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.

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.

Temperature dependence of semi-hard iron silicide rare-earth free magnet.

Searching for rare-earth free permanent magnet attracts extensive research interests due to diverse technological applications and other subtle issues. Here, the temperature dependent magnetic properties of Fe5SiC structure are explored. The Fe5SiC has a critical temperature of 710 K with perpendicular magnetic anisotropy. The magnetic anisotropy constant and coercive field are monotonically decreased with increasing temperature. For instance, the magnetic anisotropy constant is 0.42 MJ m-3at zero Kelvin and reduced to 0.24 MJ m-3and 0.06 MJ m-3at 300 K and 600 K. The coercive field becomes 0.7 T at 0 K. With increasing the temperatures, it is suppressed to 0.42 T and 0.20 T at 300 K and 600 K, respectively. Overall, the Fe5SiC system has a (BH)maxof 417 kJ m-3at zero Kelvin. The (BH)maxis decreased at high temperature. Nonetheless, we obtain the (BH)maxof 234 kJ m-3at 300 K. Since the Fe5SiC shows better permanent magnetic (PM) property than the conventional ferrites and also CeCo5. This finding may indicate that the Fe5SiC can be a potential candidate as a Fe-based gap PM between ferrite and Nd-Fe-B (or Sm-Co) at room temperature.

Enhanced Curie temperature in partially decorated CrSnSe3 monolayer with alkali metals (Li, Na, and K).

A two-dimensional ferromagnetic layer with a large Curie temperature is highly desired for spintronics applications. Herein, we investigated the effect of the partial decoration of the CrSnSe3 monolayer with alkali metals Li, Na, and K on the structure, electronic and magnetic properties. The calculated formation energy, phonon dispersion curves, and ab initio molecular dynamics indicated that the decorated CrSnSe3 layers are stable and can be fabricated. The Li, Na, and K decorated systems display semiconducting band features, with bandgaps of 0.53, 0.55, and 0.55 eV, respectively, with the HSE06 hybrid functional. We found a ferromagnetic ground state and an in-plane magnetic anisotropy of -2.12, -2.42, and -2.39 meV per cell in the Li, Na, and K-decorated systems, respectively. Based on Monte Carlo Simulations, we obtained largely enhanced Curie temperatures of 241, 256, and 265 K in the Li, Na, and K decorated systems, respectively. Our findings suggest that the decorated layers could be used as potential candidates for spintronics applications.

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.

Strain tuning of the Curie temperature and valley polarization in two dimensional ferromagnetic WSe2/CrSnSe3heterostructure.

Magnetic proximity effect can be used to tailor the magnetic ground state and valley polarization in the monolayer of transition metal dichalcogenides. Thus, we explore the effect of biaxial tensile and compressive strain on valley polarization in the WSe2/CrSnSe3heterostructures with different stacking orders systematically. The indirect band gaps in the two most stable stackings; hollow (0.27 eV) and top (0.33 eV) were further enhanced to 0.35 eV under tensile strain while suppressed to almost 0.13 eV under compressive strain. The heterostructures had a FM ground state with a total magnetic moment per unit cell of 6µBin pristine as well as strained structures. In hollow stacking and compressively strained structures, we obtained a perpendicular magnetocrystalline anisotropy, while the top stacking and tensile strain structures had small in-plane anisotropy. An enhancement was found in Curie temperature from 73 K in pristine to 128 K in a 6% tensile strained structure. The valley splittings found in pristine hollow (4 meV) and top (9 meV) stacked heterostructures were further enhanced to 29 meV and 22 meV at 5% compressive strain respectively. This enhancement was attributed to the increased spatial dependence of the charge density along K+and K-directions of the Brillouin zone, which give rise to the different local dipolar fields at these valleys. Our results suggest that strain could be an effective way to control or tune the valley splitting in WSe2/CrSnSe3heterostructures.

Determination of Dy substitution site in Nd2-xDyxFe14B by HAADF-STEM and illustration of magnetic anisotropy of "g" and "f" sites, before and after substitution.

Nd2Fe14B and Nd2-xDyxFe14B (x = 0.25, 0.50) particles were prepared by the modified co-precipitation followed by reduction-diffusion process. Bright field scanning transmission electron microscope (BF-STEM) image revealed the formation of Nd-Fe-B trigonal prisms in [- 101] viewing zone axis, confirming the formation of Nd2Fe14B/Nd2-xDyxFe14B. Accurate site for the Dy substitution in Nd2Fe14B crystal structure was determined as "f" site by using high-angle annular dark field scanning transmission electron microscope (HAADF-STEM). It was found that all the "g" sites are occupied by the Nd, meanwhile Dy occupied only the "f" site. Anti-ferromagnetic coupling at "f" site decreased the magnetic moment values for Nd1.75Dy0.25Fe14B (23.48 µB) and Nd1.5Dy0.5Fe14B (21.03 µB) as compared to Nd2Fe14B (25.50 µB). Reduction of magnetic moment increased the squareness ratio, coercivity and energy product. Analysis of magnetic anisotropy at constant magnetic field confirmed that "f" site substitution did not change the patterns of the anisotropy. Furthermore, magnetic moment of Nd2Fe14B, Nd2-xDyxFe14B, Nd ("f" site), Nd ("g" site) and Dy ("f" site) was recorded for all angles between 0° and 180°.

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.

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.

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.

Magnetic anisotropy and Curie temperature of two-dimensional VI3 monolayer.

Recently, it was reported that the VI3 had a Mott insulator nature and also displayed the structural and magnetic phase transition at low temperature. Here, we explored the magnetic properties of the two-dimensional (2D) monolayer structure using the density functional theory. We found that the 2D VI3 had an enhanced lattice constant compared with that in the bulk structure. Besides, the 2D monolayer had an indirect band gap of 0.98 eV, and this band gap was increased (decreased) with tensile (compressive) strain up to ±3%. The monolayer structure had a ferromagnetic ground state and this nature was preserved under both tensile and compressive strains. We obtained that the monolayer structure had a perpendicular magnetic anisotropy energy of 0.29 meV/cell. The perpendicular magnetic anisotropy still remained even after applying the tensile and compressive strains although the magnitude of magnetic anisotropy was slightly changed. Using the Metropolis Monte Carlo simulations, we found that the monolayer had a Curie temperature of 46 K. This Curie temperature was increased to 57 K with 3% tensile strain whereas it was decreased to 35 K with 3% compressive strain. Overall, we found that the magnetic property of 2D VI3 monolayer was robust under the strain.

High Curie temperature and strain-induced semiconductor-metal transition with spin reorientation transition in 2D CrPbTe3 monolayer.

One of the major obstacles for Cr-based 2D materials such as CrI3, CrSiTe3 and CrGeTe3 for spintronics applications is their low Curie temperature. Herein, we investigated the strain-induced magnetic properties of 2D CrPbTe3 (CPT) monolayer belonging to members of the Cr-based 2D family. We explored the possibility of the fabrication of 2D layer through the mechanical stability, dynamical stability, formation energy, cohesive energy and thermal stability calculations. We found ferromagnetic ground state and the pristine CrPbTe3 monolayer had an indirect band gap of 0.25 eV with an in-plane magnetic anisotropy of -1.37 meV cell-1. The Curie temperature was 110 K and this is much larger than that of CrI3, CrSiTe3 and CrGeTe3. Under 4% tensile strain, the band gap was increased to 0.45 eV and the Curie temperature was increased to 150 K. We found strain-induced semiconductor-metal transition at 3% compressive strain and also spin reorientation transition from in-plane to perpendicular magnetic anisotropy at 4% compressive strain, and the perpendicular magnetic anisotropy energy was almost three times larger than that of the CrGeTe3 layer. Our finding may suggest that the CrPbTe3 system can be utilized for spintronics and straintronics applications.

Magnetic properties of Fe16-x (Ta/W) x N2 ternary alloy: first principles and atomistic simulations.

Using the first principles method, we explored the magnetic properties of Fe16-x (Ta/W) x N2 alloy. We calculated the alloy formation energy, dynamical and thermal stabilities and proved the possibility of materialization of Fe16-x (Ta/W) x N2 alloy. The Fe14Ta2N2 had a Curie temperature of 1020 K while it was 950 K in Fe14W2N2. We found greatly enhanced uniaxial anisotropy in ternary alloy systems. Particularly, the Fe14W2N2 alloy has a uniaxial anisotropy constant of 2.68 MJ m-3 which is almost a five times enhancement of the value of 0.57 MJ m-3 in the Fe16N2. The increase in the uniaxial anisotropy resulted in an enhancement of the coercive field (H C). The Fe14W2N2 had a coercive field of 27 kOe at 300 K and this is comparable to that of 25.6 kOe in Dy-doped Nd based magnet. Besides, the (BH)max of 54.5 MGOe in Fe14W2N2 alloy was even larger than the value of 40 MGOe in Dy-doped Nd-based magnet. Overall, we propose that the Fe16-x (Ta/W) x N2 can be a potential rare-earth-free permanent magnet.

Two-dimensional graphitic carbon nitride (g-C4N3) for superior selectivity of multiple toxic gases (CO, NO2, and NH3).

Using first principles calculations, we investigated the possibility of selecting multiple toxic gases using one substrate material. Here, we explored the transport property of H2, N2, O2, CO, CO2, NO2, and NH3 gas molecules on two-dimensional graphitic carbon nitride (2D g-C4N3). The homonuclear molecule such as H2, N2, and O2 has very weak adsorption energy (equal to or less than 0.1 eV) and also CO2 has an adsorption energy of 0.23 eV. In the typical toxic gas molecule adsorption systems, we found an appreciable charge transfer. In CO and NH3 adsorption systems, the charge transfer of 0.397 and 0.418 electrons from the molecule to the substrate was found, while the NO2 molecule gained 0.124 electrons from the substrate. Due to this large amount of charge transfer, we obtained large adsorption energies of 4.57, 1.29, and 1.93 eV in CO, NO2, and NH3 systems. Moreover, through the I-V curve calculations, we found large difference in the current. The calculated current was 21, 13.11, and 16.16 µA for CO, NO2, and NH3 systems at the bias voltage of 0.5 V. Our results imply that the 2D g-C4N3 can be a superior substrate material for sensing of multiple toxic gases.

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.

Pressure-induced ferromagnetism and enhanced perpendicular magnetic anisotropy of bilayer CrI3.

Using the first principles calculations, we explored the pressure dependent magnetic properties of CrI3 bilayer for AB and AA type stacking. We found an anti-ferromagnetic (AFM) ground state in both pristine bilayer systems with indirect band gaps of 1.71 and 1.68 eV for AB and AA type system. However, the transition from AFM to ferromagnetic (FM) state was achieved with the pressure and the band gap was decreased although the indirect band gap nature remains unchanged. We obtained a substantially enhanced perpendicular magnetic anisotropy with pressure. In the pristine AFM bilayer, both Cr and iodine atoms almost equally contributed to the perpendicular magnetic anisotropy. However, the contribution from the iodine atoms was almost four times larger than the Cr contribution with increasing the pressure. Particularly, the interface iodine atoms played a major role for the enhancement of the perpendicular magnetic anisotropy. We found that both AB and AA type bilayer displayed the same tendency.

Antiferromagnetic coupling of van der Waals ferromagnetic Fe3GeTe2.

Among two-dimensional (2D) layered van der Waals materials, ferromagnetic 2D materials can be useful for compact low-power spintronic applications. One promising candidate material is Fe3GeTe2 (FGT), which has a strong perpendicular magnetic anisotropy and relatively high Curie temperature. In this study, we confirmed that an oxide layer (O-FGT) naturally forms on top of exfoliated FGT and that an antiferromagnetic coupling (AFC) exists between FGT and O-FGT layers. From a first-principles calculation, oxide formation at the interface of each layer induces an AFC between the layers. An AFC causes a tailed hysteresis loop, where two-magnetization reversal curves are included, and a negative remanence magnetization at a certain temperature range.

Size-Dependent Critical Temperature and Anomalous Optical Dispersion in Ferromagnetic CrI3 Nanotubes.

Using first principles calculations, we explored the magnetic and optical properties of chromium(III) iodide (CrI3) nanotubes (NTs) by changing their chirality and diameter. Here, we considered six types of NTs: (5,0), (5,5), (7,0), (10,0), (10,10), and (12,0) NTs. We found that both zigzag and armchair NTs had a ferromagnetic ground with a direct band gap, although the band gap was dependent on the chirality and diameter. Using the Monte Carlo simulation, we found that the Curie temperatures (Tc) exhibited chirality and diameter dependence. In zigzag NTs, the larger the tube diameter, the larger the Tc, while it decreased with increasing diameter in the armchair tube. We found that the Tc was almost doubled when the diameter increased two-fold. This finding may guide development of room temperature ferromagnetism in zigzag NTs. We also found that the CrI3 NTs displayed anisotropic optical properties and anomalous optical dispersion in the visible range. Specifically, the (10,0) zigzag NT had a large refractive index of 2 near the infrared region, while it became about 1.4 near blue light wavelengths. We also obtained large reflectivity in the ultraviolet region, which can be utilized for UV protection. Overall, we propose that the CrI3 NTs have multifunctional physical properties for spintronics and optical applications.

A 2D ferromagnetic semiconductor in monolayer Cr-trihalide and its Janus structures.

Using first principles calculations, we explored the physical properties of two-dimensional (2D) Cr-trihalide X3-Cr2-X3 or CrX3 (X = Cl, Br, I) and its Janus monolayers X3-Cr2-Y3 (X and Y = Cl, Br, I) where X ≠ Y. We found that Janus monolayer X3-Cr2-Y3 materials are dynamically stable and that it is feasible to synthesize X3-M2-Y3 2D-crystals. Both pristine and Janus layers have intrinsic two-dimensional ferromagnetic semiconducting band structures; the largest band gap of 2.3 eV was found in Cl3-Cr2-Cl3, while the band gaps decreased in heavier halide systems. Using Monte Carlo simulations, we found that the Curie temperatures (Tc) showed the same feature of strong dependence on the X and Y halides. In non-polar systems with X = Y, we found no dipole moment, while the polar systems with X ≠ Y had induced dipole moments. Thus, the pristine layer has the same function on both sides, while the Janus layer displays dissimilar work functions in two different surface directions; this was related to the dipole moment and the value of electronegativity. We found that both pristine and Janus layer systems displayed rather weak frequency-dependent dielectric functions. Thus, the variation of the refractive index with frequency was small, and almost zero reflectivity was found in all systems.