First principles predictions of structural, electronic and topological properties of two-dimensional Janus Ti2N2XI (X = Br, Cl) structures.

Motivated by the report of the giant Rashba effect in ternary layered compounds BiTeX, we consider two Janus structured compounds Ti2N2XI (X = Br, Cl) of the same ternary family exhibiting a 1 : 1 : 1 stoichiometric ratio. Broken inversion symmetry in the Janus structure, together with its unique electronic structure exhibiting anti-crossing states formed between Ti-d states and strong spin-orbit coupled I-p states, generates large Rashba cofficients of 2-3 eV Å for these compounds, classifying them as strong Rashba compounds. The anti-crossing features of the first-principles calculated electronic structure also result in non-trivial topology, combining two quantum phenomena - Rashba effect and non-trivial topology - in the same materials. This makes Janus TiNI compounds candidate materials for two-dimensional composite quantum materials. The situation becomes further promising by the fact that the properties are found to exhibit extreme sensitivity and tunability upon application of uniaxial strain.

First principles insights into the relative stability, electronic and catalytic properties of core-shell, Janus and mixed structural patterns for bimetallic Pd-X nano-alloys (X = Co, Ni, Cu, Rh, Ag, Ir, Pt, Au).

The three well-known orderings of the two constituting atomic species in a bimetallic nano-alloy - core-shell, Janus and mixed structural patterns - may be interconvertible depending on the synthesis conditions. Using first principles electronic structure calculations in the present work, we look for the microscopic origin for such structural transformation considering eight Pd-related bimetallic nano-alloys. Our analysis shows that it is the change in atom-atom covalency that is responsible for such structural transformation. Our study also reveals that the three patterns are distinctly identified in terms of total orbital hybridization. Finally, we have analyzed the trend in the relative catalytic activity for the three structures of each bimetallic nano-alloy using the d-band model. Our analysis indicates that the trend in the catalytic activity for the bimetallic Pd-X nano-alloys seems to be intermediate to those of the pristine Pd and Pt nano-clusters possessing similar structure and equal number of total atoms. Among the studied binary nano-alloys, the bimetallic Pd-Ni nano-alloy appears as the most suitable binary pair to develop a non-Pt catalyst.

Thermochemical stability, and electronic and dielectric properties of Janus bismuth oxyhalide BiOX (X = Cl, Br, I) monolayers.

Ultrathin monolayers of bismuth oxyhalide materials BiOX (X = Cl, Br, I) grown along 〈001〉 are studied using first-principles density functional theory. Both pristine BiOX and Janus (X, X' = Cl, Br, I) monolayers are investigated by analyzing their structural stability using formation enthalpy and phonon density of states. On the other hand, their thermochemical reactivity is understood from their surface energy trends in symmetric and asymmetric terminations. The theoretically measured optical band gaps and fundamental band gaps of these Janus monolayers are compared with their pristine counterparts BiOX and BiOX' as well as to the known experimental measurements. All of the possible Janus monolayers possess structural, electronic and optical properties intermediate to the corresponding properties of the two associated pristine BiOX and BiOX' monolayers. According to the formation enthalpy, stabilization is equally favorable for all the monolayers, whereas the lowest surface energy is found for BiOCl0.5Br0.5, leading to excellent thermochemical reactivity which is consistent with recent experimental measurements. The frequency dependent dielectric functions are simulated in the density functional perturbation theory limit, and the optical band gaps are estimated from the absorption and reflectance spectra, and are in excellent agreement with the known experimentally measured values. High frequency dielectric constants of these materials with 2D symmetry are estimated from G 0 W 0 calculations including local field and spin-orbit effects. The larger dielectric constants and wider differences in the charge carriers' effective masses also provide proof that this new class of 2D materials has potential in photo-electrochemical applications. Thus, fabricating Janus monolayers of these oxyhalide compounds would open up a rational design strategy for tailoring their optoelectronic properties, which may offer guidance for the design of highly efficient optoelectronic materials for catalysis, valleytronic, and sensing applications.

Impact of bi-axial strain on the structural, electronic and optical properties of photo-catalytic bulk bismuth oxyhalides.

The structural, electronic and optical properties of bulk bismuth oxyhalides, BiOX (X = F, Cl, Br, and I), were studied using state-of-the-art density functional theory (DFT)-based calculations. The effects of compressive and tensile strains on the in-plane lattice parameters were analyzed to better understand their good performance in photo-catalytic applications. Our present first-principles calculations show that at least 4% in-plane bi-axial compressive strain over the experimental lattice parameters of BiOF is needed for phonon stability of this material, whereas other BiOX systems can accept up to 2% in-plane bi-axial compressive strain and retain their dynamical stability. On the other hand, 2% in-plane tensile strain breaks the structural stability of all bulk BiOX structures. Tuning the electronic band structures with such external compressive strain indeed helps to enhance the separation of charge carriers due to larger electron-hole effective mass differences in the BiOBr and BiOI structures. The optical properties are discussed from their calculated absorption spectra and optical conductivity within independent particle approximations. The average values of the calculated optical band gaps are in the range of 3.8-3.9 eV, 3.3-3.4 eV, 2.7-2.8 eV and 1.7-1.8 eV for the unstrained and compressive strained structures, respectively, of the BiOF, BiOCl, BiOBr and BiOI materials, which are reasonably good compared to their known experimental ultra-violet visible spectroscopy measured data.

First principles study of bimetallic Ni13-nAgn nano-clusters (n = 0-13): Structural, mixing, electronic, and magnetic properties.

Using spin polarized density functional theory based calculations, combined with ab initio molecular dynamics simulation, we carry out a systematic investigation of the bimetallic Ni13-nAgn nano-clusters, for all compositions. This includes prediction of the geometry, mixing behavior, and electronic properties. Our study reveals a tendency towards the formation of a core-shell like structure, following the rule of putting Ni in a high coordination site and Ag in a low coordination site. Our calculations predict negative mixing energies for the entire composition range, indicating mixing to be favored for the bimetallic small sized Ni-Ag clusters, irrespective of the compositions. The magic composition with the highest stability is found for the NiAg12 alloy cluster. We investigate the microscopic origin of a core-shell like structure with negative mixing energy, in which the Ni-Ag inter-facial interaction is found to play a role. We also study the magnetic properties of the Ni-Ag alloy clusters. The Ni dominated magnetism consists of parallel alignment of Ni moments while the tiny moments on Ag align in anti-parallel to Ni moments. The hybridization with the Ag environment causes reduction of Ni moment.

Study of morphology effects on magnetic interactions and band gap variations for 3d late transition metal bi-doped ZnO nanostructures by hybrid DFT calculations.

Using density functional theory (DFT) based electronic structure calculations, the effects of morphology of semiconducting nanostructures on the magnetic interaction between two magnetic dopant atoms as well as a possibility of tuning band gaps have been studied in the case of the bi-doped (ZnO)24 nanostructures with the impurity dopant atoms of the 3d late transition metals-Mn, Fe, Co, Ni, and Cu. To explore the morphology effect, three different structures of the host (ZnO)24 nano-system, having different degrees of spatial confinement, have been considered: a two dimensional nanosheet, a one dimensional nanotube, and a finite cage-shaped nanocluster. The present study employs hybrid density functional theory to accurately describe the electronic structure of all the systems. It is shown here that the magnetic coupling between the two dopant atoms remains mostly anti-ferromagnetic in the course of changing the morphology from the sheet geometry to the cage-shaped geometry of the host systems, except for the case of energetically most stable bi-Mn doping, which shows a transition from ferromagnetic to anti-ferromagnetic coupling with decreasing aspect ratio of the host system. The effect of the shape change, however, has a significant effect on the overall band gap variations of both the pristine as well as all the bi-doped systems, irrespective of the nature of the dopant atoms and provides a means for easy tunability of their optoelectronic properties.

Enhanced magnetism of Cu(n) clusters capped with N and endohedrally doped with Cr.

The focus of our work is on the production of highly magnetic materials out of Cu clusters. We have studied the relative effects of N-capping as well as N mono-doping on the structural stability and electronic properties of the small Cu clusters using first principles density functional theory based electronic structure calculations. We find that the N-capped clusters are more promising in producing giant magnetic moments, such as 14 µB for the Cu6N6 cluster and 29 µB for the icosahedral Cu13N12 cluster. This is accompanied by a substantial enhancement in their stability. We suggest that these giant magnetic moments of the capped Cun clusters have relevance to the observed room temperature ferromagnetism of Cu doped GaN. For cage-like hollow Cu-clusters, an endohedral Cr-doping together with the N-capping appears as the most promising means to produce stable giant magnetic moments in the copper clusters.

Structural, electronic and magnetic properties of transition metal binary alloy clusters with isoelectronic components: case study with MnmTcn, TimZrn and MnmRen.

With the goal of achieving an understanding of the properties of bimetallic alloy clusters having atoms of two isoelectronic elements, we have studied the structural, electronic and magnetic properties of MnmTcn, MnmRen and TimZrn clusters with m + n = 13 (n = 0, 1, 4, 6, 9, 12, 13), using first-principles density functional calculations. MnmTcn and MnmRen represent clusters of isoelectronic series with a half-filled d shell, while TimZrn represents an isoelectronic cluster series of early transition metals. Mn-rich alloy clusters are found to prefer compact structures and isoelectronic Tc-rich or Re-rich alloy clusters are found to adopt open structures. In contrast, TimZrn clusters are all found to stabilize in compact structures, irrespective of being Ti-rich or Zr-rich. This change in behavior between two isoelectronic series is found to be driven by differences in hybridization effects, due to differences in the evolution of the relative energy positions of the d level with respect to the s and p levels upon moving from 3d to 4d or 5d elements. This effect further competes with the magnetization effect to decide the morphology of the alloy clusters. Focusing on the magnetic properties of the studied clusters, we find that the single Tc atom substituted alloy cluster exhibits markedly improved magnetic properties compared to that of pure Mn clusters.

Study of structural stability and electronic structure of nonstoichiometric CdS nano clusters from first principles.

Structural stability of small sized nonstoichiometric CdS nano clusters between zincblende and wurtzite structures has been investigated using first-principles density functional calculations. Our study shows that the relative stability of these two structures depends sensitively on whether the surface is S-terminated or Cd-terminated. The associated band gap also exhibits non-monotonic behavior as a function of cluster size. Our findings may shed light on contradictory reports of experimentally observed structures of CdS nano clusters found in the literature.