Browsing by Subject "Band gap"

Now showing 1 - 8 of 8

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
Does the donor-acceptor concept work for designing synthetic metals? III. theoretical investigation of copolymers between quinoid acceptors and aromatic donors
(2006) Salzner, U.; Karaltı, O.; Durdaǧi, S.
Homopolymers of quinoxaline (QX), benzothiadiazole (BT), benzobisthiadiazole (BBT), thienopyrazine (TP), thienothiadiazole (TT), and thienopyrazinothiadiazole (TTP) and copolymers of these acceptors with thiophene (TH) and pyrrole (PY) were investigated with density functional theory. Theoretical band-gap predictions reproduce experimental data well. For all but six copolymers, band-gap reductions with respect to either homopolymer are obtained. Four of the acceptors, BBT, TP, TT, and TTP, give rise to copolymers with band gaps that are smaller than that of polyacetylene. BBT and TTP copolymers with PY in 1:2 stoichiometry are predicted to be synthetic metals. Band-gap reductions result from upshifts of HOMO energies and much smaller upshifts of LUMO values. The smallest band gaps are predicted with TTP, since changes in LUMO energies upon copolymerization are particularly small. The consequence of the small interactions between LUMO levels of donor and acceptor are vanishingly small conduction bandwidths. © Springer-Verlag 2006.
Open Access
Dynamics of sliding and loading bilayer graphene
(2022-09) Edun, Benjamin
Sliding and loading bilayer graphene is investigated using the Tight Binding method. We develop interaction-distance dependent tight binding parameter functions to allow for the calculation of band structure for different interacting distances. We investigate the band structures of sliding graphene and loading bilayer graphene, in which case the latter consists of varying interlayer distances between monolayers. We show that based on developed parameter models, band splittings can be seen to emerge in the band structures, which follow different patterns for different sliding directions. As expected we con rm that for varying vertical interlayer distances, monolayer graphene band structure is the limit for both AA and AB stacking con gurations. By applying a quadratic energy model to the curvature of the band structures in the vicinity of the K-point for AB stacking con guration we predict the effective mass of electrons and holes in bilayer graphene, and electrons in monolayer graphene. We also show the pattern of change of effective mass with respect to changing interlayer distance, and try to investigate where our prescribed quadractic energy model breaks down, as we approach the limit of the monolayer band structure.
Open Access
Effect of coumarin concentration on the physical properties of CdO nanostructures
(Elsevier, 2014-05) Sahin, B.; Bayansal, F.; Yuksel, M.; Bıyıklı, Necmi; Cetinkara, H. A.
In this work, nanostructured CdO films with different coumarin contents in the growth solution were fabricated on glass substrates by the SILAR method. The effects of coumarin content in the bath on optical, structural and morphological properties were studied by means of (UV–vis) spectrophotometer, SEM and XRD analysis. The analysis showed that the band gaps, surface morphologies and XRD peak intensities of the CdO films were found to change with coumarin content. A change in the band gap energy can be attributed to the improvement in crystallinity of the samples. XRD analysis showed that, the films have poly-crystalline structures with decent crystallinity levels.
Open Access
Investigation of the effect of donor-acceptor substitution on band gap, band width, and conductivity
(Elsevier, 2001) Salzner, U.
Polymers of two donor-acceptor systems, 3-cyano,3′-hydroxybithiophene 1 and 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b′],3,4-ethylenedioxythiophene (CDM-EDOT) 2, were analyzed with density functional theory. As predicted by perturbation theory, interactions between donors and acceptors with very different energy levels are greatly reduced compared to those between fragments with similar energy levels. This leads to localized states and bands with little dispersion. For poly-1 these localized states lie below the valence band and above the conduction band. For 2 localized unoccupied levels lie within the band gap. These acceptor levels account for the high electron affinity of poly-2 and allow for self-doping. Self doping explains the increased intrinsic conductivity of poly-2, the localized nature of the low lying MOs rationalizes the low mobility of n-type carriers in poly-2.
Open Access
Plasmonic band gap cavities
(2008) Kocabaş, Aşkın
Surface plasmon polaritons (SPP’s) are trapped electromagnetic waves coupled to free electrons in metals that propagate at the metal-dielectric interfaces. Due to their surface confinement and potential in sub-wavelength optics, SPP’s have been extensively studied for sensing and nanophotonic applications. Dielectric structures and metallic surfaces, both periodically modulated, can form photonic band gaps. Creating a defect cavity region in the periodicity of dielectrics allows specific optical modes to localize inside a cavity region. However, despite the demonstration of numerous plasmonic surfaces and unlike its dielectric counterparts, low index modulation in metallic surfaces limits the formation of plasmonic defect cavity structures. This thesis describes new approaches for plasmonic confinement in a cavity through the use of selective loading of grating structures as well as through the use of Moiré surfaces. In our first approach, we demonstrate that a high dielectric superstructure can perturb the optical properties of propagating SPPs dramatically and enable the formation of a plasmonic band gap cavity. Formation of the cavity is confirmed by the observation of a cavity mode in the band gap both in the infrared and the visible wavelengths. In addition to the confinement of SPP’s in the vertical direction, such a cavity localizes the SPP’s in their propagation direction. Additionally, we have demonstrated that such biharmonic grating structures can be used to enhance Raman scattering and photoluminescence (PL). Using biharmonic grating structure 105 times enhancement in Raman signal and 30 times enhancement in PL were measured. Furthermore, we show that metallic Moiré surfaces can also serve as a basis for plasmonic cavities with relatively high quality factors. We have demonstrated localization and slow propagation of surface plasmons on metallic Moiré surfaces. Phase shift at the node of the Moiré surface localizes the propagating surface plasmons in a cavity and adjacent nodes form weakly coupled plasmonic cavities. We demonstrate group velocities around v = 0.44c at the center of the coupled cavity band and almost zero group velocity at the band edges can be achieved. Furthermore, sinusoidally modified amplitude about the node suppresses the radiation losses and reveals a relatively high quality factor for plasmonic cavities.
Open Access
Selenium in diketopyrrolopyrrole-based polymers: influence on electronic properties and charge carrier mobilities
(Wiley-VCH Verlag, 2014) Dhar, J.; Kanimozhi, C.; Yaccobi-Gross, N.; Anthopoulos, T. D.; Salzner, U.; Patil, S.
Diketopyrrolopyrrole (DPP)-based p-conjugated copolymers with thiophene have exceptionally high electron mobilities. This paper investigates electronic properties and charge carrier mobilities of selenophene containing analogues. Two new copolymers, with alternating thiophene DPP (TDPP) and selenophene DPP (SeDPP) units, were synthesized. Two side-chains, hexyl (Hex) and triethylene glycol (TEG) were employed, yielding polymers designated as PTDPPSeDPP-Hex and PTDPPSeDPP-TEG. Selenophene systems have smaller band gaps, with concomitant enhancement of the stability of the reduced state. For both polymers, ambipolar mobilities were observed in organic fieldeffect transistors (OFET). Grazing incidence X-ray diffraction (GIXD) data indicates preferential edge-on orientation of PTDPPSeDPP-TEG, which leads to superior charge transport properties of the TEG substituted polymer, as compared to its Hex analogue. Time-dependent-density functional theory (TDDFT) calculations corroborate the decrease in the optical band gap with the inclusion of selenophene. Ambipolar charge transport is rationalized by exceptionally wide conduction bands. DSCF calculations confirm the larger electron affinity, and therefore the greater stability, of the reduced form of the selenophene-containing DPP polymer in presence of chloroform.
Open Access
Theoretical analysis of poly(difluoroacetylene) PDFA
(Elsevier, 2003) Salzner, U.
Due to the π-donating ability of fluorine, fluorosubstitution has a remarkable effect on the band structure of polyacetylene (PA). Valence and conduction band edges decrease in energy, leading to narrower valence and wider conduction bands. Ionization potential and electron affinity of PDFA are predicted to be about 1.5 eV higher than those of PA. This indicates that PDFA is an excellent candidate for an n-type conductor. PDFA tends to adopt non-planar structures but the energy of planarization is only 5.44 kcal/mol. Alternating difluoroethylene and ethylene units yield planar polymers with decreased band gaps. Ionization potential and electron affinity of the mixed polymer are between those of the homopolymers. © 2003 Elsevier Science B.V. All rights reserved.