Çetin Kılıç

n-type doping of oxides by hydrogen

First-principles total-energy calculations suggest that interstitial hydrogen impurity forms a shallow donor in SnO2, CdO, and ZnO, but a deep donor in MgO. We generalize this result to other oxides by recognizing that there exist a “hydrogen pinning level” at about 3.0±0.4 eV below vacuum. Materials such as Ag2O, HgO, CuO, PbO, PtO, IrO2, RuO2, PbO2, TiO2, WO3, Bi2O3, Cr2O3, Fe2O3, Sb2O3, Nb2O5, Ta2O5, FeTiO3, and PbTiO3, whose conduction band minimum (CBM) lie below this level (i.e., electron affinity>3.0±0.4 eV) will become conductive once hydrogen is incorporated into the lattice, without reducing the host. Conversely, materials such as BaO, NiO, SrO, HfO2, and Al2O3, whose CBM lie above this level (i.e., electron affinity<3.0±0.4 eV) will remain nonconductive since hydrogen forms a deep impurity.

Pressure-induced interlinking of carbon nanotubes

We predict new forms of carbon consisting of one- and two-dimensional networks of interlinked single-wall carbon nanotubes, some of which are energetically more stable than van der Waals packing of the nanotubes on a hexagonal lattice. These interlinked nanotubes are further transformed with higher applied external pressures to more dense and complicated stable structures, in which curvature-induced carbon

Reversible band-gap engineering in carbon nanotubes by radial deformation

{\mathrm{sp}}^{3}

Variable and reversible quantum structures on a single carbon nanotube

rehybridizations are formed. We also discuss the energetics of the bond formation between nanotubes and the electronic properties of these predicted novel structures.

Combined hybrid functional and DFT+U calculations for metal chalcogenides.

We present a systematic analysis of the effect of radial deformation on the atomic and electronic structure of zigzag and armchair single wall carbon nanotubes using the first-principle plane wave method. The nanotubes were deformed by applying a radial strain, which distorts the circular cross section to an elliptical one. The atomic structure of the nanotubes under this strain are fully optimized, and the electronic structure is calculated self-consistently to determine the response of individual bands to the radial deformation. The band gap of the insulating tube is closed and eventually an insulator-metal transition sets in by the radial strain which is in the elastic range. Using this property a multiple quantum well structure with tunable and reversible electronic structure is formed on an individual nanotube and its band lineup is determined from first principles. The elastic energy due to the radial deformation and elastic constants are calculated and compared with classical theories.

Doping of chalcopyrites by hydrogen

The band gap of a semiconducting single wall carbon nanotube decreases and eventually vanishes leading to metalization as a result of increasing radial deformation. This sets in a band offset between the undeformed and deformed regions of a single nanotube. Based on the superlattice calculations, we show that these features can be exploited to realize various quantum well structures on a single nanotube with variable and reversible electronic properties. These quantum structures and nanodevices incorporate mechanics and electronics.

Vibrations of the cubane molecule: inelastic neutron scattering study and theory

In the density-functional studies of materials with localized electronic states, the local/semilocal exchange-correlation functionals are often either combined with a Hubbard parameter U as in the LDA+U method or mixed with a fraction of exactly computed (Fock) exchange energy yielding a hybrid functional. Although some inaccuracies of the semilocal density approximations are thus fixed to a certain extent, the improvements are not sufficient to make the predictions agree with the experimental data. Here, we put forward the perspective that the hybrid functional scheme and the LDA+U method should be treated as complementary, and propose to combine the range-separated Heyd-Scuseria-Ernzerhof (HSE) hybrid functional with the Hubbard U. We thus present a variety of HSE+U calculations for a set of II-VI semiconductors, consisting of zinc and cadmium monochalcogenides, along with comparison to the experimental data. Our findings imply that an optimal value U(*) of the Hubbard parameter could be determined, which ensures that the HSE+U(*) calculation reproduces the experimental band gap. It is shown that an improved description not only of the electronic structure but also of the crystal structure and energetics is obtained by adding the U(*) term to the HSE functional, proving the utility of HSE+U(*) approach in modeling semiconductors with localized electronic states.

Quantum point contact on graphite surface

First-principles total-energy calculations for hydrogen impurities in CuInSe2 (CIS) and CuGaSe2 (CGS) show that H+ takes up the Cu–Se bond center position, whereas H0 and H− take up tetrahedral interstitial site next to In (in CIS) or Ga (in CGS). Hydrogen creates a negative-U center (i.e., H0 is never stable), with a (+/−) transition level at Ec−0.39 eV in CIS, and Ec−0.57 eV in CGS. However, once combined with the 2VCu−+IIICu2+ complex, hydrogen forms shallower centers with transition levels at Ec−0.15 eV in CIS, and Ec−0.39 eV in CGS. We conclude that hydrogen could convert CIS to n type, but not CGS.

Crystal and electronic structure of BiTeI, AuTeI, and PdTeI compounds: A dispersion-corrected density-functional study

Cubane C H is an immensely strained molecule whose C-C-Cbond angle is 908 rather than 109.58 as expected for 88 sp 3 bonding of carbon. We have measured the intramolecular vibrational spectrum of cubane using inelastic neutron scattering. The neutron data are used to test the transferability of various phenomenological potentials and tight-binding models to this highly strained molecule. Unlike these models, first-principles calculations of the INS spectrum both energy .

Doping-induced spin-orbit splitting in Bi-doped ZnO nanowires

Department of Physics, Bilkent University, Bilkent 06533, Ankara, Turkey.The conductance through a quantum point contact created by a sharp and hard metal tip onthe graphite surface has features which to our knowledge have not been encountered so far inmetal contacts or in nanowires. In this paper we first investigate these features which emergefrom the strongly directional bonding and electronic structure of graphite, and provide a theoreticalunderstanding for the electronic conduction through quantum point contacts. Our study involves themolecular-dynamics simulations to reveal the variation of interlayer distances and atomic structureat the proximity of the contact that evolves by the tip pressing toward the surface. The effectsof the elastic deformation on the electronic structure, state density at the Fermi level, and crystalpotential are analyzed by performing self-consistent-field pseudopotential calculations within thelocal-density approximation. It is found that the metallicity of graphite increases under the uniaxialcompressive strain perpendicular to the basal plane. The quantum point contact is modeled by aconstriction with a realistic potential. The conductance is calculated by representing the currenttransporting states in Laue representation, and the variation of conductance with the evolution ofcontact is explained by taking the characteristic features of graphite into account. It is shown thatthe sequential puncturing of the layers characterizes the conductance.73.40.Cg, 73.23.Ad, 62.20.DcI. INTRODUCTION