Magnetically Ordered Transition-Metal-Intercalated WSe2.

Introducing magnetic behavior in nonmagnetic transition metal dichalcogenides is essential to broaden their applications in spintronic and nanomagnetic devices. In this article, we investigate the electronic and magnetic properties of transition-metal-intercalated tungsten diselenide (WSe2) using density functional theory. We find that intercalation compounds with composition of T1/4WSe2 (T is an iron-series transition-metal atom) exhibit substantial magnetic moments and pronounced ferromagnetic order for late transition metals. The densities of states of the T atoms and the magnetic moments on the W sites indicate that the moments of the intercalated atoms become more localized with increasing atomic number. A large perpendicular magnetocrystalline anisotropy of about 9 meV per supercell has been found for Fe1/4WSe2. Furthermore, using mean field theory, we estimated high Curie temperatures of 660, 475, and 379 K for Cr, Mn, and Fe, respectively. The predicted magnetic properties suggest that WSe2 may have applications in spin electronics and nanomagnetic devices.

Halogenated MOF-5 variants show new configuration, tunable band gaps and enhanced optical response in the visible and near infrared.

Inspired by recent experimental fabrication of mono-halogenated versions of Metal-Organic Framework MOF-5 (i.e., X-MOF-5, X = F to I) and some experimentally known fully halogenated MOF compounds, we systematically studied frameworks incorporating full halogenation of the BDC linkers of the prototypical Iso-Reticular Metal-Organic Framework (IRMOF) series, exemplified by MOF-5. Using quantum chemistry calculations, we find that halogenation leads to a 90° rotation of the aryl group, which is mainly ascribed to overcrowding between halogen atoms and the carboxyl and benzene ring and strong repulsion among in-plane atoms/groups. The 90° configuration decreases the repulsion, and maximizes the stabilization energy, and is therefore more stable than 0° configuration. We find that the band gap can be tuned from 4.1 to 1.5 eV as we go from F, Cl, Br, to I. This extends the optical response of these experimentally accessible materials through the visible and infrared region. We have also considered a broader range of new materials that substitute various metals for Zn. Totally, 70 materials were systematically examined computationally including (M4O)(BDC-Z4)3 (M = Zn, Cd, Be, Mg, Ca, Sr, Ba; Z = H, F, Cl, Br, I). For the full range of materials, we calculate band gaps of 4.2 to 1.0 eV, corresponding to a threshold of absorption of 290-1240 nm. Four selected materials were tested for stability using short 5 ps molecular dynamics simulations up to 600 K. The new materials with the smallest band gaps could potentially be used in near-infrared (NIR) light-emitting devices. Other properties, e.g., bulk moduli, formation energy, chemical bonding, and optical properties, were also investigated. The present results may provide new materials for use as novel photocatalysts, photoactive materials for photovoltaic cells, or functional devices in nanoelectronics and optoelectronics.

Tunable magnetism in metal adsorbed fluorinated nanoporous graphene.

Developing nanostructures with tunable magnetic states is crucial for designing novel data storage and quantum information devices. Using density functional theory, we investigate the thermodynamic stability and magnetic properties of tungsten adsorbed tri-vacancy fluorinated (TVF) graphene. We demonstrate a strong structure-property relationship and its response to external stimuli via defect engineering in graphene-based materials. Complex interplay between defect states and the chemisorbed atom results in a large magnetic moment of 7 µB along with high in-plane magneto-crystalline anisotropy energy (MAE) of 17 meV. Under the influence of electric field, spin crossover effect accompanied by a change in the MAE is observed. The ascribed change in spin-configuration is caused by the modification of exchange coupling between defect states and a change in the occupation of d-orbitals of the metal complex. Our predictions open a promising way towards controlling the magnetic properties in graphene based spintronic and non-volatile memory devices.

Thermodynamic stability of a bi-layer of copper nitride on Cu(100) surface.

Ultrathin insulating films composed of a few atomic layers are being extensively used for controlling the electronic coupling of nanostructures deposited on a substrate. Ultrathin film, for example, a single layer of Cu(2)N deposited on a Cu(100) surface (known as Cu(2)N/Cu(100) surface) has been used to determine the spectral properties of nanomagnets using scanning tunneling spectroscopy. However, recent experiments that measure spin relaxation times in a single atom suggest that the single layer of Cu(2)N does not provide efficient electronic decoupling. In this work, we study the thermodynamic stability of a bi-layer of copper nitride on the Cu(100) surface. We calculate adsorption and co-adsorption energies of Cu and N as a function of their concentration on the Cu(2)N/Cu(100) surface using density functional theory. We find that the adsorption and co-adsorption energies of Cu and N on the Cu(2)N/Cu(100) surface are of the order of a few eV. This suggests that the bi-layer of copper nitride is thermodynamically stable on the Cu(100) surface. We also find that the work function of N-adsorbed Cu(2)N/Cu(100) increases with the N concentration, suggesting a better insulating character of the bi-layer of copper nitride on the Cu(100) surface.

Interplay between bonding and magnetism in the binding of NO to Rh clusters.

We have studied the binding of NO to small Rh clusters, containing one to five atoms, using density functional theory in both spin-polarized and non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters than it does to Rh(100) or Rh(111), suggesting that Rh clusters may be good catalysts for NO reduction. However, binding to NO also quenches the magnetism of the clusters. This (local) effect results in reducing the magnitude of the NO binding energy, and also washes out the clear size-dependent trend observed in the nonmagnetic case. Our results illustrate the competition present between the tendencies to bond and to magnetize, in small clusters.

Symmetries, vibrational instabilities, and routes to stable structures of clusters of Al, Sn, and As.

We investigate the stability of small clusters using density functional theory to compute the total energy, forces, and vibrational frequencies using linear response. We exhibit an efficient and computationally low-cost route to finding stable structures, by starting with high-symmetry structures and distorting them according to their unstable modes. We illustrate this by application to 4-, 6-, and 13-atom clusters of Al, Sn, and As. This technique also naturally elucidates the origins of stability of the lower symmetry structures, which is variously due to the linear or pseudo Jahn-Teller effect, combined with a lowering of various contributions to the total energy. We show that the situation is more complex than has generally been appreciated.