查看更多>>摘要：Global structural searches performed for sodium amalgam compounds reveal six new and stable Na-Hg stoichiometries (e.g., NaHg3, NaHg4, Na2Hg, Na4Hg, Na5Hg and Na6Hg) under ambient and high-pressure conditions. With increasing Hg content in the compounds, the structure topology of Hg evolves from isolate atom (NamHgn, m/n >= 3, 0D), linear chains (Na2Hg, 1D), puckered honeycomb layers (Na3Hg2, 2D), diamond networks (NaHg, 3D), dodecahedron (NaHg2, 3D), to tetrakaidecahedron (NaHg3, 3D). Electronic structure analysis shows that Hg can attain higher negative oxidation states, transferring more than one electron from Na atoms to the Hg 6p orbitals. In NamHgn (m/n < 3) compounds, the covalent Hg-Hg interactions are found stemming from the sp hybridization. In Na4Hg, quasi-zero-dimensional (0-D) electride is found with the electrons located within the octahedrons of Na in the lattice. The present results establish the richness of sodium amalgam stoichiometries under ambient and high-pressure conditions.
Macedo, Luis Eduardo LeiteKleger, AaronMeunier, VincentGirao, Eduardo Costa...
9页
查看更多>>摘要：Two-dimensional allotropes of carbon are the subject of intense research in an effort to fine tune nanocarbon's physical properties for their insertion into operational nanoscale devices. C57 and C65 lattices are examples of such proposed networks. They are hypothetically formed by the concatenation of acepentalene blocks and are shown to display a metallic behavior. Here, we use density functional theory to investigate the electronic properties of ribbons whose lattices are composed of these two 2D nanocarbons. These systems share the common feature with their parent structures of displaying a metallic behavior. However, they are also found to host spin-polarized states, thereby offering opportunities for their applications in spintronics. Furthermore, one of the 2D parent structures is found to also allow a non-trivial spin distribution, as well as corrugated phases, which was not previously reported for this system. Finally, the structural and electronic properties calculated for the C57 and C65 systems are rationalized in terms of a resonance model.
查看更多>>摘要：Density functional theory calculations were adopted to systematically investigate the adsorption and diffusion behaviors of sodium and aluminum over TiB2 surfaces or in TiB2 crystal to characterize the interaction mechanism between sodium and TiB2 cathode in aluminum reduction cells. Results suggest that Na and Al will stably adsorbe on the low-index TiB2 (0 0 0 1) surface, and the presence of vacant defects can significantly strengthen this adsorption. The migration of Na and Al over pristine TiB2 is anisotropic, with the largest energy barrier of 0.024/0.32 eV for Na and 0.28/1.57 eV for Al over Ti/B-terminated surfaces. The Ti vacancy in Ti-terminated surface is more effective to hinder Na and Al migration with the large diffusion barriers of 0.36 eV for Na and 2.07 eV for Al. Specially, for the B-terminated surface, B vacancy will promote the Na and Al diffusion with the lower barriers. Additionally, it is difficult for Na and Al to form interstitial defects and diffuse in covalent TiB2 crystal. Given these results, compared to graphite cathode, the sodium prefers to deposit on TiB2 surface, and the strong interaction between sodium and TiB2 promotes the early process of sodium penetration. On the other hand, the smoother landscape for Na diffusion on the TiB2 surface suggests the decreased stability of aluminum liquid, so that the current efficiency of aluminum reduction cell will decrease.
查看更多>>摘要：Though lead halide perovskites hold impressive optoelectronic properties, because of the bottleneck of lead toxicity, searching for the lead free perovskites is highly demanding for their commercial applications in the optoelectronic devices. Alongside of the lead free perovskites, two dimensional (2D) organic-inorganic hybrid halide perovskites (OIHHPs) are getting great attention as photovoltaic materials due to their air stability over the three dimensional(3D) counterpart and high performance. Herein, we have investigated recently synthesized lead free 2D Ruddlesden-Popper (RP) phase OIHHPs (X-PEA)2SnI4 (PEA=phenylethylammonium and X=H, para-F, meta-F, ortho-F) along with (PEA)(2)Sn1-xGexI4 where x = 0.5 and 1. Here studied all perovskites are direct band gap semiconductors with band gap in the solar energy region with high absorption coefficients. We have calculated the charge carrier effective masses along X-gamma-X and Y-gamma-Y and finally we have estimated maximum theoretical photoconversion efficiency (PCE) for all which reaches up to 25.58% making them suitable for their applications in high performance photovolatic devices.
查看更多>>摘要：First-principles molecular dynamics is employed to describe the atomic structure of amorphous SiN, a non-stoichiometric compound belonging to the SixNy family. To produce the amorphous state via the cooling of the liquid, both the Car-Parrinello and the Born-Oppenheimer approaches are exploited to obtain a system featuring sizeable atomic mobility. At high temperatures, due to the peculiar electronic structure of SiN, exhibiting gap closing effects, the Car-Parrinello methodology could not be followed since non-adiabatic effects involving the ionic and electronic degrees of freedom do occur. This shortcoming was surmounted by resorting to the Born-Oppenheimer approach allowing to achieve significant ionic diffusion at T = 2500 K. From this highly diffusive sample, an amorphous state at room temperature was obtained with a quenching rate of 10 K/ps. Four different models were created, differing by their sizes and the thermal cycles. We found that the subnetwork of atoms N has the same environment than in the stoichiometric material Si3N4 since N is mostly threefold coordinated with Si. Si atoms can also be found coordinated to four N atoms as in Si3N4, but a substantial fraction of them forms homopolar bonds with one, two, three and even four Si. Our results are not too dissimilar from former models available in the literature but they feature a higher statistical accuracy and refer more precisely to room temperature as the reference thermodynamical condition for the analysis of the structure in the amorphous state.
查看更多>>摘要：Tight binding models are widely used in large scale electronic structure calculations of nanostructures. Their atomistic nature makes them flexible, but also means the computational cost increases rapidly with system size. The large number of calculations required to design nanostructures makes computational efficiency desirable. We have developed a method to increase computational speed while retaining most of its accuracy. The method is based on the use of supercells and zone folding combined with a truncation of the Hamiltonians to only include states close to the band-edges. We apply the method to model the band edge energies of a GaAs/AlAs quantum well grown along the [110]-directions with 3D and 2D periodic boundary conditions as well as the density of states and dielectric function of the quantum well. We typically find a speed-up of ten times with only a small loss of accuracy of the calculation result.
NSTL
Elsevier