Browsing by Subject "Energy gap"

Now showing 1 - 20 of 58

Open Access
Ab initio study of Ru-terminated and Ru-doped armchair graphene nanoribbons
(Taylor and Francis, 2012) Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.; Özbay, Ekmel
We investigate the effects of ruthenium (Ru) termination and Ru doping on the electronic properties of armchair graphene nanoribbons (AGNRs) using first-principles methods. The electronic band structures, geometries, density of states, binding energies, band gap information, and formation energies of related structures are calculated. It is well founded that the electronic properties of the investigated AGNRs are highly influenced by Ru termination and Ru doping. With Ru termination, metallic band structures with quasi-zero-dimensional, one-dimensional and quasi-one-dimensional density of states (DOS) behavior are obtained in addition to dominant one-dimensional behavior. In contrast to Ru termination, Ru doping introduces small but measurable (12.4 to 89.6meV) direct or indirect band gaps. These results may present an additional way to produce tunable band gaps in AGNRs.
Open Access
Accurate method for obtaining band gaps in conducting polymers using a DFT/hybrid approach
(American Chemical Society, 1998) Salzner, U.; Pickup, P. G.; Poirier, R. A.; Lagowski, J. B.
DFT calculations on a series of oligomers have been used to estimate band gaps, ionization potentials, electron affinities, and bandwidths for polyacetylene, polythiophene, polypyrrole, polythiazole, and a thiophene - thiazole copolymer. Using a slightly modified hybrid functional, we obtain band gaps within 0.1 eV of experimental solid-state values Calculated bond lengths and bond angles for the central ring of sexithiophene differ by less than 0.026 Å and 0.7° from those of the sexithiopnene crystal structure. IPs and EAs are overestimated by up to 0.77 eV compared to experimental bulk values. Extrapolated bandwidths agree reasonably well with bandwidths from band structure calculations.
Open Access
All-chalcogenide glass omnidirectional photonic band gap variable infrared filters
(AIP Publishing, 2009) Kondakci, H. E.; Yaman, M.; Koylu, O.; Dana, A.; Bayındır, Mehmet
We report on the design, fabrication, and characterization of spatially variable infrared photonic band gap filter that consists of thermally evaporated, high refractive indexcontrast, amorphous chalcogenide glass multilayers. Due to graded thickness structure, the filter exhibits a position dependent stop band and a cavity mode ranging from 1.8 to 3.4 μm3.4 μmwavelengths. Reflection measurements on the variable filter agree well with theoretical calculations. These results pave the way to low-loss infrared mirrors, filters, spectral imaging, and miniaturized spectrometers at infrared region.
Open Access
All-chalcogenide variable infrared filter
(SPIE, 2009) Kondakcı, H. Esat; Köyle, Özlem; Yaman, Mecit; Dana, Aykutlu; Bayındır, Mehmet
We present the design, fabrication, characterization of spatially variable infrared filter and a demonstration of the filter as a simple infrared spectrometer. A varying photonic band gap filter which consists of thermally evaporated, high refractive index contrast amorphous chalcogenide glass multilayers, makes the structure suitable to be used as spectrometer. Due to graded thickness structure, the filter exhibits a position dependent stop band and a cavity mode ranging from 2 to 8 μm wavelengths. It is demonstrated that the filter can be used to detect absorption peaks of common gases in the cavity mode range of the filter. © 2009 SPIE.
Open Access
Band gap and optical transmission in the Fibonacci type one-dimensional A5B6C7 based photonic crystals
(Wiley-VCH Verlag, 2015) Simsek S.; Koc, H.; Palaz S.; Oltulu, O.; Mamedov, A. M.; Özbay, Ekmel
In this work, we present an investigation of the optical properties and band structure calculations for the photonic crystal structures (PCs) based on one-dimensional (1D) photonic crystal. Here we use 1D A5B6C7(A:Sb; B:S,Se; C:I) based layers in air background. We have theoretically calculated photonic band structure and optical properties of A5B6C7(A:Sb; B:S,Se; C:I) based PCs. In our simulation, we employed the finite-difference time domain (FDTD) technique and the plane wave expansion method (PWE) which implies the solution of Maxwell equations with centered finite-difference expressions for the space and time derivatives. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Band gap structure of elliptic rods in water for a 2D phononic crystal
(Springer Verlag, 2017) Oltulu, O.; Mamedov, A. M.; Özbay, Ekmel
The propagation of acoustic waves in two-dimensional sonic crystals (SC) is studied theoretically. Effects of elliptical rod orientations on the acoustic band gaps in periodic arrays of rigid solid rods embedded in a polar liquid are investigated. We have found that the pass bands and forbidden bands of the sonic crystals can be changed by utilizing the rotational anisotropy of the structure factor at different rotation angles of the scatterers. The plane wave expansion (PWE) method is used to calculate the band structure. The variation of the absolute band gap was also investigated as a function of any filling fraction at a fixed orientation of the elliptical columns. The gap-tuning effect can be controlled by the rotational asymmetry and eccentricity of the scatterers.
Open Access
Band Structure and Optical Properties of Kesterite Type Compounds: First principle calculations
(Institute of Physics Publishing, 2017) Palaz S.; Unver H.; Ugur G.; Mamedov, Amirullah; Özbay, Ekmel
In present work, our research is mainly focused on the electronic structures, optical and magnetic properties of Cu2FeSnZ4 (Z = S, Se) compounds by using ab initio calculations within the generalized gradient approximation (GGA). The calculations are performed by using the Vienna ab-initio simulation package (VASP) based on the density functional theory. The band structure of the Cu2FeSnZ4 ( Z = S, Se) compounds for majority spin (spin-up) and minority spin (spin-down) were calculated. It is seen that for these compounds, the majority spin states cross the Fermi level and thus have the metallic character, while the minority spin states open the band gaps around the Fermi level and thus have the narrow-band semiconducting nature. For better understanding of the electronic states, the total and partial density of states were calculated, too. The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant for Cu2FeSnZ4 (Z = S, Se) compounds were also calculated. © Published under licence by IOP Publishing Ltd.
Open Access
Bio-insprired optoelectronic digital nose for breath analysis
(2011) Bayındır, Mehmet; Yıldırım, Adem; Yaman, Mecit; Vural, Mert
A novel electronic nose device is presented that can be used in disease diagnostics by exhaled breath analysis. Exhaled breath contains more than a thousand organic compounds that can be analysed to insect various diseases and metabolic activity. The novel device is an electronic nose, based on photonic bandgap fibers that can selectively guide infrared radition inside a hollow core plastic fiber. Instead of a laser line source, a broadband balackbody source is used that exploits the filtering/ guiding properties of the fibers to scan the whole mid-infrared region, making it high selectivity of volatile organic compounds possible. In addition waveguiding inside the fiber enhances the electromagnetic radiation intensity, resulting in improved infrared absorption cross-section. The fiber electronic nose can be integrated and deployed as a portable electronics device to point-of-care institutes.
Open Access
Confined optical phonon effects on the band gap renormalization in quantum wire structures
(Elsevier Science, 1999) Bennett, C. R.; Güven, K.; Tanatar, Bilal
We consider the different approximations for the bandgap renormalization (BGR) within the random phase approximation (RPA), the quasi-static limit and the plasmon-pole approximation, and compare with the full result. We then include bulk optical phonons and also the phonon confinement using the phonons from the dielectric continuum (DC) model. We show that the results are very similar except at low densities where the quasi-static results overestimate the renormalization.
Open Access
Coupled optical microcavities in one-dimensional photonic bandgap structures
(Institute of Physics Publishing, 2001) Bayındır, Mehmet; Kural, C.; Özbay, Ekmel
We present a detailed theoretical and experimental study of the evanescent coupled optical microcavity modes in one-dimensional photonic bandgap structures. The coupled-cavity samples are fabricated by depositing alternating hydrogenated amorphous silicon nitride and silicon oxide layers. Splitting of the eigenmodes and formation of a defect band due to interaction between the neighbouring localized cavity modes are experimentally observed. Corresponding field patterns and the transmission spectra are obtained by using transfer matrix method (TMM) simulations. A theoretical model based on the classical wave analogue of the tight-binding (TB) picture is developed and applied to these structures. Experimental results are in good agreement with the predictions of the TB approximation and the TMM simulations.
Open Access
Density functional theory investigation of substituent effects on building blocks of conducting polymers
(Elsevier, 1999) Salzner, U.
Substituted heterocyclic dimers were calculated employing density functional theory (DFT) and analyzed with the natural bond orbits method (NBO). Substitution in 3- and 4-positions leads to parallel shifting of HOMO and LUMO but does not reduce energy gaps. For bridge dimers, HOMO-LUMO gaps correlate with π-electron densities in the carbon backbone and energy gap reduction correlate with the strength of π-π* interactions from the backbone to the bridging group. Alternating donor-acceptor groups do not reduce energy gaps and lead to systems with average HOMO and LUMO levels compared to the parent molecules.
Open Access
Dynamic nonlinear optical processes in some oxygen-octahedra ferroelectrics: first principle calculations
(Taylor & Francis Inc., 2015) Simsek S.; Koc, H.; Palaz S.; Oltulu, O.; Mamedov, A. M.; Özbay, Ekmel
The nonlinear optical properties and electro-optic effects of some oxygen-octahedra ferroelectrics are studied by the density functional theory (DFT) in the local density approximation (LDA) expressions based on first principle calculations without the scissor approximation. We present calculations of the frequency- dependent complex dielectric function and the second harmonic generation response coefficient over a large frequency range in tetragonal and rhombohedral phases. The electronic linear electro-optic susceptibility is also evaluated below the band gap. These results are based on a series of the LDA calculation using DFT. The results for are in agreement with the experiment below the band gap and those for are compared with the experimental data where available. © 2015 Taylor & Francis Group, LLC.
Open Access
Effect of chalcogens on electronic and photophysical properties of vinylene-based diketopyrrolopyrrole copolymers
(American Chemical Society, 2015) Dhar, J.; Mukhopadhay, T.; Yaacobi-Gross, N.; Anthopoulos, T. D.; Salzner, U.; Swaraj, S.; Patil, S.
Three vinylene linked diketopyrrolopyrrole based donor−acceptor (D−A) copolymers have been synthesized with phenyl, thienyl, and selenyl units as donors. Optical and electronic properties were investigated with UV−vis absorption spectroscopy, cyclic voltammetry, near edge X-ray absorption spectroscopy, organic field effect transistor (OFET) measurements, and density functional theory (DFT) calculations. Optical and electrochemical band gaps decrease in the order phenyl, thienyl, and selenyl. Only phenyl-based polymers are nonplanar, but the main contributor to the larger band gap is electronic, not structural effects. Thienyl and selenyl polymers exhibit ambipolar charge transport but with higher hole than electron mobility. Experimental and theoretical results predict the selenyl system to have the best transport properties, but OFET measurements prove the thienyl system to be superior with p-channel mobility as high as 0.1 cm2 V−1 s −1.
Open Access
Effect of reactor pressure on optical and electrical properties of InN films grown by high-pressure chemical vapor deposition
(Wiley - V C H Verlag GmbH & Co. KGaA, 2015) Alevli, M.; Gungor, N.; Alkis, S.; Ozgit Akgun, C.; Donmez, I.; Okyay, Ali Kemal; Gamage, S.; Senevirathna, I.; Dietz, N.; Bıyıklı, Necmi
The influences of reactor pressure on the stoichiometry, free carrier concentration, IR and Hall determined mobility, effective optical band edge, and optical phonon modes of HPCVD grown InN films have been analysed and are reported. The In 3d, and N 1s XPS spectra results revealed In-N and N-In bonding states as well as small concentrations of In-O and N-O bonds, respectively in all samples. InN layers grown at 1 bar were found to contain metallic indium, suggesting that the incorporation of nitrogen into the InN crystal structure was not efficient. The free carrier concentrations, as determined by Hall measurements, were found to decrease with increasing reactor pressure from 1.61×1021 to 8.87×1019 cm-3 and the room-temperature Hall mobility increased with reactor pressure from 21.01 to 155.18 cm2/Vs at 1 and 15 bar reactor pressures, respectively. IR reflectance spectra of all three (1, 8, and 15 bar) InN samples were modelled assuming two distinct layers of InN, having different free carrier concentration, IR mobility, and effective dielectric function values, related to a nucleation/interfacial region at the InN/sapphire, followed by a bulk InN layer. The effective optical band gap has been found to decrease from 1.19 to 0.95 eV with increasing reactor pressure. Improvement of the local structural quality with increasing reactor pressure has been further confirmed by Raman spectroscopy measurements. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Effects of perfluorination on thiophene and pyrrole oligomers
(2010) Salzner, U.
The effect of perfluorination on thiophene and pyrrole oligomers in neutral, cationic, and anionic states was investigated with density functional theory at the (TD)B3P86-30%/6-31G* level. For the title compounds fluorination leads to planarization. For pyrroles a band gap reduction of 0.58 eV results, as unsubstituted pyrroles are nonplanar and disordered in the solid state. For thiophene the band gap is slightly increased as long thiophene oligomers are almost planar. Ionization energies and electron affinities increase upon fluorination by 0.65 and 0.60 eV for polythiophene and by 0.45 and 0.90 eV for polypyrrole. Conduction band widths increase by 0.5 for polythiophene and by 0.7 eV for polypyrrole. Spectra of charged (doped) forms are almost identical to those of the parent systems. Like parent systems, fluorinated oligomers with chain lengths of more than six rings develop a third UV absorption that increases in strength and decreases in energy upon chain length increase.
Open Access
Electronic structure of half-metallic ferromagnet Co2MnSi at high-pressure
(Springer New York LLC, 2010) Gökoǧlu, G.; Gülseren, O.
In this study, first principles calculation results of the half-metallic ferromagnetic Heusler compound Co2MnSi are presented. All calculations are based on the spin-polarized generalized gradient approximation (σ-GGA) of the density functional theory and ultrasoft pseudopotentials with plane wave basis. Electronic structure of related compound in cubic L21 structure is investigated up to 95 GPa uniform hydrostatic pressure. The half-metal to metal transition was observed around ∼70 GPa together with downward shift of the conduction band minimum (CBM) and a linear increase of direct band gap of minority spins at Γ-point with increasing pressure. The electronic density of states of minority spins at Fermi level, which are mainly due to the cobalt atoms, become remarkable with increasing pressure resulting a sharp decrease in spin polarization ratio. It can be stated that the pressure affects minority spin states rather than that of majority spins and lead to a slight reconstruction of minority spin states which lie below the Fermi level. In particular, energy band gap of minority spin states in equilibrium structure is obviously not destroyed, but the Fermi level is shifted outside the gap.
Open Access
Enhancement of optical switching parameter and third-order optical nonlinearities in embedded Si nanocrystals: A theoretical assessment
(Elsevier, 2008) Yildirim, H.; Bulutay, C.
Third-order bound-charge electronic nonlinearities of Si nanocrystals (NCs) embedded in a wide band-gap matrix representing silica are theoretically studied using an atomistic pseudopotential approach. Nonlinear refractive index, two-photon absorption and optical switching parameter are examined from small clusters to NCs up to a size of 3 nm. Compared to bulk values, Si NCs show higher third-order optical nonlinearities and much wider two-photon absorption-free energy gap which gives rise to enhancement in the optical switching parameter.
Open Access
Experimental demonstration of highly confined photonic crystal based waveguides
(IEEE, 2001) Bayındır, Mehmet; Özbay, Ekmel; Temelkuran, B.; Sigalas, M. M.; Soukoulis, C. M.; Biswas, R.; Ho, K. M.
The bending and guiding of the electromagnetic (EM) waves in highly confined waveguides was demonstrated. The electromagnetic waves were constructed by removing a single rod from a perfect three layer-by-layer photonic crystals. A layer-by-layer dielectric photonic crystal based on square shaped alumina rods was used with center-to-center separation of 1.12 cm. The results suggested the use of the layer-by-layer photonic crystal structure in the design of optoelectronic integrated circuits.
Open Access
Experimental investigation of layer-by-layer metallic photonic crystals
(Institution of Electrical Engineers, 1998-12) Temelkuran, B.; Altug, H.; Özbay, Ekmel
The authors have investigated the transmission properties and defect characteristics of layer-by-layer metallic photonic crystals. They have demonstrated experimentally that the metallicity gap of these crystals extends to an upper band-edge frequency, and no lower edge was detected down to 2 GHz. The defect structures built around these crystals exhibited high transmission peak amplitudes (100%) and high Q factors (2250). The crystals with low filling ratios (around 1-2%) were tested and were still found to possess metallic photonic crystal properties. These crystals exhibited high reflection rates within the metallicity gap and reasonable defect mode characteristics. A power enhancement factor of 190 was measured for the electromagnetic (EM) wave within planar cavity structures, by placing a monopole antenna inside the defect volume. These measurements show that detectors embedded inside a metallic photonic crystal can be used as frequency selective resonant cavity enhanced (RCE) detectors with increased sensitivity and efficiency when compared to conventional detectors.
Open Access
Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis
(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, Salim
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 predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).