Modifying a D-A-π-A-D HTM system for higher hole mobility by the meta-substitution strategy to weaken the electron-donating ability of the donor unit: a DFT study.

Tuning the electron-donating ability (EDA) of the donor units of hole transporting materials (HTMs) is an efficient strategy to modulate the optoelectronic properties of HTMs. Based on this strategy, we first theoretically investigated the effects of the EDA of donor units on D-A-π-A-D architectural HTMs. The results show that the enhanced EDA of the donor unit leads to larger hole reorganization energy and poorer molecular stability of HTMs. In contrast, meta-substitution of side groups is an effective strategy to reduce the EDA of the donor unit. We found that the application of the meta-substitution strategy in the D-A-π-A-D system not only successfully improves the molecular stability, but also achieves higher hole mobility by promoting the electronic coupling between the molecular dimers and decreasing the hole reorganization energies simultaneously. Studies on interfacial properties indicate that intermolecular coupling also synergistically enhances the interfacial charge extraction performance and reduces carrier recombination. In conclusion, by utilizing the meta-substitution strategy to reduce the EDA of donor units on D-A-π-A-D architectural HTMs, we successfully designed four superior performance HTMs mD1, mD2, mD3, and mD4.

Ferromagnetic vanadium disulfide VS2 monolayers with high Curie temperature and high spin polarization.

The structural, electronic, and magnetic properties of vanadium disulfide VS2 monolayers were investigated using first-principles calculations and Monte Carlo (MC) simulations. The results of molecular dynamics simulations and phonon dispersion showed that the VS2 monolayer has good dynamic and thermodynamic stabilities. Based on the results of the band structure, we also explore the effect of carrier concentrations on the spin gap, spin polarization and the direction of the easy magnetic axis. Our results demonstrated that doping an appropriate amount of holes can cause the reversal of the easy magnetic axis and maintain nearly 100% spin polarization, which greatly improves the application possibility of the VS2 monolayer as a spintronic device. The contribution of different orbits to the spin-orbit coupling (SOC) effect is given in magnetocrystalline anisotropy energy, which provides a theoretical basis for explaining the origin of magnetic crystal anisotropy. Based on the MC simulations, we also showed the influences of different parameters (carrier concentrations, magnetic field and crystal field) on the magnetothermal properties of the VS2 monolayer. It is found that the increase of hole doping concentrations can promote the increase of the Curie temperature, while the increase of electron doping concentrations will greatly weaken the Curie temperature. Furthermore, according to the influences of different parameters on the Curie temperature and spin polarization, we conclude that a suitably enhanced magnetic field and appropriate hole concentrations will not only make the system maintain high spin polarization, but also make the system exhibit ferromagnetic properties above room temperature.

A first-principles study on the electronic, piezoelectric, and optical properties and strain-dependent carrier mobility of Janus TiXY (X ≠ Y, X/Y = Cl, Br, I) monolayers.

Janus transition metal dichalcogenide monolayers (TMDs) have attracted wide attention due to their unique physical and chemical properties since the successful synthesis of the MoSSe monolayer. However, the related studies of Janus monolayers of transition metal halides (TMHs) with similar structures have rarely been reported. In this article, we systematically investigate the electronic properties, piezoelectric properties, optical properties, and carrier mobility of new Janus TiXY (X ≠ Y, X/Y = Cl, Br, I) monolayers using first principles calculations for the first time. These Janus TiXY monolayers are thermally, dynamically, and mechanically stable, and their energy bands near the Fermi level (EF) are almost entirely contributed by the central Ti atom. Besides, the Janus TiXY monolayers exhibit excellent in-plane and out-of-plane piezoelectric effects, especially with an in-plane piezoelectric coefficient of ∼4.58 pm V-1 for the TiBrI monolayer and an out-of-plane piezoelectric coefficient of ∼1.63 pm V-1 for the TiClI monolayer, suggesting their promising applications in piezoelectric sensors and energy storage applications. The absorption spectra of Janus TiXY monolayers are mainly distributed in the visible and infrared regions, implying that they are fantastic candidates for photoelectric and photovoltaic applications. The obtained carrier mobilities revealed that TiXY monolayers are hole-type semiconductors. Under uniaxial compressive strain, the hole mobilities of these monolayers are gradually improved, indicating that TiXY monolayers have potential applications in the field of flexible electronic devices.

Electronic and thermoelectric properties of semiconducting Bi2SSe2 and Bi2S2Se monolayers with high optical absorption.

Bismuth telluride (Bi2Te3) and its derivatives are often focused on as thermoelectric materials around room temperature. In this work, we theoretically predicted two new types of Bi2Te3-based two-dimensional materials Bi2SSe2 and Bi2S2Se using density functional theory (DFT) combined with Boltzmann transport theory. The thermal, dynamic, and mechanical stabilities of Bi2SSe2 and Bi2S2Se monolayers are confirmed using ab initio molecular dynamics (AIMD) simulations, phonon dispersion, and elastic constant calculations. The phonon transport properties, including lattice thermal conductivity, group velocity, Grüneisen parameter, converged scattering rate, and phonon lifetimes contributed by different branches, are systematically investigated. The electronic and thermoelectric properties, including carrier mobility (µ), Seebeck coefficient (S), electrical conductivity (σ), power factors, and figure of merit (zT) along the zigzag and armchair directions as a function of carrier concentration at different temperatures, are also investigated. It is found that the Bi2SSe2 and Bi2S2Se monolayers have moderate indirect band gaps (0.92 eV and 1.08 eV at the PBE level, respectively) and low lattice thermal conductivities (4.35 and 5.37 W m-1 K-1 at 300 K, respectively). The largest zT values of Bi2SSe2 and Bi2S2Se monolayers are 0.50 and 0.28 at 300 K and 1.39 and 0.93 at 700 K for p-doping types, respectively. The Bi2SSe2 and Bi2S2Se monolayers are predicted to show high optical absorption peaks at 8 × 105 cm-1 in the visible and near-UV light region, respectively. Our results indicate that both Bi2SSe2 and Bi2S2Se could be promising candidates in energy conversion, solar cells, and optoelectronic devices.

An Efficient Dopant for Introducing Magnetism into Topological Insulator Bi2Se3.

In this work, we obtained an effective way to introduce magnetism into topological insulators, and successfully fabricated single crystal C-Bi2Se3. The structural, electrical and magnetic properties of non-magnetic element X (B, C and N) doped at Bi, Se1, Se2 and VDW gap sites of Bi2Se3 were studied by the first principles. It is shown that the impurity bands formed inside the bulk inverted energy gap near the Fermi level with C doping Bi2Se3. Due to spin-polarized ferromagnetic coupling, the time inversion symmetry of Bi2Se3 is destroyed. Remarkably, C is the most effective dopant because of the magnetic moment produced by doping at all positions. The experiment confirmed that the remnant ferromagnetism Mr is related to the C concentration. Theoretical calculations and experiments confirmed that carbon-doped Bi2Se3 is ferromagnetic, which provides a plan for manipulating topological properties and exploring spintronic applications.

Electron Transport of the Nanojunctions of (BN) n (n = 1-4) Linear Chains: A First-Principles Study.

We applied the density functional theory and nonequilibrium Green's function method (DFT + NEGF) to investigate the relationship between the conductance and chain length in the stretching process, the asymmetric coupling of contact points, and the influence of positive and negative biases on the electron transport properties of the nanojunctions formed by the coupling of (BN) n (n = 1-4) linear chains and Au(100)-3 × 3 semi-infinite electrodes. We find that the BN junction has the lowest stability and the (BN)2 junction has the highest stability. Under zero bias, the equilibrium conductance decreases as the chain length increases; px and py orbitals play a leading role in electron transport. In the bias range of -1.6 to 1.6 V, the current of the (BN) n (n = 1-4) linear chains increases linearly with increasing voltage. Under the same bias voltage, (BN)1 has the largest current, so its electron transport property is the best. The rectification effect reflects the asymmetry of the structure of BN linear chains themselves and the asymmetry of coupling with the Au electrode surfaces at both ends. With the chain length increasing, the transmission spectrum near E f is suppressed, the tunneling current decreases, and the rectification ratio increases. (BN)4 molecular junctions have the largest rectification ratio, reaching 13.32 when the bias voltage is 1.6 V. Additionally, the Au-N strong coupling is more conducive to the electron transport of the molecular chain than the Au-B weak coupling. Our calculations provide an important theoretical reference for the design and development of BN linear-chain nanodevices.

Biaxial Tensile Strain-Induced Enhancement of Thermoelectric Efficiency of α-Phase Se2Te and SeTe2 Monolayers.

Thermoelectric (TE) materials can convert waste heat into electrical energy, which has attracted great interest in recent years. In this paper, the effect of biaxial-tensile strain on the electronic properties, lattice thermal conductivity, and thermoelectric performance of α-phase Se2Te and SeTe2 monolayers are calculated based on density-functional theory and the semiclassical Boltzmann theory. The calculated results show that the tensile strain reduces the bandgap because the bond length between atoms enlarges. Moreover, the tensile strain strengthens the scatting rate while it weakens the group velocity and softens the phonon model, leading to lower lattice thermal conductivity kl. Simultaneously, combined with the weakened kl, the tensile strain can also effectively modulate the electronic transport coefficients, such as the electronic conductivity, Seebeck coefficient, and electronic thermal conductivity, to greatly enhance the ZT value. In particular, the maximum n-type doping ZT under 1% and 3% strain increases up to six and five times higher than the corresponding ZT without strain for the Se2Te and SeTe2 monolayers, respectively. Our calculations indicated that the tensile strain can effectively enhance the thermoelectric efficiency of Se2Te and SeTe2 monolayers and they have great potential as TE materials.

Structural and electronic transport properties of a SiC chain encapsulated inside a SiC nanotube: first-principles study.

Silicon carbide (SiC) chains and silicon carbide nanotubes (SiCNTs), as promising one-dimensional nanostructures, have potential applications in more controllable nanoelectronic devices. In this paper, we design a completely new hybrid structure with encapsulation of a linear SiC chain inside a SiCNT, using first-principles calculation and the non-equilibrium Green's function formalism to systematically investigate the structural stability and electronic properties, particularly the quantum transport properties. It is found that, due to the nanotube-chain interaction, the stability of this structure is mainly provided by the charge transfer from the hosting tube to the guest chain. Furthermore, the transport properties of the hybrid structure confirm that encapsulation of a SiC chain within a SiCNT can significantly enhance the electronic transport of the component system in a wide range of high voltage. The distance and the unique coupling configuration between the encapsulated system and the electrodes are demonstrated to be other important factors that affect the transport behaviours. We hope that our study of encapsulation may offer a significant starting point for the design of new materials related to low-dimensional SiC nanostructures and possibly open a novel path towards stability and conductivity enhancement.

Electronic and Magnetic Properties of 5d1, 5d2, and 5d3 Double Perovskites Ba2MOsO6 (M = K, Ca, and Sc): Ab Initio Study.

We perform detailed investigations on the electronic and magnetic properties in double perovskites Ba2MOsO6 (M = K, Ca, and Sc), with formal valences of Os7+ (5d1), Os6+ (5d2), and Os5+ (5d3), respectively, using first-principles calculations. To understand the effects of Coulomb interaction ( U), spin-orbit coupling (SOC), and magnetic order, we carry out different calculations within density functional theory. It is shown that SOC and U energy not only provide the magneto crystalline anisotropies but also significantly affect the size of the local moments and the magnetic structures in these compounds. The electronic configuration of 5d1 and 5d2 of Os in Ba2MOsO6 (M = K and Ca) have the metal-insulator transition (MIT) as the direction of the local moment changes, while the electronic structure of half-filled 5d3 orbitals of Os in Ba2ScOsO6 is insulator, independent of the local moment direction. Our results indicate that both SOC and U interactions are necessary in enlarging the band gaps and putting these compounds into the MIT correlated insulators.

Quantum anomalous/valley Hall effect and tunable quantum state in hydrogenated arsenene decorated with a transition metal.

The quantum anomalous Hall (QAH) effect is superior to the quantum spin Hall (QSH) effect, which can avoid the inelastic scattering of two edge electrons located on one side of a topological nontrivial material, and thus it has attracted both theoretical and experimental interest. Here, we systematically investigate the lattice structures, and electronic and magnetic properties of hydrogenated arsenene decorated with certain transition metals (Cr, Mo and Cu) based on density-functional theory. A unique QAH effect in Mo@AsH is predicted, whose Chern number (C = 1) indicates only one chiral edge channel located on its one side. Then, we prove that this QAH effect realization is closely related with band inversion, which is the competitive result between its spin-orbit coupling (SOC) strength and exchange field. The quantum state of Mo@AsH can also be tuned by an external strain, similar to SOC, and it is noted that its increased topological gap of about 35 meV under 5.0% tensile strain, is large enough to realize the QAH effect at room-temperature. Additionally, the quantum valley Hall effect in Cu@AsH contributed by the inequality of AB sublattices is also found. Our results reveal the physical mechanism to realize the QAH effect in TM@AsH and provide a platform for electrically controllable topological states, which are highly desirable for nanoelectronics and spintronics.

Ab Initio Study of Ionized Water Radical Cation (H2O)8+ in Combination with the Particle Swarm Optimization Method.

The structures of cationic water clusters (H2O)8+ have been globally explored by the particle swarm optimization method in combination with quantum chemical calculations. Geometry optimization and vibrational analysis for the 15 most interesting clusters were computed at the MP2/aug-cc-pVDZ level and infrared spectrum calculation at MPW1K/6-311++G** level. Special attention was paid to the relationships between their configurations and energies. Both MP2 and B3LYP-D3 calculations revealed that the cage-like structure is the most stable, which is different from a five-membered ring lowest energy structure but agrees well with a cage-like structure in the literature. Furthermore, our obtained cage-like structure is more stable by 0.87 and 1.23 kcal/mol than the previously reported structures at MP2 and B3LYP-D3 levels, respectively. Interestingly, on the basis of their relative Gibbs free energies and the temperature dependence of populations, the cage-like structure predominates only at very low temperatures, and the most dominating species transforms into a newfound four-membered ring structure from 100 to 400 K, which can contribute greatly to the experimental infrared spectrum. By topological analysis and reduced density gradient analysis, we also investigated the structural characteristics and bonding strengths of these water cluster radical cations.

Ab initio investigation of structure, stability, thermal behavior, bonding, and infrared spectra of ionized water cluster (H2O)6.

The low-lying isomers of cationic water cluster (H2O)6+ have been globally explored by using particle swarm optimization algorithm in conjunction with quantum chemical calculations. Compared with previous results, our searching method covers a wide range of structural isomers of (H2O)6+ and therefore turns out to be more effective. With these local minima, geometry optimization and vibrational analysis are performed for the most interesting clusters at second-order Møller-Plesset (MP2)/aug-cc-pVDZ level, and their energies are further refined at MP2/aug-cc-pVTZ and coupled-cluster theory with single, double, and perturbative triple excitations/aug-cc-pVDZ level. The interaction energies using the complete basis set limits at MP2 level are also reported. The relationships between their structure arrangement and their energies are discussed. Based on the results of thermal simulation, structural change from a four-numbered ring to a tree-like structure occurs at T ≈ 45 K, and the relative population of six lowest-free-energy isomers is found to exceed 4% at some point within the studied temperature range. Studies reveal that, among these six isomers, two new-found isomers constitute 10% of isomer population at 180 K, and the experimental spectra can be better explained with inclusions of the two isomers. The molecular orbitals for six representative cationic water clusters are also studied. Through topological and reduced density gradient analysis, we investigated the structural characteristics and the bonding strengths of these water cluster radical cations.

Study of the thermodynamic properties of CeO2 from ab initio calculations: the effect of phonon-phonon interaction.

The thermodynamic properties of CeO2 have been reevaluated by a simple but accurate scheme. All our calculations are based on the self-consistent ab initio lattice dynamical (SCAILD) method that goes beyond the quasiharmonic approximation. Through this method, the effects of phonon-phonon interactions are included. The obtained thermodynamic properties and phonon dispersion relations are in good agreement with experimental data when considering the correction of phonon-phonon interaction. We find that the correction of phonon-phonon interaction is equally important and should not be neglected. At last, by comparing with quasiharmonic approximation, the present scheme based on SCAILD method is probably more suitable for high temperature systems.

Density functional theory investigation of the phonon instability, thermal equation of state and melting curve of Mo.

The phonon instability and thermal equation of state of Mo are extensively investigated using density functional theory. The calculated phonon dispersion curves agree well with experiments. Under compression, we captured a large softening in the transverse acoustic (TA) branches of body-centred cubic Mo. When the pressure is raised to 716 GPa, the frequencies along Γ-N in the TA branches soften to imaginary frequencies, indicating structural instability. For face-centred cubic Mo, the phonon calculations predicted the stability by promoting the frequencies from imaginary to real. Within quasi-harmonic approximation, we predicted the thermal equation of state and some other properties including the thermal expansion coefficient α, product αK(T), heat capacity C(V), entropy S, Grüneisen parameter γ and Debye temperature Θ(D). The melting curves of Mo were also obtained successfully.

Lattice dynamics and thermodynamics of molybdenum from first-principles calculations.

We calculated the phase transition, elastic constants, full phonon dispersion curves, and thermal properties of molybdenum (Mo) for a wide range of pressures using density functional theory. Mo is stable in the body-centered-cubic (bcc) structure up to 703 +/- 19 GPa and then transforms to the face-centered close-packed (fcc) structure at zero temperature. Under high temperature and pressure, the fcc phase of Mo is more stable than the bcc phase. The calculated phonon dispersion curves accord excellently with experiments. Under pressure, we captured a large softening along H-P in the TA branches. When the volume is compressed to 7.69 A(3), the frequencies along H-P in the TA branches soften to imaginary frequencies, indicating a structural instability. When the pressure increases, the phonon calculations on the fcc Mo predict the stability by promoting the frequencies along Gamma to X and Gamma to L symmetry lines from imaginary to real. The thermal equation of state was also investigated. From the thermal expansion coefficient and the heat capacity, we found that the quasiharmonic approximation was valid only up to about melting point at zero pressure. However, under pressure, the validity can be extended to a much higher temperature.