Simone S. Alexandre

Stability, geometry, and electronic structure of the boron nitride B36N36 fullerene

We apply first-principles calculations to study the structural and electronic properties of a boron nitride fullerene-like cluster, B36N36. This cluster has shape and dimensions comparable to those of a single-shelled BN fullerene recently produced in an electron-beam irradiation experiment. The calculations show that B36N36 is energetically less favorable than C60, when both are compared to nanotube structures. This is consistent with the experimental difficulty to obtain BN fullerenes. On the other hand, B36N36 presents a large energy gap, larger in fact than that of a BN nanotube of the same diameter. This is an indication that the molecule is a stable one, once it is formed.

Correlated magnetic states in extended one-dimensional defects in graphene.

Ab initio calculations indicate that while the electronic states introduced by tilt grain boundaries in graphene are only partially confined to the defect core, a translational grain boundary introduces states near the Fermi level that are very strongly confined to the core of the defect, and display a ferromagnetic instability. The translational boundary lies along a graphene zigzag direction and its magnetic state is akin to that which has been theoretically predicted to occur on zigzag edges of graphene ribbons. Unlike ribbon edges, the translational grain boundary is fully immersed within the bulk of graphene, hence its magnetic state is protected from the contamination and reconstruction effects that have hampered experimental detection of the magnetic ribbon states. Moreover, our calculations suggest that charge transfer between grain boundaries and the bulk in graphene is short ranged, with charge redistribution confined to ~5 Å from the geometric center of the 1D defects.

Room temperature compressibility and diffusivity of liquid water from first principles.

The isothermal compressibility of water is essential to understand its anomalous properties. We compute it by ab initio molecular dynamics simulations of 200 molecules at five densities, using two different van der Waals density functionals. While both functionals predict compressibilities within ~30% of experiment, only one of them accurately reproduces, within the uncertainty of the simulation, the density dependence of the self-diffusion coefficient in the anomalous region. The discrepancies between the two functionals are explained in terms of the low- and high-density structures of the liquid.

Synthesis of designed conductive one-dimensional coordination polymers of Ni(II) with 6-mercaptopurine and 6-thioguanine.

Calculations performed with the goal of designing suitable electrical conductive [M(6-MP)(2)](n) (M = transition metal, 6-MP = 6-mercaptopurinato) one-dimensional coordination polymers suggested that metal ions such as Ni(II) could provide suitable materials. In this work, direct hydrothermal reactions between 6-mercaptopurine (6-MPH) and the analogous 6-thioguanine (6-ThioGH) with NiSO(4).6H(2)O yield the compounds [Ni(6-MP)(2)](n).2nH(2)O [1] and [Ni(6-ThioG)(2)](n).2nH(2)O [2]. The X-ray structures confirm that both compounds present similar structures based on one-dimensional chains in which the deprotonated nucleobases act as the bridging ligands connecting the metal ions by short distances. Electrical measurements at room temperature confirm the conductor character of both coordination polymers. The small differences found in these measurements have been rationalized with the help of density functional theory calculations. Preliminary adsorption studies on surfaces for 1 have allowed characterization of single chains on mica and graphite. The results obtained suggest the potential use of coordination polymers on nanomaterials for molecular electronics.

Metallicity in individual MMX chains.

We present an ab initio study of the structural and electronic properties of the halogen-bridged MMX single polymer [Pt2(CH3CS2)4I]n and of various possible modifications of its sequence, in the framework of density functional theory. The computed band structure of the infinite regular polymer reveals a net metallic character; this evidence is compatible with the outcome of recent measurements done in the solid phase at room temperature. By taking the regular [Pt2(CH3CS2)4I]n polymer as our reference system, we analyzed the origin and the robustness of the metallic state along the chain with respect to a large set of geometrical and chemical perturbations of the subunits. In particular, we considered partial substitutions of the metal, halide, and dithiocarboxylate ligand subunits, as well as structural strain, defects, and magnetic effects. Our results demonstrate that the metallic character of single MMX chains is very resistant to a wide range of possible distortions that can occur in reality.

Magnetism of two-dimensional defects in Pd: Stacking faults, twin boundaries, and surfaces

Careful first-principles density functional calculations reveal the importance of hexagonal versus cubic stacking of closed packed planes of Pd as far as local magnetic properties are concerned. We find that, contrary to the stable face centered cubic phase, which is paramagnetic, the hexagonal close-packed phase of Pd is ferromagnetic with a magnetic moment of 0.35

Design of molecular wires based on one-dimensional coordination polymers

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DNA-psoralen: Single-molecule experiments and first principles calculations

/atom. Our results show that two-dimensional defects with local hcp stacking, like twin boundaries and stacking faults, in the otherwise fcc Pd structure, increase the magnetic susceptibility. The (111) surface also increases the magnetic susceptibility and it becomes ferromagnetic in combination with an individual stacking fault or twin boundary close to it. On the contrary, we find that the (100) surface decreases the tendency to ferromagnetism. The results are consistent with the magnetic moment recently observed in small Pd nanoparticles, with a large surface area and a high concentration of two-dimensional stacking defects.

Magnetic states of linear defects in graphene monolayers: Effects of strain and interaction

The authors report the results of ab initio calculations for the structural and electronic properties of one-dimensional coordination polymers with the general formula [M(6-MP)2]n (where 6-MP=6-mercaptopurinate, and M=MnII, FeII, CoII, NiII, and CuII). A common stable structure, consistent with the experimental data for [Cd(6-MP)2]n, is found for all metal cations studied, with the exception of MnII. Polymers containing FeII, NiII, and CoII are found to be ferromagnetic semiconductors, while [Cu(6-MP)2]n shows a Peierls-unstable paramagnetic metallic phase that undergoes a transition to a ferromagnetic semiconductor one under small stretching.

CCDC 751781: Experimental Crystal Structure Determination

The authors measure the persistence and contour lengths of DNA-psoralen complexes, as a function of psoralen concentration, for intercalated and crosslinked complexes. In both cases, the persistence length monotonically increases until a certain critical concentration is reached, above which it abruptly decreases and remains approximately constant. The contour length of the complexes exhibits no such discontinuous behavior. By fitting the relative increase of the contour length to the neighbor exclusion model, we obtain the exclusion number and the intrinsic intercalating constant of the interaction. Ab initio calculations are employed in order to provide an atomistic picture of these experimental findings.