Kailun Yao

Nine new phosphorene polymorphs with non-honeycomb structures: A much extended family

We predict a new class of monolayer phosphorus allotropes, namely, ε-P, ζ-P, η-P, and θ-P. Distinctly different from the monolayer α-P (black) and previously predicted β-P (Phys. Rev. Lett. 2014, 112, 176802), γ-P, and δ-P (Phys. Rev. Lett. 2014, 113, 046804) with buckled honeycomb lattice, the new allotropes are composed of P4 square or P5 pentagon units that favor tricoordination for P atoms. The new four polymorphs, together with five additional hybrid polymorphs, greatly enrich the phosphorene structures, and their stabilities are confirmed by first-principles calculations. In particular, the θ-P is shown to be equally stable as the α-P (black) and more stable than all previously reported phosphorene polymorphs. Prediction of nonvolatile ferroelastic switching and structural transformation among different polymorphs under strains points out their potential applications via strain engineering.

Synthesis of magnetite nanoparticles in W/O microemulsion

huazhong univ sci & technol, dept phys, wuhan 430074, peoples r china. huazhong univ sci & technol, state key lab laser technol, wuhan 430074, peoples r china. chinese acad sci, int ctr mat phys, shenyang 110015, peoples r china. tongji med coll, wuhan 430030, peoples r china.;liu, zl (reprint author), huazhong univ sci & technol, dept phys, wuhan 430074, peoples r china;wangxin207@sohu.com klyao@hust.edu.cn

Half-metallic sp-electron ferromagnets in rocksalt structure: The case of SrC and BaC

First-principles calculations are employed to study the structural and magnetic properties of SrC and BaC in four different phases: rocksalt, CsCl, zinc blende, and NiAs. The obtained results indicate that both compounds show half-metallic behavior in all phases except the CsCl-type structure. Among them the rocksalt structure is found to be the most stable one with a robust half-metallic characteristic with respect to the lattice compression and expansion. The magnetic phase transition temperature Tc of the rocksalt phase is calculated employing both the mean-field approximation and the random-phase approximation. The predicted Tc values of both compounds are considerably above room temperature.

High-efficient thermoelectric materials: The case of orthorhombic IV-VI compounds.

Improving the thermoelectric efficiency is one of the greatest challenges in materials science. The recent discovery of excellent thermoelectric performance in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of orthorhombic IV-VI compounds GeS,GeSe,SnS, and SnSe based on the first-principles combined with the Boltzmann transport theory, we show that the Seebeck coefficient, electrical conductivity, and thermal conductivity of orthorhombic SnSe are in agreement with the recent experiment. Importantly, GeS, GeSe, and SnS exhibit comparative thermoelectric performance compared to SnSe. Especially, the Seebeck coefficients of GeS, GeSe, and SnS are even larger than that of SnSe under the studied carrier concentration and temperature region. We also use the Cahills model to estimate the lattice thermal conductivities at the room temperature. The large Seebeck coefficients, high power factors, and low thermal conductivities make these four orthorhombic IV-VI compounds promising candidates for high-efficient thermoelectric materials.

Preserving the half-metallicity at the surfaces of rocksalt CaN and SrN and the interfaces of CaN/InN and SrN/GaP: a density functional study.

Recent theoretical studies indicate that metastable rocksalt CaN, SrN, and BaN exhibit half-metallic ferromagnetism (Volnianska and Boguslawski 2007 Phys. Rev. B 75 224418; Gao et al 2008 Phys. Lett. A 372 1512), and further experiments confirm the existence of self-assembled metastable CaN nanostructures (Liu et al 2008 Surf. Sci. 602 1844). We here use the first-principles method based on density functional theory to investigate the structural, electronic, and magnetic properties of the (111) surfaces of CaN and SrN and the interfaces of CaN/InN(111) and SrN/GaP(111). The surface stability from the calculated surface energy indicates that the N-terminated (111) surface is more stable than the Ca (Sr)-terminated (111) surface in the N-rich environment. For CaN and SrN, both anion- and cation-terminated (111) surfaces preserve the half-metallic characteristics of the bulk. Interfacial studies indicate that the half-metallicity of bulk CaN is retained in two of the four possible configurations of the CaN/InN(111) interface, while for the interface of SrN/GaP(111) only one interfacial configuration shows half-metallicity. Furthermore, we assess the interfacial adhesive strength for all the possible different configurations of the interfaces studied here by calculating the interface adhesion energies.

Spin Seebeck Effect and Thermal Colossal Magnetoresistance in Graphene Nanoribbon Heterojunction

Spin caloritronics devices are very important for future development of low-power-consumption technology. We propose a new spin caloritronics device based on zigzag graphene nanoribbon (ZGNR), which is a heterojunction consisting of single-hydrogen-terminated ZGNR (ZGNR-H) and double-hydrogen-terminated ZGNR (ZGNR-H2). We predict that spin-up and spin-down currents flowing in opposite directions can be induced by temperature difference instead of external electrical bias. The thermal spin-up current is considerably large and greatly improved compared with previous work in graphene. Moreover, the thermal colossal magnetoresistance is obtained in our research, which could be used to fabricate highly-efficient spin caloritronics MR devices.

The transport properties and new device design: the case of 6,6,12-graphyne nanoribbons

By performing first-principle quantum transport calculations, we studied the transport properties of three kinds of 6,6,12-graphyne nanoribbons with different edges and different cutting directions. The nanoribbon with zigzag edges shows metallic properties and the spin-polarized currents show an obvious negative differential resistance effect, the other two nanoribbons terminated by a phenyl ring are semiconductors and spin-unpolarized. We also designed several nanowire devices based on these 6,6,12-graphyne nanoribbons, such as rectifier, spin filter diode, spin FET and spin caloritronics devices. These results indicate that 6,6,12-graphyne is a potential candidate for spintronics and spin caloritronics.

Surface sp half-metallicity of zinc-blende calcium monocarbide

Recent studies by Gao et al. [Phys. Rev. B 75, 174442 (2007)] indicate zinc-blende CaC, SrC, and BaC exhibit robust sp half-metallic ferromagnetism with Curie temperatures higher than room temperature. Here we further investigate the surface electronic and magnetic properties of CaC by using the first-principles full-potential linearized augmented plane-wave method. The (001) surfaces terminated with Ca and C, respectively, and the (110) surface terminated with both Ca and C are considered. We discuss the surface stabilities from the calculated relaxed surface energies. Electronic structure calculations indicate that the half-metallicity is destroyed for both the Ca- and C-terminated (001) surfaces; however, the (110) surface preserves the half-metallic characteristic of the bulk CaC. We further reveal that the atomic magnetic moments of the (001) surfaces are greatly different from the bulk values, but the difference of atomic magnetic moments between the (110) surface and the bulk CaC is very small.

Electric properties of BiFeO3 films deposited on LaNiO3 by sol-gel process

Sol-gel process was adopted to prepare BiFeO3 films. Pure phase BiFeO3 films were deposited on LaNiO3 coated Si (111) substrates at various annealing temperatures of 450–600°C. The films annealed at 450–600°C are (110) and (110) biaxis preferential oriented. Below 550°C, the remnant polarization increases with the annealed temperature. The film annealed at 550°C has the largest double remnant polarization of 12.8μC∕cm2. For the film annealed at 600°C, small double remnant polarization of 2Pr=4.6μC∕cm2 was observed for its low breakdown electric field. Besides, the electric property is enhanced with the annealing temperature below 550°C and it is deteriorated for the film annealed at 600°C. Large dielectric constant and low leakage conduction were obtained by the improvement of preparation technology.

Preparation and electrochromic properties of nanocrystalline WO3 thin films prepared by pulsed excimer laser ablation technique

Tungsten oxide thin films with different stoichiometry have been deposited on In2O3:SnO2 glass (ITO) and Si wafer by pulsed laser deposition (PLD) technique at different deposition temperatures and oxygen pressure. The structural properties of WO3 thin films were analyzed by using a scanning accessory of transmission electron microscope (STEM). X-ray diffraction (XRD), Fourier transform infrared spectrum (FT-IR), Raman spectrum (RS). Thin films deposited at 200 degreesC on ITO showed amorphous structure, those deposited at 300 and 400 degreesC on ITO showed nanocrystalline triclinic structure. As for samples deposited at 400 degreesC, in which the crystals keep nanocrystal line, cyclic voltammograms at sweep rate of 50 mV/s shows that no long-term degradation was noted at least up to 1000 cycles, and durability was verified to 8000 cycles in the voltage range between -1.2 and 1.4 V. Its open and porous structure is suitable for Li-ions going into and out, and induced improved cycle stability and reversibility.