Showing papers by "Pushan Ayyub published in 2011"

Highly enhanced hard x-ray emission from oriented metal nanorod arrays excited by intense femtosecond laser pulses

Abstract: We report a 43-fold enhancement in the hard x-ray emission (in the 150--300 keV range) from copper nanorod arrays (compared to a polished Cu surface) when excited by 30-fs, 800-nm laser pulses with an intensity of ${10}^{16}$ W/cm${}^{2}$. The temperature of the hot electrons that emit the x rays is 11 times higher. Significantly, the x-ray yield enhancement is found to depend on both the aspect ratio as well as the cluster size of the nanorods. We show that the higher yield arises from enhanced laser absorption owing to the extremely high local electric fields around the nanorod tips. Particle-in-cell plasma simulations reproduce these observations and provide pointers to further optimization of the x-ray emission.

Role of annealing conditions on the ferromagnetic and dielectric properties of La2NiMnO6

Abstract: La2NiMnO6 (LNMO) was prepared by a combustion method followed by heating at high temperature. Subsequently, the preformed LNMO was annealed in air, oxygen, or N2 atmosphere and characterized by powder x-ray diffraction (XRD), neutron diffraction, superconducting quantum interference device magnetometry, and dielectric analysis. Structural studies by XRD and neutron diffraction revealed the coexistence of partially cation disordered monoclinic (31%) and rhombohedral (69%) phases in the sample annealed in air. However, the sample annealed in oxygen shows about 50:50% of monoclinic and rhombohedral phases. Relaxor-like behavior with relative permittivity of the order of 104 was observed in the sample annealed in air, while relative permittivity decreases to about 200 in samples annealed in oxygen atmosphere. The magnetic properties indicate a well-defined ferromagnetic phase in the oxygen-annealed sample compared to a feeble ferromagnetic signature in the air-annealed one. The dielectric and ferromagnetism of LNMO samples have been related to formation and annihilation of oxygen vacancies.

Novel hexagonal polytypes of silver: growth, characterization and first-principles calculations

Abstract: We report a study of the relative effects of dimensional and kinetic constraints on the stabilization of metastable, polytypic forms of metallic silver. We show that the hexagonal 4H polytype (hitherto observed only in size-constrained systems) can be produced in the form of bulk thin films by suitably slowing down the growth kinetics. Further, using extremely slow growth conditions, we have been successful in depositing a novel, two-dimensional, metastable polytype (2H) of silver, which is highly reactive (easily oxidized) and has a density 23% lower than normal silver. First-principles calculations based on density functional theory confirm that the 4H structure is relatively stable. However, local stability analysis via a determination of the phonon dispersion of the 2H structure reveals that it is only marginally stable with an energy surface that is rather flat or weakly varying with respect to many of the modes. This makes a large contribution to the configurational entropy and is probably the reason for the metastability of the observed 2H polytype with an unusually large lattice constant along the c-direction.

Conjugation of cytochrome c with hydrogen titanate nanotubes: novel conformational state with implications for apoptosis

Abstract: We show that hydrogen titanate (H2Ti3O7) nanotubes form strongly associated reversible nano-bio-conjugates with the vital respiratory protein, cytochrome?c. Resonance Raman spectroscopy along with direct electrochemical studies indicate that in this nano-bio-conjugate, cytochrome?c exists in an equilibrium of two conformational states with distinctly different formal redox potentials and coordination geometries of the heme center. The nanotube-conjugated cytochrome?c also showed enhanced peroxidase activity similar to the membrane-bound protein that is believed to be an apoptosis initiator. This suggests that such a nanotube?cytochrome?c conjugate may be a good candidate for cancer therapy applications.

Field emission from hydrogen titanate nanotubes

Abstract: Hydrothermally synthesized hydrogen titanate (H2Ti3O7) nanotube meshes and arrays exhibit excellent field emission characteristics. The turn-on field is as low as 1.4 V μm−1 for the mesh and 2.6 V μm−1 for the array, while the electric field corresponding to an emission current density of 10 μA cm−2 is 2 V μm−1 (mesh) and 3.8 V μm−1 (array). The H2Ti3O7 nanotube mesh has one of the lowest reported turn-on voltages and highest enhancement factors. The emission current also shows good long term stability. We attribute the efficient field emission to the presence of mid-gap states arising from the negative surface charge on the H2Ti3O7 nanotubes.

Growth of aligned ZnO nanorod arrays from an aqueous solution: effect of additives and substrates.

Abstract: We report a simple, versatile, low cost fabrication technique for synthesizing nanorod arrays whose architecture is suited for many applications spanning the nanometer to micrometer range. Specifically, we have covered the range of nanorod diameter from 50 to 1200 nm. From a detailed study of the growth parameters involved in the synthesis of the ZnO nanorod arrays from an aqueous solution, we report, in particular, the effects of varying the capping agent, substrate and substrate-seeding. We find that seeding the substrate and selecting the appropriate capping agent play the most crucial roles in the alignment of nanorod arrays. Our study on the use of different precursor materials and varied substrates for the growth of ZnO nanorod arrays should lead to an enhanced understanding of the controllable growth of ZnO crystals and nanostructures.