Four-Phonon Enhanced the Thermoelectric Properties of ScSX (X = Cl, Br, and I) Monolayers.

Recently, the FeOCl-type two-dimensional materials have attracted significant attention owing to their versatile applications in fields such as thermoelectricity and photocatalysis. This study aims to systematically investigate the thermoelectric properties of ScSX (X = Cl, Br, and I) monolayers by a combination of the first-principles calculations and the machine-learning interatomic potential approach. These monolayers are indirect semiconductors with band gaps of 3.22 (ScSCl), 3.27 (ScSBr), and 2.87 eV (ScSI), respectively. The lattice thermal conductivity is decreased by 25.72% (20.90%), 44.05% (40.00%), and 30.96% (34.76%) for ScSCl, ScSBr, and ScSI along the x-axis (y-axis) when the four-phonon scattering is introduced, indicating its important role in phonon transport. Anharmonic phonon scattering yields high Grüneisen parameter and scattering rate values, hence causing these low lattice thermal conductivities. Additionally, the large Seebeck coefficients and electrical conductivities of n-type doped ScSX monolayers contribute to their excellent power factors (24.69, 25.66, and 24.99 mW/K2·m for ScSCl, ScSBr and ScSI at 300 K, respectively). Based on the excellent power factor and low thermal conductivity, the maximum values of the figure of merit are calculated to be 2.68, 3.39, and 3.21 for ScSCl, ScSBr, and ScSI monolayers at 700 K, respectively. Our research provides valuable insights into the phonon thermal transport of ScSX monolayers and suggests a promising approach to address high-order anharmonicity.

Exploring the structural stability and electrochemical performance of B doped T-graphene nanotubes from first-principles calculations.

The structural stability and electrochemical performance of intrinsic and B doped T-graphene nanotubes with different tube lengths are systematically studied by using first-principles calculations within the framework of density functional theory (DFT). The results show that with the increase of tube length, the adsorption energy of both intrinsic and B doped T-graphene nanotubes exhibits regular oscillations, and B doping is beneficial for elevating the adsorption ability of T-graphene nanotubes. The density of states show that intrinsic T-graphene nanotubes are zero band gap semiconductors, and the orbitals' electronic states cross the Fermi level to form a p-type semiconductor, indicating that B doping greatly improves the conductivity of the system. The results of migration behavior demonstrate that B doping can effectively reduce the diffusion barrier of lithium ions on their surface, especially in B doped T-graphene nanotubes with a tube length of N = 1, resulting in more effective migration behavior and excellent rate performance. These findings provide a theoretical basis for the development and application of negative electrode materials for lithium-ion batteries.

Elucidating the effects of B/Al doping on the structure stability and electrochemical properties of silicene using DFT.

Using first-principles calculation based on density functional theory, the effects of B, Al and B-Al doping on the structural stability and electrochemical properties of silicene were systematically studied, and their potential as anode materials for lithium ion batteries was evaluated. The calculated results of formation energy indicate that the doped system has good stability. The charge density difference and density of states show that doping can improve the conductivity of silicene, and enhance the interaction with Li. Moreover, on the surface of B, Al and B-Al doped silicene, the diffusion barriers of the most easily migrated path for Li ions are 0.22 eV, 0.19 eV, and 0.21 eV, respectively, suggesting that all doped systems have good Li ion migration rates. And the open circuit voltage is between 0.40 V and 0.54 V, which is relatively stable and low. Therefore, B, Al and B-Al doping can effectively regulate the structural stability and electrochemical performance of silicene, which provides a theoretical basis for the experimental preparation of excellent silicene anode materials.

Enhanced Photocatalytic Activity of Two-Dimensional Polar Monolayer SiTe for Water-Splitting via Strain Engineering.

A two-dimensional (2D) polar monolayer with a polarization electric field can be used as a potential photocatalyst. In this work, first principle calculations were used to investigate the stability and photocatalytic properties of 2D polar monolayer SiTe as a potential promising catalyst in water-splitting. Our results show that the 2D polar monolayer SiTe possesses an indirect band gap of 2.41 eV, a polarization electric field from the (001) surface to the (001¯) surface, a wide absorption region, and a suitable band alignment for photocatalytic water-splitting. We also discovered that the photocatalytic activity of 2D polar monolayer SiTe could be effectively tuned through strain engineering. Additionally, strain engineering, particularly compressive strain in the range from -1% to -3%, can enhance the photocatalytic activity of 2D polar monolayer SiTe. Overall, our findings suggest that 2D polar monolayer SiTe has the potential to be a promising catalyst for photocatalytic water-splitting using visible light.

Understanding the different effects of 4d-transition metals on the performance of Li-rich cathode Li2MnO3 by first-principles.

The poor cycling performance of Li-rich cathode Li2MnO3, a promising cathode for next-generation Li-ion batteries, limits its commercial applications. Transition metal (TM) doping is widely applied to optimize the electrochemical performance of Li2MnO3, where the d valence electrons of the TM play a crucial role. Nevertheless, the rule of the doping effect of TM with various numbers of d electrons has not been well summarized. In this work, 4d-TMs (Zr, Nb, Mo, Ru and Rh) are selected as dilute doping elements for Li2MnO3 to evaluate their effect on the performance of Li2MnO3 through first-principles calculations. The calculations indicate that as the number of 4d electrons increases, the doped TM transforms from an electrochemically inert state (Zr and Nb) to an electrochemically active state (Mo, Ru and Rh) in Li2MnO3. Meanwhile, the orbital hybridization between the 4d electrons of the TM and the 2p electrons of O becomes stronger from Zr to Rh, which promotes the co-oxidation of the TM and O for charge compensation and alleviates the excessive oxidation of O, thus enhancing the stability of O. Moreover, the oxidation of the doped TM and lattice Mn during charging can trigger a decrease in the initial average delithiation potential. Although the 4d-TMs exhibit slight promoting or inhibiting effects on Li diffusion, no obvious rule related to the number of d electrons has been found. Our work highlights the rule of the doping effect of TMs with different 4d electrons on the electrochemical performance of Li2MnO3 and would facilitate a better design of Li2MnO3 cathode materials.

Self-Doping Based Facet Junctions and Oxygen Vacancies in Ferroelectric Bi3TixNb2-xO9 Nanosheets for Boosting Photocatalytic Degradation and Antibacterial Activity.

Constructing facet junction in semiconductor photocatalysts has been demonstrated as an effective method to promote charge-carrier separation and suppress carrier recombination. Herein, we proposed a novel but facile self-doping strategy to regulate the crystal facet exposure ratio in ferroelectric Bi3TixNb2-xO9 single-crystalline nanosheets, thereby optimizing its facet junction effect. Through tuning the atomic ratio of Ti and Nb, the exposure ratio of {001} and {110} crystal planes in Bi3TixNb2-xO9 nanosheets can be delicately modulated, and more {110} facets were exposed with the increase of the Ti/Nb atomic ratio as evidenced by the X-ray diffraction and scanning electron microscopy results. A facet junction between {110} and {001} crystal planes was verified based on the density functional theory calculation and photodeposition experiment results. Photogenerated electrons tend to accumulate in {110}, while holes gathered in {001} crystal planes. Owing to the optimal facet junction effect, the sample of Ti1.05 shows the most efficient charge-carrier separation and transportation compared to Ti0.95 and Ti1.00 as supported by the photoluminescence, surface photovoltage, photoelectrochemistry, and electron paramagnetic resonance (EPR) results. In addition, the oxygen vacancy arising from the inequivalent substitution of Nb5+ by Ti4+ as proved by X-ray photoelectron spectroscopy and EPR results and the enhanced ferroelectricity supported by P-E loops can also assist charge-carrier separation and migration. Benefiting from these properties, Ti1.05 outperformed Ti0.95 and Ti1.00 in the photodegradation of organic dye and antibiotic molecules. Meanwhile, the excellent antibacterial activity of Ti1.05 under visible light was also demonstrated by the Escherichia coli sterilization experiment. This work not only presents a novel pathway to adjust the facet junction but also provides new deep insights into the crystal facet engineering in ferroelectrics as photocatalysts.

Tuning the structural stability and electrochemical properties in graphene anode materials by B doping: a first-principles study.

The first-principles method of density functional theory (DFT) is used to study the structural stability and electrochemical properties of B doped graphene with concentrations of 3.125%, 6.25% and 18.75% respectively, and their lithium storage mechanism and characteristics are further studied. The results show that the doped systems all have negative adsorption energy, indicating that the structures can exist stably, and the adsorption energy of lithium ions on graphene decreases with the increase of B doping concentration. Among them, the B6C26 structure has the lowest adsorption energy and can adsorb more lithium ions. The density of states indicates that doping with B can increase the conductivity of graphene greatly. Subsequently, the CI-NEB method to search for the transition state of the doped structure is used, showing that the B6C26 structure has the lowest diffusion barrier and good rate performance. Therefore, these findings provide a certain research foundation for the development and application of lithium-ion battery anode materials.

High-Output Lotus-Leaf-Bionic Triboelectric Nanogenerators Based on 2D MXene for Health Monitoring of Human Feet.

As versatile energy harvesters, triboelectric nanogenerators (TENGs) have attracted considerable attention in developing portable and self-powered energy suppliers. The question of how to improve the output power of TENGs using cost-effective means is still under vigorous investigation. In this paper, high-output TENGs were successfully produced by using a simple and low-cost lotus-leaf-bionic (LLB) method. Well-distributed microstructures were fabricated via the LLB method on the surface of a polydimethylsiloxane (PDMS) negative triboelectric layer. 2D MXene (Ti3C2Tx) and graphene were doped into the structured PDMS to evaluate their effects on the performance of TENG. Owing to merits of the MXene doping and microstructures on the PDMS surface, the output power of MXene-doped LLB TENGs reached as high as 104.87 W/m2, which was about 10 times higher than that of graphene-doped devices. The MXene-doped LLB TENGs can be used as humidity sensors, with a sensitivity of 4.4 V per RH%. In addition, the MXene-doped LLB TENGs were also sensitive to human body motions; hence, a foot health monitoring system constructed by the MXene-doped LLB TENGs was successfully demonstrated. The results in this work introduce a way to produce cost-effective TENGs using bionic means and suggest the promising applications of TENGs in the smart monitoring system of human health.

Daylight-White-Emitting and Abnormal Thermal Antiquenching Phosphors Based on a Layered Host SrIn2(P2O7)2.

Single-phase white-emission phosphors possess a judicious usage potential in phosphor-converted white-light-emitting diodes (WLEDs). Recently, numerous efforts have been made toward the development of new patterns of white-emitting phosphors that achieve excellent quantum yield, superior thermal stability, and applaudable cost effectiveness of WLEDs. Finding suitable single-component white phosphor hosts to provide an ideal local environment for activators remains urgent. Inspired by the original discovery of the promising host MgIn2(P2O7)2 (MIP) and its structural dependence on alkali-metal cations, we synthesized a brand-new phosphor host, SrIn2(P2O7)2 (SIP), via the traditional solid-state reaction. Its crystal structure was determined using an ab initio analysis and the Rietveld method. It belongs to a monoclinic unit cell with the space group C2/c. Besides, SIP exhibits a special layered three-dimensional framework in which the monolayer [SrO10]∞ was surrounded by a bilayer [In2P4O14]∞ made of the InO6 octahedra and P2O7 groups. A series of pure SIP:Tm3+,Dy3+ phosphors with tunable blue-white-yellow emission were prepared by adjusting the dopant concentration and utilizing the Tm3+-Dy3+ energy transfer. The daylight-white-emitting phosphor SIP:0.01Tm3+,0.04Dy3+ (the correlated color temperature is 4448 K) exhibits an abnormal thermal antiquenching property, and the emission intensity of 423 K reaches 103.7% of the initial value at 300 K. On the basis of the temperature-dependent lattice evolution and microenvironment analysis, the reduction of ß and lattice distortion can lead to lower asymmetry of the activators and benefit the compensation of trapped-electron thermal activation. In this work, an integration study was carried out on the crystal structure of the new matrix, the occupation of the luminescent center, the interaction of different activators in the host, and the distortion degree of the local structure for the activators, which is of great practical sense for producing a novel single-matrix white phosphor possessing superior thermal endurance for UV-light-stimulated WLEDs.

Two-dimensional polarized MoTe2/GeS heterojunction with an intrinsic electric field for photocatalytic water-splitting.

The construction of van der Waals heterostructures based on 2D polarized materials is a unique technique to achieve enhanced photocatalytic performance. We have investigated the intrinsic electric field and photocatalytic properties of the MoTe2/GeS heterostructure via first-principles calculations. The results showed that a dipole-induced electric field induced by the GeS monolayer and an interface-induced electric field induced by the interface between the GeS monolayer and the MoTe2 monolayer emerge in the 2D polarized MoTe2/GeS heterostructure. The dipole-induced electric field contributes mainly to the total intrinsic electric field. Moreover, the 2D polarized MoTe2/GeS heterostructure possesses many excellent and distinguished photocatalytic performance parameters, such as a direct semiconductor bandgap of 1.524 eV, a wide light spectrum ranging from the ultraviolet to near-infrared region with a high absorption coefficient (about 106 cm-1), a total intrinsic electric field, which reduces the probability of the recombination of photo-generated electron-hole pairs effectively, and a suitable band alignment for the water-splitting reaction. These indicate that the 2D polarized MoTe2/GeS van der Waals heterostructure is a potential novel high-efficient photocatalyst for water-splitting.

Benefits of Ga, Ge and As substitution in Li2FeSiO4: a first-principles exploration of the structural, electrochemical and capacity properties.

Herein, the feasibility of Fe substitution by Ga, Ge and As in Li2FeSiO4 in modulating its structural, mechanical, electrochemical, capacity and electronic properties was systematically studied via first-principles calculations based on density functional theory within the generalized gradient approximation with Hubbard corrections (GGA+U). The calculated results show that Ga, Ge and As doping can effectively reduce the range of the cell volume change during Li+ removal, improving the Li+ detachment ability and cycle stability of the system. Meanwhile, the calculated mechanical properties including modulus ratio, B/G, and Poisson ratio, ν, indicate that the doped systems of Ga, Ge and As exhibit excellent mechanical properties. In addition, besides the increase in theoretical average deintercalation voltage induced by the Ga dopant when more than one Li+ ion is removed in the formula unit, the doping of Ga, Ge and As all reduce the theoretical average deintercalation voltage in the process of Li+ extraction. Especially in the case of doping of Ge, when 0.5 Li+ is removed from LiFe0.5Ge0.5SiO4, the theoretical average deintercalation voltage only increases by 0.19 V compared with the case of the removal of one Li+ in Li2Fe0.5Ge0.5SiO4, which causes the cathode material to have a longer and more stable discharge platform. Moreover, in the process of Li+ removal, the doping of Ga, Ge and As can effectively participate in the charge compensation of the system, and Ge and As can provide further charge, increasing the capacity of the Li2FeSiO4 cathode material considerably.

Drivers toward a Low-Carbon Electricity System in China's Provinces.

Decarbonization of the power sector is one of the most important efforts to meet the climate mitigation targets under the Paris Agreement. China's power sector is of global importance, accounting for ∼25% of global electricity production in 2015. The carbon intensity of China's electricity is still much higher than the global average, but the country has made important strides toward a low-carbon transition based on two main pillars: improvement of energy efficiency and decreasing the share of fossil fuels. By applying a decoupling indicator, our study shows that 21 provinces achieved a "relative decoupling" of carbon emissions and electricity production and the remaining nine provinces achieved "absolute decoupling" between 2005 and 2015. We updated China's emission factors based on the most recent data by also considering the quality of imported coal and compared our results with the widely used Intergovernmental Panel on Climate Change coefficients to show the sensitivity of results and the potential error. Our decomposition analysis shows that improvement of energy efficiency was the dominant driver for decarbonization of 16 provincial power sectors, while the access to low-carbon electricity and substitution of natural gas for coal and oil further accelerated their decarbonization.

Distinct green electroluminescence from lead-free CsCuBr2 halide micro-crosses.

Low-dimensional, lead-free, and cuprous-based halide compounds of Cs3Cu2Br5 micro-rods and CsCuBr2 micro-crosses (MCs) were synthesized via a simple solution method. The CsCuBr2 MCs were quite stable in air. Distinct green electroluminescence at 527 nm originating from CsCuBr2 MCs was observed at a low driving voltage of less than 3 V.

Phase Stability and Mechanical Properties of Al8Fe4RE via First-Principle Calculations.

We report on the phase stability, elastic, electronic, and lattice dynamic properties of 17 Al8Fe4RE (RE = Sc, Y, La⁻Lu) intermetallic compounds (IMCs) using first-principle calculations. The calculated lattice constants coincided with the experimental results. The calculated enthalpy formation indicated that all the 17 IMCs are stable. The elastic constants and various moduli indicated that Al8Fe4RE can be used as a strengthening phase due to its high Young's modulus and shear modulus. The 3D surfaces of Young's modulus for Al8Fe4RE showed anisotropic behavior, and the values of hardness for the IMCs were high (about 14 GPa). The phonon spectra showed that only Al8Fe4Y had a soft mode, which means the other IMCs are all dynamically stable.

Enhanced photocatalytic activity for water splitting of blue-phase GeS and GeSe monolayers via biaxial straining.

The structural, electronic, dipole-induced internal electric field, optical and photocatalytic properties of monolayer GeS and GeSe under external biaxial strain were investigated by using first-principles calculations. The monolayer GeS and GeSe are indirect semiconductors with the band gaps of 3.265 eV and 2.993 eV, respectively. The band alignment of the monolayer GeS and GeSe manifests the photocatalytic activity for water splitting. Especially, it is effective to tune the properties including structures, band gaps, surface potential difference, dipole moment P, dipole-induced internal electric field, absorption and photocatalytic activity of the monolayer GeS and GeSe via biaxial strain. Our results suggest that monolayer GeS and GeSe possess photocatalytic properties for water splitting, and strain engineering, especially tensile strain, can enhance the photocatalytic activity under ultraviolet and visible light.

Quantum anomalous Hall effect and topological phase transition in two-dimensional antiferromagnetic Chern insulator NiOsCl6.

By doing calculations based on density functional theory, we predict that the two-dimensional anti-ferromagnetic (AFM) NiOsCl6 as a Chern insulator can realize the quantum anomalous Hall (QAH) effect. We investigate the magnetocrystalline anisotropy energies in different magnetic configurations and the Néel AFM configuration is proved to be ground state. When considering spin-orbit coupling (SOC), this layered material with spins perpendicular to the plane shows properties as a Chern insulator characterized by an inversion band structure and a nonzero Chern number. The nontrivial band gap is 37 meV and the Chern number C = -1, which are induced by a strong SOC and AFM order. With strong SOC, the NiOsCl6 system performs a continuous topological phase transition from the Chern insulator to the trivial insulator upon the increasing Coulomb repulsion U. The critical U c is indicated as 0.23 eV, at which the system is in a metallic phase with [Formula: see text]. Upon increasing U, the E g reduces linearly with C = -1 for 0 < U < U c and increases linearly with C = 0 for U > U c . At last we analysis the QAH properties and this continuous topological phase transition theoretically in a two-band [Formula: see text] model. This AFM Chern insulator NiOsCl6 proposes not only a promising way to realize the QAH effect, but also a new material to study the continuous topological phase transition.

Toward near-white-light electroluminescence from n-ZnO nanocrystals/n-Si isotype heterojunctions via an AZO spectral scissor.

A strategy to realize ZnO-based near-white-light electroluminescence (EL) was proposed by utilizing and regulating the intrinsic defect-related emissions of solution-processed ZnO nanocrystals (NCs). Prototype near-white light-emitting diodes (LEDs) based upon this strategy were demonstrated by using n-ZnO NCs/n-Si isotype heterojunctions. The emission color of the n-ZnO NCs/n-Si isotype heterojunction LEDs was tuned toward near white by using an Al-doped ZnO (AZO) spectral "scissor" which can tailor the green light more severely, rather than the blue or red light. Moreover, quantum size effect was clearly observed in both the photoluminescence (PL) and EL spectra via the redshift of the near-band-edge UV emission of the ZnO NCs. The strategy using AZO spectral "scissors" to regulate the VO-related green emission of ZnO may present a promising pathway to realize ZnO-based white-light LEDs.

Study of low-modulus biomedical ß Ti-Nb-Zr alloys based on single-crystal elastic constants modeling.

CALPHAD-type modeling was used to describe the single-crystal elastic constants of the bcc solution phase in the ternary Ti-Nb-Zr system. The parameters in the model were evaluated based on the available experimental data and first-principle calculations. The composition-elastic properties of the full compositions were predicted and the results were in good agreement with the experimental data. It is found that the ß phase can be divided into two regions which are separated by a critical dynamical stability composition line. The corresponding valence electron number per atom and the polycrystalline Young׳s modulus of the critical compositions are 4.04-4.17 and 30-40GPa respectively. Orientation dependencies of single-crystal Young׳s modulus show strong elastic anisotropy on the Ti-rich side. Alloys compositions with a Young׳s modulus along the <100> direction matching that of bone were found. The current results present an effective strategy for designing low modulus biomedical alloys using computational modeling.

Physical properties of thallium-tellurium based thermoelectric compounds using first-principles simulations.

We present a study of the thermodynamic and physical properties of Tl(5)Te(3), BiTl(9)Te(6), and SbTl(9)Te(6) compounds by means of density functional theory based calculations. The optimized lattice constants of the compounds are in good agreement with the experimental data. The electronic density of states and band structures are calculated to understand the bonding mechanism in the three compounds. The indirect band gaps of BiTl(9)Te(6) and SbTl(9)Te(6) compounds are found to be equal to 0.256 and 0.374 eV, respectively. The spin-orbit coupling has important effects on the electronic structure of the two semiconducting compounds and should therefore be included for a good numerical description of these materials. The elastic constants of the three compounds have been calculated, and the bulk modulus, shear modulus, and Young's modulus have been determined. The change from ductile to brittle behavior after Sb or Bi alloying is related to the change of the electronic properties. Finally, the Debye temperature and longitudinal, transverse, and average sound velocities have been obtained.