Localizing Oxygen Lattice Evolutions Eliminates Oxygen Release and Voltage Decay in All-Mn-Based Li-Rich Cathodes.

All-Mn-based Li-rich cathodes Li2 MnO3 have attracted extensive attention because of their cost advantage and ultrahigh theoretical capacity. However, the unstable anionic redox reaction (ARR), which involves irreversible oxygen releases, causes declines in cycling capacity and intercalation potential, thus hindering their practical applications. Here, it is proposed that introducing stacking-fault defects into the Li2 MnO3 can localize oxygen lattice evolutions and stabilize the ARR, eliminating oxygen releases. The thus-made cathode has a highly reversible capacity (320 mA h g-1 ) and achieves excellent cycling stability. After 100 cycles, the capacity retention rate is 86% and the voltage decay is practically eliminated at 0.19 mV per cycle. Attributing to the stable ARR, samples show reduced stress-strain and phase transitions. Neutron pair distribution function (nPDF) measurements indicate that there is a structure response of localized oxygen lattice distortion to the ARR and the average oxygen lattice framework is well-preserved which is a prerequisite for the high cycle reversibility.

Two Dimensional MOene: From Superconductors to Direct Semiconductors and Weyl Fermions.

The number of semiconducting MXenes with direct band gaps is extremely low; thus, it is highly desirable to broaden the MXene family beyond carbides and nitrides to expand the palette of desired chemical and physical properties. Here, we theoretically report the existence of the single-layer (SL) dititanium oxide 2H-Ti2O MOene (MXene-like 2D transition oxides), showing an Ising superconducting feature. Moreover, SL halogenated 2H- and 1T-Ti2O monolayers display tunable semiconducting features and strong light-harvesting ability. In addition, the external strains can induce Weyl fermions via quantum phase transition in 2H-Ti2OF2 and Ti2OCl2 monolayers. Specifically, 2H- and 1T-Ti2OF2 are direct semiconductors with band gaps of 0.82 and 1.18 eV, respectively. Furthermore, the carrier lifetimes of SL 2H- and 1T-Ti2OF2 are evaluated to be 0.39 and 2.8 ns, respectively. This study extends emerging phenomena in a rich family of 2D MXene-like MOene materials, which provides a novel platform for next-generation optoelectronic and photovoltaic fields.

Superconductivity and topological states in hexagonal TaC and NbC.

Materials with superconductivity and a nontrivial band structure near the Fermi level are promising candidates in realizing topological superconductivity. Using first-principles calculations, we systematically investigated the stability, mechanical properties, superconductivity, electronic structures, and topological states of hexagonal TaC and NbC. The results show that they are stable and have excellent mechanical properties. We predicted that these two carbides are strong electron-phonon coupling superconductors with superconducting transition temperatures of 14.8 and 17.1 K, respectively. Strong coupling is mainly dominated by in-plane Ta/Nb atomic vibrations and in-plane Ta/Nb-dxy/dx2-y2 electronic orbitals. The electronic structure calculations demonstrate that a nodal line and a triply degenerate point coexist when not including the spin-orbit coupling (SOC) effect. After including the SOC effect, the nodal line is gapped. The complicated surface states are also calculated and need further experiments to verify. The present results indicate that the hexagonal TaC and NbC are potential candidates as topological superconductors, and pave the way towards exploring the superconductivity and topological materials in condensed matter systems.

Superconductivity in Mo-P compounds under pressure and in double-Weyl semimetal Hex-MoP2.

Based on first-principles calculations, we predict five global stable molybdenum phosphorus compounds in the pressure range of 0-300 GPa. All of them display superconductivity with different transition temperatures. Meanwhile, we find that a metastable crystal hex-MoP2, crystallized in a noncentrosymmetric structure, is a double-Weyl semimetal and the Weyl point is in the H-K path. The long Fermi arcs and the topological surface states, which can be observed by angle-resolved photoemission spectroscopy, emerge at the (100) surface below the Fermi level. Furthermore, we find that the superconductivity in hex-MoP2 can be enhanced by carrier doping. Due to the breaking of inversion symmetry, the unconventional spin-triplet pairing coexists with spin-singlet pairing in channel . Based on our theoretical model, there are the superconducting band gaps in both pairings. Our work provides a new platform of hex-MoP2 for studying both topological double-Weyl semimetal and superconductivity.

Low thermal conductivity and high performance anisotropic thermoelectric properties of XSe (X = Cu, Ag, Au) monolayers.

Combining density functional theory (DFT) and semi-classic Boltzmann transport theory, we report the thermoelectric (TE) performance of a family of two-dimensional (2D) group IB-selenides XSe (X = Cu, Ag, Au). The results show that these monolayers exhibit small and anisotropic phonon velocities (0.98-3.84 km s-1), large Grüneisen parameters (up to 100), and drastic phonon scattering between the optical and acoustic phonons. These intrinsic properties originate from strong phonon anharmonicity and suppress the heat transport capacity, resulting in low lattice thermal conductivities (12.54 and 1.22 W m-1 K-1) along the x- and y-directions for a CuSe monolayer. Among our studied monolayers, the 2D CuSe monolayer possesses the most remarkable TE performance with ultrahigh ZT (3.26) for n-type doping along the y-direction at 300 K. CuSe monolayer can achieve higher thermoelectric conversion efficiency at a lower synthetic preparation cost than the expensive AgSe and AuSe monolayers, and our work provides a theoretical basis for paving the way for further experimental studies.

Theoretical prediction of superconductivity in two-dimensional MXenes of molybdenum carbides.

Theoretically and experimentally, MXenes consisting of Mo and C have aroused much interest due to superconductivity in their films and even monolayer forms. Here, based on first-principles calculations, we systematically calculate the electronic structure, phonon dispersion, and electron-phonon coupling (EPC) of monolayer Mo2C (both T- and H-phases), Mo3C2, and Mo3C3. The results show that H-MoxCy (x = 2 or 3, y = 1-3) always have lower total energies than their corresponding T phase and other configurations. All these two-dimensional (2D) molybdenum carbides are metals and some of them display weak phonon-mediated superconductivity at different superconducting transition temperatures (Tc). The Mo 4d-orbitals play a critical role in their electronic properties and the Mo atomic vibrations play a dominant role in their low-frequency phonons, EPC, and superconductivity. By comparison, we find that increasing the Mo content can enhance the EPC and Tc. Besides, we further explore the impact of strain engineering on their superconducting related physical quantities. With increasing biaxial stretching, the phonon dispersions are gradually softened to form some soft modes, which can trigger some peaks of α2F(ω) in the low-frequency region and evidently increase the EPC λ. The Tc of H-Mo2C can be increased up to 11.79 K. Upon further biaxial stretching, charge density waves may appear in T-Mo2C, H-Mo3C2, and H-Mo3C3.

Ultralow thermal conductivity and anisotropic thermoelectric performance in layered materials LaMOCh (M = Cu, Ag; Ch = S, Se).

In layered materials with the stacking axis perpendicular to the basal plane, anharmonicity strongly affects phonon propagation due to weak interlayer coupling, which is helpful to reduce the lattice thermal conductivity and improve the thermoelectric (TE) performance significantly. By combining first-principles calculations and the Boltzmann transport equation, we systematically analyzed and evaluated the lattice thermal conductivity and TE properties of LaMOCh (M = Cu, Ag; Ch = S, Se). The results indicate that these layered materials exhibit ultralow lattice thermal conductivities of 0.24-0.37 W m-1 K-1 along the interlayer direction at room temperature. The low lattice thermal conductivities have been analyzed from some inherent phonon properties, such as low acoustic phonon group velocity, large Grüneisen parameters, and a short phonon relaxation time. Originating from their natural layered crystal structure, the thermal and electronic transports (i.e., thermal conductivity, Seebeck coefficient, and electrical conductivity) are both highly anisotropic between their intralayer and interlayer directions. Finally, we obtained ZT values of 1.17 and 1.26 at 900 K along the interlayer direction for n-type LaCuOSe and LaAgOSe, respectively. Generally, LaMOSe exhibit larger anisotropy than LaMOS, in both n- and p-types of doping. Our findings of low thermal conductivities and large anisotropic TE performances of these layered systems should stimulate much attention in BiCuOSe and alike layered TE families.

Quadruple-layer group-IV tellurides: low thermal conductivity and high performance two-dimensional thermoelectric materials.

Through first-principles calculations, we report the thermoelectric properties of two-dimensional (2D) hexagonal group-IV tellurides XTe (X = Ge, Sn and Pb), with quadruple layers (QL) in the Te-X-X-Te stacking sequence, as promising candidates for mid-temperature thermoelectric (TE) materials. The results show that 2D PbTe exhibits a high Seebeck coefficient (∼1996 µV K-1) and a high power factor (6.10 × 1011 W K-2 m-1 s-1) at 700 K. The lattice thermal conductivities of QL GeTe, SnTe and PbTe are calculated to be 2.29, 0.29 and 0.15 W m-1 K-1 at 700 K, respectively. Using our calculated transport parameters, large values of the thermoelectric figure of merit (ZT) of 0.67, 1.90, and 2.44 can be obtained at 700 K under n-type doping for 2D GeTe, SnTe, and PbTe, respectively. Among the three compounds, 2D PbTe exhibits low average values of sound velocity (0.42 km s-1), large Grüneisen parameters (∼2.03), and strong phonon scattering. Thus, 2D PbTe shows remarkable mid-temperature TE performance with a high ZT value under both p-type (2.39) and n-type (2.44) doping. The present results may motivate further experimental efforts to verify our predictions.

Inelastic Electron Tunneling in 2H-Ta_{x}Nb_{1-x}Se_{2} Evidenced by Scanning Tunneling Spectroscopy.

We report a detailed study of tunneling spectra measured on 2H-Ta_{x}Nb_{1-x}Se_{2} (x=0∼0.1) single crystals using a low-temperature scanning tunneling microscope. The prominent gaplike feature, which has not been understood for a long time, was found to be accompanied by some "in-gap" fine structures. By investigating the second-derivative spectra and their temperature and magnetic field dependencies, we were able to prove that inelastic electron tunneling is the origin of these features and obtain the Eliashberg function of 2H-Ta_{x}Nb_{1-x}Se_{2} at an atomic scale, providing a potential way to study the local Eliashberg function and the phonon spectra of the related transition-metal dichalcogenides.

Thermal transport properties of monolayer MoSe2 with defects.

Two-dimensional (2D) molybdenum diselenide (MoSe2) as one of the ultrathin transition metal dichalcogenides (TMDs) has attracted considerable attention because of its potential applications in thermoelectric and nano-electronic devices. Here, the thermal conductivity of monolayer MoSe2 and its responses to simulated size and defects are studied by nonequilibrium molecular dynamics simulations. With the increase of sample length, the thermal conductivity of monolayer MoSe2 nanoribbons exhibits an enhancement whereas it is insensitive to the width. At room temperature, the thermal conductivities of monolayer MoSe2 along armchair and zigzag directions are 17.758 and 18.932 W (m K)-1, respectively, which are consistent with previous results. The impact of defects on thermal conductivity has also been studied by varying the concentration of the vacancy from 0.1% to 0.5%. The results show that an increase of the defect concentration will greatly suppress the thermal conductivity. The 0.5% defect concentration with a Mo vacancy can result in a thermal conductivity reduction of ∼43%. Such a study would provide a good insight into the tunable thermal transport for potential applications of not only monolayer MoSe2, but also many other TMDs.

First-principles study of thermal transport properties in the two- and three-dimensional forms of Bi2O2Se.

Recently, an air-stable layered semiconductor Bi2O2Se has been synthesized [Nat. Nanotechnol., 2017, 12, 530; Nano Lett. 2017, 17, 3021]. It possesses ultrahigh mobility, semiconductor properties, excellent environmental stability and easy accessibility. Here, we report on the thermal transport properties in monolayer (ML), bilayer (BL), and bulk forms of Bi2O2Se using density-functional theory and the Boltzmann transport approach. The results show that the ML exhibits better thermal transport properties than the BL and bulk. The intralayer opposite phonon vibrations greatly suppress the thermal transport and lead to an ultralow lattice thermal conductivity of ∼0.74 W m-1 K-1 in the ML, which has a large band gap of ∼2.12 eV, a low value of average acoustic group velocity of ∼0.76 km s-1, low-lying optical modes of ∼0.54 THz, strong optical-acoustic phonon coupling, and large Grüneisen parameters of ∼5.69. The size effect for all three forms is much less sensitive due to their short intrinsic phonon mean free path (MFP).

Tetragonal and trigonal Mo2B2 monolayers: two new low-dimensional materials for Li-ion and Na-ion batteries.

In this study, we report two new Mo2B2 monolayers and investigate their stabilities, electronic structures, lattice dynamics, and properties as anode materials for energy storage by using the crystal structure prediction technique and first-principles method. The calculated phonon spectra and electrical structures indicate that our predicted tetragonal and trigonal Mo2B2 (tetr- and tri-Mo2B2) monolayers possess excellent electronic conductivity and great stability. The adsorption energies of Li/Na on them are negative enough to ensure stability and safety under operating conditions. Besides, tetr-Mo2B2 possesses a theoretical specific capacity of ∼251 mA h g-1 for both Li- and Na-ion batteries (LIBs and NIBs), while tri-Mo2B2 possesses ∼251 and ∼188 mA h g-1 for LIBs and NIBs, respectively. The diffusion energy barriers of Li/Na over tetr- (0.029/0.010 eV) and tri- (0.023/0.013 eV) Mo2B2 are very small, indicating their excellent charge/discharge capability. In addition, the low electrode potential of Li/Na-intercalated tetr- and tri-Mo2B2 is beneficial to their performance as anode materials. This work is of great importance for widening the families of both anode materials for LIBs/NIBs and two-dimensional transition metal borides.

Novel two-dimensional tetragonal vanadium carbides and nitrides as promising materials for Li-ion batteries.

Two-dimensional (2D) materials, owing to their unique properties, have shown great potential for energy storage. In this work, we predict two types of new 2D transition metal carbides and nitrides, namely, tetragonal V2C2 and V2N2 (tetr-V2C2 and tetr-V2N2) monolayer sheets. Comprehensive first-principle calculations show that these two 2D systems exhibit dynamic (thermal) stabilities and intrinsic metallic nature. Compared with the commercialized graphite anode material, tetr-V2C2 and tetr-V2N2 monolayer sheets exhibit lower Li diffusion barrier of 89 and 94 meV, higher theoretical capacity of 412 and 425 mA h g-1 and lower average open circuit of 0.468 and 0.583 V, respectively. Combining those advanced features, our proposed tetr-V2C2 and tetr-V2N2 monolayer sheets are both promising candidates as anode materials for lithium-ion batteries (LIBs) in the future.

First-principles calculations of thermal transport properties in MoS2/MoSe2 bilayer heterostructure.

Bilayer transition metal dichalcogenide heterostructures obtained by vertical stacking have attracted considerable attention because of their potential applications in thermoelectric and optoelectronics devices. The thermal transport behavior plays a pivotal role in assessing their functional performance. Here, we systematically investigate the thermal transport properties of the MoS2/MoSe2 bilayer heterostructure (MoS2/MoSe2-BH) by combining first-principles calculations and Boltzmann transport theory (BTE). The results show that the thermal conductivity of MoS2/MoSe2-BH at room temperature is 25.39 W m-1 K-1, which is in-between those of monolayer MoSe2 and MoS2. According to our calculated orbital-resolved phonon dispersion curves, Grüneisen parameters, phonon group velocity and relaxation time, we find that the acoustic and low-frequency optical branches below 172.65 cm-1 have strong coupling and contribute mainly to the lattice thermal conductivity. Compared with free standing monolayer MoS2 and MoSe2, the lattice thermal conductivity of MoS2/MoSe2-BH is influenced by the weak van der Waals interlayer interactions.

Understanding of transition metal (Ru, W) doping into Nb for improved thermodynamic stability and hydrogen permeability: density functional theory calculations.

Hydrogen solubility and diffusivity are the key features of hydrogen permeable membrane materials. To characterize the hydrogen permeation performance of NbTM (TM = W, Ru) phases, their hydrogen diffusion coefficient and solution coefficient, thermodynamic stability and chemical bonding are studied by a series of first principles calculations. The phonon spectra and elastic constants show that NbTM is dynamically stable. The TM-H chemical bonds have an ionic/covalent mixed character and are stronger than the Nb-H bond. The preferential diffusion paths of H in both Nb16H and Nb15TMH are from a tetrahedral interstitial site (TIS) to another TIS. The TM doping in Nb16H lowers the solubility and energy barrier of H diffusion and enhances the H diffusion coefficient (D), with Nb16RuH exhibiting the highest D value for TIS to TIS diffusion (2.14 × 10-8 m2 s-1) at 600 K. This study shows that alloying and temperature could significantly affect the solubility and diffusivity of hydrogen in Nb. Moreover, TM doping could greatly improve the hydrogen diffusion performance with good control of hydrogen embrittlement.

Novel structures of two-dimensional tungsten boride and their superconductivity.

Two-dimensional (2D) superconductors, which can be widely applied in optoelectronic and microelectronic devices, have gained renewed attention in recent years. Based on the crystal structure prediction method and first-principles calculations, we obtain four novel 2D tungsten boride structures of tetr-, hex-, and tri-W2B2 and hex-WB4 and investigate their bonding types, electronic properties, phonon dispersions and electron-phonon coupling (EPC). The results show that both tetr- and hex-W2B2 are intrinsic phonon-mediated superconductors with a superconducting transition temperature (Tc) of 7.8 and 1.5 K, respectively, while tri-W2B2 and hex-WB4 are normal metals. We demonstrate that carrier doping as well as biaxial strain can soften the low-frequency phonon modes and enhance the strength of the EPC. While the Tc of tetr-W2B2 can be increased to 15.4 K under a compressive strain of -2%, the Tc of hex-W2B2 can be enhanced to 5.9 K by a tensile strain of +4%. With the inclusion of spin-orbit couping (SOC), the value of Tc decreases by 38.5% in our systems. Furthermore, we explore the stabilities and mechanical properties of tetr- and hex-W2B2 and indicate that they may be prepared by growing on ZnS(100) and ZnS(111), respectively. Our findings provide novel 2D superconducting materials and will stimulate more efforts in this filed.

Square transition-metal carbides MC6 (M = Mo, W) as stable two-dimensional Dirac cone materials.

Searching for new two-dimensional (2D) Dirac cone materials has been popular since the exfoliation of graphene. Herein, based on density functional theory, we predict a novel family of 2D Dirac cone materials in square transition-metal carbides MC6 (M = Mo, W) which show inherent stability confirmed by phonon spectrum analysis and ab initio molecular dynamics calculations. The Dirac point, located exactly at the Fermi level, mainly arises from the hybridization of M-dz2,x2-y2 and C-pz orbitals which gives rise to an ultrahigh Fermi velocity comparable to that of graphene. Moreover, strong spin-orbit coupling related to M-d electrons can generate large band gaps of 35 and 89 meV for MoC6 and WC6 monolayers, respectively, which allows MC6 materials to be operable at room temperature (26 meV), as candidates for nanoelectronics in the upcoming post-silicon era. The conceived novel stable metal-carbon framework materials provide a platform for designing 2D Dirac cone materials.

Hexagonal Ti2B2 monolayer: a promising anode material offering high rate capability for Li-ion and Na-ion batteries.

Combining the first-principles density functional method and crystal structure prediction techniques, we report a series of hexagonal two-dimensional transition metal borides including Sc2B2, Ti2B2, V2B2, Cr2B2, Y2B2, Zr2B2, and Mo2B2. Their dynamic and thermal stabilities are testified by phonon and molecular dynamics simulations. We investigate the potential of the two-dimensional Ti2B2 monolayer as an anode material for Li-ion and Na-ion batteries. The Ti2B2 monolayer possesses high theoretical specific capacities of 456 and 342 mA h g-1 for Li and Na, respectively. The very high Li/Na diffusivity with an ultralow energy barrier of 0.017/0.008 eV indicates an excellent charge-discharge capability. In addition, good electronic conductivity during the whole lithiation process is found by electronic structure calculations. The very small change in volume after the adsorption of one, two, and three layers of Li and Na ions indicates that the Ti2B2 monolayer is robust. These results highlight the suitability of Ti2B2 monolayer as well as the other two-dimensional transition metal borides as excellent anode materials for both Li-ion and Na-ion batteries.

Superconductivity in two-dimensional phosphorus carbide (ß0-PC).

Two-dimensional (2D) boron has been predicted to show superconductivity. However, intrinsic 2D carbon and phosphorus have not been reported to be superconductors, which has inspired us to study the superconductivity of their mixture. Here we performed first-principles calculations for the electronic structure, phonon dispersion, and electron-phonon coupling of the metallic phosphorus carbide monolayer, ß0-PC. The results show that it is an intrinsic phonon-mediated superconductor, with an estimated superconducting temperature Tc of ∼13 K. The main contribution to the electron-phonon coupling is from the out-of-plane vibrations of phosphorus. A Kohn anomaly on the first acoustic branch is observed. The superconducting related physical quantities are found to be tunable by applying strain or by carrier doping.

Lattice dynamics and chemical bonding in Sb2Te3 from first-principles calculations.

Pressure effects on the lattice dynamics and the chemical bonding of the three-dimensional topological insulator, Sb2Te3, have been studied from a first-principles perspective in its rhombohedral phase. Where it is possible to compare, theory agrees with most of the measured phonon dispersions. We find that the inclusion of relativistic effects, in terms of the spin-orbit interaction, affects the vibrational features to some extend and creates large fluctuations on phonon density of state in high frequency zone. By investigations of structure and electronic structure, we analyze in detail the semiconductor to metal transition at ∼2 GPa followed by an electronic topological transition at a pressure of ∼4.25 GPa.