Browsing by Author "Akman, N."
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Item Open Access Atomic and electronic structures of doped silicon nanowires: A first-principles study(2007) Durgun, Engin; Akman, N.; Ataca, C.; Çıracı, SalimWe have investigated the atomic and electronic structures of hydrogen saturated silicon nanowires doped with impurity atoms (such as Al, Ga, C, Si, Ge, N, P, As, Te, Pt) using a first-principles plane wave method. We considered adsorption and substitution of impurity atoms at the surface and also their substitution at the core of the nanowire. In the case of adsorption to the surface, we determined the most energetic adsorption geometry among various possible adsorption sites. All impurities studied lead to nonmagnetic ground state with a significant binding energy. Impurity bands formed at high impurity concentration are metallic for group IIIA and VA elements but are semiconductor and modify the band gap for group IVA and VIA elements. While low substitutional impurity concentration leads to usual n - and p -type behaviors reminiscent of bulk Si, this behavior is absent if the impurity atom is adsorbed on the surface. It is shown that the electronic properties of silicon nanowires can be modified by doping for optoelectronic applications.Item Open Access Confined states in multiple quantum well structures of Sin Gen nanowire superlattices(American Physical Society, 2007) Akman, N.; Durgun, Engin; Cahangirov, S.; Çıracı, SalimMechanical properties, atomic and energy band structures of bare and hydrogen-passivated Sin Gen nanowire superlattices have been investigated by using first-principles pseudopotential plane-wave method. Undoped, tetrahedral Si and Ge nanowire segments join pseudomorphically and can form superlattice with atomically sharp interface. We found that Sin nanowires are stiffer than Gen nanowires. Hydrogen passivation makes these nanowires and Sin Gen nanowire superlattice even more stiff. Upon heterostructure formation, superlattice electronic states form subbands in momentum space. Band lineups of Si and Ge zones result in multiple quantum wells, where specific states at the band edges and in band continua are confined. The electronic structure of the nanowire superlattice depends on the length and cross section geometry of constituent Si and Ge segments. Since bare Si and Ge nanowires are metallic and the band gaps of hydrogenated ones vary with the diameter, Sin Gen superlattices offer numerous alternatives for multiple quantum well devices with their leads made from the constituent metallic nanowires.Item Open Access Functionalization of silicon nanowires with transition metal atoms(American Physical Society, 2008) Durgun, Engin; Akman, N.; Çıracı, SalimThis paper investigates atomic structure, mechanical, electronic, and magnetic properties of silicon nanowires (SiNW) using first-principles plane-wave calculations within density-functional theory. We considered bare, hydrogen-terminated, and 3d -transition metal (TM) adsorbed SiNWs oriented along [001] direction. We also studied Cr interstitial impurity. Nanowires of different sizes are initially cut from the bulk Si crystal in rodlike forms, and subsequently their atomic structures are relaxed before and also after the termination of surface dangling bonds by hydrogen atoms. We first presented an extensive analysis of the atomic structure, stability, elastic, and electronic properties of bare and hydrogen-terminated SiNWs. The energetics of adsorption and resulting electronic and magnetic properties are examined for different levels of 3d -TM atom coverage. Adsorption of TM atoms generally results in magnetic ground state. The net magnetic moment increases with increasing coverage. While specific SiNWs acquire half-metallic behavior at low coverage, at high coverage ferromagnetic nanowires become metallic for both spin directions, and some of them have very high spin polarization at the Fermi level. Our results suggest that the electronic and spintronic devices with conducting interconnects between them can be fabricated on a single SiNW at a desired order. We believe that our study will initiate new research on spintronic applications of SiNWs.Item Open Access Half-metallic silicon nanowires: First-principles calculations(American Physical Society, 2007) Durgun, Engin; Çakır, D.; Akman, N.; Çıracı, SalimFrom first-principles calculations, we predict that specific transition metal (TM) atom-adsorbed silicon nanowires have a half-metallic ground state. They are insulators for one spin direction, but show metallic properties for the opposite spin direction. At high coverage of TM atoms, ferromagnetic silicon nanowires become metallic for both spin directions with high magnetic moment and may have also significant spin polarization at the Fermi level. The spin-dependent electronic properties can be engineered by changing the type of adsorbed TM atoms, as well as the diameter of the nanowire. Present results are not only of scientific interest, but also can initiate new research on spintronic applications of silicon nanowires. © 2007 The American Physical Society.Item Open Access Hydrogen storage capacity of titanium met-cars(IOP Publishing Ltd., 2006) Akman, N.; Durgun, Engin; Yildirim, T.; Çıracı, SalimThe adsorption of hydrogen molecules on the titanium metallocarbohedryne (met-car) cluster has been investigated by using the first-principles plane wave method. We have found that, while a single Ti atom at the corner can bind up to three hydrogen molecules, a single Ti atom on the surface of the cluster can bind only one hydrogen molecule. Accordingly, a Ti8C12 met-car can bind up to 16H2 molecules and hence can be considered as a high-capacity hydrogen storage medium. Strong interaction between two met-car clusters leading to the dimer formation can affect H2 storage capacity slightly. Increasing the storage capacity by directly inserting H 2 into the met-car or by functionalizing it with an Na atom have been explored. It is found that the insertion of neither an H2 molecule nor an Na atom could further promote the H2 storage capacity of a Ti8C12 cluster. We have also tested the stability of the H2-adsorbed Ti8C12 met-car with ab initio molecular dynamics calculations which have been carried out at room temperature.