Anlong Kuang

Density-functional study of small neutral and cationic bismuth clusters Bin and Bin+(n=2–24)

Density-functional theory with scalar-relativistic pseudopotential and a generalized gradient correction is used to calculate the neutral and cationic Bi(n) clusters (2< or =n< or =24), with the aim to elucidate their structural evolution, relative stability, and magnetic property. The structures of neutral Bi clusters are found to be similar to that of other group-V elemental clusters, with the extensively studied sizes of n=4 and 8 having a tetrahedron and wedgelike structure, respectively. Generally, larger Bi clusters consist of a combination of several stable units of Bi(4), Bi(6), and Bi(8), and they have a tendency to form an amorphous structure with the increase of cluster sizes. The curves of second order energy difference exhibit strong odd-even alternations for both neutral and cationic Bi clusters, indicating that even-atom (odd-atom) sizes are relatively stable in neutral clusters (cationic clusters). The calculated magnetic moments are 1micro (B) for odd-atom clusters and zero for even-atom clusters. We propose that the difference in magnetism between experiment and theory can be greatly improved by considering the orbital contribution. The calculated fragmentation behavior agrees well with the experiment, and for each cationic cluster the dissociation into Bi(4) or Bi(7) (+) subclusters confirms the special stability of Bi(4) and Bi(7) (+). Moreover, the bond orders and the gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital show that small Bi clusters would prefer semiconductor characters to metallicity.

The spin and orbital moment of Fen (n = 2–20) clusters

Complementary to the recent experimental finding that the orbital magnetic moment is strongly quenched in small Fe clusters [M. Niemeyer, K. Hirsch, V. Zamudio-Bayer, A. Langenberg, M. Vogel, M. Kossick, C. Ebrecht, K. Egashira, A. Terasaki, T. Möller, B. v. Issendorff, and J. T. Lau, Phys. Rev. Lett. 108, 057201 (2012)], we provide the theoretical understanding of the spin and orbital moments as well as the electronic properties of neutral and cation Fen clusters (n = 2-20) by taking into account the effects of strong electronic correlation, spin-orbit coupling, and noncollinearity of inter-atomic magnetization. The generalized gradient approximation (GGA)+U method is used and its effluence on the magnetic moment is emphasized. We find that without inclusion of the Coulomb interaction U, the spin (orbital) moments have an average value between 2.69 and 3.50 μB/atom (0.04 and 0.08 μB/atom). With inclusion of U, the magnetic value is between 2.75 and 3.80 μB/atom (0.10 and 0.30 μB/atom), which provide an excellent agreement with the experimental measurements. Our results confirm that the spin moments are less quenched, while the orbital moments are strongly quenched in small Fe clusters. Both GGA and GGA+U functionals always yield collinear magnetic ground-state solutions for the fully relaxed Fe structures. Geometrical evolution, as a function of cluster size, illustrates that the icosahedral morphology competes with the hexagonal-antiprism morphology for large Fe clusters. In addition, the calculated trends of ionization potentials, electron affinities, fragment energies, and polarizabilities generally agree with respective experimental observations.

Hybrid Density Functional Study on Mono- and Codoped NaNbO3 for Visible-Light Photocatalysis.

Monodoping with Mo, Cr, and N atoms, and codoping with Mo-N and Cr-N atom pairs, are utilized to adjust the band structure of NaNbO3 , so that NaNbO3 can effectively make use of visible light for the photocatalytic decomposition of water into hydrogen and oxygen, as determined by using the hybrid density functional. Codoping is energetically favorable compared with the corresponding monodoping, due to strong Coulombic interactions between the dopants and other atoms, and the effective band gap and stability for codoped systems increase with decreasing dopant concentration and the distance between dopants. The molybdenum, chromium, and nitrogen monodoped systems, as well as chromium-nitrogen codoped systems, are unsuitable for the photocatalytic decomposition of water by using visible light, because defects introduced by monodoping or the presence of unoccupied states above the Fermi level, which promotes electron-hole recombination processes, suppress their photocatalytic performance. The Mo-N codoped NaNbO3 sample is a promising photocatalyst for the decomposition of water by using visible light because Mo-N codoping can reduce the band gap to a suitable value with respect to the water redox level without introducing unoccupied states.

Geometrical structure and spin order of Gd13 cluster

The spin-polarized generalized gradient approximation to the density-functional theory has been used to determine the lowest energy structure, electronic structure, and magnetic property of Gd(13) cluster. Our results show that the ionic bonding is combined with the covalent characteristics in stabilizing the Gd cluster. The ferrimagnetic icosahedron is found to be the lowest energy configuration, in which the centered Gd atom couples antiferromagnetically with the rest Gd atoms surrounding it. No spin non-collinear evidence has been detected in our calculations. It is identified that the local magnetic moments of Gd atom are about 8 μ(B) regardless of geometrical structure. Finally, the comprehensive electronic structure analyses show that the indirect long-range magnetic coupling between the polarized 4f is mediated by the polarization of 5d, 6s, and 6p conduction electrons, which is the typical Ruderman-Kittel-Kasuya-Yosida interactions.

BiOX/BiOY (X, Y = F, Cl, Br, I) superlattices for visible light photocatalysis applications

The BiOX/BiOY (X, Y = F, Cl, Br, I, X ≠ Y) systems have been investigated as possible visible light photocatalysts in contrast with the BiOX (X = F, Cl, Br, I) systems by using hybrid density functional calculations. All the BiOX/BiOY systems have indirect bandgaps, and all the bandgaps of BiOX/BiOY systems we considered are between the bandgaps of BiOX and BiOY systems. The calculated bandgaps for BiOF/BiOCl, BiOF/BiOBr, BiOF/BiOI, BiOCl/BiOBr, BiOCl/BiOI, and BiOBr/BiOI are respectively 3.86, 3.41, 2.74, 2.99, 2.30, and 2.23 eV. The maximum absorption wavelength increases in the order of BiOF, BiOF/BiOCl, BiOCl, BiOF/BiOBr, BiOCl/BiOBr, BiOBr, BiOF/BiOI, BiOCl/BiOI, BiOBr/BiOI, and BiOI. The conduction band edges for all the BiOX/BiOY systems originate from Bi 6p states, but the valance band edges are contributed by different electronic states. Besides, the relative positions of X p states and Y p states for BiOX/BiOY systems are different, which should be attributed to the different p orbital energies of X and Y atoms. Due to the conduction band maximum is lower than the hydrogen reduction potential, all the BiOX and BiOX/BiOY systems are thermodynamically unfavorable for hydrogen production. Meanwhile, owing to the suitable bandgaps and band edge positions, the BiOF/BiOI, BiOCl/BiOBr, BiOCl/BiOI, and BiOBr/BiOI superlattices are possible visible light photocatalysts for degradation of organic pollutants.

The effect of disorder on the electronic and magnetic properties of Mn2CoAl/GaAs heterostructures

We study the effect of disorder, including swap and antisite, on the electronic and magnetic properties of heterostructures by using extensive first-principles calculations within density functional theory. Thirteen kinds of swap disorders and sixteen kinds of antisite disorders are proposed and studied comprehensively. Our calculation reveals that disorders at the interface have low formation energies, indicating that disorders are most likely to appear at the interface instead of the deep layer. Among all kinds of disorders, Mn1(Al) (where the interface Mn is occupied by an Al atom) and Mn1(As) (where the interface Mn is occupied by an As atom from a GaAs slab) antisite disorders possess the lowest formation energies. This shows that the interface Mn has a higher probability of being replaced by an Al atom, and that an As atom from a GaAs slab easily diffuses into a Mn2CoAl slab and occupies the position of the interface Mn. Moreover, further study on the interface electronic structure reveals that interface spin polarization suffers dramatic reduction due to Mn1(Al) and Mn1(As) antisite disorders. It can be deduced that the interface state, together with Mn1(Al) and Mn1(As) antisite disorders, may be the main causes of the low TMR ratio of Mn2CoAl/GaAs heterostructures.

Density functional calculations for structural, electronic, and magnetic properties of gadolinium-oxide clusters

Gadolinium-oxide clusters in various sizes and stoichiometries have been systematically studied by employing the density functional theory with the generalized gradient approximation. The clusters in bulk stoichiometry are relatively more stable and their binding energies increase with the increasing size. Stoichiometric (Gd2O3)n clusters of n = 1–3 prefer cage-like structures, whereas the clusters of n = 4–30 prefer compact structures layered by wedge-like units and exhibit a rough feature toward the bulk-like arrangement with small disorders of atomic positions. The polyhedral-cages analogous to carbon-fullerenes are stable isomers yet not the minimum energy configurations. Their stabilities can be improved by embedding one oxygen atom or a suitable cage to form core-shell configurations. The mostly favored antiferromagnetic couplings between adjacent Gd atoms are nearly degenerated in energy with their ferromagnetic couplings, resulting in super-paramagnetic characters of gadolinium-oxide clusters. The...

Electronic structural and magnetic properties of Mn5Ge3 clusters.

Theoretical understanding of the stability, ferromagnetism, and spin polarization of Mn5Ge3 clusters has been performed by using the density functional theory with generalized gradient approximation for exchange and correlation. The magnetic moments and magnetic anisotropy energy (MAE) have been calculated for both bulk and clusters, and the enhanced magnetic moment as well as the enlarged MAE have been identified in clusters. The most attractive achievement is that Mn5Ge3 clusters show a fine half-metallic character with large energy scales. The present results may have important implications for potential applications of small Mn5Ge3 clusters as both emerging spintronics and next-generation data-storage technologies.

ZnO/MoX2 (X = S, Se) composites used for visible light photocatalysis

Hybrid density functional has been adopted to investigate the structural, electronic, and optical properties of ZnO/MoS2 and ZnO/MoSe2 composites as compared with the results of ZnO, MoS2, and MoSe2 monolayers. The results indicate that MoS2 and MoSe2 monolayers could contact with monolayer ZnO to form ZnO/MoS2 and ZnO/MoSe2 heterostructures through van der Waals (vdW) interactions. The calculated bandgap of ZnO/MoS2 (ZnO/MoSe2) is narrower than that of ZnO or MoS2 (MoSe2) monolayers, facilitating the shift of light absorption edges of the composites towards visible light in comparison with bare ZnO and MoX2 monolayers. Through the application of strain, the ZnO/MoS2 and ZnO/MoSe2 composites which own suitable bandgaps, band edge positions, efficient charge separation, and good visible light absorption will be promising for visible light photocatalytic water splitting. These results provide a route for design and development of efficient ZnO/MoS2 and ZnO/MoSe2 photocatalysts for water splitting.

Magnetic moment and magnetic anisotropy of Ge3Mn5 thinfilms on Ge(111) substrate: A density functional study

Magnetism and magnetic anisotropy energy (MAE) of the Ge3Mn5 bulk, free-standing surface, and Ge3Mn5(001)|Ge(111) thinfilms and superlattice have been systemically investigated by using the relativistic first-principles electronic structure calculations. For Ge3Mn5 adlayers on Ge(111) substrates within Mn1 termination, the direction of magnetization undergoes a transition from in-plane at 1 monolayer (ML) thickness (MAE = -0.50 meV/ML) to out-of-plane beginning at 3 ML thickness (nearly invariant MAE = 0.16 meV/ML). The surficial/interfacial MAE is extracted to be 1.23/-0.54 meV for Mn1-termination; the corresponding value is 0.19/1.03 meV for Mn2/Ge-termination; the interior MAE is averaged to be 0.09 meV per ML. For various Ge3Mn5 systems, the in-plane lattice expansion and/or interlayer distance contraction would enhance the out-of-plane MAE. Our theoretical magnetic moments and MAEs fit well with the experimental measurements. Finally, the origination of MAE is elucidated under the framework of second-order perturbation with the electronic structure analyses.