Xue Jiang

Mechanical and electronic properties of B12-based ternary crystals of orthorhombic phase

Using first-principles calculations, the structural, mechanical and electronic properties of the experimentally synthesized B(12)-based ternary crystals (AlMgB(14), AlNaB(14), AlLiB(14), Mg(2)B(14), MgSi(2)B(12), MgC(2)B(12), Li(2)Si(2)B(12) and Li(2)C(2)B(12)) have been investigated. The theoretical equilibrium lattice constants of these crystals agree with the experimental values. The Vickers hardness (H(v)) estimated from the theoretical Youngs moduli ranges from 20 to 30 GPa, and the MgC(2)B(12) compound (H(v) = 31.4 GPa) is harder than α-boron. Based on the electron density of states and Mulliken population analysis, the origination of hardness and interaction between the interstitial atoms and the B(12) framework were discussed. Scaled bond order of the B-B bonds was used to interpret the hardness of these B(12)-based ternary compounds. The crystal hardness is primarily determined by the B(12) icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron transfer from metal to B atoms. We also calculated the ideal tensile strength of AlMgB(14) and MgC(2)B(12) and found that the <001> and <010> directions are their cleavage directions under tensile strains, respectively.

Structural stability, mechanical and electronic properties of cubic BCxN crystals within a random solid solution model

We propose a random solution model for cubic BC(x)N (0.21<x<19.28) crystals and compute the formation energies, elastic moduli and bandgaps for different compositions. Significant deviations of the elastic moduli and lattice parameters from the predictions of Vegards law reveal that the BC(x)N solid solutions are not a simple mixing of diamond and cubic-BN. The computed bandgaps are substantially lower than those of diamond and BN. Compared to BC(2)N, the BC(x)N solids with higher carbon content (x>2) exhibit better structural stability and higher elastic moduli, making them more attractive as potential superhard materials.

Bottom-up design of 2D organic photocatalysts for visible-light driven hydrogen evolution

To design two-dimensional (2D) organocatalysts, three series of covalent organic frameworks (COFs) are constructed using bottom-up strategies, i.e. molecular selection, tunable linkage, and functionalization. First-principles calculations are performed to confirm their photocatalytic activity under visible light. Two of our constructed 2D COF models (B1 and C3) are identified as a sufficiently efficient organocatalyst for visible light water splitting. The controllable construction of such COFs from suitable organic subunit, linkage, and functional groups paves the way for correlating band edge alignments and geometry parameters of 2D organic materials. Our theoretical prediction not only provides essential insights into designing 2D-COF photocatalysts for water splitting, but also sparks other technological applications for 2D organic materials.

Mechanical and electronic properties of diamond nanowires under tensile strain from first principles

The atomic and electronic structures, heat of formation, Youngs modulus, and ideal strength of hydrogenated diamond nanowires (DNWs) with different cross-sections (from 0.06 to 2.80 nm(2)) and crystallographic orientations ((100), (110), (111), and (112)) have been investigated by means of first-principles simulations. For thinner DNWs (cross-sectional area less than 0.6 nm(2)), preferential growth orientation along (111) is observed. The Youngs modulus and ideal strength of these DNWs decrease with decreasing cross-section and show anisotropic effects. Moreover, the band gap of DNWs is sensitive to the size, crystallographic orientation and tensile strain, implying the possibility of a tunable gap. The effective mass at the edges of the conduction band and valence band are also obtained. These theoretical results are helpful for designing novel optoelectronic devices and electromechanical sensors using diamond nanowires.

Giant magnetic anisotropy of a 5d transition metal decorated two-dimensional polyphthalocyanine framework

Magnetic properties of a 5d transition-metal adatom decorated two dimensional (2D) polyphthalocyanine framework (TM@Pc) are systematically investigated by means of first-principles calculations. Giant perpendicular magnetic anisotropy with a MAE of up to 20.7 meV is found in Re@Pc. After decorating with a homonuclear transition metal adatom, overturning of the easy axis is demonstrated and the magnetic anisotropy energy can be increased to over 40 meV for Os2@Pc and Ir2@Pc. The origin of magnetic anisotropy, structural stability and magnetic coupling behavior is also discussed. All the results show that these 2D organic materials can serve as promising candidates for future magnetic storage devices.

MBene (MnB): a new type of 2D metallic ferromagnet with high Curie temperature

We extend the 2D MXene family into the boride world, namely, MBenes. High-throughput calculations screen twelve MBenes with excellent stability. Among them, 2D MnB MBene exhibits robust metallic ferromagnetism (∼3.2 μB per Mn atom) and high Curie temperature (345 K). After functionalization with the -F and -OH groups, the ferromagnetic ground state of 2D MnB is well preserved. The Curie temperature is increased to 405 and 600 K, respectively, providing a novel and feasible strategy to tailor the TC of 2D magnetic materials.

Optimizing the thermoelectric transport properties of BiCuSeO via doping with the rare-earth variable-valence element Yb

Herein, we propose for the first time a novel recipe to improve the thermoelectric properties of BiCuSeO by doping with variable valence elements. Taking Yb-doped BiCuSeO as a typical example, the dimensionless figure of merit (ZT) is optimized by simultaneously doping with Yb2+ to increase the carrier concentration and by introducing Yb3+ to increase the carrier mobility. The mechanisms of valence fluctuation, the increase in carrier concentration and the increase in mobility are investigated by combining experimental results with first-principles calculations. By coupling high electronic conductivity, medium Seebeck coefficient, and low thermal conductivity, a maximum ZT value of 0.62 is achieved at 823 K for Bi0.7Yb0.3CuSeO, which is 1.55 times higher than that of pristine BiCuSeO. These findings undoubtedly provide a new strategy for the rational design of high-ZT thermoelectric materials.

Prediction of huge magnetic anisotropies in 5d transition metallocenes

The stability, electronic structure and non-collinear magnetic properties of a series of 5d metallocenes, namely, two cyclopentadienyl (Cp) rings sandwiched with a single 5d transition metal atom, are investigated. Our first-principles calculations reveal that Cp rings not only provide a suitable ligand environment for metal atoms, but also result in tunable magnetism depending on the transition metal element. Among them, HfCp2 and WCp2 show a high preference for the magnetization axis perpendicular to the Cp plane, with large magnetic anisotropy energies (MAEs) around 10 meV. We further consider triple decker metallocenes (M2Cp3), and find a huge MAE of above 60 meV in Ta2Cp3. The orbital energy split and shifts induced by composition change in metallocenes is mainly responsible for the significant MAE enhancement. By choosing a suitable crystal field for transition metal atoms, we pave a feasible pathway for designing promising building blocks of future magnetic storage devices.