Junling Meng

Compositionally tunable Cu2Sn(SxSe1−x)3 nanocrystals: facile direct solution-phase synthesis, characterization, and scalable procedure

In this paper, we report a facile, low-cost synthesis of Cu2Sn(SxSe1−x)3 colloidal nanocrystals (NCs) by heating a mixture of metal salts, sulfur powder, selenium powder, oleylamine and dodecanethiol. The composition of the Cu2Sn(SxSe1−x)3 NCs could be tuned across the x range from 0 to 1 by modulating the molar ratio of the S/Se precursors. The lattice parameters (a and c) of the Cu2Sn(SxSe1−x)3 NCs, calculated from X-ray diffraction patterns, were consistent with Vegards law, confirming the formation of homogeneous nanocrystals. The X-ray diffraction and transmission electron microscopy results indicated that the as-prepared Cu2Sn(SxSe1−x)3 NCs had a monoclinic structure. The UV-visible absorption spectra of the Cu2Sn(SxSe1−x)3 NCs revealed that the band gap of the nanocrystals could be tailored from 1.55 to 1.87 eV by decreasing the Se content. Additionally, compared with the more commonly used hot injection method, the procedure developed here is highly suitable for large-scale colloidal nanocrystal production, which we tested by performing a gram-scale synthesis.

Quantitative determination of Eliashberg function and evidence of strong electron coupling with multiple phonon modes in heavily overdoped (Bi,Pb)(2)Sr2CuO6+delta

It is known that the parity of a reflection amplitude can either be even or odd under a mirror operation. Up to now to our knowledge, all the parities of a reflection amplitude in the one-mode energy region have been even under a mirror operation. In this paper, we give an example of odd parity for Andreev reflection (AR) in a three-terminal graphene-supercondutor hybrid system. We found that the parity is even for the Andreev retroreflection and odd for specular Andreev reflection (SAR). We attribute this remarkable phenomenon to the distinct topology of the band structure of graphene and the specular Andreev reflection involving two energy bands with different parity symmetry. As a result of the odd parity of SAR, the SAR probability of a four-terminal system with two superconducting leads (two reflection interfaces) can be 0 even when the system is asymmetric due to the quantum interference of two ARs.

First-principles study of crystal structural stability and electronic and magnetic properties in LaMn(7)O(12).

The crystal structure, electronic and magnetic properties of LaMn(7)O(12) ((LaMn(3)(3+))(A)Mn(4)(3+)O(12)) are investigated by GGA (LSDA) and GGA + U (LSDA + U) (0.0 ≤ U ≤ 5.0 eV) methods. Based on two experimentally refined structures (distinguished by the distortion parameter Δ, namely S(I) (Δ = 8.5 × 10(-5)) and S(II) (Δ = 25.0 × 10(-4))), GGA and GGA + U with U < 3.0 eV calculations indicate that S(I) with a small distortion is the lowest-energy crystal structure while GGA + U with 3.0 ≤ U ≤ 5.0 eV calculations show that S(II) with a larger distortion is the ground-state crystal structure. Within the LSDA method, S(II) is always the ground-state structure no matter if U is considered or not. There are two independent magnetic sublattices: Mn(3+) within the A site and Mn(3+) within the B site. First, it is predicted that A-site Mn(3+) ions are preferably AFM-coupled in G-type (antiferromagnetically coupled in three directions). Based on this result, four magnetic configurations (FM-[Formula: see text], AFM1-[Formula: see text], AFM2-[Formula: see text] and AFM3-[Formula: see text]) are designed, and their total energies are calculated. Our results demonstrate that AFM2 and AFM3 are the lowest magnetic state, respectively, for S(I) and S(II). Correspondingly, LaMn(7)O(12) is metallic with no orbital ordering at AFM2 for S(I) while it is an insulator with orbital ordering at AFM3 for S(II). Thus, modulation of the distortion parameter Δ, e.g. by chemical doping, could be employed as a new avenue to induce a magnetic phase transition and the corresponding metal-to-insulator transition in LaMn(7)O(12).

Synergistic effects of intrinsic cation disorder and electron-deficient substitution on ion and electron conductivity in La1-xSrxCo0.5Mn0.5O3-δ (x = 0, 0.5, and 0.75).

The effects of intrinsic cation disorder and electron-deficient substitution for La1-xSrxCo0.5Mn0.5O3-δ (LSCM, x = 0, 0.5, and 0.75) on oxygen vacancy formation, and their influence on the electrochemical properties, were revealed through a combination of computer simulation and experimental study. First-principles calculations were first performed and found that the tendency of the oxygen vacancy formation energy was Mn(3+)-O*-Mn(4+) < Co(2+)-O*-Co(3+) < Co(2+)-O*-Mn(4+), meaning that antisite defects not only facilitate the formation of oxygen vacancy but introduce the mixed-valent transition-metal pairs for high electrical conductivity. Detailed partial density of states (PDOS) analysis for Mn on Co sites (MnCo) and Co on Mn sites (CoMn) indicate that Co(2+) is prone to being Co(3+) while Mn(4+) is prone to being Mn(3+) when they are on antisites, respectively. Also it was found that the holes introduced by Sr tend to enter the Co sublattice for x = 0.5 and then the O sublattice when x = 0.75, which further promotes oxygen vacancy formation, and these results are confirmed by both the calculated PDOS results and charge-density difference. On the basis of microscopic predictions, we intentionally synthesized a series of pure LSCM compounds and carried out comprehensive characterization. The crystal structures and their stability were characterized via powder X-ray Rietveld refinements and in situ high-temperature X-ray diffraction. X-ray photoelectron spectroscopy testified to the mixed oxidation states of Co(2+)/Co(3+) and Mn(3+)/Mn(4+). The thermal expansion coefficients were found to match the Ce0.8Sm0.2O2-δ electrolyte well. The electrical conductivities were about 41.4, 140.5, and 204.2 S cm(-1) at doping levels of x = 0, 0.5, and 0.75, and the corresponding impedances were 0.041, 0.027, and 0.022 Ω cm(2) at 850 °C, respectively. All of the measured results testify that Sr-doped LaCo0.5Mn0.5O3 compounds are promising cathode materials for intermediate-temperature solid oxide fuel cells.

Luminescence Mechanistic Study of BaLaGa3O7:Nd Using Density Functional Theory Calculations.

BaLaGa3O7:Nd (BLGO:Nd) has been investigated as a laser crystal material for about three decades. In the present work, the luminescence mechanism of BLGO:Nd is clarified by density functional theory (DFT) calculations. Structural optimization was first performed on the constructed supercell to obtain the equilibrium geometry. On the basis of the optimized crystal, the electronic structures of the BLGO host (without and with single defects) and the BLGO:Nd phosphor (without and with neighboring defects) were comprehensively investigated. Three important features are revealed by theoretical analyses. First, single defects in BLGO have little effect on the light emission, although the impurity levels appeared within the band gap. Second, luminescence can be realized by the introduction of Nd ions. Calculations of optical properties demonstrated that parity-forbidden transitions among the 4f levels are partially allowed because the mixing of 4f and 5d configurations occurs at higher empty 4f levels. It is thus clear that the electronic transitions between occupied 4f and empty 4f-5d states are electric-dipole-allowed. Therefore, light emission in BLGO:Nd can be achieved in the electronic transition process of Nd 4f electrons → empty 4f-5d levels → empty 5d levels → Nd 4f levels. The neighboring intrinsic defects play only an auxiliary role in prolonging the decay time. Third, co-doping of Tb in BLGO:Nd is considered to be beneficial to luminescence in theory because of its shallow to deep distribution of impurity orbitals in the band gap. Therefore, BLGO:Nd co-doped with other lanthanide ions will offer guidelines in the search for the best luminescent materials.

Strong-correlated behavior of 4f electrons and 4f5d hybridization in PrO2

Bringing oxygen atoms from infinite, passing equilibrium until short enough distances, we aim to reveal the 4f5d electron bonding property and its relevance to the peculiar physical properties within PrO2 based on both accounting for electron Coulomb repulsion and spin-orbit coupling effects in combination with Wannier function methods. The microscopic mechanism of static Janh-Teller distortions and the physical insight into the dynamic Jahn-Teller effects are clarified. Peculiarly, the magnetic coupling is suggested to be via 4f-5d-O2p-5d-4f pathway in PrO2, and the coupling between spin and orbital ordering of 4f electrons is for the first time disclosed. The 5d orbitals, hybridized with 4f electrons, are found to play important roles in these processes.

Tuning charge transfer in the LaTiO3/RO/LaNiO3 (R = rare-earth) superlattices by the rare-earth oxides interfaces from a first-principles calculation

We investigate the internal charge transfer at the isopolar interfaces in LaTiO3/RO/LaNiO3 (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) superlattices by means of density functional theory calculations. The charge transfer from Ti sites to Ni sites in all superlattices is induced by the electronegativity difference between the elements Ti and Ni, and the lanthanide oxides interfaces can modulate the amount of charge transfer. Comparison of the perovskite heterostructures with the different rare-earth interfaces shows that increasing the deviations of bond angles from 180.0° and the oxygen motions near the interfaces enhance charge transfer. The 4f electrons themselves of rare-earth elements have faint influences on charge transfer. In addition, the reasons why our calculated 4f states of Sm and Tm elements disagree with the experimental systems have been provided. It is hoped that all the calculated results could be used to design new functional nanoelectronic devices in perovskite oxides.We investigate the internal charge transfer at the isopolar interfaces in LaTiO3/RO/LaNiO3 (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) superlattices by means of density functional theory calculations. The charge transfer from Ti sites to Ni sites in all superlattices is induced by the electronegativity difference between the elements Ti and Ni, and the lanthanide oxides interfaces can modulate the amount of charge transfer. Comparison of the perovskite heterostructures with the different rare-earth interfaces shows that increasing the deviations of bond angles from 180.0° and the oxygen motions near the interfaces enhance charge transfer. The 4f electrons themselves of rare-earth elements have faint influences on charge transfer. In addition, the reasons why our calculated 4f states of Sm and Tm elements disagree with the experimental systems have been provided. It is hoped that all the calculated results could be used to design new functional nanoelectronic devices in perovskite oxides.

Insight into the Mechanism of the Ionic Conductivity for Ln-Doped Ceria (Ln = La, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, and Tm) through First-Principles Calculation

Oxygen vacancy (VO) formation energy and its migration barrier are two determining factors for the effectiveness of solid electrolytes (SEs) in solid oxide fuel cells (SOFCs). In this work, a series of aliovalent rare-earth-doped ceria (Ln xCe1- xO2-δ, Ln = lanthanides) compounds serving as SEs are comprehensively and comparatively calculated, through which the determinant factors for oxygen vacancy formations and their migration activity are figured out at an atomistic level via the first-principles calculations with the consideration of electronic correlations. Initially, it is found that the oxygen vacancy formation energies of the Ln-doped ceria are largely reduced in contrast to the undoped ceria (CeO2-δ), which obviously agree with the literature. Then, the migration activity of an oxygen vacancy in Ln xCe1- xO2-δ is closely correlated to the association energies of Ln-VO, in which the different 4f5d bonding properties for different Ln ions should be taken into account. Additionally, the analysis of charge difference gradient (CDG) is revealed to be the intrinsic driving force for oxygen vacancy migration. We hope that our investigation provides a microscopic insight into the oxygen vacancy defect physics, and it is also a benefit for the design of more advanced relevant functional materials.

Theoretical Study on the Negative Thermal Expansion Perovskite LaCu3Fe4O12: Pressure-Triggered Transition of Magnetism, Charge, and Spin State

The A-site ordered negative thermal expansion material LaCu3Fe4O12 (LaCFO) was comprehensively investigated by using first-principles calculations. A pressure-triggered crystal structural phase transition from space group Im3̅ (No. 204) to Pn3̅ (No. 201) and magnetic transformation from a G-type antiferromagnetic (G_AFM) ground state to ferrimagnetic (FerriM) coupling were observed in LaCFO via gradual compression of the equilibrium volume. Correspondingly, the Fe-Cu intersite charge transfer from Fe to Cu 3dxy orbital, expressed as 4Fe3+ + 3Cu3+ → 4Fe3.75+ + 3Cu2+, was simulated along with the magnetic phase transformation from the G_AFM configuration to the FerriM state. Intriguingly, the Fe charge disproportionation, formulated as 8Fe3.75+ → 5Fe3+ + 3Fe5+, appeared and was attributed to the strong hybridization between Fe 3d and O 2p orbitals in the FerriM state when the volumes were substantially compressed up to less than or equal to 80%V. Meanwhile, the external hydrostatic pressure also leads to a spin flip from a high-spin Fe3+ antiferromagnetically arranged LaCu3+3Fe3+4O12 Mott insulator at low pressure and goes through a FerriM LaCu2+3Fe3.75+4O12 half-metal to a low-spin FerriM coupled LaCu2+3Fe3+5/2Fe5+3/2O12 metal at high pressure. Therefore, the crossover from high spin to low spin is responsible for the charge disproportionation in LaCFO. Essentially, the charge transfer and spin flip originate from the discontinuous changes of metal-oxygen bond lengths and angles in the compressed atomic structure. Finally, the negative thermal expansion behavior and mechanism of LaCFO were theoretically examined and clearly revealed.