Biomedical subjects

Sohrab Ismail-Beigi

Publications and source records attributed to Sohrab Ismail-Beigi.

5 recordsLinked to original sources

PubMed2005

Theory and ab initio calculation of radiative lifetime of excitons in semiconducting carbon nanotubes.

We present a theoretical analysis and first-principles calculation of the radiative lifetime of excitons in semiconducting carbon nanotubes. An intrinsic lifetime of the order of 10 ps is computed for the lowest optically active bright excitons. The intrinsic lifetime is, however, a rapid increasing function of the exciton momentum. Moreover, the electronic structure of the nanotubes dictates the existence of dark excitons near in energy to each bright exciton. Both effects strongly influence measured lifetime. Assuming a thermal occupation of bright and dark exciton bands, we find an effective lifetime of the order of 10 ns at room temperature, in good accord with recent experiments.

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PubMed2005

Self-trapped excitons in silicon dioxide: mechanism and properties.

Irradiating silica produces self-trapped excitons (STEs) that spontaneously create atomic-scale distortions on which they localize themselves. Despite enduring interest in STEs and subsequent defects in this key technological material, the trapping mechanism and geometry remain a mystery. Our ab initio study of STEs in alpha-quartz using a many-electron Green's function approach answers both questions. The STE comprises a broken O-Si bond with the hole localized on the defected oxygen and the electron on the defected silicon atom in a planar sp2 conformation. The results further explain quantitatively the measured STE spectra.

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PubMed2005

Photoisomerization of azobenzene from first-principles constrained density-functional calculations.

Despite considerable work in the field, the precise mechanism for the photoisomerization of azobenzene, C(12)H(10)N(2), is still an open issue. Early theoretical studies of the problem indicated that isomerization occurs through an in-plane inversion path, and this has been used to explain recent time-resolved UV-visible spectroscopy measurements. On the other hand, a number of recent theoretical studies have concluded that a torsion of the N-N bond ("rotation path") is probably the most favorable mechanism for photoisomerization involving the first excited state. We have performed first-principles calculations using constrained density-functional theory (DFT) and time-dependent DFT in the local-density approximation, with results that also favor the rotation path mechanism. Our results are compared with other analyses, primarily based on configuration interaction.

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PubMed2004

Excitonic effects and optical spectra of single-walled carbon nanotubes.

Many-electron effects often dramatically modify the properties of reduced dimensional systems. We report calculations, based on an ab initio many-electron Green's function approach, of electron-hole interaction effects on the optical spectra of small-diameter single-walled carbon nanotubes. Excitonic effects qualitatively alter the optical spectra of both semiconducting and metallic tubes. Excitons are bound by approximately 1 eV in the semiconducting (8,0) tube and by approximately 100 meV in the metallic (3,3) tube. These large many-electron effects explain the discrepancies between previous theories and experiments.

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PubMed2003

Excited-state forces within a first-principles Green's function formalism.

We present a new first-principles formalism for calculating forces for optically excited electronic states using the interacting Green's function approach with the GW Bethe-Salpeter-equation method. This advance allows for efficient computation of gradients of the excited-state Born-Oppenheimer energy, allowing for the study of relaxation, molecular dynamics, and photoluminescence of excited states. The approach is tested on photoexcited carbon dioxide and ammonia molecules, and the calculations accurately describe the excitation energies and photoinduced structural deformations.

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