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We study the structural stability and electronic properties of new classes of DNA-like inorganic double helices of the type A2B2XY (A = Si-Pb, B = Cl-I, and XY = PN and SiS) by employing first principles density functional theory (DFT) calculations including van der Waals interactions. In these quaternary double helices PN or SiS forms the inner helix while the AB helix wraps around the inner helix and the two are interconnected. We find that the bromides and iodides of Ge, Sn, and Pb as well as Pb2Cl2PN form structurally stable double helices while Ge2I2SiS as well as bromides and iodides of Sn and Pb have stable double helices. The atomic structures of different double helices have been analyzed in detail to understand the stability of these systems as there is up to about 80\% difference in the interatomic distances in the two helices which is remarkable. Also in these new classes of double helices there is polar covalent bonding in the inner helix due to heteroatoms. We have calculated the DDEC6 partial atomic charges and bond orders which suggest strong covalent bonding in the inner helix. The electronic structure reveals that these double helices are semiconducting and in many cases the band gap is direct. Furthermore, we have studied the effects of doping and found that hole doping is the most appropriate way to tuning their electronic properties. © the Owner Societies 2018.

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Physical Chemistry Chemical Physics

Atomic structure and electronic properties of A2B2XY (A = Si-Pb, B = Cl-I, and XY = PN and SiS) inorganic double helices: First principles calculations

In this study, the evolution of C60F18 molecules on a Cu(001) surface was studied by means of scanning tunneling microscopy and density functional theory calculations. The results showed that fluorinated fullerenes (tortoise-shaped polar C60F18) decay on Cu(001) surfaces by a step-by-step detachment of F atoms from the C60 cage. The most favorable adsorption configuration was realized when the F atoms of C60F18 pointed towards the Cu surface and six F atoms were detached from it. The results also showed that a further decay of C60F12 molecules strongly depended on the initial C60F18 coverage. The detached F atoms initially formed a two-dimensional (2D) gas phase which then slowly transformed into F-induced surface structures. The degree of contact between the C60F12 molecules and the Cu(001) surface depended on the density of the 2D gas phase. Hence, the life-time of fluorinated fullerenes was determined by the density of the 2D gas phase, which was affected by the formation of new F-induced structures and the decay of C60F12 molecules. © 2018, Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.

Nano Research

Real-time decay of fluorinated fullerene molecules on Cu(001) surface controlled by initial coverage

We studied the stability of several borophene layers on an Al(111) surface and found a structure called 9R using ab initio calculations. This layer competes with χ3 and β12 borophene layers and is made up of boron nonagons that form a network of hexagonal boron double chains. Remarkably, it has no B6 hexagon unlike other borophene layers. All three layers lie significantly lower in energy than the honeycomb layer recently reported on the Al(111) surface [W. Li, et al., Sci. Bull., 2018, 63, 282]. We discuss the structural stability and electronic structures of different borophene layers in light of the role of the filling factor f of boron atoms in boron hexagons in a honeycomb layer as well as charge transfer from the Al substrate to the borophene layer as obtained from the Bader charge analysis. The electron localization function shows that the 9R layer has two-center bonding within the nonagon rings and three-center bonding between the rings. Calculations of the phonon spectra show that a free 9R layer is dynamically stable raising the hope of its isolation. The electronic structure shows that in all cases the borophene layer is metallic. © The Royal Society of Chemistry.

Nanoscale

Borophene layers on an Al(111) surface-the finding of a borophene layer with hexagonal double chains and B9 nonagons using ab initio calculations

Thin layers of inorganic halide perovskites An+1MnX3n+1 (n = 1-6, A= Cs, M = Pb and Sn, and X = Cl, Br, and I) have been studied in orthorhombic and cubic phases along with layers of monoclinic CsSnCl3. It is found that one-unit-cell-thick layers have low stability except the monoclinic CsSnCl3 for which formation energy is slighly less than the bulk value. However, Cs2PbI4 is unstable in both cubic and orthorhombic phases. The formation energy for n &gt; 3 becomes comparable to bulk, but the inclusion of spin-orbit coupling is found to be important for the stabilization particularly for layers with Pb. Importantly, layers of environmentally friendly Sn-based systems have similar values of formation energy in the orthorhombic and cubic phases as well as similar band gaps which make them good materials for solar cell applications as temperature range changes during their operation. The studied 66 cubic and orthorhombic nanosystems have direct band gap (0.6-2.9 eV) using generalized gradient approximation for the exchange-correlation functional, but the use of the HSE06 method increases the band gap. The reduced dimensionality leads to elongation (contraction) of MX6 octahedra perpendicular (parallel) to the plane of the layers and an increase in the band gap. The presence of surface makes the hybridization between M s and X p orbitals near the valence band maximum stronger than in bulk which is good for light absorption. The effective mass of the electrons and holes is very light which augers well for the transport properties. Lead-based systems have larger band gap, and these can be useful in applications such as light-emitting diodes. © 2018 American Chemical Society.

Journal of Physical Chemistry C

Atomic and Electronic Structure of Two-Dimensional Inorganic Halide Perovskites An +1M n X3 n +1 (n = 1-6, A = Cs, M = Pb and Sn, and X = Cl, Br, and I) from ab Initio Calculations

Ab initio calculations on one, two, and three layers of SnS and SnSe compound semiconductors show that they all have indirect band gap similar to bulk and it varies in the range of ∼0.5–1.6 eV within the generalized gradient approximation due to quantum confinement as well as structural relaxations. In two-dimensional structures, the difference between the direct and the indirect band gap is very low and in the case of a SnS bilayer, this difference is minimum. Further, the band gaps calculated with HSE06 functional are in good agreement with the experimental results available for bulk and single layer. The total and projected densities of states show that the top of the valence band arises from the hybridization of Sn 5s and chalcogen p valence orbitals while the bottom of the conduction band has predominantly Sn 5p character. The effective mass of electrons and holes are found to be small. These features are similar to the recently discovered perovskite materials for photovoltaics with high efficiency and suggest that SnS and SnSe layered materials are also promising for photovoltaics. Further calculations on bulk and layers of GeS and GeSe are reported and the results are compared with those of SnS and SnSe structures. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Physica Status Solidi (B) Basic Research

Bandgap engineering in semiconducting one to few layers of SnS and SnSe

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We study the stability of drum-shaped transition metal (TM)-doped boron clusters, M@Bn with n = 14 and 16, and M = 3d, 4d, and 5d TM atom using ab initio calculations. Our results show that drum-shaped M@B14 clusters are favored for M = Cr, Mn, Fe, Co, and Ni, while in other cases, open conical or bowl shaped structures become more favorable. The isoelectronic Ni@B14 and Co@B14- clusters have large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps and these are magic clusters. Their stability has been correlated with the occurrence of magic behavior with 24 valence electrons in a disk jellium model, while for Fe@B14 case the drum structure is deformed and the stability occurs at 22 delocalized valence electrons. The bonding nature in these clusters has been studied by analyzing the electron density at bond and ring critical points, the Laplacian distribution of the electron density, the electron localization function, the source function, and electron localization-delocalization indices, all of which suggest two- and three-center σ bonding within and between the two B7 rings, respectively, and hybridization between the TM d orbitals and the π bonded molecular orbitals of the drum. The infrared and Raman spectra of these magic clusters show all real frequencies, suggesting the dynamical stability of the drum-shaped structures. There is a low frequency mode associated with the M atom. Results of the electronic spectra of the anion clusters are also presented that may help to identify these species in future experiments. Further, we discuss the stability of 24 delocalized valence electron systems Mn@B16 anion, Fe@B16, Co@B16 cation, and other related clusters. Assembly of Co@B14 clusters has been shown to stabilize a carbon nanotube-like nanotube of boron with Co atomic nanowire inside while a nanotube of boron with triangular network has been obtained with the assembly of Fe@B16 drum-shaped clusters. Both the nanotubes are metallic. © 2017 American Chemical Society.

Chemistry

(SoNS) School of Natural Sciences

Electronic Origin of the Stability of Transition-Metal-Doped B14 Drum-Shaped Boron Clusters and Their Assembly into a Nanotube

Results of ab initio pseudopotential calculations are presented concerning the atomic structure and magnetic properties of GaN doped with selected rare earth atoms. Effects of codoping these materials with Si are also discussed. It has been found that the doping of a Eu atom on a Ga site in bulk GaN creates significant local deformation and it costs 1.84eV. However, the addition of a Si atom makes Eu doping in bulk GaN energetically favorable as strain from an oversized Eu atom and an undersized Si atom is compensated. Therefore codoping of Si facilitates doping of rare earths in GaN. The excess charge due to codoping of Si tends to cause Eu to be in a 2+ state and the magnetic moment on Eu ion is enhanced to 7μB. Further studies have shown that Eu atoms tend to cluster with interatomic separation of about 5Å and there is ferromagnetic coupling. For rare earth atoms in nanoparticles, ab initio calculations have been performed on (GaN)n (n=12, 16, 22, and 32) nanoclusters with Eu, Gd, and Nd atoms substituting on Ga sites. These studies have yielded preferred atomic structures and magnetic behavior. Cage structures of these GaN nanoclusters were found to be lower in energy as compared to bulk fragments. Specific results show that Eu-doping in GaN nanoparticles is favorable compared with bulk GaN since a large fraction of atoms in a nanocluster lie on the surface where strain can be lower. Codoping of Si further facilitates Eu doping as in the case of bulk GaN. © 2016 Elsevier Ltd. All rights reserved.

Rare Earth and Transition Metal Doping of Semiconductor Materials: Synthesis, Magnetic Properties and Room Temperature Spintronics

Energetics, atomic structure, and magnetics of rare earth-doped GaN bulk and nanoparticles

Ab initio calculations on hard/soft (FePt)m/(FeCo)n, (m = 4, 6, 8 and n = 2-2m) magnetic superlattices show that the B2 type FeCo layers become anisotropic with varying interlayer spacing and enhanced magnetic moments. The average magnetic moment in superlattices is higher than in bulk FePt, resulting in high maximum energy product for (FePt)4/(FeCo)8 which is nearly double the calculated value for bulk FePt. The calculation of the magnetic anisotropy energy shows that the optimal thickness of the soft magnetic phase for good permanent magnet behaviour of the superlattice is less than ∼2 nm. © 2016 Author(s).

AIP Advances

Optimum thickness of soft magnetic phase in FePt/FeCo permanent magnet superlattices with high energy product and large magnetic anisotropy energy

Boron atomic clusters show several interesting and unusual size-dependent features due to the small covalent radius, electron deficiency, and higher coordination number of boron as compared to carbon. These include aromaticity and a diverse array of structures such as quasi-planar, ring or tubular shaped, and fullerene-like. In the present work, we have analyzed features of the computed electron density distributions of small boron clusters having up to 11 boron atoms, and investigated the effect of doping with C, P, Al, Si, and Zn atoms on their structural and physical properties, in order to understand the bonding characteristics and discern trends in bonding and stability. We find that in general there are covalent bonds as well as delocalized charge distribution in these clusters. We associate the strong stability of some of these planar/quasiplanar disc-type clusters with the electronic shell closing with effectively twelve delocalized valence electrons using a disc-shaped jellium model. B10, B7P, and B8Si, in particular, are found to be exceptional with very large gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, and these are suggested to be magic clusters. © 2016 The Royal Swedish Academy of Sciences.