Browsing by Subject "Boron compounds"

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    Isotope Effect and Phonon Softening in Superconducting Borocarbides and Boronitrides
    (1998) Hakioǧlu, Tuğrul; Ivanov, V. A.
    The isotope effect in the recently disvovered class o superconductors LuNi2B2C and La3Ni2B2N3 is investigated in the context of electron-squezed phonon interaction renormalizing the Ni-d electron-electron correlations. Squeezed phonon mode originates from the anharmonic character of the tetragonal Ni-B structure and is polarized in the vortical direction to the Ni layers. The isotope effect arises as a result of the zero point motion of the Ni-Ni d-electron hopping amplitude dominantly due to this vertical phonon mode. Within this model the isotope exponent is calculated to be αB ≤ 0.20 as compared to the recently found experimental value αexp B=0.27∓0.10. Finally, the phonon frequency softening predicted by our model eletron-phonon interaction is discussed within the context of recent experiments on the relevant boron A1g softening.
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    Phenylethynyl-BODIPY oligomers: bright dyes and fluorescent building blocks
    (2009) Cakmak, Y.; Akkaya, E. U.
    Boradiazaindacene dyes were converted into phenylethynyl-BODIPY oligomers via a cycle of reactions, notably including Sonogashira couplings. As expected, as the number, n, of repeating units increases, peak absorption and emission wavelengths are shifted to the red end of the visible spectrum, albeit with smaller increments as n increases. Decyl groups help to keep the solubility remarkably high, and in addition to being very bright red-emitting fluorophores, their rigid rod-like structures could allow their use as functional building blocks. © 2009 American Chemical Society.
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    Phonon squeezing in superconducting borocarbides
    (Elsevier BV * North-Holland, 1994) Hakioglu, T.; Ivanov, V. A.; Shumovsky, A. S.; Tanatar, Bilal
    The recently discovered superconductor LuNi2B2C is investigated in the context of strong electron-electron correlations modulated by a squeezed phonon mode propagating in the perpendicular direction to the layers with longitudinal polarization. The squeezed phonons arise in the anharmonic lattice expansion since the linear electron-phonon interaction vanishes due to the structure of the NiB4 tetrahedra. The observed weak isotope effect and small dependence of Tc on pressure is qualitatively understood within the framework of this model. © 1994.
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    A sensitive and selective ratiometric near IR fluorescent probe for zinc ions based on the distyryl-bodipy fluorophore
    (2008) Atilgan, S.; Ozdemir, T.; Akkaya, E. U.
    (Chemical Equation Presented) A novel distyryl-substituted boradiazaindacene (bodipy) dye with an emission peak moving hypsochromically from 730 to 680 nm on Zn(II) ion binding seems to be promising as one of the very few water-soluble fluorescent chemosensors emitting in the near IR region. © 2008 American Chemical Society.
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    Synthesis of symmetrical multichromophoric bodipy dyes and their facile transformation into energy transfer cassettes
    (2010) Bozdemir, O. A.; Cakmak, Y.; Sozmen F.; Ozdemir, T.; Siemiarczuk, A.; Akkaya, E. U.
    Multichromophoric boron-dipyrromethene (Bodipy) dyes synthesized on phenylene-ethynylene platforms have been be converted to energy transfer cassettes in a one-step chemical transformation. Excitation energy transfer processes in these highly symmetrical derivatives were studied in detail, including time-re-solved fluorescence spectroscopy techniques. Excitation spectra and the emission lifetimes suggest efficient energy transfer between the donor and acceptor chromophore. These novel energy transfer cassettes, while highlighting a short-cut approach to similar energy transfer systems, could be useful as large pseudo-Stokes shift multichromophoric dyes with potential applications in diverse applications.
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    Tetrastyryl-bodipy dyes: convenient synthesis and characterization of elusive near IR fluorophores
    (2009) Buyukcakir, O.; Bozdemir, O. A.; Kolemen, S.; Erbas, S.; Akkaya, E. U.
    1,3,5,7-Tetramethyl-Bodipy derivatives undergo Knoevenagel-type condensations with aromatic aldehydes to ultimately yield tetrastyryl-Bodipy derivatives. The resulting dyes absorb and emit strongly In the near IR. As the versatility of the Bodipy dyes are fully appreciated, these new tetrastyryl dyes are likely to be featured In a variety of functional supramolecular systems. © 2009 American Chemical Society.
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    Theoretical and spectroscopic investigations on the structure and bonding in B-C-N thin films
    (2009) Bengu, E.; Genisel, M. F.; Gulseren, O.; Ovali, R.
    In this study, we have synthesized boron, carbon, and nitrogen containing films using RF sputter deposition. We investigated the effects of deposition parameters on the chemical environment of boron, carbon, and nitrogen atoms inside the films. Techniques used for this purpose were grazing incidence reflectance-Fourier-transform infrared spectroscopy (GIR-FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). GIR-FTIR experiments on the B-C-N films deposited indicated presence of multiple features in the 600 to 1700 cm- 1 range for the infrared (IR) spectra. Analysis of the IR spectra, XPS and the corresponding EELS data from the films has been done in a collective manner. The results from this study suggested even under nitrogen rich synthesis conditions carbon atoms in the B-C-N films prefer to be surrounded by other carbon atoms rather than boron and/or nitrogen. Furthermore, we have observed a similar behavior in the chemistry of B-C-N films deposited with increasing substrate bias conditions. In order to better understand these results, we have compared and evaluated the relative stability of various nearest-neighbor and structural configurations of carbon atoms in a single BN sheet using DFT calculations. These calculations also indicated that structures and configurations that increase the relative amount of C-C bonding with respect to B-C and/or C-N were energetically favorable than otherwise. As a conclusion, carbon tends to phase-segregate in to carbon clusters rather than displaying a homogeneous distribution for the films deposited in this study under the deposition conditions studied.