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Distinguishing from traditional working environments, we propose the harsh gamma radiation method to study the stability and reliability of the emerging two-dimensional (2D) quantum material tungsten diselenide (WSe2). Transmission electron microscopy studies showed clear chemical modulation with an atomically sharp interface, indicating that the selenium vacancy content increased with the irradiation dose. The WSe2 crystal could be transitioned into an n-doped semiconductor due to the anion vacancies created by radiation. Changes in the lattice vibrational modes induced by the passivation of oxygen was captured via Raman spectroscopy. The frequency shifts in both in-plane and out-of-plane modes are dependent linearly on the selenium vacancy content. The friction of WSe2 increases with the irradiation dose. Electrostatic properties were investigated by measuring the surface potential via Kelvin probe force microscopy. Due to different environments, molecular collisions lead to an increase in the concentration of vacancy defects, which made our results different from those previously reported. The first principles calculation suggests that an increase in the selenium vacancy population is generally accompanied by a transition from a direct gap material to an indirect one. This opens up a new venue to engineer the optical, frictional and electronic properties of transition metal dichalcogenides using irradiation.

Low-dimensional ferroelectricity and Dirac materials with protected band crossings are fascinating research subjects. Based on first-principles calculations, we predict the coexistence of spontaneous in-plane polarization and novel 2D emergent fermions in dynamically stable quadruple-layer (QL) XSbO$_2$ (X= Li, Na). Depending on the different polarization configurations, QL-XSbO$_2$ can exhibit unconventional inner-QL ferroelectricity and antiferroelectricity. Both ground states harbor robust ferroelectricity with enhanced spontaneous polarization of 0.56 nC/m and 0.39 nC/m for QL-LiSbO$_2$ and QL-NaSbO$_2$, respectively. Interestingly, the QL-LiSbO$_2$ possesses two other metastable ferroelectric (FE) phases, demonstrating the first 2D example with multiple FE orders. The ground FE phase can be flexibly driven into one of the two metastable FE phases and then into the antiferroelectric (AFE) phase. During this phase transition, several types of 2D fermions emerge, for instance, hourglass hybrid and type-II Weyl loops in the ground FE phase, type-II Weyl fermions in the metastable FE phase, and type-II Dirac fermions in the AFE phase. These 2D fermions are robust under spin-orbit coupling. Notably, two of these fermions, e.g., an hourglass hybrid or type-II Weyl loop, have not been observed before. Our findings identify QL-XSbO$_2$ as a unique platform for studying 2D ferroelectricity relating to 2D emergent fermions., 5 figures

We investigate the transport properties of bulk Ca2YZ (Y = Au, Hg; Z = As, Sb, Bi, Sn and Pb) by a combination method of first-principles and Boltzmann transport theory. The focus of this article is the systematic study of the thermoelectric properties under the effect of a spin–orbit coupling. The highest dimensionless figure of merit (ZT) of Ca2AuAs at optimum carrier concentration are 1.23 at 700 K. Interestingly enough, for n-type Ca2HgPb, the maximum ZT are close to each other from 500 K to 900 K and these values are close to 1, which suggests that semimetallic material can also be used as an excellent candidate for thermoelectric materials. From another viewpoint, at room temperature, the maximum PF for Ca2YZ are greater than 3 mW m−1 K−2, which is very close to that of ∼3 mW m−1 K−2 for Bi2Te3 and ∼4 mW m−1 K−2 for Fe2VAl. However, the room temperature theoretical κl of Ca2YZ is only about 0.85–1.6 W m−1 K−1, which is comparing to 1.4 W m−1 K−1 for Bi2Te3 and remarkably lower than 28 W m−1 K−1 for Fe2VAl at same temperature. So Ca2YZ should be a new type of promising thermoelectric material at room temperature.

Atom-thick van der Waals heterostructures with nontrivial physical properties tunable via the magnetoelectric coupling effect are highly desirable for the future advance of multiferroic devices. In this work on LaCl/In2Se3 heterostructure consisting of a 2D ferromagnetic layer and a 2D ferroelectric layer, reversible switch of the easy axis and the Curie temperature of the magnetic LaCl layer has been enabled by switching of ferroelectric polarization in In2Se3. More importantly, magnetic skyrmions in the bimerons form have been discovered in the LaCl/In2Se3 heterostructure and can be driven by an electric current. The creation and annihilation of bimerons in LaCl magnetic nanodisks were achieved by polarization switching. It thus proves to be a feasible approach to achieve purely electric control of skyrmions in 2D van der Waals heterostructures. Such nonvolatile and tunable magnetic skyrmions are promising candidates for information carriers in future data storage and logic devices operated under small electrical currents., Electrical control of topological spin textures is a major goal of spintronics. Here, the authors combine LaCl, a ferromagnet, and In2Se3, a ferroelectric, to create a multiferroic heterostructure, where bimerons, a topological spin-texture, can be created and annihilated by ferroelectric switching.

Perovskite oxide interfaces have become a platform for the realization of many unexpected physical properties in recent years, while effective electric control of magnetism is promising for future spintronic devices. In this work, external electric field is applied to control the physical properties of LaFeO3/SrTiO3 perovskite oxide heterostructures investigated by first-principles calculations based on density functional theory (DFT). We find that external electric field can effectively modulate the band dispersion around the Fermi level of both the n-type and the p-type interfaces. Negative electric field can weaken the band shift, even leading to a two-dimensional electron gas, which is accompanied by a reversible insulator-to-metal/half-metal transition at the n-type interface. Moreover, the n-type interface perfectly realizes a transformation from G-type antiferromagnetic to ferrimagnetic at −0.3 V/A. Meanwhile, the electric field also regulates the electronic structure of the p-type interface of the heterostructure. Our work mainly focus on using electric field to tune the interfacial electronic structure and magnetic properties of perovskite oxides heterostructures, and further pave the way to the experimental design of novel magnetoelectric interface materials.

SrTiO3-based transition-metal oxide heterostructures with superconducting, ferromagnetic, ferroelectric, and ferroelastic properties exhibit high application potential in the fields of energy storage, energy conversion, and spintronic devices. Meanwhile, high effective (charge)-Seebeck coefficient materials composed of a ferromagnetic layer and SrTiO3 insulator layer have been achieved but we still have blocks to pursuing high spin-Seebeck coefficient materials. Here, we use first-principles calculations combined with spin-resolved Boltzmann transport theory to investigate the spin- and effective-Seebeck coefficients in the LaX(X = Mn and Co)O3/SrTiO3 superlattice. Compared with the LaMnO3/SrTiO3 superlattice, LaCoO3/SrTiO3 with ferromagnetic ordering has high spin polarization, relatively low valence valley degeneracy but high effective mass. Utilizing these characteristics, the maximum spin-Seebeck coefficient of LaMnO3/SrTiO3 is −152 μV K−1 at 450 K along the cross-plane direction, while LaCoO3/SrTiO3 reaches −247 μV K−1 under the same conditions. Interestingly, the spin- and effective-Seebeck coefficients are amazingly consistent with each other below 200 K, which indicates that one spin channel (spin-up or spin-down) dominates the carrier transport, and the other one (spin-down or spin-up) is filtered out. These characteristics are mainly associated with the magnetic MnO2/CoO2 layers with distinct dxy and dz2 orbitals near the Fermi level. Our results clarify the relationship of spin- and effective-Seebeck coefficients and indicate that SrTiO3-based transition metal oxide heterointerfaces are a key candidate for spin caloritronics.

The previous experimental work showed that n-type low-temperature (LT) SnSe doped with BiCl3 simultaneously exhibited larger Seebeck coefficient (S), higher electrical conductivity (σ), and lower thermal conductivity (κ) compared with polycrystalline LT SnSe. Here, the first-principles method was used to explore how the independent adjustment to the tangled thermoelectric (TE) transport coefficients of n-type LT SnSe was achieved and whether there exist the similar adjustment mechanisms in n-type high-temperature (HT) SnSe. Cl doping increases the electron-pockets and the total density of states (TDOS) near the Fermi level (EF). This results in a high σ and an obvious decrease in S. Whereas, Bi doping introduces two half-filled bands into the band gap. The two bands originate from the 6s “lone-pairs” of Bi atoms and the sigma bonds formed by Bi atoms and their nearest Se atoms. This contributes to maintaining a relative large S and a low κ, but this cannot effectively enhance σ. Fortunately, the coexistence of Cl ions and Bi ions can make Bi/Cl codoped LT (or HT) SnSe set a high σ and a relatively large S in one through independent controlling the electronic structures and charge distribution near the EF. Further, we predict that Bi/Cl co-doping is also an effective strategy to improve the TE performance of HT SnSe.

The magneto-Seebeck effect has recently attracted considerable attention because of its novel fundamental physics and future potential application in spintronics. Herein, employing first-principles calculations and the spin-resolved Boltzmann transport theory, we have systematically investigated the electronic structures and spin-related transport properties of Co2FeAl/MgO/Co2FeAl multilayers with parallel (P) and anti-parallel (AP) magnetic alignment. Our results indicate that the sign of tunneling magneto-Seebeck (TMS) value with Co2/O termination is consistent with that of the measured experimental result although its value (-221%) at room temperature is smaller than the experimental one (-95%). The calculated spin-Seebeck coefficients of the Co2/O termination with P and AP states and the FeAl/O termination with the AP state are all larger than other typical Co2MnSi/MgO/Co2MnSi heterostructures. By analyzing the geometries, electronic structures, and magnetic behaviors of two different terminations (Co2/O and FeAl/O terminations), we find that the two terminations in the interface region form anti-bonding and bonding states, reconstructing the energy gap, changing the magnetic moment of O atoms, and improving the spin-polarization (-82%). This phenomenon can be ascribed to the charge transfer and hybridization between Co/Fe 3d and O 2p states, which also results in a bowknot orbital shape of Co atoms with Co2/O termination and an ankle shape of Co atoms with FeAl/O termination far away from the interface. Moreover, there are spin-splitting transmission gaps with the Co2/O-termination around the Fermi level, while the transmission gaps with the FeAl/O-termination are closed and thus show a typical metallic character. Our findings will guide the experimental design of magneto-Seebeck devices for future spintronic applications.

Designing two-dimensional (2D) materials with magnetic and topological properties has continuously attracted intense interest in fundamental science and potential applications. Here, on the basis of first-principles calculations, we predict the coexistence of antiferromagnetism and Dirac nodal loop (NL) in the monolayer MnB, where the band crossing points are very close to the Fermi Level. Remarkably, a moderate strain can induce an antiferromagnetic to ferromagnetic phase transition, driving the monolayer MnB to a ferromagnetic metal with Weyl NLs. Such a type of topological quantum phase transition has not been observed before. In addition, the symmetry-protected properties of the two types of NLs as well as the magnetic critical temperatures are investigated. The controllable magnetic and topological order in monolayer MnB offers a unique platform for exploring topological quantum phase transitions and realizing nanospintronic devices.

Two-dimensional (2D) ferromagnetic materials with high Curie temperatures (TC) and tunable physical properties are crucial to the development of nanoscale spintronics devices. Here, we investigate the newly synthesized iodinene using first-principles calculations. Our results show that doping carriers in monolayer and bilayer iodinene can easily introduce itinerant ferromagnetism due to a flatband structure near the Fermi level, and that the associated TC is higher than room temperature. Moreover, we find that a structural phase transition can be achieved through the application of moderate tensile strain for both monolayer and bilayer iodinene. The magnetic moment, Curie temperature, band structure, carrier effective mass, and optical absorption can be changed significantly through this phase transition, and the transition can also lead to a magnetic phase transition with an appropriate doping concentration. Our work provides a feasible approach for designing 2D magnetic materials with potential for application in microelectronics devices.

The nanoscale characteristics of semiconducting transition metal dichalcogenides (TMDCs) are largely determined by their photonic, mechanical, magnetic, thermal, and electronic properties, which can be modulated by adjusting thickness and radiation treatments. In this paper, gamma-rays were applied to irradiate the materials with one to six layers, based on which a comparison was drawn of the frictional and electrical properties before and after irradiation. The changes on a few-layer WSe2 were investigated using Raman spectroscopy, photoluminescence spectroscopy, scanning electron microscopy, transmission electron microscopy, force friction microscopy, and Kelvin probe force microscopy. Under the context of irradiation, there was a phenomenon found different than previously reported. The friction force of WSe2 nano-flakes increased from monolayer to bilayer, decreased at tri-layer, and then increased on a continued basis with thickness. It is suggested that the gamma-ray irradiation treatment could be effective in improving frictional and electronic properties. The range of change to the surface contact potential difference (CPD) was narrowed, and the stability of the device surface potential was enhanced. The continuum mechanics theory was applied to explore the friction force variation between different thickness layers. Based on the puckering effect of tip-flake adhesion, the friction force was determined by bending stiffness. The thermal treatment of WSe2 nanoflakes had a significant impact on the CPD between the sample and the test tip. After thermal treatment, the surface potential increased from one to five layers with thickness. These phenomena were explained in detail. The research contributes to enriching nanotribology and electrical theory in addition to promoting the use of semiconducting TMDCs for nano-components’ design.

Ba 3 Al 2 As 4 exhibits an unusual anisotropic electrical conductivity, that is, the electrical conductivity along the chain is smaller than those along other two directions. The results is conflict with previous conclusion for Ca 5 M 2 Pn 6 . Earlier studies on Ca 5 M 2 Pn 6 showed that a higher electrical conductivity could be obtained along the chain. The band decomposed charge density is used to explain such unusual behavior. Our calculations indicate the existence of a conductive pathway near the Fermi level is responsible for the electrons transport. Further, the Ba–As bonding of Ba 3 Al 2 As 4 has some degree covalency which is novel for the Zintl compounds.

As one of desirable spin-gapless semiconductors, Heusler alloy CoFeCrGa is a potential candidate for semiconductor spintronic applications. However, the absence of explicit investigation on transport properties opens a great way for our theoretical research. In this study, we report on the spin-Seebeck coefficient of quaternary Heusler alloy CoFeCrGa and CoFeCrAl from ab initio density functional theory and Boltzmann transport calculations. Interestingly, CoFeCrGa is a spin-gapless semiconductor while CoFeCrAl acts as a half-metallic ferrimagnet. These two effects produce different spin-Seebeck coefficients of 6.76 μV/K and -0.57 μV/K at room temperature, respectively. By modulating chemical potential and temperature, the spin-Seebeck coefficient can be boosted to -11.7 μV/K under µ = -0.15 eV for CoFeCrGa, and -16.2 μV/K with µ = 0.13 eV for CoFeCrAl, both at 400 K. We demonstrate that the mechanism of the variation of the Seebeck coefficient depending upon the chemical potential and temperature is related to bond lengths, electronic structures, magnetic properties, and transmission. These characteristics of CoFeCrGa and CoFeCrAl offer perspectives for their spin-transport devices.

The highest thermoelectric figure of merit (ZT) value of 2.6 has been experimentally determined in p-type single crystalline SnSe. In this work, we propose possible method to further enhance the thermoelectric performance of SnSe by comparing different thermoelectric performances of p- and n-type SnSe using the first-principles method and the semiclassical Boltzmann theory. There are relatively stronger intra-layer interactions between Sn and Se atoms for the layered single crystalline SnSe. The Sn p and s electrons near the Fermi level are pushed away along the a-direction because of the anti-bonding interaction between Sn and Se atoms. The pushed away electrons would hinder holes transport in the a-direction. This leads to the lowest electrical conductivity and ZT value along the a-direction for the p-type single crystalline SnSe. For n-type SnSe, the Sn p and Se p electrons are primarily distributed along the a-direction near the Fermi level. This would lead to high electrical conductivity and large Seebeck coefficient simultaneously in the a-direction. Combined with its ultralow thermal conductivity in the a-direction, the ZT value (∼3.1 at 770 K by a rough estimation) of n-type SnSe along the a-direction is probably much higher than that of p-type one. The peak values of S 2 σ τ of the p-type SnSe correspond to different carrier concentrations along the a- and b-directions. This is one reason for the low experimental peak ZT value of 0.6 for the p-type polycrystalline SnSe at 750 K. For the n-type SnSe, the S 2 σ τ can attain their peak values almost at the same carrier concentration along the a-, b-, and c-directions. This suggests that the power factor value of n-type polycrystalline SnSe would be much higher than that of p-type one at 770 K.

Engineering atomic-scale native point defects has become an attractive strategy to improve the performance of thermoelectric materials. Here, we theoretically predict that Ag-Mg antisite defects as shallow acceptors can be more stable than other intrinsic defects under Mg-poor‒Ag/Sb-rich conditions. Under more Mg-rich conditions, Ag vacancy dominates the intrinsic defects. The p-type conduction behavior of experimentally synthesized α-MgAgSb mainly comes from Ag vacancies and Ag antisites (Ag on Mg sites), which act as shallow acceptors. Ag-Mg antisite defects significantly increase the thermoelectric performance of α-MgAgSb by increasing the number of band valleys near the Fermi level. For Li-doped α-MgAgSb, under more Mg-rich conditions, Li will substitute on Ag sites rather than on Mg sites and may achieve high thermoelectric performance. A secondary valence band is revealed in α-MgAgSb with 14 conducting carrier pockets.

The previous experimental work showed that Hf- or Zr-doping has remarkably improved the thermoelectric performance of FeNbSb. Here, the first-principles method was used to explore the possible reason for such phenomenon. The substitution of X (Zr/Hf) atoms at Nb sites increases effective hole-pockets, total density of states near the Fermi level (EF), and hole mobility to largely enhance electrical conductivity. It is mainly due to the shifting the EF to lower energy and the nearest Fe atoms around X atoms supplying more d-states to hybrid with X d-states at the vicinity of the EF. Moreover, we find that the X atoms indirectly affect the charge distribution around Nb atoms via their nearest Fe atoms, resulting in the reduced energy difference in the valence band edge, contributing to enhanced Seebeck coefficients. In addition, the further Bader charge analysis shows that the reason of more holes by Hf-doping than Zr in the experiment is most likely derived from Hf atoms losing less electrons and the stronger hybridization between Hf atoms and their nearest Fe atoms. Furthermore, we predict that Hf/Sn co-doping may be an effective strategy to further optimize the thermoelectric performance of half-Heusler (HH) compounds.

The structural, electronic, and thermoelectric properties of InSe nanotubes (InSeNTs) are investigated using first-principle calculations based on density functional theory (DFT). The thermoelectric transport coefficients of InSeNTs at room temperature are calculated within the semiclassical Boltzmann theory. Calculated total energies show that (2,2) InSeNT is more stable than other ones. As a consequence of high Seebeck coefficient and reasonable electrical conductivity, its thermoelectric powerfactor with respect to relaxation time is much larger than those of other studied InSeNTs and is nearly 10 times larger than that of BiSb nanotube. The light and heavily bands appear together around the Fermi level in (2,2) InSeNT, inducing its high Seebeck coefficient and reasonable electrical conductivity. (2,2), (4,0), and (6,0) InSeNTs are metallic. (3,3), (4,4), (6,6), and (8,0) InSeNTs are semiconducting. It is found that each Se atom in the semiconducting InSeNTs is coordinated by two In atoms, and that in ...

By introducing a suitable barrier, carrier filtering can yield a high Seebeck coefficient by filtering out electrons (or holes) with low energy. To understand carrier filtering in a PbTe/SrTe interface, the first-principles method and semiclassical Boltzmann theory are used to investigate electronic structures and Seebeck coefficients of PbTe/SrTe (110) and (100) interfaces. The PbTe/SrTe heterostructure is found to be a type-I interface that can form an energy barrier that filters low-energy carriers. Such carrier filtering induces a large in-plane Seebeck coefficient of ∼277 μV K−1 (T = 600 K) with a fixed carrier concentration of 1.6 × 1020 cm−3. This large in-plane Seebeck coefficient is attributed to the Te-p states and the strong asymmetry of the transmission. Additionally, the values of the Seebeck coefficient of p-type PbTe/SrTe (110) are larger than those of the n-type one, and the electronic properties of the PbTe/SrTe (100) interface are similar to those of the PbTe/SrTe (110) interface.

BiCuSeO is an attractive material for its high temperature stability, high Seebeck coefficient, and low lattice thermal conductivity. However, its electrical conductivity is low. To enhance the thermoelectric properties of BiCuSeO further, we investigated the material's electronic structure and transport property using first-principles calculations. We determine that the low electrical conductivity may originate from strong ionic bonding and without a Cu–Se conductive pathway forming near the Fermi level. Moreover, although p-type BiCuSeO has a high thermopower due to the high density of states near the Fermi level, its electrical conductivity is relatively low because no conductive pathway for carrier transport exists along the c axis. In contrast, a conductive pathway is formed between Bi and Cu atoms at the conduction-band minimum along the c axis. This feature would lead to high electrical conductivity for n-type BiCuSeO. With relatively high electrical conductivity and slightly decreased thermopower, n-type BiCuSeO exhibits a higher power factor than that of p-type BiCuSeO. The maximum ZT value could be improved by 32% for n-type BiCuSeO compared with the prevailing p-type doping approach at 920 K.

Density functional theory calculations by using both generalized gradient approximation (GGA) method and the GGA with considering strong correlation effect (GGA+U) were performed to elucidate the effect of Ce-4f orbit on the electronic structure of ZnO. It is found that after the cerium incorporation, a new localized band appears between the valence and conduction bands, which corresponds to the majority spin of Ce-4f states. It is this localized band that constructs a bridge between the valence and conduction states, which will improve the optical performance of ZnO. ZnO:Ce is a degenerate semiconductor. The strong correlation effect is very important for the 4f orbit of the Ce atom in ZnO:Ce. The mismatch of the majority and minority spin for the Ce-4f, Ce-5d states and the spin-polarized holes in O-2p states induced by Ce doping leads to the presence of the magnetic order for ZnO:Ce. We also studied the band structure and optical properties of ZnO:Ce with lacking one electron and two electrons, respect...