Decoupling 1D and 2D features of 2D sp-nanoribbons-the megatom model.

The dependence of the electron energy band gap on the width of an-nanoribbon is investigated using a generalization of the 1D tight binding model for a chain of atoms. Within the proposed generalization, small linear atomic formations along lines perpendicular to the 2D ribbon axis are modeled as single large atoms calledwhose properties depend on the type, the size and the atomic conformation. Replacement of a 1D chain of atoms by that of the megatoms is accompanied by the incorporation of zeroth order 2D features into the 1D model approximation of the nanoribbon.

Magnetism versus band-gap relationship in diluted magnetic semiconductors: megatom impurity behavior of the magnetic dopant complexes.

An analysis ofnumerical results obtained for the total energy of diluted magnetic semiconductors (DMSs) doped with dopant formations of various structural and spin conformations consisting of 2-4 3D transition metal (TM atoms) has revealed that a dopant formation acts as large impurity atom i.e., as a, in a reverse analogy to the process of the adsorption of-atoms onto metallic surfaces.

Codoping induced enhanced ferromagnetism in diluted magnetic semiconductors.

The experimentally observed-magnetism and its subsequent attribution to the presence of structural and topological defects has opened the way for engineering the magnetic properties of diluted magnetic semiconductors (DMSs) and transition metal oxides (TMOs). Doping and codoping constitute the most commonly used processes (either experimentally or theoretically) for developing and studying this type of defect-induced magnetism. The focus of the present review is to highlight the basic features of the defect magnetism which have been observed over diverse systems, while emphasizing the local, holistic and synergistic response of the host materials to their doping and investigating their role in the development of the magnetic coupling (MC) that is developed among the magnetic dopants.

High temperature stability, metallic character and bonding of the SiBN planar structure.

The family of monolayered SiBN structures constitute a new class of 2D materials exhibiting metallic character with remarkable stability. Topologically, these structures are very similar to graphene, forming a slightly distorted honeycomb lattice generated by a union of two basic motifs with AA and AB stacking. In the present work we study in detail the structural and electronic properties of these structures in order to understand the factors which are responsible for their structural differences as well as those which are responsible for their metallic behavior and bonding.

Estimation of-exchange constants revisited.

We present a new computational method for estimating the-exchange constant,Jeffsp-d, applicable to transition metal doped diluted magnetic semiconductors, transition metal oxides, and 2D- and 3D- dichalcogenides. The proposed method is based on results describing the variation of the magnetic features of a doped system with the variation of its magnetization density (). The results forJeffsp-d(M)obtained with the proposed method are compared with the corresponding results,Jeffsp-d(ΔEVBM), obtained from estimations of the spin electron orbital splitting, Δ, at the valence band maximum (VBM).

Goodenough-Kanamori rules applied to wurtzite crystals.

Goodenough-Kanamori (GK) criteria have provided a significant contribution to our understanding of the importance of the symmetry and the electron orbital characteristics in the development of the magnetic superexchange coupling [antiferromagnetic (AFM) or ferromagnetic (FM)] applied primarily to systems with bond angles of 180° and 90°. In the present work, we quantify and apply the GK criteria to wurtzite systems. Our approach is based on calculations of (i) the spin electron densities of the anions which are first nearest neighbors (1nn) to the magnetic dopants and, (ii) the generalized exchange integrals which are derived by investigating the electronic properties of the systems under a magnetization density constraint.

Data driven modeling of magnetism in dilute magnetic semiconductors: correlation between the magnetic features of diluted magnetic semiconductors and electronic properties of the constituent atoms.

We propose an efficient machine learning based approach in modeling the magnetism of diluted magnetic semiconductors (DMSs) leading to the prediction of new compounds with enhanced magnetic properties. The approach combines accurate ab initio methods with statistical tools to uncover the correlation between the magnetic features of DMSs and electronic properties of the constituent atoms to determine the underlying factors responsible for the DMS-magnetism. Taking the electronic properties of different DMS systems as descriptors to train different regression models allows us to achieve a speed up of several orders of magnitude in the search for an optimum combination of the host semiconductor and the dopants with enhanced magnetic properties.

Antiferromagnetic successive superexchange interactions underlying ferromagnetic couplings in codoped diluted magnetic semiconductors.

Our recent works have revealed that the magnetic coupling among the magnetic codopants in diluted magnetic semiconductors and doped transition metal oxides has a strong local feature. This was attributed to successive spin polarizations induced by the codopants to their neighboring anion ligands. In the present work, we analyze and refine the successive spin polarization based magnetic coupling using results of ab initio calculations and assign the magnetic coupling among the magnetic codopants to a combination of superexchange and double-exchange interactions.

Universal features underlying the magnetism in diluted magnetic semiconductors.

Investigation of a diverse variety of wide band gap semiconductors and metal oxides that exhibit magnetism on substitutional doping has revealed the existence of universal features that relate the magnetic moment of the dopant to a number of physical properties inherent to the dopants and the hosts. The investigated materials consist of ZnO, GaN, GaP, TiO, SnO, SnN, MoS, ZnS and CdS doped with 3d-transition metal atoms. The primary physical properties contributing to magnetism include the orbital hybridization and charge distribution, the d-band filling, d-band center, crystal field splitting, electron pairing energy and electronegativity.

Successive spin polarizations underlying a new magnetic coupling contribution in diluted magnetic semiconductors.

We propose a new type of magnetic coupling (MC) that is found in diluted magnetic semiconductors (DMSs). The origin of this is found to be the result of charge transfer processes followed by successive spin polarizations (SSPs) along successive cation-anion segments which include the impurities. The basic process underlying the SSP-based MC (SSP-MC) is the sharing of a single spin orbital by two neighboring impurities.

Informatics guided discovery of surface structure-chemistry relationships in catalytic nanoparticles.

A data driven discovery strategy based on statistical learning principles is used to discover new correlations between electronic structure and catalytic activity of metal surfaces. From the quantitative formulations derived from this informatics based model, a high throughput computational framework for predicting binding energy as a function of surface chemistry and adsorption configuration that bypasses the need for repeated electronic structure calculations has been developed.

New visible light absorbing materials for solar fuels, Ga(Sbx)N1-x.

A novel visible-light-absorbing dilute alloy, Ga(Sbx)N1-x is synthesized by metal organic chemical vapor deposition (MOCVD) for solar hydrogen production. Significant bandgap reduction of GaN, from 3.4 eV to 1.

Band alignment and optical absorption in Ga(Sb)N alloys.

We extend the theory of band alignment proposed by Harrison to ternary and quaternary heteropolar semiconductors. Combining this with first-principles density functional theory incorporating the LDA/GGA+U formalism (LDA: local density approximation; GGA: generalized gradient approximation) can result in useful electronic structure predictions for new alloys. The practicality of this is demonstrated by application to the Ga(Sbx)N1-x alloys, where the feasibility of water splitting reaction under visible light irradiation is discussed.

The synergistic character of the defect-induced magnetism in diluted magnetic semiconductors and related magnetic materials.

In this work, we introduce a new perspective in explaining the origin of magnetism in dilute magnetic semiconductors, carbon-based materials and other related materials. According to our proposal, the magnetism in these materials is the result of the synergistic action of defect-induced electronic processes mostly of local character which can provide magnetic moments and develop a ferromagnetic coupling among them. This synergy is realizable via appropriate codoping which appears as a general and generic approach.

Magnetic anisotropy and engineering of magnetic behavior of the edges in Co embedded graphene nanoribbons.

Using first-principles calculations, we demonstrate the existence of anisotropic ferromagnetic interactions in Co embedded graphene nanoribbons (GNRs). Spin polarization of the edge states is found to alter significantly compared to the metal-free cases. Our findings can all be well-justified as the output of the interplay between the development of an induced spin polarization in the neighborhood of the Co atoms and the maintaining of the polarization picture of the Co-free GNR.

Symmetry-switching molecular Fe(O2)n(+) clusters.

Experimental and theoretical studies based on mass spectrometry, collision-induced dissociation, and ab initio calculations are performed on the formation and stability of FeO(n)(+) clusters, as well as on their structural, electronic, and magnetic properties. In the mass spectra, clusters with an even number of oxygen atoms show increased stability, most prominently for FeO(10)(+). The extra stability of this cluster is confirmed by measurements of fragmentation cross sections through crossed molecular beam experiments.

Ferromagnetic interactions in hosted bipartite materials--generalized-double-exchange and generalized-superexchange interactions.

Defect-induced magnetism in dilute magnetic semiconductors challenges our understanding of magnetism in solids. Theories based on conventional superexchange or double-exchange interactions cannot explain long range magnetic order at concentrations below the percolation threshold in these materials. On the other hand, the codoping-induced magnetism, which can explain magnetic interactions below the percolation threshold, has eluded explanation.

Defect-induced defect-mediated magnetism in ZnO and carbon-based materials.

The recent discovery of magnetism in a variety of diverse non-magnetic materials containing defects has challenged conventional thinking about the microscopic origin of magnetism in general. Especially intriguing is the complete absence of d electrons that are traditionally associated with magnetism. By a systematic microscopic investigation of two completely dissimilar materials (namely, ZnO and rhombohedral-C(60) polymers) exhibiting ferromagnetism in the presence of defects, we show that this new phenomenon has a common origin and the mechanism responsible can be used as a powerful tool for inducing and tailoring magnetic features in systems which are not magnetic otherwise.

Identification of descriptors for the CO interaction with metal nanoparticles.

The design and performance optimization of future nanocatalysts will depend on our understanding of adsorbate-metal interactions. Using first principle calculations, we identify suitable descriptors, namely, the coordination number and curvature angle of the surface Au atoms, capable of predicting the CO binding strength on every site of Au nanoparticles. Our results unravel how the size, shape, and symmetry of nanoparticles affect their electronic properties and, consequently, their interaction with CO.

Surface conductivity of hydrogenated diamond films.

The experimentally observed high surface conductivity of hydrogenated diamond films is explained through ab initio results as well as model calculations based on the tight-binding molecular dynamics method. Our results support the previously reported experimental results indicating that the surface conductivity of the hydrogenated diamond surfaces is due to the surface adsorption of a H(3)O(+) monolayer. Specifically, it is shown that the presence of the H(3)O(+) adlayer results in the formation of an electrostatic surface dipole moment which makes the potential of the surface H layer effectively more attractive.

Oscillatory band gap behavior in small diameter Si-clathrate nanowires.

Electronic structure analysis of small cagelike silicon nanowires is carried out and reveals many surprising features. The band gap values for all the nanowires are found to be smaller than their bulk counterparts. The most intriguing aspect appears to be the alternating sequence of direct and indirect band gaps as the diameter changes.

Structure, stability, and quantum conductivity of small diameter silicon nanowires.

Structures and energetics of various types of silicon nanowires have been investigated using both quantum and classical molecular dynamics simulations to determine the most stable forms. The tetrahedral type nanowires have been found to be the most stable and, surprisingly, the polycrystalline forms of nanowires, while having the smallest surface to bulk ratio, are found to be the least stable. We also show that the cagelike nanowires have greater thermal stability than the tetrahedral nanowires.

Orbital magnetism: pros and cons for enhancing the cluster magnetism.

The discrepancy seen in the experimental and theoretical results on the magnetic moment of a small magnetic cluster has been attributed to the contribution arising from orbital magnetism. In this Letter we show that the magnetic states with large orbital magnetic moment are not always energetically favorable; they could, however, be realizable by coating the cluster or deposing it on appropriate substrates. More importantly, our work shows that the crucial factors that determine the cluster magnetism are found to be the intrinsic, and consequently, the extrinsic properties of the constituent atoms of the cluster.

Magnetic enhancement and magnetic reduction in binary clusters of transition metal atoms.

Electronic and magnetic properties of small binary clusters containing one or two transition metal atoms are investigated using ab initio calculations with a view to explain the experimentally observed magnetic enhancement/reduction in these systems. As the present investigations do not rely on spin-orbit effects, our results reveal the enhancement or reduction in the magnetic moment to depend on two main factors; namely geometry and, most importantly, the d-band filling. The results can be used as a guide in the experimental synthesis of high density magnetic grains.

Degradation of inter-atomic bonds during structural phase change in intermediate Ni-clusters (Ni39-Ni49).

Results based on a symmetry- and spin-unrestricted tight-binding molecular-dynamics study are presented for the ground-state geometries of intermediate Ni(n), n in [39,49], clusters. A structural phase change is found to take place around n=43 during which a structural transition from fcc/hcp structure to icosahedral one is observed. This is in good agreement with recent experimental findings.