric filed dependent valley polarization in 2D WSe2/CrGeTe3 heterostructure.

Valleytronics in 2D transition metal dichalcogenides (TMDs) has received extensive interest as a promising field for information processing, storage, and logic operation applications. Here, we have investigated the electric field dependent valley polarization of 2D WSe2/CrGeTe3 heterostructure. The WSe2/CrGeTe3 system has an indirect band gap of 0.253 eV without spin-orbit coupling (SOC), and this is reduced to 21 meV with SOC. The WSe2/CrGeTe3 system has a weak perpendicular magnetic anisotropy of 0.05 meV cell-1, and the critical temperature is 38 K. The magnitude of the valley polarization is 3 meV without an electric field. For instance, we obtain the valley polarization of 9 meV if the electric field of 0.4 V Å-1 is applied from the CrGeTe3 to the WSe2, but it becomes 0.5 meV if the electric field direction is reversed even at the same intensity. We have found that the charge redistribution happens depending on the electric field direction. So, we attribute this feature to the electric field dependent valley polarization of the 2D WSe2/CrGeTe3 heterostructure.

Giant spin Seebeck effect in two-dimensional ferromagnetic CrI3 monolayer.

Spin Seebeck effect (SSE) is a key factor in the spin caloritronics field, and extensive studies have been performed for potential spin thermoelectric modulator device applications. However, the performance of spin current generation was not high enough, and this is due to the weak yield of the SSE. Despite the many studies for the SSE in bulk materials, no reports are available yet in the pure two-dimensional (2D) ferromagnetic material. Hereby, we investigated the SSE of two-dimensional ferromagnetic CrI3 using the Boltzmann transport approach allowing diffusive scattering. We obtained a giant effective spin Seebeck effect of 1450 µV K-1, and this value is at least 4-5 times larger than previously reported values in bulk systems. Therefore, our finding may suggest that 2D CrI3 can be a potential material to open another perspective for 2D materials in the spin caloritronics field.

Anomalous in-plane lattice thermal conductivity in an atomically thin two-dimensional α-GeTe layer.

Low lattice thermal conductivity is one of the most important physical quantities for phononic device applications. Thus, we investigated the in-plane lattice thermal conductivity of mono- and bi-layer α-GeTe systems. The lattice thermal conductivity of the monolayer system along the zigzag direction was 0.43 (W mK-1) while it was 0.21 (W mK-1) along the armchair direction at 300 K, and the lattice thermal conductivity mostly originated from the out-of-plane acoustic mode. In the bilayer system, it was significantly suppressed to 0.044 (W mK-1) and 0.047 (W mK-1) along the zigzag and armchair directions, respectively, at 300 K. Particularly, the out-of-plane acoustic mode in the bilayer had a tremendous Grüneisen parameter and this led to an ultralow in-plane lattice thermal conductivity in the bilayer, and the optical mode dominated the contribution to the lattice thermal conductivity. Our findings may raise intriguing issues regarding the thermoelectric effect, heat insulators, and phononic device applications, and stimulate further experimental studies to verify our theoretical predictions.

First-Principles Investigation of Simultaneous Thermoelectric Power Generation and Active Cooling in a Bifunctional Semimetal ZrSeTe Janus Structure.

Traditional thermoelectric materials often face a trade-off between efficient power generation (high ZT) and cooling performance. Here, we explore the potential of achieving simultaneous thermoelectric power generation and cooling capability in the recently fabricated bulk ZrSeTe Janus structure using first-principles density functional theory (DFT). The layered ZrSeTe Janus structure exhibits a semimetal character with anisotropic transport properties along the in-plane and out-of-plane directions. Our DFT calculations, including the explicit calculation of relaxation time, reveal a maximum ZT of ~0.065 in the out-of-plane direction at 300 K which is one order of magnitude larger than that in the in-plane direction (ZT~0.006). Furthermore, the thermoelectric cooling performance is also investigated. The in-plane direction shows a cooling performance of 13 W/m·K and a coefficient of performance (COPmax) of ~90 with a temperature difference (ΔT) of 30 K, while the out-of-plane direction has a cooling performance of 2.5 W/m·K and COPmax of ~2.5. Thus, the out-of-plane current from the thermoelectric power generation can be utilized as an in-plane current source for active heat pumping. Consequently, we propose that the semimetal ZrSeTe Janus structure can display bifunctional thermoelectric properties for simultaneous thermoelectric power generation and active cooling.

High Thermoelectric Performance in Hexagonal 2D PdTe2 Monolayer at Room Temperature.

Motivated by the recent fabrication of the hexagonal PdTe2 monolayer, we investigated the thermoelectric properties of the hexagonal and pentagonal PdTe2 structures using two approaches. The pentagonal monolayer has not been synthesized yet. The hexagonal layer had an indirect band gap of 0.17 eV while the pentagonal structure had an indirect band gap of 1.18 eV. By applying the semiempirical Wiedemann-Franz law to calculate the electronic thermal conductivity, we found that both hexagonal and pentagonal structures had a very high ZT, more than 3. However, the Wiedemann-Franz law underestimated the electronic thermal conductivity, and this resulted in a high ZT. Thus, we employed the Boltzmann transport equation for the electronic thermal conductivity. At high temperature (>500 K), the pentagonal PdTe2 structure showed a better thermoelectric performance than the hexagonal structure. However, both structures displayed the same ZT of 0.8 at 300 K. We propose that the hexagonal PdTe2 can be a potential high performance thermoelectric material at room temperature.