• H S Bhatti

      Articles written in Pramana – Journal of Physics

    • Laser-induced down-conversion parameters of singly and doubly doped ZnS phosphors

      H S Bhatti Rajesh Sharma N K Verma

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      Singly and doubly doped ZnS phosphors have been synthesized using flux method. Laser-induced photoluminescence has been observed in ZnS-doped phosphors when these were excited by the pulsed UV N2 laser radiation. Due to down-conversion phenomenon, fast phosphorescence emission in the visible region is recorded in milliseconds time domain for ZnS:Mn while in the case of ZnS:Mn:killer (Fe, Co and Ni) the lifetime reduces to microseconds time domain. Experimentally observed luminescent emission parameters of excited states such as, lifetimes, trap-depth values and decay constants have been reported here at room temperature. The high efficiency and fast recombination times observed in doped ZnS phosphors make these materials very attractive for optoelectronic applications.

    • Phonon dispersions in graphene sheet and single-walled carbon nanotubes

      Dinesh Kumar Veena Verma H S Bhatti Keya Dharamvir

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      In the present research paper, phonons in graphene sheet have been calculated by constructing a dynamical matrix using the force constants derived from the second-generation reactive empirical bond order potential by Brenner and co-workers. Our results are comparable to inelastic X-ray scattering as well as first principle calculations. At $\Gamma$ point, for graphene, the optical modes (degenerate) lie near 1685 cm$^{−1}$. The frequency regimes are easily distinguishable. The lowfrequency ($\omega \to 0$) modes are derived from acoustic branches of the sheet. The radial modes can be identified with $\omega \to 584$ cm$^{−1}$. High-frequency regime is above 1200 cm$^{−1}$ (i.e. ZO mode) and consists of TO and LO modes. The phonons in a nanotube can be derived from zone folding method using phonons of a single layer of the hexagonal sheet. The present work aims to explore the agreement between theory and experiment. A better knowledge of the phonon dispersion of graphene is highly desirable to model and understand the properties of carbon nanotubes. The development and production of carbon nanotubes (CNTs) for possible applications need reliable and quick analytical characterization. Our results may serve as an accurate tool for the spectroscopic determination of the tube radii and chiralities.

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