Browsing by Subject "First principles calculations"

Now showing 1 - 4 of 4

Open Access
Formation and functionalization of boron phosphide monolayers
(2015-09) Hallıoğlu, Lütfiye
Since the synthesis of graphene with its unique properties has increased the focus on novel two dimensional (2D) materials, successively new 2D materials from either layered or non-layered materials have been synthesized following the advances in thin film growth and characterization techniques. Hexagonal boron nitride (h-BN) is the runner-up material, which is structurally stable in hexagonal honeycomb form. h-BN is an insulator whereas, it is a good thermal conductor. However, the electronic and structural properties of these 2D materials are very susceptible to doping and adsorption, as such, these properties can be altered extensively. Therefore, we have examined the phosphorization of h-BN with varying concentrations, which leads to stable 2D boron phosphide at the ultimate limit. The lattice constant of the BN16 gap semiconductor with impurity characteristics of adsorbants. Also, we have shown that except for Al and Ga, these impurity adatoms carry small amount of magnetic moment in moderate temperatures. In addition, we have studied the substitution of monolayer BP with Group III-IVV atoms. Based on our calculations, we have found that C and N can substitute P atom under ambient conditions. Nonetheless, only N atom selectively substitute for P atom, whereas C atom substitutes both for B and P giving rise to possible chemical etching of monolayer BP in the presence of excess C atom. Substitution of C for B/P results in metallic state in monolayer BP, while substitution of N for P leaves monolayer BP direct gap semiconductor. It is also found that none of these substitutions makes substrate magnetic. Using state-of-the-art computational tools based on the Density Functional Theory( DFT), we have calculated the structural and electronic properties of phosphorization of monolayer h-BN and doped monolayer h-BP.
Open Access
Gate induced monolayer behavior in twisted bilayer black phosphorus
(IOP Publishing, 2017) Sevik, C.; Wallbank, J. R.; Gülseren, O.; Peeters, F. M.; Çakir, D.
Optical and electronic properties of black phosphorus strongly depend on the number of layers and type of stacking. Using first-principles calculations within the framework of density functional theory, we investigate the electronic properties of bilayer black phosphorus with an interlayer twist angle of 90°. These calculations are complemented with a simple k p model which is able to capture most of the low energy features and is valid for arbitrary twist angles. The electronic spectrum of 90° twisted bilayer black phosphorus is found to be x-y isotropic in contrast to the monolayer. However x-y anisotropy, and a partial return to monolayer-like behavior, particularly in the valence band, can be induced by an external out-of-plane electric field. Moreover, the preferred hole effective mass can be rotated by 90° simply by changing the direction of the applied electric field. In particular, a +0.4 (-0.4) V A-1 out-of-plane electric field results in a ~60% increase in the hole effective mass along the y (x) axis and enhances the m*y /m*x (m*x /m*y) ratio as much as by a factor of 40. Our DFT and k p simulations clearly indicate that the twist angle in combination with an appropriate gate voltage is a novel way to tune the electronic and optical properties of bilayer phosphorus and it gives us a new degree of freedom to engineer the properties of black phosphorus based devices. © 2017 IOP Publishing Ltd.
Open Access
Oscillatory exchange coupling in magnetic molecules
(IOP Publishing, 2007) Sevincli, H.; Senger, R. T.; Durgun, Engin; Çıracı, Salim
Recently, first-principles calculations based on the spin-dependent density functional theory (DFT) have revealed that the magnetic ground state of a finite linear carbon chain capped by two transition metal (TM) atoms alternates between ferromagnetic and antiferromagnetic configurations depending on the number of carbon atoms. The character of indirect exchange coupling in this nanoscale, quasi-zero-dimensional system is different from those analogous extended structures consisting of magnetic layers separated by a non-magnetic spacer (or magnetic impurities in a non-magnetic host material) and a formulation based on an atomic picture is needed. We present a tight-binding model which provides a theoretical framework to the underlying mechanism of the exchange coupling in molecular structures. The model calculations are capable of reproducing the essential features of the DFT results for the indirect exchange coupling and the atomic magnetic moments in the TM-Cn-TM structures as functions of the number of carbon atoms. In nanostructures consisting of a few atoms the concepts of extended wavefunctions and the band theory lose their validity, and hence the oscillatory exchange coupling turns out to be a consequence of quantum interference effects due to the spin-dependent onsite and hopping energies. © IOP Publishing Ltd.
Open Access
A theoretical study on silicon and III-V compound nanotubes
(TÜBİTAK, 2005) Durgun, Engin; Çıracı, Salim
In this paper we present a theoretical study on single-wall silicon and III-V compound nanotubes. First principles plane wave calculations within density functional theory are used to predict energetics and electronic structures of armchair and zigzag nanotubes. The stability of tubular structures is further investigated at finite temperature by ab initio molecular dynamics calculations. Our results indicate that (n,0) zigzag and (n,n) armchair single-wall Si nanotubes are stable for n ≥ 6. Mechanically, the Si nanotubes are radially soft, however they are strong against axial deformations. Electronic analysis showed that zigzag nanotubes are metallic for n ≤ 11, but they show semiconducting behavior for larger radii. On the other hand, all armchair nanotubes are metallic. (8,0) single wall nanotube has been chosen as prototypes for AlP, GaN, and GaAs compounds and we found that they are semiconducting and stable at room temperature. © TÜBİTAK.