Li-ion intercalation-driven control of two-dimensional magnetism in van der Waals FePS3 bilayers.

Manipulating two-dimensional (2D) magnetism in layered van der Waals (vdW) materials like FePS3 (FPS), with its wide-ranging applications in flexible spintronic devices, poses a persistent challenge. Through first-principles calculations, we have achieved reversible ferrimagnetic (FiM, FePS3 bilayer) ↔ antiferromagnetic (AFM, 1Li-intercalated FePS3 bilayer) ↔ ferromagnetic (FM, 2Li-intercalated FePS3 bilayer) phase transitions by using a Li-ion intercalation method. Intercalated Li ions significantly enhance the Fe-3d and S-3p hybridization and reduce the Fe-Fe, Li-Fe, Li-S, and Li-P bond lengths. The manipulation of 2D magnetism in Li-intercalated FPS bilayers can be attributed to the charge transfer between two FPS monolayers mediated by Li ions. Moreover, this study offers insights into the underlying physical mechanisms that govern the variations of electronic structures, 2D magnetism, magnetic anisotropy energy, and exchange couplings. Our reversible Li-ion intercalation permits straightforward de-intercalation using a two-step route, thereby reinstating the initial magnetic order of the FPS bilayer. Our purpose-designed FPS bilayer with different Li concentrations and robust exchange coupling not only enriches the Li-intercalation physics in the FPS system but also offers a general pathway for manipulating 2D magnetism in Fe-based vdW trisulfides.

The Isoelectronic Dopant in the Z-Scheme SnS2/ß-As Heterostructure Enhancing Photocatalytic Overall Water Splitting.

The catalytic field aims to decrease reaction barriers, accelerate reaction processes, and enhance the selectivity toward a target product. This study uses first-principles calculations to design a modified direct Z-scheme SnS2/ß-As heterostructure as a potential photocatalyst for overall water splitting. Our previous investigations have demonstrated that the SnS2/ß-As heterostructure can realize a hydrogen evolution reaction (HER) under light, while the oxygen evolution reaction (OER) follows a pathway involving the intermediate HOOH*. Interestingly, by substituting an S atom of SnS2 with a Se or Te atom, the rate-determining step of the OER is significantly reduced from 3.76 eV to 2.56 or 2.22 eV. Moreover, the OER can occur directly without the transition via HOOH*. Isoelectronic doping effectively trades off the adsorption strength of OER intermediates and promotes the OER process. This work highlights the dual benefits of isoelectronic doping, namely lowering the reaction barrier of the rate-determining step and promoting the selectivity of end products. These findings provide insights into the rational design of high-efficiency photocatalysts for water splitting.

The polarized electric field of CdTe/B4C3 heterostructure efficiently promotes its photocatalytic overall water splitting.

Inspired by natural photosynthesis, two-dimensional van der Waals (vdW) heterostructures are considered as promising photocatalysts for solar-driven water splitting and they attract ever-growing interest. A type-II vdW hetero-photocatalyst (CdTe/B4C3) integrating the polarized CdTe into metal-free B4C3 was constructed, which could achieve solar-driven spontaneous overall water splitting at pH = 0-7 and exhibit a high solar-to-hydrogen (STH) efficiency of 19.64%. Our calculation results show that the interlayer interaction between the CdTe and B4C3 monolayers in the heterostructure creates an interfacial electric field enhanced by the intrinsic dipole of polarized CdTe, which accelerates the effective separation of photogenerated carriers and makes the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) take place separately on the B4C3 and CdTe layers. Furthermore, the CdTe/B4C3 heterostructure has decent band edge positions to promote the redox reaction to decompose water due to the significant electrostatic potential difference in the CdTe/B4C3 heterostructure and it could trigger spontaneous redox reaction under light at pH = 0-7. This work is helpful for us to design type-II heterojunction photocatalysts with high efficiency of photogenerated carrier separation for overall water splitting.

Machine Learning Accelerated Discovery of Promising Thermal Energy Storage Materials with High Heat Capacity.

Thermal energy storage offers numerous benefits by reducing energy consumption and promoting the use of renewable energy sources. Thermal energy storage materials have been investigated for many decades with the aim of improving the overall efficiency of energy systems. However, finding solid materials that meet the requirement of high heat capacity has been a grand challenge for material scientists. Herewith, by training various machine learning models on 3377 high-quality data from full density functional theory (DFT) calculations, we efficiently search for potential materials with high heat capacity. We build four traditional machine learning models and two graph neural network models. Cross-comparison of the prediction performance and model accuracy was conducted among different models. The deeperGATGNN model exhibits high prediction accuracy and is used for predicting the heat capacity of 32,026 structures screened from the open quantum material database. We gain deep insight into the correlation between heat capacity and structure descriptors such as space group, prototype, lattice volume, atomic weight, etc. Twenty-two structures were predicted to possess high heat capacity, and the results were further validated with DFT calculations. We also identified one special structure, namely, MnIn2Se4, with space group no. 227 (Fd3̅m), that exhibits extremely high heat capacity, even higher than that of the Dulong-Petit limit at room temperature. This study paves the way for accelerating the discovery of novel thermal energy storage materials by combining machine learning with minimal DFT inquiry.

Magneto-Seebeck effect in Co2FeAl/MgO/Co2FeAl: first-principles calculations.

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.

Ag-Mg antisite defect induced high thermoelectric performance of α-MgAgSb.

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.

Optimizing the Dopant and Carrier Concentration of Ca5Al2Sb6 for High Thermoelectric Efficiency.

The effects of doping on the transport properties of Ca5Al2Sb6 are investigated using first-principles electronic structure methods and Boltzmann transport theory. The calculated results show that a maximum ZT value of 1.45 is achieved with an optimum carrier concentration at 1000 K. However, experimental studies have shown that the maximum ZT value is no more than 1 at 1000 K. By comparing the calculated Seebeck coefficient with experimental values, we find that the low dopant solubility in this material is not conductive to achieve the optimum carrier concentration, leading a smaller experimental value of the maximum ZT. Interestingly, the calculated dopant formation energies suggest that optimum carrier concentrations can be achieved when the dopants and Sb atoms have similar electronic configurations. Therefore, it might be possible to achieve a maximum ZT value of 1.45 at 1000 K with suitable dopants. These results provide a valuable theoretical guidance for the synthesis of high-performance bulk thermoelectric materials through dopants optimization.

Tuning magnetism by biaxial strain in native ZnO.

Magnetic ZnO, one of the most important diluted magnetic semiconductors (DMS), has attracted great scientific interest because of its possible technological applications in optomagnetic devices. Magnetism in this material is usually delicately tuned by the doping level, dislocations, and local structures. The rational control of magnetism in ZnO is a highly attractive approach for practical applications. Here, the tuning effect of biaxial strain on the d(0) magnetism of native imperfect ZnO is demonstrated through first-principles calculations. Our calculation results show that strain conditions have little effect on the defect formation energy of Zn and O vacancies in ZnO, but they do affect the magnetism significantly. For a cation vacancy, increasing the compressive strain will obviously decrease its magnetic moment, while tensile strain cannot change the moment, which remains constant at 2 µB. For a singly charged anion vacancy, however, the dependence of the magnetic moment on strain is opposite to that of the Zn vacancy. Furthermore, the ferromagnetic state is always present, irrespective of the strain type, for ZnO with two zinc vacancies, 2VZns. A large tensile strain is favorable for improving the Curie temperature and realizing room temperature ferromagnetism for ZnO-based native semiconductors. For ZnO with two singly charged oxygen vacancies, 2Vs, no ferromagnetic ordering can be observed. Our work points the way to the rational design of materials beyond ZnO with novel non-intrinsic functionality by simply tuning the strain in a thin film form.

A class of rare antiferromagnetic metallic oxides: double perovskite AMn3V4O12 (A = Na(+), Ca(2+), and La(3+)) and the site-selective doping effect.

We have investigated the structural, electronic, and magnetic properties of A-site-ordered double-perovskite-structured oxides, AA'3B4O12 (A = Na, Ca, and La) with Mn and V at A' and B sites, respectively, using first-principle calculations based on the density functional theory. Our calculation results show that the antiferromagnetic phase is the ground state for all the compounds. By changing the A-site ions from Na(+) to Ca(2+) and then to La(3+), the transfer of charge between Mn and O ions was changed from 1.56 to 1.55 and then to 1.50, and that between the V and O ions changed from 2.01 to 1.95 and then to 1.93, revealing the cause for the unusual site-selective doping effect. Mn 3d electrons dominate the magnetic moment and are localized, with an intense hybridization with O 2p orbitals, which indicates that the magnetic exchange interaction between Mn ions is mediated through O and that the super exchange mechanism will take effect. These materials have a large one-electron bandwidth W, and the ratio of the on-site Coulomb repulsion U to W is less than the critical value (U/W)c, which leads to metallic behavior of AMn3V4O12. This is further evidenced by the large number of free electrons contributed by V at the Fermi surface. These calculations, in combination with the reported experimental data, prove that these double perovskites belong to the rare antiferromagnetic metallic oxides.

Theoretical investigation of the effects of doping on the electronic structure and thermoelectric properties of ZnO nanowires.

The effects of doping ZnO nanowires with Al, Ga and Sb on their electronic structure and thermoelectric properties are investigated by first-principles calculations. We find that the band gap of ZnO nanowires is narrowed after doping with Al and Ga, while band gap broadening is observed in Sb doped ZnO nanowires. The lattice thermal conductivity of ZnO nanowires is obtained based on the Debye-Callaway model. The thermoelectric properties of ZnO nanowires were calculated using the BoltzTraP code. The results show that there exists an optimal carrier concentration yielding the maximum value of ZT for Al, Ga and Sb doped ZnO nanowires at room temperature. The maximum value of ZT, 0.147, is obtained for Ga doped ZnO nanowires, when the carrier concentration is 3.62 × 10(19) cm(-3). The figure of merit ZT of Sb doped ZnO nanowires is higher than that of Ga doped ZnO nanowires when the temperature is between 400 K and 1200 K. We also find that Al doped ZnO nanowires always have poor thermoelectric properties, which means that the Al dopant may not be the optimal choice for ZnO nanowires in thermoelectric applications.

Strong ultraviolet luminescence of ZnO thin films with nanowall-network structures.

ZnO thin films with three different types of surface structures, smooth, large nanowall-networks, and small nanowall-networks, were grown by a vapor phase transport process. The XRD patterns of these samples showed a high c-axis orientation. Photoluminescence spectra of these samples showed that the intensities of ultraviolet (UV) emission from ZnO thin films having a surface with nanowall-network structures were enhanced by 1-2 orders of magnitude, the linewidths of UV emission were reduced, and the peak positions blueshifted significantly compared with the ZnO film having a smooth surface. The greatly enhanced UV emission was attributed to high crystalline quality with the oriented growth, and whispering gallery mode resonance in the nanowall-formed cavities.