Fatih Ersan

Electronic structure of BSb defective monolayers and nanoribbons

In this paper, we investigate two- and one-dimensional honeycomb structures of boron antimony (BSb) using a first-principles plane wave method within the density functional theory. BSb with a two-dimensional honeycomb structure is a semiconductor with a 0.336 eV band gap. The vacancy defects, such as B, Sb, B + Sb divacancy, and B + Sb antisite disorder affect the electronic and magnetic properties of the 2D BSb sheet. All the structures with vacancies have nonmagnetic metallic characters, while the system with antisite disorder has a semiconducting band structure. We also examine bare and hydrogen-passivated quasi-one-dimensional armchair BSb nanoribbons. The effects of ribbon width (n) on an armchair BSb nanoribbon and hydrogen passivation on both B and Sb edge atoms are considered. The band gaps of bare and H passivated A-Nr-BSb oscillate with increasing ribbon width; this property is important for quantum dots. For ribbon width n = 12, the bare A-Nr-BSb is a nonmagnetic semiconductor with a 0.280 eV indirect band gap, but it becomes a nonmagnetic metal when B edge atoms are passivated with hydrogen. When Sb atoms are passivated with hydrogen, a ferromagnetic half-metallic ground state is observed with 2.09μB magnetic moment. When both B and Sb edges are passivated with hydrogen, a direct gap semiconductor is obtained with 0.490 eV band gap with disappearance of the bands of edge atoms.

Electronic and magnetic properties of monolayer α-RuCl3: a first-principles and Monte Carlo study

Recent experiments revealed that monolayer α-RuCl3 can be obtained by a chemical exfoliation method and exfoliation or restacking of nanosheets can manipulate the magnetic properties of the materials. In this paper, the electronic and magnetic properties of an α-RuCl3 monolayer are investigated by combining first-principles calculations and Monte Carlo simulations. From first-principles calculations, we found that the spin configuration of FM corresponds to the ground state for α-RuCl3, however, the other excited zigzag oriented spin configuration has an energy of 5 meV per atom higher than the ground state. The energy band gap is found to be 3 meV using PBE functionals. When the spin-orbit coupling effect is taken into account, the corresponding energy gap is determined to be 57 meV. We also investigate the effect of the Hubbard U energy terms on the electronic band structure of the α-RuCl3 monolayer and revealed that the band gap increases approximately linearly with increasing U value. Moreover, spin-spin coupling terms (J1, J2, and J3) have been obtained using first-principles calculations. By benefiting from these terms, Monte Carlo simulations with a single site update Metropolis algorithm have been implemented to elucidate the magnetic properties of the considered system. Thermal variations of magnetization, susceptibility and also specific heat curves indicate that monolayer α-RuCl3 exhibits a phase transition between ordered and disordered phases at the Curie temperature of 14.21 K. We believe that this study can be utilized to improve two-dimensional magnetic materials.

Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: A first-principles perspective to recent synthesis

Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were...

Adsorption of alkali and alkaline earth metal atoms and dimers on monolayer germanium carbide

First-principles plane wave calculations have been performed to study the adsorption of alkali and alkaline earth metals on monolayer germanium carbide (GeC). We found that the favourable adsorption sites on GeC sheet for single alkali and alkaline earth adatoms are generally different from graphene or germanene. Among them, Mg, Na and their dimers have weakly bounded to GeC due to their closed valence electron shells, so they may have high mobility on GeC. Two different levels of adatom coverage ( and ) have been investigated and we concluded that different electronic structures and magnetic moments for both coverages owing to alkali and alkaline earth atoms have long range electrostatic interactions. Lithium atom prefers to adsorbed on hollow site similar to other group-IV monolayers and the adsorption results in metallisation of GeC instead of semiconducting behaviour. Na and K adsorption can induce 1 total magnetic moment on GeC structures and they have shown semiconductor property which may have potential use in spintronic devices. We also showed that alkali or alkaline earth metal atoms can form dimer on GeC sheet. Calculated adsorption energies suggest that clustering of alkali and alkaline earth atoms is energetically favourable. All dimer adsorbed GeC systems have nonmagnetic semiconductor property with varying band gaps from 0.391 to 1.311 eV which are very suitable values for various device applications.

The effect of vacancies and the substitution of p-block atoms on single-layer buckled germanium selenide

Single-layer GeSe is a new candidate in the two-dimensional family of materials. In our recent study, we showed that GeSe can form a stable buckled honeycomb structure (b-GeSe) and is a semiconductor with a 2.29 eV band gap. This paper investigates the effect of point defects of both hole (Ge, Se) and substitution doping of p-block elements, in single-layer b-GeSe, based on first principles plane wave calculations within spin-polarized density functional theory. In the case of the substitution process, we present an extensive analysis of the effects of substituting atoms (Al, As, Cl, P, C, N, Ge or Se, Si, B, F, Ga and S) on the electronic and magnetic properties of the b-GeSe phase. Our results show that nonmagnetic and semiconducting b-GeSe can be half-metallized by Ge vacancies, while it remains a semiconductor with Se vacancies with a decreasing band gap value. The results of the substitution process can be categorized by the group number in the periodic table. b-GeSe remains a nonmagnetic semiconductor upon the substitution of defects with group IVA and VIA atoms on either the Ge or Se position of the b-GeSe structure. On the other hand, the results show that the influence of group IIIA and VIIA atoms is obvious, as these atoms raise the net magnetic moments (1 μB to 3 μB) of the new b-GeSe system. In particular, the system shows half-metallicity when the Se atoms are replaced with group IIIA atoms. The system has a net magnetic moment when substituting group VA atoms for Se atoms, whereas it does not when substituting them for Ge atoms (except for N). We believe that these results are useful for the further functionalization of b-GeSe with point defects.

Diffusion quantum Monte Carlo and density functional calculations of the structural stability of bilayer arsenene

We have studied the structural stability of monolayer and bilayer arsenene (As) in the buckled (b) and washboard (w) phases with diffusion quantum Monte Carlo (DMC) and density functional theory (DFT) calculations. DMC yields cohesive energies of 2.826(2) eV/atom for monolayer b-As and 2.792(3) eV/atom for w-As. In the case of bilayer As, DMC and DFT predict that AA-stacking is the more stable form of b-As, while AB is the most stable form of w-As. The DMC layer-layer binding energies for b-As-AA and w-As-AB are 30(1) and 53(1) meV/atom, respectively. The interlayer separations were estimated with DMC at 3.521(1) Å for b-As-AA and 3.145(1) Å for w-As-AB. A comparison of DMC and DFT results shows that the van der Waals density functional method yields energetic properties of arsenene close to DMC, while the DFT + D3 method closely reproduced the geometric properties from DMC. The electronic properties of monolayer and bilayer arsenene were explored with various DFT methods. The bandgap values vary significantly with the DFT method, but the results are generally qualitatively consistent. We expect the present work to be useful for future experiments attempting to prepare multilayer arsenene and for further development of DFT methods for weakly bonded systems.

T-ZrS nanoribbons: structure and electronic properties

Recently, monolayer and few layers of trigonal phases of zirconium disulfide (T-ZrS) sheets were obtained experimentally on hexagonal boron nitride using an evaporation technique. On the basis of these previous results, we report the structural and electronic properties of armchair nanoribbons (ANRs) and zigzag nanoribbons (ZNRs) of T-ZrS by means of density functional theory. According to our results, both ANRs and ZNRs are nonmagnetic semiconductors similar to a two-dimensional T-ZrS monolayer. The semiconducting character is not altered by termination of the edge atoms with hydrogen. The band gaps are associated with the ribbon widths and edge structures. The band gaps of bare and H-terminated ANR-ZrS decrease exponentially, whereas the band gaps of ultra-narrow zigzag nanoribbons oscillate slightly with increasing ribbon width. Although the band gaps of bare ANRs approach that of 2D T-ZrS, other structures have larger band gaps than the monolayer with increasing ribbon width. The cohesive and formation energies of bare ANRs and ZNRs converge rapidly to that of the 2D T-ZrS structure with increasing ribbon width.