Chuanyun Xiao

Half-metallicity in organic single porous sheets.

The unprecedented applications of two-dimensional (2D) atomic sheets in spintronics are formidably hindered by the lack of ordered spin structures. Here we present first-principles calculations demonstrating that the recently synthesized dimethylmethylene-bridged triphenylamine (DTPA) porous sheet is a ferromagnetic half-metal and that the size of the band gap in the semiconducting channel is roughly 1 eV, which makes the DTPA sheet an ideal candidate for a spin-selective conductor. In addition, the robust half-metallicity of the 2D DTPA sheet under external strain increases the possibility of applications in nanoelectric devices. In view of the most recent experimental progress on controlled synthesis, organic porous sheets pave a practical way to achieve new spintronics.

Pd and Ag dimers and tetramers adsorbed at the MgO(001) surface: a density functional study

We have studied computationally the adsorption properties of small Ag and Pd clusters deposited on the MgO(001) surface. The calculations were carried out employing a gradient-corrected density functional approach; the oxide surface was represented by model clusters embedded in a large array of point charges. Supported Ag and Pd dimers and tetramers were investigated in order to identify the preferred adsorption sites and the modifications induced in the cluster by the interaction with the substrate. All metal clusters adsorb in proximity of oxygen centers. An adsorption mode with the molecular axis parallel to the surface is the most stable one for Pd2 while Ag2 prefers an upright adsorption mode. Various isomer structures of the supported metal tetramers have been considered. In general, the most stable gas phase structure is also the preferred one upon adsorption. This suggests that the metal–metal bonding prevails over the metal–MgO interaction.

Geometric and Electronic Structure of WSiN(N = 1–6, 12) Clusters

Geometry optimizations of WSiN (n = 1–6, 12) clusters are performed using the B3LYP/ LanL2DZ method for a sequence of different spin states, changing from spin singlet to spin septet conditions. The resulting equilibrium structures are discussed under the aspects of geometric features, cluster internal charge transfer and magnetic properties. It is shown that the W impurity in the SiN environment generally acts as an electron acceptor. However, the charge on the W atom, as obtained by natural population analysis, can be sensitively tuned through the variation of the spin constraint from S = 0 to S = 3. The resulting geometries of WSin (N = 3–6) are compared with the known ground state structures of SiN+1 (N = 3–6), and substitutional geometries are identified for N = 3 and N = 5. The nonzero spin states of WSiN are shown to display different patterns of magnetic order, corresponding to uniform and to alternating atomic spin orientations within the cluster. Highly compact Oh and D6h structures are identified as stable geometries of WSi6 and of the experimentally detected unit WSi12, respectively. Comparison is made with the cluster series MoSiN(N = 1–6) and CuSiN(N = 1–6,12).

Lithium-doped MOF impregnated with lithium-coated fullerenes: A hydrogen storage route for high gravimetric and volumetric uptakes at ambient temperatures

We theoretically demonstrated that by the impregnation of Li-decorated IRMOF-10 with Li-coated C(60), the hydrogen storage capacity is improved to be 6.3 wt% and 42 g L(-1) at 100 bar and 243 K. Both the gravimetric and volumetric hydrogen uptakes reach the 2015 DOE target at near ambient conditions.

Charge transfer mechanism in Cu-doped silicon clusters: a density functional study

Abstract The geometric and bonding properties of Si n Cu ( n =4, 6) clusters were studied using the ab initio and density functional methods. Three isomers for Si 4 Cu and two isomers for Si 6 Cu were found. The structure of the most stable isomer for Si n Cu keeps the framework of corresponding Si n cluster nearly unchanged. Charge transfer in Si n Cu was examined to proceed from Cu to Si atoms. Population analysis indicates that the 3d shell of Cu atom is only slightly disturbed in Si n Cu and that Cu behaves like an alkali metal atom. The bonding nature in Si n Cu was analyzed by correlating the HOMO of Si n Cu to the LUMO of corresponding Si n cluster, and a dominant s–p or p–p bonding as well as a weak d–p bonding was found for the Cu–Si interaction in Si n Cu. This bonding nature is quite different from the interaction of 3d metals on silicon surfaces or 3d impurities in bulk silicon, where the bonding involving Si 3p and M 3d overlap is dominant. Remarkable similarity is found both in the geometry and in the bonding nature between the corresponding isomers of Si n Cu and Si n Na, confirming the alkali-metal-like behavior of Cu in Si n Cu.

Two-Dimensional Hexagonal Transition-Metal Oxide for Spintronics

Two-dimensional materials have been the hot subject of studies due to their great potential in applications. However, their applications in spintronics have been blocked by the difficulty in producing ordered spin structures in 2D structures. Here we demonstrated that the ultrathin films of recently experimentally realized wurtzite MnO can automatically transform into a stable graphitic structure with ordered spin arrangement via density functional calculation, and the stability of graphitic structure can be enhanced by external strain. Moreover, the antiferromagnetic ordering of graphitic MnO single layer can be switched into half-metallic ferromagnetism by small hole-doping, and the estimated Curie temperature is higher than 300 K. Thus, our results highlight a promising way toward 2D magnetic materials.

Geometric structures and stabilities of CuSin clusters (n=8,10,12)

Abstract The geometric structures and stabilities of CuSi n ( n =8,10,12) clusters were studied using a hybrid density functional method (B3LYP). Eight isomers were found for CuSi 8 , 21 isomers for CuSi 10 and five isomers for CuSi 12 . The stabilities of CuSi n can be well related to the stabilities of Si n . The Si frameworks are kept nearly unchanged and Cu is located at a substitutional site in many isomers of CuSi n , especially for CuSi 10 . The obtained results can be used to understand well the observations in mass spectrometric experiments. The high abundance of CuSi 10 in the experimental mass spectrum is attributed to the enhanced stability of CuSi 10 over neighboring clusters and the existence of a large number of low-lying isomers.

Quantum Monte Carlo characterization of small Cu-doped silicon clusters: CuSi4 and CuSi6

The relative energies, binding energies, and adsorption energies of three CuSi4 and two CuSi6 clusters have been computed in the fixed-node diffusion Monte Carlo (FNDMC), CASSCF, and B3LYP DFT methods. These results are compared with the earlier Hartree–Fock (HF) and B3LYP DFT investigations of these systems by two of us [C. Xiao and F. Hagelberg, J. Mol. Struct.: THEOCHEM 529, 241 (2000)]. The very close energy level spacing of the isomers under consideration confirms the previous work of Xiao and Hagelberg. The FNDMC results show some qualitative discrepancies with B3LYP DFT, and HF findings. They also confirm the appropriateness of the B3LYP DFT method for the prediction of the most stable CuSi4 isomer, while the CASSCF method compares more favorably with FNDMC for adsorption energies than B3LYP DFT.

Enhancing magnetic vacancies in semiconductors by strain

Although cation-vacancies can induce localized magnetic moments in semiconductors, the collective magnetism is impeded by low vacancy concentration. To improve the vacancy concentration, we study the effect of external hydrostatic strain on the vacancy formation energy. Our first-principles calculations discover that vacancy formation energy is significantly reduced in ionic semiconductors with the monotonic volume contraction, while only slightly decreased in covalent semiconductors. Especially for ZnO, the equilibrium concentration of cation-vacancies has been improved by 109 times. We predicted that strain can be used to produce “d0 magnetism” in ionic semiconductors much easier in experiments.

The theoretical search for half-metallic material: The non-stoichiometric peroskite oxide Sr2FeCoO6−δ

The non-stoichiometric peroskite oxide Sr2FeCoO6−δ is investigated and predicted to be half-metallic material using density-functional calculations. The results reveal that Sr2FeCoO5 shows antiferromagnetic half-metallic behavior and exhibits magnetic moment ordering with the magnetic moments of 2.97 and 3.72 μB on Fe(1) and Fe(2) sites, 2.96 and 3.20 μB on Co(1) and Co(2) sites antiparallel to those of Fe, and about 0.05 μB on O sites parallel to those of Co, respectively. Moreover, Sr2FeCoO4 and Sr2FeCoO6 both have half-metallic character. This hints that Sr2FeCoO6−δ possesses half-metallic nature in a large range of δ.