Yelda Kadioglu

Electronic and geometric structure of AuxCuy clusters studied by density functional theory

We have determined the stable structures of AuCun, Au2Cun, Au3Cun and AuxCu8-x clusters. It has been observed that AuCun, Au2Cun and Au3Cun systems have two-dimensional (2D) structures up to six atoms and they become three-dimensional (3D) afterwards. AuxCu8-x clusters favor 3D structures till the Au7Cu1 cluster. We have found a lowest energy isomer of Au6Cu2 from the literature. Bond lengths, binding energies, density of states (DOS), highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) gaps, ionization potential (IP) and electron affinity (EA) have been calculated for these structures using the first principles density functional theory (DFT) within the generalized gradient approximation (GGA) and the local density approximation (LDA). Generally, we have observed the overlap between s electrons of Cu and p electrons of Au near the Fermi level. Charge transfers are calculated by using the Lowdin analysis. It is observed that one Cu atom does not significantly modify the clusters which have more gold atoms. It is also seen that these clusters generally have nonmagnetic properties and results are consistent with the hybridization between s and d orbitals of Au in AuxCu8-x clusters.

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.

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.

Modification of electronic structure, magnetic structure, and topological phase of bismuthene by point defects

This paper reveals how electronic, magnetic structure and topological phase of 2D, single-layer structures of bismuth are modified by point defects. We first showed that free standing, single-layer, hexagonal structure of bismuth, named as h-bismuthene exhibits non-trivial band topology. We then investigated interactions between single foreign adatoms and bismuthene structures, which comprise stability, bonding, electronic and magnetic structures. Localized states in diverse location of the band gap and resonant states in band continua of bismuthene are induced upon the adsorption of different adatoms, which modify electronic and magnetic properties. Specific adatoms result in reconstruction around the adsorption site. Single and divacancies can form readily in bismuthene structures and remain stable at high temperatures. Through rebondings Stone-Whales type defects are constructed by divacancies, which transform into a large hole at high temperature. Like adsorbed adatoms, vacancies induce also localized gap states, which can be eliminated through rebondings in divacancies. We also showed that not only optical and magnetic properties, but also topological features of pristine h-bismuthene can be modified by point defects. Modification of topological features depends on the energies of localized states and also on the strength of coupling between point defects.

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.