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

The dependence of the electron energy band gap on the width of ansp-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 calledmegatomswhose 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. We use this model to investigate the oscillating band gap of ansp-nanoribbon in terms of the ribbon's width. Results are presented for the width dependence of the energy gap of the zig-zag Si2BN nanoribbons.

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

An analysis ofab initionumerical 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 amegatom, in a reverse analogy to the process of the adsorption ofsp-atoms onto metallic surfaces. As a result, thed-orbitals of the magnetic dopant formation (themegatom) become hybridized with thesp-bands of the host anions thus creating a number of impurity states which are reflected in the changes of the band gap of the DMS establishing an implicit relationship between the band gap and magnetism. Additional findings also indicate that: (i) the total magnetic momentMtot(α)and the band gapegap(α)which characterize a DMS with a dopant formation in spin conformation (α) do not vary independently from each other but instead form one composite system parameter. (ii) The per dopant-pair magnetic interactions in dopant formations consisting of more than two dopants are smaller than those obtained for an isolated dopant-pair. These are demonstrated with results obtained for GaN doped with 3D-TM dopant formations.

Codoping induced enhanced ferromagnetism in diluted magnetic semiconductors.

The experimentally observedd0-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.Ab initiocomputational results elucidate the local aspects of the MC (charge and spin transfers between dopants and their first nearest neighboring anion ligands) and their relation with holistic processes which are reflected in the band structure, and the shifts of both thed- andp-band centers of the doped material (compared to the undoped one). In view of these results the MC between the magnetic dopants is framed within the newly proposed successive spin polarization and the defect-induced defect-mediated models. The similarities found in the magnetic characteristics between the codoped DMSs/TMOs and the magnetic multilayer systems lends further support to these models which introduce new contributions to the MC that are competitive with the existing classical ones (superexchange, double exchange,s-d&p-dcouplings etc).

High temperature stability, metallic character and bonding of the Si2BN planar structure.

The family of monolayered Si2BN 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. Their high temperature stability is demonstrated by the calculations of finite temperature phonon modes which show no negative contributions up to and beyond 1000 K. Presence of the negative thermal expansion coefficient, a common feature of one-atom thick 2D structures, is also seen. Comparison of the two motifs reveal the main structural differences to be the differences in their bond angles, which are affected by the third nearest neighbor interactions ofcis-transtype. On the other hand, the electronic properties of these two structures are very similar, including the charge transfers occurring between orbitals and between atoms. Their metallicity is mainly due to thepzorbitals of Si with a minor contribution from thepzorbitals of B, while the contribution from thepzorbitals of N atoms is negligible. There is almost no contributions from the Npzelectrons to the energy states near the Fermi level, and they form a band well below it. I.e., thepzelectrons of N are localized mostly at the N atoms and therefore cannot be considered as mobile electrons of thepzcloud. Moreover, we show that due to the relative positions in the energy axis of the atomic energies of thepzorbitals of B, N and Si atoms, the density of states (DOS) of Si2BN can be considered qualitatively as a combination of the DOS of planar hexagonal BN (h-BN) and hypothetically planar silicene (ph-Si). As a result, the Si2BN behaves electronically at the Fermi level as slightly perturbed ph-Si, having very similar electronic properties as silicene, but with the advantage of having kinetic stability in planar form. As for the bonding, the Si-Si bonds are covalent, while theπback donation mechanism occurs for the B-N bonding, in accordance with the B-N bonding in h-BN.

Estimation ofsp-dexchange constants revisited.

We present a new computational method for estimating thesp-dexchange 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 (M). 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, ΔEVBM, at the valence band maximum (VBM). The latter is estimated in two ways; either directly from plots of the band structure calculations or by calculating the energy difference between the band-centers of the spin-up and spin-down electron density of states of the doped systems. Despite the inherent drawbacks in these two estimation methods for ΔEVBM, they lead to equivalent results and the correspondingJeffsp-d(ΔEVBM)are in good agreement with theJeffsp-d(M)ones.Ab initioresults obtained for the 2D-MoS2doped with 3d-series transition metals are presented to demonstrate the validity and applicability of the proposed computational schemes for obtainingJeffsp-d. The proposed methods can be utilized as useful tools in the search of new materials for spintronics and valleytronics applications.

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. We demonstrate that the magnetization constraint can be used as a probe in investigating the magnetic properties of the materials under magnetization constraints. Our results indicate that the GK criteria applied to diluted magnetic semiconductors and transition metal oxides of wurtzite structures always lead to a FM coupling between two 1nn dopants of the same type. This is justified by ab initio calculations obtained for ZnO and GaN doped with 3d-transition metal dopants.

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. We demonstrate this by analyzing a large set of descriptors for a wide range of systems and show that only 30% of these features are more likely to contribute to this property. We also show that training regression models with the reduced set of features to predict the total magnetic moment of new candidate DMSs has reduced the mean square error by about 20% compared to the models trained using the whole set of features. Furthermore, our results indicate that the predictive power of our method can be improved even more by extending our descriptor set.

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. In particular, it is shown that antiferromagnetic successive superexchange interactions can lead to a ferromagnetic coupling between two magnetic dopants mediated by a suitable codopant with the latter forming a ferromagnetic double exchange coupling with its first nearest neighbor anions which couple it with the magnetic cations. This is exemplified by ab initio results for the magnetic coupling of two Co-dopants in the presence of a mediated Cu codopant in the environment of various hosts, namely ZnO, GaN, GaP, TiO2, CdS and SnO2. Additional results for other codopant pairs in various hosts are also presented.

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, TiO2, SnO2, Sn3N4, MoS2, 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. These features specify the strength of the spin-polarization induced by the dopants on their first nearest neighboring anions which in turn specify the long range magnetic coupling among the dopants through successively induced spin polarizations (SSP) on neighboring dopants. The proposed local SSP process for the establishment of the magnetic coupling among the TM-dopants appears as a competitor to other classical processes (superexchange, double exchange, etc). Furthermore, these properties can be used as a set of descriptors suitable for developing statistical predictive theories for a much larger class of magnetic materials.

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. As such, it can be considered as a localized double exchange as it is not mediated by free carriers. SSP-MC can be either ferromagnetic (SSP-FMC) or antiferromagnetic (SSP-AFMC) and, as demonstrated here, the SSP-FMC can be significantly enhanced via codoping; it can act in competition with superexchange and/or double and/or p-d exchange interactions. While the SSP-MC is not directly related to the magnitude of the magnetic moments of the impurities, it depends strongly on the energy difference of the host and impurity d-band centers, the difference of their electronegativities and rather weakly on the coupling interactions between them as well as between the cations and their mediating anions. The validity of the proposed SSP-MC as a new type of magnetic coupling is demonstrated by ab initio results for DMSs, namely ZnO, GaN, GaP, TiO2 and MoS2 monodoped (with Co, Cu and Mn) and codoped (with Co-Cu-Co and Mn-Cu-Mn).

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 1.8 eV, is observed, with a low (2%) incorporation of antimonide, and the lattice expansion is in agreement with our first-principles calculations. The band edges of Ga(Sbx)N1-x are found to straddle the water redox potentials showing excellent suitability for solar water splitting.

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. In the present report, we use ab initio results to demonstrate that in a diverse sample of systems including codoped ZnO, GaN, TiO(2) and carbon-based materials, the ferromagnetic coupling that is developed among the doped (or defect-induced) magnetic moments results from the interaction of spin-polarized neighborhoods centered at the defect sites. Our results also give evidence that bipartite codopant configurations can further enhance the ferromagnetic features of these systems significantly.

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. Based on our results, we propose an efficient pathway for graphene-based spintronics applications.

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. In addition, the calculations indicate a structural phase transition at this size, and most importantly, the FeO(n)(+) clusters show unique magnetic features, exhibiting isoenergetic low-spin (LS) and high-spin (HS) ground states. In the LS state, the magnetic moments of the O atoms adopt an antiferromagnetic alignment with respect to the magnetic moment of Fe(+), whereas in the HS state, the alignment is ferromagnetic. FeO(10)(+) is the largest thermodynamically stable complex, with the highest magnetic moment among the FeO(n)(+) clusters (13 µ(B) in HS).

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. In this work we propose that defect-induced magnetism in codoped non-magnetic materials can be viewed within a molecular generalization of the atomic double-exchange and superexchange interactions applied to an arbitrary bipartite lattice hosted by (or embedded in) defect-free non-magnetic materials. In this view, the crucial factor for the development of magnetism appears to be the defect complementarity of the codopants. We demonstrate this by taking ZnO and GaN (the most widely studied doped oxide and nitride magnetic semiconductors, respectively) as host materials and perform theoretical calculations using ab initio methods after codoping them with transition metal impurities for a variety of configurations. Our results indicate that the magnetic coupling among the induced and/or doped magnetic moments takes the form of an interaction among spin-polarized molecular units which is facilitated by the formation of the hosted bipartite codopant structures. The universality of the proposed mechanism is further supported by earlier results referring to the rhombohedral C(60)-based polymers.

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. Importantly, these descriptors can be successfully applied to other metals using structural inputs from experiments and/or molecular modeling.

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. Based on our findings, we propose a general recipe for developing ferromagnetism in new materials of great technological interest. The recipe is quite general, although its realization is system specific. In each case, the required basic step is to find two synergistic codopants, one for providing the unpaired electrons and the other for facilitating the ferromagnetic coupling.

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. This, in turn, ignites charge transfer from the diamond lattice to the surface layer creating, thus, the necessary charge carriers (holes) for the observed high conductivity.